The Effect of Cooling Rate and Degassing on Microstructure and Mechanical Properties of Cast AZ80 Magnesium Alloy

The effect of casting cooling rate and degassing on the mechanical properties of cast AZ80 alloy is investigated. A water-cooled copper mould with a wedge geometry is chosen for the casting, which provides different cooling rates at different sections. The casting is performed with and without the addition of hexachloroethane degassing agent. Macrohardness, hot compression, quasi-static tensile, and stress-controlled cyclic tests are performed to characterize the mechanical properties of the cast samples. The microstructure of the material is studied using optical microscopy and X-ray computed tomography methods. The connection between the microstructure and mechanical test results shows that the morphology of primary α-Mg and porosity defects affect the mechanical properties of cast AZ80. The casting cooling rate controls both of these microstructural features. Higher cooling rates result in a finer dendritic morphology of α-Mg that improves mechanical properties and creates a finer and more discrete morphology of β intermetallic particles. The fine dendritic morphology results in smaller interdendritic regions, which in turn prevents pore coalescence and negates the detrimental effect of porosity on the quasi-static tensile and cyclic properties. In lower cooling rates, where the coarse grain structure of the cast material cannot prevent pore coalescence, degassing benefits the mechanical properties, especially fatigue resistance, by reducing the volume fraction of porosity defects. Moreover, the hot deformation behavior of cast samples shows that the addition of the degassing agent and casting with a higher cooling rate improves the range of deformation conditions where cracking will not occur.

本研究探讨了铸造冷却速率与除气工艺对铸造AZ80镁合金力学性能的影响。选用带有楔形几何结构的水冷铜模进行铸造，该模具可在不同截面处提供差异化的冷却速率。铸造过程分别设置添加与不添加六氯乙烷除气剂两组工况。通过宏观硬度测试、热压缩试验、准静态拉伸试验及应力控制循环试验，对铸造试样的力学性能进行表征。采用光学显微镜与X射线计算机断层成像（X-ray computed tomography）技术对材料的微观组织进行分析。微观组织与力学测试结果的关联分析表明，初生α-Mg相的形貌与孔隙缺陷共同影响铸造AZ80镁合金的力学性能，而铸造冷却速率对这两类微观结构特征均具有调控作用。更高的冷却速率可使α-Mg相形成更细小的枝晶形貌，不仅优化了合金的力学性能，同时使β金属间化合物颗粒呈现更细小且分散的分布形态。细小的枝晶形貌会缩小枝晶间区域，进而抑制孔隙合并，抵消孔隙缺陷对准静态拉伸与循环性能的不利影响。在冷却速率较低的工况下，铸造材料的粗晶组织无法抑制孔隙合并，此时添加除气剂可通过降低孔隙缺陷的体积分数，改善合金的力学性能，尤其是抗疲劳性能。此外，铸造试样的热变形行为研究显示，添加除气剂并采用更高冷却速率进行铸造，可拓宽合金不发生开裂的变形工况范围。