The skin effect in a high pressure die cast Mg-La alloy

Cross-sectional microhardness maps of high pressure die cast-to-shape specimens of a Mg- -3.44mass%La alloy with either rectangular or circular cross-section were produced. A harder layer, or "skin", was observed near the casting surface in both cross-sectional shapes. The higher hardness values were ascribed to the finer solidification microstructure and higher volume fraction of eutectic in the skin. The lower hardness values observed at the centre, or "core", of the cross-section were ascribed to the larger average grain size resulting from the aggregation of externally solidified grains and dispersed microporosity. The skin was non- -uniform in both depth and hardness due to local homogeneities of the grain microstructure caused by the scattered presence of large externally solidified grains near the surface, for both geometries, but especially for the rectangular one. The global and local features of the skin appear related to the distribution of the externally solidified grains as determined by the solidification pattern characteristic to geometry of the die cross-section.