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thesis entitled

Laser Cladding of Hard—Facing Materials onto

A181 1045 Steel for Patterned Cutting-Die Design
presented by

Yiping EU

has been accepted towards fulfillment
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MaSter'SJegree in Materials Science

 

 

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LASER CLADDING OF HARD-FACING MATERIALS ONTO
A181 1045 STEEL FOR PATTERNED CUTTING-DIE DESIGN

By

Yiping Hu

A THESIS

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

MASTER OF SCIENCE
Department of Materials Science and Mechanics
Michigan State University

East Lansing, MI 48824

1996

ABSTRACT

LASER CLADDING OF HARD-FACIN G MATERIALS ONTO
AISI 1045 STEEL FOR PATTERNED CUTTING-DIE DESIGN

By

Yiping Hu

In conventional rotary cutting-die manufacturing process, the cost of materials is
high, the process is time-consuming, production efficiency is very low, and therefore
production costs are extremely high. Also, the life of rotary cutting dies is rather short due
to the segregation and ﬂake morphology of carbides in D2 alloy, commonly used for die
making. For this reason, it is necessary to seek a new tool material, with a hard surface, a
tough core, and with wear-resistant properties, to form patterned cutting blades to prolong
the life of rotary cutting dies. Alternatively, we can investigate an alternative
manufacturing process to produce such cutting dies at a signiﬁcantly reduced production
cost. Laser cladding appears to be such a process.

In our present research, a new 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 suitable for depositing

high quality cladding tracks, along complicated geometric paths, on ﬂat or curved

surfaces. A simple but realistic model relating the clad thickness and processing
parameters is also developed, and the various relationships between the clad thickness
and process parameters are discussed in detail. The calculated results of this model are in
good agreement with the experimental ones. Finally, a novel process is successfully
developed for directly depositing hard-facing tracks, along complicated geometric paths,
on AISI 1045 steel substrates for manufacturing plate or rotary cutting dies. Fully
functional prototype of industrial rotary cutting dies, for pilot testing, has been
successfully produced by this process. This new technique is being implemented in die-

making industry.

ACKNOWLEDGEMENTS

I wish to thank my thesis advisor, Prof. K. Mukherjee, for his academic guidance,
advice, and ﬁnancial support throughout the duration of this work. I would also like to
extend my sincere appreciation to Prof. P. Y. Kwon and Prof. K. N. Subramanian for their
invaluable suggestions for improving this thesis.

Sincere gratitude is also extended to my wife and my parents for their
encouragement, and spiritual support.

I would like to thank Dr. C. W. Chen, Dr. S. Kudapa, and J. Howard for their
friendship and help. I would also like to thank Dr. Yu Zhang, in Bemal Division, Stevens
Graphics Inc. for having the coaxial nozzle manufactured, and providing CPM 10V and

CPM 15V alloy powders as well as AISI 1045 steel plates and rollers.

iv

TABLE OF CONTENTS

LIST OF TABLES viii
LIST OF FIGURES ix
1 INTRODUCTION 1
1.1 Background and Motivation ...................................... 1
1.2 Sc0pe ....................................................... 4
2 REVIEW OF LASER SURFACE TREATMENT PRINCIPLE 6
2.1 Principle of Laser Operation. .................................... 6
2.1.1 The Nd:YAG laser ..................................... 7
2.1.2 The C02 laser ......................................... 8
2.2 Laser-Beam Mode ............................................. 9
2.3 Laser Parameters .............................................. 11
2.3.1 Laser-Beam Diameter ................................... 11
2.3.2 Laser-Beam Power ..................................... 11
2.3.3 Power Density. ....................................... 12
24 Laser Beam-Material Interaction .................................. 12
3 LASER SURFACE TREATMENT 15
3.1 Laser Solid-Transformation Hardening ,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 15
32 Laser Remelting. . ............................................ 16
33 Laser Alloying ................................................ 16
3.4 Laser DiSpersing. ............................................. 17
35 Laser Cladding. .............................................. 18
3.5.1 General Description. ................................... 18
352 Scope of Laser Cladding ................................. 18
3.5.3 Processing Parameters ................................... 19

3-5-3-1 Absorptivity. ................................ 19

35-3-2 Traverse Speed ............................... 20

3.5.3.3 Powder Feeding Techniques ,,,,,,,,,,,,,,,,,,,,, 21

35-34 Shielding Gas. ............................... 22

3.5.4 Advantages of Laser Cladding Process ,,,,,,,,,,,,,,,,,,,,,, 22

3.5.5 Limitations of Laser Cladding Process ,,,,,,,,,,,,,,,,,,,,,, 23

3.5.6 Applications of Laser Cladding Process ,,,,,,,,,,,,,,,,,,,,, 23

4 EXPERIMENTAL PROCEDURES 25
4.1 General Description ............................................ 25
42 Choice of materials ............................................ 26
4.2.1 Substrate Materials ..................................... 26
4.2.2 Clad Materials ........................................ 27

4.3 Laser Cladding System ......................................... 30
4.3.1 Laser Generator ....................................... 30
4.3.2 Beam Delivery System .................................. 30
4.3.3 Powder Feeding System ................................. 32
4.3.3.1 Powder Feeder ............................... 34

4.3.3.2 Powder-Flow Splitter ,,,,,,,,,,,,,,,,,,,,,,,,,, 34

4.3.3.3 Coaxial Nozzle ............................... 36

4.3.4 CAM System .......................................... 38

44 Laser cladding ................................................ 38
4.5 Post-Clad Treatment ............................................ 43
4.6 Microstructures and Hardness Measurements ,,,,,,,,,,,,,,,,,,,,,,,, 43
5 RESULTS AND DISCUSSION 44
5.1 Measurement of Powder Utilization Coefﬁcient ,,,,,,,,,,,,,,,,,,,,,, 44
5.2 Relationships between Clad Width and Process Parameters ,,,,,,,,,,,,, 51
5.3 Process Modeling ............................................. 57
53.1 Assumptions .......................................... 57
5.3.2 Basic Equation ........................................ 57

5.3.3 Energy-Density Equation ................................ 59

vi

5.3.4 Clad Thickness-Energy Density Equation ,,,,,,,,,,,,,,,,,,,, 60

5.4 Correlation Derived from Basic Equation ,,,,,,,,,,,,,,,,,,,,,,,,,,, 61
5.4.1 Relationship between Clad Thickness and Traverse Speed, , , , , , 61
5.4.2 Relationship between Clad Thickness and Spot Size, ,,,,,,,,,, 61

5.4.3 Relationship between Clad Thickness and Powder Feed Rate, .. . . 62

5.5 Correlation Derived from the Energy-Density Equation ,,,,,,,,,,,,,,,,, 66
5.5.1 Relationship between Energy density and Spot Size ,,,,,,,,,,,, 66

5.5.2 Relationship between Energy density and Traverse Speed, ,,,,,, 66

5.5.3 Relationship between Energy density and Laser Power ,,,,,,,,,, 67

5.6 Correlation between Clad Thickness and Energy Density ,,,,,,,,,,,,,,, 71
5.7 Comparisons of Calculated and Experimental Results ,,,,,,,,,,,,,,,,,, 72
5.7.1 Clad Thickness and Traverse Speed ,,,,,,,,,,,,,,,,,,,,,,,, 72

5.7.2 Clad Thickness and Spot Size ............................. 75

5.7.3 Clad Thickness and Powder Feed Rate ,,,,,,,,,,,,,,,,,,,,,, 77

5.7.4 Clad Thickness and Energy Density ,,,,,,,,,,,,,,,,,,,,,,,, 79

5.8 Correlation between Penetration Depth and Energy Density ,,,,,,,,,,,,, 81
5.9 Microstructure ............................................... 83
5.9.1 Geometric Characteristics of A Single Cladding Track ,,,,,,,,,, 83
5.9.2 Vanadium CarbideS. ................................... 92

5.10 Microhardness of A Single Clad Track ,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 93
5.11 Manufacturing of Patterned Rotary Cutting Die ,,,,,,,,,,,,,,,,,,,,,,, 96
6 CONCLUSIONS 103
BIBLIOGRAPHY 106

vii

LIST OF TABLES

1 Chemical compositions of substrate and clad materials ,,,,,,,,,,,,,,,,,,,,, 27
2 Characteristics of the RF exited C02 laser ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 3O
3 CNC machine speciﬁcations .......................................... 38
4 Measurements of powder utilization coefﬁcient 0t ,,,,,,,,,,,,,,,,,,,,,,,,, 46
5 Correlation between clad width and processing parameters ,,,,,,,,,,,,,,,,,, 52
6 Processing parameters used to develop the correlation between clad thickness

and reciprocal of traverse speed ....................................... 73
7 Processing parameters used to develop the correlation between clad thickness
and reciprocal of spot size ............................................ 76

8 Processing parameters used to develop the correlation between clad thickness

and powder feed rate ................................................ 78
9 Processing parameters used to develop the correlation between clad thickness

and energy density .................................................. 80
10 Processing parameters used to develop the correlation between penetration depth

and energy density .................................................. 81
11 Processing parameters for producing rotary cutting dies ,,,,,,,,,,,,,,,,,,,,, 97

viii

LIST OF FIGURES

1 Transverse mode patterns in cylindrical and rectangular coordinates [5] ,,,,,,,, 10
2 Wear resistance of CPM 10V and conventional tool steels at indicted

hardness [80]. CPM lOV* (hardened) is subjected to quenching, which

increases its hardness, and thus the extra wear resistance ,,,,,,,,,,,,,,,,,,,, 28

3 Charpy C—notch impact properties of CPM 10V and conventional tool steels at

indicated hardness [80] ............................................. 29
4 A schematic of laser cladding system ................................... 31
5 A view of side powder feeding system .................................. 33
6 A view of cone-shaped powder stream .................................. 35
7 A schematic of transverse section of a coaxial nozzle ,,,,,,,,,,,,,,,,,,,,,, 36
8 A view of single cladding tracks with smooth surface on a steel plate ,,,,,,,,,, 41
9 A view of intersecting hard-facing cladding tracks directly deposited on a steel

roller ............................................................ 42
10 Plot of powder utilization coefﬁcient vs. traverse speed ,,,,,,,,,,,,,,,,,,,,, 49
11 Plot of powder utilization coefﬁcient vs. powder feed rate ,,,,,,,,,,,,,,,,,,, 49
12 Plot of powder utilization coefﬁcient vs. laser spot size ,,,,,,,,,,,,,,,,,,,,, 50
13 Clad width as a function of traverse speed ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 55
14 Clad width as a function of energy density ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 55
15 Clad width as a function of powder feed rate ,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 56
16 Parabolic shape of the cross-section of a clad track ,,,,,,,,,,,,,,,,,,,,,,,, 58
17 Clad thickness as a function of reciprocal of traverse speed ,,,,,,,,,,,,,,,,,, 63
18 Clad thickness as a function of reciprocal of spot size ,,,,,,,,,,,,,,,,,,,,,, 64
19 Clad thickness as a function of powder feed rate ,,,,,,,,,,,,,,,,,,,,,,,,,, 65
20 Energy density as a function of reciprocal of spot size ,,,,,,,,,,,,,,,,,,,,,, 68

ix

21

22

23

24

25

26

27

28
29

3O

31

32

33

34

35

Energy density as a function of reciprocal of traverse speed ,,,,,,,,,,,,,,,,,, 69
Energy density as a function of laser power ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 70
Clad thickness as a function of energy density ,,,,,,,,,,,,,,,,,,,,,,,,,,,, 71
Comparison of calculated and experimental relationship between clad thickness

and reciprocal of traverse speed. (a) v < 8.4 mm/sec. (b) v 2 8.4 mm/sec ,,,,,,, 74
Comparison of calculated and experimental relationship between clad thickness

and reciprocal of Spot size ........................................... 77
Comparison of calculated and experimental relationship between clad thickness

and powder feed rate ................................................ 79
Comparison of calculated and experimental relationship between clad thickness

and energy density ................................................. 80
Penetration depth as a function of energy density ,,,,,,,,,,,,,,,,,,,,,,,,,, 82
Three basic single clad bead proﬁles. (a) Deep penetration clad bead. (b) Clad

bead with a reasonable width to height aspect ratio. (c) Parabolic clad bead ,,,,, 85
Optical micrographs of cross section of clad tracks with no macroscopic cladding

defects. (a) v = 3 mm/sec. (b) v = 3.9 min/sec. (c) v = 4.9 mm/sec. (d) v = 5.8
mm/sec. (e) v = 6.6 mm/sec. Other processing parameters are as follows:

P = 2073 watts, D = 1.8mm, and Fp = 0.167 gm/sec ,,,,,,,,,,,,,,,,,,,,,,,, 86
Effect of laser power on clad formation. (a) P = 2073 watts. (b) P = 1755 watts.

