L T... k. »

fax; .1:
r .A I: .
awfiwifig

f, .

Waugh». .. 3.
rvwkfirm‘mr.“ .

7. i
. 1t.

5......5a .
”was? . ..
Eh

mam

3339:.
5 .
13...? LN:
£5- . ti.

 

yiflifiéflmhm . raga“? mum“... «M...» . rmflgwgfih

32%.

mfififvfl

”agigfizfiafim

 

THEmS

 

l///////////7’7"’W WWW/W???“
LIB HARY 3 iM/Ifligz/éflfiV/m
MiChiQan State 9
University

 

 

 

This is to certify that the

thesis entitled

Laser Die-Less Forming of Superplastic

Materials

presented by

Ning Hu

has been accepted towards fulfillment
of the requirements for

Master's Materials Science

degree in

 

 

 

 

Date July 2, 1998

0-7639 MS U is an Affirmative Action/Equal Opportunity Institution

 

PLACE IN RETURN BOX to remove this checkout from your record.
TO AVOID FINES return on or before date due.
MAY BE RECALLED with earlier due date if requested.

 

DATE DUE

DATE DUE

DATE DUE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1M WWpGS-QM

LASER DIE-LESS FORMING OF SUPERPLASTIC
MATERIALS

By
Ning 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

1998

ABSTRACT

LASER DIE-LESS FORMING OF SUPERPLASTIC
MATERIALS

By

Ning Hu

Traditional forming of a “superplastic” sheet can be accomplished by using a gas
pressure to blow, or using vacuum to draw the sheet into a die cavity, when the

temperature of the sheet reaches the superplastic deformation temperature.

An innovative study on the feasibility of using high energy laser to deform plastic and
superplastic materials was conducted in the High Energy Laser Materials Processing
Laboratory HELP of Michigan State University. This novel superplastic sheet forming
technique is vastly different from the conventional superplastic forming process. In this
study, a high energy C02 laser was controlled to perform heating, at selected and
programmed locations, on a superplastic sheet which is situated on a box containing a gas
at higher than ambient pressure, or vacuum. Both the geometry of the heating path and
the amount of heat input could be precisely implemented through computer numerical
control ( CNC ). Convex or concave bulges, which will delineate a desired shape, can be

obtained.

With the experimental results, the feasibility of laser die-less superplastic forming has
successfully demonstrated the economic and practical potential in complex shape
forming. It is even possible to control not only the final thickness, but also the deformed
shape by using this technique. Since the quality of the workpiece can be controlled at any

region of processing without the use of a die, a complex fabrication seems possible.

DEDICATION

To my parents and my husband:

for their understanding, encouragement, and support

iv

ACKNOWLEDGEMENTS

I want to express my sincere appreciation and gratitude to Prof. K. Mukherjee, my
advisor, for his guidance, encouragement, and financial support throughout the period of
this study. I also would like to thank Prof. T.R.Bieler, who provided a lot of valuable
advice and theoretical instruction for my research. Special thanks to Prof. J. P. Lucas and

Prof. P. Y. Kwon for their invaluable suggestions for improving this thesis.

Sincere gratitude is extended to Dr. C.W.Chen, for his specific technical assistance and
patient help. Helpful suggestions received from Mr. Yiping Hu, and Mr. J. K. Park are

gratefully acknowledged.

TABLE OF CONTENTS

LIST OF TABLES viii
LIST OF FIGURES ix
1 INTRODUCTION .............................................................................. 1
2 SUPERPLASTICITY & SUPERPLASTIC FORMING
2.1 History of Superplasticity ............................................................. 4
2.2 Properties of Superplastic Materials ................................................ 5
2.3 Theory of Superplasticity ............................................................. 6
2.4 Requirement for Superplasticity ....................................................... 9
2.5 Types of Superplastic Materials ..................................................... 10
2.6 Superplastic Forming Techniques ................................................... 10
3 PRINCIPLE OF LASER & LASER MATERIALS PROCESSING
3.1 Principle of Laser Operation ........................................................ 15
3.1.1 Radiation of Light Quantum .................................................... 15
3.1.2 Stimulated Emission ............................................................. 16
3.2 C02 Laser .............................................................................. 20
3.3 The Characteristic of Laser Beam ................................................... 22
3.3.1 The Property of Continuos Beam ............................................... 22
3.3.2 The “ Mode” of Laser Beam ..................................................... 23
3.4 Interaction of Laser Beam with Materials ......................................... 25
3.5 Laser Applications in Materials Processing ....................................... 29
3.5.1 Laser Cutting ....................................................................... 29
3.5.2 Laser Welding ..................................................................... 30
3.5.3 Laser Surface Treatment ......................................................... 30
3.5.4 Laser Forming ( Bending ) ....................................................... 32
4 LASER DIELESS SUPERPLASTIC METAL FORMING ........................... 33
4.1 The Function of Laser Beam ......................................................... 33
4.1.1 Laser Beam Power ................................................................ 34
4.1.2 Laser Beam Diameter ............................................................ 34

vi

4.1.3 Beam Traverse Speed ............................................................ 36

4.1.4 Beam Scanning Track ............................................................ 37

4.2 Forming Force ......................................................................... 37

5 EXPERIMENTAL PROCEDURE ......................................................... 39
5.1 Experimental Setup ................................................................... 39
5.1.1 C02 Laser ........................................................................... 39
5.1.2 CAM System ........................................................................ 42

5.1.3 Superplastic Forming System .................................................... 43

5.1 .4 Measurement System .............................................................. 45

5.2 Preparation of Testing Materials .................................................... 45
5.3 General Description of Experiments ................................................ 48

6 RESULTS AND DISCUSSION .............................................................. 50
6.] Investigation of Laser Energy Effect ............................................... 50
6.1.1 The Influence of the Laser Parameters .......................................... 50
6.1.1.1 Small Beam Diameter at Different Traverse Speed ...................... 51

6.1.1.2 Large Beam Diameter ........................................................ 54

6.2 Measurement of Temperature Distribution ....................................... 56
6.2.1 Two Points Measurement ( dynamic state ) ................................... 60

6.2.2 Center Point Measurement ( static state ) ...................................... 64
6.2.3 Four Points Measurement ( static state ) ....................................... 66

6.3 Evaluation of Microstructure ........................................................ 74
6.3.1 Grain Growth at Furnace Heat Treatment ..................................... 74

6.3.2 Grain Growth during Laser Process Heating .................................. 80

6.4 Measurement of Strain and Strain Rate ........................................... 80
6.5 The Variation of the Thickness ...................................................... 85

7 SUMMERY ....................................................................................... 91
BIBLOIGRAPHY ................................................................................. 93

vii

LIST OF TABLES

Superplastic properties of several alloys ................................................... 11
The Characteristics of RF excited C02 laser .............................................. 42
Specifications of the CNC machine ........................................................ 43
Composition and selected mechanical properties of GK 45 alloy ...................... 48
The parameters used in the experiment for testing energy absorption ................. 52

viii

LIST OF FIGURES

l. The relationship between stress and strain rate ............................................ 7
2. The relationship between strain rate sensitivity exponent, m, and strain rate ......... 7
3. A schematic view of sequence for superplastic forming process ........................ 12
4. Spontaneous photon emission ............................................................... 16
5. Photon stimulated transition of an electron and photon emission ....................... 17

6. Balance of absorption and emission in steady state:

(a) stimulated emission; (b) absorption; (c) spontaneous emission ..................... 18
7. A vibration mode of C02 atoms .............................................................. 21
8. Energy levels of C02 laser .................................................................... 21
9. Transverse-mode patterns in rectangular and cylindrical symmetry .................... 24

10. Surface temperature of A] as a function of time for various absorbed energy.

The dotted line shows the shape of the Gaussian laser beam mode.

Melt depth as a function of time with velocity of the liquid-solid interface ........... 27
11. Reflectivity as a function of wavelength for several metals ........................... 28
12. Schematic diagram of CNC laser assisted superplastic forming ........................ 35

13. Schematic diagram showing the vacuum chamber used in conjunction

with laser process .............................................................................. 40

14. A photograph showing the laser nozzle, chamber and temperature

measurement setup ............................................................................. 41
15a. A schematic diagram showing the condition of vacuum forming ..................... 44
15b. A schematic diagram showing the condition of argon gas forming ................... 44

ix

16.

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

A schematic diagram showing the setup for temperature measurement .............. 45

A schematic diagram showing the laser spot diameter, locus of the spot

for generating a dome ....................................................................... 49
A Photograph of six patterns showing the surface condition of laser

energy absorption by using small laser beam diameter and with

different traverse speed. Testing material was 7475 aluminum alloy ................. 53

A photograph of a sample showing the surface condition of laser
energy absorption with a circle pattern in 7475 aluminum alloy ...................... 55
The temperature variation with 20mm laser spot diameter, at 350 w
power output, traverse speed 0.42 mm/s ................................................... 57
The temperature variation with 20mm laser spot diameter, at 470 w
power output, traverse speed 0.42 mm/s ................................................. 58
The temperature variation with 20mm laser spot diameter, at 470 w
power output, traverse speed 0.105 mm/s ................................................ 59
Two points measurement. 230 w power output and 21.16 mm/s
traverse speed at 5 mm spot diameter .................................................... 61
Two points measurement. 230 w power output and 8.47 mm/s
traverse speed at 5 mm spot diameter ................................................... 62
Two points measurement. 230 w power output and 4.23 mm/s
traverse speed at 5 mm spot diameter ................................................... 63

One point measurement. 345-370 w power output and 20mm

27.

28.

29.

30.

31.

32.

33.

34.

35.

36.

37.

38.

39.

40.

41.

42.

spot diameter at static state ................................................................ 64

One point measurement. 470-490 w power output and 20mm
spot diameter at static state ................................................................ 65

One point measurement. 570 w power output and 20mm

spot diameter at static state ................................................................ 65
Four points measurement. Power output is 490 w ...................................... 68
Four points measurement. Power output is 550 w ...................................... 68
Temperature distribution at 70 seconds, cover gas 0.5 scfh ........................... 69
Temperature distribution at 70 seconds, cover gas 3.0 scfh ........................... 69
Temperature distribution at 350 w power output ....................................... 71
Temperature distribution at 490 w power output ....................................... 72
Temperature distribution at 530 w power output ....................................... 73

A photograph showing the grain size at 450°C, 40 minutes furnace annealing. ....75
A photograph showing the grain size at 500°C, 40 minutes furnace annealing. ....76
A photograph showing the grain size at 550°C, 40 minutes furnace annealing. 77
A photograph showing the original grain size of GK 45 alloy ........................ 78
The curve showing the grain size growth tendency at 40 minutes

furnace heating condition .................................................................. 79
A photograph showing the grain size at the bottom of a deformed

dome in GK 45 sample ..................................................................... 81
A set of deformation profiles, taken at 20 minutes interval

during the processing ....................................................................... 82

xi

43.

45.

46.

47.

48.

49.

The length elongation of the cross section with process time ......................... 82

A sketch of the cross section showing the thickness variance
with vacuum forming system ............................................................... 86
A sketch of the cross section showing the thickness variance
with argon gas forming system ............................................................ 86
A photograph showing the cross section appearance forming
by vacuum .................................................................................... 88
A photograph showing the cross section appearance forming
by argon gas .................................................................................. 88
A sketch of showing the deformation process by a laser beam

scanning around a circle and stable irradiation at the center ........................... 87

A photograph of a dome produced by LSF in GK 45 alloy. A square-mesh

grid pattern is projected on the sample to help visualize the curvature ............... 90

xii

Chapter 1

INTRODUCTION

In the current superplastic sheet forming industry, all products are fabricated by
compressing between two matching tooling dies to obtain a desired shape. Superplastic
forming has been considered as a mature processing technique for many years. This is
evident by the large amount of technical literature related to superplasticity and

superplastic forming.

However, the conventional methods for making both male and female dies are always
time-consuming and expensive, especially for prototyping or small batch production. One
reason is that the high cost of materials and labor in making die. Furthermore, the product
cycle is dominated by the time spent on die making. Another reason is that the parts
loading and unloading are complicated by heated forming dies, removing a formed part

involves a cooling down period.

