本文同時以數值與實驗方法探討脈衝雷射點銲不鏽鋼薄板時，銲點溫度的變化及不同雷射操作參數對銲點尺寸大小、形狀、縱橫比的影響。在數值分析方面，將雷射能量視為三維的高斯熱源分佈並求得點銲過程中，銲點的溫度變化及點銲完成後，銲點的大小。本研究主要的參數計有雷射能量、雷射脈衝時間、雷射入射角等。在實驗方面，主要分三部分，首先利用金相方法量測點銲完成後，銲點的大小，並與數值結果作比對。接下來，使用高速照相機紀錄點銲過程中，銲點的動態變化。最後，建立一套紅外線溫度量測系統，以觀測點銲過程中溫度的變化。由結果可證明利用提出的熱源模式所得到數值結果與實驗結果相當吻合。本研究得知，銲點的尺寸大小與雷射能量、雷射脈衝時間、雷射入射角等有很大的相關性。

Laser spot welding on a stainless steel plate was investigated numerically and experimentally. A numerical method was applied to predict the dimensions of fusion zone and temperature distribution in the welding process. In the numerical approach, a three-dimensional heat source equation is used to model laser beam intensity distribution, which is assumed to be a Gaussian distribution in the radial direction and exponential decay in the penetration direction. The parameters of the pulsed Nd:YAG laser spot welding include pulse energy, pulse duration, and incident angles of laser beam. Experiments were also conducted in the study. The characteristic lengths of welded spot were measured by metallographic method, and then, the dynamical behavior of the laser welding process was visualized by a high-speed video camera. Finally, the temperature variations during the laser-spot welding process were measured by an infrared pyrometer system. It is demonstrated that the numerical results by proposed model agree well with experimental observations in predicting the characteristic lengths of welded spots. From this study, it is found that weld dimensions is a strong function of incident angles of laser beam, laser energy, and pulse duration time.

CONTENTS ………………………………i
LIST OF TABLES …………………………iv
LIST OF FIGURES………………………v
NOMENCLATURE ………………………xii
摘要………………………………………xv
ABSTRACT ……………………………xvi
CHAPTER 1 INTRODUCTION………1
1.1Background ………………1
1.2Literature Review ………2
1.3Motivations and Objectives ………11
CHAPTER 2 NUMBERICAL ANALYSIS …14
2.1Model for Thermal Analysis.…………14
2.2Mathematical Description of the Model…15
2.3Governing Equation Validation in the Micro-Scale Problem…16
2.4Heat Source Distribution………………17
2.5Effective Focal Radius…………………18
2.6Effective Penetration Depth……………19
2.7 Grid Independent and Time Step Setting in Vertical Welding…20
2.7.1 Grid Independent Test……………20
2.7.2 Time Step Setting…………………22
2.8Model Validation in Vertical Welding……22
CHAPTER 3 EXPERIMENTAL APPARATUS………39
3.1Laser Welding System ……………………39
3.1.1 Laser Source ………………………39
3.1.2 Workstation for Laser Welding.……40
3.1.3 The Power Detector and Meter………41
3.1.4 Metallographic Method Applications of Welded Spot… 42
3.2 Digital High-Speed Camera………………42
3.3 Infrared Pyrometry…………………………43
3.3.1 Infrared (IR) Pyrometer……………… 43
3.3.2 Basic Theory of Pyrometry……………43
3.3.3 Components of IR Pyrometer………46
3.3.4 Pyrometer Design…………………48
3.3.5 Calibration of IR Pyrometer System……49
CHAPTER 4 RESULTS AND DISCUSSION…65
4.1Computational Results and Discussion…65
4.1.1 Three-Dimension Thermal Analysis Model…65
4.1.2 The Characteristic Lengths of Welded Spot…65
4.1.3 Influence of Laser Energy on Size and Aspect Ratio of Welded Spot…………65
4.1.4 Influence of Laser Incident Angle on Size and Aspect Ratio of Welded Spot……………………67
4.1.5 Influence of Laser Pulse Duration Time on Size and Aspect Ratio of Welded spot…68
4.1.6 Volume Analysis of Welded Spot…………69
4.1.7 Summary………………………70
4.2 Experimental Results and Discussion………71
4.2.1 Environments and Laser Parameters……72
4.2.2 Influence of Laser Energy and Laser Incident Angle on Size of Welded Spot……………………72
4.2.3 Shape and Aspect Ratio of Welded Spot …74
4.2.4 Volume Analysis of Welded Spot…………76
4.2.5 Comparisons with Numerical Results…77
4.2.6 Summary……………………………77
4.3 Visualization of Laser-Induced Plume Processing in Pulsed Laser Welding………………………78
4.3.1 Environments and Laser Parameters…… 79
4.3.2 Influence of Laser Energy on Size of Plume, Plume Sustained Time, and Solidification Time of Weld Pool...79
4.3.3 Influence of Laser Pulse Duration Time on Size of Plume, Plume Sustained Time, and Solidification Time of Weld Pool……81
4.3.4 Summary…………82
4.4 Temperature Measurements During Pulsed Laser Welding …82
4.4.1Environments and Laser Parameters……… 82
4.4.2Influence of Laser Energy and Pulse Duration Time on Plume Sustained Time………………83
4.4.3Measured Temperature Variations Compared with Numerical Result…………………………85
4.4.4Surface Temperature Measurements at Different Laser Operating Parameters and Aiming Position…87
4.4.5 Summary…………………………88
CHAPTER 5 CONCLUSIONS AND FUTURE WORK…145
5.1 Conclusions…………………………145
5.2 Future Work…………………………146
REFERENCES …………………………148

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