viii
TABLE OF CONTENTS
CHAPTER
TITLE
PAGE
DECLARATION
DEDICATION
ACKNOWLEDGEMENT
ABSTRACT
ABSTRAK
TABLE OF CONTENTS
LIST OF TABLES
LIST OF FIGURES
LIST OF SYMBOLS
iii
iv
v
vi
vii
viii
xi
xii
xiv
1
INTRODUCTION
1.1
Overview
1.2
Problem statement
1.3
Objectives of study
1.4
Scope of study
1.5
Significance of study
1
1
3
3
4
5
2
LITERATURE REVIEW
2.1
Introduction
2.2
Previous models of laser welding
2.3
Pulsed Nd:YAG Laser Welding Processes
2.3.1 Nd:YAG laser advantages
2.3.2 Laser power
2.3.3 Focal point position
2.4
Welding material
6
6
6
9
9
10
13
14
3
THEORY
3.1
Introduction
3.2
Nd:YAG laser
3.2.1 Pulsed Nd:YAG laser
3.3
Gaussian Beam
17
17
17
18
19
ix
3.4
3.5
3.6
Depth of Focus
Interaction of laser radiation with materials
Reflectivity of metals
23
24
26
4
RESEARCH METHODOLOGY
4.1
Introduction
4.2
Experimental setup
4.2.1 Laser source
4.2.2 Fiber optic delivery system
4.2.3 Welding workstation
4.3
Welding materials
4.3.1 Material preparation
4.4
Welding measurement
4.4.1 Measurement calibration
4.5
Welding characterization
4.6
Theoretical consideration and mathematical modeling
30
30
32
34
36
36
37
38
39
41
42
43
5
THEORETICAL ANALYSIS ON Nd:YAG LASER
MICRO WELDING
5.1
Introduction
5.2
Mathematical model of a weld penetration depth
5.3
Mathematical model of laser beam penetration
44
44
44
54
EXPERIMENTAL ANALYSIS OF A SPOT WELD
60
6.1
6.2
60
60
61
6
6.3
6.4
Introduction
Penetration depth and width of a spot weld
6.2.1 Penetration depth and weld width for various
laser peak powers
6.2.2 Penetration depth and weld width for various
pulse durations
6.2.3 Penetration depth and weld width for various
focus point positions
6.2.4 Penetration depth and weld width for various
angles of incidence
6.2.5 Penetration depth and weld width for various
number of pulse shots
Welding characterization
Laser output characteristics
65
66
68
71
73
77
x
7
8
TENSILE STRENGTH ANALYSIS OF A WELD JOINT
79
7.1
7.2
7.3
7.4
7.5
79
79
81
82
84
Introduction
Tensile strength of a single spot welding
Tensile strength of a seam welding
Tensile strength for various number of spots
Tensile strength of stainless steel control sample
CONCLUSION
8.1
Summary
8.2
Suggestions for future work
85
85
86
REFERENCES
APPENDICES A-C
87
98
xi
LIST OF TABLES
TABLE
NO.
DESCRIPTION
4.1
Laser parameters for Unitek Miyachi LW10E ultra compact
pulsed Nd:YAG laser
Properties of stainless steel and Invar
4.2
PAGE
35
38
xii
LIST OF FIGURES
FIGURE
NO.
DESCRIPTION
2.1
2.2
2.3
Output characteristics of various welding laser sources
Depth/width ratio versus peak power density
Comparison between the experimental results and the model for 6mm
bead-on-plate laser welding weld size
Thermal conductivity (Wm-1 0C-1) and specific heat (J kg-1 0C-1) as a
function of temperature (0C)
Energy level of Nd:YAG laser
Parameters of the laser pulse
Focused laser beam onto material surface
Rayleigh range and beam waist for Gaussian beam
The depth of focus (DOF) of laser beam
Absorbed laser radiation into materials
Reflectivity as a function of wavelength for several metals
Parallel and perpendicular-polarized light
Spectral, directional reflectivity of platinum at 2µm
Flow chart of the research methodology
Laser Welding System (LW4000S)
Experimental setup
Nd:YAG laser schematic diagram
Laser welding workstation
Stainless steel with thickness 1.0mm
Optical Microscopy equipped with CCD video camera.
