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THESIS

Z
2000

 

LIBRARY

Michigan State
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This is to certify that the

dissertation entitled

Laser Direct Fabrication of 3-D Components

presented by

Yiping Hu

has been accepted towards fulfillment
of the requirements for

Ph.D. degree in Materials Science

 

 

and Engineering

    

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Date May 4, 2000

 

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moo clam-41059.14

LASER DIRECT FABRICATION OF 3-D COMPONENTS

By

Yiping Hu

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Materials Science and Mechanics

2000

ABSTRACT
LASER DIRECT FABRICATION OF 3-D COMPONENTS
By

Yiping Hu

A novel laser direct deposition of composite materials has been investigated to
manufacture 3—D components. This process allows for either the direct production of an
industrial component or rapid, and low-cost fabrication of tooling dies and injection
molds.

In present research, an integrated powder feeding system is designed for laser
cladding. It can provide a stable, continuous, and accurate powder feeding rate, and
deliver the powder stream coaxially into the molten pool under the laser beam. The
experimental results showed that this powder feeding system is suited for depositing
high quality cladding tracks, along complicated geometric paths, on flat or curved
surfaces. Due to the change of powder feeding mechanism, the characteristic of cone-
shaped powder stream and interaction between the laser beam and the powder stream
have also been investigated.

Although laser cladding is a complicated process, based on reasonable
approximations, a simple but realistic process model, relating the clad thickness and
processing parameters, is successfully developed. The calculated results of this model are

in good agreement with the experimental ones.

A novel process for manufacturing tooling dies, with required geometric patterned
cutting blades, has been successfully developed directly by laser cladding process. Fully
functional prototype of an industrial rotary cutting die, for pilot testing, has been
produced. This new process has been implemented in tooling-die manufacturing industry.
In addition, various geometric laminated 3-D components have been directly produced by
the same process. The laminated composites have relatively smooth surface and good
planner interfaces, are free of pores and cracks, and good metallurgical bonding with the
substrate.

Finally, the measurement of the thermal cycle, and the temperature distributions
for laser cladding process has been made. Within optimized operation window, a wide
range of processing parameters, and various clad materials have been used in laser
cladding experiments. The clad-zone temperatures, under various processing conditions,
lies between 1650 °C and 1800 °C. These results are in good agreement with those
obtained by optical pyrometer and CCD techniques. In addition, in both cases of laser

remelting and cladding processes, the temperature gradients along y-direction have

Gaussian distribution.

Copyright by
Yiping Hu
2000

To my lovely wife Zanping and daughter Dianna.

ACKNOWLEDGMENTS

I wish to thank my dissertation advisor, University Distinguished Professor K.
Mukherjee, for his academic guidance, advice, and financial support throughout the
duration of this research work. I would also like to extend my sincere appreciation to my
dissertation committee members: Professor J. Lucas and Professor K.N. Subramanian of
Department of Materials Science and Mechanics, and Professor S.D. Mahanti of Physics
and Astronomy Department, Michigan State University, for their invaluable advice and
suggestions in conducting this research work.

My sincere gratitude is extended to my parents for their encouragement, and
spiritual support. Special thanks are due to my uncle, Dr. Kuang Y. Chen, who
generously supports me. I would also like to thank Dr. C.W. Chen for his kind assistance

and valuable technical advice. I want to thank Drs. S. Kudapa and J.K. Park for their

friendship and help.

vi

TABLE OF CONTENTS

LIST OF TABLES .................................................................................... x

LIST OF FIGURES ................................................................................. xi

INTRODUCTION ..................................................................................... 1

1. Background and Motivation .............................................................. 1

2. Scope ........................................................................................ 4
CHAPTER 1

REVIEW OF LASER SURFACE TREATMENT PRINCIPLE ............................... 6

1.1 Basic Principle of Laser Operation ...................................................... 6

1.1.1 Radiation Transitions .......................................................... 7

1.1.2 Spontaneous and Stimulated Emission ...................................... 8

1.2 Laser-Beam Mode ........................................................................ 10

1.3 Laser Parameters ......................................................................... 14

1.3.1 Laser-Beam Diameter ........................................................ 14

1.3.2 Laser-Beam Power ............................................................ 15

1.3.3 Power Density ................................................................. 16

1.4 Industrial Lasers .......................................................................... 17

1.4.1 The NszAG Laser .......................................................... 18

1.4.2 The C02 Laser ............................................................... 20

1.5 Laser Beam-Material Interaction ...................................................... 23
CHAPTER 2

LASER SURFACE TREATMENT .............................................................. 25

2.1 Laser Solid—Transformation Hardening ............................................... 25

2.2 Laser Remelting .......................................................................... 27

2.3 Laser Surface Alloying .................................................................. 27

2.4 Laser Dispersing ......................................................................... 28

2.5 Laser Surface Cladding ................................................................. 29

2.5.1 General Description .......................................................... 29

2.5.2 Scope of Laser Cladding ................................................... 29

2.5.3 Processing Parameters ....................................................... 30

2.5.3. 1 Absorptivity ...................................................... 30

2.5.3.2 Traverse Speed ................................................... 32

2.5.3.3 Power Feeding Techniques .................................... 32

2.5.3.4 Shielding Gas .................................................... 33

2.5.4 Advantages of Laser Cladding Process .................................... 34

2.5.5 Limitations of Laser Cladding Process .................................... 34

vii

2.5.6 Applications of Laser Cladding Process ................................... 35

CHAPTER 3
EXPERIMENTAL PROCEDURES ............................................................. 37
3.1 General Description ..................................................................... 37
3.2 Choice of Materials .....................................................................
37
3.2.1 Substrate Materials ............................................................ 38
3.2.2 Clad Materials ................................................................ 39
3.3 Laser Cladding System .................................................................. 44
3.3.1 C02 Laser ...................................................................... 45
3.3.2 Beam Delivery System ...................................................... 45
3.3.3 Powder Feeding System ...................................................... 45
3.3.3.1 Powder Feeder ................................................... 48
3.3.3.2 Powder-Flow Splitter ........................................... 48
4.3.3.3 Coaxial Nozzle ................................................... 48
3.3.4 CAM System .................................................................. 51
3.4 Laser Cladding ........................................................................... 51
3.5 Temperature Measurement ............................................................. 52
3.6 Post-Clad Treatment ..................................................................... 55
3.7 Microstructures and Hardness Measurements ....................................... 55
3.8 Hardness Movement ..................................................................... 56
CHAPTER 4
RESULTS AND DISCUSSION .................................................................. 44
4.1 Characteristics of Cone-Shaped Powder Stream .................................... 44
4.2 Desirable Powder Morphology and Particle-Size Distribution .................... 60
4.3 Effect of Powder Feeding Systems on Clad Quality ................................ 61
4.4 Measurement of Powder Utilization Coefficient .................................... 62
4.5 Relationships between Clad Width and Process Parameters ....................... 66
4.6 Process Modeling ........................................................................ 69
4.6. 1 Assumptions ................................................................... 69
4.6.2 Basic Equations ............................................................... 70
4.6.3 Energy Density Equation .................................................... 71
4.6.4 Clad Thickness-Energy Density Equation ................................. 72
4.7 Relationships Derived from Basic Equation .......................................... 73
4.7.1 Relationship Between Clad Thickness and Traverse Speed ............ 73
4.7.2 Relationship Between Clad Thickness and Powder Feed Rate. . . . . 74
4.8 Correlation Derived from the Energy Density Equation ............................ 75
4.8.1 Relationship between Energy Density and Spot Size .................... 75
4.8.2 Relationship between Energy Density and Traverse Speed ............ 76
4.8.3 Relationship between Energy density and Laser Power ................. 77
4.9 Correlation between Clad Thickness and Energy Density ......................... 79
4.10 Relationship Between Penetration Depth and Energy Density .................... 79

viii

4.8.3 Relationship between Energy density and Laser Power ................. 77

4.9 Correlation between Clad Thickness and Energy Density ......................... 79
4.10 Relationship Between Penetration Depth and Energy Density .................... 79
4.11 Evaluation Standards of Laser Cladding Quality .................................... 81
4.12 Overlapping Rate ........................................................................ 85
4.13 Dilution .................................................................................... 86
4.14 Calculation of Deposition Rate ........................................................ 88
4.15 Geometric Characteristics of Single Cladding Tracks .............................. 89
4.16 Laser Direct Fabrication of 3-D Components ........................................ 92
4.16.1 Tooling Dies ................................................................... 92
4.16.1.1 Fabrication of Tooling Dies .................................. 92
4.16.1.2 Microstructure of Clad Bead ................................. 94
4.16.1.3Microhardness of Clad Bead ...................................... 94
4.16.2 Laminated Metal Matrix Composites (LMMCs) ......................... 99
4.16.2.1 Direct Fabrication of LMMCs ................................. 99
4.16.2.2 Microstructure of the Cross Section of LMMCs .......... 104
4.16.2.3 X—Ray Dot Mapping ........................................... 107
4.16.2.4 Microhardness Measurement of LMMCs .................. 110
4.17 Temperature
Measurement ............................................................ 111
4.17.1 Temperature Distributions without Powder Feeding ................... 111
4.17.2 Temperature Distributions for Laser Cladding Process ................ 114
CHAPTER 5
CONCLUSIONS .................................................................................. 121
BIBLIOGRAPHY ................................................................................. 126

ix

3.1

3.2

3.3

4.1

4.2

LIST OF TABLES

Chemical compositions of substrates and clad materials ............................... 39
Characteristics of the RF exited C02 laser ................................................ 45
CNC machine specifications ................................................................ 51
Processing parameters for producing rotary cutting dies ............................... 93
Processing parameters used for the laser cladding operations ........................... 100

1.1

1.2

1.3

1.4

1.5

1.6

3.1

3.2

3.3

3.4

3.5

3.6

3.7

LIST OF FIGURES

Types of emission processes between two energy levels [13] .......................... 9
Transverse mode patterns in cylindrical and rectangular coordinates [15] ............ 11
Variation of relative intensity and percentage total power with radius for a

Gaussian TEMoo beam [18]. Note that for the vertical line representing e",

the value should read 0.71 instead of 0.59 ................................................ 13
Laser materials interaction energy diagram [21] .......................................... 17
Energy level of Nd: YAG laser [24] ....................................................... 19
Three vibrational modes and various energy levels for CO2 laser [25] ............... 21

An overview of powder morphology. (a) CPM10V tool steel. (b) 304L
stainless steel ................................................................................. 40

Wear resistance of CPM 10V and conventional tool steels at indicted hardness
[80]. CPM 10V* (hardened) is subjected to quenching, which increases its
hardness, and thus the extra wear resistance .............................................. 42

Charpy C-notch impact properties of CPM 10V and conventional tool steels
at indicated hardness [80] .................................................................. 42

A schematic drawing of the laser cladding system [4] ................................... 44

An overview of powder feeding systems. (a) Lateral powder feeding system. (b)

Integrated coaxial powder feeding system ................................................. 47
A schematic drawing of the transverse section of a coaxial nozzle [10] ............... 49
A view of cone-shaped powder stream irradiated by laser beam ........................ 50

xi

3.8

An overview of specimen dimension, thermocouple layout and location [102]. . . .54

4.1 A view of cone-shaped powder stream. (a) Without laser beam. (b) With laser

beam. Process parameters are as follows: laser power is 2.1 kw, laser spot size

2.9 mm, powder feed rate 0.25 g/sec, and cover gas flow rate 0.85

m3/hr ............................................................................................ 58
4.2 Effect of cover gas flow rate on cone~shaped powder stream. (a) Without laser

beam irradiation. (b) With laser beam irradiation. Cover gas flow rate 3.4

m3/hr. Other processing parameters are the same as those of Figure

4.1 ............................................................................................. 59
4.3 A view of circular cladding tracks on a solid cylinder. Cladding track 2 is an

example of poor quality clads due to higher cover gas flow rate ..................... 60
4.4 A view of desirable powder morphology for laser cladding process .................. 61
4.5 Effect of powder feeding systems on cladding quality. (a) Rectangular

cladding tracks with smooth surface. (b) Perpendicular cladding tracks with

rough surface via lateral powder feeding system ........................................ 62
4.6 Plot of powder utilization coefficient vs. traverse speed ................................ 64
4.7 Plot of powder utilization coefficient vs. powder feed rate ............................. 65
4.8 Plot of powder utilization coefficient vs. laser spot size ................................ 65
4.9 Clad width as a function of traverse speed ................................................ 67
4.10 Clad width as a function of energy density ............................................... 68
4.11 Clad width as a function of powder feed rate ............................................ 68
4.12 Parabolic shape of the cross-section of a single cladding track ........................ 70
4.13 Clad thickness as a function of traverse speed ............................................ 74

xii

4.14 Clad thickness as a function of powder feed rate ......................................... 75

4.15 Energy density as a function of reciprocal of spot size .................................. 76
4.16 Energy density as a function of reciprocal of traverse speed ............................ 77
4.17 Energy density as a function of laser power ............................................... 78
4.18 Clad thickness as a function of energy density ........................................... 80
4.19 Penetration depth as a function of energy density ........................................ 80

4.20 Overview of the transverse section of cladding tracks or layers. (a) High
dilution. (b) Optimized condition. (0) Porosity. ((1) Lack of fusion. (e)
Inclusion. (f) Cracking A: Clad. B: Base metal ........................................... 82

4.21 Schematic drawing of the transverse section of multi-track overlapping .............. 86

4.22 Schematic drawing of cladding and intermixing zones for calculating dilution
ratio ........................................................................................... 87

4.23 Optical microstructure of the transverse section of a cladding track, showing a
low intermixing zone ......................................................................... 88

4.24 Schematic drawing of multi-track overlapping to produce a platform layer by
laser cladding process ....................................................................... 89

4.25 Schematic drawing of transverse geometric shapes of single cladding tracks.
(a) Small aspect ratio. (b) Desirable aspact ratio ........................................ 91

4.26 An overview of lack of fusion forming under the following condition: two
adjacent tracks with a small aspect ratio ovrelap. Unetched .......................... 91

4.27 An overview of near-net-shaped cutting blades deposited on a steel roller by
laser cladding process. (a) As-cladded condition. (b) After machining [4] ............ 93

xiii

4.28 Overview of carbides in CPM 10V and D2 alloys. (3) Fine vanadium carbide
uniformly dispersed in the matrix in CPM 10V tool steel. (b) Segregation and
flake morphology of coarse chromium carbides in D2 alloy ........................... 95

4.29 SEM micrograph of clad bead, showing morphology of vanadium carbides ............ 96

4.30 Microhardness profiles of the cladding track. (a) As-clad. (b) As-clad +
tempering. (c) As -clad + tempering + liquid nitrogen treatment .................... 97

4.31 Photos of LMMC samples. (a) Buildup coupon (sample 2). (b) Square

(sample 3).(c) Saddle prototype (sample 4) with smooth surface via laser
cladding ....................................................................................... 101

4.32 A view of LMMC cylinder prototype produced by laser cladding (sample 5).

(a) is a good sample, and (b) and (c) show some forming defects as discussed
in this section ................................................................................. 103

4.33 An overview of the cross section of LMMC samples 6 and 7. A and C are
tool steel CPM10V layers, while B and D are stainless steel layers. E is heat-
affected zone [87] ......................................................................... 105

4.34 Higher magnified optical micrographs of laser cladded stainless steel 304L
layer. (a) Cellular dendrite structure. (b) The same sample, but higher

magnified micrograph shows the precipitated carbides dispersed in the
austenite matrix ............................................................................ 106

4.35 Overview of X-ray dot mapping of sample 6. (a) SEM micrograph of 304
stainless steel-1045 steel interface zone. (b) Chromium distribution in the

inter-diffusion zone. (c) Nickel distribution in the inter-diffusion
zone .......................................................................................... 108

4.36 X-ray dot mapping of sample 6. (a) SEM micrograph of CPMlOV tool steel
and 88304L stainless steel interface zone. (b) Chromium distribution in the
inter-diffusion zone. (c) Nickel distribution. (d) Vanadium distribution .......... 109

4.37 Vickers hardness profile of LMMC sample 6 ........................................... 110

xiv

4.38 A temperature distribution of a laser beam and substrate interaction without
powder feed ................................................................................. 1 13

4.39 Temperature distributions on the surface of the substrate as a function of the
distance from laser beam center along y-direction [102] ............................... 1 13

4.40 Surface temperature at laser beam center as a function of laser power input [102]. 114

4.41 Temperature distributions of laser cladding process under given process
conditions [102] ............................................................................. 1 16

4.42 Temperature gradients along y direction [102] ........................................... 116

4.43 Temperature distribution of laser cladding process under laser beam center.

(a) CPMlOV tool steel. (b) SS304L stainless steel. (c) IN-738LC nickel-base
superalloy. (d) Carbide blend ............................................................. 1 17

4.44 Optical macrograph of the cross section of a single cladding track, and its
processing conditions are given in Figure 5.43(a) [102] .............................. 120

XV

INTRODUCTION

1. Background and Motivation

I have been investigating a novel laser direct deposition of composite materials for
manufacturing 3-D components in the past six (6) years. This process allows for either the
direct production of an industrial component or rapid, and low-cost fabrication of tooling
dies and injection molds. If this brand-new process can be successfully developed, it will
significantly reduce production costs and time in getting a functional prototype or product
ready for use.

Cutting, stamping, and corrugating dies have been widely used in packaging,
automobile, and printing industries, and the like. For example, rotary cutting and scoring
dies are extensively applied in packaging industry to cut and fold cereal and soft beverage
containers, and cut facial tissues, paper money, credit cards, high way tickets, and disk
floppies, and so on. The cutting speed is fast, and impact loads or shear stresses are large,
and thus the service life of rotary cutting dies is usually very short. Typically, such rotary
cutting dies can work for a few months depending on the chemical composition,
hardness, and other physical or chemical properties of the materials to be cut.

In conventional cutting-die manufacturing process, both die body and cutting
blades are made of an expensive tool steel, such as D2 and M2. In order to prevent the
distortion of cutting blades, the entire die body is first quenched to obtain the hardness of
approximately HRC 60, then the hardened body is machined by electrical discharge
machining (EDM), or photochemical etching to produce the cutting blades. Thus, for the

conventional process, the cost of materials is high, the process is time-consuming and

complicated, the production efficiency is very low, and the overall production costs are
extremely high. Moreover, conventional tool steel ingots often inherit the pronounced
segregation and flake morphology of carbides, and give rise to a coarse non-uniform
rnicrostructure. This causes poor mechanical properties along the transverse direction. The
larger the tool steel ingots, the poorer is their mechanical properties. For this reason, the
service life of cutting dies, made of conventional tool steel D2, is relatively short.

In order to greatly improve the durable life of cutting and stamping dies, a new
wear-resistant tool material with hard surface and tough core, and/or an innovative
process has to be sought to form patterned cutting blades, which can withstand imposed
service loads without deformation, fracture, and excessive wear. Based on the practical
application of cutting dies, only the cutting blades are used for cutting functions.
Therefore, the die body made entirely of an expensive tool steel D2 is completely
wasteful. It is highly desirable to develop a novel processing technique to produce such
cutting and stamping dies at a significantly reduced production cost.

Laser surface cladding offers the potential to deposit new materials with required
mechanical and physical properties onto various metallic substrates. In this process, an
alloy powder or metal/carbide mixtures are allowed to interact with the laser beam on a
substrate surface to form a rapidly solidified cladding layer, without affecting the
performance of bulk material. The chemical compositions of the clad layer is usually very
close or equal to those of the alloy powder used because the dilution of the cladding layer
can be controlled by choosing optimized process variables. By laser cladding, high
hardness and high wear-resistant, high corrosion-resistant, and high oxidation-resistant

coatings, in the geometric contours and dimensions of cutting blades or a component, can

be deposited on selected areas and/or in a precisely defined pattern on a variety of
metallic substrates [1-7]. In addition, laser surface cladding has many advantages over
other competing techniques, such as plasma spraying, flame spraying, and tungsten inert
gas (TIG) welding. Compared to plasma spraying, the laser process offers a good bonding
between the coatings and the substrate, and a lower level of porosity. Compared to TIG
welding, the laser surface cladding has the advantage of smoother coating surface, lower
dilution, more economic use of expensive alloys, lower thermal distortion, less
machining, higher process flexibility, and better suitability for on-line process control and
automatic production [8, 9]. It is the advantages and the process flexibility of laser
cladding technique that we have employed to directly fabricate functional tooling dies or
3-D components at a sharply reduced production cost, compared with the conventional
process [10]. I have already done pioneer work in this field, and opened a novel
application field of laser cladding process in tooling-die manufacturing industry.
Compared with the conventional cutting-die manufacturing processes, this new
technology, in which cutting blades with required geometric patterns and dimension, are
directly deposited by laser cladding onto a substrate, has many advantages:
1. Material cost can be greatly reduced because an expensive tool steel die body can be
replaced by a cost-effective structural steel, such as A181 1045 or 4140 steels.
2. Production costs can also be significantly reduced due to the elimination of costly

heat treatment, and no or greatly reduced demand for electric discharge machining

(EDM).

2.

Durability of cutting and stamping dies is expected to be significantly improved
because the wear resistant property of clad materials is much better than that of the
tool steel D2, currently used in industry.
Production efficiency can be much higher than that for the conventional process due
to the eliminated or greatly reduced demand on the EDM, and the elimination of the
costly tool steel heat treatment.
Such a processing technique can also be used for repairing worn or damage cutting
blades.
The new processing technology is more flexible because different substrates and clad
materials, with desired mechanical and physical properties, can be chosen for various
applications.

