Melt flow velocity in high pressure die casting: its effect on microstructure and mechanical properties in an Al-Si alloy

Analysis of as cast tensile specimens of Australian alloy CA313 (an equivalent of A380) made using the high pressure die casting process at three different melt flow velocities has revealed that increasing melt velocity resulted in a finer microstructure and a reduced pore volume fraction and, consequently, better mechanical properties. Mechanical properties are shown to increase more than proportionately with reducing pore volume fraction and the reason for this is demonstrated to be the contribution from the more refined microstructure. The gate used in the investigation was unusual in that its geometry encouraged and prolonged the shearing of the melt. It is proposed that the microstructure was refined by flow shear, the rate of which increased with melt velocity and consequently enhanced the fragmentation of the externally solidified alpha-Al grains and inclusions such as oxides, cold flakes and gas bubbles.