Bands of positive macrosegregation and porosity commonly follow the surface contour of components produced by high-pressure die casting (HPDC). In this article, Al alloy AlSi7Mg and Mg alloys AZ91 and AM60 were cast into tensile test bars using cold-chamber (cc) HPDC. Microstructural characterization revealed that externally solidified crystals (ESCs) are not necessary for defect band formation, and that defect bands can form both near to and relatively far from any surface layer of different microstructure. The defect bands were 140 to 240 lm thick. In addition to defect-band-related macrosegregation, the castings also contained inverse segregation and surface segregation. Defect bands are shown to have the characteristics of the dilatant shear bands reported in past rheology studies, indicating that defect bands form due to strain localization in partially solid material during the HPDC process.

In cold-chamber high-pressure die castings (HPDC), the microstructure consists of coarse externally solidified crystals (ESCs) that are commonly observed in the central region of cross sections. In the present work, controlled laboratory scale casting experiments have been conducted with particular emphasis on the flow and solidification conditions. An A356 aluminum alloy was used to produce castings by pouring semi-solid metal through a steel die. Microstructures similar to those encountered in HPDC have been produced and the resulting microstructure is found to depend on the melt and die temperature: (1) the fraction of ESCs determines the extent of migration to the central region; (2) a maximum packing determines the area fraction of ESCs in the center; and (3) the die temperature affects the position of the ESCs-a higher die temperature can induce a displaced ESC distribution. It is found that the migration of crystals to the central region requires a flow, which is constrained at all melt/die interfaces. Furthermore, potential lift mechanisms are discussed. An assessment of the Saffman lift force on individual particles shows it has no significant effect on the migration of ESCs.

Understanding microstructure formation during high-pressure die casting (HPDC) is important for the effective quality control of high-pressure diecast aluminum-alloy components for highintegrity applications. In this study, two HPDC-specific aluminum alloys, AlSi4MgMn and AlMg5Si2Mn, were cast into tensile test bars by cold-chamber (CC) HPDC. The microstructures of the tensile bar specimens were characterized at different length scales, from the scale of the casting to the scale of the eutectic interlamellar spacing. The results show that the salient ascast microstructural features, e.g., externally solidified crystals (ESCs), defect bands, the surface layer, grain size distribution, porosity, and hot tears were similar for both alloys. The formation of these features can be understood by considering the influence of flow and solidification during each stage of the HPDC process.