本研究主要藉由數值模擬的方式探討選擇性雷射熔融製程(Selective Laser Melting，SLM)中熔池行為對表面形貌的影響。為了實現並計算金屬粉末受雷射加熱後的物理現象，該模型必須考慮三維空間下的亂序粉床排列、雷射加熱、熱傳模型、材料相變化和流體流動等。其中，在表面形貌的計算是採用流體體積法(Volume of Fluid，VOF)作為自由液面的處理方式。所有的數學方程式皆是利用商用數值模擬軟體FLOW-3D進行差分化計算得到。
在製程參數探討的部分，模擬結果顯示當增加掃描速度時，表面形貌會隨之變平坦，但當掃描速度超過1150 mm/s時，表面又再次轉為粗糙。而當雷射功率提高時，表面形貌則是逐漸變粗糙，其原因可以從熔池體積和熔池壽命兩者與表面形貌的關係說明。因此，透過參數實驗發現在低熔池體積和適當的熔池壽命下，便可以獲得良好的SLM表面品質。
另外，本研究也透過一組對照模型說明在SLM模擬中考慮keyhole現象和亂序粉床的重要性，亦可從模擬結果得知當發生鑰孔(keyhole)熱傳時，熔池凝固過程因為馬蘭哥尼效應(Marangoni Effect)造成的的表面形貌變化。以上的模擬結果都有相對應的實驗驗證，也都得到很好的相似性。因此，該數值模型也能被用來作為一個SLM表面形貌預測及製程參數優化模型。

A three dimensional selective laser melting process simulation is developed to investigate (1) the influence of normalized process parameters on surface morphology and (2) the melt-pool behavior of a randomly-distributed powder bed with keyhole formation by Nd-YAG laser. Results showed that when the scanning speed was increased, the surface morphology initially became flatter, but then roughness developed again at high speed case. Further, as the laser power was increased, the surface morphology gradually roughened. To better describe the surface morphology phenomenon according to different laser parameters, the melt pool volume and melt pool lifetime were also investigated. With these two factors constrained, a fine surface could be obtained with a low melt pool volume and proper lifetime (approximately 100 µs to 130 µs). Also, to show the importance of evaporation during laser melting, the melt-pool temperature, melt-pool dimensions and the surface morphology are used as metrics for comparison. Through simulation, the transition from keyhole formation to the final convex surface at a local area was discovered. The simulation results are all validated via good agreement with the experiment.

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