Microstructure and mechanical properties of foundry Al-Si-Cu-Hf alloy

Solidification microstructure, precipitation microstructure, and mechanical properties of foundry Al-Si-Cu-Hf alloy were investigated to elucidate the role of Hf during the solidification and precipitation processes. In the as-casting condition, α-Al dendrites, eutectic Si phases, and intermetallic compounds (Al2Cu, the needle-like β-Fe and Chinese-script α-Fe) were observed in the base alloy. However, no β-Fe was detected in the modified alloy, strongly indicating that Hf addition promotes the formation of the α-Fe phase but suppress the formation of the β-Fe phase. Furthermore, Hf addition can result not only in the formation of Hf-containing intermetallic compounds with different morphologies, which were identified as Si2Hf, but also, more importantly, in a significant decrease in the size, number, density, and volume fraction of porosity. After treatment with solution at 520 °C for 20 h, the formation of thermally stable Si2Hf precipitate was observed. After aging treatment at 180 °C up to 72 h, a delayed aging hardening was observed, which may be because Hf addition suppressed the formation of θ' precipitate. Clearly, Hf addition in Al-Si-Cu alloy can modify the solidification microstructure and the precipitation microstructure, which is believed to be beneficial for mechanical properties. Indeed, a significant increase in elongation by 51.4% without a sacrifice of strength was obtained, compared with the unmodified Al-Si-Cu alloy. This investigation highlights the importance of a well-controlled solidification microstructure and precipitation microstructure for the improvement of mechanical properties, in particular to elongation.