The effect of ultrasonic melt treatment on macro-segregation and peritectic transformation in an Al-19Si-4Fe alloy

It is well documented that ultrasonic melt treatment (USMT) can refine dendritic and eutectic microstructures during solidification, but much less attention has been paid to the effect of USMT on macro-segregation and intermetallic transformations. In this research, macro-segregation and primary Fe-containing intermetallic peritectic transformations in an Al-19 wt pct Si-4 wt pct Fe alloy were investigated without and with USMT. Macrostructural examination showed that in the absence of USMT the ingot revealed considerable non-uniform distribution of both the primary Fe-containing intermetallic and primary Si particles, whereas the ingot with USMT exhibited near homogeneous distribution of both primary phases, i.e., reduced macro-segregation. The beneficial effect of USMT on relieving macro-segregation was further examined using quantitative microstructural metallography and the results indicated that the area fraction, number density, and size distribution of both primary phases became essentially uniform across the ingot after USMT. USMT further exerted a significant impact on the constitution of the primary Fe-containing intermetallics, where complex particles of δ-Al3FeSi2/β-Al5FeSi were prominent without USMT, while few δ-Al3FeSi2 particles were observed after USMT and the primary Fe-containing intermetallics existed mostly as the single-phase β-Al5FeSi. The underlying reason was attributed to the reduction in the size of the primary δ-Al3FeSi2 particles which ensures the complete transformation of most primary δ-Al3FeSi2 particles to the peritectic β-Al5FeSi phase