在雷射焊接(Laser Welding, LW)中，金屬的蒸發效應會影響熔池中之元素成分含量，導致銲接過程中熔池的物理現象、熱和質量的傳遞過程有所差異。在本研究中，建立含Knudsen Layer蒸發模型的三維雷射銲接燒熔數值模型進行模擬。其中質量傳遞以多相流流體體積法(Volume of Fluid, VOF)計算空氣相與金屬相，並以物種方程式(Species equation)計算合金中各元素含量。熔池動態行為方面，考慮表面張力(Surface tension)、馬倫格尼力(Marangoni force)、固液化模型(Solidification model)及反衝力(Recoil force)。熔池熱傳包含雷射熱(Laser heat)及蒸發熱(Evaporation heat)。並以Knudsen Layer蒸發模型的觀點，計算反衝壓力、蒸發熱及各元素蒸發量。將上述所提之動態行為，除了固液化模型，以源項形式作用於自由液面上。
除了Knudsen Layer蒸發模型之外，在相同工況下建立含Clausius-Clapeyron蒸發模型的模擬，比較兩種蒸發模型的模擬結果。由於Knudsen Layer模型的材料沸點(1940 K)比Clausius-Clapeyron模型的材料沸點(2720 K)低，導致前者的飽和壓力和蒸發熱啟動溫度較後者低，造成前者反衝壓力和蒸發熱的值皆比後者大。由模擬結果得知，前者的熔池下凹尺寸比後者深，且前者的熔池上填高度比後者高。藉由計算熔池的質量佩克萊數(Péclet number，Pe)分析熔池中鎂的濃度分布，可以發現熔池中質量的對流特徵時間較擴散快，因此在熔池表面蒸發後的鎂，可透過對流效應在熔池中快速混和均勻。利用無因次焓與鎂損失量的關係，比較模擬結果與實驗數據，發現模擬結果相比於實驗更符合無因次焓的趨勢，因此可以無因次焓的觀點懷疑實驗的正確性。比較本研究與參考文獻中的模擬結果，發現本研究的熔池因加入多相流流體體積法和反衝壓力讓熔池起伏，使熔池溫度和蒸發面積減少，與實驗有差距。而文獻中的模擬以平坦表面和不加反衝壓力的方式處理熔池表面，使熔池溫度和蒸發面積增加，以符合實驗的鎂損失量，但因實驗本身的正確性問題，應對其模擬的合理性有所疑惑。

This study focuses on developing a three-dimensional numerical model for laser welding, including the Knudsen Layer evaporation model to simulate the physical phenomena, heat, and mass transfer during the process. In mass transfer, the multiphase flow is calculated by VOF (Volume of Fluid) method, and the specific component in the alloy is calculated by species equation. This model also considers dynamic behaviors like surface tension, Marangoni force, solidification model, and recoil force, the dynamic behaviors mentioned above are added on the free surface in source term.
To compare the differences the Knudsen Layer evaporation model with the Clausius-Clapeyron evaporation model, the simulation is performed under the same conditions. The results show that using Knudsen Layer model can get higher values of recoil pressure and evaporation heat, leading to a deeper depression depth and a higher height of the molten pool. Moreover, the study analyzes the concentration distribution of magnesium in the molten pool and finds that the convection effect is significant in mixing magnesium uniformly and quickly.
This study also compares its simulation results with the experiment data. By using the relationship between the dimensionless enthalpy and the magnesium loss, we found that the experiment data is not consistent with the theory of the dimensionless enthalpy. Besides, this study compares its simulation results with the research work that treats the free surface as a flat surface without considering the effect of recoil pressure. The differences in the simulation methods result in variations in fluid motion, temperature, and evaporation area of the molten pool. The study concludes by the dimensionless enthalpy theory that the reasonableness of both the research simulations and the experimental data is in doubt.
In summary, this study provides insights into the complex physical phenomena, heat, and mass transfer that occur during laser welding. It highlights the differences between the evaporation models and the importance of the characteristics to achieve reasonable results.

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