Effect of casting defects on mechanical properties of AZ91D magnesium alloy castings

Die cast AM60 and AZ91D magnesium alloys show a large variation in tensile strength and ductility within a group of castings produced under the same average process conditions and chemistry. It is, therefore, important to understand the factors that affect such variability in the mechanical properties. In an earlier contribution, we presented quantitative microstructural and fractographic data on AM60 alloy castings and their correlation with the variability in the mechanical properties. In this contribution, we present our quantitative data on variability in mechanical properties of AZ91D magnesium alloy castings to establish quantitative correlation with microstructural and fractographic parameters. For this purpose, the fracture surfaces of the tensile test specimens and the bulk microstructures have been quantitatively characterized. In these cast microstructures, porosity (shrinkage and gas and/or air) is a dominant type of defect. A method to quantify porosity on the fracture surface and in the bulk microstructure has been developed and quantitative relationships between porosity on the fracture surface and the tensile ductility as well as UTS is established.