Effect of Melt Purification Treatment on the Solidified Microstructure of Hypereutectic Al-Si Alloys

In this paper, hypereutectic Al-Si alloys were purified by a composite sodium salt, and the solidified microstructure of hypereutectic Al-Si alloys before and after melt purification was studied by OM, SEM/EDS, XRD, and TEM. The results showed that the Fe content decreased from 0.73 wt% to 0.23 wt% after purification with the Na salt. This also prevented an increase in heterogeneous nucleation. The Si content remained almost unchanged, which excluded the possibility of removing Si from the molten Na salt. According to DSC analysis, Na salt purification increased the undercooling degree of the melt, so that the hypereutectic Al-Si alloy melt was deeply undercooled to the pseudo eutectic region, and a nearly-perfect pseudo eutectic structure was obtained. Moreover, it was confirmed that NaAlSi changed the growth of eutectic Si, thereby changing its microstructure. At the same time, nanoscale spherical precipitates formed by trace Fe in the alloy could significantly promote the nucleation of eutectic Si.