压铸镁合金压室预结晶组织实验研究、超声处理及模拟仿真

The externally solidified crystals (ESCs) in the shot sleeve have a significant influence on the final microstructure and performance of magnesium alloys during cold-chamber high pressure die casting (HPDC) process. In this project, a series of magneisum die casting experiments will be conducted to study the formation mechanism of the ESCs in the shot sleeve, as well as the effects of the HPDC parameters on the ESCs and the fianl microstructure of magnesium alloy die castings, especially focusing on the grain size, the amount and distribution and the morphology of the ESCs in the die castings. Based on the experimental studies and understanding of the ESCs of high pressure die cast magnesium alloys, the project will also devote to explore efficient methods to refine the ESCs. A process with ultrasonic treatment on the melt during the fast shot phase is proposed to refine the grain size of the ESCs,special ultrasonic probs will be designed to be installed in the die casting die and experiments will be conducted to study the effects of the inlet method, the power, the frequency of the ultrasonic waves and the ultrasonic treatment time on the ESCs and the final microstructure of die cast magnesium alloys. Besides, numerical simulation works will be conducted in this project regarding the prediction of the microstructure of die cast magnesium alloys, which contain the establishment of the nucleation model suitable for the HPDC process, as well as the growth model for magnesium alloys with hcp crystal structure. The goal of the simulation works is to simulate the evolution of the microstructure of die cast magnesium alloys considering the ESCs in the shot sleeve, and offer guidance for the control and optimization of the HPDC process from both therical analysis and practical application point of view.

镁合金高压铸造过程中压室中的预结晶组织对压铸件最终凝固组织及使用性能具有重要的影响。项目将针对压铸镁合金，开展一系列压铸实验，研究镁合金压室预结晶现象，掌握镁合金压铸工艺参数对压室预结晶组织及铸件最终凝固组织，特别是晶粒大小、压室预结晶组织的含量及分布形态的影响规律。基于镁合金压铸实验对压室预结晶组织的认知，探索压室预结晶组织的改善及控制措施，项目提出在压铸过程中高速充填阶段应用超声处理细化压室预结晶组织，设计开发用于压铸模具的超声探头，研究超声引入方式，超声功率、频率及持续时间等工艺参数对镁合金压铸预结晶组织和铸件最终凝固组织的影响规律。同时，采用数值模拟方法，建立镁合金压室预结晶组织及铸件凝固组织的形核模型及生长模型，实现考虑压室预结晶的压铸镁合金凝固组织模拟，采用模拟方法研究压室预结晶组织的形成及压铸工艺对压室预结晶组织的影响，为优化压铸工艺控制提供理论及实际指导作用。

压铸件的压室预结晶组织及最终凝固组织对镁合金实际产品的使用性能具有着至关重要的影响，而镁合金压铸件的这些组织结构和形貌特征主要受到具体压铸工艺条件的控制。本项目围绕镁合金压铸工艺参数优化、压铸件微观组织结构改善及其力学性能提升的研究目标，以AZ91D、AM60和AM50压铸镁合金及其高压铸造工艺为研究对象，通过开展压室预结晶组织相关的系列实验，研究了镁合金压铸过程中压室预结晶组织的形成规律以及压铸工艺条件（低速速度、高速速度、浇注温度及模具温度等）对镁合金组织（晶粒大小、取向、分布和形貌等）的影响规律，明确了超声处理对镁合金压室预结晶组织的细化效果。同时，利用同步辐射X射线断层扫描技术，表征了镁合金枝晶的三维形貌特征并研究了其取向选择问题。基于实验研究结果，建立了镁合金压铸预结晶组织及枝晶组织的形核及生长模型，通过计算机数值模拟仿真的方法，模拟得到了接近于实际镁合金压铸件的凝固组织，模拟研究结果和实验所获结果一致。项目的研究工作对于镁合金压铸工艺的优化、压室预结晶组织的调控、枝晶组织形貌的认识以及铸件力学性能的改善具有重要的理论指导意义和工程应用价值。