Abstract
The influence of Ca and P addition, and cooling rate on the Sr-modified eutectic Si morphology of Al-12.6%Si alloy was investigated using commercial grade and high purity grade Al-Si alloy. One hundred grams each of the two types of alloys were melted in a high purity alumina crucible and cast into two metallic molds to change their cooling rate at 2K/s and 60K/s respectively. Their macro- and micro-structure were observed using an optical microscope, an SEM, an EDX-EPMA and a WDX-EPMA.
P in the commercial grade samples, combined with the Ca and Sr to form the nucleus of the polyhedron Si, and decreased in Sr content in the melt, as well as the modification ratio.
High purity Al-Si alloy contains very low P content, therefore the influence of Ca on modification is small. Nevertheless, the Ca reacts with Si to form Sr contained Ca-silicide, causing the modification ratio to decrease locally.