(c) P = 1567 watts. (d) P = 1110 watts. Other processing parameters:

v = 8.4 mm/sec, D = 2.3 mm, and Fp = 0.167 gm/sec ,,,,,,,,,,,,,,,,,,,,,,, 89
A magniﬁed optical micrograph of ﬁgure 31 ((1), showing the lack of fusion

between the clad and substrate ........................................ 91
A view of carbides in CPM 10V and D2 alloys. (a) Fine vanadium carbide

particles uniformly dispersed in the matrix in CPM 10V clad. (b) Segregation

and ﬂake morphology of carbides in D2 alloy ,,,,,,,,,,,,,,,,,,,,,,,,,,,, 93
Microhardness proﬁles of the cladding track. (a) As-clad. (b) As-clad + tempering.
(c) As -clad + tempering + liquid nitrogen treatment ,,,,,,,,,,,,,,,,,,,,,,, 96
A view of intersecting hard-facing tracks, with smooth surfaces, uniform clad

thickness, and width, deposited on a AISI 1045 steel ﬂat plate,

36 A view of the hard-facing tracks, directly deposited onto a AISI 1045 steel roller

by laser cladding process ............................................ 100
37 Hard-facing tracks, with complicated geometric patterns, deposited on a AISI

1045 steel roller via laser cladding ..................................... 101
38 A view of hard-facing tracks, with complex geometric patterns, deposited on a

steel roller by laser cladding after surface grinding ,,,,,,,,,,,,,,,,,,,,,,,, 102

xi

Chapter 1

INTRODUCTION

1.1 Background and Motivation

In packaging industry, rotary cutting dies have been widely used to cut cereal and
soft beverage boxes, facial paper towels, paper money, credit cards, and disk floppies, and
so on. The cutting speed is so fast, and impact loads and shear stresses are so large that
the service life of rotary cutting dies is very short. Typically, such rotary cutting dies can
work for one or two months depending on the chemical compositions, hardness, and
other properties of the materials to be cut.

In conventional rotary cutting-die manufacturing process, the die-block and the
cutting blades are made of an expensive tool steel D2. In order to prevent the distortion of
the cutting blades, the entire die-block is ﬁrst hardened to obtain the hardness of HRC 60,
then the hardened block is machined by electrical discharge machining (EDM) to produce
the cutting blades. Thus, for the conventional process, the cost of materials is high, the
process is time—consuming, the production efﬁciency is very low, and therefore the
overall production costs are extremely high. As mentioned before, the working
environment of rotary cutting dies is very severe. Because of pronounced ingot
segregation, conventional tool steels often contain a coarse, nonuniform microstructure

accompanied by poor transverse properties. Therefore, the life of the rotary cutting dies

made of D2 alloy is rather short due to the segregation and ﬂake morphology of carbides
in D2 alloy.

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

Laser surface cladding offers the potential to deposit new materials with special
properties onto different substrates. By this process, high wear-resistant, corrosion-
resistant, and oxidation-resistant coatings, in the shape of cutting blades, can be deposited
on selected areas and/or in a precisely deﬁned pattern on a variety of substrates. For
example, metal carbides, or metal/carbide mixtures can be injected to laser-generated
molten pool, on a substrate, to form a composite layer on the surface to improve wear
property of tool steel, and prolong the surface life of forging dies [1-2]. However, the
base materials are still very expensive. To date, no published literature describes any
technology for manufacturing plate and rotary cutting dies by laser cladding process. It is
this observation that led us to investigate the feasibility of developing such a new
technique.

From our laser material processing experience, we realized the potential

application of the laser cladding process in manufacturing cutting dies. Through our

arduous efforts, and repeated analysis of experimental variables, we have succeeded in
developing a novel technology for manufacturing plate and rotary cutting dies. Fully
functional prototype of industrial cutting dies, for pilot testing, has been successfully
produced by this process. Also, this new technology has greater manufacturing ﬂexibility
because different substrates and reinforcement materials, with required mechanical
properties, can be chosen to adapt to the different applications.

Compared with the conventional cutting-die manufacturing processes, this
technology, in which cutting blades, with complicated geometric patterns, are directly
deposited by laser cladding onto a substrate, has many advantages:

0 Material cost can be greatly reduced because an expensive tool steel body of the die-

block can be replaced by a cost-effective structural steel.

Production costs can also be signiﬁcantly reduced due to the elimination of costly heat

treatment, and the reduced demand on the EDM.

o Durability of the cutting die is expected to be signiﬁcantly improved because the wear
resistant property of hard-facing material is much better than that of the tool steel D2,
currently used in industry.

0 Production efﬁciency could be much higher than that of the conventional process due
to the reduced demand on the EDM, and the elimination of the costly tool steel heat
treatment.

0 Such a processing technique can also be used for repairing the worn cutting blades.

0 The new processing technology could be more ﬂexible because different substrates

and clad materials, with desired mechanical properties, could be chosen for various

applications.

1.2 Scope

The aims of this research are to develop a processing parametric model based on
the physical phenomena and geometric features of single cladding tracks in laser cladding
process, and to investigate the feasibility of developing a novel process technique for
manufacturing plate and rotary cutting dies by laser cladding process. The detailed
requirements are as follows:

0 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.

0 Perform preliminary experiments to obtain initial processing parameters, such as laser
power, spot size, traverse speed, powder feed rate, and shielding gas ﬂow rate.

0 Develop a simple but realistic model, relating the maximum clad thickness and
processing parameters, discuss in detail the various relationships between the clad
thickness and different processing variables, and determine the minimum energy
density for clad formation under the experimental condition.

0 Characterize the microstructures of cladding layer, interface, and heat-affected zone
by using optical microscopy, and measure the microhardness of cross section of the
samples under the following conditions: 1) as-clad, 2) as-clad + tempering, and 3) as-
clad + tempering + liquid nitrogen treatment.

0 Eliminate any unacceptable defects in the clads, such as porosity, microcracks, and
poor metallurgical bonding between the clad and substrate by modifying the process

parameters and other related factors.

Based on the basic study, investigate the feasibility of depositing a high wear-resistant
alloy powder, CPM 15V, on AISI 1045 medium carbon steel for making plate or

rotary cutting dies, with complicated geometric patterned cutting blades, by laser

cladding process.

Chapter 2

REVIEW OF LASER SURFACE TREATMENT PRINCIPLE

2.1 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 coherent, controllable, and
intense. It is these properties that make such lasers as Nd:YAG and C02 competing heat
sources to be widely used for materials surface modiﬁcations.

Every laser is composed of three main components: amplifying medium or active
medium, means of excitation, and an optical resonator [3—4]. The amplifying medium
determines the laser wavelength and the type of excitation required. A medium is
potentially used as an optical frequency ampliﬁer if the population of its energy levels can
be altered so that there are more emissions than absorptions per unit time. That is so
called the population inversion, which is absolutely necessary for ampliﬁcation of light to
occur. Otherwise, a beam of light directed through the active medium with photon energy
will be absorbed.

In terms of the 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 Nd:YAG and C02 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 Nd:YAG crystal to lase at 1.06 mm, and the latter is used 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 the combination of the
ampliﬁer and resonator is at the threshold for lasing. Upon reaching or exceeding
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.

2.1.1 The Nd:YAG Laser

The Nd:YAG laser is by far the most commonly used type of solid-state laser.
Neodymium-doped yttrium aluminum garnet (Nd: YAG) 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 Nd:YAG, 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 [5-7].

When exited by light from an electric discharge lamp 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 mm. The most
efﬁcient laser operation is obtained when the optical resonator is designed for 1.06 um.
This is the wavelength used in Nd:YAG lasers for materials interactions.

Recently, Nd:YAG lasers become of interest for materials surface treatments
because multi-kilowatt systems now are on the market. The maximum power for
Nd:YAG 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.
2.1.2 The C02 Laser

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. When the excited nitrogen molecules collide with ground state C02
molecules, the nitrogen molecules lose their energy and return to the ground state. The
C02 molecules in the ground state are excited to the higher energy state by this
bombardment. The excited C02 molecules take downward transitions to the lower energy
state, producing the emission of radiation at 10.6 um wavelength. 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 C02 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, it operates in the infrared wavelength [4, 5, 8].
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 C02 lasers are

commercially available with output power up to 25 kW.
2.2 Laser-Beam Mode

The spatial distribution of the electromagnetic field inside the optical cavity of a
laser is described by resonant electromagnetic modes, which is refered to as the
conﬁgurations of electromagnetic ﬁeld inside a laser resonator cavity. By convention, the
modes are indexed as TEan, meaning transverse electromagnetic mode with m number
of radial zero ﬁelds and n number of angular zero ﬁelds. The Figure 1 illustrates some
TEan modes in cylindrical and rectangular coordinates [4]. Beam divergence, beam
diameter, and intensity distribution along the plane of the laser beam's cross section are
governed by the transverse modes. The lowest order mode is TEMoo, which has a
Gaussian-like intensity distribution. The laser beam in TEMoo can be focused to the
smallest spot diameter on a workpiece surface in order to achieve the highest power
density. A laser beam with TEMms has a ring of intense energy surrounding a circle with
no intensity, and this mode is commonly referred to as a "donut" mode. For laser surface
treatment processes, low-order laser beams are usually used due to more uniform

intensity distribution.

10

3000.

01f 10 11~ 20

. SPOT
- C D 0.; ﬁ‘ '{ SIZE
01 02 03 64
CYLINDRICAL

00 1 O 20 30

. . . ‘

.00. .
.00. I

.. CI. 0.00 ;
11 21 33 04
RECTANGULAR

Figure 1- Transverse mode patterns in cylindrical and rectangular coordinates [4].

11

2.3 Laser Parameters

2.3.1 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. One deﬁnition has been established that 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 l/e2
beam diameter is usually adopted [9].

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. For a Gaussian beam, the diffraction limited focal spot size, dmin, is expressed by
the relationship: dmm = 1.270. / D, where "f" is the focal length of the focusing optics, and
"X" is the wavelength of the laser beam, and "D" is unfocused beam diameter. The actual
value will be larger due to aberrations and other imperfections of the focusing optic
system.