There is also a problem with the cost of the large-scale superplastic production. Further,
the situation of clamping load and thermal stresses, encountered during heat-up and cool-
down, can cause permanent distortions in tooling dies, so the tooling die needs to be
frequently replaced. Based on the features of conventional superplastic forming process,
it would be desirable if at least prototypes could be produced without need of the tooling

die. This would increase the speed of marketing.

Recently, the laser materials processing techniques are being developed for numerous
industrial applications. The processing that combines superplastic forming and laser
processing techniques was developed to overcome the shortcomings brought by the
conventional processing techniques. In this technique, plastic deformation of a
superplastic metal and alloys can be produced with little mechanical contact,

consequently, the hard tooling requirement is significantly reduced.

Through the experiments in this project, the feasibility of the laser dieless superplastic
forming has been demonstrated. In this novel laser processing technique, laser energy
was employed to work as a heating resource and localized deformation was generated by
a slow scanning speed of a laser beam and gas pressures. With respect to the stationary
laser beam, the target of the laser beam is moved by the computer numerical control table,

which can be programmed in a CAD/CAM package.

Laser dieless superplastic forming, for manufacturing, is not yet sufficiently developed
for a large scale production. However, it seems clearly possible to produce a variety of

complex geometric shapes with this technique.

Compared with conventional superplastic forming processes, laser dieless superplastic

forming has the following advantages:

For rapid prototyping, and limited production, the cost of complicated devices and
tooling die are not necessary.

This economical processing procedure can increase the competitiveness of products.
The deformation of a complex geometric shape, can be generated just by
programming a desired pattern.

The temperature, for getting correct deformation, can be controlled by adjusting the
laser power parameters, and laser beam diameter.

The final thickness, at any region of the final shape, can be controlled by patterning

the deformation.

Chapter 2

SUPERPLASTICITY & SUPERPLASTIC FORMING

2.1 History of Superplasticity
It is often thought that superplasticity is a recently discovered phenomenon. In fact the

phenomenon of the superplasticity was reported about eighty - five years ago, and the first

paper on superplasticity was published in 1912, by Bengough [1].

However, the history of superplasticity could be traced back to the early Bronze period
around 2200 BC in Turkey. At that time, the amount of ancient arsenic bronzes has been
considered to contain up to 10 wt %, as used in early Bronze. Such alloys could be
superplastic, since they were two-phase alloys with very small grain size. According to
modern superplasticity theory, it may have developed the required stable, fine-grained
superplastic structure, during hand forging into intricate shapes. From about 300 BC. to
the late 19th Century, the composition of ancient steels, used in Damascus, have been
discovered to be very similar to the ultrahigh carbon steel, used in modern time, which
have typical superplastic characteristics. In Bengough’s report[1], he described his
discovery as a process in which “ a certain special brass as if were pulled out to a fine
point, just like pulling a glass would give rise to an enormous elongation ”. The concept
of “amorphous cement” theory was discussed in a following paper by Rosenhain and
Ewen [2]. The early stages for superplasticity were done in the late 1920’s by Jenkins,
[3]. After 1945, superplasticity became a common term in science coined by Bochvar and

Sviderskaya [2]. Up to 1962, superplasticity was described as a firm concept based on

the comprehensive review of Underwood[3], and the early experimental work of
Backofen and Cowokers [3,4]. Recently, research on superplasticity has progressed
much further. Its scientific merit in the context of fundamental flow and failure
mechanism, as well as its technological significance in forming process, have been

studied.

By now, it is very clear that superplasticity is a terminology used to describe the
exceptional ductility that certain metals can exhibit when deformation conditions are
proper. It is also the deformation process that produces essentially the neck-free

elongation of many hundreds of percents in metallic materials deformed in tension [5,6].

Recently, it was recorded that in a certain metal alloy system, tensile elongation of
thousands of percent has been produced. For instance, a superplastic elongation of 4,850
% was demonstrated in a Pb-62 wt % Sn alloy [7], and 5,550 % is demonstrated in a
commercial aluminum bronze [8,9]. The current world record is about 7,7550% in the

Pb-Sn system [10], and most recently an elongation record of 8000 % has been reported

in the commercial aluminum bronze [11].

2.2 Properties of Superplastic Materials
As mentioned above, the basic definition of superplasticity is the capability of certain
alloys and even certain ceramic materials to develop extremely high tensile elongation at

elevated temperatures and under controlled deformation rates [12-16]. Thus, sometimes

it behaves as if it were a polymer or glass. Another characteristic is that the forces and
stresses required to cause deformation, can be as little as ‘/2 to 1/20th of that or the
conventional alloy under the sameiconditions [17]. These two factors which are, high
tensile elongation and low flow stress, provide the exceptional potential for superplastic

deformation.

2.3 Theory of Superplasticity

It is well known that a major characteristic of a superplastic material is its strong
resistance to necking deformation. The resistance of necking, which leads to a
phenomenon of an extremely high tensile elongation at certain conditions is best
measured by the value of the strain rate sensitivity exponent m. The term m is related

to the rate of change of flow stress with strain rate. It is expressed in the following form:

_ 31nd

 

alné

where o is the flow stress and. 8 is the strain rate.

The analytical relationship between ln 0 and ln 5 is shown in Figure 1.

From Fig.1, several important features are be recognized. First, the slope of the curve
represents the strain rate sensitivity exponent m. Second, this plot reveals three distinct
regions labeled as I, II and III, respectively. It is obvious that the strain rate sensitivity is

maximum in the intermediate strain rate - region II, because the slope of the

 

 

I I
r I rr ' ill

I l

I l

6' l l
a: I I
.2 ' l
l l

I

I l

'09 é{

Figure l. The relationship between stress, 0, and strain rate 8 [12]

 

—

T
I
I

--q

I
.4— Superplasticity —>
range

Ilb Ill

     

   

 

--——-_—- -

  

 

 

I
t
I
l
i
I
i
I
L l
‘ it
109 6t
log ét

---

 

Figure 2. The relationship between strain rate sensitivity exponent, m, and

strain rate 8 [12]

curve is steeper than the slopes in the regions I and HI. From the expression of the strain
rate sensitivity exponent, the higher the m value, the larger the amount of stress under a
certain strain rate. Therefore it indicates that the higher the m value, the stronger the
resistance to necking deformation. Due to this reason, a large m value will be the desired
situation for neck stability. The region II is associated with optimum superplasticity,
since it is related to a grain boundary sliding mechanism. The m value of region 11 is
equal or higher than 0.5 which meets the requirement of superplasticity. The higher
strain rate located in the region III, can be correlated with dislocation creep mechanism,
which is controlled by climb of edge dislocations. Usually, the m value is 0.2 or less. Up
to now, the origin of the low strain rate region I, is not exactly clear because the
experimental evidence in this region is often contradictory. But it is supposed that region

I may be associated with Coble creep [3].

The value of m for a superplastic material is not a constant function of strain rate. The
influence of variables on the strain rate sensitivity exponent, m, has been considered as

strain rate, microstructure and temperature. Generally, m has a maximum value at a

critical strain rate and normally the curve of m versus ln 8 looks like a bell shaped

curve as shown in Figure 2. Although in some cases of superplastic deformation, m
has been reported to be independent [18.19.25], and in some cases it increases [20,27] or
decrease with [28,29] increasing strain rate. However, usually, it has a maximum value
at a critical strain rate region [20-23]. Figure 2 shows clearly that beyond the critical

strain rate or region III, and I, m decreases with strain rate at region IIb, and increases

with strain rate at region IIa. Comparing with the Figure 1, optimal superplastic
deformation could be obtained in region II. On the other hand, m also increases with

decreasing grain size [20,22,25] and with increasing temperature [21, 24-26].

2.4 Requirement for Superplasticity

Since there is a significant difference between normal materials and superplastic materials
for ductility and tensile elongation, it is well understood that there would be some
fundamental requirements for superplasticity. Studies on this objective have concluded
three prime conditions for the display of superplasticity: (1) grain size must be very fine
(normally less then 10 um), and equal-axed grain size should be stable during
deformation; (2) the deformation temperature usually is higher than one half of the
melting point of the matrix in Kelvin, (3) and the rate of straining must be controlled

between 10%1 and 10'2 s" [3,4,17].

There are just a relatively few metals or alloys which can demonstrate superplasticity.
Most metals do not maintain a fine grain size at elevated temperatures, unless the grain
growth can be inhibited by some microstructural features. But the grain size also will
increase when the temperature reaches more than one half the melting point, and therefore

superplastic materials must be few in numbers [4,6].

2.5 Types of Superplastic Materials

Only some ferrous and nonferrous metals and alloy systems possess the potential for
demonstrating superplastically. Such as titanium and aluminum alloys have a great
potential for superplasticity. The application of superplastic forming techniques have

been developed extensively for several materials, listed in the Table 1 [30,31].

2.6 Superplastic Forming Techniques

The term of superplastic forming means a typical process that utilizes this unique
characteristic of superplasticity as illustrated in Figure 3 [5]. As shown in the schematic
view, the superplastic blank is inserted between two die elements or tools, one of which is
configured to the part requirement, so the lower die is the configuration die. The upper
die is utilized to apply the gas pressure, to cause superplastic forming. The tooling and
the superplastic blank are then placed between heated elements, which applies the heat on
the superplastic blank to a suitable temperature for superplastic forming. A clamping
pressure is applied to this sandwich of heated platens, and a set of tooling contains the gas
pressure. The gas pressure is then imposed over the top of the sheet causing is to blow
form or stretch form into the die cavity. The critical factor in superplastic forming is that
the rate of applied pressure must be controlled because it reflects directly into the strain

rate imposed on the superplastic blank.

10

Table 1 .

Superplastic Properties of Several Alloys [7]

 

 

 

Test Strain Strain rate Elongation
Alloy temperature rate sensitivity %
0C 0F s'1 m

Aluminum:
AL-4.5Zn-4.5Ca 550 1120 8 x 10'3 0.5 600
A1—6 tolOZn-l.5Mg-0.22r 550 1120 10'3 0.9 1500
Al-5.6Zn-2Mg-l.5Cu- 516 961 2 x 104 08-09 800-1200
0.2Cr (7475)
Al-6Cu-0.52r(Supra1100) 450 840 10'3 0.3 1000
A1-6Cu-0.35Mg-0.14Si 451 840 10'3 0.3 900
(Supral 220)
Al-4Cu-3Li-0.52r 450 840 5 x 10'3 0.5 900
Al-3Cu-2Li-lMg-0.22r 451 930 1.3 x 10'3 0.4 878
Titanium:
Ti-6Al-4V 840-870 1.3 x 10“ 0.75 7501170
Ti-6Al-5V . 850 1560 5 x 10“ 0.70 7001100
Ti-6Al-28n-42r-2Mo 900 1650 2 x 10“ 0.67 538
Ti-4.5Al-5Mo—l.5Cr 901 1600 2 x 10“ 0.63-0.81 >510
Ti-6Al-4V-2Ni 902 1499 2 x 104 0.85 720
Ti-6Al-4V-2Co 903 1499 2 x 10“ 0.53 670
Ti-6Al-4V-2Fe 815 1499 2 x 10“ 0.54 650
Ti-5A1-2.58n 1000 1830 2 x 10“ 0.49 420
Nickel:
IN 199 (PM) 1010 1850 0.5 1000
Iron:
Fe-1.6C ( +1.5Cr) 650 1202 10“ 0.46 1200
Fe—26Cr-6.5Ni (IN 744) 900 1652 5 x 10‘4 -- 1000
Zinc: Zn-22Al 200 392 10'2 0.5 2000

 

 

 

 

 

11

 

   
 
  
         

Tooling

Superplastic
sheet

Resistance-heated
ceramic upper platen

% Resistance-heated

ceramic lower platen

      

     

Mllllll

    

-\___ _.-_ _7 ../ \:
/////l/Im-mm\\\\\\\\\\

          

      

iii-I|\\.\\\\\\\\\

 
     
  

Figure 3. The schematic view of sequence for superplastic forming process [5]

12

The forming temperature is also important in superplastic forming, since the temperature

variation in a forming die is a primary source of localized thinning.

