Abstract
► Microstructure and hardness of Al–Si–Cu–Mg type 319 alloys. ► Formation of Mg2Si, Q-Al5Mg8Cu2Si6, π-Al8Mg3FeSi6, Al2Cu phases. ► Effects of alloy type, Mg content, cooling rate, heat treatment (T6,T7). ► Mg, Cu, and high cooling rate (DAS 24μm) improve hardness in T6 condition. ► Experimental alloys show higher hardness than industrial alloys.
Castings were prepared from both experimental and industrial 319 alloy melts containing 0–0.6wt% Mg. Test bars were cast in two different cooling rate molds, a star-like permanent mold and an L-shaped permanent mold, with DASs of 24μm and 50μm, respectively. The bars were tempered at 180°C (T6 treatment) and 220°C (T7 treatment) for 2–48h. The results showed that Mg content, aging conditions, and cooling rate have a significant effect on the microstructure of both experimental and industrial alloys and, consequently, on the hardness. The addition of Mg resulted in the precipitation of the β-Mg2Si, Q-Al5Mg8Cu2Si6, π-Al8Mg3FeSi6 and of the block-like θ-Al2Cu phases. The Mg and Cu, as well as the higher cooling rates improved the hardness values, especially in the T6heat-treated condition, whereas the addition of Sr decreased these values.