基于多场耦合的Mg-Zn-Cu-Zr合金热裂及流动行为机理研究

As the application of light-weight Mg alloys in the automobile, the newly developed Mg alloys mainly focused on element alloying and mechanical properties. However, investigation on hot tearing susceptibility and fluidity are very limited. The hot tearing and misrun have become important reason which caused the low yield and high cost during actural production. In this project, the hot tearing susceptibility and fluidity of Mg-Zn-Cu-Zr alloys are investigated by experimental methods. The influence of Zn/Cu content、microstructure、initial mold temperature and pouring rate on hot tearing susceptibility and fluidity are studied in depth using OM、SEM、TEM and X-Ray methods. Residual stresses near the hot cracks were investigated by STRESS-SPEC neutron diffractometer. The project will mainly reveal the microscopic mechanism of hot tearing initiations and propagations based on multi-field coupling analysis on temperature field、flow field、defect field and thermal-stress, etc. The hot tearing susceptibility and fluidity of Mg-Zn-Cu-Zr alloys are also simulated by ProCAST software according to the experimental results in order to explore the feasibility of predicting hot cracks and fluidity. The results of project will not only provide theoretical guidance to optimize the composition of Mg-Zn-Cu-Zr alloys and casting process parameters, but also establish the fundamental study on solidification.

目前汽车轻量化镁合金铸件的研究多集中在合金化及力学性能等方面，对铸造过程中合金的热裂倾向性及流动性的研究较少，在实际生产中热裂和浇不足已成为镁合金铸件成品率低和成本高的重要原因。本项目以热裂较为敏感的Mg-Zn-Cu-Zr合金作为研究对象，通过凝固实验研究，运用OM、SEM、TEM和X-Ray等现代分析手段，深入研究Zn/Cu含量、微观组织、初始模具温度及浇铸速率对Mg-Zn-Cu-Zr合金热裂及流动性的影响规律。利用STRESS-SPEC中子衍射技术分析热裂区域残余应力的分布。基于温度场、流场、缺陷场及热应力等多场耦合分析，重点揭示热裂萌生及其扩展微观机制。结合实验结果，采用ProCAST软件对Mg-Zn-Cu-Zr合金热裂及流动行为进行模拟，探索预测合金热裂和流动性的可行性。本项目研究结果将为Mg-Zn-Cu-Zr合金的成分及铸造工艺参数优化提供理论指导，也为凝固理论研究奠定基础。

本项目中，通过自制的“T”型热裂模具和流动性模具分别研究了Cu元素的含量和模具温度对Mg-(6-7)Zn-xCu-0.6Zr合金的热裂敏感性和流动性能的影响，并利用石蜡渗透法测定合金热裂纹体积。基于修正的Clyne-Davies热裂模型对合金热裂倾向性进行了准确的预测。通过XRD、TEM鉴别了合金的微观物相，并结合SEM和EPMA对合金的显微组织进行了观察。实验结果表明随着Cu元素的加入，晶粒得到细化，晶界上的共晶相增多，共晶相在凝固末期对分离的枝晶起到补缩的作用，降低合金热裂倾向性。研究结果表明合金微观裂纹的形成是液膜、凝固收缩补偿和晶间搭桥共同作用的结果。同时，合金随着Cu元素的加入，枝晶相干温度和凝固区间随着Cu元素含量的增加而降低，形成新的低熔点共晶相，增加合金的流动性能。本项目的研究结果将对汽车用镁合金以及扩展镁合金应用领域提供理论依据和实验支持。