The effect of the gate shape on the microstructural characteristic of the grain size of Al–Si alloy in the semi-solid die casting process

Semi-solid forming is an attractive process for fabricating sound, weight-reduced automotive parts. Therefore, in this work, die systems of varied gate shape, gate width, and gate thickness were designed to investigate the filling characteristics of semi-solid material with a controlled solid fraction. When semi-solid materials with controlled solid fraction of 50% filled inside die cavity, the causes of liquid segregation according to the separation phenomena of solid particles and liquid materials were investigated. The parts were characterized for eutectic segregation, a segregation band, cracks and shrinkage hole on the part surface. It was found that all these defects could be solved by the selection of the optimal casting speed and selection of gate shape when the semi-solid material with a controlled solid fraction completely filled the die cavity. The correlation between liquid segregation and a gate system has been synthetically investigated with experimental results and theoretical results. The hardness in the primary α zone and the eutectics microstructural zone were measured by nano indentation. The relationship between liquid segregation and hardness was also investigated.