2.3.2 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.

12

2.3.3 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: Pd =
P/A, where Pd is the power density, P is the incident laser-beam power, and A is the
irradiation area. The power density is a main parameter for laser surface treatment 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 2., the material, and the surface conditions of the interaction zone. The
situation would be typical 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 case would correspond to the pro-
cesses 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.
2.4 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

l3

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 surface treatments. In practice, most metals behave in this
manner for light wavelengths exceeding about 0.5 um , which includes both YAG and
C02 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,
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 material’s absorbance. It has been found that the absorbance 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 [9]. 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

14

um wavelength at the same incident angle, the absorptivity varies from 3.6 to 43%. In
addition, increase in the roughness of a metal surface also can enhance the absorptivity by
a factor of 1.2 to 1.5.

Although metals severely reﬂect C02 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 um wavelength of C02 laser
beams, surface coatings, such as colloidal graphite coating and paint coating, and surface
oxidation by chemical or heat treatment processes [10—1 1] are widely used in industry. By

this process, the absorptivity can be increased to values of 70 to 80%.

Chapter 3

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 [12], it can extend solid solution, form metastable phases,
reﬁne the grain size, and hence improve the wear resistant, corrosion resistant, and
oxidation 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:

3.1 Laser Solid-Transformation Hardening

The laser surface hardening is usually used to harden the selected surface region
of iron-base alloys. Laser-induced transformation hardening is based on the principle: the

surface layers of an alloy is heated locally to transform to austenite and is then rapidly

15

16

quenched by conducting heat to the bulk material. This leads to the formation of a hard
martensitic layer. By this process, laser heat treatment can improve the wear resistance of
the treated material [13-17], Because a certain volume fraction of the martensitic phase
transformation at the surface layers of a substrate is contributed to an increase of hardness

in these surface regions.
3.2 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 cooling and soli-
diﬁcation rates can lead to the formation of ﬁne and homogeneous microstructures. 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 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 [18-22], aluminum alloys [23-24],

low and medium carbon steels [25-26], and tool steels [27-28].
3.3 Laser Surface Alloying

Unlike laser remelting, laser surface alloying is usually used to change the
chemical composition and microstructures of the surface layers of a substrate by
simultaneous surface melting and injection of alloying elements, and hence creates a new
surface alloy with the novel structures and required properties, such as wear resistance,

corrosion resistance, and high-temperature oxidation resistance. The chemical

17

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 to achieve the required structures and properties.
By laser alloying process, the high temperature in the melt pool can dissolve the stable
phases like intermetallic compounds, the convection in the melt pool causes a good
interrnixing 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
[29-37], aluminum alloys [38-39] and titanium alloys [40-41].

3.4 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 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, M02C 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 [42-47],
aluminum alloys [48-49], and titanium alloys [50-52], and wear resistance of the surface

layers has been signiﬁcantly improved. Because high percentage of carbides may lead to

18

the reduction in ductility of the composite layers, the dispersing process possibly causes

cracking [53].
3.5 Laser Surface Cladding

3.5.1 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 characteristics of uniform coating
thickness and smooth surface, porosity- and cracks-free, minimum dilution and distortion,
and good metallurgical bonding with the substrate.
3.5.2 Scope of Laser Cladding

For laser cladding process, different kinds of ferrous and non—ferrous alloys 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 [54-59], laser cladding with nickel
base alloys is used to improve the wear resistance of aluminum alloys [60-61], and laser
cladding with Ni-Cr-Al-Hf improves the high temperature oxidation resistance of nickel-

base superalloys [62-63].

19

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 [64-69].

3.5.3 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, laser-beam spot size and shape, laser absorptivity of the substrate, traverse
speed, powder feed rate, and shielding gas ﬂow rate. Laser parameters are discussed in
Chapter 2, and the remaining processing parameters are described as follows:
3.5.3.1 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 material. It has
been found that, because the infrared absorption of metals mainly depends on the
conductive absorption by free electrons, the absorptivity is a function of the electrical
resistivity of the substrate [11]. 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 [9, 11]. Therefore, the effective

2O

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 [9]. 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 um wavelength, and is six times

more than that for CO2 laser with 10.6 um 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.
3.5.3.2 Traverse Speed

Traverse speed is the velocity at which the substrate moves with respect to the
stationary laser beam. The ratio of the 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. 0n 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

21

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.

3.5.3.3 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 [70-71]. This process was used in the early stages of
development of laser surface alloying and cladding. However, two-step process has many
drawbacks, such as weak 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 powder is 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 [72-73] or injecting the powder into the molten pool through a
coaxial nozzle along with the laser beam [74-75]. By using the coaxial nozzle, which was

designed by researchers at High Energy Laser Processing Laboratory, Michigan State

22

University, the efﬁciency of powder utilization is up to 80%, 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. If other
variables are held constant, the clad thickness increases linearly, and the dilution
decreases with the increase of the powder feed rate.
3.5.3.4 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
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.

3.5.4 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

23

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 [76-
77].

3.5.5 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 suitable for
manufacturing costly and special parts. Obviously, it is this reason that limits the wider
applications of laser cladding process in consumer product industries.

3.5.6 Applications of Laser Cladding Process

Laser materials processes, such as laser cutting, laser welding, and laser heat
treatment, have been successfully implemented in many industrial applications. However,
in most cases, laser surface cladding is still in various stages of experimental
development. Some real-world applications of laser cladding in industry are described as
follows:

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 ap-
plication is the cladding of interlocking edges of turbine blades at 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 [78]. Since

24

then, this new technique has been widely used to rebuild or repair the worn turbine blades
and vanes.

Another application introduced by Toyota Motor Company is the laser cladding
process for engine valve seats [79]. 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.

Chapter 4

EXPERIMENTAL PROCEDURES

4.1 General Description

This research mainly consists of the two parts: one is to develop a simple but
realistic model that is relating the maximum clad thickness with process variables, and
the other is to deposit hard-facing tracks along complex geometric patterns, onto AISI
1045 (medium carbon) steel roller for fabricating patterned rotary cutting die by laser
cladding process.

In this research, designing a powder feeding system, measuring the powder
utilization coefﬁcient, and determining the relationship between the clad width and
processing parameters, were considered. The practical goal was to determine optimal
process parameters for laser cladding of CPM 10V and CPM 15V on AISI 1045 steel
plate and roller.

In order to reach this goal, a combination of CAD (computer-assisted
design)/CAM (computer-assisted manufacturing) process and a laser cladding system (a
laser generator, a beam delivery system, a close-loop powder feeder, a powder-ﬂow
splitter, and a special coaxial nozzle) were used to precisely deposit the hard-facing tracks
in a preprogramming geometric pattern, onto an AISI 1045 steel roller for fabricating a

rotary cutting die

25

26

4.2 Choice of Materials

The aim of laser surface cladding is to deposit the intended material with the
required mechanical and physical properties on 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.

4.2.1 Substrate Material

As mentioned in Chapter 1, in the conventional rotary cutting-die process, the
entire die block and cutting blades are made of an expensive tool steel D2. In practical
application, only cutting blades are used for cutting functions. Therefore, the die block
made of tool steel is not necessary, and completely wasteful. Also, the heat treatment cost
of tool steel die block is very expensive. In order to greatly reduce the cost of material
itself, AISI 1045 medium carbon steel plates and rollers were used as the substrates in
this study. The steel plate was used in modeling work, and its dimension was 100 mm x
50 mm x 12 mm. In addition, a 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.
The substrate surface was ground to remove oxidation and rust before laser cladding
process. For convenience, the chemical compositions of AISI 1045 steel and clad

materials are listed in Table l.

27

Table 1- Chemical composition of substrate and clad materials (wt. pct)

 

Materials Fe Cr V C Mn Mo Si 8
1045 steel bal. --- --- 0.43-0.5 0.6-0.9 --- --- 0.05
CPM 10V* bal. 5.25 9.75 2.45 0.50 1.30 0.90 0.09
CPM 15V* bal. 5.25 14.5 3.40 0.50 1.30 0.90 0.07

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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

In order to signiﬁcantly improve the wear resistance of cutting blades and prolong
the life of rotary cutting dies, the clad materials used in this research were commercially
available tool steel powders, CPM 10V and CPM 15V, which are made by the Crucible
Particle Metallurgy (CPM) process. The particle size for CPM 10V was -325 mesh (S 44
um), and for CPM 15V was +200 mesh (2 74 um). The chemical compositions of CPM
10V and CPM 15V are listed in Table 1.

CPM 10V 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.

CPM 15V 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.

28

As a result of pronounced ingot segregation, conventional tool steels often contain a
coarse, nonuniform microstructure accompanied by inferior mechanical properties along the
transverse direction. However, tool steels CPM 10V and CPM 15V 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 2 and 3 [80].

700

 

 

O)
8
1
r

 

$ 0"
8 8
1 I
r i

Crossed-cylinder wear teat, 107 MPa

 

 

 

 

 

 

 

 

 

 

200 -—
100 --
0 I I i J1; %
Tool steel 02 M2 CPM10V cpmovr
HRC 62 64 so 63

Figure 2- Wear resistance of CPM 10V and conventional tool steels at indicated
hardness [80]. CPM 10V* (hardened) is subjected to quenching, which

increases its hardness, and thus the extra wear resistance.

29

50

 

&
o
1

 

OJ
0

 

 

 

Charpy C-notch impact test, J
N
O

i
o
I
I

 

 

 

 

 

 

 

 

 

 

O i i :
Tool steel D2 M2 CPM1OV CPM1OV
HRC 62 64 60 63

 

Figure 3- Charpy C-notch impact properties of CPM 10V and

conventional tool steels at indicated hardness [80].

30

4.3 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 4.

4.3.1 CO2 Laser

A continuous wave C02 laser operating in the TEMop mode, with an output power
range of 200 to 2500 watts was used for laser cladding experiments. A 2500 L model
Triumph (Triumph Industrial Lasers Inc., MA) laser, with the characteristics given in Table

2, was used. For the purpose of this experiment, the laser was operated in a continuous

wave (cw) mode.

Table 2- Characteristics of the RF excited C02 laser

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Parameter Value/Type
Type of Laser Fast ﬂow coaxial C02 laser
Mode of the beam TEMOI.
Output stability i2%
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 urn
Gas consumption He: 64 Uhr
N2: 12 Uhr
C022 3 VIII

 

4.3.2 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

31

 

 

 

 

 

 

 

 

 

 

 

 

- __ @595 feeder
::=== A
LASER laser beam I
I" , powder
I” I' ‘ stream
”1 powder
co-axial II Splitter
laser ll
head I to powder
I Inletot

 

 

co-oxlal

shielding gas tie-d
powder stream 3 II

 

 

 

 

substrate

' é moving
CNC table direction

 

 

 

 

Figure 4- A schematic of laser cladding system.

32

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.
4.3.3 Powder Feeding System

The quality of laser cladding tracks really depends upon the processing variables,
and the 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 center, will lead to a significant variation of the geometry, thickness, and
smoothness of the cladding layer.

