采用低压铸造工艺生产的铝合金复杂薄壁铸件在航空、航天、汽车等行业有着广泛的应用。铸件的微观组织对最终使用性能有着重要的影响。在实验基础上借助于计算机数值模拟技术对铝合金复杂薄壁铸件的凝固及微观组织形成过程进行研究，能够辅助优化生产工艺，改善铸件质量，缩短研制周期。论文针对ZL114A合金的凝固特点，建立了ZL114A合金微观组织演化的物理模型。在对平板、阶梯铸件实验数据进行统计分析的基础上结合凝固动力学、热力学分析建立了ZL114A合金形核模型并给出了相应的形核参数。对传统CA方法进行改进，综合考虑了固液两相溶质再分布、固液界面溶质平衡、界面曲率、各向异性等因素建立了铝硅合金枝晶生长演化的数学模型。此外，考虑颗粒第二相与基体合金相互作用以及界面能等因素，建立了有SiC颗粒第二相存在情况下铝硅合金微观组织生长演化的二维及三维模型。针对薄壁复杂铸件的凝固特点，建立了铝硅合金铸件宏观传热及微观组织形核、生长数值算法，并对枝晶优先生长方向以及共晶组织的形成进行了处理。应用建立的铝硅合金微观组织数理模型及数值算法，对Al-7.0%Si合金凝固微观组织演化进行了模拟，再现了单等轴晶、多等轴晶的生长过程。为了考察模型对定向凝固条件下枝晶生长的模拟能力，利用透明合金直接观察实验研究了柱状枝晶的生长过程及枝晶主要特征值，并与解析理论和数值模拟结果进行了对比验证。应用建立的有第二相存在情况下铝硅合金微观组织演化模型对SiC颗粒第二相与枝晶的相互作用过程进行了数值模拟，研究了不同颗粒体积分数、冷却速度等因素对颗粒分布及微观组织的影响并与阶梯铸件实验结果进行了对比。应用开发的铝硅合金低压铸造模拟软件，对航空启动机叶轮、汽车轮毂以及发动机缸体铸件的宏观凝固过程及关键部位的微观组织进行了预测，给出了相应部位的晶粒形貌、晶粒尺寸及二次枝晶臂间距并与实验值进行了对比，模拟与实验结果吻合较好。
Thin-walled aluminum alloy castings prepared by low pressure casting are applied extensively in automobile, aeronautic and astronautic industry. Microstructure of castings is vital to their final performance. Studying the solidification process and the formation of casting microstructure with the aid of computer numerical simulation based on experimental study is helpful to optimize production technology, improve casting quality and reduce research & design cycle. According to the solidification behavior of cast ZL114A alloy, physical model was established to describe the microstructure evolution of Al-Si alloy. Based on the statistical analysis of plate-shaped and step-shaped sample casting experimental data and thermodynamic and kinetics analysis, nucleation model of ZL114A alloy was established and corresponding model parameters were given. A modified Cellular Automaton(CA) method was presented to simulate the dendrite growth of Al-Si alloy，which takes into account the solute redistribution in both liquid and solid, the solid/liquid interface solute conservation, interface curvature and the growth anisotropy. Besides, two dimensional and three dimensional models of Al-Si alloy microstructure evolution with SiC particle secondary phase were presented, which consider the interaction between additive particle phase and matrix alloy and interface energy.Macro heat, nucleation and growth algorithm of real Al-Si alloy casting have been founded according to the features of complex and thin-walled castings. The dendrite growth with different preferential growth direction and the formation of eutectic phase was treated. The developed physical and mathematical model and numerical algorithm were applied to simulate the microstructure evolution of Al-7.0%Si (mass fraction) alloy, reproducing the growth of single and multi equiaxed dendrites with different crystallographic orientation. The experiment of dendrite growth direct observation with transparent alloy was applied to study the process of columnar dendrite growth and related key characteristic parameters. The experimental results were compared with analytical ones and numerical simulated ones, respectively and they agreed well.The interaction between SiC particle secondary phase and matrix dendrite and the process of matrix alloy microstructure formation including particle distribution were simulated with the developed model of Al-Si alloy microstructure evolution with SiC particle secondary phase. The effects of particle volume fraction and cooling rate on particle distribution and microstructure were studied, and the simulated results were compared with those got in step-shaped sample casting experiments. The developed software for Al-Si alloy during low pressure casting was applied to predict macro solidification process and the microstructure at typical positions for real engineering castings including aircraft priming impeller, automobile wheel and cylinder body. The grain morphology, grain size and secondary dendrite arm space of corresponding positions were predicted and agreed well with experimental ones.
