High pressure die casting of aluminium and magnesium alloys some comparisons of microstructure formation

The solidification characteristics related to the cold chamber high pressure die casting process (HPDC) and the alloys control the formation of grains and defects and eventually the mechanical properties of the castings. Significant amounts of pre-solidified metal flow into the die cavity during cold chamber HPDC that commonly consists of individual crystals, termed externally solidified crystals (ESCs). The semi-solid metal fills the die cavity and, eventually, the microstructure consists of coarse ESCs and fine grains solidified in-situ. The prevalence of ESCs in the castings takes a variety of distributions. Another striking feature in the microstructure is banded defects that form in the final stages of the process. The nature of the bands depends on different parameters but often consist of segregated eutectic and/or porosity. In the present paper the microstructure formation in HPDC of aluminium alloy A356 is compared with a magnesium alloy AM60B with particular focus on pre-solidification and defect bands.