Microstructure evolution and phase transformation of traditional cast and spray-formed hypereutectic aluminium-silicon alloys induced by heat treatment

Microstructural evolution and phase transformation induced by different heat treatments of the hypereutectic aluminium-silicon alloy, Al-25Si-5Fe-3Cu (wt%, signed as 3C), fabricated by traditional cast (TC) and spray forming (SF) processes, were investigated by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy and X-ray diffraction techniques. The results show that Al₇Cu₂Fe phase can be formed and transformed in TC- and SF-3C alloys between 802–813 K and 800–815 K, respectively. The transformation from β-Al₅FeSi to δ-Al₄FeSi₂ phase via peritectic reaction can occur at around 858–870 K and 876–890 K in TC- and SF-3C alloys, respectively. The starting precipitation temperature of δ-Al₄FeSi₂ phase as the dominant Fe-bearing phase in the TC-3C alloy is 997 K and the exothermic peak about the peritectic transformation of δ-Al₄FeSi₂→β-Al₅FeSi is not detected in the present DSC experiments. Also, the mechanisms of the microstructural evolution and phase transformation are discussed.