Scope

The aim of this research is to investigate the possibility and the feasibility of

developing a novel technique for directly manufacturing cutting dies, and rapidly

prototyping industrial components by laser cladding process. The detailed research scopes

are described as follows:

1.

Design a new powder feeding system to provide a stable, continuous, and accurate
powder feeding rate, and deliver the powder stream coaxially into the molten pool on
the surface of a substrate under the laser beam.

Investigate the effect of powder feeding systems (lateral and coaxial powder feeding
mechanisms) on the laser cladding quality and geometry.

Perform preliminary experiments to obtain initial processing parameters, such as laser

power, laser spot size, traverse speed, powder feed rate, and shielding gas flow rate.

4. Develop a simple but realistic process model, governing various relationships
between the clad thickness and different processing variables, and determine the
minimum energy density for clad formation under the experimental condition.

5. Establish an embodied standard for evaluating laser cladding quality, and discuss the
mechanisms of forming cladding defects.

6. Characterize the microstructures of cladding layer, interface, and heat-affected zone
by using optical microscopy (0M) and scanning electron microscopy (SEM).

7. Eliminate any unacceptable defects in the clads, such as porosity, microcracks, and
poor metallurgical bonding between the clad and the substrate by modifying the
process parameters and other related factors.

8. Based on the fundamental study, investigate the feasibility of directly depositing 3-D
components with required geometric patterns and dimension by laser cladding
process.

9. Measure the rnicrohardness of the samples produced by laser cladding, and compare
these experimental values with those of tool steel D2 in order to support this brand-
new process.

10. A successful laser cladding process is a rapidly solidifying process. Due to high
cooling rate, fine microstructure is expected to be obtained. For above-mentioned
special application purpose, the temperature distributions of clad zone, heat-affected
zone, and substrate during laser cladding, will be investigated by using high

temperature thermocouples.

Chapter 1

REVIEW OF LASER SURFACE TREATNIENT PRINCIPLE

1.1 Basic Principle of Laser Operation

The laser, which is an acronym for light amplification by stimulated emission of
radiation, is a source of optical frequency radiation which is monochromatic, coherent,
controllable, and intense. It is these properties that make such lasers as NszAG
(Neodymium-doped yttrium aluminum garnet ) and CO2 competing heat sources, which
are being widely used for materials processing.

Laser requires the combination of population inversion, stimulated emission, and
amplification. An amplifying medium or active medium, a means of excitation, and an
optical resonator are the essential components for lasing to take place [11, 12]. The
amplifying medium determines the laser wavelength and the type of excitation required.
A medium can be potentially used as an optical frequency amplifier if the population of
its energy levels can be changed so that there are more emissions than absorptions per
unit time. This phenomenon is so called the population inversion, which is absolutely
necessary for amplification of light to occur. Otherwise, a beam of light directing through
the active medium with photon energy will be absorbed.

In terms of different energy state schemes, population inversion requires
excitation of atoms or molecules to the higher laser level by pumping methods or means
of excitation. Pumping methods includes optical pumping, gas discharge pumping, and
chemical reaction pumping. The use of pumping methods really depends on the active

medium. For NszAG and CO2 lasers, optical pumping and gas discharge pumping are

employed, respectively, to create excited atoms or the passages of an electrical discharge
through gaseous media to produce collisionally excited atoms or molecules. The former is
used to excite an NszAG crystal to lase at 1.06 um, and the latter is employed to excite
a C02 gas laser at 10.6 um.

The optical resonator consists of a system of mirrors. Such a device causes the
light emitted parallel to its axis to reflect back and forth through the amplifier. When the
amplifier gain is equal to the round trip loses in the resonator, then the combination of the
amplifier and the resonator is at the threshold for lasing. Upon reaching or exceeding the
threshold the light in the resonator traveling parallel to its axis is amplified many times.
The fraction removed on each pass by transmission through the output coupler results in a
laser beam. Such multiple use of the amplifier leads to highly energetic laser outputs.
1.1.1 Radiation Transitions

When an electron orbiting around the nucleus undergoes transition in energy
levels, it either spontaneously emits or absorbs energy. These emission and absorption
processes are known as radiation transitions. Associated with each transition is a specific

quantum of energy and wavelength of the radiation, given by the following relation:

E,-E,. =hv =— (1.1)

where E; and E, are the energies at the excited state and the lower state, respectively, h is
the Planck's constant (6.6 x 10'34 j-sec), c is the velocity of light (3 x 108 m/s), 1 is the

wavelength in meters, and v is the frequency of the emitted radiation. hv is the photon

energy.

Two main methods are commonly used to pump or energize electrons in a laser
cavity: (1) photon energy transfer (e.g., flash tube) and (2) transfer of energy by colliding
with excited electrons or molecules in an electrical field. The result of either type of
excitation is to pump an electron in a ground state to a higher energy level by absorbing a
quantum of energy as described by Equation (1.1). This process eventually creates an
inverted state. It has been found that energy absorption depends on material properties
and the intensity of the radiation, while spontaneous emission is governed only by the
internal properties of the material. Einstein had postulated that an additional type of

radiation with proper intensity is responsible for laser operation. This is known as

stimulated emission
1.1.2 Spontaneous and Stimulated Emission

An atom in an upper energy level spontaneously decays to a lower level, emitting
a quantum of energy difference as a photon with the appropriate frequency to satisfy
Equation (1.1). This release of photon energy is known as fluorescent or spontaneous
emission. The emitted photon has arbitrary phase and direction, as shown in Figure 1.1.
When an atom in an upper energy level is stimulated by incoming radiation of frequency
v,-,- satisfying Equation (1.1) to give up its energy and fall to a lower energy level. The
result of this process is that one photon interacts with the atomic system and two photons
emerge. Both photons have the same frequency and travel in the same direction. Thus, the
light intensity has been amplified during the stimulated emission process. It is the

stimulated emission process which is responsible for laser operation.

s. n.

 

E2 "‘
Absorption Stimulated

omission

Figure 1.1 Types of emission processes between two energy levels [13].

Absorption of energy of light traveling in the z direction through a material with
densities of atoms N,- and N} in two energy levels is given by [14]
11 = I0 exp(- 02) (1.2)
where I2 and lo are intensities before and after absorption, a is the absorption coefficient

and is given (for a Gaussian line shape) by

_ c2 ln2ffli
a_[479‘21 III/if] (1.3)

 

where r is the average fluorescent life time (relaxation time) of the material, c is the

velocity of light in the material, f is the frequency, and Af is the half-width of the

Gaussian line.
Similarly, amplification of light energy by stimulated emission is given by
I2 = I0 exp(+ gz) (1.4)

where g is the coefficient of gain and is given (for a Gaussian line) by

__ c2 ln2(_1_V_L
g‘iwzr aw] (1'5)

 

The similarity of the forms for a and g indicates that absorption and stimulated emission

are inverse processes. The net effect of these two expressions can be written as

_ 02 ln2{_l_ __ l 6
W Ml >2

If N.- > Nj, the light intensity will increase as it travels; otherwise, light intensity

 

will decrease. The condition N, > N; should to be satisfied for lasing to occur. This
condition is called "population inversion". The population inversion is simply having
more electrons in a high-lying energy level than in the lower-lying energy level. Equation
(1.6) indicates that, without the population inversion, there will be a net absorption at
frequency f. With the population inversion, there will be light amplification by stimulated
emission of radiation. The process of stimulated emission can cause photon yield
amplification that travels in a particular direction. Thus, a preferential direction is
established by placing mirrors at both ends of an optical cavity (resonator).
1.2 Laser-Beam Mode

The output of the laser is often characterized by its spatial and temporal
coherence. Spatial coherence describes the phase relation in a plane perpendicular to the
direction of beam propagation, temporal coherence indicates the phase relation in the
direction of the beam. The spatial distribution of the electromagnetic field inside the
optical cavity of a laser is described by resonant electromagnetic modes, which is referred
to as the configurations of electromagnetic field inside a laser resonator cavity. By
convention, the laser-beam profile is normally characterized by its transverse
electromagnetic mode (TEM) and represented in the form TEan, where the subscripts m

and n (small integers) represent the number of nodes/nulls in the directions orthogonal to

10

the beam propagation. Thus, intensity distribution, beam diameter, and beam divergence
along the plane of the laser beam's cross section are governed by the transverse modes.
Figure 1.2 displays the intensity distribution in the plane perpendicular to the plane in

which the mode distribution lies.

. SPOT
- a a do a‘ '{me
01 02 03 O4
CYLINDRICAL

00 1 0 20 30

g. .0. .

 

":’ {=3

.. 0.. :00: N;

11 21 33 04
RECTANGULAR

Figure 1.2 Transverse mode patterns of gas lasers in cylindrical and rectangular
coordinates [1 5].

The transverse beam modes can be represented by either Cartesian coordinates or
cylindrical coordinates. For rectangular symmetry, the notation TEMW. is interpreted as
the number of nulls which occur in the spatial pattern in each of the two orthogonal
directions, transverse to the direction of beam propagation [16]. For cylindrical
symmetry, the integer m indicates the number of nulls in a radial direction, and n

indicates half the number of nulls in an azimuthal direction. The modes marked with an

11

asterisk are linear superposition of two modes rotated by 90° about the central axis [17].
In Figure 1.2, TEMOI. mode is the linear combination of TEMm and TEMlo modes. A
laser beam with TEMOI. has a ring of intense energy surrounding a circle with no
intensity, and this mode is commonly referred to as the doughnut mode.

The above discussion about the mode patterns is related to gas lasers, such as the
He-Ne laser and the C02 laser. Solid-state lasers such as the Nd: YAG laser and Nd: glass
laser etc., exhibit more complicated spatial patterns, which are not easily describable in
simple mathematical terms.

For laser surface treatment processes, low-order laser beams are usually used due
to more uniform intensity distribution. Therefore, it is often desirable to obtain laser
operation in the lowest order mode known as IBM» mode (Gaussian mode). Compared
with other higher order transverse modes, TEMoo mode has uniform intensity distribution,
and low beam divergence. A plot of Gaussian intensity distribution (power per unit area
vs. radial distance) is illustrated in Figure 1.3. The plot shows that TEMoo mode has
maximum intensity at its center with exponential fall-off (decay) of the square of radius
of the beam.

The intensity of the Gaussian mode as a function of radius r from the center of the
beam is expressed by

I (r) = Io exp(%2r2) (1.7)
where 10 is the intensity of the beam at the center, and r0 is the so-called Gaussian beam

radius, where the intensity is reduced from its central value by a factor (22 or e. It is more

common to use the standard of fiof the peak intensity as the Gaussian beam radius. In

12

this case, the spot radius r0 implies that at r = r0, I =%=0.135 I0, i.e., approximately
86% of the total power is included. In the case of LL’ I = 0.368 Io, which indicates that

about 63% of the total power is accounted.

 

0.3.. -20

 

Rel-them};
2
l
3
Montcalm

 

 

 

 

 

 

 

 

oz - ~ 00
a“
10%

0 059 1 m M 210°

Relative radius f;

Figure 1.3 Variation of relative intensity and percentage total power with radius for a
Gaussian TEMoo beam [18]. Note that for the vertical line representing e",

the value should read 0.71 instead of 0.59.
One of the important characteristics of laser radiation is the highly directional,
collimated nature of the beam. The collimation is so important because it means that the
energy canied by laser beam can easily be collected and focused to a small area. In terms

of diffraction theory of light, for a monochromatic, circular light beam, the angle of

divergence 0 is given approximately by the equation

_ i
0-122(1)) (1.8)

13

where 6 is the wavelength of the light, D the diameter of the aperture through which the
light emerges. Equation (1.8) implies that the divergence angle 6 is inversely proportional
to the beam size. Of all TEMm modes of laser beam, only Gaussian beam is truly
diffraction-limited (the minimum value of the divergence angle 6 set by Equation (1.8))
[19]. In other words, the Gaussian beam retains its Gaussian form as the beam transmits
through optical systems. This represents the best case. Compared with the Gaussian
beam, other TEMW, modes exhibit larger beam divergence angles because they consist of
two or more individual beams with different phases.
1.3 Laser Parameters
1.3.1 Laser-Beam Diameter

Laser-beam diameter is one of the two important variables determining the power
density for a given incident laser-beam power. Because of the nature of the laser beam
and the definition of the beam diameter, it is very difficult to measure the beam diameter
for high-power laser beams. As has been stated before, the Gaussian beam diameter can
be defined as the diameter where the laser power has dropped to 1/e2 or 1/e of the central
value. The beam diameter defined on the basis of 1/e2 of the central value contains almost
86% of the total power, whereas the diameter defined on the basis of 1/e of the central
value contains slightly over 60% of the total power. Therefore, the lie2 beam diameter is
usually adopted [20].

Because the spatial distribution of energy in the laser beam affects the focusing
characteristics of the beam, the laser beam can not be focused to one point on a workpiece
surface. The diffraction-limited focal spot size, assuming no aberrations from the optical

lens, is expressed by

14

d = 2(1 22%) f ‘/(2M +1) = 2.44(%),/(2M +1) (1.9)

where d is the focused beam diameter, f the focal length of the focusing optics, 2. the
wavelength of the laser beam, D the unfocused beam diameter, and M the number of
oscillating modes.

For a Gaussian beam, M = 0, hence

d = 2.44(if—) (1.10)
D
In fact, the following relation is more commonly used to describe the focused spot

diameter of a Gaussian beam

2:52
IED

(1.11)
The actual value will be larger than the focused beam diameter calculated by Equation
(1.11) due to aberrations and other imperfections of the focusing optic system.
1.3.2 Laser-Beam Power

Laser-beam power is one of the main processing variables for laser surface
treatment. Incident laser power and laser-beam diameter determine the power density in
the interaction zone, and laser power and traverse speed determine the treatable clad

volume per unit time for a given powder feeding rate. The total power P of the Gaussian

beam is given by following equation

= 700210
2

P (1.12)

15

1.3.3 Power Density
Power density is defined as the incident laser-beam power per unit area.

According to this definition, power density can be expressed by the relationship
P
P = — 1.13
d A ( )

where P4 is the power density, P is the incident laser-beam power, and A is the irradiation
area. For a circular laser beam, for example, the irradiated area A = in}. The power
density is a main parameter for laser materials processes since it determines the process
functions, efficiency, and quality.

For power density values up to about 104 W/cmz, the absorbed energy depends on
the wavelength ,1, the material, and the surface conditions of the interaction zone. In
general, this range of power density is suited for solid transformation hardening.

When power density values increase to the order of 105 W/cmz, the material in the
interaction zone begins to melt in a hollow seam. This power density is typically used for
the laser processes of conduction welding, surface melting, alloying, and cladding.

The powder density values in the order of 106 W/cm2 leads to the formation of a
vapor hole which is approximately focused laser spot size ("keyhole") surrounded by
molten material. This situation will correspond to the processes of drilling and deep
penetration welding.

Figure 1.4 clearly shows a relationship between power density and laser materials

processes.

16

 

1o1 \

\\ \ \ \x
109 \\\\\\OCK\\ \\ \ \
\§HAR0£N1NG\\ \\ \ \specmc ENERGY. chm2

\\\\\\\\\ \\ \\ \\\ \\
\10:\\\\\\\1o4\ \\ \\\\1\:\

\ LASE\RGLAZ\\I\\NG \:\\\\\: Pfixgggm § \ \\\

\\\\\\\\\\\\\ >

d

d
O .
N

 

‘

POWER DENSITY -- was»2

d

%

 

 

 

T\RANSFO\RMATI\ON'
N1
1 l \\ mole Na
11 . 10'5 10" 10'2

INTERACTION TIME - ac

Figure 1.4 Laser materials interaction energy diagram [21].

1.4 Industrial Lasers

Lasers are being used in manufacturing industries ranging from automobile and
tooling-die to gas turbines. They can be coupled with CAD/CAM systems to produce
functional prototypes and industrial components economically. The various types of
lasers commercially available include solid-state lasers (Ruby and Nd: YAG lasers), gas
lasers (He-Ne and CO2 lasers), and semiconductor (diode) lasers (GaAs laser), and so on.
Of the various types of laser systems, Nd: YAG and CO2 laser are widely used in

fabrication of industrial components from metals, alloys, and composites.

l7

1.4.1 The Nd:YAG Laser

The most commonly used solid-state laser is the 1.06 pm Nd: YAG (Yttrium
aluminum garnet, Y 3.415012) laser. Neodymium-doped yttrium aluminum garnet (Nd:
YAG) crystal possesses a combination of properties uniquely favorable for laser
operation. The YAG host is hard, of good optical quality, and has a high thermal
conductivity. Furthermore, the cubic structure of YAG favors narrow fluorescent
linewidth, which results in high gain and low threshold for laser operation. In NszAG,
about 1% trivalent yttrium is substituted by trivalent neodymium, so charge compensation
is not required. While Nd3+ ion exhibits a satisfactorily long fluorescence lifetime and
narrow fluorescence linewidths, and possesses a terminal state for the laser transition
sufficiently high above the ground state so that continuous-wave operation at room
temperature is readily feasible. Therefore, this ion is usually incorporated as a dopant,
called lasing medium. Due to special properties of trivalent neodymium, a higher power
level has been obtained from Nd lasers than from any other four-level material [22, 23].

A simplified energy level diagram of the Nd3+ ions in the YAG crystal is
illustrated in Figure 1.5, with conventional spectroscopic notations for the energy levels.
All the excited higher levels relax non-radiatively to 4F3/2 level. The 1.06 pm laser
transition is between the 4F3,2 and 4111/2 level. The lower laser transition level, 4111/2 is
2000 cm" above the ground state (419/2) and will normally be unpopulated at room
temperature. This indicates that the Nd: YAG laser operates on a four -level system and
that the threshold for lasing is low.

The Nd: YAG laser is excited by light from an electric discharge lamp. Xenon

flash lamps and krypton filled discharge lamps are most commonly used for excitation

18

purpose. The excited Nd3+ ions in the YAG host in a proper optical resonator can cause
the stimulated emission at a number of frequencies within three different groups of
transitions centered at 0.9, 1.06, and 1.35 um. The most efficient laser operation is
obtained when the optical resonator is designed for 1.06 um. This is the wavelength used
in NszAG lasers for materials interactions.

Recently, NszAG lasers become of interest for materials surface treatments
because multi-kilowatt systems now are available on the market. The maximum power

for NszAG sources that can be purchased is about 2.5 kW in continuous-wave mode or

about 3 kW average power in a pulsed laser system.

Pimped Levels

 

f-----------------a

 

\

s \ Non-radiative Decay

 

 

Optical
Pumping

 

 

 

 

 

 

 

Figure 1.5 Energy level of Nd: YAG laser [24].

19

1.4.2 The CO2 Laser

Among many types of gas lasers, the most important molecular laser is carbon
dioxide laser. Just as its name implies, the carbon dioxide laser uses C02 gas as its active
medium. It has been found that the carbon dioxide molecule is a linear molecule, and
three atoms lie in a straight line with the carbon atom locating in the middle. Therefore,
there are three different types of vibrations which can occur in the carbon dioxide
molecules. These vibration modes are called as symmetric vibration, bending vibration
and asymmetric vibration respectively, which are illustrated schematically in Figure 1.6
[25].

Symmetric vibration mode V]: the carbon atom remains stationary, and the
oxygen atoms oscillate about their equilibrium positions as illustrated by the arrows. At
any given time, the oxygen atoms are moving in opposite directions along the line of
symmetry.

Bending vibration mode V2: all the atoms are moving in a plane perpendicular to
the line of symmetry. The carbon atom moves in one direction while the two oxygen
atoms move in the opposite direction.

Asymmetric vibration mode V3: all the atoms move along the line of symmetry.
At any given time, the carbon atom is moving in a direction opposite to the two oxygen
atoms.

For the different vibration modes mentioned above, the energy associated with
them is quantized, based on the principle of quantum mechanics. In the ground
vibrational state, there are no vibrational motions. The excited states correspond to

excitation of one or more quanta of vibrational energy. The molecule can vibrate in more

20

than one mode at the same time, and thus it can have more than one quantum of vibration
energy in each mode. For convenience, the notation adopted to denote the states is in the
form (ijk), where i, j, and k are integers denoting the quanta of vibrational energy in each

of the three modes. The integer i corresponds to the symmetric vibration, j to the bending

vibration, and k to the asymmetric vibration.

    

(001)

_ 0.2

3

Energy (ev)

Energy level (wavennmbel') an"

E

 

 

 

 

 

 

 

 

V3

 

Figure 1.6 Three vibrational modes and various energy levels for CO2 laser [25].

21

Figure 1.6 Three vibrational modes and various energy levels for CO2 laser [25].

In carbon dioxide lasers, carbon dioxide, nitrogen, and helium gases are mixed in
the laser tube. The gas tube is placed in an optical resonator. When the gas discharge is
induced by applying the electric field to the gas tube, the electronic bombardment and the
ionic collision with helium ions excite the nitrogen molecules to their vibrational
quantum states (001). When the excited nitrogen molecules collide with ground state
(000) C02 molecules, the nitrogen molecules transfers their energy from the collisions to
CO2 molecules, and then return to the ground state. The CO2 molecules in the ground
state are excited to the higher energy state by this bombardment. When the excited C02
molecules take downward transitions to the lower energy state, the transitions will
produce a population inversion between the (001) state and the (100) state, as shown in
Figure 1.6. Although there is another state (020), which shares a common upper state with
the (001) state, the probability of transition to the (020) state is only about one-twentieth
to the (100) state. The most of laser operation in C02 laser proceeds at the wavelength of
10.6 pm. The addition of helium to the gas mixture can also increase the output power.
Because helium gas tends to deplete the CO2 population on the lower laser level (010) by
collisions and tends to keep the gas mixture cool due to the high mobility of helium
atoms. Because of the relaxation effect of helium, carbon dioxide molecules in the lower
energy level eventually return to their ground states. This process cools the CO2 gas and
reduces losses due to absorption of 10.6 um light by lower to upper laser level transitions.

