Effect of (Mn + Cr) addition on the microstructure and thermal stability of spray-formed hypereutectic Al-Si alloys

Microstructures and thermal stability of hypereutectic Al-Si alloys with or without (Mn + Cr) addition, prepared via Spray Forming technique, are studied and compared with traditional cast alloys with same composition, using scanning electron microscopy with energy diffraction spectrum, X-ray diffraction, transmission electron microscopy and differential scanning calorimeter. The results show that the Fe-bearing and primary silicon phases in SF-3C alloy can be refined to less than 10 μm, especially in SF-MC21 alloy the Fe-bearing phase is refined into uniformly distributed α-Al(Fe,Mn,Cr)Si phase particles with sizes smaller than 5-6 μm, contributing to the decrease/elimination of the deleterious effect of needle-like Fe-bearing phases. The results of different heat treatments show SF-MC21 alloy possesses excellent thermal stability than SF-3C alloy which is unstable below 750 K for the coarsening of β-Al5FeSi phase and formation of Al7Cu2Fe phase. The study indicates that both the existence of thermodynamically stable α-Al(Fe,Mn,Cr)Si particles and the increase of solidus temperature of SF-3C alloy induced by adding (2Mn + 1Cr) elements contribute to the high thermal stability of SF-MC21 alloy. Contemporarily, combined the phase reactions or transformation occurred during the melting and solidification processes of both spray-formed hypereutectic Al-Si alloys, the microstructure formation of spray-formed alloys is discussed.