激光增材制造過程中微結構及其演化與制造參數之間關聯的計算預測，已成為基于增材制造的材料/結構設計和開發過程的重要組成部分。選區激光燒結（SLS）增材制造過程中，微結構演化極度依賴于局部溫度的急劇變化，故而常規的等溫相場模型很難適用于SLS的模擬。

來自德國達姆施塔特工業大學的終身教授胥柏香領導的團隊，與南京航空航天大學的青年千人易敏教授合作，建立了一種熱力學自洽的非等溫相場模型，用以研究SLS增材制造中微結構的演化。該模型考慮微結構與熱傳導的強耦合，SLS局部極高溫導致的局部熔化，以及激光-粉末相互作用。并提出了一種類似于求解最小著色數問題的新解決方案，結合晶粒追蹤方法，該方案可僅用8個序參量來模擬具有多達200個晶粒的系統。本研究還使用了基于LM算法的非線性優化方法，同時擬合模型與實驗中表面能，晶界能隨溫度變化的趨勢，以獲取用于非恒溫相場的模型參數。特別地，將該非等溫相場模型用于SLS處理316L不銹鋼粉末的研究，揭示了激光功率和掃描速度對孔隙率、表面形貌、溫度分布、晶粒幾何形狀以及致密度等微觀結構特征的影響規律，并證實了該模型可用于預測SLS過程中致密化因子與激光比能量之間的關聯。該工作提出的非等溫相場模型及其相關高效數值方法，有望用于SLS增材制造的大規模計算模擬。

該文發表于npj Computational Materials5:81 (2019)。

3D non-isothermal phase-?eld simulation of microstructure evolution during selective laser sintering

Yangyiwei Yang, Olav Ragnvaldsen, Yang Bai, Min Yi & Bai-Xiang Xu

During selective laser sintering (SLS), the microstructure evolution and local temperature variation interact mutually. Application of conventional isothermal sintering model is thereby insufficient to describe SLS. In this work, we construct our model from entropy level, and derive the non-isothermal kinetics for order parameters along with the heat transfer equation coupled with microstructure evolution. Influences from partial melting and laser-powder interaction are also addressed. We then perform 3D finite element non-isothermal phase-field simulations of the SLS single scan. To confront the high computation cost, we propose a novel algorithm analogy to minimum coloring problem and manage to simulate a system of 200 grains with grain tracking algorithm using as low as 8 non-conserved order parameters. Specifically, applying the model to SLS of the stainless steel 316L powder, we identify the influences of laser power and scan speed on microstructural features, including the porosity, surface morphology, temperature profile, grain geometry, and densification. We further validate the first-order kinetics of the transient porosity during densification, and demonstrate the applicability of the developed model in predicting the linkage of densification factor to the specific energy input during SLS.