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

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 mu m and 50 mu m, respectively. The bars were tempered at 180 degrees C (T6 treatment) and 220 degrees C (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 beta-Mg2Si, Q-AL(5)Mg(8-)Cu(2)Si(6), pi-Al8Mg3FeSi6 and of the block-like 0-Al2Cu 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. (C) 2010 Elsevier Ltd. All rights reserved.