Recently, a number of methods has been reported for superplastic materials forming [2,
5]. Each method has a unique capability and develops a unique set of forming
characteristics. They are: (I) blow forming, (2) vacuum forming, (3) deep drawing,

(4) superplastic forming Idiffusion bonding, (5) forging, (6) extrusion and (7) dieless

drawing.

Blow and vacuum forming are basically the same process in which a gas pressure is
imposed on the superplastic diaphragm, causing the material to form into the die
configuration. In vacuum forming, the applied pressure is limited to atmospheric
pressure, and the forming rate and capability are therefore limited. With blow forming,
additional pressure is applied from a gas pressure reservoir, and the only limitations are
related to the pressure of the gas rating of the system, and the pressure of the gas source.
Generally, large and complex parts can be formed by this method, because it has the
advantage of no moving die components and dose not require mated die components.
Multiple parts can be formed in a single process cycle, thus permitting an increase in the

production rate for some parts.

Thermal-forming methods, adopted from plastic technology, sometimes use a moving or

adjustable die member in conjunction with a gas pressure or vacuum. These techniques

13

provide many ways of producing different shapes of parts, and are effective for

controlling the thinning characteristics of the finished part.

For deep-drawing process, it depends on strain hardening to achieve the required
forrnability and to prevent thinning and rupture during forming. We know that
superplastic materials do not strain harden to any great extent, but they depend on the

high strain rate hardening for their forming characteristics.

Recent developments in superplastic forming have been demonstrated that a number of
unique processes are available if joining methods, such as diffusion bonding can be
combined with superplastic forming. This technique involves the use of a minimum of
four sheets to make a sandwich panel. The external sheets are then bonded, the inner two
sheets are then welded to define the core structure, and finally the core is expended to

bond with the external sheets and to complete the formation of sandwich structure.

Forging is a closed die process for the manufacture of precise shapes. It is usually a high
temperature process and occurs under conditions in which low flow stress is encountered.
Superplastic alloys have very low stresses at low strain rate and although compressive
tests have shown that frictional constraints can lead to pressure build up, the effect is
much less marked than in compressive deformation of conventional alloys. Thus it can be
anticipated that if sufficient time is allowed then the use of superplastic alloys in closed
die forging should result in excellent die filling with efficient reproduction of very fine

detail.

14

Chapter 3

PRINCIPLE OF LASER & LASER MATERIALS
PROCESSING

3.1 Principle of Laser Operation

Lasers have evolved into an important element of advanced technology since T. H.
Maiman created the first working laser in 1960 [33]. Laser is an acronym for light
amplification by stimulated emission of radiation. Due to the highly directional,
monochromatic and coherent nature of light emitted from a laser, many uses have been
found for laser devices in ways never before imagined. Examples include powerful,
effective tools developed for material processing such as in welding and cutting of steel,
inspection of parts and checking dimensional tolerances, and many others. Other uses for
lasers have been found in the field of medicine as a precise surgical tool, in chemistry for

analytical techniques, and in electronics for communications and information processing

[33, 44].

3.1.1 Radiation of Light Quantum

Light is a form of electromagnetic radiation exhibiting both wave and particle-like
characters. Experiments originating in the later 19°‘ century led to the development of a
branch of physics called quantum mechanics which describes light energy in terms of

quantized units called photons. The energy of a photon is related to its frequency by the

relation

15

'J

E = hv = hc/l
Where v is the frequency, c is the velocity of light, it is the wavelength, and h is Planck’s

constant and has the value: h = 6.627x10'34 Joule-sec = 6.626x10'27 erg-sec.

3.1.2 Stimulated Emission
Emission of light from an atom occurs when an electron makes a transition from one
energy state to another lower energy state. For one electron shifting from one orbit at

energy level E2 to E1, the energy of the emitted photon would be: hv = E2 —E1.

 

Ez

hv

 

 

‘ Ej hV=E2_E1

Figure 4. Spontaneous photon emission due to electron shift from a higher to lower
energy level [41] .

In a material at a finite temperature, there will be some electrons that are thermally
excited to a higher energy levels through a random process. These electrons will fall
back to lower energy levels, emitting photons. This process is classified as spontaneous
emission because of its random nature. The rate at which electrons fall from some upper
energy level E2 is proportional to the population of electrons at this energy level. Given
an initial number of electrons No at an upper energy level, and with no replenishing of the

supply, the population of electrons n2 would exponentially decay. That is,

16

mm = No eXp(-0rt) ( 1)

Where No is an initial populatio of electrons at E2, and a is some decay constant.

Electrons can also be stimulated to fall from a higher to lower energy level. A photon

field of the proper frequency provides this stimulus such that the energy of the photons

corresponds to the difference in energy levels in which the electron is transitioning

 

 

 

between.
0 o o o o 0 E2
W
thZ W hV|2
W ,
‘ Photon field
0 o o 0 ll. 0 o o E:

Figure 5. Photon stimulated transition of an electron and photon emission [41 ].

The rate of stimulated emission will be proportional to the population of electrons at
energy level E2 and the energy density of the incoming photon field, which we will

denote as p12.

Along with stimulated emission, absorption can also occur in which electrons are

promoted from energy level E. to a higher energy level E2. This process would also be

17

proportional to the energy density of the photon field p12 and the population of electrons

In at energy level E1.

Now consider a steady state condition in which absorption is balanced by emission of
electrons. Let the rate of absorption be denoted as B12n1p12, the rate of stimulated
emission as Bzin2 p12, and the rate of spontaneous emission as A21n2. The coefficients

B12, B21 and A21 are called Einstein coefficients. Einstein described the steady state

 

relation:

Aznz 4' 132102912 (2)

1312111912

Absorption spontaneous + stimulated

emission emission

The corresponding diagram illustrate as Figure 6:

 

 

 

 

 

"2
l O O O O :52
"M
321 "2 P(V12) BIZ "1P(V12) A21 "2
M
P012)
0 O l: O O O O O O O 0 ll 0
ME!
(8) (b) (C)
"1

Figure 6. Balance of absorption and emission in steady state:

(a) stimulated emission; (b) absorption; (c) spontaneous emission. [41]

18

Because the stimulated emission corresponds to a definite energy transition, the emitted
light is of uniform frequency, i.e., monochromatic. And because the stimulated emission

would all be in phase, the emitted light is coherent.

From equation (2) we can deduce the condition necessary for increased stimulated
emission. Obviously increasing the photon energy density p12 will increase stimulated
emission, although absorption will likewise increase. Photon density increases can be
achieved by the use of optical resonators. The other choice is to increase the electron
population n2. However, we recall from statistical and thermal physics that the Bolltzman

factor gives the ratio of electron populations at two different energy levels as

.1

n =exp[ Ez-E, =ex -hV12
n| kT kT

Assuming the density of available states is the same at both levels. k is Bolltzman’s
constant and it is equal to 1.38 x 10'23 Joules/K. Thus It; would normally be larger than
n2 as expected. Therefore, to increase stimulated emission significantly over absorption
we would want n2 to exceed 111. This condition is known as population inversion, or as a

negative temperature condition.

19

3.2 CO2 Laser

The laser, by definition, is a device that amplifies light by means of stimulated emission
of radiation. In practice, a laser is generally used as a source or generator of radiation
and the generator is constructed by adding a feedback mechanism in the form of mirrors

to the light amplifier.

Amongst many types of lasers, the most important molecular laser is carbon dioxide laser
[35]. Which means the CO2 gas is employed as its active medium. The principle of
lasing phenomenon of C02 laser can be considered as vibration energy, which provides
various energy levels for laser transitions. It is known that the carbon dioxide molecule is
a linear molecule and three atoms lie in a straight line with the carbon atom locates in the
middle; Therefore, there are three different types of vibrations, which can occur in the
carbon dioxide molecules. It is called as symmetric vibration, bending vibration and

asymmetric vibration, which are illustrated schematically in Figure 7.

For the different vibration modes shown above, the energy associated with them is
quantized. The molecule can vibrate in more than one mode at the same time and it can
have more than one quantum of vibration energy in each mode. This feature leads to a
simplified energy level schematic diagram shown in Figure 8, which is the most
important mechanism for CO2 laser operation. The population inversion in the CO2 laser
is produced through collisions with nitrogen molecules and high power output of CO2
laser is obtained by adding nitrogen to the mixture. The nitrogen molecule is excited by

collisions with electrons, and then transfers their energy from the collisions to CO2

20

ENERGY ev

OXYGEN CARBON OXYGEN

Figure 7. A vibration mode of C02 atoms [33]

  

 

 

0 3 _ M _ MANY ROTATIONAL
_ } SUBLEVELS
(00-1) VIBRATIONAL LEVEL

g LASER TRANSITIONS
o ,_ IO.6 I‘m

0.2 - '2‘ >
j; 9£um
8 l
2 g “0‘01
0 a: .

0 | __ g 5 ‘02 °’ COLLISIONAL

- a 3 RELAXATION 8

d t! (Ol'Ol RADIATIVE DECAY

 

 

N2 LEVELS (:02 LEVELS

Figure 8. Energy levels of the CO2 laser [33]

21

molecules. This produces a population inversion between the (001) state and the (100)
state. Although there is another state (020), which shares a common upper state with the
(001) state, the probability of transition to the (020) state is only about one-twentieth of
that to the (100) state. The most of laser operation in C02 laser proceeds at the
wavelength of 10.6 pm. The addition of helium to the gas mixture can also increase the
output power. Because helium is a kind of gas, which tends to deplete the population on
the lower laser level by collisions and tends to keep the gas mixture cool due to the high
mobility of helium atoms. The cooling function of helium plays an important role in
keeping the C02 lasers maintain a high power output situation. Therefore, in general, a
C02 laser actually operates by utilizing a mixture of C02 , N2, and He together to obtain
a high power output. This is the reason why C02 lasers are widely used in industries, and
the CO2 laser has been developed to produce a high power output up to 25kW recently.

[79]

3.3 The Characteristic of a Laser Beam

3.3.1 The Property of Continuous Beam

A laser with a continuous beam means that the output beam is constant, or the power
output is measured as a constant wattage, so it is often called as "CW" laser. Many kind
of gas laser can provide "CW" outputs. Since the gas can be operated to flow through the
lasing chamber not only to replenish the gas but also to provide cooling function since the
increase of the temperature of the gas mixture will decrease the power output. For

example, in CO2 laser, adding helium to the gas mixture in order to keep the gas mixture

22

as cool as possible, CW laser needs a relatively long period to establish beam intensity at
constant output. Usually, the intensity is described as "power density", which has been
defined in ANSI standard (the American National Standards Institute) Z 136.1 as below:
" Quotient of the radiant flux incident on an element of the surface containing the point at
which irradiant is measured, divided by the area of that element. Unit: watts per square

centimeter, W/cm-2" [32].

3.3.2 The "Mode" of Laser Beam

The mode of laser beam is another important characteristic, which reflects the beam
cross-sections, or spatial patterns. The distinctive spatial patterns are termed as
“transverse modes”. It describes the functional dependence of the electromagnetic field
on X and Y coordinates. They are knotted as TEan modes, where the subscripts m and
n represent the numbers and times the electric (or magnetic) field crosses the X and Y
axes. In fact, the TEM mode indicates the amplitude distribution of the electric field in a

plane perpendicular to the axis of the laser resonator. Some transverse mode patterns are

presented in Figure 9.

From the Figure 9, it is obvious that the lowest order mode TEMoo, which is called as
Ganssion distribution, has maximum power density no matter in rectangular or
cylindrical symmetry patterns because of its small beam divergence angle. Therefore, it is
the desirable mode for laser operation since the power density distribution is more

uniform than higher order modes. Actually, the TEMop mode, which is often called the

23

 

00 IO 20 30

on ‘0’ 3'1: 2
0 40‘ :17: 5.3.
II 2| 33 04

00 01* 10 11* 20

- Q! Q'O :%
- ' ‘ '.§ ‘3’
01 02 03

Figure 9. Transverse-mode patterns in rectangular symmetry ( upper) and in
cylindrical symmetry ( lower). [34]

24

"donut mode" and is made up of a combination of TEM” and TEM“), and it is commonly

used in gas laser such as CO2 laser operating at or above lKW range.

