【摘要】 基于Al-Bi过偏晶合金凝固通过难混溶区阶段产生的液-液相分解及分离的运动行为,采用两相体积平均法,对质量、动量、能量、组分及液滴密度守恒方程进行数值模拟,计算中考虑了形核、扩散长大、Stokes运动及Marangoni运动等多种复杂物理现象的耦合作用,分析了两相运动速度、第二相尺寸分布、第二相体积分数分布以及液滴密度分布对过偏晶合金凝固的显微组织演变及宏观偏析的影响。结果表明,过偏晶合金凝固过程中显微组织演化在不同阶段的主要影响因素不同:凝固初始阶段主要以形核和扩散长大为主;凝固中期和后期第二相迁移运动行为将逐渐占主导作用。凝固过程中,铸件顶角位置首先获得过冷度驱动形核,并以较快的形核速率达到最大形核密度。随着凝固过程不断地进行,第二相小液滴受到的Marangoni力约为Stokes粘滞阻力的两倍,开始由铸件顶角和边缘低温区向中心高温区聚集。凝固时间为1 s时,铸件顶角和边缘第二相小液滴的生长直径和第二相体积分数大于铸件中心位置,而凝固时间为5和7 s时,第二相小液滴直径随铸件中心距离变化的曲线斜率随凝固过程的进行而逐渐变缓,长大速率逐渐变慢。更多还原

【Abstract】 Basing on the mechanism of liquid-liquid decomposition and separation of Al-Bi hypermonotectic alloys,conservation equations of mass,momentum,energy,species and droplet density were simulated using two-phase volume averaging approach during solidification of the hypermonotectic alloys in the miscibility gap.In the simulation,several simultaneous physical phenomena were considered,including nucleation,diffusion growth,and Stokes and Marangoni motion.The main factors that influenced the microstructural evolution and macrosegregation were also analyzed,including the effects of two phases moving velocity,second phase size distribution,second phase volume fraction distribution,and second phase droplet density distribution.The simulation results indicated that the microstructural evolution in different stages possessed different main influencing factors.Nucleation and diffusion-control growth played a dominating role in the first stage,and then the second phase moving behavior gradually became more important.The undercooling firstly served as the driving force for the nucleation,and the maximal second phase density was reached by higher nucleation rate in the casting corners during solidification.The Marangoni force was two times bigger than the Stokes viscous resistance,and drove the second phase from the cool corners and surface regions to the hot casting center during solidification processing.At 1 s,the droplet diameter and volume fraction of the second phase in the corners and surface regions were higher than those in the center,but at 5 and 7 s,the curve slope of the droplet diameter of the second phase changing with the distance from casting center became smooth,and the rate of growth became slow.更多还原