Microstructures and phase formation of cast and spray-formed Al-25Si-5Fe-3Cu alloys

A hypereutectic Al-Si alloy with 25% (mass fraction) Si was prepared by traditional cast and spray forming processes, and its microstructures, melting and precipitation processes were studied using SEM(EDS), XRD and DSC methods. The results show that the cast alloy contains coarse plate primary silicon phase and coarse needle-like Fe-bearing phase, but the sizes and morphologies of coarse primary silicon phase and coarse needle-like Fe-bearing phase can be improved by spray forming process so as to improve the properties of the hypereutectic Al-Si alloy. Al matrix, primary silicon phase and Al₂Cu phase are present in both the cast and spray-formed alloys, but the difference is that the main Fe-bearing phase in the cast alloy is Al₄FeSi₂ as well as β-AI₅FeSi phase does in the spray-formed alloy. The main reason is that the cooling rate of the as-deposited preform decreases due to the existence of mushy layer on the preform surface so that the phase transformation about Al₄FeSi₂ phase can be occurred and the eutectic microstructure increases, and β-Al₅FeSi phase can dominate Fe-bearing phase in the spray-formed alloy. The melting and solidification process of phases in the spray-formed alloy was discussed via DSC experiment.