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 beta-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 beta-Al(5)FeSi phase was converted to Chinese script Al(8)Mg(3)FeSi(6) phase. Neither the acicular beta-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 beta-Al(5)FeSi phase. The morphologies of Chinese script Al(8)Mg(3)FeSi(6) and acicular beta-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 lambda' precipitates (Al(5)Cu(2)Mg(8)Si(6)) after T6-tempering treatment. The large amount of lambda' 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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