Microstructural characteristics of the surface layer in high-pressure die castings

A thin layer of distinct microstructure, commonly called the surface layer, is often present near the surface of high-pressure die cast (HPDC) components. Although the surface layer formation has been previously investigated, its salient microstructural features in recent studies are significantly different. Therefore, this research aims to gain a better understanding of the surface layer’s microstructural characteristics and the surface layer formation. Microstructural analysis was performed on complex-geometry, industrial-part Al-11Si-2Cu-1Fe (ADC12) and Zn-4Al (Zamak3) HPDC specimens and Al-5Mg-2Si-0.7Mn U-shaped HPDC specimens. The characterized microstructural features suggest that the surface layer starts to form during die filling and becomes stagnant at the die wall. When the inner material moves past the surface layer, primary crystals in the inner material migrate away from the surface layer, resulting in a region adjacent to the surface layer containing only solute-enriched liquid. The solute-enriched liquid in this region subsequently solidifies into the surface-layer edge.