3.4 Interaction of Laser Beam with Materials

In laser materials processing applications, the principle of interaction of the laser with
material is to utilize lasers emitting photons with relatively low energy. For example, the
energy of C02 laser photons is only 0.12 eV and the photons come from YGA laser have
about 1.2 eV energy [35]. Thus, the electrons excited by C02 laser or YGA laser
radiation does not have enough energy to be removed from the metal surface. They return
to an equilibrium state when excited electrons are scattered by lattice defects, such as
dislocation, grain boundaries, and other discontinues. The excess energy will be released
to convert electronic energy obtained from the beam of incident photons to heat energy,
which is used for laser materials processing. In fact, when the laser radiation shoot on the
surface of a workpiece, a part of radiation is absorbed, and most parts of that are
reflected. Just the absorbed part is capable of being effective in raising the energy state of
the electrons within the metal so that the excited electrons collide with lattice atoms to

create heat energy.

The result of heat transfer by laser radiation on the surface of metal lead to the surface
temperature quickly rising to its melting temperature. This phenomenon is of interest for
Welding, cutting and other such applications. The temperature distribution of the metal
Surface interacted with laser radiation depends on the heat flow in the material. The heat

flow depends on the thermal conductivity K and the specific heat of the material. The

25

heating rate is an important factor, and it can be expressed as the term of K/cp, where p
is material density. This term has the characteristic of a diffusion coefficient since its
dimension is cmzlsec. Hence the heat flow can be calculated by using the usual heat

diffusion equation.

The Figure 10 illustrates a surface temperature of Aluminum as a function of time for
various absorbed energy densities [39]. The reflectivity of a metal surface is another
important parameter in laser materials processing. It is very sensitive to the status of the
surface such as impurities, oxide layers and the nature of the metals as well as the
wavelength of laser beam. The reflectivity of several metals as a function of wavelength

is shown in Figure 11.

From Fig. 11, it is obvious that aluminum has a very high reflectivity through the visible
spectrum, but ferrous metal such as steel and Nickel alloys have relatively lower
reflectivity throughout the entire spectrum. Another phenomenon shown in Fig.1] is that
the reflectivity of the surface is especially important for CO2 laser due to its wavelength
at 10.6 pm. It is difficult to couple the energy from the optical beam into the workpiece.
Therefore, how to improve the absorption of infrared CO2 laser energy is the first
problem need to be solved. Up to now, there are couple of treatments used for enhancing
the absorptivity of metals at 10.6 um wavelength of CO2 laser. In industrial application,

surface coating, and surface oxidation have been used to increase absorption [36, 37]. It

26

 

—- 42100

1
l

1 800

.. - 1500

1
l

1200

    

0.8 New2

\:
0.6 J/cm2

.Jl'm
000

 

SURFACE TEMPERATURE (K')

800

 

 

 

 

# i 300
.L
2 r
0.8J/cm b l
I .— \
I \
I \
I \
A I O ’ -t
g I \
E I 0.6J/cm2 \
t ’ I ‘\ \
3 l ,’ \ \
5 I I \ \
3 ’ I \ \
0.5 '- ’ I \ \ —
l, \ \
I , 0.4mm2 \ \
'I I\ \ \ 15m/s
I] ’ \ \
I, ,’ \ 24 m/s \ 23_m/s \
’ 0 so 100 150
TIIE (one)

Figure 10. Surface temperature of Al as a function of time for various absorbed energy.
The dotted line shows the shape of the Gaussian laser beam mode. (upper)
Melt depth as a function of time with velocity of the liquid-solid interface.
(lower) [ 39]

27

 

 

    

 

 

 

1.0
0.8-
E 0.6-
2
.u-
3
‘11 o_4— A - POLISHED SILVER
‘5; B - COPPER
C - ALUMINUM
0.2- O- NICKEL
E- CARBON STEEL
0.0 1 1 1 l 1 1 1 1 l
0.2 0.4 0.6 1.0 2 4 6 8 IO 20

WAVELENGTH (MICRONSI

Figure 11. Reflectivity as a function of wavelength for several metals. [35]

28

has also been partially improved by roughing, etching, and baking at certain temperatures

through our experiments [38].

3.5 Laser Applications in Materials Processing

Comparing to a conventional thermal source, a laser can deliver very high power density
to a localized region on a workpiece. For example, a metal can not be melted by a 200 w
light bulb but a 200 w continuos laser can do that. Therefore the laser has become a
source of energy that can be concentrated by a series of lens (or mirrors), achieve very

high power density at the focal spot. This ability provides many potential applications in

materials processing technology.

3.5.1 Laser Cutting

Laser can be used to cut a wide variety materials. Laser cutting will cause a vaporization
of materials along a path, which is created by a moving laser beam, and it leads to
separation of the pieces. Because C02 laser with 10.6 1.1m wavelength beam can be
manufactured with extremely high continuos wave power densities. It is most commonly
used in industrial cutting. Not only in cutting of metals, but also in nonmetals materials
such as ceramics, plastics, cloth, wood and paper, and glass [40,43,47]. Gas jetting is
usually associated with metal cutting either to enhance the removal with a reactive gas,
such as oxygen, or with an inert gas to protect flammables. Because of high reflectivity,

some metals that are efficiently cut with the CO2 lasers, such as aluminum and

titanium[46, 47,52, 54 ].

29

3.5.2 Laser Welding

Laser welding technology has been considered ideally suitable for welding plastic
materials, ceramics, glass and metallic materials. Because the localized nature of the
laser beam, the heat affected region can be kept to a minimum size so that the welding
zone is able to keep the surface clean and unaffected. The technique of laser welding is to
utilize the rapid interaction of material with a focussed laser beam. A big difference
between the melting temperature on the surface and an ambient temperature inside the
substrate forms a steep temperature gradient, which is an important principle of creating
rapid solidification. The power density, traverse speed, interaction time and pressure of
cover gas are main parameters influencing the solidification and quench rates that
influence the quality of welding. Compared with some established techniques, there is
some advantage in laser welding. For example, no material contacts the workpiece, so
that there is no contamination. The heat-affected zone is very small. It is especially
important for the cases where a weld must be made near a heat sensitive element.
Another example is that laser welding can be performed under the ordinary atmosphere,
which is big cost saving. Because of these advantages, laser welding is gaining more

and more acceptances for performing welding tasks [48-51, 58, 59].

3.5.3 Laser Surface Treatment

Laser surface treatment includes laser hardening, remelting, alloying and cladding
processes. Recently, these techniques are widely applied in industry due to their interest
features. A high power density is created on a small area on the top of the alloy so that a

very rapid heating and cooling occurs near the surface of materials. For the laser

30

hardening ferrous alloys, a phase transformation can occur from austenite to martensite
due to vary rapid cooling. After transformation, the hardness on the surface layer
increases because a certain volume fraction undergoes the phase transformation, which

lead to a surface hardening [39, 41, 47, 55, 57].

For laser remelting, its purpose is to improve the wear resistance and corrosion resistance
of the surface of materials. This is because that the microstructure of the surface can be
changed due to the rapid self-quenching process. Laser alloying is another way of
changing the chemical composition and microstructure of the surface. Simultaneously,
with surface melting, an alloying element can be injected to obtain a new alloying layer.
Majority of alloying elements like C, Ni, Cr, Mo, W, Co, Mn, and Ti are usually used for

improving properties of the surface in steels [60, 61, 62], aluminum alloys [63, 64], and

titanium alloys [65, 66].

Laser cladding is also widely used in industry for obtaining a high hardness, high wear
resistance, high corrosion resistance and high-temperature resistance without affecting
the substrate material. The mechanism of laser cladding is that an alloy powder or
metal/carbide mixtures, such as SiC, TiC, and VC [67], are injected along with a laser
beam, resulting in a rapidly solidified cladding layer which coats the surface of the
substrate. Generally, a perfect clad has characteristics of uniform coating thickness and

smooth surface, minimum dilution and distortion, as well as good metallurgical bonding

with the substrate.

31

3.5.4 Laser Forming (Bending)

Although laser forming is a relatively new technique comparing with other laser materials
processing, but it is also a big development in this field of study. It could potentially be
used for tool-less manufacturing parts necessaries for automobile and aerospace industry
[68, 70, 71, 73]. The microscopic scale applications of laser forming are also involved
in stereolithography [72], and micro-optical components field [74]. However, the
principle mechanism of laser forming, in the microscopic scale, is that the sheet metal is
heated by the laser beam, and the forming temperature is controlled by adjusting the
power output parameters and path feed rate. The gravitational force, a positive or a

negative pressure work as forming stresses, which influences the shape change operation.

In this research, we have demonstrated, for the very first time, that a laser-aided 3-D

forming of a superplastic material is possible. The review part of this thesis describe this

development.

32

Chapter 4

LASER DIELESS SUPERPLASTIC METAL FORMING

Laser Die-less Superplastic Forming (LSF) is a novel laser processing technique. To our
best knowledge, such a forming process has not been reported to date in any of the recent
reviews [ 75-77 ]. The new possibility of this technology can be realized by combining
the innovative "tool" - laser beam and metal forming without any mechanical contact.
Comparing with conventional superplastic forming, which is accomplished by using die
pressing or extruding to force the metal sheet into a die cavity when the temperature of
the sheet reaches deformation temperature, the hard tooling is not required. Therefore, it
is high technology process and an economical processing technique especially for rapid

prototyping and small batch productions.

4.1 The Function of Laser Beam

The utilization of high-energy laser beam for plastic and superplastic forming opens
another application in laser materials processing field. The physical fundamentals of laser
action, during forming process, is based on the localized heating of the surface of a metal
sheet. A controlled laser beam is applied at selected, and preprogrammed locations on a
superplastic sheet. The sheet metal is heated by the laser beam, and the deformation
shape is obtained by a slow scanning laser beam along a programmed pattern. It is

possible to achieve an ideal forming component by adjusting the parameters such as laser

33

power, laser beam size and transverse speed of the beam. Figure 12 is a schematic

diagram of CNC laser assisted superplastic forming.

4.1.1 Laser Beam Power
Laser beam power is one of the primary factors, which influences the quality of laser
processing. The combination of incident laser power and relative beam size constructs the

power density within the radiation region.

In laser superplastic forming process, the interaction between laser beam and metal sheet
provides the heat to metal sample. Therefore, the laser energy works as a heating source
for reaching a deformation temperature. However, the amount of laser energy required to
achieve a suitable deformation temperature is the most critical issue. Due to a high
reflectivity of Aluminum alloy to the infrared CO2 laser radiation, the key point for
inducing laser energy to create a required temperature on the surface of workpiece is to
manage to improve the surface condition. There are some methods for improving the

absorption condition have been tried, such as roughing, etching, polishing, anodizing and

painting [77, 78].

4.1.2 Laser beam diameter
Laser beam diameter is another important variable, which influences the power output.
This means that the power density will vary with this variable directly. Theoretically, the

light emitted by laser is confined to a rather narrow cone, with a diameter "d", but as the

34

Computer

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

l:

 

    

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[:1 Laser
r—Lgr
‘0 E.
Q ./Optics
Pattern
Superplastic \ / //
Pressure
Box
1 Gas inlet
CNC Table

 

 

 

 

Figure 12. Schematic diagram of CNC laser assisted superplastic forming

35

beam moves through space, it slowly diverges to a bigger diameter “(1”. Simply, the
relation between them is expressed as
D' = l 0 + d

Where 1 is the traversing distance of laser beam, 0 is a small divergence angle. The CW
gas laser generally has the smallest values of beam divergence. So in fact, the laser
beam diameter is usually adjusted by changing the distance between the beam jet (beam
head) to the surface of the workpiece. In addition, since the characteristic of laser beam
"mode", it is also an obvious factor, which affect the laser energy distribution in the
interaction zone. As described as before, the TEMoo "mode" or Gassion Distribution is an
ideal mode because its uniform energy distribution in the spot area. For CO2 laser
generator, TEM 01- mode is commonly used for laser materials processing. For laser
forming application, TEM 0;- mode is also effective for creating a uniform energy

distribution on the surface of the metal sheet because of its "donut" pattern.

