本研究利用數值模擬軟體ANSYS FLUENT 15.0建立三維暫態的多層掃描DED加工數值熱分析模型，以加工三層金屬薄層來探討多層DED之製程現象，並透過工研院的DED機台進行實驗驗證，證明模擬結果具有準確性。研究中探討雷射功率及雷射掃描速率對多層金屬薄層幾何尺寸及冷卻速率的影響性與趨勢變化，接續再從節省製程時間、節省能源消耗與降低冷卻速率此三項來衡量各種製程參數組合的優劣，並歸納出最佳化製程參數。為了優化多層金屬薄層的堆疊外型，本研究中也歸納出雷射功率的調降趨勢可提供予實驗者參考。最後依據所選用的最佳化製程參數，探討加工停滯時間、基板預熱和單、雙向加工掃動方式的影響性。
研究結果顯示當以固定的製程參數來加工多層金屬薄層時，每層的堆疊高度會隨加工層數增加而增加，而冷卻速率則相反；當雷射功率1800 W、雷射掃描速率8 mm/s時最能節省製程時間及能源，且對產品的結構強度和金相最具助益；調降雷射功率來優化多層堆疊外形的結果中，第一、二層間的雷射功率與堆疊高度的差異最大，而第二、三層間的差異小，明顯呈現出趨緩的趨勢；加工停滯時間的長短對改善製程的影響有限；基板預熱能有效增加金屬薄層幾何尺寸，並可降低冷卻速率和溫度梯度，有利於柱狀晶生長與增強結構強度；最後以單向掃動方式來加工多層金屬薄層，較有利於維持成品外觀在堆疊高度上的一致性。

In this research, a numerical model for multi-layer scanning of Directed Energy Deposition (DED) process for SS316L is constructed to study transient thermal phenomenon under primary process parameters. The geometrical sizes and cooling rate of multi-layer metal thin layers are investigated under various combination of laser power and scanning speed. It is discovered that the layer height will increase with layer number when maintaining the same process parameters. Moreover, judged from the view of time-saving, energy-saving and reducing cooling rate, the optimal combination of process parameters is determined. Furthermore, in order to obtain superior deposition appearance, the trends of reducing laser power for each layer are proposed.
It is found that laser power of 1800 W and scanning speed of 8 mm/s are the optimal combination of process parameters, which is most beneficial for time saving, energy saving, promoting the growth of columnar grains and reducing cracking possibility. To further improve the manufacturing problems induced by thermal field, the effects of idle time, substrate preheating and scanning direction are studied. Results indicate that idle time has no obvious effects on process. Substrate preheating could increase geometrical sizes of metal thin layers, and obviously reduce cooling rate and temperature gradient. The single scanning direction is helpful for maintaining the conformity of deposition appearance.

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