Since the useful transitions in a carbon dioxide laser are based on vibrational
molecular quantum states, the transition energies are low. Therefore, carbon dioxide laser

operates in the infrared wavelength.

22

Although the transition energies are low, the transition energy per unit time is
large or the operational power is high. Today high power continuous-wave CO2 lasers are
commercially available with output power up to 25 kW.

1.5 Laser Beam-Material Interaction

In metals, the incident laser radiation is predominantly absorbed by free electrons
in the conduction band. The photon of the incident laser radiation has relatively low
energy. For example, the energy of C02 laser photons is only 0.12 eV while the photons
obtained from the Nd:YAG laser have about 1.2 eV of energy. When a laser beam strikes
a metal surface, heat transfer occurs by photon/electron interactions with free or bound
electrons. These interactions raise the energy state of the electrons in the conduction
band. The excited electrons collide with other photons and electrons, and give back their
radiation by spontaneous emission. In this way, the electronic energy derived from the
photons of the incident laser radiation is converted into the heat energy. It is this heat
energy that is used for laser materials processes. In practice, most metals behave in this
manner for light wavelengths exceeding about 0.5 um, which includes both YAG and
CO2 lasers. Previous studies have shown that most of this absorption occurs in the
shadow surface layer of metals, typically a few atomic layers, so that laser absorption in
metals is truly a surface effect.

The interaction of high-power laser radiation with metals is capable of providing a
novel and useful processing tool. However, it is difficult to obtain effective energy
coupling between the incident laser radiation and workpiece due to the high reflection of
metals. At room temperature, most pure metals and alloys absorb the less of the incident

laser radiation, about one percent for copper and tens of percent for steels, for example,

23

so that most of incident laser power is reflected from the metal surface. In order to yield
an effective process, it is necessary to obtain the adequate laser energy at the workpiece,
and couple a fraction of the incident laser energy as high as possible into the materials.

The efficiency with which a material absorbs the incident laser radiation is
defined as the materials absorptivity. It has been found that the absorptivity of a surface is
a function of the materials, nature of the surface, the level of oxidation, temperature, and
wavelength and power density of the incident laser radiation [26]. Therefore, improving
the nature of a metal surface or use of lasers with shorter wavelengths can greatly enhance
its absorption. For a C02 laser beam with 10.6 um wavelength at normal incidence, the
absorptivity of most metals is less than 10%, while for a Nd:YAG laser beam with 1.06
um wavelength at the same incident angle, the absorptivity is up to 30%. In addition,
increase in the roughness of a metal surface also can enhance the absorptivity by a factor
of 1.2 to 1.5 due to surface contamination caused by machining process.

Although metals severely reflect CO2 laser beams, up to now, the lasers used for
material surface modifications mainly are C02 lasers due to their high available power
level. In order to improve the absorptivity of metals at 10.6 tun wavelength of CO2 laser
beams, surface coatings, such as colloidal graphite coating and paint coating, and surface
oxidation by chemical or heat treatment processes [27, 28] are widely used in industry.

By these processes, the absorptivity can be increased to the values of 70 to 80%.

24

Chapter 2

LASER SURFACE TREATMENT

The use of high-power laser beams has opened a novel category of processing
opportunities in metal surface treatment and composite material fabrication. Laser
materials processing is characterized by concentrating an extremely high power density
on a small area of the alloy surface to achieve very rapid heating and cooling of the near
surface of materials. During laser irradiation, with or without alloy powder injection, the
substrate surface can be modified by changing either its alloy composition and structure,
or only its structure. Because laser surface treatment (LST) belongs to a kind of rapid
melting and solidifying process, it can extend solid solution, form metastable phases,
refine the grain size, and hence improve the wear resistant, corrosion resistant, and
oxidation resistant properties of the treated materials.

Depending upon the main processing variables, power density and interaction
time, different laser surface treatment processes, such as laser hardening, remelting,
surface alloying, dispersing, and cladding, have been developed. The definition and
function of laser surface modifications are succinctly described as follows:

2.1 Laser Solid-Transformation Hardening

The laser surface hardening is usually used to harden the selected surface area of
iron-base alloys such as mild carbon steel, tool steel, and cast iron, etc. A basic feature of
laser-induced transformation hardening is so called self-quenching. Such a process is
based on the basic principle similar to the conventional heat treatment: the surface layers

of an alloy is heated locally to transform to austenite, and is then rapidly quenched by

25

conducting heat to the bulk material (self-quenching). This process leads to the formation
of a certain volume fraction of the martensitic phase transformation at the surface layers
of a substrate. To produce large temperature gradients, short dwell times are necessary in
order to avoid too much temperature rise in deeper layers. Determined by laser power
input, spot size, and traverse speed, typical dwell times during the process are
approximately between 0.1 and 5 seconds. As a result, the total heat input of the
workpiece is low, and thus its distortion can be minimized.

Typically, laser heat treatment produces two zones: one is the hardened zone, and
the other is the transition zone. The hardened zone is referred as to the area with a desired
hardness and a hardened depth. The hardened zone is surrounded by a transition zone, a
region of less effectively hardened material. Reduced effectiveness of treatment is caused
incomplete heating. Compared with the conventional heat treatment process, laser solid-
transformation hardening has following advantages [29]:

0 Low energy input to the workpiece results in low distortion upon heating.

0 Self-quenching into the cold rigid bulk of the workpiece causes little distortion upon
cooling.

0 Energy can be more efficiently used due to local heating of the treated surface area.

0 Process is more flexible because laser beam can be directed to reach the complex
geometric parts of a workpiece.

0 Laser heat treatment can often achieve a higher hardness due to rapid heating and
cooling.

0 External quenchants are not needed due to the process characteristic so that fumes and

dirt from heating and quenching can be minimized.

26

Due to the above advantages, laser heat treatment has been widely used in
manufacturing industries to improve the wear resistance and fatigue properties of the
treated materials or components [30-34], such as camshaft, crankshaft, extruder shaft,
gear, and the like.

2.2 Laser Remelting

Laser remelting can improve the wear resistance and corrosion resistance of the
thin surface layers of different alloys by homogenizing and refining the surface
microstructure through rapid self-quenching. Through this process, high heating and soli-
dification rates can generate supersaturated solutions, metastable phases, or amorphous
structures. For this reason, the properties of the treated surface layers can be improved.
Taking tool steels for example, due to the high solidification rates, carbon diffusion is
suppressed, and the high temperature phases with their increased solubility limits are
quenched down to the room temperature. The resulting fine and homogeneous
microstructure of the remelted tool steel leads to the improvement of wear and corrosion
resistance. The metallic materials suitable for laser remelting process include cast iron
[35-39], aluminum alloys [40, 41], low and medium carbon steels [42, 43], and tool steels
[44, 45]. In comparison to laser-induced transformation hardening, the laser remelting
process requires a higher power density. This density can be achieved by increasing laser
power input at a given laser-spot size.

2.3 Laser Surface Alloying

Unlike laser remelting process, laser surface alloying is usually used to change

both chemical compositions and microstructures of the surface layers of a substrate by

simultaneous surface melting and injection of alloying elements, and hence creates a new

27

surface alloy with the novel structures and required properties, such as wear resistance,
corrosion resistance, and high-temperature oxidation resistance. The chemical
composition of the formed surface alloy not only depends on the composition of the
materials involved, but also on the degree of mixture. In other words, the base material
and the added materials must be adapted each other to achieve the required structures and
properties. By laser alloying process, the high temperature in the melt pool can dissolve
the stable phases like interrnetallic compounds, and the convection in the melt pool
causes a good intermixing of the molten substrate with the alloying elements added, and
the rapid solidification leads to the formation of fine and homogeneous structures. Laser
surface alloying with alloying elements, such as C, Ni, Cr, Mo, W, Co, Mn, and Ti, is
usually used to improve the wear resistance and corrosion resistance of different kinds of
steels [46-54], aluminum alloys [55, 56] and titanium alloys [57, 58].
2.4 Laser Dispersing

Laser dispersing is a special type of alloying process. During this process, the
carbide particles with a high melting point are injected into the melt pool so that
composite layers form after rapid solidification. The formed composite layers mainly
consist of the injected hard particles uniformly dispersed in the metallic matrix with a
lower melting point. In order to obtain a good wear behavior of the layers produced by
laser dispersing, a low dissolution of the injected hard particles is necessary. For laser
dispersing, metal carbides, such as WC, TiC, VC, Mo2C and SiC, are usually used as hard
particles. A significant number of publications report that this process has been developed
to inject hard carbide particles into different metal substrates, such as steels [59-64],

aluminum alloys [65, 66], and titanium alloys [67-69], and wear resistance of the surface

28

layers has been significantly improved. Because high percentage of carbides may lead to
the reduction in ductility of the composite layers, the dispersing process possibly causes
cracking [70].
2.5 Laser Surface Cladding
2.5.1 General Description

Laser surface cladding is a process in which an alloy powder or metal/carbide
mixtures are allowed to interact with the laser beam on the surface of the substrate to
form a rapidly solidified cladding layer with the required properties, such as high
hardness, high wear resistance, high corrosion resistance, and high-temperature oxidation
resistance, without affecting the performance of the bulk material. Unlike laser alloying,
the chemical composition of the clad layer is usually very close to that of the alloy
powder used because the dilution of the cladding layer can be controlled by choosing
optimized process variables. Generally, a good clad has following characteristics: uniform
coating thickness and smooth surface, free of porosity and cracks, minimum dilution and
distortion, and good metallurgical bonding with the substrate.
2.5.2 Scope of Laser Cladding

For laser cladding process, different kinds of ferrous and non-ferrous alloys as
well as superalloys are usually used as the substrates, and the clad materials used really
depend on the application purposes. For example, laser cladding with reinforcement
materials, such as Fe-Cr-C-W, Co-Cr-C-W, Ni-Cr-B-Si or Fe-Cr—Ni, is widely used to
improve the wear and corrosion resistance of plain and mild carbon steels [71-76]. Laser

cladding with nickel base alloys is used to improve the wear resistance of aluminum

29

alloys [77, 78]. Laser cladding with Ni-Cr-Al-Hf improves the high temperature oxidation
resistance of nickel-base superalloys [79, 80].

Laser cladding by carbide/metal composite powder on steels, aluminum, nickel,
and titanium base alloys has shown its application potential for becoming a widely used
hard-facing technique. Optimized combinations of basic material, metallic binder, and
hard material allow the composite layers to be produced. The composite powder is
usually made up of hard particles, such as SiC, WC, TiC, and VC, and ductile cobalt or
nickel base alloys, and the produced cladding layers display high wear-resistant and
corrosion-resistant properties [81-86].

In addition, laser cladding process is being used to produce functionally gradient
composites or laminated metal-matrix composites [87, 88]. Such a process is displaying a
broad application potentials in manufacturing industries.

2.5.3 Processing Parameters

Both applications and efficiency of laser surface treatments strongly depend on
process variables. The main process parameters for laser cladding include incident laser-
beam power and spacious energy distribution, laser-beam spot size and shape, laser
absorptivity of both substrate and clad materials, traverse speed, powder feed rate, and
shielding gas flow rate, and so on. Laser parameters are discussed in Chapter 1, and the
remaining processing parameters are described in detail as follows:
2.5.3.1 Absorptivity

As has been stated before, for an optically opaque material, the absorptivity is
referred to as the fraction of the laser energy absorbed by the substrate and the clad

materials. It has been found that, because the infrared absorption of metals mainly

30

depends on the conductive absorption by free electrons, the absorptivity is a function of
the electrical resistivity of the substrate. It was found that the absorbtivity is proportional

to the square root of electrical resistivity, and expressed as follows [28]:
A=112. p, (2.1)

where A is the absorptivity, and p, is the electrical resistivity.

Equation (2.1) shows that the smaller the electrical resistivity, the lower is the
absorptivity. That is why most metallic materials, particularly copper and aluminum, have
smaller values of intrinsic absorbtivity. On the other hand, the apparent (extrinsic)
absorptivity at a metal surface is considerably modified by its surface mechanical and
chemical properties, such as roughness and oxidation [20, 28]. Therefore, the effective
absorptivity also is a function of the surface condition of the substrate. Increased
roughness or oxidation of the metal surface can greatly enhance the coupling of laser
energy. In addition to the influence of the electrical resistivity and the surface condition,
the absorptivity is related to the laser-beam wavelength [20]. In metals, absorbtivity
increases for shorter-wavelength laser beam. For example, absorptivity of the polished
steel surface is about 30% for Nd:YAG laser with 1.06 pm wavelength, and is six times
more than that for CO2 laser with 10.6 ttm wavelength. If the surface is oxidized, the
absorptivity can increase to the value of 60% for CO2 laser radiation.

In laser surface alloying and cladding processes, alloy powder has to be used. It is
provided by either blowing the powder into the interaction zone or preplacing the powder

on the surface of the substrate. In both cases, energy absorption has been observed to be

significantly improved.

31

2.5.3.2 Traverse Speed

Traverse speed is the velocity at which the substrate moves with respect to the
stationary laser beam. The ratio of the laser-beam diameter to the traverse speed defines
the interaction time. For a given spot size, the reduction in traverse speed increases the
interaction time. This will lead to the increase of the dilution, if other variables such as
laser—beam power and powder feed rate, are held constant. On the other hand, variation of
traverse speed has a marked effect on the dimensions of the cladding tracks. The clad
width slightly increases and the clad thickness rapidly increases as the traverse speed
decreases. When the power density and powder feed rate are held constant, the traverse
speed has an upper and a lower limits. The high speed limit will cause cladding to cease
due to insufficient energy input to melt the substrate surface, and low speed limit will
lead to the high dilution or even the distortion of the substrate due to the high energy
input. Therefore, for laser cladding process, suitable traverse speed has to be selected to

ensure the mechanical properties of the cladding layer and the metallurgical bonding

between the clad and the substrate.
2.5.3.3 Powder Feeding Techniques

There are two techniques to supply the alloy powder for laser surface alloying and
cladding. One is the preplaced powder technique, and the other is the blown powder
technique. The former is called the two-step process, while the latter is called the one-step
process.

For two-step process, loose or paste-bound powder is preplaced on the substrate
surface before laser irradiation [89, 90]. This process was used in the early stages of

development of laser surface alloying and cladding. However, two-step process has many

32

drawbacks, such as weak metallurgical bonding with the substrate, uncontrollable clad
thickness, and insufficient operation flexibility, so that it is not suitable for an automatic
production environment.

For one-step process, alloy or carbide/metal composite powders are transported by
an inert gas stream, usually argon or helium, and injected through a nozzle directly into
the center of the laser beam/molten pool on the moving substrate. This method of powder
supply is widely used due to its high process quality and flexibility. There are two
manners to deliver the alloy powder into the molten pool: laterally injecting the powder
into the molten pool [91, 92] or injecting the powder into the molten pool through a
coaxial nozzle along with the laser beam [93, 94]. By using the coaxial nozzle, which was
designed by researchers at High Energy Laser Processing (HELP) Laboratory, Michigan
State University, the efficiency of powder utilization is up to 80% [95], and the localized
areas of components of complex geometry can be deposited with better control on clad
thickness and dilution by carefully selecting the processing parameters.

Powder feed rate is defined as the total powder amount supplied per unit time.
The variation of powder feed rate greatly affects the clad thickness and dilution as well as
its quality. If other variables are held constant, the clad thickness increases linearly, and
the dilution decreases with the increase of the powder feed rate.
2.5.3.4 Shielding Gas

The shielding gas is usually used to protect the optical components from fume and
spatter, and thus to ensure effective transmission of the laser beam to the interaction zone.
The shielding gas also protects the molten material from oxidation. For laser surface

alloying and cladding process, an inert gas such as helium or argon is usually used as the

33

shielding gas. However, since helium gas is expensive, argon gas is used as the shielding
gas to reduce the production costs for laser cladding process.

Shielding gas flow rate has a great effect on the clad quality. Increasing the cover
gas flow rate helps reduce the dilution of the clad by enhancing cooling rate (other
processing parameters remaining constant). However, a high cover gas flow rate leads to
the formation of porosity inside the cladding layer due to bubble entrapment. Therefore,
the optimum shielding gas flow rate has to be selected to ensure the clad quality.

2.5.4 Advantages of Laser Cladding Process

Laser surface modification has many advantages over other competing techniques,
such as plasma spraying, flame spraying, and tungsten inert gas (TIG) welding. Compared
to plasma spraying, the laser process offers a good bonding between the coatings and the
substrate, and a lower level of porosity. Compared to TIG welding, the laser surface
cladding has the advantage of smoother coating surface, lower dilution, more economic
use of expensive alloys, lower thermal distortion, less machining, higher process
flexibility, and better suitability for on-line process control and automatic production.
2.5.5 Limitations of Laser Cladding Process

Laser surface treatment has many advantages, but it has some limitations. A main
characteristic of all laser surface treatments is that the treated region is a narrow strip. For
this reason, larger surface areas to be treated can be finished only by overlapping tracks.
From the point of view of cost-effectiveness, laser surface treatments are just suitable for
manufacturing costly and special industrial components. Obviously, it is this reason that

limits the wider applications of laser cladding process in consumer product industries.

34

2.5.6 Applications of Laser Cladding Process

Laser cladding technique was invented more than two decades ago. Since then,
many researchers around the world, have been dedicating their investigation. One of such
studies is production of surface coating, when laser is the tool for the surface coating
work. Due to such great efforts, both theoretical and applicable aspects of interaction
between a high energy laser beam and a solid material are approaching closer to the real-
world applications. Some real-world applications of laser cladding in manufacturing
industries are described as follows:

Mah [96] employed the directed light fabrication (DLF) process to deposit various
complicated geometric components by laser cladding. The parts have straight sides,
smooth contours, and sharp comers. This novel process is being substituted for the
conventional process to fabricate the functional prototype of a newly developed product.

In tooling-die manufacturing industry, Mazumder et al. [97] directly deposited
H13 tool steel powders by laser cladding to prototype injection molding and trimming
dies. It was reported that the production period of such tooling dies was greatly shortened,
and thus the production costs were dramatically reduced. Concurrently, Hu et al. [10]
utilized a similar technique to deposit highly wear-resistant tool steel CPMISV on the
mild steel die body to form near-net-shaped cutting blades, and manufactured cutting and
stamping dies at a sharply reduced production cost, compared with conventional
manufacturing process.

In aerospace industry, it is well recognized that Rolls Royce is a pioneer in taking
advantage of the many benefits of laser cladding for production purpose. The first and the

most well-known application is the cladding of interlocking edges of turbine blades at

35

Rolls Royce. A Cobalt-based alloy was deposited on the Nickel-based superalloy blades
by laser cladding process using a 2 kW C02 laser, and the one-step powder feeding
technique [98]. Since then, this new technique has been widely used to rebuild or repair
the worn turbine blades and vanes.

In automobile industry, Toyota Motor Company developed a novel laser cladding
process for producing engine valve seats [99]. By this process, the valve seats were
directly formed on the cylinder head by cladding copper-based alloy powder on the
aluminum substrate. It was reported that the temperature of the valves and their seats was
reduced, the overall engine performance was improved, and the durability of the engine
was enhanced three times in comparison to the conventional process of the valve seat
preparation.

Now laser cladding process has demonstrated a great application potential in
various manufacturing industries. The combination of laser cladding technique and
advanced CAD (computer-aided design)/CAM (computer-aided manufacturing) system
can be employed for rapidly prototyping industrial components, manufacturing tooling

dies with required geometric shapes and larger dimensions, and repairing both aerospace

and industrial turbine blades and vanes.

36

Chapter 3

EXPERIMENTAL PROCEDURES

3.1 General Description-

This research mainly consists of the following parts: (1) design an integrate
coaxial powder feeding system; (2) develop a simple but realistic process model which
governs various relationships between the maximum clad thickness and the process
variables; (3) direct fabrication of 3-D components with different geometric patterns by
laser cladding; (4) measure the temperature distributions of molten pool, heat-affected
zone, and substrate during laser cladding process.

A combination of CAD (computer-assisted design)/CAM (computer-assisted
manufacturing) systems and an integrated laser cladding system were used in this
experiment. Thus, the reinforced materials can be precisely deposited onto metallic
substrates in a preprograrnmed geometric patterns, in order to reach the above-mentioned
research goal.

3.2 Choice of Materials

The aim of laser surface cladding is to deposit reinforced materials with the
required mechanical and physical properties onto different metallic substrates in order to
alter their surface properties, such as wear resistance, corrosion resistance, and high-
temperature oxidation resistance. Typically, cost-effective structural materials like plain
or mild carbon steel, and aluminum alloy are used as substrates. The clad materials used

depend on the application purposes.