So far, lateral powder feeding system has been widely used in laser surface
alloying and cladding [73, 79]. However, based on our laser 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 because the powder material is
brought laterally into the interaction zone. This positioning is very critical, and a small
deviation will greatly decrease the powder catchment efﬁciency (to be deﬁned later), and
lead to a bad quality of cladding tracks. With side powder feeding systems, cladding
tracks are generally rough, and the thickness and width of tracks are not uniform.
Secondly, side feeding system, once set up, might work well for a linear cladding track,
i.e., x- or y- motion only, but does not work well for complex geometric patterns
requiring bidirectional motions. 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 12-15 mm, so the distance from the laser nozzle tip to workpiece
requires at least 20 mm, as shown in Figure 5. This gap is so large that shielding gas is

not fully able to protect the cladding surface from oxidation. If the cover gas ﬂow rate is

33

increased, the powder is blown away. For these reasons, it is necessary to develop a new
powder feeding system, which produces high stability, continuity, and accuracy of

powder ﬂow.

 

Figure 5- A view of side powder feeding system.

34

Few publications have reported the design of a nozzle which has a coaxial powder
ﬂow [74-75]. However, one of the designs has a main drawback, the powder stream
interacts with the laser beam inside the nozzle. As a result, molten or partially molten
particles are delivered to the surface of a substrate. Because of this, the nozzle tip
frequently clogs up and/or burns out. In our new design, the cone-shaped powder stream
(its vertex lies on the specimen surface) interacts with the laser beam just on the surface
of a substrate, and therefore the coaxial nozzle (equipped with water-cooling system)
performs very well for laser surface alloying and cladding for on-line production. The
new powder delivery system designed includes a close-loop powder feeder, a powder
splitter, and a coaxial nozzle, as shown in Figure 4.
4.3.3.1 Powder Feeder

A close-loop powder feeder (METCO model 9MP) was used for providing a stable,
continuous, and accurate powder feeding rate. It utilizes a weight-loss metering system to
control the powder feed rate. The powder feeder works as a ﬂuidized-bed metering
mechanism, and can consistently deliver alloy powder at rates as low as 0.07 gm/sec.
4.3.3.2 Powder-Flow Splitter

The powder feeder transports the alloy powder, through a hose, to the powder-ﬂow
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
coaxial nozzle, where they converge to form a cone-shaped powder stream with the same
central axis as the laser beam. This cone of powder ﬁnally reaches the laser-generated

molten pool on the surface of a workpiece, as shown in Figure 6.

35

 

Figure 6- A view of cone-shaped powder stream.

36

4.3.3.3 Coaxial Nozzle

Figure 7 is a schematic diagram of the transverse section of our coaxial nozzle. The
speciﬁcations in this diagram were developed by us, based on extensive preliminary
investigations in High Energy Laser Processing Laboratory. The ﬁnal mechanical drawing

and actual fabrication of the coaxial nozzle were performed by the Bemal Division of

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Stevens International.
I V
\ I ' §,' Inner nozzle
3/
z : .s ’4
A‘
— Water jacket
I 16
132.00 H. 2\
/
/ .
/ ~ Middle nozzle
' II / ~/ /
I / J
”a // .
1'1 5 , Powder stream In (4)
3 , / I
, /
. External nozzle
it59°?'_99_'_"_t_ .. -
;—— — — —— -— - —.—— Surface of workpiece
15.00 5.00

Figure 7- A schematic of the transverse section of a coaxial nozzle.

37

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 it prevents the particles from ﬂowing up the lens by the positive
shielding gas pressure.

The middle and the external nozzles form a cone-shaped circumferential channel
with 27° cone angle, where four parts of powder streams converge as has been stated before.
It should be pointed out that, on the bottom surface, the gap between the middle and
external nozzles is really a critical design parameter. If this gap is too large, the powder
streams can not converge, and the powder transportation is then by free fall. Another
characteristic of this nozzle is that spacers can be inserted to upper place on the middle
nozzle for adjusting defocus distance which controls spot diameter. For this coaxial nozzle,
the minimum spot size on the specimen surface is 1.8 mm. As shown in Figure 7, the gap
from the bottom surface of the coaxial nozzle to the specimen surface is ﬁxed at 5 m. At
this setting, the powder utilization coefﬁcient exceeds 80% (This can be stated as the
catchment efﬁciency of 0.8).

During laser cladding process, a considerable amount of laser radiation is reﬂected
to the bottom surface of the nozzle, and heats the nozzle to a relatively high temperature.
Therefore, the 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.

This new powder delivery system avoids all of the drawbacks of a lateral powder
feeding system. 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

38

quality of cladding tracks, with smooth surface, and uniform thickness and width. Also, this
system allows deposition of complex geometric patterns on a ﬂat, or a curved surface for
different applications, such as fabrication of rotary cutting dies, repair of turbine blades, and
prototyping of forging dies.
4.3.4 CAM System

Computer-assisted manufacturing (CAM) system was used for laser cladding
experiments. CAM system consists of a high precision X-Y table with a computer controller
(General Numeric Inc.), and its speciﬁcations are given in Table 3. The specimen mounted
on X-Y table was translated under a stationary laser beam. The combination of CAM

system with two linear axes and step motor was utilized to fabricate rotary cutting dies.

Table 3- CNC 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

 

 

 

4.4 Laser Cladding

Laser cladding experiments were carried out in two separate sets of experiments:
one was to deposit tool steel CPM 10V on AISI 1045 medium carbon steel ﬂat plates for
modeling work, and the other was to deposit tool steel CPM 15V on AISI 1045 steel rollers

for fabrication of patterned rotary cutting dies.

39

To date, no published literature has reported that tool steels, CPM 10V and CPM
15V, are used as hard-facing materials for manufacturing plate and rotary cutting dies by
laser cladding process. Therefore, wide ranges of experiments were accomplished to
measure the powder utilization coefﬁcient, determine the correlation between the clad width
and process parameters, develop various relationships between the maximum clad thickness
and process parameters, and investigate the optimal process variables for fabricating rotary
cutting dies for industrial application.

The main processing parameters important for laser cladding are laser power, laser
spot diameter and shape, traverse speed, powder feed rate, and shielding gas ﬂow rate.
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
min/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 1.27 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.

Laser cladding with injection of powder materials is the most complex case of
laser material processes because it is related to numerous process variables, and involves
various physical and metallurgical phenomena. Therefore, modeling the laser cladding
process is a difﬁcult task. It is not possible to predict the inﬂuence of an individual
parameter on the whole cladding process. In practice, several parameters have to be
varied simultaneously in order to change one characteristic of the deposited layer. For

instance, obtaining clads of the same height at different traverse speeds requires to modify

40

the amount of powder injected into the molten pool and the laser power or the laser spot
diameter.

Figure 8 shows an overview of a single cladding track on a AISI 1045 steel ﬂat plate. The
cladding track, with 80 mm in length, 4 mm in width, and 1.5 mm in height, is smooth in
appearance, and upon testing, found to be metallurgically sound. The process parameters
corresponding to this sample are as follows: laser power = 2073 watts, spot size = 3.4 mm,
traverse speed = 3.9 mm/sec, powder feed rate = 0.167 gm/sec, and shielding gas (argon)
ﬂow rate = 1.27 m3/hr.

Figure 9 displays a view of intersecting hard-facing cladding tracks on a AISI 1045 steel
roller with 50.8 mm in diameter. The cladding tracks, with smooth surface, uniform
thickness and width, were found to have no macrocracks in crossover region. The
experimental result showed that the curvature of a substrate has no effect on the quality of
circumferential clad formation. Thus, this new process has opened a novel application
field of laser surface cladding, through which complex geometric patterns on steel
substrates can be deposited for manufacturing cutting and/or stamping dies. The
parameters used for the sample shown in Figure 9 are the same as those of the sample
mentioned above. T 00] steel CPM 10V was used as the hard-facing material. It should be
pointed out that, for a middle circumferential cladding track, shielding gas ﬂow rate was
increased from 1.27 to 1.7 m3/hr, and as result of some of alloy powder was blown away,

causing the bad quality cladding track.

41

 

Figure 8- A view of a single cladding track with smooth surface on a steel plate.

(A: clad. B: substrate)

42

 

Figure 9- A view of intersecting hard-facing cladding tracks

directly deposited on a steel roller.

43

4.5 Post-Clad Treatment

Tempering treatment was carried out in a bench-top Lindberg 58114 wire element
furnace with a microprocessor-base controller. In terms of the heat treatment speciﬁcation
of tool steel CPM 10V, laser cladding samples were double tempered at 540 °C for 15
minutes each, and then cooled to the room temperature. 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 obviously increased.

4.6 Microstructures and Hardness Measurements

The laser produced samples 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 microscopy (0M). The equipment used for
microstructural observation was given as follows:

0 Olympus PM-IOAK optical microscope.
0 Olympus PME-3 optical microscope.

The microhardness measurements of laser cladding samples under different
treatment conditions were carried out using a LECO M-400-G1 microhardness tester. A
1000-g load was used for the measurement, and the indentation time was set at 15
seconds. The hardness measurements were taken across the cladding layer, heat-affected

zone, and substrate.

Chapter 5

RESULTS AND DISCUSSION

5.1 Measurement of Powder Utilization Coefficient

Powder utilization coefﬁcient "or" 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 0t is not only related
to the type of feeding mechanism, such as side feeding system or coaxial feeding system,
but also to material characteristics like the type of powders, and nature of substrates, etc.
Generally, the or value for coaxial feeding system is much higher than that for side
feeding system (typically, or value falls between about 0.3 and 0.5) due to the precise
deposition of the powder with respect to the center of the laser beam.

Other material factors can be divided into intrinsic and extrinsic properties of the
clad and substrate materials. Intrinsic pr0perties include thermal conductivity, thermal
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. A large substrate

45

provides a large heat sink and therefore reduces the pre-heating effect at the laser-molten-
pool front. This may lead to the decrease in powder utilization efﬁciency. As mentioned
in Chapter 2, the substrate surface condition has a signiﬁcant effect on the absorption of
the incident laser radiation, and hence affects the clad formation.

For the simple modeling work, CPM 10V alloy powder and AISI 1045 steel plate
were used as the hard—facing material and substrate, respectively. Numerous experiments
with various process conditions were carried out to measure the powder utilization
coefﬁcient or. The powder utilization coefﬁcient 0t 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 W1 = weight of the substrate, and W2 = total weight of the substrate and the
clad, then Wc = W2-Wl is the net clad weight within scanning time. If Fp = power feed
rate, then Wp = Fpt, which is the total weight of powder delivered within the scanning
time "".t The powder utilization coefﬁcient or is then obtained from: or = Wc / Wp_ The
processing parameters used to measure or value are listed in Table 4.

Based on the results in Table 4, a few conclusions can be made as follows:

0 For a given laser power and spot size, when traverse speed varies in a range of about 5
to 10.5 mm/sec, a value is approximately equal to 0.7 and 0.8 for each process
condition, as shown in Figure 10. 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, or value is also rather close to 0.8, shown in Figure 11. This

means that the ratio WC / Fp is a constant when scanning time "t" is ﬁxed because the

46

net clad weight increases as the increase of powder feed rate. It should be noted that
there is a threshold powder feed rate. Poor clad may be produced if powder feed rate
is below the threshold due to poor coupling between the powder and the substrate.

Figure 12 shows that or value is related to spot size. When laser-beam diameter D =
1.8 mm, average or 5 0.71. While D 2 2.3 mm, average or s- 0.80. For convenience,

when D 2 2.3 mm, or E 0.80 is employed in this model.

