Metallurgical parameters controlling the microstructure and hardness of Al-Si-Cu-Mg base alloys

摘要

Castings were prepared from both experimental and industrial 319 alloy melts containing 0-0.6 wt% 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 24 urn and 50 urn, respectively. The bars were tempered at 180 ℃ (T6 treatment) and 220 ℃ (T7 treatment) for 2-48 h. 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-Mg_2Si, Q-Al_5Mg_(8-) Cu_2Si_6,π-Al_8Mg_3FeSi_6 and of the block-like θ-Al_2Cu phases. The Mg and Cu, as well as the higher cooling rates improved the hardness values, especially in the T6 heat-treated condition, whereas the addition of Sr decreased these values.