37

3.2.1 Substrate Materials

As has been stated in INTRODUCTION, in the conventional manufacturing
process of rotary cutting die, the entire die body and cutting blades are made of an
expensive tool steel D2 or M2. However, in practical application, only cutting blades are
used for cutting functions. Therefore, the die body made of tool steel is not necessary, and
completely wasteful. In addition, the heat treatment cost of tool steel die body is also
extremely expensive. In order to greatly reduce the costs of material itself and production,
AISI 1045 medium carbon steel plates and rollers were used as the substrates in this
study. The steel plates were used in modeling work and prototype of 3-D components,
and its dimension was 100 mm x 50 mm x 6.35 mm. In addition, the steel roller was used
for fabrication of the patterned rotary cutting die, and its dimension was 305 mm in
length, and 101.6 mm in diameter. For all research projects, the substrate surfaces were
polished by using a sandpaper to remove the oxidation layer and also to roughen the
surface for improving coupling rate of the laser energy before laser cladding process. For
convenience, the chemical compositions of AISI 1045 and 4140 steels, and Inconel
738LC (IN-738LC) nickel-based superalloy as well as reinforced materials are listed in
Table 3.1.

In the investigation of measuring temperature distribution, AISI 4140 steel plates

were used as the substrate, which had dimensions of 75 mm x 50 mm x 6.35 mm.

38

Table 3.1 Chemical compositions of substrates and clad materials (wt. pct)

 

 

 

 

 

 

 

 

 

 

Materials Fe Cr V C Mn Mo
1045 steel bal. --- --- 0.43-0.5 0.6-0.9 ---
4140 steel bal. 0.8-1.1 -- 0.38-0.43 0.75-1.1 0.15-0.25
AISI 304L bal. 18.5 -- 0.024 1.44 --
CPM10V* bal. 5.25 9.75 2.45 0.50 1 .30
CPMISV“ bal. 5.25 14.5 3.40 0.50 1.30
IN 738LC 0.50 max 15.70-16.30 -- 0.09-0. 13 0.20 max 1.50-2.00
Materials Si Ni S P Co W
1045 steel -- -- 0.05 --- -- --
4140 steel 0.15-0.3 -- 0.04 0.035 -- --
AISI 304L 0.55 9.5 0.019 -— -- --
CPMIOV 0.90 --- 0.09 --- --- ---
CPM15V 0.90 --- 0.07 --- -- --
IN-738LC** 0.30 max bal. 0.015 max -- 8.00-9.00 2.40-2.80
Materials Al Ti Ta Nb B Zr
1045 steel --- --- -- -- --- ---
4140 steel -- -- -- -- -- --
AISI 304L —- -— — — _ .—
CPMlOV --- --- -- --- --- ---
CPM15V -- -- -- -- -- --
IN-738LC 3.20-3.70 3.20-3.70 1 50-200 0.60-1.10 0.007 -0.012 0.03-0.08

 

 

* The chemical compositions of CPM 10V and CPM 15V clad materials are provided by
Crucible Materials Corporation.

** The chemical compositions of IN-738LC clad material are provided by Praxair Surface

Technologies.

3.2.2 Clad Materials

In order to significantly improve the wear resistance of cutting blades and prolong

the service life of tooling dies, the clad materials used in this research were commercially

available tool steel powders, CPMIOV and CPM15V, which are made by the Crucible

Particle Metallurgy (CPM) process. The particle size for CPMIOV was -270/+325 mesh (44

- 55 um), and for CPM15V was -140/+200 mesh (74 -100 pm). The chemical compositions

39

of CPM 10V and CPM15V are listed in Table 3.1. For convenience, morphology of clad

materials is shown in Figure 3.1.

 

Figure 3.1 An overview of powder morphology. (a) CPMlOV tool steel. (b) 304L
stainless steel.

CPM10V was designed as a tough, air hardening alloy with added high carbon and
vanadium for superior wear resistance, toughness, and strength for cold and warm tooling
applications. It is these properties that make it an excellent candidate to replace other
conventional wear-resistant materials in cold work tooling applications, particularly where
tool toughness is a problem.

CPM15V contains 50% more hard vanadium carbides in its microstructure than
CPM 10V, to provide even higher wear resistance. Therefore, it is intended for applications
requiring exceptional wear resistance.

As a result of pronounced ingot segregation, conventional tool steels often contain a
coarse, non-uniform microstructure accompanied by inferior mechanical properties along
the transverse direction. However, tool steels CPMlOV and CPM15V produced by the
CPM process exhibit very homogeneous microstructures with uniformly dispersed spherical
carbides. These steels are characterized by exceptional wear resistance and relatively good
impact toughness in comparison with conventional tool steels, such as D2 and M2, as

shown in Figures 3.2 and 3.3 [100].

41

r,
8

 

I

 

I

Crossed-cylinder wear test, td MPa
. s a s a s s

 

 

 

 

 

 

l I I

DZ M2 CPM10V CPM10V*
HR 62 6 60 63

 

.4
_8_

Figure 3.2 Wear resistance of CPMIOV and conventional tool steels at indicated
hardness [100]. CPM10V* (hardened) is subjected to quenching, which
increases its hardness, and thus the extra wear resistance.

50

 

B 8 8

Charpy C-notch impact test, J
3

 

 

 

 

 

 

 

 

 

 

0 1
Tool D2 M2 CPM10V CPM10V
HR 62 6 60 63

 

4L.

1
I

Figure 3.3 Charpy C-notch impact properties of CPMIOV and conventional tool steels at
indicated hardness [100].

42

AISI type 304L stainless steel is an austenitic Cr—Ni stainless steel. It has good
corrosion resistance, and high ductility. 304L stainless steel also has excellent drawing,
forming, and spinning properties. In addition, its low carbon content means less carbide
precipitation in the heat-affected zone during the welding process and a lower
susceptibility to intergranular corrosion.

A laminated metal matrix composite (IMMC) is defined as a composite material
with alternate layers of a high-modulus, high-strength material as the strengthener, and a
deformable ductile material as the matrix. Such a composite material can be directly
deposited by laser cladding technique. In this study, 304L stainless steel and CPMIOV tool
steel have been used as clad materials. The particle size for 304L stainless steel powder
ranges from -l40/+325 mesh (44 - 100 um). Its chemical compositions are listed in Table
3.1, and powder morphology is shown in Figure 3.1. The composite materials have been
produced in such a way that the laminates consist of alternating stainless steel and tool steel
layers or one stainless steel and two tool steel layers. Thus, the composite materials are
characterized by hard surface, wear-resistance, and tough core, and possibly used to
fabricate cutting blades of cold tooling dies, which require to withstand imposed service
loads without premature deformation, fracture, and excessive wear.

The reinforced material used for repairing industrial and aviation gas turbine
blades was commercially available IN-738LC superalloy thermal spay powder. Its particle
size falls between -200 and +325 mesh (44 - 74 um). IN-738LC filler material was
produced by Praxair Surface Technologies. The chemical compositions provided by the

manufacturer are given in Table 3.1.

43

3.3 Laser Cladding System
Laser cladding system mainly consists of a continuous-wave C02 laser, a set of
optical mirrors, a powder feeding system, and a computer-assisted manufacturing (CAM)

system. The experimental setup is shown in Figure 3.4.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

powder feeder
LASER |
= I I powder
‘ -—-—! I I I T (— stream
| I | powder splitter
coaxial I I I .
laser head | I four powder
I I outlets
focus lens
cover gas -> i- wder flow
powder flow> ll! 5 p0
LEl computer

 

 

 

l 1
clad /
substrat
P <:——— moving direction

I CNC table

 

 

 

Figure 3.4 A schematic drawing of the laser cladding process [4].

3.3.1 CO2 Laser

A continuous wave CO2 laser operating in the TEMOI. mode, with an output power
range of 200 to 2500 watts was used for laser cladding experiments. The characteristics of a
2500 L model Triumph (Triumph Industrial Lasers Inc.) laser are given in Table 3.2. For
the purpose of this research, the CO2 laser was operated in a continuous wave (cw) mode.

Table 3.2 Characteristics of the RF excited CO2 laser

 

 

 

 

 

 

 

 

 

 

 

 

 

Parameters ValuelType

Type of Laser Fast flow coaxial CO2 laser
Mode of the beam TEMm.
Output stability :l:2%
Beam diameter (raw) z 18 mm
Excitation RF excited
Temporal mode Continuous and pulsed (1-10 kHz)
Maximum output power 2800 watts (continuous)
Wave length 10.6 pm
Gas consumption He: 64 l/hr

N22 12 VIII

C02: 3 Vin

 

 

 

3.3.2 Beam Delivery System

Optical devices mainly consist of a beam bender mirror and a transmissive ZnSe
focal lens with a focal length of 190.5 mm. Laser beam generated by laser machine travels
along its optical axis to a beam bender mirror, then vertically reflects down to a focusing
lens, and finally reaches the surface of a substrate for laser treatment process.
3.3.3 Powder Feeding System

The quality of laser cladding tracks depends upon the processing variables, and
position, stability, continuity, and accuracy of the powder feeding. Even a small deviation

of the powder flow rate, and the location of powder delivered with respect to laser beam

45

center, will lead to a significant variation of the geometry, thickness, and smoothness of
cladding tracks [101].

Lateral powder feeding system, as shown in Figure 3.5(a), has been commonly
used for laser surface alloying and cladding [91, 92]. However, in terms of our laser
alloying, and cladding experience, such a feeding system has many drawbacks. First, it is
difficult to align the location of the powder delivery with respect to the laser beam center.
This positioning is very critical, and a small deviation will greatly decrease the powder
utilization efficiency, and lead to a poor quality cladding tracks. Generally, the powder
utilization efficiency for lateral feeding system is rather low, and falls between about 0.3
and 0.5, depending on the positioning accuracy of powder delivered with respect to laser
beam center, and the divergence of powder stream. With side powder feeding systems,
preheating or premelting of powder does not occur, as a result, cladding tracks are
generally rough, and both thickness and width of tracks are not uniform. Second, side
feeding system, once set up, might work reasonably well for a linear cladding track, but
does not work for complex geometric patterns requiring bidirectional motions. Third,
typically, the inclination angle between the delivery tube and horizontal plane is 25-35°,
and the distance from the delivery tube tip to the interaction zone is about 12-15 mm, so
the distance from the laser nozzle tip to workpiece requires a clearance of at least 20 mm.
This gap is so large that the shielding gas is not fully able to protect the cladding surface
from oxidation. If the cover gas flow rate is increased to overcome this problem, the
powder is blown away. For these reasons, it is necessary to design a new powder feeding
system, which produces high stability, continuity, and accuracy of powder flow, while,

avoiding oxidation, under an actual production condition.

46

The integrated powder delivery system designed in this research, includes a close-

loop powder feeder, a powder splitter, and a coaxial nozzle, as shown in Figure 3.5(b).

 

Figure 3.5 An overview of powder feeding systems. (a) Lateral powder feeding system. (b)
Integrated coaxial powder feeding system.

47

3.3.3.1 Powder Feeder

A close-loop powder feeder (MET C0 Model 9MP) was used for providing a stable,
continuous, and accurate powder feeding rate. The heart of the 9MP powder feeder is the
proven fluidized-bed, powder regulating concept in which a carrier gas pressure
differential is used to deliver the powder at precise flow rates. A microprocessor based
weight-loss metering system continuously closed-loop monitors and controls powder feed
rates. This system is capable of consistently delivering alloy powder at rates as low as 0.07
gm/sec.
3.3.3.2 Powder-Flow Splitter

The powder feeder transports the alloy powder, through a hose, to the powder
splitter by using pressurized argon gas (called carrier gas). The powder stream is split into
four streams of powder flow. These four streams are then carried through four tubes to the
circumferential channel between the middle and the external nozzles.
3.3.3.3 Coaxial Nozzle

Figure 3.6 is a schematic diagram of the transverse section of our coaxial nozzle.
The specifications of this design, however, were developed by researchers in High Energy
Laser Processing (HELP) Laboratory, Michigan State University, based on our extensive
preliminary investigations.

The coaxial nozzle consists of an inner nozzle, a middle nozzle, an external nozzle,
and a water jacket for cooling. The inner nozzle has a channel, through which the focused
laser beam passes, and a shielding gas flows. Positive pressure of this shielding gas prevents
the particles from flowing up to the lens. As shown in Figure 3.6, the focal point of laser

beam is positioned at the tip of the inner nozzle to allow minimum opening for most

48

effective protection of the lens. The shielding gas also protects the cladding layer from

oxidation.

 

 

 

 

 

 

 

\ ' Innernozzle
l l
l l
'9 Waterjacket
fit 1
fl I
/ 1
a" Middlemzzle
/
' /
5 ‘1': Powder stream in (4)
3 / ’ ‘
' E Externalnozzle
Emery.--—
__..____.-— -—1——Surfaceofworkpiece

6.0

Figure 3.6 A schematic drawing of the transverse section of a coaxial nozzle [10].

The middle and the external nozzles form a cone-shaped circumferential channel,
with 27° cone angle, where four parts of powder streams meet and converge to form a cone-
shaped powder stream with the same central axis as the laser beam. Figure 3.7 shows the
nature of powder flow through the coaxial nozzle. This cone of powder finally interacts
with the laser beam on the surface of a workpiece to form cladding tracks. As a result of
precise deposition of the powder with respect to the center of the laser beam, the powder

utilization coefficient for our coaxial feeding system is up to about 80%. More details about

49

measuring the powder utilization coefficient are discussed in Chapter 4. Since the working
distance, from the bottom surface of the coaxial nozzle to the specimen surface, is fixed at
about 6-8 mm, spacers (each is 5 mm thick) are inserted between the middle nozzle and the
nozzle block for adjusting defocus distance, which controls laser spot diameter on the work

piece. Therefore, the range of spot diameter is between 1.8 mm and 3.4 mm.

 

Figure 3.7 A View of cone-shaped powder stream irradiated by laser beam.

During laser cladding process, a considerable amount of laser radiation is reflected
to the bottom surface of the nozzle, which heats it to a relatively high temperature.
Therefore, a circulating-water cooling system is necessary to prevent overheating of the
nozzle. From our study, this nozzle, equipped with cooling system, can work very well for a
continuous long-time use, for manufacturing patterned cutting-dies via laser cladding.

This integrated powder delivery system avoids all of the drawbacks of a lateral

powder feeding system. It can provide a highly stable, continuous, and accurate powder feed

50

rate, and deliver the powder stream precisely into the molten pool on the substrate to form
high quality cladding tracks. Also, this system allows deposition of complex geometric
patterns on a planar, or a curved surface for different applications, such as fabrication of
cutting and stamping dies, repair of turbine blades, and rapid prototyping of industrial
components. By adjusting the pressure of the coaxial cover gas, the amount of preheating of
powder can also be controlled.
3.3.4 CAM System

Computer-assisted manufacturing (CAM) system was used for laser cladding
experiments. CAM system consists of a high precision X-Y motion tables with a computer
controller (General Numeric Inc.), and its specifications are given in Table 3. The specimen
mounted on X-Y tables was translated under a stationary laser beam. The combination of
CAM system with two linear axes and step motor was utilized to fabricate various 3-D

components.

Table 3.3 CN C machine specifications

 

 

Description Value
Range of motion 1.27 m x 1.27 m

 

Maximum traverse speed 0.254 m/sec
Maximum contour speed 0.127 m/sec

 

 

 

 

 

Accuracy 0.0166 cm/m
Resolution 0.0127 cm
Repeatability 0.00254 cm

 

 

 

3.4 Laser Cladding
Laser cladding experiments were carried out in three separate sets of experiments:
(1) deposit tool steel CPMIOV on AISI 1045 medium carbon steel plates for modeling

work; (2) alternatively deposit stainless steel 304L and CPMIOV on the 1045 steel plate to

51

fabricate 3-D components; (3) coat CPMIOV on AISI 4140 steel plate for measuring
temperature distribution during laser cladding.

To date, no published literature has reported that tool steels, CPMIOV and
CPM15V, are used as hard-facing materials for manufacturing tooling dies by laser cladding
process. Therefore, wide ranges of experiments were accomplished to fully develop such a
brand-new and cost-effective process. The powder utilization coefficient was measured. The
correlations between the clad width and process parameters were determined. Various
relationships between the maximum clad thickness and process parameters were developed.
And optimal process variables were investigated for fabricating 3-D components for real-
world application.

In addition, the process parameters used in this research covered a wide range:
Laser power varied from 1.1 to 2.4 kw; laser spot diameter from 1.8 to 3.4 mm; traverse
speed from 3 to 14.3 mm/sec; and powder feed rate from 0.1 to 0.25 gm/sec. Argon was
used as the shielding gas, and its flow rate was fixed at 0.85 m3/hr, which ensured to prevent
the deposited coatings from oxidation during laser cladding. The detailed process

parameters are given in different sections in order to clearly explain and discuss different

relationships.
3.5 Temperature Measurement

Two types of thermocouples were utilized for measuring temperature distributions
during laser cladding process [102]. One is Tungsten-Rhenium thermocouple (C type),
and the other is Platinum-Rhodium thermocouple (R type). The upper limits for
temperature measurement of C-type and R-type thermocouples, respectively, are about

2400 °C and 1480 °C. Both types of thermocouples have 0.127 mm diameter.

52

Liu et al. [103] used k—type thermocouple to measure the temperature distributions
beyond welding zone, that is, heat-affected zone and base material. For convenience, we
have adopted some part of his experimental setup. Four small holes with 1.88-mm diameter,
were drilled from its bottom almost through top surface of a specimen in order to measure
the temperature of clad zone. The top thin layer (10 pm) is very important for the
temperature measurement of clad zone because, without this thin layer, laser beam will
directly irradiate the thermocouple junctions, and thus easily burn the thermocouples out.
The specimen dimension, and thermocouple layout and location are shown in Figure 3.8.
The C-type thermocouple is located at the laser beam center, and three R-type
thermocouples at the indicated positions. In addition, the thermocouples were inserted into
ceramic tubes, which have 1.59 mm outside diameter and 0.40 mm inside diameter, to
prevent the contact between two thermocouple wires. The thermocouple junctions were
welded to the bottom of the blind holes using a capacitive discharge welding facility.
Finally, the ceramic tubes were fixed to the workpiece using OMEGABOND 200 in order

to strengthen the connection between the thermocouples and the specimen.

53

Y
PA

Moving direction /

r

 

 

 

6.35 m I

 

75mm

4».

Thermocouple 0: X=0, and Y=0, Thermocouple 2: X=-5 mm, and Y=-2 mm,
Thermocouple 1: X=5 mm, and Y=l mm, Thermocouple 3:X=0, and Y=-4 mm (remelting),
or X=0, and Y=—5 mm (cladding).

 

 

 

   

2
Omegabond 00 Ceramic tube

 

To computer

 

C type thermocouple

Figure 3.8 An overview of specimen dimension, thermocouple layout and location [102].

3.6 Post-Clad Treatment

Tempering treatment was carried out in a bench-top Lindberg 58114 wire element
fumace with a microprocessor-base controller. It has a 1473 K temperature rating, and a
stability of -5/+5 K at steady state. In terms of the heat treatment specification of tool steel
CPM 10V, laser-deposited samples were tempered at 813 K (540 °C) for 15 minutes, and
then cooled to the room temperature. This process repeated one more time (so called
“double tempering”). The aim of tempering treatment in this study was to release the
thermal stresses caused by laser cladding process.

Subzero treatment was carried out in liquid nitrogen. The tempered sample was
dipped into liquid nitrogen for 15 minutes. This treatment led to the transformation of the
retained austenite to martensite, and therefore the hardness was obviously increased.

3.7 Microstructural Characterization

The laser-cladded samples, with or without thermal treatments, were sectioned,
mounted, polished, and etched using standard metallographic techniques. The
microstrutures of the cladding layer, interface, and heat-affected zone were characterized by
optical microscope (OM) and scanning electron microscope (SEM). The facilities used for
microstructural observation was given as follows:

0 Olympus PM-lOAK optical microscope.
- Olympus PME-3 optical microscope.
0 Hitachi scanning electron microscope.
In order to reveal the microstructural feature for each clad and base materials,

several chemical and electrolytic reagents were used to prepare the specimens for

55

microstructural observation. Table 3.6 lists the procedure used for specimen preparation

and chemical reagents.

Table 3.6 Chemical reagents and specimen preparation procedures

 

 

 

Technique Procedure Reagent Material Feature
Mechanical
polish
Optical Chemical etch #1 98 ml Mild and tool General
Microscopy (2% Nital) methanol steels microstructure
2 ml H3PO4
Scanning Chemical etch General
Electron (Marble) #2 10 g CuS04 304L stainless microstructure
Microscopy 50 ml HCl steel
50 ml H20
Chemical etch General
(Kalling) #3 1.5 g CuClz IN 738LC microstructure
33 ml ethanol superalloy
33 ml HCl
33 ml H20

 

 

 

 

 

 

3.8 Hardness Measurements

The microhardness measurements of laser cladding samples under different
treatment conditions were carried out using a LECO M-400-G1 rrricrohardness tester.
Different loads were used for the hardness measurement, and the indentation time was set
at 15 seconds. The hardness measurements were taken across the cladding layer, heat-

affected zone, and substrate at increments of 0.1 mm.

56

Chapter 4

RESULTS AND DISCUSSION

4.1 Characteristics of Cone-Shaped Powder Stream

In the past few years, coaxial powder feeding method has been used in laser
cladding process. Due to great changes in powder feeding mechanism, laser beam-materials
interaction results have been improved. This interaction has a great effect on the temperature
of clad zone. Therefore, it is necessary to discuss the characteristic of the powder stream
first. Figure 4.1(a) shows the nature of powder flow through the nozzle without laser beam
irradiation. The powder stream is uniformly distributed on the cone surface. As a result of
special design of our coaxial nozzle, cone-shaped powder stream interacts with laser beam
at about 3.5 mm from the nozzle exit, and is preheated to rather high temperature, or even
partially or fully molten before reaching the molten pool of a substrate, as shown in Figure
4.1(b). It was reported that after the powder stream interacts with the laser beam during its
motion from the nozzle exit to the workpiece surface, only approximately 40% of the laser
energy is transmitted to the workpiece [104]. This process is very complex, and involved in
many factors, such as spatial distribution of laser beam and power density, velocity and
density distribution of the powder stream, powder property, interaction time between the

laser beam and the powder stream, and cover gas flow rate, etc [102, 105].

