Title Page
Contents
Abstract 7
1. Introduction 13
1.1. Research Background 13
1.2. Laser Forming Mechanism 16
1.2.1. Temperature Gradient Mechanism (TGM) 16
1.2.2. Buckling Mechanism (BM) 20
1.2.3. Upsetting Mechanism (UM) 21
1.2.4. Elastic Expansion Mechanism (EEM) 23
1.2.5. Coupling Mechanism (CM) 24
1.3. Research Trends and Applications 27
1.4. Research Motivation and Objectives 30
1.5. Outline of the Thesis 31
2. Theory of Finite Element Analysis 33
2.1. Introduction 33
2.2. Finite Element Method (FEM) 33
2.3. Thermal Problem 34
2.3.1. Heat diffusion equation 35
2.3.2. Finite element formulation 39
2.4. Structural Problem 40
2.4.1. Stress equation 40
2.4.2. Finite element formulation 42
2.5. Summary 47
3. Finite Element Modeling and Experimental Verification of Laser Forming Process 48
3.1. Introduction 48
3.2. 3-D Finite Element Modeling 49
3.2.1. 3D part and material properties 51
3.2.2. Element selection 55
3.2.3. Heat source 55
3.2.4. Boundary conditions 57
3.2.5. Finite element analysis result 58
3.3. Experimental Verification 60
3.4. Summary 61
4. Finite Element Analysis of One-time Laser Scanning 62
4.1. Introduction 62
4.2. Analysis of laser parameter effect 62
4.2.1. Effect of Laser Power 62
4.2.2. Effect of Laser Scanning Speed 64
4.2.3. Effect of Line Energy 66
4.3. Analysis of specimen size effect 70
4.3.1. Effect of Sheet Length 72
4.3.2. Effect of Sheet Width 72
4.3.3. Effect of Sheet Thickness 76
4.4. Summary 76
5. Analysis of Scanning Pattern Effect in Multi-time Laser Scanning 78
5.1. Introduction 78
5.2. Two-time Scanning Analysis 80
5.3. Three-time Scanning Analysis 85
5.4. Summary 89
6. Conclusions and Recommendations 93
6.1. Conclusions 93
6.2. Recommendations 94
6.2.1. Finite element models 94
6.2.2. Simulation of complex shapes 94
References 95
Appendix 100
Appendix A. Overview of Laser 100
Appendix B. Lists of ABAQUS Computer Program 103
요약 116
Table 1.1. Summary of laser forming mechanisms 26
Table 3.1. Chemical composition of DP980 steel 49
Table 3.2. Parameters and values involved in laser forming process 50
Table 4.1. Final bending angles obtained with various line energies 68
Table 4.2. Sets of sheet length and width for size effect analysis 70
Table 5.1. Different scanning patterns in the two-time laser scanning analysis 79
Table 5.2. Different scanning patterns in the three-time laser scanning analysis 85
Fig. 1.1. Schematic of a straight-line laser forming process 14
Fig. 1.2. Temperature gradient mechanism of laser forming: (a) Laser... 18
Fig. 1.3. Flow chart of laser forming process. 19
Fig. 1.4. Buckling mechanism of laser forming: (a) generation of thermal... 22
Fig. 1.5. Upsetting mechanism of laser forming 23
Fig. 1.6. Elastic expansion mechanism of laser forming 24
Fig. 1.7. Applications of the laser forming process 29
Fig. 2.1. Heat transfer modes involved in the laser forming process 38
Fig. 2.2. Heat transfer modes involved in a 3-D solid Ω 38
Fig. 2.3. Stress acting on a small area: (a) Normal and shear stresses; and... 41
Fig. 2.4. External tractions acting on a point P 42
Fig. 3.1. Optical microscopic structures of DP980 steel 49
Fig. 3.2. Schematic of laser forming process 50
Fig. 3.3. Stress-strain curves against temperature of DP980 high strength steel 52
Fig. 3.4. Young's modulus against temperature of DP980 high strength steel 52
Fig. 3.5. Poisson's ratio against temperature of DP980 high strength steel 53
Fig. 3.6. Thermal conductivity against temperature of DP980 high strength steel 53
Fig. 3.7. Specific heat against temperature of DP980 high strength steel 54
Fig. 3.8. Local mesh refinement technique for 3-D metal sheet part 54
Fig. 3.9. Gaussian-distributed laser heat source model 56
Fig. 3.10. FEM simulation of the laser forming process 58
Fig. 3.11. Temperature histories at the center point of the steel sheet 59
Fig. 3.12. Final bending angles along the width direction of the steel sheet 59
Fig. 3.13. Experimental equipment of laser forming system 60
Fig. 3.14. A DP980 steel sheet deformed by laser forming system: (a) top... 61
Fig. 4.1. Temperature comparisons at the center point of the steel sheet... 63
Fig. 4.2. Final bending angle comparisons under different laser powers 64
Fig. 4.3. Temperature comparisons at the center point of the steel sheet... 65
Fig. 4.4. Final bending angle comparisons under different scanning speeds 66
Fig. 4.5. Final bending angle comparisons under different line energies 69
Fig. 4.6. Relationship between final bending angles and line energies 69
Fig. 4.7. Effect of the sheet length on peak temperature 71
Fig. 4.8. Effect of the sheet length on final bending angles 71
Fig. 4.9. Effect of the sheet width on peak temperature 74
Fig. 4.10. Effect of the sheet width on final bending angles 74
Fig. 4.11. Effect of the sheet thickness on peak temperature 75
Fig. 4.12. Effect of the sheet thickness on final bending angles 75
Fig. 5.1. Mises stress distributions after the cooling process of the first laser... 79
Fig. 5.2. Different scanning patterns in the two-time laser scanning analysis 81
Fig. 5.3. Final bending angles for different scanning patterns in two-time... 82
Fig. 5.4. Stress directions of the finite elements after the cooling process of... 82
Fig. 5.5. (a) Temperature distributions at the center point of the steel sheet in the two-time scanning analysis: Case 1 83
Fig. 5.5. (b) Temperature distributions at the center point of the steel sheet in the two-time scanning analysis: Case 2 84
Fig. 5.6. Different scanning patterns in the three-time laser scanning... 87
Fig. 5.7. Final bending angles for different scanning patterns in three-time... 88
Fig. 5.8. (a) Temperature distributions at the center point of the steel sheet in the three-time scanning analysis: Case 1 90
Fig. 5.8. (b) Temperature distributions at the center point of the steel sheet in the three-time scanning analysis: Case 2 91
Fig. 5.9. Final bending angle comparisons for different scanning times 92