Evaluation of surface chemical segregation of semi-solid cast aluminium alloy A356

Abstract

In order for SSM forming to produce homogeneous properties in a casting, it is important that there is a uniform distribution of the primary grains. Besides producing a sound casting free of porosity, the amount of liquid segregation must be minimized. The surface liquid segregation phenomenon was investigated on high pressure die cast (HPDC) A356 alloy. SSM slurries were prepared using the CSIR Rheocasting System and plates of 4mm × 80mm × 100mm were HPDC. The chemical composition depth profile from the surface was determined using optical emission spectroscopy (OES) and glow discharge optical emission spectroscopy (GDOES). It was found that a 0.5-1.0 mm eutectic rich layer existed on the surface of the alloy. The thickness of the segregation layer depended on the location on the casting. It was found that this layer was insignificant close to the gate of the casting but was relatively consistent over most of the plate. Although this segregation layer did not impact on the bulk mechanical properties, hardness tests did reveal that this region had significantly higher hardness values which may have a considerable impact on the fatigue properties