Section thickness and the skin effect in high pressure die cast Mg-Al Alloys

Abstract

Microhardness cross sectional maps of cast-to-shape tensile specimens of thickness 1 and 5 mm have been determined for a Mg-I2%AI binary alloy. For the 5 nun thick specimen the hardness was generally higher near the surface and at the comers of the cross-section in comparison with that at the centre. This difference was accounted for by the coarser solidification microstructure and the concentration of porosity at the casting's core. In contrast, no large differences in hardness were found across the section for the I mm thick specimen. The mapping showed a more or less well defined harder surface layer, albeit discontinuous and asymmetrical, for the 5 mm specimen but little differentiation of the skin for the I mm specimen. For both specimens the hardness appeared patchy and non unifonn over the entire cross section. These features of the microhardness maps are largely explained by the non unifonnity of the grain microstructure