57

Cone-shaped
' powder stream

 

Figure 4.1 A view of cone-shaped powder stream. (a) Without laser beam. (b) With laser
beam. Process parameters are as follows: laser power is 2.1 kw, laser spot size
2.9 mm, powder feed rate 0.25 g/sec, and cover gas flow rate 0.85 m3/hr.

Cover gas flow rate has a great effect on cone-shaped powder stream. When cover
gas flow rate is increased from 0.85 to 3.4 m3/hr, it exerts a larger force on cone-shaped
powder stream along its central axis, and squeezes the powder stream out, as shown in
Figure 4.2(a). 0n the other hand, with higher cover gas flow rate, a larger impact force

develops, and more powder bounces up from a workpiece surface in all directions, as

displayed in Figure 4.2(b). This can lead to decrease in powder utilization efficiency, and

58

formation of poor quality cladding tracks. Circular cladding track 2 in Figure 4.3 is an
example of poor quality clads due to higher cover gas flow rate. It has been found that, for
our specially designed coaxial powder feeding system, the upper limit of cover gas flow rate

for laser cladding process is about 1.7 m3/hr.

 

Figure 4.2 Effectlof cover gas flow rate on cone-shaped powder stream. (a) Without laser
beam 1rradiation. (b) With laser beam irradiation. Cover gas flow rate 3.4 m3/hr.
Other processing parameters are the same as those of Figure 4.1.

59

 

Figure 4.3 A view of circular cladding tracks on a solid cylinder. Cladding track 2 is an
example of poor quality clads due to higher cover gas flow rate.

4.2 Desirable Powder Morphology and Particle-Size Distribution

Material factors can be divided into intrinsic and extrinsic properties of the clad
and the substrate materials. Intrinsic properties include thermal conductivity and
diffusivity, specific heat, latent heat, reflectivity, and mass density of the powder and the
substrate. These intrinsic properties affect the molten-pool temperature, and temperature
distribution in the substrate and thereby affecting clad formation. The extrinsic properties
include particle size, particle-size distribution, particle shape, and the surface condition of
the substrate. A powder suitable for laser cladding must have a proper particle-size
distribution and particle shape for it to efficiently absorb laser energy and form a molten

pool. Spherical particles are preferable for smooth powder delivery.

Figure 4.4 shows a spherical powder morphology. To the best of our knowledge,
this is a desirable powder, and best suited for laser cladding process. The particle-size

distribution for the powder falls between a range of 53 to 125 um.

 

Figure 4.4 A View of desirable powder morphology for laser cladding process.

4.3 Effect of Powder Feeding Systems on Clad Quality

Figure 4.5(a) shows an overview of single cladding tracks with rectangular contours
(x and y translations) on an AISI 1045 steel plate. The cladding track, which is 2.3 mm in
width, and 1.3 mm in height, is smooth in appearance, and upon testing, found to be
metallurgically sound. Figure 4.5(b) displays a macrograph of cladding tracks perpendicular
each other, which were produced by using lateral powder feeding system. As seen in Figure
4.5(b), cladding tracks look very rough. When the powder delivery direction was

perpendicular to the substrate-moving direction, no clad was formed because no molten pool

61

on the substrate was produced during laser cladding process. It is this drawback of the side
powder feeding system that limits the real-world applications of laser surface alloying and

cladding in consumer product industries.

ii
1
i

":92

"1 manner
.1. IItIIIQll r 1 I

 

Figure 4.5 Effect of powder feeding systems on cladding quality. (a) Rectangular cladding
tracks with smooth surface. (b) Perpendicular cladding tracks with rough
surface via lateral powder feeding system.

4.4 Measurement of Powder Utilization Coefficient

Powder utilization coefficient a is defined as the ratio of the amount of powder

participating in cladding to the total amount of powder provided by the feeding system

within the duration of laser irradiation on the substrate. The value of dis not only related

62

to the type of feeding mechanism, such as side feeding system or coaxial feeding system,
but also to material characteristics like the type, particle size, and particle shape of
powders, and the nature and dimension of substrates, etc. Generally, the a value for
coaxial feeding system is much higher than that for side feeding system (for side feeding,
typically, a value falls between about 0.3 and 0.5 in comparison with about 0.8 for

coaxial case) due to the precise deposition of the powder with respect to the center of the

laser beam.
Many experiments with various process conditions, were carried out to measure
the powder utilization coefficient a The powder utilization coefficient a was computed

by setting a powder feed rate, and then recording the laser scanning time, and weighing

the sample before and after laser treatment.

If W, is weight of the substrate, and W2 total weight of the substrate and the clad,
then W, = W2-W1 is the net clad weight within scanning time t. If 1“,, is power feed rate,
then Wp = Fpt, is the total weight of powder delivered within the scanning time t. The
powder utilization coefficient a is then obtained from this ratio: a = WJWP, Based on
experimental results, a few relationships have been observed:

0 For a given laser power, spot size, and powder feed rate, when traverse speed varies
in a range of ~ 5 to ~10.5 mm/sec, or value is approximately equal to 0.7 and 0.8 for
each process condition, as shown in Figure 4.6. This shows that or value is not related
to traverse speed.

0 For a given laser power, spot size, and traverse speed, when powder feed rate varies

from 0.1 to 0.25 gm/sec, avalue is also rather close to 0.8, shown in Figure 4.7. This

63

means that the ratio Well“p is a constant when scanning time t is fixed because the net
clad weight increases with the increase of powder feed rate.

Figure 4.8 shows that a value is related to spot size. The smaller the laser-beam
diameter, the less amount of powder laser beam catches, and therefore the lower the
powder utilization efficiency. When laser-beam diameter D = 1.8 mm, average a 5

0.71. While D 2 2.3 mm, average as 0.80.

 

 

 

 

 

—o—P=2073 watts, 0:2.3 mm,

0.95 ~~ Fp=0.25 gm/s.
E — a- - P=1567 watts. D=1.8 mm,
.3 0.9 *r Fp=0.1gmls.
11:
§ 0.85 --
C ‘_/—4
.9
a 0.8 -- ‘Ar’ '
'13 0.75 --
h-
8
g 0.7 -- D” ''''' “D' ------ -r:r
a

0.65 «-

0.6 i 1 l 1 .g 4.
4 5 6 7 a 9 10 11
Traverse speed (mm/s)

Figure 4.6 Plot of powder utilization coefficient vs. traverse speed.

 

 

 

 

 

-o—P=2073 watts, D=2.9 mm,
v:6.6 rum/s.
E - D- - P=2370 watts, 0:2.9 mm.
3 v=8.4 mmls.
§ 0.9 -~
g
is”
“3
g 0.8 -~
3
0.
0.7 1 1 1 1
0.05 0.1 0.15 0.2 0.25 0.3

Powder feed rate (gm/sec)

Figure 4.7 Plot of powder utilization coefficient vs. powder feed rate.

 

 

 

 

E 0931567 watts
52 0.9 -. IP=2073watts
g AP=2370 watts
C
Q A
'15 0.3 -- I
g
'5
§ 0.7 1— °
8.
81 -.
E 0.6
3

0.5 1 1

1.5 2 2.5 3

Laser spot size (mm)

Figure 4.8 Plot of average powder utilization coefficient vs. laser spot size.

65

4.5 Relationships between Clad Width and Process Parameters

Theoretically, clad width is primarily dictated by the laser-beam diameter, the

integrated beam diameter or the scanning beam amplitude. However, the clad width is

also related to traverse speed, energy density (PIMP/70v), and powder feed rate. In

order to determine the relationships between the clad width and processing parameters,

several experiments were carried out under various processing conditions. Based on

experimental results, the following points can be made:

With other variables held constant, clad width linearly decreases as the traverse speed
increases, as shown in Figure 4.9. When traverse speed accelerates to about 8.5
mm/sec, clad width is approximately equal to laser-beam diameter for each process
condition. This speed is called threshold traverse speed under experimental
conditions. Below threshold traverse speed, cladding track is wider than laser-spot

size. Lines (1), (2) and (3) in Figure 4.9 are governed by the following equations

respectively,
w=—0.l9v+4.64 (4.1)
w = —0.19v + 4.25 (4.2)
w = —0.20v + 3.93 (4.3)

Figure 4.10 is a diagram of clad width versus energy density. When laser power, spot
size, and powder feed rate are kept constant, clad width increases with increase in

specific energy. This is because sufficient energy is available to melt clad material

situated at a distance wider than the beam itself.

66

0 Figure 4.11 shows that, with other variables held constant, cladding track gently
widens as powder feed rate increases. If the energy input is sufficient, the large

powder feed rate will lead to a wider cladding track.

 

 

 

 

 

4
3.5 ~~
’e‘
g 3 «4»-
.C
5
3 ‘A
U 2.5 -_ D P32370 watts. D= 2.9 mm. \ . \
8 Fp=0.2s gm/s. - \ .(3)
\ O
X P=2073 watts. D=2.3 mm. 1
2 + Fp=0.167 gm/s.
A 1922073 watts, D=1.8 mm.
Fp=0.167 gm/s.
1.5 1 1 1 1 1 1
2 3 4 5 6 7 8 9
Traverse speed (mm/sec)

Figure 4.9 Clad width as a function of traverse speed.

67

Clad width (mm)

Clad width (mm)

 

 

 

 

 

 

 

 

4
A P=2073 watts, 0:23 mm,
Fp=0.167 gm/s.
3.5 «—
3 -_
2.5 1 1 1 1 1
100 150 200 250 300 350 400
Energy density (J/mmz)
Figure 4.10 Clad width as a function of energy density.
4
-D-P=2073 watts, D=2.9 mm.
v=5.8 rum/s.
3.5 «~
3 ~1—
2.5 1 1 1 1
0.05 0.1 0.15 0.2 0.25 0.3
Powder feed rate (gm/sec)

Figure 4.11 Clad width as a function of powder feed rate.

68

4.6 Process Modeling

The laser cladding is a very complicated process involving the following physical
and metallurgical phenomena: (1) laser-beam energy and material interaction, (2) heat
transfer in the clad, and the substrate, (3) rapid melting and solidification, and (4) clad
material and substrate material reaction and inter-diffusion. For this reason, process
modeling of the laser—cladding process is a difficult task. A few models for laser cladding
process have thus far been developed by other authors [106-108]. These models involved
various physics concepts and complicated mathematical calculations, and help understand
phenomena and mechanisms during laser cladding, but were not suited for rapidly
evaluating optimized processing parameters. The following model is developed based on
the geometric features of the transverse section of single cladding tracks, physical
properties of alloy powder itself, and processing parameters. Due to a simple, but realistic
approximations, this process model can be easily used to evaluate the optimal processing
conditions for on-line production and manufacturing.

4.6.1 Assumptions

The model is based on the following assumptions:

1. Power density is suitable for laser cladding process, where the clad track is sound and
free of cladding defects, such as porosity, microcracks, poor metallurgical bond, and
high dilution.

2. Geometry of the transverse section of single cladding tracks is parabolic arch.

3. When traverse speed v < ~8.5 mm/sec, Equations (4.1), (4.2) and (4.3) are used to

calculate the clad width, which is inserted into related formula to calculate the

69

maximum clad thickness. While traverse speed varies from ~85 to ~14.5 mm/sec,
laser-beam diameter is substituted for the clad width in the model.
4. Laser-spot diameter D = 1.8 mm, as 0.70, while D 2 2.3 mm, as 0.80.

4.6.2 Basic Equation

Net clad weight is a function of powder utilization coefficient, powder feed rate,

and irradiation time, so it can be expressed as

WC = ant (4.4)
where W, is net clad weight (gm), apowder utilization coefficient, F ,, powder feed rate
(gm/sec), and t irradiation time (see) (related to scanning speed).

According to assumption 2, the cross-section is approximately represented by a

parabolic arch, as shown in Figure 4.12, and its area A is expressed as

 

 

 

 

 

 

 

Figure 4.12 Parabolic shape of the cross-section of a single cladding track.

A = §wh (4.5)

where w is clad width (mm), and h the maximum clad thickness (mm).

70

From geometric shape of a clad track, and physical properties of the alloy itself,

the net clad weight can be written as
2
W, = §wh(vt)p (4.6)
where v is traverse speed (mm/sec), and p alloy density (gm/cm3). For tool steel CPM

10V, p = 7.418 gm/cm3. The term,-:—wh(vt) , is actually the volume of a single cladding

track.

3 p ( . )

Equation (4.7) is a basic equation, relating the maximum clad thickness and

various process parameters.
4.6.3 Energy Density Equation
Energy density is defined as [21]
E a = P41 (4.8)
where E; is energy density (J/mmz), Pd power density (watts/mmz), and 1' time (sec)

required to move a distance of the laser beam diameter along the traverse direction.

Power density is expressed as
P
P = — 4.9
.1 A: ( )

where P is laser power (watts), and A1 irradiation area (mmz).
For simplicity, assume that the shape of laser spot is circular (for our TEMm-
mode laser beam, the shape actually is doughnut), then irradiation area A 1 = 7502/4, where

D is laser beam diameter (mm).

71

Substituting A 1 into Equation (4.9), we can obtain

 

4 P
P = 4.10
d er2 ( )
The scanning time rcan be expressed as
r = 2 (4.1 1)
v

Substituting Equations (4.10) and (4.11) into Equation (4.8), one has

— ii- (4.12)

E—
derv

The above equation shows that the energy density is directly proportional to laser

power, and inversely proportional to spot size or traverse speed.

4.6.4 Clad Thickness-Energy Density Equation

Equations (4. 7) and (4. 12) can be rewritten as, respectively,

 

3aF
h = p (4.13)
2va
_l_ = FL (4.14)
Dv 4P

Based on assumption 3, when v 2 8.4 mm/sec, w = D. Then substituting Equation (4.14)

into Equation (4.13), we can obtain

aF
3” P E (4.15)

 

72

For a particular alloy powder, if laser power and powder feed rate are fixed, the clad
thickness is directly proportional to energy density, which depends on the spot size and
traverse speed.

4.7 Relationships Derived from Basic Equation

Equation (4. l 3)

3an
— 2va

 

shows that clad thickness is related to powder feed rate, spot size (w = D, when v 2 8.4

mm/sec), and traverse speed, while the laser power is held constant.

4.7.1 Relationship between Clad Thickness and Traverse Speed
In Equation (4.13), for a given powder (fixed mass density p), if laser power, spot

size, and powder feed rate are kept constant, then Equation (4.13) can be simplified as
h = — (4.16)

where C, is a constant depending upon process variables and alloy density.

Equation (4.16) shows that the clad thickness is inversely proportional to traverse
speed. The faster the traverse speed, the less is the powder input per unit length, and
therefore the thinner the cladding track. For example, substituting a: 0.70, Fp = 0.167
gin/sec, and p = 7.418 x 10’3 gin/mm3 into Equation (4.13), it becomes: h = 23.59/wv.
When traverse speed varies from 3 to 6.6 mm/sec, the corresponding clad thickness can

be calculated using Equation (4.3) and this simplified relationship. Figure 4.13 displays

the calculated and experimental plots, which show that clad thickness is inversely

73

proportional to traverse speed in both cases. Also, experimental and calculated results

match very well.

 

 

 

 

 

 

2.5

P=2073 watts, D=1.8 mm,
Fp=0.167 gm/s.
(calculated)

E - - - -Experlmental

e 2 ‘

33

3

5

.9

5

E 1.5 —

O

1 I I I I
2 3 4 5 6 7

Traverse speed (mm/sec)

Figure 4.13 Clad thickness as a function of traverse speed.

4.7 .2 Relationship between Clad Thickness and Powder Feed Rate
If laser power, spot size and traverse speed are kept constant, Equation (4.13)

becomes

h = €sz (4. 17)

where C2 is a constant which is related to process parameters and alloy density.
Equation (4.17) reveals that clad thickness and powder feed rate have a linear
relation. The larger the powder feed rate, the more is the powder input per unit length,
and higher will be the clad thickness. However, for a fixed energy density, if the powder

feed rate is too high, it is possible to cause lack of fusion in cladding layer, or produce

poor metallurgical bonding between the cladding layer and the substrate.

74

Inserting a= 0.8, w = D = 2.9 m, V = 8.4 mm/sec, and p = 7.418 x 10'3 gm/mm3
into Equation (4.13), then it becomes h = 6.64 Fp. Theoretical and experimental results

are displayed in Figure 4.14. As shown in Figure 4.14, clad thickness linearly increases as

powder feed rate increases.

 

 

 

 

 

 

2
P=2370 watts, D=2.9 mm,
v=8.4 mm/s. (calculated)
E - G- - Experimental
E 1.5 ~- D
8
2
x
.9
5
s 1 1
0
a O
0.5 1 1 1 1
0.05 0.1 0.15 0.2 0.25 0.3

Powder feed rate (gm/sec)

Figure 4.14 Clad thickness as a function of powder feed rate.

4.8 Correlation Derived from the Energy Density Equation

Equation (4.12) displays that the energy density is related to spot size, laser

power, and traverse speed.
4.8.1 Relationship between Energy Density and Spot Size

The laser power and traverse speed are kept constant, and then Equation (4.12) is

simplified as

Ed =— (4.18)

where C3 is a constant.

75

Equation (4.18) displays that energy density is inversely proportional to spot size.
In other words, energy density and the reciprocal of the spot size have a linear relation, as
shown in Figure 4.15. For very low values of energy density, it is possible to create
cladding defects, such as lack of fusion and poor metallurgical bonding, or even

debonding. This is because insufficient energy is available to melt the substrate surface

after melting powder particles.

 

   
  
  

 

 

 

350
P=2370 watts, v=4.9 mm/s.
------ P=2370 watts, v=6.6 mm/s.
300 -~
_ . — - P=2370 watts. v=8.4 mmls.
117‘
E 250 .. (2). a ' '
3 -' ' '
E
10
5 200 _,_ (3) ,1
“o I' I
o I I.’
”C1 150 "" I I
100 ~~
50 i

 

0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6

Reciprocal of spot size (mm")

Figure 4.15 Energy density as a function of reciprocal of spot size.

4.8.2 Relationship between Energy Density and Traverse Speed

When the laser power and spot size are fixed, Equation (4.12) is reduced to

E, =— (4.19)

where C, is a constant.

76

The above equation demonstrates that the energy density is inversely proportional
to traverse speed. The faster the traverse speed, the less the interaction time or dwell time,
and the smaller the energy density. The relation between energy density and the
reciprocal of traverse speed is displayed in Figure 4.16. It shows that energy density
linearly increases as the reciprocal of traverse speed increases.

In terms of Equation (4.10), when laser power is fixed, power density is related to
spot size, and not related to traverse speed. Therefore, it is more reasonable to use energy

density to explain phenomena that happen during laser cladding than power density.

 

 

 

 

 

 

300
P:2370 watts. 0:23 mm.

250 ._ - - - - P=2370 watts, D=3.4 mm.
E
3 200 -~
.12:
«n
c:
8
g 150 ..
2
1.1.1

100 «-

50 1 . .
0.05 0.1 0.15 0.2 0.25

Reciprocal of traverse speed (smm")

Figure 4.16 Energy density as a function of reciprocal of traverse speed.

4.8.3 Relationship between Energy Density and Laser Power

If spot size and traverse speed are held constant, then Equation (4.12) becomes

77

E d = CSP (4.20)

where C5 is a constant.
Equation (4.20) shows that energy density and laser power have a linear relation.
The higher the laser power, the larger the energy density, as shown in Figure 4.17. If the
energy density is too high, it is possible to create high dilution which can change the
properties of cladding layer. This is because, for a given powder feed rate, greater energy
delivered per unit area causes greater substrate melting and change in clad alloy

composition.

 

280

 

D=1.8 mm, v=6.6 mm/s (1)

- - - - D=3.4 mm, v=6.6 mm/s
230 ~~

‘21

Energy density (Jmm'z)
8
o

 

 

 

 

30 1 .
500 1000 1500 2000 2500

Laser power (watts)

Figure 4.17 Energy density as a function of laser power.

78

4.9 Relationship between Clad Thickness and Energy Density

From Equations (4.12) and (4.13), we know that, for fixed laser power and spot
size, energy density is only a function of traverse speed. While, for fixed laser power,
spot size, and powder feed rate, clad thickness is also a function of traverse speed.
Therefore, both clad thickness and energy density are inversely proportional to traverse
speed. The faster the traverse speed, the lower the energy density, and thinner the clad
track.

Substituting a = 0.8, P = 2370 watts, D = 3.4 mm, Fp = 0.25 gm/sec, and p =
7.418 x 10'3 gm/mm3 into Equations (4.12) and (4.15), respectively, we obtain: E4 =
887.5/v and h = 0.0134 Ed. When traverse speed varies from 8.4 to 14.3 mm/sec, the
corresponding clad thickness can be calculated using the above two simplified
relationships. Figure 4.18 represents calculated and experimental results between the clad
thickness and the specific energy, respectively. They show that the clad thickness and
energy density have a linear relationship. Also, the calculated result is in good agreement
with the experimental result.
4.10 Relationship between Penetration Depth and Energy Density

If traverse speed and spot size in Equation (4.12) are kept constant, energy density
is only a linear function of laser power. For a given powder feed rate, the higher is the
laser power, the higher is the energy density, thus more energy is transferred to the
substrate, and therefore deeper is the heat-affected zone. The plot of penetration depth

versus energy density is shown in Figure 4.19 The penetration depth linearly increases

with increase in energy density.

