Microstructure and mechanical properties of melt-conditioned high-pressure die-cast Mg-Al-Ca alloy

A new shape casting process, melt-conditioned high-pressure die-casting (MC-HPDC) was developed. In this process, liquid metal was conditioned under intensively forced convection provided by melt conditioning with advanced shear technology (MCAST) unit before being transferred to a conventional cold chamber high-pressure die-casting (HPDC) machine for shape casting. The effect of melt conditioning was investigated, which was carried out both above and below the liquidus of the alloy, on the microstructure and properties of a Mg-Al-Ca alloy (AZ91D+2%Ca (mass fraction), named as AZX912). The results show that many coarse externally-solidified crystals (ESCs) can be observed in the centre of conventional HPDC samples, and hot tearing occurs at the inter-dendritic region because of the lack of feeding. With the melting conditioning, the MC-HPDC samples not only have considerably refined size of ESCs but also have significantly reduced cast defects, thus provide superior mechanical properties to conventional HPDC castings. The solidification behaviour of the alloy under different processing routes was also discussed.