Ductility and the skin effect in high pressure die cast Mg-Al alloys

The prominent microstructural features of high pressure die cast Mg-Al alloys are presented. The high pressure die cast Mg-Al alloys examined typically exhibit a finer grain size, a region near the casting surface termed the skin that has a finer structure, a distinctive band of porosity below the skin and a central region with a high fraction of large primary magnesium dendrites. Direct measurement of the as-cast grain size of AZ91D and AM60B has been obtained by EBSD analysis. The depth of the skin region in these alloys has been quantified by several techniques: microhardness traverses; the location of porosity bands; and the variation of the fraction of large primary a grains. The use of these features to define a skin depth is discussed