選擇性雷射熔化(SLM)是一種積層製造技術，可以通過沿著粉末床在預設掃描路線上對粉末形式的指定材料進行雷射掃描和燒結來製造三維工件。該過程涉及移動的雷射熱源照射粉末床，引起局部的瞬間熱質傳遞，並發生相變，即在熔化的粉末池中熔化和固化。在 SLM 過程中，雷射點附近的高溫梯度會產生很高的殘餘應力，從而導致製造零件失效。為了在合適的計算成本下，充分研究複雜的傳輸過程，本研究提出了一種新穎的準暫態數值模型（跳躍模型）。該模型包括一個定義為橢圓高斯雷射點的體積熱源，沿著掃描路徑跳動。首先，進行暫態熱模擬以獲得用於預測成型外貌的溫度分佈。比較從模擬與實驗獲得的成型外貌溫度分布說明了跳躍模型的準確性和效率。獲得的溫度分佈被用作熱負荷輸入以進行熱機械分析。隨後，熱機械模擬可以預測工件中的殘餘應力。跳躍模型可以有效地預測工件的成型外貌和殘餘應力。此外，計算時間在引入準暫態熱源於熱模擬模型減少60%，而熱機械模擬會減少68%。雖然跳躍模型可以模擬成型外貌和殘餘應力，但無法很精準地預測整體熔池特性。為了有效地預測熔池尺寸和特性，本研究開發了一個全面性的模型。最初，進行數值模擬以獲得在SLM過程中的瞬時熔池溫度和速度分佈。然後將模擬結果與理論分析結合，以開發一個全面性模型。使用過往發表的實驗數據和熔池特性驗證了模型，其雷射功率範圍在 130 W 到 310 W。根據歸一化焓值來確定從傳導模式到鎖孔模式的轉變。該模型可以有效地預測傳導和鎖孔模式下熔池的歸一化高度和面積。

The selective laser melting process is an additive manufacturing technique that can fabricate three-dimensional workpieces by laser scanning and sintering of designated material in powder form on a preset scanning route along a bed of powders. This process involves a moving laser heat source irradiating powder bed which causes local transient mass and heat transfer with phase change, i.e., melting and solidifying, in a pool of melted powders. The high temperature gradient near the laser spot during the SLM process generates high residual stress that can lead to failure of fabricated parts. To fully investigate such a complicated transport process with reasonable computation cost, a novel quasi-transient numerical model (hopping model) is proposed in this study. The model includes a volumetric heat source defined as an elliptical Gaussian laser spot that hops along the scanning path. Initially, a transient thermal simulation is performed to acquire temperature distribution for predicting the molding appearance. The comparison of molding appearance of the workpieces obtained from the simulation with that from experiments illustrates the accuracy and efficiency of the hopping model. The obtained temperature distribution is then utilized as thermal loading inputs to perform the thermo-mechanical analysis. Subsequently, thermo-mechanical simulation can predict the residual stress in the workpieces. The hopping model can effectively predict the molding appearance and residual stresses in the workpieces. Moreover, the computational time is also reduced by introducing a quasi-transient heat source in thermal simulation (60%) as well as in thermo-mechanical simulation (68%). Although the hopping model could simulate the molding appearance and residual stress, the overall melt pool characteristics could not be predicted with high precision. To effectively predict melt pool dimensions and characteristics, a comprehensive model is thus developed. Initially, a numerical simulation is performed to obtain transient melt pool temperature and velocity distribution during SLM. The results are then coupled with theoretical analysis to develop a comprehensive model. The model is validated using experimental data and melt pool characteristics from previous publications over a wide range of laser power, from 130 W to 310 W. The transition from conduction mode to keyhole mode is identified in terms of normalized enthalpy. This model could effectively predict the normalized height and area of the melt pool for both the conduction and keyhole modes.

Abstract---------------------------------------------------------iv
Table of Contents------------------------------------------------v
List of Figures--------------------------------------------------vii
List of Tables---------------------------------------------------x
List of Symbols--------------------------------------------------xi
Chapter 1 – Introduction----------------------------------------1
1.1 Background---------------------------------------------------1
1.2 Literature review--------------------------------------------4
1.2.1 Experimental investigations--------------------------------4
1.2.2 In-situ temperature measurement----------------------------5
1.2.3 Numerical investigations-----------------------------------6
1.3 Motivation and objectives------------------------------------9
Chapter 2 – Numerical Modeling----------------------------------12
2.1 Thermal simulation-------------------------------------------12
2.1.1 Governing equations----------------------------------------14
2.1.2 Marangoni force and recoil pressure------------------------14
2.1.3 Material properties----------------------------------------15
2.1.4 Numerical models-------------------------------------------18
2.1.5 Nusselt number correlation---------------------------------23
2.2 Thermo-mechanical simulation---------------------------------26
2.2.1 Governing equations----------------------------------------28
2.2.2 Boundary conditions----------------------------------------29
2.2.3 Element birth and death------------------------------------30
2.2.4 Material properties----------------------------------------31
Chapter 3 – Experimental Details--------------------------------32
3.1 Sample preparation-------------------------------------------32
3.1.1 Single track samples---------------------------------------32
3.1.2 Multi-track samples----------------------------------------34
3.2 Temperature measurement--------------------------------------36
3.3 Electron Back Scatter Diffraction (EBSD) measurement---------37
Chapter 4 – Results and Discussions-----------------------------38
4.1 Thermal simulation-------------------------------------------38
4.1.1 Molding appearance from experiment-------------------------38
4.1.2 Temperature distribution of IR measurement and full transient model------------------------------------------------------------41
4.1.3 Quasi-transient model--------------------------------------45
4.1.4 Peak temperature and stripe width--------------------------46
4.1.5 Computational resources------------------------------------49
4.2 Thermo-mechanical simulation---------------------------------50
4.2.1 Comparison between effective-transient and quasi-transient model------------------------------------------------------------50
4.2.2 Computational resources------------------------------------52
4.2.3 Temperature and stress distribution------------------------53
4.2.4 Transition temperature and stress variation----------------55
4.2.5 EBSD analysis----------------------------------------------59
4.2.6 Stress analysis for non-overlapping specimen---------------61
4.3 Melt pool characteristics------------------------------------65
4.3.1 Experimental results---------------------------------------65
4.3.2 Temperature and velocity distribution----------------------67
4.3.3 Theoretical analysis---------------------------------------68
4.3.4 Comparison of normalized melt pool dimensions--------------73
Chapter 5 – Conclusions and Future scope------------------------76
5.1 Conclusions--------------------------------------------------76
5.2 Future scope-------------------------------------------------78
References-------------------------------------------------------79

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