Effects of grain refiner additions on the grain structures in HPDC A356 castings

Abstract

In the cold chamber high pressure die casting process (HPDC) solidification begins when the metal is poured into the shot sleeve and impinges on the relatively cold shot sleeve wall and plunger. Therefore, a mixture of liquid and externally solidified crystals (ESCs) is injected into the die cavity. The mechanisms that control the formation of ESCs are not fully understood. In the work presented here, the microstructures of thin walled A356 aluminium alloy die castings have been investigated. The castings were produced by varying the melt superheat and constitutional conditions. It was found that the area fraction of ESCs (fsESC) increases when decreasing the melt superheat; a low superheat generates coarser, more globular ESCs, whilst a larger superheat results in branched, dendritic crystals; additions of Ti in solution increase the fsESC and additions of Al–5Ti–1B grain refiner increase the number of globular, coarse ESCs and generate a finer grain size in the casting. The results are discussed with special emphasis on the shot sleeve solidification conditions and the mechanisms that control the formation of the ESCs.