4.1.3 The Beam Traverse Speed
A successful laser-aided superplastic deformation just can occur at a certain condition

with a critical combination of optimum parameters including power output, beam

diameter and traverse speed.
Due to the high thermal conductivity of Aluminum alloy, it is difficult to establish a

relatively stable temperature distribution around forming zone. The heat is applied within

small area at a time, a proper sequence of heating zones could be mapped out individually

36

and the sum of these locally deformed areas will result in the final shape of the element.
For avoiding a non-uniform deformation, this is caused by combining much localized
deformation. An effective way to avoid that is to decrease the beam traverse speed, and
increase the beam spot size to provide sufficient energy, which can be transferred to
heat at a certain time for keeping the temperature distribution as uniform as possible

within the heating zones.

4.1.4 Beam Scanning Track

The feature of the laser dieless superplastic forming is that no mechanical contact exist.
The geometry of the heating path and the amount of the heat input have been precisely
implemented through designing a beam scanning track, which is programmed by using
CAM/CAD package to run the computer controlled table under the laser beam. It is
possible to realize a dieless superplastic forming by moving laser beam scanned on the
target of forming part. According to its mechanism, the beam scanning track akin to the

function of tooling die used in conventional superplastic forming technique.

4.2 Forming Force

The term of "die-less" has indicated that in laser superplastic forming technique, a
mechanical contact is not required. The stress needed to deform the sheet metal is applied
by a vacuum or an inner gas in a cavity container. Comparing with the conventional
superplastic forming technique, using gas as forming "die" instead of mechanical forming
die is an innovate idea, which provide a more uniform stress distributing throughout the

action region of the workpiece. It is a highly desirable feature, since the comers of the

37

forming parts usually have the thinnest section in conventional superplastic forming
especially for a complex part. There are some shortcomings of the gas forming. For
example, the applied vacuum is limited to the pressure of atmosphere, or the inner gas
added in the cavity chamber could not be put too much due to the resistance to pressure
of the chamber. It has also a strong potential to provide a sufficient forming force to
produce complex shapes component without the use of dies. Furthermore, this technique

will offer a prospect to control the thickness uniformly at any part of the final shape.

38

Chapter 5

EXPERIMENTAL PROCEDURE

In this research, a high energy laser was employed as a heating resource to aid in shaping
superplastic sheet metal. The heated superplastic sheet was deformed by either applying a
vacuum or by an argon gas pressure. The desirable deformation shapes could be obtained
by programming the computer numerically controlled (CNC) code, which drives the CNC
table to move the sample with respect to the laser beam. 80, the feature of the laser-
dieless forming processing, is that a deformation temperature, could be controlled by
adjusting the power of laser energy, and the geometry of the heating path could be

precisely controlled through computer assisted manufacturing (CAM) system.

5.1 Experimental Setup

The equipment used in the experiments, consist of a continuos wave CO2 laser generator,
a computer - assisted manufacturing ( CAM ) system, and a forming chamber. The

experimental setup is shown in Figure 13 and Figure 14.

5.1.1 C02 Laser
A 2500-L model Triumph ( Triumph Industrial Laser Inc., MA ) laser in the TEM 01* M
is being used in this study. The characteristics are given in Table 2. For superplastic

forming experiments, the amount of the heat transferred from laser energy to the work

39

—
'
— \ Laser Head

1:5:

   

Vacuum

Port
Vacuum

\\\s a L\\\\\\

/

7//////////////////// Ill/1

I CNC Table I

Figure 13. Schematic diagram showing the vacuum chamber used in conjunction
with laser processing

40

 

Figure 14. A photograph showing the laser nozzle, the chamber, and the
temperature measuring setup.

4]

piece is an important parameter. According to the temperature measurement, the output

power range of 200 to 500 watts was used during this processing.

Table 2. The Characteristics of the ( TRUMPF 2500 W) RF exited C02 laser

 

 

 

 

 

 

 

Parameter Value/'1‘ ype

Type of Laser Fast flow coaxial C02 laser

Mode of the beam TEM op

Output stability :1: 2 %

Beam diameter (raw) z 18 mm

Excitation RF exited

Temporal mode Continuous and pulsed ( l-lOkHz)

 

Maximum output power 2800 watts (continuous )

 

 

Wave length 10.6 1.1m

Gas consumption He: 64 Uhr
N2: 12 Uhr
CO2: 3l/hr

 

 

 

 

5.1.2 CAM System
A computer - assisted manufacturing ( CAM ) system was used for laser dieless
superplastic forming experiments. This system consists of a high precision X-Y table

controlled by a computer numerically controller that is called CNC table machine.

42

( General Numeric Inc.) Its specifications are given in Table 3. Generally, for laser
materials processing, the specimen is fixed on the X—Y table which can move under a
stationary laser beam. The designed pattern on the metal sheet is realized by moving X-Y

table. A CAD package is used to design the scanning path.

Table 3. Specifications of the CNC machine

 

 

 

 

 

 

 

 

Description Value

Range of motion 1.27 m x 1.27 m
Maximum contour speed 0.127 m/sec
Maximum traverse speed 0.254 m/sec
Accuracy 0.0166 cm/m
Resolution 0.0127 cm
Repeatability 0.00254 cm

 

 

 

5.1.3 Superplastic Forming Process

A novel superplastic forming system was designed by our lab. It consists of a vacuum
pump, a vacuum chamber ((11 4.0 inch x 5.0 inch ) with cooling water tubes, which are
wrapped around the wall of the chamber. A vacuum gage or a pressure gage could be
connected to the chamber. The chamber was made of steel, and the capability of loading
pressure is over 2.0 bar, which is the highest pressure that was used in the experiments.

The forming system setup is shown in Figure 15a, and 15b, where Figure 15a shows the

43

 

 

 

 

 

 

 

 

 

 

 

\
\W

/////////////

Figure 15a. A schematic diagram showing the condition of vacuum forming

 

 

 

aaaaaaaa m

 

 

 

 

\\

Argon %
Gas

%

////////

Figure 15b. A schematic diagram showing the condition of argon gas forming

§\\\\\\\\\j\S
\\

 

 

 

 

 

 

 

 

 

 

conditions that vacuum was applied, and the Figurele sketched the conditions for an

argon gas pressure induced superplastic forming.

5.1.4 Measurement System

For the temperature measurement, we used a computer data acquisition system, which is
connected with four pair of thermocouples. The thermocouples were connected to points
on different locations from the center of the metal sheet. This is to measure the
temperature distribution and to view the inference of thermal conductivity of the sample.

A sketch of the temperature measurement system is shown in Figure 16.

A profiler was used to trace the deformation progress of metal sheet during different

stages of the process.

An OLYMPUS PME-3 optical microscope was used to evaluate the microstructure and
grain size of the samples. For the measurement of the thickness variance, a precision

optical measuring tool was used for the cross-section analysis.

5.2 Preparation of the Testing Materials

We used different kinds of testing materials in the experiments. However, since the
requirement for superplastic deformation, relatively a few materials are able to qualify.
Thus, our range of choice is relatively narrow. In this research, the 7475 aluminum alloys,

5083 aluminum alloys, and GK 45 aluminum alloys were tested.

45

Stationary
laser beam

Thermocouples

 

 

 
    
 
 

  

 

 

 

 

Computer
data
acquisition
system
1 3mm
1
30mm
é a
50mm

 

 

Figure 16. A schematic diagram showing the setup for temperature measurement

46

In addition, the 718-Nickel based alloy, and a thermal-plastic polymeric sheets were also
tested. After preliminary tests of various materials, we focused on GK 45 aluminum

superplastic alloy, that has the right properties for this process.

It is well known that aluminum has a very low absorption of infrared C02 laser energy.
Since the aluminum sheets, received from Japan, were cool-rolled, they exhibited
shinning surfaces. At first, the absorption of infrared CO2 laser energy on their surfaces

was a problem.

In order to improve the surface condition of the aluminum sheet for higher absorption of

laser energy, different methods were used. First, a high temperature black paint was

applied on the surface. However, the paint easily peel off from the surface during the

laser heating and processing. This shows that the bonding between the black paint and the

surface was very poor. Second, before applying black paint, the surface of the sample

was roughened with a coarse emery paper, and then sprayed with the high temperature

black paint, This procedure was also not effective for getting stable condition. Finally, a

very strong and cohesive bonding was achieved by using the following steps:

1. Etching the surface of the sample prior to application of black paint where etching
reagent was Barker’s reagent.

2. Painting an uniform coat of high temperature black paint .

3. Curing the sample at 200°C, for 2 hours and cooling it in the air.

The chemical composition, and selected mechanical properties of GK 45 aluminum alloy

are listed in Table 4.

47

Table 4. Composition and selected mechanical properties of GK 45 alloy

 

Compositions Yield Tensile Strain Rate Deformation
‘ (wt %) Strength Strength Temperature
Al Mg Mn Cr Zr
Bal 4.5 0.6 0.2 * 125 MPa 285 MPa 10‘~10’3s" 500~550°c

 

 

 

 

 

 

 

 

 

* Not provided by the manufacturer

5. 3 General Description of Experiments

The first step of the experiment was to check the laser system, including processing in the
Helium, Carbon Dioxide and Nitrogen gases. Further, turning on the cooling machine,
and checking the optics. After checking, if this conditions were normal, then turning on
the laser beam and adjusting output power. Next step is to install the sample on the top
of vacuum chamber, and the chamber is then fixed onto the CNC table. The CNC table
could be moved by controlling the CNC code transferred from the program. These
different geometric shapes of deformation were obtained by moving the CNC table, with

respect to the stationary laser beam.

Designing a deformation trace is an important procedure in which a desirable geometric
pattern was generated by using a CAD ( computer assisted design ) package. A CAM

(computer assisted manufacturing ) program was then used to translate the CAD drawing
to G-code program used by the CNC controller. The CNC table was driven by the CNC

controller so that the sample on the top of the chamber which fixed on the CNC table

48

could be moved with respect to the stationery laser beam. The Figure 17 illustrates a
simple pattern to get a dome-shaped deformation. The heating trace, for such a case, a
circular pattern, and the *** of the dome is defined along the edge of the laser beam
trace. A steady-state superplastic forming temperature, for the alloy, can now be

established by adjusting the laser beam scanning speed, and laser power.

 

SpotDia.
=20mm

 

 

 

 

Figure 17. A schematic diagram showing the laser spot size, and the locus of the spot for

generating a dome.

At the end of every program execution, a surface profiler was used to measure the

progress of the defamation, and to see how much total deformation had been achieved.

49

Chapter 6

RESULTS AND DISCUSSION

6.1 Investigation of Laser Energy Effect

In any Laser-Aided materials processing technique, such as laser cutting, laser welding,
and laser cladding as well as laser treatment, the amount of the laser energy absorbed by
the material is a critical parameter. Especially for laser dieless superplastic forming, the
amount of energy absorption from laser plays an important role in heating and cooling of
the sheet metal. Thus, how much laser energy, which would be effective on the surface of
the aluminum sheet, was initial consideration in this case. The high thermal conductivity

of aluminum alloy is a critical component of this process.

6.1.1 The Influence of the Laser Parameters to Deformation

The power output, and the laser energy absorption are primary factors which influence
the deformation. Laser dieless forming means that the laser will be the only heat source,
which can raise the temperature to a proper region. In general, energy density is an
important parameter used to evaluate the effectiveness of energy parameters in laser
materials processing. Therefore, the concept of the “energy density” was introduced to
design our experiments.

From the definition of the energy density
Ed = P4 1', (6-1)
Where E4 . energy density( J/cmz), P4 is the power density, and 1' is the interaction time

(see), which is defined as the time required to move a distance of the laser beam diameter

along the transverse direction.

50

Likewise, power density is defined as
[’4 = P/A, (6‘2)

where the P is the laser power (watts) and A is the irradiation area (cmz). Combining
equation [6-1] and [62], energy density of circular density laser beam can also be
expressed as follow. for a circular beam:

Ed=4PI1tDV (6-3)
where D is laser spot diameter (cm), and v is beam traverse speed (cm/sec).
It is obvious that the energy density is related to the beam diameter D, beam traverse
speed v, and power output P. Finding a better combination of these parameters, for

creating a possible deformation, will be our next concern.

