Effect of Mo Addition on Microstructural Evolution of Al–Si alloys Containing High Iron Impurity

The increasing iron content in secondary aluminum sources promotes the formation of Fe-rich intermetallic phases in Al alloys. Among these, the β-Al₅FeSi phase, which predominates in Al–Si casting alloys, is particularly detrimental due to its needle-like morphology that degrades mechanical properties. In this study, the effect of molybdenum (Mo) addition on the microstructural evolution and mechanical properties of Al–7Si–2Fe alloys was systematically investigated. Mo was added in the range of 0.1–0.4 wt.% to evaluate its role in modifying Fe-rich intermetallics and enhancing alloy performance. Microstructural changes were examined using optical microscopy and scanning electron microscopy, while tensile testing was employed to assess mechanical properties. The results showed that Mo addition transformed the needle-like β-Al₅FeSi phase into more granular morphologies, thereby promoting a more uniform distribution of Fe-rich phases. At 0.1 wt.% Mo, both Chinese-script and blocky α-Al(Fe,Mo)Si phases appeared, accompanied by partial fragmentation of β-Al₅FeSi, marking the onset of phase transformation. Increasing the Mo content to 0.2 wt.% led to a microstructure dominated by compact blocky α-Al(Fe,Mo)Si phases. At higher Mo concentrations, coarse star-like α-Al(Fe,Mo)Si phases became prevalent. Correspondingly, tensile testing revealed a progressive improvement in both ultimate tensile strength and elongation with increasing Mo content. These findings demonstrate that Mo addition is an effective strategy to refine Fe-rich intermetallics and enhance the mechanical performance of Al–Si casting alloys with high Fe levels.