Table 4- Measurements of powder utilization coefﬁcient 0t

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Samples Power Traverse speed Spot size Powder feed rate
P (watts) v (mm/sec) D (m) Fp (gm/sec)
A 2073 3.0 2.3 0.167
B 2073 3.9 2.3 0.167
C 2073 4.9 2.3 0.167
D 2073 8.4 2.3 0.167
E 2073 6.6 2.3 0.25
F 2073 8.4 2.3 0.25
G 2073 10.4 2.3 0.25
Cover gas Carrier gas Scan time W. W2
Samples ﬂow rate ﬂow rate t (sec) (gm) (gm)
(m3/hr) (m3/hr)
A 1.27 0.51 19.6 89.1344 91.6340
B 1.27 0.51 16.0 91.2466 93.3100
C 1.27 0.51 13.0 87.8486 89.6642
D 1.27 0.51 5.60 80.6674 81.4038
E 1.27 0.51 7.58 82.3200 83.8366
F 1.27 0.51 7.38 103.512 105.005
G 1.27 0.51 4.57 81.4037 82.3500

 

 

 

 

 

47

Table 4 (con’d)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Samples WC=W2-W1 Wp a:WJWp Average or
(gm) (gm)
A 2.50 3.27 0.76
B 2.06 2.66 0.77
C 1.82 2.18 0.83
D 0.74 0.93 0.80 0.80
E 1.52 1.90 0.80
F 1.49 1.85 0.81
G 0.95 1.14 0.83
Samples Power Traverse speed Spot size Powder feed rate
P (watts) v (mm/sec) D (m) Fp (gm/sec)
H 1567 4.9 1.8 0.1
I 1567 6.6 1.8 0.1
J 2073 6.6 1.8 0.1
K 1567 8.4 1.8 0.1
L 1567 6.6 1.8 0.167
M 1567 8.4 1.8 0.167
Cover gas Carrier gas Scan time W1 W2
Samples ﬂow rate ﬂow rate t (sec) (gm) (gm)
(m3/hr) (m3/hr)
H 1.27 0.51 10.40 41.4985 42.2343
1 1.27 0.51 8.33 40.3514 40.9389
J 1.27 0.51 7.42 38.9380 39.4607
K 1.27 0.51 5.95 44.6333 45.0538
L 1.27 0.51 8.33 50.4055 51.4230
M 1.27 0.51 6.31 42.2131 42.9746
Samples WC=W2-W1 Wp a=Wc /Wp Average 0t
(g3) cm
H 0.74 1.04 0.71
I 0.59 0.83 0.71
J 0.52 0.74 0.70 0.71
K 0.42 0.60 0.70
L 1.02 1.39 0.73
M 0.76 1.05 0.72

 

 

 

 

 

 

 

Table 4 (con’d)

48

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Samples Power Traverse speed Spot size Powder feed rate
P (watts) v (mm/sec) D (mm) Fp (gm/sec)
N 2073 6.6 2.9 0.1
0 2370 8.4 2.9 0.1
P 2073 6.6 2.9 0.167
Q 2370 8.4 2.9 0.167
R 2073 6.6 2.9 0.25
S 2370 8.4 2.9 0.25
Cover gas Carrier gas Scan time W1 W2
Samples ﬂow rate ﬂow rate t (sec) (gm) (gm)
(m3/hr) (m3/hr)
N 1.27 0.51 7.35 37.5200 38.1130
0 1.27 0.51 5.48 39.2687 39.7213
P 1 .27 0.5 1 7.42 46.4468 47.4229
Q 1.27 0.51 5.12 40.2030 40.8991
R 1.27 0.51 8.03 42.3325 43.9976
S 1.27 0.51 5.95 40.5351 41.7500
Sarnples Wc=W2-W1 Wp oc=Wc / Wp Average a
(gm) (gm)
N 0.61 0.74 0.82
0 0.45 0.55 0.82
P 0.98 1.23 0.80 0.82
Q 0.70 0.85 0.82
R 1.67 2.01 0.83
S 1.22 1.49 0.82

 

 

 

 

 

 

 

49

 

 

 

1T—— — ——— ;_L ——7 ’ v *—
—0-— P=2073 watts, D=2.3 mm,
0-95 ‘" Fp=0.25 gm/s.
- D- - P=1567 watts. D=1.8 mm,
E 09 .- Fp=0.1 gm/s.

I
E ]
3: I
8 0.85 + .

o
'5 / ‘
a 0.8 «- Cr—f >
g I
3 0.75 «-

8
3 I
o D’ ''''''' ‘U' ‘ ' - - — - — '
n- 0 7 "" D 1‘

0.65 ..

0.6 i i i i i i i
4 5 6 7 8 9 10 11

Traverse speed (mm/s)

Figure 10- Plot of powder utilization coefficient vs. traverse speed.

1 -,_ _.___ . L,___ E_ A

 

 

 

 

 

—<>—P=2073 watts. D=2.9 mm.
v=6.6 mm/s.
- 'D- - P=2370 watts, D=2.9 mm,
E v=8.4 mm/s.
.9
Q
:8: 0.9 --
c
.Q
‘5
.E
“3 s
8
.0 0.8 «-
3 i
O
a I
0.7 i 1r i P i
0.05 0.1 0.15 0.2 0.25 0.3

Powder feed rate (gm/s)

Figure 11- Plot of powder utilization coefﬁcient vs. powder feed rate.

50

 

0.9 4-

0.8 +

0.7 «-

0.6 «—

Average powder utilization coefficient

 

 

0.5 i i
1.5 2 2.5 3

 

Laser spot size (mm)

Figure 12- Plot of average powder utilization coefﬁcient vs. laser spot size.

51

5.2 Relationships between Clad Width and Process Parameters

Theoretically, clad width is primarily dictated by the optical conﬁguration which

is deﬁned by laser-beam diameter, integrated beam diameter or scanning beam amplitude.

However, the clad width is also related to traverse speed, power density, and powder feed

rate. In order to determine the relationship between the clad width and processing

parameters, many experiments were carried out under various process conditions. The

process variables and experimental results are listed in Table 5.

Based on the results in Table 5, we can make some conclusions:

With other variables held constant, clad width modestly decreases as the traverse
Speed increases, as seen in Figure 13. When traverse speed accelerates to about 9
mm/sec, clad width is almost equal to laser-beam diameter. This speed is called
threshold traverse speed. Below threshold traverse speed, the clad is wider than spot
Size. For this speed range, the average clad width is used to calculate the maximum
clad thickness. Thus, laser-beam diameter is substituted for the clad width, in the
mOdel, when traverse speed varies from ~85 to ~14.5 mm/sec .

Clad width gently increases with the increase in energy density when other parameters
a:- e kept constant, as shown in Figure 14. This is because sufﬁcient energy is available
to melt material situated at a distance wider than the beam itself.

With other variables held constant, the clad gently widens as powder feed rate
increases, as shown in Figure 15. If the energy input is sufﬁcient, the large powder

feed rate will lead to a wider clad track.

52

Table 5- Correlation between clad width and processing parameters

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Power Traverse speed Spot size Powder feed rate Cover gas
Samples P (watts) v (mm/sec) D (m) Fp (gm/sec) ﬂow rate
(m3/hr)
1 2073 3.0 2.3 0.167 1.27
2 2073 3.9 2.3 0.167 1.27
3 2073 4.9 2.3 0.167 1.27
4 2073 5.8 2.3 0.167 1.27
5 2073 6.6 2.3 0.167 1.27
D 2073 8.4 2.3 0.167 1.27
F 2073 8.4 2.3 0.25 1.27
G 2073 10.4 2.3 0.25 1.27
Carrier gas Energy density Clad width
Samples ﬂow rate Ed (J/mmz) W (mm) B=W/D Average [3
(m3/hr)
1 0.51 382.5 3.45 1.50
2 0.51 294.3 3.35 1.46
3 0.51 234.2 3.25 1.41 1.40
4 0.51 197.9 3.12 1.36
5 0.51 173.9 3.00 1.30
D 0.51 136.6 2.38 1.03 1.03
F 0.51 136.6 2.38 1.03 1.01
G 0.51 110.3 2.30 1.00

 

 

 

 

 

 

 

 

Table 5 (con’d)

53

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Power Traverse speed Spot size Powder feed rate Cover gas
Samples P (watts) v (mm/sec) D (m) Fp (gm/sec) ﬂow rate
(m3/hr)
H 1567 4.9 1.8 0.1 1.27
I 1567 6.6 1.8 0.1 1.27
K 1567 8.4 1.8 0.1 1.27
6 2073 3 1.8 0.167 1.27
7 2073 3.9 1.8 0.167 1.27
8 2073 4.9 1.8 0.167 1.27
9 2073 5.8 1.8 0.167 1.27
10 2073 6.6 1.8 0.167 1.27
M 1567 8.4 1.8 0.167 1.27
Carrier gas Energy density Clad width
Samples ﬂow rate Ed (J/mmz) W (m) [3:W/D Average [3
(m3/hr)
H 0.51 226.2 2.15 1.19 1.15
I 0.51 167.9 2.00 1.11
K 0.51 132.0 1.80 1.00 1.00
6 0.51 488.8 3.27 1.82
7 0.51 376 3.14 1.74
8 0.51 299.3 3.00 1.67 1.64
9 0.51 252.8 2.80 1.56
10 0.51 222.2 2.56 1.42
M 0.51 132.0 2.05 1.14 1.14

 

 

 

Table 5 (con’d)

54

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Power Traverse speed Spot size Powder feed rate Cover gas
Samples P (watts) v (mm/sec) D (mm) Fp (gm/sec) ﬂow rate
(m3/hr)
11 2073 3 2.9 0.167 1.27
12 2073 3.9 2.9 0.167 1.27
13 2073 4.9 2.9 0.167 1.27
14 2073 5.8 2.9 0.167 1.27
15 2073 6.6 2.9 0.167 1.27
16 2073 3.9 2.9 0.25 1.27
17 2073 4.9 2.9 0.25 1.27
18 2073 5.8 2.9 0.25 1.27
19 2073 3.9 2.9 0.1 1.27
20 207 3 4.9 2.9 0.1 1.27
21 2073 5.8 2.9 0.1 1.27
Q 2370 8.4 2.9 0.167 1.27
S 2370 8.4 2.9 0.25 1.27
Carrier gas Energy density Clad width
Samples ﬂow rate Ed (J/mmz) W (mm) B=WID Average B
(m3/hr)
11 0.51 303.4 3.55 1.22
12 0.51 233.4 3.40 1.17
13 0.51 185.7 3.30 1.14 1.14
14 0.51 156.9 3.20 1.10
15 0.51 137.9 3.11 1.07
16 0.51 233.4 3.87 1.33
17 0.51 185.7 3.68 1.27 1.27
18 0.51 156.9 3.55 1.22
19 0.51 233.4 3.34 1.15
20 0.51 185.7 3.20 1.10 1.09
21 0.51 156.9 3.00 1.03
Q 0.51 123.9 2.97 1.02 1.02
S 0.51 123.9 3.03 1.04 1.04

 

 

Clad width (mm)

Clad width (mm)

 

55

 

 

 

 

 

 

 

 

4 _,__ 7 v v _ .A _.
—D—P=2073 watts, D=2.3 mm,
Fp=0.167 gm/s.
3 5 q_ - -A— - P=2073 watts, D=1.8 mm,
' Fp=0.167 gm/s.
A. ' x
3 A
2.5 +
2 at
1.5 i l i 1 i i a
2 3 4 5 6 7 8 9
Traverse speed (mm/sec)
Figure 13- Clad width as a function of traverse speed.
4 - —— ~~--——————-—¥ I
—D—P=2073 watts, D=2.9 mm,
Fp=0.167 gm/s.
- -A- - P=2073 watts, D=2.3 mm,
Fp=0.167 gm/s.
3.5 «-
3 A
2.5 i + .L e i ﬁi
100 150 200 250 300 350 400
Energy density (J/mmz)

Figure 14— Clad width as a function of energy density.