Optical Microscopy magnification lenses and specimen stage
Weld dimensions measurement using Matrox Inspector 2.1 software
1.0mm of length measurement using Matrox Inspector 2.1 software
FESEM and EDX system
INSTRON Series IX/s Automated Materials Tester System
A model of penetration depth of a spot weld
Model predicted penetration depth versus peak power with
various reflectivities
Model predicted enetration depth versus peak power for different
pulse durations
Model predicted penetration depth versus pulse duration for different
peak powers
Model predicted penetration depth versus peak power and pulse duration
2.4
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
4.1
4.2
4.3
4.4
4.5
4.6
4.7a
4.7b
4.8
4.9
4.10
4.11
5.1
5.2
5.3
5.4
5.5
PAGE
11
11
14
15
18
19
20
21
23
26
28
29
31
33
33
34
37
39
40
40
40
41
42
43
45
48
49
50
51
xiii
5.6a
5.6b
5.7
5.8
5.9
5.10
5.11
6.1
6.2
6.3a
6.3b
6.4a
6.4b
6.5a
6.5b
6.6
6.7
6.8a
6.8b
6.9a
6.9b
6.10
6.11
6.12
6.13
6.14
7.1
7.2
7.3a
7.3b
7.4
7.5
Model predicted penetration depth versus laser spot radius for different
laser peak powers
Model predicted penetration depth versus laser spot radius for different
pulse durations
Model predicted penetration depth versus peak power and radius of
laser beam
A schematic diagram showing the model of laser beam penetration in
stainless steel specimen
Model predicted peak power versus depth and radius of laser beam
Model predicted peak power versus depth and time
Model predicted peak power versus radius and time
Cross sections of penetration depth
Weld width
Penetration depth versus laser peak power
Weld width versus laser peak power
Penetration depth versus pulse duration
Weld width versus pulse duration
Penetration depth versus focus point position
Weld width versus focus point position
Cross section of penetration depth for different angle of incidence
Weld width at angle of incidence of 65 degrees
Penetration depth versus angle of incidence
Weld width versus angle of incidence
Penetration depth versus number of pulse shots
Weld width versus number of pulse shots
FESEM micrograph of a weld surface
(a)Formation of weld pool,
(b)Surface cracking and porosity
FESEM micrographs of a weld joint of stainless steel specimen
(a) butt joint and (b) lap joint
EDX analysis for welded stainless steel,
(a)FESEM image shows the location of EDX analysis region,
Spectrum 1 is a base material region and Spectrum 2 is a welded
material region.(b)Element composition of stainless steel,(c)Element
composition of welded stainless steel
Pulse Energy versus Pulse Duration
Peak Power versus Pulse Energy
Tensile strength test of a single spot welding
Tensile strength test of a seam welding
Tensile strength of butt joint for various number of spots
Tensile strength of lap joint for various number of spots
Maximum tensile strength for various number of spots
Tensile strength of stainless steel control sample
51
52
53
54
57
58
58
61
62
63
63
65
65
67
67
69
69
70
70
72
72
74
75
76
77
78
80
81
82
83
83
84
xiv
LIST OF SYMBOLS
a
Į
A
A ls
c
D
f
I0
J0
k
K
Lm
l
n
P
p parallel
p perpendicular
p
ȡ
r
T
Tm
T0
t
R
µ
w0
Y0
Ȝ
z
z0
Aperture of a lens
Thermal diffusivity
Applied laser power
Laser spot area
Specific heat
Depth of heat penetration
Focal length
Power density
First order of Bessel function
Wave number
Thermal conductivity
Latent heat of fusion
Weld penetration depth
Medium refractive index
Laser beam peak power
Reflectivity for parallel-polarized light
Reflectivity for perpendicular-polarized light
Reflectivity average value for unpolarized light
Material density
Laser beam radius
Temperature
Melting temperature
Room temperature
Pulse duration
Material reflectivity
Material absorption coefficient
Beam waist
Second order of Bessel function
Wavelength
Direction of depth
Rayleigh range