本研究之目的為建立適用於積層製造(Additive Manufacturing)中的定向能量沉積法(Directed Energy Deposition, DED)的數值模型，藉由商用軟體ANSYS Fluent建構一個三維暫態的單軌單層DED加工模型，以Inconel 718做為金屬基板及粉末所使用之材料，探討多項製程參數以及馬蘭格尼效應對於熔池流動行為、熔池發展趨勢以及沉積層幾何外型的影響，研究使用多相流模型(Multiphase flow model)中的流體體積法(Volume Of Fluid, VOF)追蹤金屬相與氣體相之交界面、以熱焓多孔法來進行金屬相變化過程的模擬，使其更貼近實際之物理現象、藉由在連續方程式中加入金屬粉末源項，並且以高斯分布之經驗公式進行考慮，模擬實際加工中與雷射同軸之供粉頭供應金屬粉末的情形、考慮保護氣體所產生之強制對流效應以及因表面張力梯度所導致的馬蘭格尼效應(Marangoni effect)，並透過與文獻進行沉積層幾何尺寸驗證，確立本研究所建構之模型的可靠性。
研究結果顯示，當改變雷射製程參數時，隨著單位時間輸入工件之熱能提升，熔池的發展速度會增快，且無因次參數Ma數及Pe數與熔池尺寸大致呈正相關，此外金屬沉積層之高度及寬度也會隨之成長，但沉積層之平坦度則會因能量的提升而變差；改變粉末供應之濃度時，沉積層體積會隨濃度的提升而增大，但僅有沉積高度會受到影響，沉積寬度幾乎不會改變；馬蘭格尼效應為熔池流動之主導力，對於溫度場分布及流速有極大影響，但對於最終沉積層尺寸則無太大影響；此外，使用平頂雷射進行加工時，由於其能量分布均勻的特性，與高斯雷射相比會有較為優秀之沉積建構效率，在相同的製程參數下會有較大的沉積層體積。

The purpose of this research is to develop a three-dimensional model for directed energy deposition (DED) process. In this research Inconel 718 is used as material, and the study focuses on how process parameters and Marangoni force affect the development of molten pool and formation of cladding layer. In order to enhance the accuracy of the model, volume of fluid (VOF) method is used to capture the interface of metal phase and gas phase for the multiphase flow model. Enthalpy porosity method is used to simulate the phase change of the metal phase. Powder source is assumed to be Gaussian distribution and is treated as a source term in continuity equation. The effect of force convection due to shielding gas and other force such as buoyancy force, surface tension and Marangoni force are also considered in this model by using user defined function (UDF). Through the verification with similar research, it is proved that the model is reliable in predicting the geometry of cladding layer.
The results show that with the increase in energy input per time, the development of molten pool becomes faster, and dimensionless parameters Ma and Pe are highly related to the size of the molten pool. Besides, the width and height of the cladding layer also increase. As the powder concentration increases, the volume of the cladding layer increase and the cladding height increase as well, but it has a minimum effect on cladding width. The results also show that Marangoni effect plays a significant role in the flow behavior of molten pool, but it has less influence on the geometry of the cladding layer. Moreover, by comparing the results with different laser beams, using flattop laser beam shows better cladding efficiency than using Gaussian laser beam.

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