Clad width (mm)

 

56

 

 

4 4— —— — ——~ - -
-D—P=2073 watts. D=2.9 mm,
v=5.8 mm/s.
3.5 ~-
3 -0
2.5 + + 1r 4. 4.
0.05 0.1 0.15 0.2 0.25 0.3

Powder feed rate (gm/sec)

Figure 15- Clad width as a function of powder feed rate.

57

5.3 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. The following model is
developed based on the geometric features of the cross section of the clad, physical
properties of alloy powder itself, and process parameters. Therefore, the model is based
on simple but realistic approximations, and can be used to evaluate the optimal
processing conditions for on-line production and manufacturing.

5.3.1 Assumptions

The following model is based on the assumption that power density or energy
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.
5-3.2 Basic Equation

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

and irradiation time, so it can be expressed as

We =ant, (1)

Where Wc = net clad weight (gm), or = powder utilization coefﬁcient, Fp = powder feed

rate (gm/sec), and t = irradiation time (sec) (related to scanning speed).

58

For simple modeling work, CPM 10V alloy powder is used as the hard-facing
material. As discussed earlier, the laser-spot diameter D S 2 mm, we measured or E 0.70,
while for D 2 2.3 mm, or E 0.80.

For a single clad track, we assume that the cross-section is approximately

represented by a parabolic arch, as shown in Figure 16, and its area is expressed as

 

 

 

 

O

 

 

 

Figure 16- Parabolic shape of the cross-section of a clad track

2
A=—wh, 2
3 ()

where w = track width (mm) (assume to be equal to the spot diameter), and h = the
maximum clad thickness (m).
From geometric shape of a clad track, and physical properties of the alloy itself,

the net clad weight can be written as

59

W, = %wh(vt)p , (3)
where w = clad width (mm), v = traverse speed (mm/sec), p = alloy density (gm/cm3),
and t = irradiation time (sec). For CPM 10V, p = 7.418 gm/cm3. The terrn§wh(vt) is

actually the volume of a cladding track.

Combination of Eq. (1) and Eq. (3), we can get
2 h _ 4
3w vp — an. ( )

Equation (4) is a basic equation, relating the maximum clad thickness and various
process parameters.
5.3.3 Energy Density Equation
Energy density or speciﬁc energy is deﬁned as [81]
E d = Pdr, (5)
where Ed = energy density (J/mmz), Pd = power density (watts/mmz), and t = time (sec)
required to move a distance of the laser beam diameter along the traverse direction.

Power density is expressed as
P, = —- (6)

where P = laser power (watts), and A1 = irradiation area (mmz).
For simplicity, assume that the shape of laser spot is circular (for our TEMOI.
mode laser beam, the shape actually is toroid), then irradiation area A; = 11D2 / 4, where D

= laser beam diameter (mm).

60

Substituting A1 in Eq. (6), we can obtain

 

4P
P = . 7
4 7:02 ( )
The time ’t can be expressed as
r = 2. (8)
v
Substituting Eqs. (7) and (8) in Eq. (5), one has
4 P
E =__, 9
d 7: Dv ( )

The above equation shows that the energy density is directly proportional to laser
power, and inversely proportional to spot size or traverse speed.
5.3.4 Clad Thickness-Energy Density Equation

Eqs. (4) and (9) can be rewritten as, respectively,

 

 

3aF
h = p , (10)
2va
—1-= ”Ed . (11)
Dv 4P

For simplicity, assume that w = D when v 2 8.4 mm/sec, then substitute Eq. (11)
in Eq. (10), we can obtain

aF
big—P" Ed, (12)
p

where or = powder utilization coefﬁcient, Fp = powder feed rate (gm/sec), P = laser power

(watts), and p = alloy density (gm/cm3).

61

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.
5.4 Correlation Derived from Basic Equation
Equation (10)

3an
h = ,
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. We will discuss it in
details.
5.4.1 Relationship between Clad Thickness and Traverse Speed

In equation (10), for a given powder (ﬁxed p), if powder feed rate and spot size

are kept constant, then equation (10) can be simpliﬁed as
h = —, (13)

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

Equation (13) 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 clad track. Under different experimental conditions, the linear
relationship between the clad thickness and the reciprocal of traverse speed is shown in
Figure 17.

5.4.2 Relationship between Clad Thickness and Spot Size

For ﬁxed powder feed rate and traverse speed, equation (10) can be reduced to

 

h = , (14)

where C2 is a constant for the assumed conditions.

Equation (14) shows that clad thickness is inversely proportional to the spot size.
The larger the spot size, the wider is the clad track, but the thinner is the clad track , as
shown in Figure 18.
5.4.3 Relationship between Clad Thickness and Powder Feed Rate

If the spot size and traverse speed are kept constant, and then Eq. (10) becomes

h=C3Fp, (15)

where C3 is a constant which is related to process parameters and alloy density.

Equation (15) reveals that clad thickness and powder feed rate have a linear
relation, as shown in Figure 19. The larger the powder feed rate, the more is the powder
input per unit length, and the higher 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, and produce poor metallurgical bond between cladding layer and

substrate.

Clad thickness (mm)

63

 

 

 

  
    

 

2.5
P=2073 watts, D=2.3 mm.
Fp=0.25 gm/s.
— — — P=2370 watts. D=2.9 mm. (1)
2 Fp=0.25 gm/s.
— - — - P=2370 watts. D=3.4 mm.
Fp=0.25 gm/s.
<2)
I ’ I
<2 I
1.5 (1}),
I ’ - ’ ‘ ’
I I I ' ’
1 ”P I I I . I I I
0.5 i i i i i i
0.06 0.07 0.08 0.09 0.1 0.11 0.12

Reciprocal of traverse speed (smm'1)

Figure 17- Clad thickness as a function of reciprocal of traverse speed.

 

0.13

Clad thickness (mm)

 

 

 

  

 

 

2.5
v=8.4 mm/s, Fp=0.25 gm/s.

2 - — — v=8.4 mm/s, Fp=0.167 gm/s. (1)

1.5 «-
(2),

1 ‘I’ ' ’ a ’ I ’ ’
0.5 .L i 4.

0.25 0.3 0.35 0.4 0.45

Reciprocal of spot size (mm'1)

Figure 18- Clad thickness as a function of reciprocal of spot size.

Clad thickness (mm)

65

 

 

 

 

 

2.5
(av
‘ /
2 ~» , /
. /
' /
. ' /
(2y 4’
1.5 ~~ / r I
v=8.4 mm/s.
— — — P=2370 watts. D=2.9 mm.
1 i v=8.4 mm/s.
— - — - P=2370 watts. D=3.4 mm.
v=8.4 mm/s.
0.5 + i i
0.05 0.15 0.25 0.35 0.45

Powder feed rate (gm/sec)

Figure 19- Clad thickness as a function of powder feed rate.

 

66

5.5 Correlation Derived from the Energy Density Equation

Equation (9) displays that the energy density is related to spot size, laser power,
and traverse speed.
5.5.1 Relationship Between Energy Density and Spot Size

The laser power and traverse speed are kept constant, and then Eq. (9) is

simplified as
E d = —, (16)

where C4 is a constant.

Equation (16) 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 20. 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.

5.5.2 Relationship between Energy Density and Traverse Speed

When the laser power and spot size are ﬁxed, Eq. (9) is reduced to

Ed =g, (17)
v

where C5 is a constant.
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

67

of traverse speed is displayed in Figure 21. It shows that energy density linearly increases
as the reciprocal of traverse speed increases.

In terms of equation (7), 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.

5.5.3 Relationship between Energy Density and Laser Power
If spot size and traverse speed are held constant, then Eq. (9) becomes
5, = 06p, (18)
where C6 is a constant.

Equation (18) 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 22. 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.

Energy density (Jmm'z)

68

 

   
  
  

 

350
P=2370 watts. v=4.9 mm/s.

------ P=2370 watts, v=6.6 mm/s.
300 -~

— - — - P=2370 watts, v=8.4 mm/s.
250 -~ (2) . - '
200 —- (3) - ’

’ I
I
O ' ' I
O ' I
‘ o ’ a’
I - ‘ ‘ ’
150 -~ - : , - ’
0 g ’
‘ I
I
I
' I
100 ~-
50 i ,1 i i 'r i

 

 

 

0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6

Reciprocal of spot size (mm")

Figure 20- Energy density as a function of reciprocal of spot size.

69

300

 

 

P=237O watts, D=2.3 mrn.

-—-—-P= 7 , -. .
250 ﬁ 23 Owatts p.34mm

N
O
O

150 ~~

Energy density (Jmm'z)

100 ~—

 

 

 

50 : 1 I

T

0.05 0.1 0.15 0.2 0.25

Reciprocal of traverse speed (smm'1)

 

Figure 21- Energy density as a function of reciprocal of traverse speed.

Energy density (Jmm'z)

70

 

 

 

 

 

 

280
D=1.8 rnm. v=6.6 mm/s (1)
— - - - D=3.4 mm, v=6.6 mm/s

230 at
180 er
130 ~~ ' ’

80 “r

30 t i i

500 1000 1500 2000

Laser power (watts)

Figure 22- Energy density as a function of laser power.

2500

71

5.6 Correlation between Clad Thickness and Energy Density

Equation (12) displays that clad thickness is related to powder feed rate and

energy density, which is subjected to spot size and traverse speed when laser power is

fixed. We will discuss the following case.

From equations (13) and (17), we know that, for fixed powder feed rate and spot
size, clad thickness is only a function of traverse speed. While, for fixed laser power and
spot size, energy density is a function of traverse speed. Therefore, the clad thickness and
energy density linearly vary with reciprocal of traverse speed. The faster the traverse

speed, the lower the energy density, and thinner the clad track, as shown in Figure 23.

Clad thickness (mm)

 

 

   

 

 

 

2.5
P=2073 watts, D=2.3 mm.
Fp=0.25 gm/s.
2 __ — — - P=2370 watts, D=2.9 mm,
1.5 w
l
I /
I I
1 - . , "
o_5 1, l l l l l l
60 70 80 90 100 1 10 1 20 1 30
Energy density (Jmm'z)

Figure 23- Clad thickness as a function of energy density.

140

72

5.7 Comparison of Calculated and Experimental Results

To test the validity of this model, its predictions have to be compared with the
experimental results of laser cladding. For this reason, many experiments in various
process conditions were carried out to verify this model. Laser power varied from about
1000 to about 2400 watts, and spot size from ~2 to ~35 mm, traverse speed from ~30 to
~14 mm/sec, and powder feed rate from 0.1 to 0.25 gm/sec. In each process condition, the
experimental results are in good agreement with those predicted by this simple model.
5.7.1 Clad Thickness and Traverse Speed

As mentioned before, when the traverse speed v < 8.4 mm/sec, average clad width
is used to calculate clad thickness in equation (10). When traverse speed varies from 8.4
to 14.3 mm/sec, the spot diameter is used to substitute for clad width. The process
variables and experimental results are listed in Table 6.

