The development of the skin in HPDC Mg–Al alloys

Cross sectional microhardness mappings were produced for cast-to-shape tensile specimens of seven high pressure die cast binary Mg–Al alloys, with Al content ranging from 0.47 to 11.6 mass%. A softer, well differentiated core region, became apparent as a result of the presence of large externally solidified grains (ESGs) only for concentrations above about 4 mass% Al. For these alloys the skin layer was increasingly patchy and uneven in both hardness and depth as the Al concentration increased. SEM observations showed that the ESGs are stochastically distributed near the surface in the more concentrated alloys, unlike for the leaner ones in which they are segregated preferentially to the core region. The lack of ESGs on the skin layer of the leaner alloys stems from their less permeable mushy zone during solidification.