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This study elucidates how Mg content affects the microstructure and mechanical properties of Al-14.5Si-4.5Cu alloy by adding 0.45 and 0.90 wt pct Mg. Primary silicon, eutectic silicon, acicular b-Al 5 FeSi, Al 2 Cu, and Al 5 Cu 2 Mg 8 Si 6 phases were observed under the as-cast condition in low-Mg alloy. In high-Mg alloy, a large proportion of the acicular b-Al 5 FeSi phase was converted to Chinese script Al 8 Mg 3 FeSi 6 phase. Neither the acicular b-Al 5 FeSi phase nor the Chinese script Al 8 Mg 3 FeSi 6 phase dissolved in the Al matrix during solution heat treatment. Tensile testing indicates that T6-treated high-Mg alloy containing the Chinese script Al 8 Mg 3 FeSi 6 phase is better than the T6-treated low-Mg alloy that contains the acicular b-Al 5 FeSi phase. The morphologies of Chinese script Al 8 Mg 3 FeSi 6 and acicular b-Al 5 FeSi phases affect the tensile properties of Mg-containing Al-14.5Si-4.5Cu alloys. Additionally, high-Mg alloy contains a higher concentration of Mg solute atoms in the Al matrix than low-Mg alloy, and so the former contains more k¢ precipitates (Al 5 Cu 2 Mg 8 Si 6 ) after T6-tempering treatment. The large amount of k¢ that precipitates in T6-treated high-Mg alloy may also enhance its tensile strength. The ultimate tensile strength (UTS) and elongation of the high-Mg alloy are superior to those of the low-Mg alloy.

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