79

Clad thickness (mm)

Penetration depth (mm)

1.5

 

 

   
  

 

 

 

 

 

 

 

 

P=2370 watts. D=3.4 mm,
Fp=0.25 gm/s. (calculated) .1:
- -D- - Experimental I
1 ..
0.5 1 1 1 1 1 1
50 6O 70 80 90 100 1 1 0 1 20
Energy density (Jmm'z)
Figure 4.18 Clad thickness as a function of energy density.
0.65
nv=a4 mm/s, o=2.3 mm.
Fp=0.167 gun/s.
0.6 -~
0.55 --
0.5 1 1 1
50 75 100 125 150
Energy density (J/mmz)

Figure 4.19 Penetration depth as a function of energy density.

80

4.11 Evaluation Standards of Laser Cladding Quality

The main processing parameters for laser cladding include laser power and spatial
distribution, laser spot diameter, traverse speed, powder feed rate, cover gas flow rate,
and overlapping percentage for multi-track buildup. Each parameter plays an important
role in laser cladding processing. Only if all these parameters harmonically work
together, high quality cladding track or layer can be produced.

General criteria for evaluating laser cladding quality is setup as follows: clad has
a smooth surface and an uniform width, it is free of porosity, crack and inclusion, and has
a good metallurgical bonding with the substrate but a low dilution. In addition, a good
clad has three zones: clad zone, interface zone, and heat-affected zone (HAZ). Clad zone
is a solidified molten alloy. Interface zone is an inter-diffusion zone between the clad and
the substrate. And heat-affected zone is produced by transferring laser energy to the
substrate during the process.

Due to higher laser energy input, clad in Figure 4.20(a) is sunk into the substrate
so that both chemical composition and mechanical and physical properties of
reinforcement material have been changed. Thus, buildup with a high dilution is not
desirable. As seen in Figure 4.20(b), the buildup meets the above evaluation criteria.
Therefore, this is an optimized processing condition. Figure 4.20(c) clearly shows that
there are a lot of pores inside the clad. Such a defect will adversely affect the mechanical
strength of cladding layer. Because of low laser energy input, cladding track did not
metallurgically fuse with the substrate, as shown in Figure 4.20(d). This phenomenon is
called lack of fusion. Optimized shielding gas flow rate has a great effect on cladding

quality. If cover gas flow rate is too high, then it is highly possible to produce porosity

81

because some inert gas is entrapped in the clad due to high cooling rate. On the other
hand, if it is too low, cladding zone is not completely protected. Therefore, oxides or
inclusion form inside the clad during the process, as shown in Figure 4.20(c).

For multi-layer deposition, both molten pool temperature and temperature
distribution of the buildup have to be well controlled by gradually reducing laser power
input if other process parameters are kept constant. Otherwise, it is easy to create
cracking inside the clad or heat-affected zone due to overheating and thermal cycle

fatigue. Figure 4.20(1) clearly display such a defect.

 

Figure 4.20 Overview of the transverse section of cladding tracks or layers. (a) High
dilution. (b) Optimized condition. (c) Porosity. ((1) Lack of fusion. (e)
Inclusion. (f) Cracking. A: Clad. B: Base metal.

82

(continued)

 

83

(continued)

 

84

(continued)

 

4.12 Overlapping Rate

Required dimensional buildup consists of multi-tracks or layers. For convenience,
it is necessary to define the term "overlapping rate". Schematic drawing of cladding track
overlapping is displayed in Figure 4.21. Just as it implies, overlapping rate R0 between
two adjacent tracks is defined as follows:

R0 = ¥X100% (4.21)

where D is laser spot diameter on the substrate surface (mm), D; the distance CNC table
moves with respect to the laser beam center (mm).
For example, if laser spot size D = 2 mm, and moving distance D, = 1 mm, and

then Rois 50%.

85

Overlapping rate plays a very important role in multi-track deposition because
this parameter affects laser cladding quality. When other processing parameter are kept
constant, for example, too small overlapping rate often creates such a defect as lack of
fusion between two adjacent tracks during laser cladding. Based on our experimental
data, 50% of overlapping rate is most suited for multi-track deposition. This point will be

explained in the section of prototype of 3-D components.

laser beam center cladding direction
—>

Figure 4.21 Schematic drawing of the transverse section of multi-track overlapping.

4.13 Dilution

By definition, laser cladding process is to deposit a reinforced material onto a
metallic substrate without changing its chemical composition and properties. In order to
get a good metallurgical bonding between the clad and the substrate, there must be an
interface zone or inter-diffusion zone. Theoretically, from the point of view of physical
metallurgy, a few atomic-layer inter-diffusion zone is thick enough to ensure a solid
bonding between the deposit and the base material. In practice, it is very difficult to
control laser energy input to get that ideal interface zone. For simplicity, the term dilution

is introduced here. Figure 4.22 presents the concept of dilution ratio. Dilution can be

86

defined as the change in chemical composition of the reinforced material caused by
intermixing of the melted substrate and deposit. Based on this definition, it is easy to
express the dilution Rd by the following relation:

R, = 1'“ x100% (4.22)
h +hd

C

 

where he is clad thickness without change in composition (mm), and hd intermixing
thickness between the deposit and the substrate (mm).

For laser cladding process, dilution ratio has to be controlled as low as possible in
order to maintain both composition and properties of reinforced material. To the best of
our knowledge and experimental data, tens of microns of inter-diffusion zone is

practically an ideal interface zone, as shown in Figure 4.23.

 

     
 

 

 

 

clad workpiece
hc surface
interface
substrate

 

 

 

Figure 4.22 Schematic drawing of cladding and intermixing zones for calculating
dilution ratio.

87

inter-diffusion zone

1

base material fl

Figure 4.23 Optical microstructure of the transverse section of a cladding track, showing a
low intermixing zone.

 

4.14 Calculation of Deposition Rate
Assume that a cubic coupon is directly deposited from metallic powder by laser
cladding process, then its volume V can be expressed as follows:

V = LWH (4.23)
where L, W, and H are length, width and height of the coupon, respectively. Due to the
characteristic of laser cladding, such a coupon can be produced only by overlapping
multi-tracks and multi-layers. For this reason, the term deposition rate is introduced to
describe the processing productivity. Just as its name implies, the deposition rate is
defined as the volume produced within a unit time (mmalsec). Figure 4.24 clearly shows

how multi—tracks overlap to produce a platform layer.

88

 

 

 

Figure 4.24 Schematic drawing of multi-track overlapping to produce a platform layer
by laser cladding process.

For 50 percent of overlapping rate between two adjacent tracks, and based on
Equation (4.23) and Figure 4.23, it is easy to get following relationships:
L=l=vt
W = [n - 0.5(n —1)]w (4.24)
H = mh
where l, w and h are length, width and height of a single cladding track (mm),
respectively. v is traverse speed (mm/s), and 1' time required for producing a single
cladding track (3). n is the number of multi-tracks, and m the number of deposited layers.
Substituting the above equation into Equation (4.23), one can have
V = 0.5m(n + l)whl (4.25)
Equation (4.25) is divided by total deposition time t, then deposition rate Rd is given by

_ 0.5m(n + 1)whl
t

 

R, (4.26)

The above equation can be used to roughly calculate the deposition rate.
4.15 Geometric Characteristics of Single Cladding Tracks
The clad bead geometry is related to process parameters, such as power density,

traverse speed, and powder feed rate. There are three basic single clad bead geometries:

89

deep penetration bead in Figure 4.20(a), parabolic clad bead in Figure 4.20(b), and clad
bead with a low dilution and a reasonable aspect ratio of width to height. As has been
discussed before, clad bead with high dilution rate is not desirable because both chemical
composition and properties of the reinforced material have been changed. In our newly
developed process for producing tooling dies, because the cutting blades on the rotary
cutting die is 1.5 mm high, only parabolic clad track can meet the height requirement for
the sake of cost—effectiveness.

When multi-tracks overlap to produce larger coating area, transverse geometry of
single cladding tracks, overlapping percentage, and aspect ratio are very important
parameters. In terms of our experimental results, for multi-track laser deposition, the
aspect ratio is at least 2 in order to make coating layer with a smooth surface and no
defects. Within a certain range of the aspect ratio, the larger this value is, the more it is
suited for multi-track buildup. Figure 4.25 shows two types of transverse geometric
shapes of single cladding tracks. The aspect ratio in Figure 4.25(a) is small; while the
aspect ratio in Figure 4.25(b) is reasonably large. When two adjacent tracks with a small
aspect ratio overlap, because laser energy hardly transmits through the first track to fuse
two tracks together, and then it is highly posssible to create lack of fusion in marked L
region during the laser cladding. Figure 4.26 clearly exhibits such a defect.

Based on our processing model, there are two ways to control the aspect ratio.
One way is to increase traverse speed to reduce clad thickness. In this way, the aspect

ratio can be increased. The other way is to decease powder feed rate to increase aspect

ratio.

90

 

(a) (b)

Figure 4.25 Schematic drawing of transverse geometric shapes of single cladding tracks.
(a) Small aspect ratio. (b) Desirable aspact ratio.

 

Figure 4.26 An overview of lack of fusion forming under the following condition: two
adjacent tracks with a small aspect ratio ovrelap. Unetched.

4.16 Laser Direct Fabrication of 3-D Components

Now laser cladding process is demonstrating a great application potential in various
manufacturing industries. The combination of laser cladding technique and advanced
CAD/CAM system can be employed for rapidly prototyping industrial components, and
tooling dies with required geometric shapes and larger dimensions. This section will
present some foremost-mentioned applications of laser cladding process in manufacturing
industries.
4.16.1 Tooling Dies
4.16.1.1 Fabrication of Tooling Dies

Figure 4.27(a) displays a view of rectangular hard-facing tracks on an AISI 1045
steel roller with 101.6 mm diameter [4]. High wear-resistant tool steel CPM15V was
laser deposited on the steel roller to form near-net-shaped cutting blades. By using
optimized processing parameters, it just took 45 minutes to deposit 12 pieces of near-net-
shaped cutting blades by laser cladding. Compared with the conventional production
process (it takes 7 days to finish the whole process, including both heat treatment and
machining), this new process is so efficient that production costs can be significantly
reduced. For the sake of cost-effective production, parabolic clad bead was chosen for
this particular application purpose. The cladding tracks are 1.8 mm in height and 3 mm in
width. They are subsequently machined, sharpened and honed into cutting blades, as
shown in Figure 4.27(b). Fully functional prototype of industrial rotary cutting dies, for
pilot testing, has been successfully produced by this newly developed process. The
experimental result has also demonstrated that the curvature of a substrate can be

accommodated, in our laser cladding system, to produce high quality curved cladding

92

tracks. Thus, this new technique has opened a novel application field of laser-assisted die
manufacturing, through which required geometric patterns on steel substrates can be

deposited for manufacturing cutting and/or stamping dies.

 

Figure 4.27 An overview of near-net-shaped cutting blades deposited on a steel roller by
laser cladding process. (a) As-cladded condition. (b) After machining [4].

The optimized processing parameters for directly fabricating the cutting dies are

listed in Table 4.1.

93

Table 4.1 Processing parameters of producing rotary cutting dies

 

 

 

 

 

 

 

 

 

 

 

 

 

Samples Material Power Traverse speed Spot size Powder feed rate I

I P (watts) v (mm/sec) D (m) Fp (gm/sec)

[ 1 | CPM 15v [ 2370 | 4.7 | 2.3 | 0.25 |
Samples Energy density Cover gas flow rate Carrier gas flow rate

I E. (J/mmz) (m3/hr) (1113/1111)

| 1 | 279.2 | 1.27 | 0.62 |

 

4.16.1.2 Microstructure of Clad Bead

Vanadium contents in CPM 10V and CPM 15V are up to about 10 and 15 wt. %,
respectively. Its major influence is the formation of highly wear-resistant carbides.
Vanadium carbide is one of the hardest carbides found in high speed steels. These
carbides also help maintain a very fine grain size at the high heat treating temperatures
used for high speed steels because vanadium carbide is a thennodynamically stable
interrnetallic compound. That is why highly wear-resistant tool steel CPM15V was used
to form near-net-shaped cutting blades by laser cladding process. Figure 4.28 (a) displays
that fine spherical vanadium carbide particles are uniformly dispersed in the matrix,
which consists of martensite and retained austenite. High magnification SEM micrograph
in Figure 4.29 clearly shows the morphology of vanadium carbide. Due to high cooling
rate, laser cladding process refines microstructures, and forms supersaturated solid
solutions. Therefore, strength, hardness, and ductility of deposited tracks can be greatly
improved. In comparison, the morphology of coarse eutectic chromium carbides and
precipitated Cr7C3 and 0230; carbides in D2 alloy [109] is shown in Figure 4.28(b). The
coarse chromium carbides segregate along transverse direction in D2 tool steel ingot, and

lead to poor mechanical properties.

94

 

Figure 4.28 Overview of carbides in CPM 10V and D2 alloys. (a) Fine vanadium carbide
uniformly dispersed in the matrix in CPM 10V tool steel. (b) Segregation and
flake morphology of coarse chromium carbides in D2 alloy.

95

 

Figure 4.29 SEM micrograph of clad bead, showing morphology of vanadium carbides.

4.16.1.3 Microhardness of Clad Bead

Figure 4.30 shows Vickers hardness measured from the CPMIOV clad bead to the
substrate under different treatment conditions. The sample used to measure the hardness
was made by the following processing parameters: laser power P is 2073 watts, laser spot
diameter D is 2.3 mm, traverse speed v is 3 mm/sec, and powder feeding rate FF is 0.167
gm/sec.

The hardness of the as-clad sample is about 650 Hv, which is approximately equal
to that of conventional tool steel D2 after standard heat treatment, as shown in Figure
4.30 (a).

Because retained austenite in high-alloy steels is extremely stable, it must be

destabilized by tempering above 500°C, and transformed to martensite. Also, the

96

tempering treatment can further increase the hardness of steel by secondary hardening
and the formation of hard martensite [110, 111]. For this reason, after the sample is
“double tempered” at 540°C for 15 minutes each, the hardness increases to about 700 Hv,
as shown in Figure 4.30 (b).

After the tempered sample is dipped in liquid nitrogen for 15 minutes, the

hardness increases to 850 Hv due to the transformation of the retained austenite to

martensite, as shown in Figure 4.30 (c).

 

 

 

 

 

900
a
800 «-
Interface

A 700 ‘W
>
E;
g 600 -~
13
w 500 --
.C
E, 400 --
gr Clads HAZ substrate

300 -1._

200 -—

100 1 1 1 1 1 1

0 0.5 1 1.5 2 2.5 3 3.5

Distance from surface (mm)

Figure 4.30 Microhardness profiles of the cladding track. (a) As—clad. (b) As-clad +
tempering. (c) As-clad + tempering + liquid nitrogen treatment.

97

(continued)

Vickers hardness (Hv)

Vickers hardness (Hv)

 

 

 

l I

«1-

Interface

 

I

 

 

I I

0.5 1 1.5

l
I I

2 2.5 3 3.5

Distance from surface (mm)

 

a.
8
41

(D
8
1
I

2001

I

 

Clad

l l l
I

Interface

HAZ

 

substrate

 

I l

 

I

0.5 1 1.5

l
I I I

2 2.5 3 3.5

Distance from surface (mm)

98

4.16.2 Laminated Metal Matrix Composites (LMMCs)

A laminated metal matrix composite (LMMC) is defined as a composite material
with alternate layers of a high-modulus, high-strength material as the strengthener, and a
deformable ductile material as the matrix. To the best of our knowledge and experience in
laser cladding process, such a composite material can be directly deposited from the metallic
powder by laser cladding technique. In this study, 304L stainless steel and CPMIOV tool
steel have been used as clad materials. The composite materials have been produced in such
a way that the laminates consist of alternating stainless steel and tool steel layers or one
stainless steel and two tool steel layers. Each a composite material is characterized by hard
surface, wear-resistance, and tough core, and could possibly be used to fabricate cutting
blades of cold tooling dies, which are required to withstand imposed service loads without
deformation, fracture, and excessive wear. I have conducted a series of experiments, and
succeeded in producing various geometric laminated 3-D components. The laminated
composites have relatively smooth surface and a good planer interface, are free of pores and
cracks, and good bonding between two layers.
4.16.2.1 Direct Fabrication of LMMCs

For convenience, the processing parameters used for producing samples 2-7 are
listed in Table 4.2. For samples 2-5, the laminated orders from the bottom to the top are
ABBABB and the like, and the processing parameters from the second layer through the

top layer are the same.

99

Table 4.2 Processing parameters used for the laser cladding operations

 

 

Samples laminated Power Traverse speed Powder feed Overlapping
Layers (watts) (mm/sec) rate (g/sec) percentage
(%)
2, 3, 4, 5 1“ 304L 1970 5 0.1 50
2“d CPM 10v 1260 5 0.1 50
3'cl CPM 10v 1260 5 0.1 50
6 1“ 304L 1914 3 0.12 25
2“d CPM 10v 1326 4.9 0.08 50
3rd 304L 1326 4.9 0.08 50
4th CPMIOV 1326 4.9 0.08 50
7 1“ 304L 1914 3 0.12 25
2"d CPM 10v 2073 4.9 0.12 25
3“1 304L 1914 3 0.12 25
4th CPMIOV 2073 4.9 0.12 25

 

 

Figure 4.31(a) is a view of a laminated metal matrix composite (LMMC) coupon,
alternating 304L stainless steel and two CPMIOV tool steel layers. The LMMC sample
has a smooth surface. The overlap between two adjacent tracks is about 50%. Two
separate powder hoppers and metering systems were used to fully automate to produce
the LMMC layers.

Figure 4.31(b) and (c) are an overview of square and saddle contours produced by
laser cladding process, respectively. The prototypes are 6 mm in height. The square is 38
mm in inner length, and the saddle is 26 mm and 34 mm in diameter and in length,
respectively. The deposited components consist of alternating one 304L stainless steel
and two CPMIOV tool steel layers. On each layer, three cladding tracks overlapped. They

have relatively smooth surface and no macro cladding defects.

100

 

Figure 4.31 Photos of LMMC samples. (a) Buildup coupon (sample 2). (b) Square
(sample 3). (c) Saddle prototype (sample 4) with smooth surface via laser
cladding.

101

(continued)

 

Figure 4.32(a) is an overview of a rapid prototype of a hollow cylinder by laser
cladding process. This cylinder is 16 mm in height, and 25.4 mm in inner diameter. It has
18 laminated layers of alternating one 304L stainless steel and two CPMIOV tool steel
layers. On each layer, four tracks overlapped. Stainless steel 304 is used as the transition
layer due to its good ductility and toughness. This thin transition layer plays a key role in
direct deposition of tool steel for making 3-D components with laser. Experimental
results have shown that, without this transition layer, cladding layers either debond from
the substrate or have macrocracks from the top through the bottom. Figure 4.32(b) and (c)

clearly show such defects.

102

9.

 

Figure 4.32 A view of IMMC cylinder prototype produced by laser cladding (sample 5).

(a) is a good sample, and (b) and (c) show some forming defects as discussed
in this section.

In all cases displayed above, the dimension tolerance for the components
produced in such a way is not accurate enough to be directly used, and they still need

post-clad machining. Based on our best understanding, the tolerance for the deposited

103

parts is a function of deposition rate (buildup volume per unit time), which is controlled
by laser power input, laser spot diameter, powder feed rate, and traverse speed. The
higher the deposition rate is, the worse is the tolerance, and vise versa. For samples 14,
the deposition rate is 30 cm3/hr. This deposition rate is much higher than 8.2 cm3/hr [96],
but the dimension tolerance is worse than that of components. There is a tradeoff between
the deposition rate and the dimension tolerance. If traverse speed is kept constant, and
power density (the ratio of incident laser power to irradiated area) is high enough, then
both powder feed rate and laser spot size play critical roles in both deposition rate and
tolerance control. Smaller the spot size, the lower powder feed rate, better will be the
tolerance control. For the consideration of both deposition rate and dimension tolerance,
all processing parameters for laser cladding technique have be optimized to directly
deposit industrial components at a relatively high deposition rate, and with smooth
surface, and good dimension tolerance. In order to reach this goal, there is still a lot of
research work to do in the future.
4.16.2.2 Microstructure of the Cross Section of LMMCs

Figure 4.33 is an optical macrograph of the cross section of laminated metal
matrix composites. It consists of alternating one 304L stainless steel layer and one
CPM10V tool steel. Figure 4.33(a) shows that laminated metal matrix composite is free
of pores and cracks, and has smooth and discrete interfaces. From C layer, laser power
input is reduced to ~1320 watts from ~1900 watts due to large increase of the energy
coupling rate after depositing the first layer. In this way, a minimum intennixing between
two layers can be maintained. In comparison, Figure 4.33(b) shows a macrograph of the

cross section of sample 7 with high dilution. Due to higher laser energy input from C

104

layer to A layer, compared with the corresponding layers in sample 6, stainless steel and
tool steel layers intermixed too much (that is what high dilution means), and thus
changed chemical compositions and properties of each layer to some extent. In addition,
the overlapping rate between two adjacent tracks is approximately 25 percent. So sample
7 has a relatively rough surface. The experimental results have shown that 50 percent

overlapping rate is best suited for depositing LMMCs.

