Pulsed Nd :YAG laser micro welding of stainless steel material

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TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION iii

DEDICATION iv

ACKNOWLEDGEMENT v

ABSTRACT vi

ABSTRAK vii

TABLE OF CONTENTS viii

LIST OF TABLES xi

LIST OF FIGURES xii

LIST OF SYMBOLS xiv

1 INTRODUCTION 1 1.1 Overview 1 1.2 Problem statement 3 1.3 Objectives of study 3 1.4 Scope of study 4 1.5 Significance of study 5 2 LITERATURE REVIEW 6 2.1 Introduction 6

2.2 Previous models of laser welding 6

2.3 Pulsed Nd:YAG Laser Welding Processes 9

2.3.1 Nd:YAG laser advantages 9

2.3.2 Laser power 10

2.3.3 Focal point position 13

2.4 Welding material 14

3 THEORY 17

3.1 Introduction 17

3.2 Nd:YAG laser 17

3.2.1 Pulsed Nd:YAG laser 18

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3.4 Depth of Focus 23

3.5 Interaction of laser radiation with materials 24

3.6 Reflectivity of metals 26

4 RESEARCH METHODOLOGY 30

4.1 Introduction 30

4.2 Experimental setup 32

4.2.1 Laser source 34

4.2.2 Fiber optic delivery system 36

4.2.3 Welding workstation 36 4.3 Welding materials 37 4.3.1 Material preparation 38 4.4 Welding measurement 39 4.4.1 Measurement calibration 41 4.5 Welding characterization 42

4.6 Theoretical consideration and mathematical modeling 43

5 THEORETICAL ANALYSIS ON Nd:YAG LASER MICRO WELDING

44

5.1 Introduction 44

5.2 Mathematical model of a weld penetration depth 44

5.3 Mathematical model of laser beam penetration 54

6 EXPERIMENTAL ANALYSIS OF A SPOT WELD 60

6.1 Introduction 60

6.2 Penetration depth and width of a spot weld 60

6.2.1 Penetration depth and weld width for various

laser peak powers

61

6.2.2 Penetration depth and weld width for various

pulse durations

65

6.2.3 Penetration depth and weld width for various

focus point positions

66

6.2.4 Penetration depth and weld width for various

angles of incidence

68

6.2.5 Penetration depth and weld width for various

number of pulse shots

71

6.3 Welding characterization 73

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7 TENSILE STRENGTH ANALYSIS OF A WELD JOINT 79

7.1 Introduction 79

7.2 Tensile strength of a single spot welding 79

7.3 Tensile strength of a seam welding 81

7.4 Tensile strength for various number of spots 82

7.5 Tensile strength of stainless steel control sample 84

8 CONCLUSION 85

8.1 Summary 85

8.2 Suggestions for future work 86

REFERENCES 87

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LIST OF TABLES

TABLE NO.

DESCRIPTION PAGE

4.1 Laser parameters for Unitek Miyachi LW10E ultra compact

pulsed Nd:YAG laser

35

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LIST OF FIGURES

FIGURE NO.

DESCRIPTION PAGE

2.1 Output characteristics of various welding laser sources 11

2.2 Depth/width ratio versus peak power density 11

2.3 Comparison between the experimental results and the model for 6mm

bead-on-plate laser welding weld size

14

2.4 Thermal conductivity (Wm-1 0C-1) and specific heat (J kg-1 0C-1) as a

function of temperature (0

15 C)

3.1 Energy level of Nd:YAG laser 18

3.2 Parameters of the laser pulse 19

3.3 Focused laser beam onto material surface 20

3.4 Rayleigh range and beam waist for Gaussian beam 21

3.5 The depth of focus (DOF) of laser beam 23

3.6 Absorbed laser radiation into materials

3.7 Reflectivity as a function of wavelength for several metals 26

3.8 Parallel and perpendicular-polarized light 28

3.9 Spectral, directional reflectivity of platinum at 2µm 29

4.1 Flow chart of the research methodology 31

4.2 Laser Welding System (LW4000S) 33

4.3 Experimental setup 33

4.4 Nd:YAG laser schematic diagram 34

4.5 Laser welding workstation 37

4.6 Stainless steel with thickness 1.0mm 39

4.7a Optical Microscopy equipped with CCD video camera. 40

4.7b Optical Microscopy magnification lenses and specimen stage 40

4.8 Weld dimensions measurement using Matrox Inspector 2.1 software 40

4.9 1.0mm of length measurement using Matrox Inspector 2.1 software 41

4.10 FESEM and EDX system 42

4.11 INSTRON Series IX/s Automated Materials Tester System 43

5.1 A model of penetration depth of a spot weld 45

5.2 Model predicted penetration depth versus peak power with

various reflectivities

48

5.3 Model predicted enetration depth versus peak power for different

pulse durations

49

5.4 Model predicted penetration depth versus pulse duration for different

peak powers

50

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5.6a Model predicted penetration depth versus laser spot radius for different laser peak powers

51

5.6b Model predicted penetration depth versus laser spot radius for different

pulse durations

52

5.7 Model predicted penetration depth versus peak power and radius of

laser beam

53

5.8 A schematic diagram showing the model of laser beam penetration in

stainless steel specimen

54

5.9 Model predicted peak power versus depth and radius of laser beam 57

5.10 Model predicted peak power versus depth and time 58

5.11 Model predicted peak power versus radius and time 58

6.1 Cross sections of penetration depth 61

6.2 Weld width 62

6.3a Penetration depth versus laser peak power 63

6.3b Weld width versus laser peak power 63

6.4a Penetration depth versus pulse duration 65

6.4b Weld width versus pulse duration 65

6.5a Penetration depth versus focus point position 67

6.5b Weld width versus focus point position 67

6.6 Cross section of penetration depth for different angle of incidence 69

6.7 Weld width at angle of incidence of 65 degrees 69

6.8a Penetration depth versus angle of incidence 70

6.8b Weld width versus angle of incidence 70

6.9a Penetration depth versus number of pulse shots 72

6.9b Weld width versus number of pulse shots 72

6.10 FESEM micrograph of a weld surface

(a)Formation of weld pool, (b)Surface cracking and porosity

74

6.11 FESEM micrographs of a weld joint of stainless steel specimen

(a) butt joint and (b) lap joint

75

6.12 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

76

6.13 Pulse Energy versus Pulse Duration 77

6.14 Peak Power versus Pulse Energy 78

7.1 Tensile strength test of a single spot welding 80

7.2 Tensile strength test of a seam welding 81

7.3a Tensile strength of butt joint for various number of spots 82

7.3b Tensile strength of lap joint for various number of spots 83

7.4 Maximum tensile strength for various number of spots 83

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LIST OF SYMBOLS

a Aperture of a lens

Į Thermal diffusivity

A Applied laser power

Als Laser spot area

c Specific heat

D Depth of heat penetration

f Focal length

I0 Power density

J0 First order of Bessel function

k Wave number

K Thermal conductivity

Lm Latent heat of fusion

l Weld penetration depth

n Medium refractive index

P Laser beam peak power

pparallel Reflectivity for parallel-polarized light

pperpendicular Reflectivity for perpendicular-polarized light

p Reflectivity average value for unpolarized light

ȡ Material density

r Laser beam radius

T Temperature

Tm Melting temperature

T0 Room temperature

t Pulse duration

R Material reflectivity

µ Material absorption coefficient

w0 Beam waist

Y0 Second order of Bessel function

Ȝ Wavelength

z Direction of depth

Figure

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References

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