The effect of cooling rate and degassing on microstructure and mechanical properties of cast AZ80 magnesium alloy

Abstract

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.