Coupling refining and modification effects of Sr, Mn, and Al-V-Ti-B on microstructure of Al-7Si-0.25Fe alloy

Al-Si alloys are widely used in aerospace and automotive industries due to their high specific strength and low cost. Their mechanical performance is closely related to the grain size of the α-Al matrix and the morphologies of secondary phases such as eutectic Si and Fe-containing intermetallic (Fe-IMC) phases. In this study, the synergistic effects of Sr and Mn modifiers, as well as Al-V-Ti-B grain refiner, on the microstructure of an Al-7Si-0.25Fe alloy were explored. The 0.3% Al-V-Ti-B grain refiner obviously refines the α-Al matrix, reducing grain size from 703 µm to 378 µm. The eutectic acicular Si is significantly modified into a granular morphology with the addition of 0.02% Sr. The modifying ability of Mn on Fe-IMC phases is influenced by Sr, which exhibits a poisoning effect on Mn; accordingly, a threshold Sr content is obtained. As the Sr-addition is below 0.0082%, the dominant Fe-IMC phase is the Chinese-script/blocky α-Al 15 (Fe, Mn) 3 Si 2 with a Mn/Fe mass ratio of ∼0.8. While, as the Sr-addition exceeds 0.018%, the ≥1.2 Mn/Fe mass ratio is needed to modify the undesirable acicular δ-Al 3 FeSi 2 phase.