 

Figure 4.33 An overview of the cross section of LMMC samples 6 and 7. A and C are
tool steel CPMIOV layers, while B and D are stainless steel layers. E is
heat-affected zone [87].

105

The microstructure of CPMIOV layer is shown in Figure 4.27 (a). Figure 4.34 is a
magnified optical micrograph corresponding to B layer in Figure 4.33(a). Figure 4.34(a)
shows cellular dendrite structure due to high cooling rate. It was reported that a duplex
structure of 'y- and 8—ferrite existed, and the latter is finely dispersed in the austenitic
matrix [112]. In addition, the top tool steel layer was built up on B stainless steel layer,
and then the latter was tempered at high temperature during the process. Thus, some

precipitated carbides are dispersed in an austenite matrix, as shown in Figure 4.34(b).

 

Figure 4.34 Higher magnified optical micrographs of laser cladded stainless steel 304L
layer. (a) Cellular dendrite structure. (b) The same sample, but higher
magnified micrograph shows the precipitated carbides dispersed in the
austenite matrix.

4.16.2.3 X-Ray Dot Mapping

One of important issues related to laser cladding process is the metallurgical
bonding and its strength. Surface melting condition and compatibility between the
cladding and base materials have a great effect on the quality of bonding. In order to
maintain both chemical composition and properties of cladding material, the intermixing
zone between the clad and the substrate has to be controlled as small as possible by
adjusting processing parameters. X-ray dot mapping or EDXS analysis can reveal the
alloying element distribution in the interface zone.

The laminated metal matrix composite consists of alternating 304 stainless steel
(88304) and CPMIOV tool steel layers, and its base metal is 1045 plain medium carbon
steel. The main alloying elements for 88304 are chromium (18.5 wt.%) and nickel(9.5
wt.%); while the alloying elements for CPMIOV are vanadium (9.75 wt.%) and
chromium (5.25 wt.%). The 1045 steel does not contain these alloying elements. Figure
4.35(a) is a SEM micrograph of the interface zone between the SS304 layer and the 1045
substrate. It clearly shown a sharp but discrete interface. Figure 4.35(b) and (c) display
chromium and nickel distributions near the interface zone. The existence of both
chromium and nickel elements in the interface zone has proved that laser energy melts a
substrate surface, and chromium and nickel atoms penetrate into the substrate forming a
sound metallurgical bonding with a low dilution.

Figure 4.36(a) is a SEM micrograph of the inter-diffusion zone between the
CPMIOV and the SS304 layers. Figure 4.36 (b), (c) and (d) shows chromium, nickel, and

vanadium element distribution near the inter-diffusion zone, respectively. During laser

107

cladding, these alloying element atoms inter-diffuse each other, and form a strong

metallurgical bonding.

 

Figure 4.35 Overview of X—ray dot mapping of sample 6. (a) SEM micrograph of 304
stainless steel-1045 steel interface zone. (b) Chromium distribution in the
inter-diffusion zone. (c) Nickel distribution in the inter-diffusion zone.

108

. interface ..

 

Figure 4.36 X-ray dot mapping of sample 6. (a) SEM micrograph of CPMIOV tool steel
and SS304L stainless steel interface zone. (b) Chromium distribution in the
inter-diffusion zone. (c) Nickel distribution. (d) Vanadium distribution.

4.16.2.4 Microhardness Measurement of LMMCs

Figure 4.37 is a microhardness profile of LMMC sample 6, measured from the top
layer through other three layers and interface to the substrate. 100-g and 500-g loads,
respectively, were used for the measurement of 304L stainless steel and CPMIOV tool
steel layers in order to reduce the indentation area. Vickers hardness for stainless steel
304L and tool steel CPM10V layers, respectively, is about 300 Hv and 750 Hv. 304L
stainless steel is austenitic type stainless steel, so it does not transform to martensite
during the laser cladding. That is why the hardness of 304L stainless steel layer is low. In

addition, hardness distribution on each cladding layer is quite uniform.

 

 

 

 

 
  
 

 

 

 

900 *
Tool steel layer Tool steel Layer
800
I511;
4 .
700 - '
E
,,, 600 ~
8
g 500 -
g Heat-affected zone
.3 400 1
5'
300 ‘ Substrate
200 . Stainless steel layer Stainless steel layer "
1% I T I
0 1 2 3 4

Distance from surface (mm)

Figure 4.37 Vickers hardness profile of IMMC sample 6.

110

4.17 Temperature Measurement

One of the characteristics of laser cladding process is rapid melting and
solidification. Due to high cooling rate, this process can refine microstructures, form
metastable phases, and extend supersaturated solid solutions. Therefore, strength,
hardness, and ductility of deposited layers can be greatly improved. It is challenging to
accurately measure the temperature distributions of the clad zone, heat-affected zone, and
substrate. Conventionally, optical pyrometer has been widely employed to measure a
thermal cycle during laser cladding [113, 114]. It was reported that, during laser cladding
experiments, the temperatures of the molten pool were all the same at approximately
1700 °C [113]. Recently, charge coupled device (CCD) camera has been used in laser
cladding process to perform temperature measurements of the melt pool [115].

So far, few reports have been published which present other new experimental
methods to measure the clad-zone temperature. It is this observation that encourages me to
utilize Tungsten-Rhenium high-temperature thermocouples to do so. In our study, a wide
range of processing parameters and various clad materials have been used in laser cladding
experiments to measure temperature distributions [102]. The clad-zone temperatures under
various processing conditions fall between 1650 °C and 1800 °C. In this section, this
experimental method will be described to determine the thermal cycle and temperature
distributions for laser cladding process. The processing parameters and the pertinent

technical issues will also be discussed in detail.

4.17.1 Temperature Distributions Without Powder Feeding
Figure 4.38 is a temperature distribution for laser remelting AISI 4140 steel plate.

Thermal cycles 0-3 were measured by thermocouples 0-3, respectively. When the laser

111

beam irradiates the steel plate at some distance away from the thermocouple, the
temperature gradually rises due to good thermal conductivity of the workpiece. As the laser
beam approaches closer to the thermocouple, then the surface temperature suddenly
increases, and quickly reaches the highest point. After that, the laser beam travels away from
the thermocouple, and the temperature of the workpiece surface immediately decreases. This
is a typical thermal cycle for laser beam-materials interaction. Under the processing
condition, the peak temperature for thermocouple 0, located in the center of the laser beam,
is about 1550 °C, which is 80 °C higher than the melting temperature 1470 °C of AISI 4140
steel. In the case of the laser cladding process, this experimental data can help understand
how much laser energy is required to be transmitted to the workpiece, in order to get a good
metallurgical bonding between the clad and the substrate.

Figure 4.39 shows the distribution of the thermal gradients on the workpiece surface
along y-direction under each processing condition. The experimental results clearly show

that the temperature distribution (the peak temperature for each thermal cycle) is a function

of the distance away from the laser beam center, if other parameters are kept constant. The
temperature gradients gradually decrease at points farther from the laser beam center. When
the distance is about 4 m away from the laser beam center, the temperature reaches to
~150 °C under all processing conditions.

During laser materials processing, the workpiece temperature at the laser beam
center is a function of laser power input if other processing parameters are kept constant, as

shown in Figure 4.40. The temperature increases with the increase of laser power input.

When the laser power input is ~760 watts, the surface temperature reaches up to ~1800 °C.

112

d

 

d

.oéssségsm

(0)

d

d

 

Temperature (°C)

 

 

 

 

 

 

Time (s)

Figure 4.38 A temperature distribution of a laser beam and substrate interaction without

 

powder feed [102].
1600
1400 +680wetts
+561watte
1200; +502watte
A +60%
1000-
% +357wetts
B .
8 30° ‘ Laser spot size: 1.8 mm
é : namespeed:5mmls
+- 600 -
400~
200«
0 . .

 

 

 

O
_.
N
(0
§
0|

Distance from laser beam center (mm)

Figure 4.39 Temperature distributions on the surface of the substrate as a function of the
distance from laser beam center along y direction [102].

113

 

:1 Experimental

3

— Fitting curve

Temperature (’C)
§ §

5

 

Laser spot size: 1.8 mm
800 ~ Traverse speed: 5 mm/s

 

 

 

100 150 200 250 300
Power density (W/mmz)

Figure 4.40 Surface temperature at laser beam center as a function of laser power input
[102].

4.17.2 Temperature Distribution for Laser Cladding Process

Laser cladding is a process, which involves many metallurgical and physical
mechanisms. For example, interaction between the laser beam and the material(s), rapid
melting and solidification, and inter-diffusion between the clad and the substrate
materials are the dominant mechanisms. During the laser cladding, the interaction of a
laser beam with the materials includes the processes of reflection, absorption, and

transmission. Thus, the energy balance equation governing laser cladding process may be

expressed as:

B=fi+fi+fl RN)

114

where P, is the total laser power input, Pp the laser power absorbed by powder particles, P...
the laser power absorbed by workpiece, and P, the laser power reflected by the workpiece
and the powder. When P1 is fixed, the magnitudes of PI, and P... control the energy coupling
rate. It was found that the energy coupling rate strongly depends on several factors, such as
powder supply methods (pre-placed or coaxial methods), laser parameters (laser-beam
mode, and laser-beam wavelength), and absorptivity of both substrate and filler materials
(material type and workpiece surface condition) [113].

The thermal cycles for the laser cladding process were, as shown in Figure 4.41,
measured from thermocouples 0-3. Under this processing condition, the temperature of
clad zone is ~1660 °C. The temperatures drop to ~1420 °C and ~500 °C at 1 mm and 2
mm from the laser beam center, respectively.

Our data acquisition board just has four channels to receive voltage signals during
the process, so two sets of data (the peak temperature for each thermal cycle in Figure
4.41) have been superimposed to plot a diagram of temperature versus. distance from the

laser beam center, as shown in Figure 4.42. The temperature gradients have Gaussian-like

distribution.

115

 

(0)

1600 ~
(1) Laser power: 1970 watts
1400 - Laser spot size: 1.8 mm
Traverse speed: 5 mm/s
1200 ‘ Powder feed rate: 0.167 g/s

Clad material: CPM10V

 

Temperature (’C)

 

 

 

 

 

 

Time (s)

Figure 4.41 Temperature distributions of laser cladding process under given process
conditions [102].

1800

 

Temperature (’C)

 

 

 

0 2 4 6 8 10 12
Distance from laser beam center (mm)

Figure 4.42 Temperature gradients along y direction [102].

116

The main processing parameters for the laser cladding technique include laser
power, laser spot size, traverse speed, powder feed rate, and shielding gas flow rate and so
on. Within optimized operation window (except extreme cases), all these variables have
some effect on the temperature of clad zone. A wide range of parameters, and various clad
materials were used for laser cladding to measure the temperature of the clad zone. Figure
4.43 displays a few thermal cycles for different filler materials, measured at the laser beam
center under the same processing conditions. Even if the filler materials are different, the
molten-pool temperatures fall between 1650 °C and 1800 °C. These results are in good

agreement with those measured by using an optical pyrometer and CCD technique [113,
115].

 

 

 

 

 

 

 

2000
(0)
1800 ~ 8
1600 _ Laser power: 1970 watts
Laser spot size: 1.8 mm
1400 - Traverse speed: 5 mm/s
9: Powder feed rate: 0.1 gls
8 1200 ~ Clad material: CPM10V
:3 1000 .
§ 800 -
.2
600 ~
400 ~
200 «
0 g 1 I I .
0 5 1O 15 20 25 30

Time (s)
Figure 4.43 Temperature distribution of laser cladding process under laser beam center.

(a) CPM10V tool steel. (b) SSBO4L stainless steel. (c) IN-738LC nickel-
base superalloy. (d) Carbide blend.

117

(continued)

Temperature (C)

Temperature (C)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2000
b 0’
1800 -
1600 . Laser power: 1970 watts
1400 4 Laser spot size: 1.8 mm
Traverse speed: 5 mm/s
1200 - Powder feed rate: 0.1 g/s
Clad material: $8304L
1000 q
800 ~
600 -
400 -
200 -Q
0 " 1 r
O 5 10 15 20 25 30
Time (see)
1800
c ‘0
1600 - n
Laser power: 1970 watts
1400 ~ Laser spot size: 1.8 mm
Traverse speed: 5 mm/s
1200 - Powder feed rate: 0.1 g/s
Clad material: IN-738LC
1000 -
800 ~
600 -
400 -
200 -
O " I r I I r
0 5 10 15 20 25 30

Time (sec)

118

(continued)

 

 

 

 

 

 

 

1800 (0)
d
1600 -
1400 ‘ Laser power: 1970 watts
Laser spot size: 1.8 mm
1200 1 Traverse speed: 5 mm/s
Q Powder feed rate: 0.1 g/s
2 1000 . Clad material: 60 wt.% WC
E + 40 wt.% steel
8 800 .
5
l- 600 .
400 -
200 -
o I I T T I
0 5 10 15 20 25 30
Time (sec)

The optical macrograph of the cross section of as-cladded sample with a thermal
couple is shown in Figure 4.44. It is very difficult to place both thermocouple wires and
their junctions inside the clad. Due to our experimental design, the thermocouple junction is
located in the middle of the clad. Therefore, the temperature measured this way has to be the
molten-powder temperature. This macrograph strongly supports the accuracy of our

experimental results. In addition, the clad is free of pores and cracks.

119

f’l‘hermocouple wire

 

Figure 4.44 Optical macrograph of the cross section of a single cladding track, and its
processing conditions are given in Figure 5.43(a) [102].

120

Chapter 5

CONCLUSIONS

Based on experimental results, the following conclusions can be drawn from this

research:

0 The new integrated powder delivery system designed for laser cladding process avoids

all the drawbacks of the lateral powder feeding systems. It can provide highly stable,
continuous, and accurate powder feed rate, and deliver the powder stream coaxially into
the molten pool with the laser beam to form high quality cladding tracks or layers, with
smooth surfaces, and uniform thickness and width. Also, this system is suited for
deposition of complex geometric patterns on a flat plate or a curved surface for different
applications, such as fabrication of rotary cutting dies, prototyping of forging dies, and
repair of turbine blades. Most importantly, the new powder feeding system is suitable
for on-line industrial production.

Cover gas flow rate has a great effect on cone-shaped powder stream. When cover gas
flow rate is increased from 0.85 to 3.4 m3/hr, it exerts a larger force on cone-shaped
powder stream along its central axis, and squeezes the powder stream out. Under this
condition, more powder bounces up from a workpiece surface in all directions. This will
lead to decrease in powder utilization efficiency, and formation of poor quality cladding
tracks. It has been found that, for our specially designed coaxial powder feeding system,
the upper limit of cover gas flow rate for laser cladding process is about 1.7 m3/hr.

For a given laser power and spot size, when the traverse speed varies from ~5 to ~

10.5 mm/sec, and the powder feed rate varies from 0.1 to 0.25 gm/sec, powder

121

utilization coefficient, a, is constant for each process condition. In other words, the
value of a is a constant function of traverse speed, and powder feed rate under
experimental conditions. However, it is related to the laser spot size. When the laser-
beam diameter D was 1.8 mm, the average value of 0: was 0.71. For D 2 2.3 mm,
average value of awas 0.80.

With other variables held constant, clad width linearly decreases as traverse speed
increases. When traverse speed accelerates to 8.4 mm/sec, clad width is almost equal
to laser-beam diameter. This speed is called the threshold traverse speed for this laser
cladding system. Below the threshold traverse speed, the clad is much wider than the
laser spot size. Clad width gently increases with the increase of energy density when
other parameters are kept constant. The clad gently widens as powder feed rate
increases with other variables held constant.

A simple but realistic process model is developed, relating the maximum clad
thickness and various process parameters. For a given alloy powder, if the powder
feed rate and spot size are kept constant, the clad thickness is inversely proportional to
traverse speed. While traverse speed and spot size are held constant, the clad
thickness is directly proportional to powder feed rate. In each case, the calculated
results of this model are in good agreement with experimental results.

An energy density equation, suitable for a TEMop beam mode, is developed. When
laser power and traverse speed are kept constant, the energy density is inversely
proportional to spot size. While laser power and spot size are held constant, the
energy density is inversely proportional to traverse speed. If traverse speed and spot

size are constant, the energy density is directly proportional to laser power.

122

0 Thermal penetration depth is a linear function of energy density. For fixed traverse
speed, spot size, and powder feed rate, the higher laser power, the higher is the energy
density, and more energy is transferred to the substrate, and therefore the deeper is the
heat—affected zone.

0 Overlapping rate, dilution, and deposition rate for laser cladding process have been
defined, respectively. Based on each definition, a simple but useful formula has also
been developed.

0 A novel process for manufacturing tooling dies, with required geometric patterned
cutting blades, has been successfully developed directly by laser cladding process.
Fully functional prototype of an industrial rotary cutting die, for pilot testing, has been
produced. Compared with the conventional manufacturing process, this new
technique can dramatically reduce the production costs due to the elimination of
costly heat treatment, and no or greatly reduced demand for electric discharge
machining (EDM). In addition, the durability of cutting and stamping dies can be
significantly improved because the wear resistant property of CPMISV clad material
is much better than that of the tool steel D2, currently used in industry. This new
process has been implemented in tooling-die manufacturing industry.

0 Within optimized process parameters, various geometric 3-D components have been
directly deposited by laser cladding technique. Laser deposited 3—D prototypes on steel
substrates are found to be relatively smooth and metallurgically sound. Optical
micrographs displayed that they are free of pores and cracks, and have a good

metallurgical bonding with the substrate, but with low dilution. Laminated layers have

123

planner interfaces to maintain both original chemical compositions and mechanical
properties of each cladding layer.

0 The measurement of the thermal cycle and the temperature distributions for laser
cladding process has been developed. Within optimized operation window, a wide
range of processing parameters, and various clad materials have been used in laser
cladding experiments. The clad-zone temperatures under various processing
conditions lies between 1650 °C and 1800 °C. These results are in good agreement
with those obtained by optical pyrometer and CCD technique. In addition, in both

cases of laser remelting and cladding processes, the temperature gradients along y-

direction have Gaussian-like distribution.

124

BIBLIOGRAPHY

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

BIBLIOGRAPHY

M. Cadenas, R. Vijande, H.J. Montes, and J.M. Sierra, "wear behavior of laser
cladded and plasma sprayed WC-Co coatings," Wear, vol. 212, no. 2, pp. 244-
253, 1997.

Niklas Axen, “Abrasion resistant laser composite layers on tool steels,”
Tribologia, vol. 17, no. 1, pp. 15-18, 1998.

W. Cerri, R. Maryinella, and G. P. Mor, “Laser deposition of carbide-reinforced
coatings,” surface and Coatings Technology, vol.49, 1991, pp. 4045.

Y. P. Hu, C. W. Chen, and K. Mukherjee, "Laser cladding of wear-resistant tool
steel," Advanced Materials & Processes, vol.152, no. 2, pp. 31-32, 1997.

Hiro Iwarnoto et al., "High temperature oxidation behavior of laser clad NiCrAlY
layer," Materials Science & Engineering A, vol. A241, no. 1-2, pp. 251-258, 1998.

J. De Damborenea and A. J. Vasquez, “Laser cladding of high-temperature
coatings,” Journal of Materials Science, vol. 28, no. 17, pp. 4775-4780, 1993.

Y. P. Hu, C. W. Chen, and K. Mukherjee, "Laser cladding of copper-based
composites on 6061 aluminum alloy," (invited) in The Third Pacific Rim
International Conference On Advanced Materials and Processing (M.A. Imam,
R. DeNale, S. Hanada, Z. Zhong, and D.N. Lee, Eds.), vol. 2, pp. 2169-2176,
published by TMS, 1998.

W. M. Steen, “Laser surface cladding,” in Laser Surface Treatment of Metals

(C.W. Draper and P. Mazzoldi, Eds.), pp. 369-387, Martinus Nijhoff Publishers,
1986.

O. D. D. Soares, "Applied laser tooling," in Applied Laser Tooling (O. D. D.
Soares and M. Perez-Amore, Eds.), pp. 1-24, Martinus Nijhoff Publishers, 1987.

Y. P. Hu, C. W. Chen, and K. Mukherjee, "Development of a new laser cladding
process for cutting and stamping die manufacturing," Journal of Materials
Science, vol. 33, pp. 1287-1292, 1998.

Leon-Pong, “The physics of lasers,” in Applied Laser Tooling (O. D. D. Soares
and M. Perez-Amore, Eds.), pp. 43-53, Martinus Nijhoff Publishers, 1987.

J. F. Ready, Industrial Applications of Lasers. New York: Academic Press, 1978

125

[13]

[14]

[15]
[16]
[17]
[18]
[19]
[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

J.T. Luxon, D.E. Parker, and RD. Plotkowski, Lasers in Manufacturing. New
York: IFS (Publications) Ltd. and Springer-Verlag, 1987.

W.W. Duley, C02 Lasers-Ejfects and Applications. New York: Academic Proess,
1976.

J. T. Verdeyen, Laser Electronics. Prentice-Hall, Inc., 1989.

D. C. Winburn, Lasers. Marcel Dekker, Inc., 1987.

SS. Charschan, Lasers in Industry. Van Nostrand Reinhold Company, 1972.

J .E. Harry, Industrial Application of Lasers. Mcgraw-Hill, U.K., 1974.