6.1.1.1 Small Beam Diameter at Different Traverse Speed.
Theoretically, a fast scanning speed can decrease energy density, and a small diameter

will increase the energy density according to the definition of the laser energy discussed

before.

To look at the influence of power output P and traverse speed v, a 7475 aluminum alloy
sample was first examined under a 5 mm diameter laser beam. The beam traversed speed

is listed in Table 5.

The effects of laser energy absorption on the surface of six samples, arranged from #1 to
#6, are shown in Figure 18. From this photograph, the first phenomenon is that the

samples from #1 to #5 were partially melted in varying degrees, and the #5 sample is

51

most deeply melted. The second thing is that the #6 sample looks like very little or no
melting at all. Since the energy density is related to P, the higher power is associated
with the higher energy density. So for the highest power output P, which is 430 W,
produces melting such as the appearance of the #5 sample . However, the #6 sample was
treated at the same power output as the sample #2, at 320 W, but the traverse

speed was almost four times slower than that in the #2 sample.

Table 5. The parameters. used in the experiment for testing energy absorption

 

 

 

 

 

 

 

Sample number Power output Traverse speed

# . ~ (watt) ( mm/s)

1 270 85

2 320 85

3 370 85

4 420 85

5 430 85

6 320 12

 

 

 

 

 

52

Figure 18.

 

A Photograph of six patterns showing the surface condition associated
with laser energy absorption. A small laser beam diameter with
different traverse speeds used. Testing material was 7475 aluminum
alloy.

53

Figure 19 is another photograph, showing a sample of 7475 aluminum alloy. The
scanning pattern is a circle of diameter 38.1 mm (1.5 inch). The 405 W laser output, 5
mm spot diameter, and 0.423mm/s scanning speed were used in this sample. The running
time is about 18 minuets. From the appearance of the heated surface (Figure 19), there
was no melting, and a crack formed. This observation indicates that although the
temperature was not high enough to melt the sample surface, a complicated internal stress

field developed.

6.1.1.2 Large Beam Diameter

Based on the fact that the small laser beam size could not work properly, a larger beam
spot diameter of 20 mm was tried. One pair of thermocouple was connected to the bottom
of testing sheet, which was located at the point off the sheet center about 3 mm. The

temperature variation with time are shown in Figures 20, 21 and 22.

From the curves shown in the Figures 20, 21 and 22, it is clear that the period of two
peaks (shown in Figure 20) is about 120 seconds, and in Figure 21 this is approximately
140 seconds. For a power output of about 480 W and beam traverse speed of 0.105
mm/s, the period of the peak is over 600 seconds. In addition, the beam traverse speed
alone could not achieve the highest temperature at certain power outputs. Therefore, this
is another evidence that the laser energy coupling with aluminum is a complicated task.

If the traverse speed remains at the same value, like the conditions shown in Figures 20

54

 

Figure 19. A photograph of a sample showing the surface condition associated with laser
energy absorption on a circular pattern in a 7475 aluminum alloy.
Improper thermal conditions produce a circular crack.

55

and 21, the power output would be a primary factor to influence the steady state

temperature distribution.

From the results of the two situations (small spot size and large spot size), it is seen that
for a larger spot diameter is effective as a heating source to provide an uniform

temperature distribution.

6.2 Measurement of Temperature Distribution

From the discussions in the previous section, it is clear that the temperature distribution
in the area of the laser interaction is the most important condition in the dieless
superplastic forming. In a conventional process, the heat provided by tooling die can
create a uniform temperature distribution on the workpiece. However, there are two

reasons why it is a somewhat difficult and a complex processing technique for laser
dieless forming. One is that the heat resource comes from infrared C02 laser energy,
which is highly reflected by aluminum alloy. Another reason is the high thermal
conductivity of aluminum alloy , which makes it very difficult to establish a steady state

temperature distribution.

6.2.1 Two Points Measurement (dynamic state)
Two points temperature distribution measurement is a basic idea to look at the heating

conditions. Laser energy output is set to 230 W with a 5 mm diameter spot size.

56

a:
8

 

§$§

L
I

Temperature( °C )
N
8

8

 

C

 

 

0 100 200 300 400
Time ( s)

Figure 20. The temperature variation with 20 mm laser spot diameter,
at 350 W power output, and a traverse speed at 0.42 mm/s.

57

 

Temperature(°C)
8 8 8 8 8

'8‘

 

O

 

 

0 100 200 300 400
11me(s)

Figure 21. The temperature variation with 20 mm beam spot diameter,
at 470 W power output, and a traverse speed at 0.42 mm/s.

58

 

 

 

 

a!)

500‘-
55
-400"
g
o- mo~
i
E ZD-l
.2

100»

0 1 a. . T
O 200 400 600 800 1G!)
111mm

Figure 22. The temperature variation with 20 mm beam spot diameter,
at 480 W power output, and a traverse speed of 0.105mm/s.

59

Although similar parameters have been tested in the previous section, such as small beam

and fast traverse speed.

To determine what critical parameters for power output effectiveness are, the
measurements were conducted by using two pairs of'thermocouple connected to the two
points, which are on a straight line. The thermocouples were separated from each other
by 15 mm distance. Different beam transverse speed at 230 W power output were used,

as shown in Figures 23, 24, and 25.

These graphs show that at certain power levels, the higher the scanning speed, the lower
the peak temperature. The sharp peaks of the curves are clearly due to the high thermal
conductivity of the aluminum alloy. The temperature difference between the two points

can not be eliminated no matter how fast or how slow the beam traversed.

60

 

B
A
V

8
1
V

Tomporaturo( °C)

 

 

 

«:9999

 

Oimananmananmam
nit-(s)

Figure 23. Two-point measurement. 230 W power output and
21.16 mm/s traverse speed at 5mm spot diameter.

61

 

Temperature (06)

 

 

 

 

 

Tine (e)

Figure 24. Two-point measurement. 230 W power output and
8.47 mm/s traverse speed at 5 mm spot diameter.

62

 

 

 

 

9 _

-31)..

.5.

1”" l l
5.... ll

.— .’

O 1000 am am 4111) am
We)

 

Figure 25. Two-point measurement. 230 W power output and
4.23 mm/s traverse speed at 5 mm spot diameter.

63

 

6.2.2 Center Point Measurement (static state)

Two-point temperature distribution curves have shown the influence of laser beam
scanning speed, at a fixed beam size and power output. It could not be a good heating
source for the conditions, taken such parameters as above, due to the unsteady
temperature distribution. It has also been demonstrated in Figure 18, which shows the
surface condition were treated improperly. However, what is the temperatures in the
center of the workpiece, which can more truly reflect the heating conditions. For the
center point measurement, a 20 mm diameter beam spot size was used, because in two-
point measurements the small beam spot size could not work well.

By using larger beam spot size such as 20 mm, corresponding power output parameters
were tested in the range of 345 to 370 watts, 470 to 490 watts, and at 570 watts. The

corresponding temperature curves are shown in Figures 26, 27, and 28 respectively.

 

 

 

 

 

O 100 200 300 400
Tin. (8)

Figure 26. One point measurement. Power output 345 to 370 W
spot diameter 20 mm at static state.

 

 

 

 

 

 

O 50 100 150 200 250
Time (e )

Figure 27. One point measurement. Power output 480 W,
Spot diameter 20 mm at static state.

 

 

 

 

0 50 100 150 200 250
Time ( e)

Figure 28. One point measurement. Power output 570 W,
Spot diameter 20 mm at static state.

65

From the melting point of aluminum, the deformation temperature should be above 500
°C, since the superplastic deformed temperature is higher than about one half of the
melting temperature in °K. Therefore, it is clear that when the power output was around
470 to 490 watts, the temperature of metal surface can reach up to 500 °C to 520 °C .
Based on this distribution, an initial power output parameter was determined at around
470 to 500 watts, depending on the laser generator condition and ambient temperature

situation.

6.2.3 Four Points Measurement (static state)

To get a reasonable temperature distribution at steady state, four thermocouples were
connected on the bottom of the GK 45 aluminum alloy sheet at different points. They
were located at ( l ) the center of the sample plate, ( 2 ) 13 mm off the center, it is a
critical point since the largest beam spot diameter is 20 mm. This point was located
approximately at the edge of the beam radiation zone, (3) 30 mm from the center, and
(4) 50 mm from the center. The sketch of the temperature measurement has been shown
in Figure 3, in Chapter 5. Four points were designed to be on a straight line. Figure 29
and Figure 30 show the curves of temperature vs time, tested at a laser power output of
490 W and 550 W, and the cover gas was 0.5 scfh ( standard cubic feet per hour). The
temperatures at different power setting are marked on the corresponding curves. From
the temperatures shown in the Figure 24 and 25, it is seen that when laser power output
was 490 W, the temperature at center point was about 528 °C. The temperature of the
point that was 13mm off the center point was 405 °C. When laser power output was 550

W, the temperature of the center and 13mm off point were 590 °C and 445 °C

66

respectively. It is clear that within the region of laser beam diameter of 20 mm, the
temperature zone can reach to 400 °C to 530 °C with 490 W power, and a temperature of
445 °C to 590 °C could be reached at 550 W. These figures also confirm that the thermal
conductivity is very high, because the temperature difference between the center point
and the point off 13 mm was about 125°C at 490 W. When the power was 550 W, the
temperature difference between these two points was 145 °C. These results indicate that

the temperature of the point 13mm off the center can reach to the range of 450°C to

500 °C.

Comparing the temperature measurement at the center point ( from which an initial
referenced parameters of laser power was considered as 470 W to 500 W), the results
obtained from a four-point measurement indicated that the power output can be adjusted
to around 500 W to 530 W, since the superplastic forming temperature for GK 45

aluminum alloy (AA 50832r) is about 500 °C to 550 °C.

The four-point temperature distributions ( the time was chosen at 70 seconds) are shown

in Figure 31 and 32.

Figures 31 and 32 indicate that the temperature at the center is the highest, despite the
high thermal conductivity of aluminum. At the edge of the spot size, the temperature is
within the superplastic deformation temperature, which has been discussed before, and it
is a desired criterion. Another condition is that the amount of the cover gas flow also

influences the temperature distribution. The degree of influence could be seen from the

67

600

 

   

 

 

 

528°C (1)
A 500 -
O
a.
e 400 4
3
’6
i 300 ~
E
O
I- 200 _
0
mm mom
0 - -
0 25 50 75 100 125 150

Time (e)

Figure 29. Four-point measurement. (1) center point of the plate, (2 ) 13 mm

Figure30.

off the center, ( 3 ) 30 mm off the center, and (4 ) 50 mm off the
center. Power output was 490 W.

 

590°C (1)

 
   

 

 

 

I? 5°° " 444°C (2)
T 4001
E 300
E 200 «-
1m 4..
0 e
o 50 100 150 200

Four-point measurement. (1) center point of the plate, ( 2 ):13 mm
off the center, ( 3 ) 30 mm off the center, and (4 ) 50 mm off the
center. Power output was 550 W.

68

 

 
 
  

—-e—power350w

600‘ ---l---power490w

G 500* —e—power505w

E 4004 —+—power530w
g...
52°04
100~

0 -

 

 

 

0 10 20 30 40 50 60
Iletenee(mm)

Figure 31. The temperature distribution at 70 seconds, 0.5 scflr cover gas

 

 

 

 

 

O 10 20 30 40 50 60

Figure 32. The temperature distribution at 70 seconds, 3.0 scfh cover gas

69

differences of the temperature profiles in Figure 31 and Figure 32. It is shown that the

higher the cover gas flow, the more uniform the temperature distribution.

The time independent graphs are shown in Figures 33, 34 and 35. Every graph is plotted
at certain power output conditions, and these graphs emphasize the temperature

distribution at a steady state condition.

