The role of iron in the formation of porosity in Al-Si-Cu-based casting alloys: Part III. A microstructural model

Abstract

Iron has been shown to have a significant effect on the formation of porosity and shrinkage defects in Al-Si-Cu-based foundry alloys. This is not simply a direct consequence of the physical presence of the β-Al5FeSi platelets in the microstructure, but is also due to the effect that these platelets have on the nucleation and growth of eutectic silicon. The alloy-dependent critical iron content determines when the β phase first solidifies and, hence, when it can participate in the silicon nucleation event. At critical iron contents, the β phase solidifies as the initial component of the ternary eutectic. However, at supercritical iron contents, the β phase is already well developed when ternary eutectic solidification begins, while, at subcritical iron contents, the β phase forms as a component of the ternary eutectic only after the binary Al-Si eutectic is well established. Each of these paths of microstructural evolution leads to different variations in microstructural permeability and, hence, interdendritic feedability and porosity formation. The actual porosity-forming response to these alloy-induced microstructural changes is influenced by the solidification conditions in the casting.