Chapter 1 - Introduction	4
1.1 Automotive use of energy	4
1.2 Materials for automotive bodies and structures	5
1.3 Advantages of aluminium	7
1.4 Laser welding of aluminium	8
1.5 Objectives of the proposed work	8
Chapter 2 - Literature Review	10
2.1 Types of lasers used for laser welding	10
2.1.1 Types of laser	10
2.1.2 Laser properties - power density	12
2.1.3 Laser choice	13
2.2 Laser/material interactions	13
2.2.1 Laser adsorption	13
2.2.2 Plasma formation, composition and temperature	14
2.2.3 Effect of plasma on the laser beam	17
2.2.4 Keyhole growth	18
2.2.5 Coupling	19
2.3 Physics of laser welding	19
2.3.1 Keyhole shape and physics	20
2.3.2 Weld shape	23
2.3.3 Weld pool shape and thermal field near a moving weld pool	24
2.3.4 Physics of laser welding	27
2.4 Solidification in welding	28
2.4.1 Nucleation and growth in welding	28
2.4.2 Growth of dendritic structures	28
2.4.3 Obtaining solid fraction in a solidifying microstructure	30
2.4.4 Microstructure and welding	32
2.5 Laser welding of aluminium alloys	33
2.5.1 Welding parameters	33
2.5.2 Previous welding work on the alloys used in the current study	34
2.5.3 Difficulties in laser welding	37
2.6 Solidification cracking in welds	39
2.6.1 Basics of hot cracking	39
2.6.2 Early theories of hot cracking in welds and castings	40
2.6.3 Generalised theory of hot cracking in castings and welds	42
2.6.4 Recent developments in the theory of hot cracking	44
2.6.5 Measurement of hot cracking	47
2.7 Metallurgical effects in hot cracking	51
2.7.1 Freezing range	51
2.7.2 Compositional effects	53
2.7.3 Microsegregation	60
2.7.4 Liquid films	63
2.7.5 Metallurgical dependence of hot cracking	64
2.8 Mechanical effects in hot cracking	66
2.8.1 Measurements of stresses around weld pools	66
2.8.2 Modelling the stresses around weld pools	69
2.9 Summary and scope of further work	73
Chapter 3 - Experimental approaches	75
3.1 Materials	75
3.2 Laser welding and different laser sources	76
3.3 Weld observations and optical metallography	80
3.4 SEM observations and EDX spectroscopy	80
3.5 Tensile testing	81
3.6 Cracking testing	83
3.7 Microhardness testing	83
3.8 Bulk chemical analysis	83
Chapter 4 - Laser welding using different laser sources	85
4.1 Laser characterisation	85
4.2 Weld observations and weld macrostructure	89
4.3 Weld microstructure - 2219 Al-Cu	95
4.4 Weld microstructure - 5083 Al-Mg	104
4.5 Weld microstructure - 6061 Al-Mg-Si	107
4.6 Weld microstructure and composition 7475 Al-Zn-Cu-Mg	110
4.7 Weld microstructure - 8090 Al-Li	118
4.8 Discussion of the use of different laser sources for laser welding	127
4.8.1 Laser parameters and weld speed	127
4.8.2 Weld shape	128
4.8.3 Weld area	129
4.9 Discussion of microstructures observed in laser welds	132
4.10 Summary	135
Appendix 4.A EDX Results	137
Chapter 5 - Mechanical properties and cracking of laser welds	139
5.1 Mechanical properties	139
5.1.1 Tensile properties	139
5.1.2 Microhardness results	142
5.2 Porosity	145
5.3 Correlation of mechanical properties with process and material parameters	147
5.3.1 Non-heat treatable alloy	148
5.3.2 Heat treatable alloys	149
5.3.3 Summary of weld metal mechanical properties	153
5.3.4 Filler wire	153
5.3.5 Additional factors affecting joint strengths	154
5.4 Cracking results	156
5.5 Variability of cracking results	160
5.6 Variables affecting cracking	162
5.6.1 Welding speed	162
5.6.2 True heat input	167
5.6.3 Cooling rate	170
5.6.4 Solidification range and BTR	173
5.6.5 Pulsed and continuous laser welding	179
5.6.6 Other metallurgical factors affecting cracking	182
5.7 Conclusions	183
5.8 Summary of laser welding using different lasers and different alloys	184
Appendix 5.A - Calculated cooling rates	187
Chapter 6 - Coupling and melting efficiencies for laser welds in aluminium alloys	189
6.1 Introduction	189
6.1.1 Coupling	189
6.1.2 Experimental determination of coupling	190
6.1.3 Modelling of coupling and the laser welding process	192
6.1.4 Aims of the current study	193
6.2 Measurement of coupling	193
6.2.1 Determining coupling	193
6.2.2. Measurement of thermal cycles	194
6.2.3 Prediction of thermal cycles using the Rosenthal equation	199
6.2.4 Data treatment	200
6.2.5 Values for coupling	205
6.3 Discussion	209
6.3.1 Wavelength dependence of coupling	209
6.3.2 The effect of pulsing upon coupling	210
6.3.3 Other effects upon coupling	211
6.3.4 Melting efficiencies	212
6.4 Conclusions	217
Appendix 6.A - The transient period in the Rosenthal approach	218
Appendix 6.B - Calculating cooling rates	219
Appendix 6.C - Calculating melting efficiencies	223
Chapter 7 - Conclusions and suggestions for further work	225
7.1 Conclusions	225
7.2 Further Work	228
References	231