There are some common phenomenon in Figures 33, 34 and 35. First, the temperature
distributions reach a steady state gradually after about 30 to 50 seconds. The highest
temperature can be reached at 70 seconds, and the steady state is reached within a laser
power of 350 W to 530 W. Secondly, the temperature gradient between the center point
and fourth point, which is located at about 50 m away from the center, is high when the

beam power was on.

These graphs indicate that the thermal conductivity of the AA 5083Zr aluminum alloy is

very high, which makes it difficult to establish a deforming temperature for localized

superplastic forming. However, our results show that such a process is attainable.

70

 

 

 

 

400‘ +30s

3502 +503
8.3004 +708
5250
g...
£1504
.-

100‘

50-1

0 T 1 fi' T T

0 10 20 30 40 50 60

Distance (mm)

Figure 33. The temperature distribution at 350 W power output.

71

 

Temperature (°C)

 

 

 

 

Figure 34. The temperature distribution at 490 W power output.

72

 

—e—30s
-lF-50£3
”en-70s

  
  

Temperature(°C)
. 8 '8’ ‘8’ 8 8 8 §

 

 

 

10 20 30 4O 50 60

0

Figure 35. The temperature distribution at 530 W power output

73

6.3 Evaluation of Microstructure

It is well known that a fine and stable grain size can be maintained at high temperature is
an essential requirement for superplastic deformation described as in Chapter 2. The

superplastic potential of Aluminum alloys is critically dependent upon grain size stability,

and cavitation.

6.3.1 Grain Growth at Furnace Heat Treatment

After getting a superplastically deformed sample, it is necessary to look at the grain
growth tendency to see where or not the grain size remains in a condition for further
superplastic deformation. Comparing the grain growth that occurred at the furnace heat
treatment, and those occurred at laser aided deformation, it is concluded that the

superplastic deformation is possible to produce actual parts.

First, some samples were treated in a furnace set at 450 °C, 500 °C, and 550 °C for 40
minutes. The laser heating time was usually longer because a loss of laser energy.
Figures 36, 37 and 38 show the appearance of the microstructure for samples heated at
450 °C, 500 °C, and 550 °C for 40 minutes. The grain size was averaged from
measurements taken in longitudinal and transverse directions. The corresponding grain
sizes are 4.68 urn, 6.03 um, and 8.08 um respectively. The original grain size is about
5.48 pm, which is shown in Figure 39. From these results, it is seen that the temperature
range, 450 °C to 550 °C, is proper for retaining the superplastic grain size. Many former
researches have indicated that a grain size of approximately 5 to 10 um is within the

superplastic forming range. The grain growth tendency is plotted in Figure 40.

74

 

..I...J.LV.U ,hwwhlufim
4mm funk. .MAW%
efldl It
LLT.5&$II0¢.W1\

Await”.

r. $03..

Luann...

inflow:

.1.. 1.49am. .

.48

~
.1

u-
a
a“.

 

M. .

a 40 minute furnace annealing at 450°C
75

Figure 36. A photomicrograph showing the grain size after

«Man .1 WWW...” «www.mvlwfltr‘bw1MWwywrmw‘N .M m
. b51w;thtmwskfifizmvuefi u
.1 a... 1.......5.”.mumwifimwfi..fi%m, m
. ”fin. .. 2. .1... .2... 5...:

a...
z . .2; ”aw—Lunar,

   

..

 

76

Figure 37. A photomicrograph showing the grain size after
a 40 minute furnace annealing at 500°C

‘7‘
.’ "-

at ‘1
H .5

 

Figure 38. A photomicrograph showing the grain size after a 40 minute
furnace annealing at 550°C.

77

  
  
  

  
    
    
  

( 1: _,,.,- .1
' ‘1” ‘3' “v; -..I “1:07;:
.- .131; {ago £91 1‘"

r ”290'? $711154

_ m5”; g‘ufl 53$"; (2;.
“his. WP {1%ng W‘Qfllkfi
80%% «$50. 151/, If

   

   

UH ‘0 AG}; . vi; 340“":

Figure 39. A photomicrograph showing the original grain size
of GK 45 alloy

78

 

4

Grain elze (mm)
'0 (a) h 0| 0: \l on CD

 

 

 

0 200 400 600

Temperature ( °C)

Figure 40. The curve showing the isothermal grain growth tendency at 40

minutes of furnace heating at temperature up to about 550 °C.

In above curve, the original grain size appears to be a little larger than the grain size after
furnace heating up to about 500°C. The original sheet was in a cold rolled condition, and

the grain shapes appear as elongated. After the temperature is raised, the microstructure

apparently experience a recovery.

6.3.2 Grain Growth during Laser Process Heating
Figure 41 is a photograph showing the grain size at the bottom of the dome from a laser
forming, which was processed at about 500°C, for a period of about 9.3 hours. The

applied pressure difference was about 91.44 kPa (685.8 torr). The grain size is found to

79

 

be 8.81 um. Comparing the grain growth data during furnace annealing, and during laser
forming, it is found that the grain growth of GK 45 aluminum alloy during laser

processing was a little faster than the grain growth at furnace anneal below 550°C.

6.4 Measurement of Strain and Strain Rate

The strain rate is an important indicator of the superplastic materials, and can be
controlled by processing procedures. The profiles in Figure 42 were monitored by a
surface profiler at every 20 minutes intervals during the processing. This set of
deformation profile is shown at the progress of every 20 minutes, with three different

power levels used. The length along the cross section of a sample versus time is plotted

in Figure 43.

80

 

Figure 41. A photomicrograph showing the grain size at the bottom of
a deformed dome in GK 45 sample

81

Deformation profiles at various stages

Figure 42.

 

/
Mmm

 

 

30m

 

J.

Time Power
(min) (W)
40

60
80

100
120

140 __
160

460
180
200. T...

50

220

A set of deformation profiles, taken at 20 minutes interval

during the processing

Length(mm)

51

 

 

 

T 7

0 20 40 60 80100120140160180200220240

 

T

'I'lme ( mln)

Figure 43. The length elongation of the cross section with process time.

82

The curve in Figure 43 demonstrates that the deforming progress was regularly 1 mm at
the beginning, and gradually decreased with the increasing time. Beyond the running time
of 120 minutes, there was no length difference for another 20 minuets of processing.

After increasing the power from 450 W to 460 W, the increment was made noticeable
again. At 480 W, the slope of the curve becomes gentle. For example, the variation of
the sample length from 0.5 mm to 1.0 mm occurs after increasing the power output. Thus,
these two cycles indicate that GK 45 has the potential to continue the superplastic

deformation if the energy is increased.

The strain and strain rate were calculated by measuring the variation of the thickness.
The idea of calculation for strain and strain rate by the thickness variance is that the
volume of the superplastically deformed region is unchanged. Suppose that To is the
original thickness, and T is the final average thickness of the deformed sample.
According to the principle of the volume conservation, the original volume SOT0 should
be equal to the final volume ST , that is

So To = ST, ( 6'4 )

Therefore, from equation (6-4), we can write the following expression as below:

(To — T)/ T = (S -— So)/ So ( 6-5 )
Where So is the original deformation area and S is the final average area.

The strain 8, is expressed by

€=(L-LO)/LO (6-6)

83

Under the plastic flow condition, the volume is equal to each other between deformed
and undeformed specimen. Thus,
Lvo = LA ( 6-7 )

Where L0 is original length of the original sample and L is final length of the sample.
A0 is the area of the original cross section, and A represents the area of the deformed
cross section. From equation ( 6-7 ), a similar expression to equation ( 6-5 ) follows:

(Lo-L)/L=(A—Ao)/Ao (6—8)
Comparing equation (6-5) and (6-8), and from the concept of Equation (6-6), the
conclusion is that:

(T — To)/ To = 8

So the strain 8 can be calculated through the formula of thickness variance (T - To)/ To.
By this way, it simplified the problem of strain measurement in a large scale.
Figures 44 and 45 are the sketches of the cross section of the deformed samples with
different forming stresses. The sketch of the cross section shown in Figure 42, was
taken from the sample which deformed at 91.44 kPa (685.8 torr) applied pressure and the
total forming time was 240 minutes. Figure 43 shows the appearance of the cross section
from a sample, which deformed at 200 kPa (2 bar) argon gas and the processing period
was around 110 minutes. The thickness variance along the cross section is marked on

the corresponding locations individually.

In Figure 44, the data of the original thickness of the sample, To is equal to 1.6612 mm
(0.0654 inch), and the final thickness of the cross section of the deformed sample is

averaged as T is equal to 1.3665 mm (0.0538 inch). Since 8 = [(T - To)/ To] x100%,

84

the computed strain a is equal to 17.74 %. The strain rate corresponding to 240 minutes
is,

g = 1.23 x 10‘51;1

The thickness variation shown in Figure 45 were measured from the original thickness.
Since To is equal to 1.6612 mm (0.0654 inch), and the average final thickness T is equal
to 1.3183 mm (0.0519 inch), The stain e is about 20.64 % and the relevant stain rate for

110 minutes is,

g = 3.13 x 10'5 s"

From the values of the stain rate for the above two samples, it is obvious that when the
applied stress increased, the strain rate increased also. Thus, the strain rate was not only
controlled by temperature but also by the forming stress applied on the deformed region.

Thus, for GK 45 alloy, there is a potential to increase the deformation speed as well.

6. 5 The Variation of Thickness

A perfect superplastic deformation should have a uniform thickness at any region of the
final shape. However, that is very difficult to achieve. Usually, the comers have the
thinnest gage section in conventional superplastic forming. Thus, the stress, distribution

on the deformed region would become increasingly non-uniform.

85

 

 

1 \1,\/

1.6612 1.3030

T 1.4300

Figure 44. A sketch of the cross section showing the thickness variance
with vacuum forming system.

 

 

 

 

1.3309
1.3056\
1.6612

 

 

m

 

 

 

 

Figure 45. A sketch of the cross section showing the thickness variance
with argon gas forming system.

86

Figures 46 and 47 are photographs showing the cross section of the sample which were
processed separately in vacuum forming system, and in argon gas forming systems. The
processing time and applied stress, as well as the corresponding strain rate have been
discussed in Section 6.3. The deformations were achieved in the following schematic
processing picture as showing in Figure 48. The scanning patterns were processed by
scanning around a circle, and then a stationary laser beam was used on the center of

pattern.

(1 )
—\//—\_,/—— (2)
VJ— (3)
V— (4)

 

Figure 48. A schematic sketch of showing the deformation process by a laser beam
scanning around a circle, and a stable irradiation at the center.

Initial stage is shown in (1), and the final stage is shown in (4).

87

 

Figure 46. A photograph showing a cross-section appearance of a sample
deformed by introducing vacuum.

 

Figure 47. A photograph showing a cross-section appearance of a sample
deformed by argon gas.

88

The thickness variation shown in Figure 45 looks more uniform than that shown in Figure
46. Note that argon gas can applied higher stress than vacuum system, therefore the

deformation rate was higher than the former.

The factors that contribute to the thinning characteristics are complex. For example,
thickness variations or materials inhomogeneities will cause local stress differences

leading to localized thinning.

The applied pressure and stress distribution, temperature distribution and laser beam
scanning patterns are dominant parameters to laser assisted superplastic forming
processing. This study has been the very first such results accomplished through laser
forming. Figure 49 is a photograph of such a dome produced with GK 45 alloy by laser

assisted superplastic forming as developed in this thesis.

89

 

Figure 49. A photograph of a dome produced by LSF in GK 45 alloy. A square-mesh
grid pattern is projected on the sample to help visualize the curvature.

90

Chapter 7

SUMMARIES

The following summaries can be derived from the experimental results.

The feasibility of laser, dieless superplastic forming of materials have been
demonstrated from this research. This technique involves a high energy C02 laser
beam as the dynamic heat source and a computer numerical by controlled motion
system. The laser beam scanning pattern can be programmed via CNC code, and
performed by a x-y moving table. A desired deformation shape could be obtained by
programming the beam heating geometry, and controlling the heat input through
laser.

From the investigation of microstructure of GK45 aluminum alloy, the superplastic
deformation temperature is around 500°C to 550°C, the corresponding laser power
output needed is around 460 W to 490 W, depending on the ambient temperature, and
the temperature of the cooling water.

