Effects of Cu content and Cu/Mg ratio on the microstructure and mechanical properties of Al-Si-Cu-Mg alloys

The microstructure and tensile properties of Al-Si-Cu-Mg alloys with different Cu contents and Cu/Mg ratios were investigated. The results show that the Mg2Si, 0-Al2Cu and Q-type intermetallic phases form in the as-cast Al-Si-Cu-Mg alloys depending on the Cu content and the Cu/Mg ratio. The Mg2Si is formed in the relatively low Cu/Mg ratio of alloy, while high Cu/Mg ratio promotes the formation of theta-phase and Q-phase. With the same Cu/Mg ratio, the high content of Cu and Mg elements simultaneously enhances the formation of theta-phase and Q-phase. Microstructural observation from the T6 tempered alloys reveals that the Cu content and the Cu/Mg ratio control the dissolution of primary intermetallic phases and the formation of precipitates, thus affecting the strength and the ductility. It is indicated that by controlling the Cu and the Cu/Mg ratio, the Al-6Si-2Cu-0.5Mg aged alloy with few primary compounds and a large number of fine Q' precipitates exhibits high strength and acceptable ductility. (C) 2015 Elsevier B.V. All rights reserved.