单相合金凝固过程微观组织的三维数值模拟

THREE DIMENSIONAL NUMERICAL SIMULATION OF SOLIDIFICATION MICROSTRUCTURE OF SINGLE PHASE ALLOY

基于晶粒形核和生长物理过程及热质基本传输过程,建立了单相台金凝固过程微观组织和微观偏析形成及枝晶形貌演化的三维数学模型,模型中考虑了成分过冷、曲率过冷和各向异性等重要因素.数值模拟结果表明,所建数学模型能够合理反映质点形核、晶粒生长和柱状→等轴晶转变物理过程,温度场、溶质场和微观组织形貌的模拟计算结果合理.自由枝晶生长过程的模拟结果表明,各向异性强度对枝晶生长和最终组织形貌具有重要影响:强各向异性趋于得到分支发达的树型结构,内部组织为取向平行排列规则的二次枝晶臂簇;而弱各向异性趋于得到表面凹凸不平的近似正八面体结构.模拟结果还证实枝晶内部存在小的孤立熔池,这有助于揭示微观组织中出现点偏析和微观缩松的根本原因.

Based on the physical process of nucleation and growth of grains and basic transfer equations such as heat and solute transfer equations, a mathematical model for the three-dimensional simulation of microstructure, microsegregation and free dendritic growth of single phase alloys was developed. Many factors such as constitutional undercooling, curvature undercooling and anisotropy, which have vital influence on the evolution of microstructure, were considered in the model. Simulated results show that the nucleation, growth, CET and microstructure evolution of free dendritic crystal could be predicted reasonably and the calculated results are coincident with actual phenomena. Simulations of free dendritic growth indicate that the curvature undercooling has a significant effect, on the dendritic growth and final microstructure pattern. The dendritic grain profiles, in which a great number of regular and parallel secondary dendritic arms exist, tend to be formed at high intensity of anisotropy. At the low intensity of anisotropy, however, near octahedral grain profiles with small protuberances of surface tend to be obtained. The existence of small molten pools in interdendritic areas during solidification was confirmed by the simulated results, which is helpful to understand the formation of microstructure related defects such as microsegregation and microporosity.