In order to obtain effective defamation, the influence of high thermal conductivity of
the alloy must be considered. For a laser beam diameter of 20 mm, a relative slow
laser beam traverse speed around 0.10 to 0.20 mm/s were found to be the best choice.
Since cold-rolled aluminum sheet exhibits shining surface, it has very low absorption
of laser energy. To enhance absorption of infrared C02 laser, the sheet metal has to
be prepared by reagent etching, high temperature painting, and subsequently baking

at 200 °C.

91

0 From the measurement of strain and calculation of the strain rate, a deformation
saturation is absorbed, although the deforming limit was not achieved.

0 The final thickness variance, measured from the cross section, is not very uniform.
However, it could be improved by controlling the laser heating path or by providing

higher pressure for speeding deformation rate, and therefore to increase strain rate.

92

10.

11.

12.

13.

14.

15.

16.

BIBLOGRAPHY

K.A.Padmanabhan, G.J.Davies “ Superplasticity Mechanical and Structural
Aspects, Enviromental Effects, Fundamentals and Applications “ Springer-Verlag
1980.

Oleg D. sherby and Jeffrey Wadsworth : Superplasticicty - Recent Advances and
FurureDirections . Matrials Science Vol. 33, pp. 169-221, 1989

Amiya K. Mukherjee : Superplastic in Metals, Ceramics and Interrnetallics
Plastic Deformation and fracture of materials Vol. 6 chap. 9

John 1. Duncan, Stuart P. keeler, Harmon D. Nine and Philip A. Stine : Sheet
Metal Formability. Tool and Manufacturing Engineers Handbook Vol.2
pp. 1-20, 1983

CH. Hamilton : Superplastic sheet Forming. Metals Handbook Ninth Edition.
Vol. 14 pp.852, 1988

Sokka M. Doraivelu, Lowell W. Foster, Harold L. Gegel : Superplastic Forming.
Tool and Manufacturing Engineers Handbook Vol.2 pp. 4-59, 1983

M.M. I. Ahmed and T.G.Langdon, Metall. Trans 8A, pp.1832. 1977
Y.Nakatani, T. Ohnishi and K. Higashi, J. Japan Inst. Metals 48, pp.113. 1984
K. Higashi, T. Ohnishi and Y. Nakatani. Scr. Metall. 19, pp.821. 1985

T. G. Langdon, Private Communication, on the work Y. Ma and T. G. Langdon,
University of Southern California. 1988

K. Higashi, Private Communication. 1988
J. W. Edington, K. N. Melton and C.P.Cutler, Prog. Mater. Sci. 21(2), 61 1976

O. D. Sherby and J. Wadsworth, American Society for Metals Park Deformation,
Progressing and Structure. pp. 355. 1984

O. D. Sherby and J. Wadsworth, Mater. Sci. and Tech 1,925. 1985.

W. A. Backofen, I. R. Turner and D. H. Avery, Trans. Am. Soc. Metals 57, 980
1964.

N. E. Paton and CH. Hamilton, The Metallurgical Society of AIME. Warrendale,
PA 1982 Superplastic Forming of Structural Alloys.

93

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

30.

31.

32.

33.

34.

35.

C. H. Hamilton: Superplastic Forming of Aluminum. International Show of
Technologies and Energies of the Future. Toulouse, France, 1981

Nicholson, R.B.: Electron Microscopy and the structure of materials. Berkeley,
University of California. pp.689. 1972

Baudelet, B.; Suery, M. : Mater. Sci. vol 7 pp.512. 1972

Avery, D. H.; Backfen. W.A. : Trans. Am. Soc. Met. Vol. 58 pp.551 1965
Holt, D.L.; Backfen, W.A. : Trans. Am. Soc. Met. Vol. 59 pp.755. 1966

Cline, H.E.; Alden, T.H.: Trans. Met. Soc. AIME Vol. 239 pp.710 1967

Zehr, S. W.; Backfen. W.A.: Trans. Am. Soc. Met. Vol. 61 pp.300. 1968

Lee, D.: Acta Metall. Vol. 17 pp.1057. 1969

Sagat, S.; Blenkinsop, P.A.; Taplin, D.M.R.: J. Inst. Met. Vol.100 pp.268. 1972
Marder, AR. : Trans. Met. Soc. AIME Vol. 245 pp.1337 1969

Horiuchi, R., El-Sebai, A. B. ; Otsuka, M.: Scripta Metall Vol.7 pp.1101. 1973
Stewart, M. J. :Metall. Trans. Vol. 7A pp.399. 1976

Garde, A.M., Chung,H.M., Kaussner, T.F.: Acta Metall. Vol.26. pp.153 1978

J. A. Wert, N.E. paton, C. H. Hamilton and M. W. Mahoney: Metall. Trans. A,
Vol.12A, pp.1265. 1981

BM. Watts, M.J. Stowell, B. L. Baikie and D.G.E. Owen: Met. Sci. Vol 10, pp.198
1976

D. C. Winbum , What Every Engineer Should Know About LASERS. New York and
Basel. 1987

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

J .13. Harry, Industrial Lasers and Their Applications. New York, McGraw—Hill,
1974.

Hans Koebner, Industrial Applications of Lasers. Chichester, New York. A Wiley-
Interscience. 1984.

94

36. R. C. Crafer and P. J. Oakley, “ Process and physical aspects of continuos wave

37.

38.

39.

40.

41.

42.

43.

45.

46.

47.

48.

49.

laser processing “ in Applied Laser Tooling, pp. 55 — 79, Martinus Nijhoff
Publishers, 1987.

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

K. Mukherjee, C. W. Chen and N. Hu, “ Laser Dieless Forming of Plastic and
Superplastic Materials “ in TMS conference, Cincinnati, 1996.

J. M. Poate, G. Foti, and D. C. Jacobson. Surface Modification and Alloying by
Laser, Ian, and Electron Beams. New York and London Plenum Press, 1983.

S. S. Charschan and R. Webb, “ considerations for lasers in manufacturing, “ in
Laser Materials Processing ( M. Bass, Ed. ) ch. 9, pp439-473, North-Holland Pub.
Co., 1983.

G. Chryssolouris, Laser Machining: Theory and Practice. New York: Springer-
Verlag, 1991.

Clifton W. Draper and Paolo Mazzoldi, Laser Surface Treatment of Metals.
Martinus Nijhoff Publishers, 1986.

T. Lehecka, N. C. Luhmann, Jr., W.A. Peebles, J. Goldhar, and SP. Obenschain,
“Gas Lasers, “ in Handbook of Microwave and Optical Components Vol. 3, pp. 451-
596, John Wiley & Sons, 1990.

BA. Lengyel, lasers. New York: Wiley—Interscience, 2ed., 1971.

J. T. Luxon, D. B. Parker, and PD. Plotkowski, Lasers in Manufacturing. New York:
IFS (Pub.) Ltd. And Springer-Verlag, 1987.

W. W. Duley, C 02 Lasers - Effects and Applications. New York: Academic Press,
1976.

D. C. O’Shea, W. R. Callen, and W. T. Rhodes, Introduction to lasers and Their
Applications. Reading, MA: Addison- Wesley, 1977.

B. A. Lenyel, Introduction to Laser Physics. New York: John Wiley & Sons, 1967.

K. Mukherjee and J. Mazumder, Laser Processing of Materials. Warrendale, PA:
AIME Publication, 1985.

95

50.

51.

52.

53.

54.

55.

56.

57.

58.

59.

61.

62.

63.

J. Mazumder and K. Mukherjee, Laser Materials Processing III. Warrendale, PA:
TMS Publication, 1989.

J. Mazumder and K. Mukherjee, and B. L. Mordike, Laser Materials Processing IV.
Warrendale, PA: TMS Publication, 1994.

R. Migliroe, “ Laser cutting,” in Engineered Materials Handbook, Composites, vol. 1 ,
pp. 676-680, ASM International, 1988.

R. R. Schrelber, “ Lasers: Technology advances are boosting the usefulness of CO2,
Nd: YAG, and excimer lasers in manufacturing. Part I, “ Manufacturing
Engineering, vol. 106, pp.41-44, January 1991.

R. R. Schrelber, “ Lasers: Lasers can help you cut up the competition. Part II, “
Manufacturing Engineering, vol. 106, pp. 281-290, Febrary 1991.

P. W. Milonni and J. H. Eberly, Lasers. New York: John Wiley & Sons, 1988.

I. Miyamoto, T.Ooie, Y.Hirota, and H. Maruo, “ Mechanism of laser ablation -
Ablation process and debris formation, “ in Laser Materials Processing, vol. SPIE
2306, pp. 1-1 1, Laser Institute of America, 1993.

E. M. Breinan and B. H. Kear, “ Rapid solidification laser processing at high power
density, “ in Laser Materials Processing ( M. Bass, Ed.) ch 5, pp.234-295, North-
Holland Pub. Co., 1983

Z. Sun and J. C. Ion, “ Review: Laser welding of dissimilar metal combinations, “ J.
Materials Science, vol.30, no. 17, pp.4205-4214, 1995.

A. V.L. Rocca, “ Laser applications in manufacturing, “ Scientific American, vol.
246, no.3, pp.94-103, 1982.

. S. Saending and P. Wiesner, “ Laser surface melting of cutting tool materials,” in

Laser Materials Processing vol.2306, pp.835-846. 1994.

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

C. Marsden, D.R.F. West and W. M. Steen, “ Laser Surface Alloying of Stainless
Stell with Carbon,” in Laser Surface Treatment of Metals, pp. 461-474. 1986.

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

96

65.

66.

67.

68.

69.

70.

71.

72.

73.

74.

75.

76.

77.

A. Almeida, R. Vilar, and R. Colaco, “ Laser alloying of Al and 7175 Al alloy for
Enhanced corrosion resistance, "“in Laser Materials Procesing, vol. 2306, pp. 903-
912. 1994.

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

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

I. C. Hawkes, A. M. walker, “ Applicattion of laser surface melting and alloying to
alloys based on the FE-C system,” in Laser Procewssing of Materials pp. 169-182
1985.

M. Pridham and G. Thomson, “ Laser forming: A force for the future? “ Materials
World. Nov. 1994.

M. Geiger, “ Synergy of Laser Material Processing and Metal Forming” CIRP
annals ...... manufacturing technology. Vol.43, no.2 pp.563-569. 1994.

Geifer, M. ; Vollertsen, F.; Deinzer, G., “Flexible Straightening of Car Body Shells
by Laser Forming.” S.A.E. transactions. Vol.102, no.5 pp.354-361. 1994.

Vollertsen, F., “Potentials of laser forming “ in Laser and optoelektronik. Vol 27,
no.3 1995.

Nakagawa, T; Makinouchi, A; Wei, J ; Shimizu, T., “ Application of laser
stereolithography in FE sheet- metal forming simulation.” Journal of materials

processing technology. Vol 50, no. 1-4. 1995.

Vaccari, John A. “ The promise of laser forming” American machinist. Vol.137,
no.6. 1993.

Veiko, V. P.; Yakovlev, Y. B. “ Physical fundamentals of laser forming of micro-
optical components.” Optical Engineering :vol.33, no 11. 1994.

O. D. Serby and J. Wardswoth, “ superpalsticity - Recent Advances and Future
directions,” in Material Science. vol.33, pp. 169, 1989.

P. J. Winkler, “ Superplasticity in Use: A Critical Review of its Status, Trends, and
Limits”, MRS symposium Proc., vol. 196, 1990.

N. Dahotre, Anne Hunter, and K. Mukherjee, “ Laser Hardening of W2 tool steel,” in
J. Mat. Science, vol 22, 403 1987.

97

 

78. S. Kudapa, V. Barnekov and K. Mukherjee , “ Laser Machining and Joining of
Composites” in Laser Materials Processing IV, TMS Publication, pp.223 1987.

79. Yiping Hu, “ Laser cladding of hard-facing Materials onto AISI 1045 Steel for
Patterned Cutting-Die Design” Thesis, 1996.

98

 

MICHIGAN STATE UNIV. LIBRRRIES
l“WINW”11WWWIIlllllllWHllHWl
31293017667969