The formation mechanism of the chill fine-grain layer with high supersaturation and its influence on the mechanical properties of die casting Al-7Si-0.5Mg alloy

Abstract

In this study, the chill layer with high supersaturation was observed on the surface of the Al-7Si-0.5Mg die casting tensile samples. The optional microstructure and electron backscatter diffraction (EBSD) results show that the chill layer has fine α-Al grains (≈5 μm). There is a solution gradient in die-casting aluminum alloy and the chill fine-grain layer is in a supersaturated state. The pre-solidified dendrites (PSDs) formed in the shot sleeve and the nonuniform velocity distribution of the melt are responsible for the formation mechanism of the chill fine-grain layer with high supersaturation. Under high pressure and high injection speed, the PSDs will aggregate in the core of the samples due to the Magnus effect. Moreover, the chill layer with solute enrichment can be obtained as a result of rapid cooling and incoordinate solute distribution. Consequently, the tiny Mg2Si phase could precipitate spontaneously from the supersaturated α-Al grains in the chill layer, simultaneously improving the of strength and ductility.