Effect of Si and Cu concentrations and solidification rate on soundness in casting structure in Al-Si-Cu alloys

摘要

Recent metal casting literature has focused on the effect of Al melt practices and alloy chemistry on the formation of dispersed microshrinkage porosity in the cross-section of sand castings. In the present work, an attempt is made to explain observations on the severity of microshrinkage porosity formation in wedge castings of four alloys having different Si and Cu concentrations, with cooling rates ranging from 0.75 down to 0.045℃s~(-1). The Al-9Si-1Cu alloy provided the lowest level of dispersed microshrinkage along the wedge and the Al-7Si-4Cu alloy had the highest porosity. It is concluded on the basis of interpretative thermal analysis with fraction solid calculation that the requirements for the lowest level of porosity are primary α-Al dendritic mass being low in volume and more permeable, a high volume of Al-Si eutectic phase but within a shorter freezing range, and a low volume fraction of the Cu and Mg containing phases post-Al-Si eutectic reactions.