汽车轻量化的发展对铝合金铸件的力学性能提出了更高要求。铝合金铸件的力学性能由不同尺度的微观组织决定，而微观组织的形成受凝固和热处理条件的影响。采用数值模拟技术研究铝合金铸件凝固-热处理全过程微观组织及力学性能的演变规律，对于定量理解工艺-组织-性能之间的内在联系具有重要的学术意义，同时结合实际生产开展建模和模拟研究，进行模型实用化开发与应用，对于企业铝合金铸件的生产向高质量和低成本方向发展具有重要的工程应用价值。通过分析Al-7Si-Mg合金凝固路径及微观组织特征，耦合Pandat软件中的热力学、动力学及多元相图数据库，建立了能够模拟初生枝晶和非规则共晶生长的多元多相CA模型。基于阶梯件不同位置的冷却曲线和凝固组织以及合理的形核假设条件，建立了适用于普通凝固条件下的Al-7Si-Mg合金等轴晶凝固形核模型。基于上述模型，模拟了Al-7Si-Mg合金枝晶和非规则共晶生长过程，分析了过冷度、溶质成分、组元间相互作用、冷却速率和固相扩散等因素对枝晶生长的影响，以及未变质和Sr变质条件下的Al-Si非规则共晶组织的变化规律。同时开展了相关实验从枝晶、共晶形貌以及凝固组织特征参数等方面进行模拟结果的验证，两者吻合较好。采用阶梯件和砂型试棒系统地研究了Al-7Si-Mg合金工艺-组织-性能之间的联系，分析了铸态组织细化程度、合金成分以及热处理条件对合金微观组织和力学性能的影响。基于实验数据的分析，建立了Al-7Si-Mg合金的屈服强度模型，并拟合得到了屈服强度与抗拉强度之间的关系式。建立了时效析出动力学模型、强化模型和应变硬化模型，开展了热处理过程微观组织和拉伸力学性能模拟及实验验证。定量分析了固溶温度、时效温度以及合金成分对合金屈服强度的影响规律，以及时效处理和二次枝晶臂间距对合金拉伸过程应力-应变曲线的影响。同时对本模型的局限性以及影响拉伸力学性能预测精度的因素进行了分析。针对铝合金铸件开展了铸造过程宏观温度场、铸态微观组织以及最终拉伸力学性能的全过程、多尺度数值模拟研究，实现了铝合金铸件工艺-组织-性能之间的有机联系，并且模拟结果和实验结果较为吻合，可为铝合金铸件生产工艺的制定和优化提供理论依据和技术支撑。
The development of automotive lightweighting has put forward higher request on the mechanical properties of aluminum alloy casting. The mechanical properties of aluminum alloy casting are determined by the microstructures in different scales, whose formation is influenced by solidification and heat treatment conditions. Using the numerical simulation technology to investigate the evolution of microstructure and mechanical properties during solidification and heat treatment processes has important academic significance in quantitatively understanding the relationship between process, structure and property. Meanwhile, carrying out the modeling and simulation through combining with the actual production, and developing industrial applicable models are of great application value for foundry enterprise to produce the high quality and low cost aluminum alloy castings.Through analyzing the solidification path and microstructure characteristic of Al-7Si-Mg aluminum alloys, and coupling with Pandat software package in combination with thermodynamic/kinetic/equilibrium phase diagram databases, a multicompent and multiphase cellular automaton (CA) model was developed to simulate the growth of dendrite and irregular eutectic. Based on the cooling curves and microstructure of step casting as well as the reasonable nucleation assumptions, a theoretical equiaxed nucleation model suitable for Al-7Si-Mg alloys under ordinary casting conditions was established. Using these models, the growth of dendrite and irregular eutectic in Al-7Si-Mg alloys was simulated. The influence of undercooling, alloying composition, interaction between solutes, cooling rate and solid diffusion etc on the dendrite growth, and the evolution of Al-Si irregular eutectic microstructure in unmodified or Sr-modified conditions were analyzed. Meanwhile, corresponding experiments were carried out to validate the simulated dendrite and eutectic morphology as well as the microstrcuture characteristic parameters, and well agreements were found between experimental and simulated results.Using the step casting and sand casting test bars, the relationship between process, structure and property in Al-7Si-Mg alloys was systematically studied, and the influence of as-cast microstructure refinement, alloy composition and heat treatment parameters on the microstructure and mechanical properties was analyzed. Based on the analysis of experimental data, a yield strength model for Al-7Si-Mg alloys was established, and the relationship between yield strength and ultimate tensile strength was obtained by data fitting.Precipitation kinetics model, strength model and strain hardening model were established, and the microstructure simulation and tensile property prediction during heat treatment process were carried out. The simulated results were vadilated by experimental results. The influence of solution temperature, aging temperature and alloying composition on yield strength, and the influence of aging treatment and as-cast microstructure refinement on the stress-strain curve during tensile process were quantitatively analyzed using these models. Finally, the limitations of present models and the factors influencing the prediction precision of tensile properties were discussed.Through process and multiscale simulations for several aluminlum castings were carried out which involvs the macroscopic temperature field calculation, as- cast microstructure simulation and the final tensile property prediction. The predicted results were consistent with the experimental results. It demonstrated that this numerical approach can well correlate the relationship between process, structure and property, and can provide the optimized process parameters for aluminum casting production.