B. A. Lengyel, Introduction to Laser Physics. John Wiley and Sons, Inc., 1967.
W. M. Steen, Laser Material Processing. Springer-Verlag London Limited, 1991.

A. Belmondo and M. Castagna, "Wear-resistant coatings by laser processing," in
Source Book on Applications of the Laser in Metalworking, pp. 310-317, ASM
International, 1981.

W. Koechner, Solid-State Laser Engineering. New York: Springer-Verlag, 1988.

DC. Brown, High-peak-power Nd: glass laser systems. New York: Springer-
Verlag, 1981.

A. Sona, "Lasers for surface engineering: Fundamentals and types," in Lasers in
Surface Engineering (N .B. Dahotre Eds.), pp. 1-34, ASM International, 1998.

S. Kudapa, Laser Beam Processing of Metal Matrix Composites. Ph.D. thesis,
Michigan State University, 1996.

W.M. Steen, and J .N. Kamalu, "Laser cutting," in Laser Materials Processing (M.
Bass, Eds.), pp. 15-112. North-Holland Publishing Company, 1983.

R. C. Crafer and P. J. Oakley, “Process and physical aspects of continuous wave
laser processing,” in Applied Laser Tooling (O. D. D. Soares and M. Perez-
Amore, Eds.), pp. 55-79, Martinus Nijhoff Publishers, 1987.

Y. Arata and I. Miyamoto, “Some fundamental properties of high power laser
beam as a heat source,” in Plasma, Electron & Laser Beam Technology (Y. Arata,
Ed.), pp.233-262, ASM International, 1986.

126

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

ED. seaman, "Laser heat-treating," in Industrial Laser Handbook, pp. 155-165,
Laser Institute of America, 1987.

T. Bell, “Surface heat treatment of steel to combat wear,” Acta Metallurgia, vol.
49, no. 3, pp. 103-111,1982.

J. Mazumder, “Laser heat treatment: the state of the art,” Journal of Metals, vol.
35, no. 5, pp.18—26, 1983.

V. G. Gregson, “Laser heat treatment,” in Laser Materials Processing (M. Bass,
Ed.), Ch. 4, North-Holland Publishing Company, 1984.

M. F. Ashby, and K. E. Easterling, “The transformation hardening of steel
surfaces by laser beam,” Acta Metallurgia, vol. 32, no. 11, pp.1935-1948, 1984.

M. Tayal and K. Mukherjee, “Thermal and microstructural analysis for laser

surface hardening of steel,” Journal of Applied Physics, vol. 75, no. 8, pp. 3855-
3861, 1994.

D. N. H. Trafford, T. Bell, J. H. P. C. Megaw, and A. D. Bransden, “Laser
treatment of gray iron,” Metal Technology, vol. 10, no. 2, pp. 69-77, 1983.

I. C. Hawkes, A. M. Walker, W. M. Steen, and D. R. F. West, “Application of
laser surface melting and alloying to alloys based on the Fe-C system,” in Laser

Processing of Materials (K. Mukherjee and J. Mazumder, Eds.), pp. 169-182
AIME, Warrendale, PA, 1985.

H. W. Bergmann, “Laser surface melting of iron-base alloys,” in laser Surface
Treatment of Metals (C. W. Draper and P. Mazzoldi, Eds.). pp. 351-368, Martinus
Nijhoff Publishers, 1986.

W. M. Steen, Z. D .Chen, and D. R. F. West, “Laser surface melting of cast irons
and alloy cast irons,” in Industrial Laser Annual Hand book I 987 (D. Belforte and
M. Levitt, Eds.), pp. 80-96.

W. J. Tomlinson and M. G. Talks, "Cavitation erosion of heat-treated low alloy
cast irons," Wear, vol. 137, no. 1, pp.143-146, 1990.

M. Gremaud, M. Carrard, and W. Kurz, “Microstructure of rapidly solidified Al-
Fe alloys subjected to laser surface treatment,” Acta Metallurgica et Materialia,
vol.38, no.12, pp. 2587-2599, 1990.

M. Pierantoni and E. Blank, “Effect of laser surface remelting and alloying on the

wear behaviour of Al-Si alloys,” Key Engineering Materials, vol. 46-47, pp. 354-
367, 1990.

127

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

J. M. Pelletier, D. Pergue, F. Fouquet, and H. Mazille, "Laser surface melting of
low and medium carbon steels. Influence on mechanical and electrochemical
properties," Journal of Materials Science, vol. 24, no. 12, pp. 4343-4349, 1989.

D. R. K. Rao, B. Venkataraman, M. K. Asundi, and G. Sundararajan, “Effect of
lasersurface melting on the erosion behaviour of a low alloy steel,” Surface &
Coatings Technology, vol. 58, no. 2, pp. 85-92, 1993.

S. Saendig and P. Wiesner, “Laser surface melting of cutting tool materials,” in
Laser Materials Processing (P. Denney, I. Miyamoto, and B. L. Mordike, Eds.),
vol. 2306, pp. 835-846. Published by Laser Institute of America, 1994.

R. Vilar, R. Colaco, and A. Almeida, “Laser surface treatment of tool steels,”
Optical and Quantum Electronics, vol. 27, no. 12, pp. 1273-1289,1995.

C. Marsden, D. R. F. West and W. M. Steen, “Laser Surface Alloying of Stainless
Steel with Carbon,” in Laser Surface Treatment of Metals (C.W. Draper and P.
Mazzoldi, Eds.), pp. 461-474. Martinus Nijhoff Publishers, 1986.

T. Chande, A. Ghose, and J. Mazumder, “surface properties and microstructures
of AISI 1016 steel laser surface alloyed with chromium and nickle,” in Laser

Processing of Materials (K. Mukherjee and J. Mazumder, Eds.), pp. 117-130.
AIME, 1985.

N. B. Dahotre, and K. Mukherjee, “Development of microstructure in laser
surface alloying of steel with chromium,” Journal of Materials Science, vol. 25,
no.1B, pp. 445-454, 1990.

T. R. Tucker, A. H. Clauer, S. L. Ream and C. T. Walkers, "Rapidly solidified
microstructures in surface layers of laser alloyed molybdenum on Fe-C substrates"
in Proc. Conf. Rapidly Solididifted Amorphous and Crystaline Alloys (B. H.
Kear, B. C. Giessen, and M. Cohen, Eds.), pp 541-545. Elsevier Science
Publishing Inc., New York, 1982.

C. W. Draper, “Laser surface alloying: the state of the art,” in Lasers in
Metallurgy (K. Mukherjee and J. Mazumder, Eds.), pp. 67-92. AIME, 1981.

J. Mazumder, “Wear Properties of Laser Alloyed and clad Fe-Cr-Mn-C Alloys,”
in Proc. ICALEO '84 (J. Mazumder, Ed.), vol. 44, pp. 159-167. Published by
Laser Institute of America, 1985.

E. MaCatterty and P. G. Moore, “Electrochemical behavior of laser-processed
metal surface,” in Laser Surface Treatment of Metals (C.W. Draper and P.
Mazzoldi, Eds.), pp. 263-295. Martinus Nijhoff Publishers, 1986.

128

[53]

[54]

[55]

[56]

[57]

[58]

[59]

[60]

[61]

[62]

[63]

[64]

L. Renaud, F. Fouquet, J. P. Millet, and J. L. Crolet, "Corrosion resistance of Fe-
Ni-Cr laser surface alloys," Surface & Coatings Technology, vol. 45, no. 1-3, pp.
449-456, 1991.

M. A. Anjos, R. Vilar, R. Li, M. G. Ferreira, W. M. Steen, and K. Watkins, “Fe-
Cr-Ni-Mo-C alloys produced by laser surface alloying,” Surface & Coatings
Technology, vol. 70, no. 2-3, pp. 235-242, 1995.

E. Gaffet, J. M. Pelletier, and S. Bonnet-Jobez, "Laser surface alloying of Ni film
on Al-based alloy," Acta Metallurgica, vol. 37, no. 12, pp. 3205-3215, 1989.

A. Almeida, R. Vilar, and R. Colaco, “Laser alloying of Al and 7175 A1 alloy for
enhanced corrosion resistance,” in Laser Materials Processing (P. Denney, I.

Miyamoto, and B. L. Mordike, Eds.), vol. 2306, pp. 903-912. Published by Laser
Institute of America, 1994.

S. Yerramareddy and S. Bahadur, “Effect of laser surface treatments on the
tribological behavior of Ti- 6Al-4V,” Wear, vol. 157, no. 2, pp. 245-262, 1992.

O. V. Akgun and O. T. Inal, “Laser surface modification of Ti-6Al-4V alloy,”
Journal of Materials Science, vol. 29, no. 5, pp. 1159-1168, 1994.

J. D. Ayers, “Particulate composite surfaces by laser processing,” in Lasers in
Metallurgy (K. Mukherjee and J. Mazumder, Eds.), pp. 115-125. AIME, 1981.

J. D. Ayers and T. R. Tucker, “Particulate TiC hardened surface by laser melt
injection,” Thin Solid Films, vol. 73, pp. 201-207, 1980.

S. Ariely, J. Shen, M. Bamberger, F. Dausiger, and H. Hugel, "Laser surface

alloying of steel with TiC," Surface & Coatings Technology, vol. 45, no.1-3, pp.
403-408, 1991.

B. Gruenenwald, E. Bischoff, J. Shen, and F. Dausinger, “Laser surface alloying
of case hardening steel with tungsten carbide and carbon” Materials Science and
Technology, vol. 8, no. 7, pp. 637-643, 1992.

D. Pantelis, H. Michaud, and M. de Freitas, “Wear behaviour of laser surface
hardfaced steels with tungsten carbide powder injection,” Surface & Coatings
Technology, vol. 57, no. 2-3, pp. 123-131, 1993.

B. S. Yilbas, R.Davies, and Z. Yilbas, “Laser alloying of metal surfaces by
injecting titanium carbide powders,” International Journal of Machine Tool &
Manufacture, vol. 29, no. 4, pp. 499-503, 1989.

129

[65]

[66]

[67]

[68]

[69]

[70]

[71]

[72]

[73]

[74]

[75]

[76]

X. B. Ziou and J.Th. M. De Hosson, “Metal-ceramic interfaces in laser coated

aluminum alloys,” Acta Metallurgica et Materialia, vol. 42, no. 4, pp.1155-1162,
1994.

J. H. Abboud and D. F. R. West, “Ceramic-metal composites produced by laser
surface treatment,” Materials Science and Technology, vol. 5, pp. 725-728, 1989.

J. A. Folkes and K. Shibata, “Laser cladding of Ti-6Al-4V with various carbide
powders,” Journal of Laser Applications, vol. 6, no. 2, pp. 88-94, 1994.

A. Weisheit and B. L. Mordike, “Laser surface alloying of titanium with VC and
MoSiz to improve wear resistance,” in Laser Materials Processing (P. Denney, I.
Miyamoto, and B. L. Mordike, Eds.), vol. 2306, pp. 923-933. Published by Laser
Institute of America, 1994.

K. Komvopoulos, “Effect of process parameters on the microstructure, geometry
and microhardness of laser-clad coating materials,” Materials Science Forum, vol.
163, no. 6, pt. 1, pp. 417-422, 1994.

K. P. Cooper and P. Slebodnick, “Recent development in lasser melt/particles
injection processing,” in Laser Materials Processing (G. Bruck, Ed.), pp. 3-16.
Springer-Verlag: IFS (Publications) Ltd., UK, 1989.

J. Choi and J. Mazumder, “Non-equilibrium synthesis of Fe-Cr-C-W alloy by

laser cladding,” Journal of Materials Science, vol. 29, no. 17, pp. 4460-4476,
1994.

A. Frenk and W. Kurz, "High speed laser cladding: solidification conditions and
micostructure of a cobalt-based alloy," Materials Science and engineering, vol.
A173, pp. 339-342, 1993.

W. Aihua, T. Zengyi, and Z. Beidi, "Laser beam cladding of slating surfaces on

exhaust valves," Welding Research Supplement, vol. 70, no. 4, pp. 106s-109s,
1991.

S. J. Matthews, “Laser fusing of hardfacing alloy powders,” in Laser in Materials
Processing (E. A. Metzbower, Ed.), pp. 138-148. ASM International, 1983.

B. C. Oberlander and E. Lugscheider, "Composition of properties of coatings
produced by laser cladding and conventional methods," Materials Science and
Technology, vol. 8, pp. 657-665, 1992.

T. Takeda, W. M. Steen, and D. R. F. West, "Laser Cladding with Mixed Powder
Feed", in Proc. conf. ICALEO '84 (J. Mazumder, Ed.), vol. 44, pp. 151-158,
Published by Laser Institute of America, 1985.

130

[77]

[78]

[79]

[80]

[81]

[82]

[83]

[84]

[85]

[86]

[87]

[88]

P. Sallamand and J. M. Pelletier, “Laser cladding on aluminium-base alloys:
Microstructural features,” Materials Science and Engineering, vol. A171, no. 1-2,
pp. 263-270, 1993.

Y. Liu, J. Koch, J. Mazumder, and K. Shibata, “Processing, microstructure, and
properties of laser-clad Ni alloy FP-5 on Al alloy AA333,” Metallurgical
Transactions, vol. 25B, pp. 425-434, 1994.

C. Ribaudo, S. Sircar, and J. Mazumder, “Laser-clad Ni7oAlzoCr7Hf3 alloys with
extended solid solution of Hf. Part II: Oxidation behavior,” Metallurgical
Transactions, vol. 20A, no. 11, pp. 2267-2277, 1989.

J. De Damborenea and A. J. Vazquez, "Laser cladding of high-temperature
coatings," Journal of Materials Science, vol. 28, pp. 4775-4780, 1993.

G. Abbas and D. R. F. West, “Laser surface cladding of stellite and stellite-Sic

composite deposits for enhanced hardness and wear,” Wear, vol. 143, no. 2, pp.
353-363, 1991.

Y. P. Pei, J. H. Ouyang, T. C. Lei, and Y. Zhou, “Microstructure of laser-clad

SiC-(Ni alloy) composite coating,” Materials Science & Engineering, vol. A194,
no. 2, pp. 219-224, 1995.

H. W Gu, “Laser cladding of Ni based WC alloy,” Jiguang Jishu/Laser
Technology, vol. 17, no. 4, pp.220-223, 1993.

E. Ramous, L. Giordano, A. Tiziani, B. Badan, and M. T. Cantello, “Laser
cladding of ceramic and metallic coatings on steel,” Key Engineering Materials,
vol. 46-47. PP. 425-433, 1990.

W. Cerri, R. Maryinella, and G. P. Mor, “Laser deposition of carbide-reinforced
coatings,” surface and Coatings Technology, vol.49, pp. 40-45, 1991.

A. Techel, A. Luft, A. Mueller, and S. Nowotny, “Production of hard metal-like

protection coatings by C02 laser cladding,” Optical and Quantum electronics, vol.
27, no. 12, pp. 1313-1318,1995.

Y. P. Hu, C. W. Chen, and K. Mukherjee, "Synthesis of laminated metal-matrix

composites by laser cladding technique," submitted to Surface and Coatings
Technology, 1999.

J .E. Smugersky and D.M. Keichner, "Laser engineered direct fabrication/manufac-
turing of metals," in Lasers in Surface Engineering (N .B. Dahotre Ed.), pp. 1-34,
ASM International, 1998.

131

[89]

[90]

[91]

[92]

[93]

[94]

[95]

[96]

[97]

[98]

[99]

D. S. Gnanamuthu, “Laser surface treatment,” in Source Book on Applications of
the Laser in Metalworking, pp. 324-358, ASM International, 1981.

J. I. Nurrninen and J. E. Smith, "Parametric evaluation of laser/clad interaction for
hardfacing applications," in Lasers in Materials Processing (E. A. Metzbower,
Ed.), pp. 94-106, ASM International, 1984.

E. M. Breinan, D. B. Snow, C. 0. Brown, and B. H. Kear, "New developments in
laser melting using continuous prealloyed powder feed," in Source Book on

Applications of the Laser in Metalworking, pp. 279-291, ASM International,
1981.

V. M. Weerasinghe and W. M. Steen, “Laser Cladding with Pneumatic Powder
Delivery,” In Lasers in Materials Processing (E. A. Metzbower, Ed.), pp. 166-
175, ASM International, 1984.

R. P. Harrison, “Laser powder welding,” Turbomachinry International, vol. 35,
no. 6, pp. 52-53, 1994.

P. A. Vetter, Th. Engel, and J. Fontaine, “Laser cladding: the relevent parameters
for process control,” in Laser Materials Processing: Industrial and
Microelectronics Applications (E. Beyer, M. Cantello, A. V. La Rocca, L. D.
Laude, F. 0. Olsen and G. Sepold, Eds.), SPIE vol. 2207, pp. 452-462, SPIE-The
International Society for Optical Engineering, 1994.

Y. P. Hu, C. W. Chen, and K. Mukherjee, "Innovative laser-aided manufacturing
of patterned stamping and cutting dies: processing parameters," Materials and
Manufacturing Processes, vol. 13, no. 3, pp. 369-387, 1998.

R. Mah, “Directed light fabrication,” Advanced Materials & Processes, vol.151,
no. 3, pp. 31-33, 1997.

J. Mazumder, J. Choi, K. Nagarathnam, K. Koch, and D. Hetzner, “The direct
metal deposition of H13 tool steel for 3-D components,” JOM, vol.49, no. 5, pp.
55-60, 1997.

R. M. Macintyre, "The use of lasers in Rolls-Royce," in Laser Surface Treatment

of Metals (C.W. Draper and P. Mazzoldi, Eds.), pp. 545-549, Martinus Nijhoff
Publishers, 1986.

Minoru Kawasaki, Koyu Takase, Sinji Kato, Masahiro Nakagawa, and Kazuhiko
Mori, “Development of engine valve seats directly deposited onto aluminum

132

[100]

[101]

[102]

[103]

[104]

[105]

[106]

[107]

[108]

[109]

[110]

cylinder head by laser cladding process,” In SAE Technical Paper Series, pp. 1-15,
Published by SAE, 1992. SAE Paper No. 920571.

K. E. Pinnow and W. Stasko, "P/M tool steels," in Metals Handbook, vol. 1, 10th
Ed. pp. 780-792, ASM International, 1990.

Y. P. Hu, C. W. Chen, and K. Mukherjee, "An analysis of the powder feeding
systems on the quality of laser cladding," in Advances in Powder Metallurgy &
Particulate Materials-I997 (R.A. McKotch and R. Webb, Eds.), vol. 3, pp.17-31,
published by Metal Powder Industries Federation, 1997.

Y. P. Hu, C. W. Chen, and K. Mukherjee, "Measurement of temperature
distributions during laser cladding process," accepted to be published by Journal
of Laser Applications, 1999.

Y.-N. Liu and E. Kannatey-Asibu, Jr., “Experimental study of dual-beam laser
welding of AISI 4140 steel,” Welding Journal, vol. 8, pp. 342-s-348-s, 1997.

P.-A. Vetter, J. Fontaine, T. Engel, L. Lagrange, and T. Marchione,
“Characterization of laser-material interaction during laser cladding process,” in
Computer Methods and Experimental Measurements for Surface Treatment
Ejfects (M. H. Aliabadi, and C. A. Brebbia, Eds.), pp. 185-194, Computational
Mechanics Publications, 1994.

J. Lin and W. M. Steen, “Design characteristic and development of a nozzle for
coaxial laser cladding,” Journal of Laser Applications, vol. 10, pp. 55-63, 1998.

A .F. A. Hoadley and M. Rappaz, "A thermal model of laser cladding by powder
injection," Metallurgical Transactions B, vol. 23B, 1992, pp. 631-640.

M. Picasso, C. F. Marsden, J .-D. Wagniere, A. Frenk, and M. Rappaz, "A simple

but realistic model for laser cladding," Metallurgical Transactions B, vol. 23B,
1994. pp. 281-291.

C. Hu and T. N. Baker, "Investigation of the temperature field developed by a

spinning beam in laser processing," Metallurgical Transactions A, vol. 27A,
1996, pp. 4039-4047.

G. Robets and R. Cary, Tool Steels, 4th edition, ASM International, 1992.

R. Colaco and R. Vilar, “Laser surface melting of bearing steels,” in Laser

Applications for Mechanical Industry (S. Martellucci, and A. Scheggi, Eds.), pp.
305-314, Kluwer Academic Publishers, 1992.

133

[111]

[112]

[113]

[114]

[115]

Q. F. Peng, Z. Shi, and I. M. Hancock, and A. Bloyce, “Energy beam surface
treatment of tool steels and their wear,” Key Engineering Materials, vol. 46-47,
pp. 229-244, 1990.

O.V. Akgun and CT. Inal, “Laser surface melting and alloying of type 304L
stainless steel Part I: Microstructural characterization,” Journal of Materials
Science, vol. 30, no. 23, pp. 6097-6104, 1995.

W. Bloehs, B. Griinenwald, F. Dausinger and H. Hiigel, “Recent progress in laser
surface treatment: I. Implications of laser wavelength,” Journal of Laser
Applications, vol. 8, pp. 15-23, 1996.

M. Ignatiev, I. Smurov, V. Martino, and G. Flamant, “High speed high spatial
resolution pyrometry in laser machining,” in ICALEO ’93 Laser Materials

processing (Paul Denny, Isamu Miyamoto, and BL. Mordike Eds.), pp. 117-126,
1993.

F. Meriaudeau, and F. Truchetet, “Control and optimization of the laser cladding

process using matrix cameras and image processing,” Journal of Laser
Applications, vol. 8, pp. 317-326, 1996.

134

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