When v < 8.4 mm/sec, inserting the relevant data in Table 6 into equation (10),
one can get a equation: h = 8.11/v, which shows that clad thickness is inversely
proportional to traverse speed. Calculated and experimental plots are shown in Figure 24
a. The slope of the calculated plot is 8.11, and the slope of the experimental plot is 6.24.
Clad thickness linearly increases with the increase of reciprocal of traverse speed, and
experimental and calculated results match very well.

When v 2 8.4 mm/sec, predicted and experimental plots are shown in Figure 24 b.
In both cases, clad thickness also linearly varies with reciprocal of traverse speed, and the

calculated result is in good agreement with the experimental result.

73

Table 6- Processing parameters used to develop the correlation
between clad thickness and reciprocal of speed

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Samples Power Traverse speed Spot size Clad width Powder feed rate
P (watts) v (mm/sec) D (mm) W (mm) Fp (gm/sec)
6 2073 3.0 1.8 2.95 0.167
7 2073 3.9 1.8 2.95 0.167
8 2073 4.9 1.8 2.95 0.167
9 2073 5.8 1.8 2.95 0.167
10 2073 6.6 1.8 2.95 0.167
D6 2370 8.40 3.4 0.25
D7 2370 10.4 3.4 0.25
D8 2370 12.5 3.4 0.25
D9 2370 14.3 3.4 0.25
Samples Energy density Cover gas ﬂow Carrier gas ﬂow Clad thickness
Ed (J/mmz) rate (m3/hr) rate (m3/hr) h (m)
6 488.8 1.27 0.51 2.35 2.39
7 376.0 1.27 0.51 1.87 1.91
8 299.3 1.27 0.51 1.57 1.60
9 252.8 1.27 0.51 1.36 1.40
10 222.2 1.27 0.51 1.21 1.23
D6 105.6 1.27 0.51 1.29 1.33
D7 85.3 1.27 0.51 1.08 1.10
D8 71.0 1.27 0.51 0.89 0.97
D9 62.0 1.27 0.51 0.80 0.84

 

 

 

 

 

 

 

Clad thickness (mm)

74

 

 

   

 

 

3
(1)
2.5 ~~
(2);:
2 .
P=2073 watts, W=2.95 mm,
1.5 —~ Fp=0.167 gm/s. (calculated)
— D- - P=2073 watts, W=2.95 mm,
Fp=0.167 gm/s.
(experimental)
1 T l l l t
0.1 0.15 0.2 0.25 0.3 0.35 0.4

Reciprocal of traverse speed (s/mm)

Figure 24- Comparison of calculated and experimental relationship

between clad thickness and reciprocal of traverse speed.
(a). v < 8.4 mm/sec. (b). v 2 8.4 mm/sec.

75

Figure 24 (con'd)

1.5

 

   
 

P=237O watts, D=3.4 mm.
Fp=0.25 gm/s.
(calculated)

Clad thickness (mm)

- G- - P2370 watts, D=3.4 mm,
Fp=0.25 gm/s.
(experimental)

 

 

 

0.5 i i i i i i
0.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13

Reciprocal of traverse speed (smm")

5.7.2 Clad Thickness and Spot Size

For ﬁxed traverse speed and powder feed rate, clad thickness is inversely
proportional to spot size, as described in equation (14). The process variables and
experimental results are given in Table 4. In terms of the relevant data in Table 7,

predicted and experimental plots are shown in Figure 25. As seen in Figure 25, clad

76

thickness linearly increases with the increase of reciprocal of spot size. In other words,
the larger the spot size, the thinner the clad track. Also, the predicted and experimental

results match reasonably well.

Table 7- Processing parameters used to develop the correlation
between cladding thickness and reciprocal of spot size

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Samples Power Traverse speed Spot size Powder feed rate
P (watts) v (mm/sec) D (m) FD (gm/sec)
F 2073 8.4 2.3 0.25
S 2370 8.4 2.9 0.25
D6 2370 8.4 3.4 0.25
Energy density Cover gas Carrier gas Clad thickness
Samples Ed (J/mmz) flow rate ﬂow rate h (m)
(m3/hr) (m3/hr)
F 136.6 1.27 0.51 1.70 1.75
S 123.8 1.27 0.51 1.47 1.50
D6 105.6 1.27 0.51 1.29 1.33

 

 

 

 

 

 

 

77

 

 

   

 

 

 

2.5
v=8.4 mm/s, Fp=0.25 gm/s.
(calculated)
1
- D- - v=8.4 mm/s, Fp=0.25 gm/s. ( )

2 ** (experimental)
E
E. PM:
U)
a)
a)
g 1.5 ~»
.2 ’ a
5 D I
'0
_c_u
0

1 -

0.5 . 1 .
0.25 0.3 0.35 0.4 0.45

Reciprocal of spot size (mm'1)

Figure 25- Comparison of calculated and experimental relationship

between clad thickness and reciprocal of spot size.

5.7 .3 Clad Thickness and Powder Feed Rate

If spot size and traverse speed are kept constant, equation (15) shows that clad
thickness is directly proportional to powder feed rate. The process parameters and
experimental results are listed in Table 8. Inserting the related value in Table 8 in
equation (15), its corresponding plot (1) is displayed in Figure 26, while experimental
plot (2) in Figure 26. As shown in Figure 26, clad thickness linearly increases as powder

feed rate increases.

78

Table 8- Processing parameters used to develop the correlation
between clad thickness and powder feed rate

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Samples Power Traverse speed v Spot size Powder feed rate
P (watts) (mm/sec) D (m) Fp (gm/sec)
O 2370 8.4 2.9 0.1
Q 2370 8.4 2.9 0.167
S 2370 8.4 2.9 0.25
Energy density Cover gas Carrier gas Clad thickness
Samples Ed (J/mmz) flow rate ﬂow rate h (m)
(m3/hr) (m3/hr)
0 123.8 1.27 0.51 0.60 0.64
Q 123.8 1.27 0.51 0.96 1.04
S 123.8 1.27 0.51 1.47 1.50

 

 

 

 

 

 

 

79

 

 

P=237O watts, D=2.9 mm,
v=8.4 mm/s. (calculated)

— G- - P=2370 watts, D=2.9 mm,
v=8.4 mm/s.
1.5 ~~ (experimental) (2)2
/ l

Clad thickness (mm)

 

 

 

0.5 i : l .
0.05 0.1 0.15 0.2 0.25 0.3
Powder feed rate (gm/sec)

 

Figure 26- Comparison of calculated and experimental relationship

between clad thickness and powder feed rate.

5.7.4 Clad Thickness and Energy Density

Based on the process variables and experimental results listed in Table 9, plots (1)
and (2) in Figure 27 shows that calculated and experimental results between the clad
thickness and energy density, respectively. Figure 27 shows that the clad thickness and
energy density have a linear relationship, as discussed in section 5.6. Also, Figure 27

shows that the calculated result is in good agreement with experimental result.

80

Table 9- Processing parameters used to develop the correlation
between clad thickness and energy density

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

     

 

 

Samples Power Traverse speed Spot size Powder feed rate
P (watts) v (mm/sec) D (mm) Fp (gm/sec)
F 2073 8.4 2.3 0.25
G 2073 10.4 2.3 0.25
B5 2073 12.5 2.3 0.25
B6 2073 14.3 2.3 0.25
Energy density Cover gas Carrier gas Clad thickness
Samples Ed (J/mmz) flow rate flow rate h (m)
(m3/hr) (m3/hr)
F 136.6 1.27 0.51 1.70 1.75
G 110.3 1.27 0.51 1.47 1.50
BS 91.8 1.27 0.51 1.14 1.16
B6 80.2 1.27 0.51 1.00 1.02
2.5
P=2073 watts, D=2.3 mm,
Fp=0.25 gm/s.
(calculated) (1 )
2 4 — - — - P=2073 watts, D=2.3 mm,
E Fp=0.25 gm/s.
é (expenmenta) (2), .D
(I) I ’
a , ’
5 1.5 -.
.2
E.
g a
6 , , ' U
1 -~ :r '
0.5 . . . l l i
70 80 90 100 110 120 130 140
Energy density (Jmm'z)

Figure 27- Comparison of calculated and experimental relationship

between clad thickness and energy density.

 

81

5.8 Correlation between Penetration Depth and Energy Density

If traverse speed and spot size in equation (9) 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 the deeper is the heat-affected zone. Process variables and experimental
results are listed in Table 10, and the plot of penetration depth versus energy density is
shown in Figure 28. The penetration depth linearly increases with increase in power
density. When laser power decreases to about 1000 watts, energy density is so low (73.2
J/mmz) that lack of fusion between the cladding layer and the substrate is observed. The
corresponding energy density is called the threshold energy density in this particular
cladding system. Energy density has to be higher than the threshold energy density to
ensure a good metallurgical bonding with the substrate.

Table 10- Processing parameters used to develop the correlation
between penetration depth and energy density

 

Samples Power Traverse speed Spot size Powder feed rate
P (watts) v (mm/sec) D (m) Fp (gm/sec)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

D 2073 8.4 2.3 0.167

31 1755 8.4 2.3 0.167

32 1567 8.4 2.3 0.167

33 1110 8.4 2.3 0.167

Samples Energy density Cover gas Carrier gas Penetration
Ed (J/mmz) ﬂow rate ﬂow rate depth
(m3/hr) (m3/hr) hp (mm)

D 136.6 1.27 0.51 0.62

31 115.7 1.27 0.51 0.60

32 103.3 1.27 0.51 0.58

33 73.2 1.27 0.51 0.53

 

 

 

 

 

 

 

Penetration depth (mm)

82

 

 

 

 

 

0.65
-D—v=8.4 mm/s, D=2.3 mm,
Fp=0.167 gm/s.
0.6 ~~
0.55 ~~
0.5 l
50 100 150

Energy density (J/mmz)

Figure 28- Penetration depth as a function of energy density.

83

5.9 Microstructure

One outstanding feature of laser cladding process is rapid melting and
subsequently rapid solidiﬁcation. The resultant phases in cladding layers therefore are
usually metastable. By laser cladding process, the high cooling rate may extend the
metastable solubility, reﬁne the grain size, and form unique nonequilibrium
microstructures different from those predicted from the equilibrium-phase diagram.
Therefore, the wear-resistant, corrosion-resistant, and oxidation-resistant properties of the
treated materials can be attained depending on clad material.

5.9.1 Geometric Characteristics of A Single Cladding Track

The clad bead geometry is related to process parameters, such as power density,
traverse speed, and powder feed rate. Figure 29 shows three basic single clad bead
proﬁles. The deep penetration bead, as shown in Figure 29 (a), is produced at a higher
power density and/or lower powder feed rate. The bead with a very low dilution and a
reasonable width to height aspect ratio can be achieved with the correct balance among
power density, traverse speed, and powder feed rate, as shown in Figure 29 (b). This
shape allows multiple clad layers to be built up by overlapping single clad track for
producing composite material layers and prototyping complicated geometric parts.
Parabolic bead with a minimum dilution is produced at higher powder feed rate and/or

lower traverse speed, as shown in Figure 29 (c).

84

 

Figure 29- Three basic single clad bead proﬁles. (a) Deep penetration clad bead.
(b) Clad bead with a reasonable width to height aspect ratio.
(c) Parabolic clad bead. (A: clad. B: heat-affected zone)

85

Figure 29 (con'd)

 

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. Figure 30 is
a set of optical micrographs of the cross section of single clad tracks. The process

variables are as follows: laser power = 2073 watts, spot size = 1.8 mm, and powder feed

86

rate = 0.167 gm/sec, and traverse speed varies from 3 to 6.6 mm/sec. Typically, three
zones in a good clad track are observed: clad zone, interface zone, and heat—affected
zone. The clad zone is the solidiﬁed molten alloy. The interface is a transitional zone
between the clad zone and the substrate. The heat-affected zone is produced by the laser
energy transferred to the substrate. The clad tracks look smooth, are free of porosity and

macrocracks, and have complete fusion with the substrate but with minimum dilution.

 

Figure 30- Optical micrographs of cross section of clad tracks with no macroscopic
cladding defects. (a) v = 3 mm/sec. (b) v = 3.9 mm/sec. (c) v = 4.9 min/sec.
(d) v = 5.8 mm/sec. (e) v = 6.6 mm/sec. Other processing parameters are as

follows: P = 2073 watts, D = 1.8 mm, and Fp = 0.167 gin/sec.

87

Figure 30 (con'd)

 

88

Figure 30 (con'd)

 

89

Figure 31 is another set of optical micrographs of cross section of single clad
tracks, and shows the effect of laser power on the clad formation. The corresponding
process parameters: traverse speed = 8.4 mm/sec, spot size = 2.3 mm, and powder feed
rate = 0.167 gm/sec, and laser power varies from 1110 to 2073 watts. As mentioned
before, when laser power decreases to 1110 watts, lack of fusion between the clad and
substrate is observed, as shown in Figure 31 (d). Figure 32 is a higher magniﬁed optical
micrograph corresponding to Figure 31 (d). It clearly shows the debonding between the
clad and substrate because energy density is not high enough to melt the substrate. The
corresponding energy density is called the threshold energy density in this particular

cladding system.

 

Figure 31- Effect of laser power on clad formation. (a) P = 2073 watts. (b) P = 1755
watts. (c) P = 1567 watts. (d) P = 1110 watts. Other processing parameters:
v = 8.4 mm/sec, D = 2.3 mm, and FD = 0.167 gm/sec.

90

Figure 31 (con'd)

 

91

Figure 31 (con'd)

 

Figure 32- A magniﬁed optical micrograph of ﬁgure 31 ((1), showing the lack of fusion
between the clad and the substrate (B: clad. A: HAZ. D: debonding).

92

5.9.2 Vanadium Carbides

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 thermodynamically stable
intermetallic compound. Figure 33 (a) displays that ﬁne vanadium carbide particles are
uniformly dispersed in the matrix, which consists of martensite and retained austenite.
Compared with the segregation and ﬂake morphology of carbides in D2 alloy, as shown
in Figure 33 (b), it is expected that wear resistant property of the cutting blades, which are
made of CPM 15V by laser cladding process, will be signiﬁcantly improved in the

present pI'OCCSS.

 

93

 

Figure 33- A view of carbides in CPM 10V and D2 alloys. (a) Fine vanadium
carbide particles uniformly dispersed in the matrix in CPM 10V
clad. (b) Segregation and ﬂake morphology of carbides in D2 alloy.

5.10 Microhardness of A Single Clad Track

Figure 34 shows Vickers hardness measured from the CPM 10V clad to the
substrate under different treatment conditions. The sample used to measure the hardness
was made by the following processing parameters: laser power P = 2073 watts, D = 2.3

m, V = 3 mm/sec, and PP: 0.167 gm/sec.

94

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 34
(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
tempering treatment can further increase the hardness of the steel by secondary hardening
and the formation of hard martensite [82-83]. 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 34 (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 34 (c).

900 *

95

 

800

700

600 ~~

500 ~~

400 +

Vickers hardness (Hv)

300 +

200 ~~

Interface

Clads HAZ

 

substrate

 

 

100

1 1 1
I l I

1.5 2 2.5
Distance from surface (mm)

1
I l

0.5

 

3.5

 

Vickers hardness (Hv)
01 a)
8 8

cat:-
88

200 -~

    

Interface

 

 

 

100

l I J l
I l l l

—i>—

0.5 1 1 .5 2 2.5
Distance from surface (mm)

 

3.5

 

96

1 000

 

C Interface

900

800 *-

700 4

600 ~~

500 —~

400 4~

Vickers hardness (Hv)

 

300 4
substrate

200 er

 

 

100 i i i i i +
0 0.5 1 1.5 2 2.5 3 3.5
Distance from surface (mm)

 

Figure 34- Microhardness proﬁles of the cladding track. (a) As-clad. (b) As-clad +

tempering. (c) As-clad + tempering + liquid nitrogen treatment.

5.11 Manufacturing of Patterned Rotary Cutting Die

The other signiﬁcant part of this research was aimed at depositing hard-facing
tracks, with complicated geometric patterns, onto a AISI 1045 medium carbon steel roller
to make rotary cutting die by laser cladding process. Based on the basic study, we
investigated this feasibility in order to reach the ﬁnal goal of this project.

Figure 35 shows a view of multiple hard-facing intersecting clad tracks on a ﬂat
plate. Cladding tracks are up to 1.5 mm thick, and 4.0 mm wide. They are smooth in

appearance, metallurgically sound, and free of macrocracks in crossover region. The

In

97

process parameters used to produce this sample are given as follows: laser power = 2400
watts, spot size = 3.4 mm, traverse speed = 3.9 mm/sec, and powder feed rate = 0.167 gm
/sec. Tool steel CPM 10V is used as the hard-facing material. This new process can be
utilized to fabricate patterned ﬂat plate cutting dies.

Finally, we tried to deposit hard-facing tracks, with complex geometric patterns,
on a medium carbon steel roller by laser cladding process. The dimension of AISI 1045
steel roller is 305 mm (12”) in length, and 101.6 mm (4”) in diameter. Highly wear-
resistant alloy CPM 15V is used as the clad material. CAD/CAM system is employed to

fabricate a patterned rotary cutting die. The process variables are listed in Table 11.

Table 11- Processing parameters of producing rotary cutting dies

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Samples Material Power Traverse speed Spot size Powder feed rate
P (watts) v (mm/sec) D (m) Fp (gm/sec)

A CPM 15V 2370 4.7 2.3 0.25

B* CPM 15V 2370 4.7 2.3 0.25
Samples Energy density Cover gas ﬂow rate Can'ier gas ﬂow rate

Ed (J/mmz) (m3/hr) (m3/hr)
A 279.2 1.27 0.62
B* 279.2 1.27 0.62

 

 

 

 

 

 

* Sample B was preheated to 150 °C before laser cladding process.

Figure 36 is a view of the hard-facing tracks, directly deposited onto a A181 1045
steel roller by laser cladding process. Figure 37 is another view of hard-facing tracks,
with complex geometric patterns, deposited on a rotary cutting roller after laser cladding
process (sample A). Hard-facing patterns with smooth surface are 1.8 mm high, and 3

mm wide, but macrocracks in crossover region were observed. With regard to the

98

macrocracks observed in Figure 37, the two reasons may be considered. The residual
tensile stresses may result from high thermal gradients by laser cladding process and
different thermal expansion behavior of the materials involved. When the residual
thermal stresses exceed the fracture stress of the clad, cracks will be created. Material
factor is intrinsic property of material itself, and can not be changed. The only factor
which can be changed is the high thermal gradients, which lead to the reduction in
thermal stresses. A simple but effective method is to preheat the workpiece to a certain
temperature.

In this study, the AISI 1045 steel roller is preheated to 150 °C, and complex hard-
facing patterns are deposited on it by laser cladding. Through examining the sample by
using an OLYMPUS PM-lOAK optical microscope just after laser cladding, no cracks in
crossover region were observed. A fully functional prototype of an industrial cutting die,
for pilot testing, has been successfully produced by this process (sample B), as shown in
Figure 38. This new technique for manufacturing patterned rotary cutting dies is so

successful that it is about to be implemented in cutting—die making industry.

 

99

 

l
a
i

. -_.

‘ 22* *-.«--~v:s~.-~

 

Figure 35- A view of intersecting hard-facing tracks, with smooth surfaces, uniform

cladding thickness, and width, deposited on a AISI 1045 steel ﬂat plate.

 

Figure 36- A view of the hard-facing tracks, directly deposited onto

a AISI 1045 steel roller by laser cladding process.

101

 

Figure 37— Hard-facing tracks, with complicated geometric patterns, deposited

on a AISI 1045 steel roller via laser cladding (sample A).

102

:0... mi ..| ' I '1'"). -

" I l!ll-i!l I-~~-'

 

 

Figure 38- A view of hard-facing tracks, with complex geometric patterns, deposited

on a steel roller by laser cladding after surface grinding (sample B).

Chapter 6

CONCLUSIONS

The following conclusions can be drawn from this research:

The new powder delivery system designed for laser cladding 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, 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, repair of turbine blades, and prototyping of forging
dies. Most importantly, the new powder feeding system is suitable for on-line industrial
production.

For a given laser power and spot size, when the traverse speed varies from about 5 to
about 10.5 mm/sec, and the powder feed rate varies from 0.1 to 0.25 gm/sec, powder
utilization coefﬁcient, on, is constant for each process condition. This shows that value
of or is not related to traverse speed, and powder feed rate. However, it is related to
spot size. When the laser-beam diameter "D" was 1.8 mm, the average value of at was
0.71. For D 2 2.3 mm, average value of or was 0.80.

With other variables held constant, clad width modestly decreases as traverse speed

increases. When traverse speed accelerates to 8.4 min/sec, clad width is almost equal

103

_ ___

104

to laser-beam diameter. This speed (for our particular system) is called the threshold
traverse speed in this cladding system. Below the threshold traverse speed, the clad is
much wider than 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 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 for ﬁxed powder feed rate and traverse speed, the clad thickness is inversely
proportional to the spot size. Finally, when 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.
A speciﬁc energy equation, suitable for a toroidal TEMm. 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.

Thermal penetration depth is a 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.

Clad bead geometry is related to the processing parameters, such as power density,

traverse speed, and powder feed rate. The deep penetration bead is produced at higher

105

power density or lower powder feed rate. The clad bead with a reasonable width to
height aspect ratio is achieved using a correct balance of power density, traverse
speed, and powder feed rate. The parabolic shape cross section, clad bead with a
minimum dilution is produced at a higher powder feed rate and/or a lower traverse
speed.

Optical micrograph displayed that ﬁne vanadium carbide particles are uniformly
dispersed in the matrix. Compared with the segregation and ﬂake morphology of
carbides in D2 alloy, it is expected that wear-resistant property of the cutting blades
(produced by laser cladding) will be signiﬁcantly improved in the present process.

A hardness of the cladding tracks up to 650 Hv has been achieved, and the hardness
increases to 850 Hv after cooling to the liquid nitrogen temperature, which causes
transformation of the retained austenite to martensite.

A novel process for manufacturing plate or rotary cutting dies, with complicated
geometric patterned cutting blades, has been successfully developed by directly laser
surface cladding. Fully functional prototype of an industrial rotary cutting die, for
pilot testing, has been successfully produced via laser cladding process. This new

technique is being implemented in die-making industry.

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