Effects of Mg Content on Microstructure and Mechanical Properties of Al-14.5Si-4.5Cu Alloy

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 β-Al₅FeSi, Al₂Cu, and Al₅Cu₂Mg₈Si₆ phases were observed under the as-cast condition in low-Mg alloy. In high-Mg alloy, a large proportion of the acicular β-Al₅FeSi phase was converted to Chinese script Al₈Mg₃FeSi₆ phase. Neither the acicular β-Al₅FeSi phase nor the Chinese script Al₈Mg₃FeSi₆ phase dissolved in the Al matrix during solution heat treatment. Tensile testing indicates that T6-treated high-Mg alloy containing the Chinese script Al₈Mg₃FeSi₆ phase is better than the T6-treated low-Mg alloy that contains the acicular β-Al₅FeSi phase. The morphologies of Chinese script Al₈Mg₃FeSi₆ and acicular β-Al₅FeSi 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 λ′ precipitates (Al₅Cu₂Mg₈Si₆) after T6-tempering treatment. The large amount of λ′ 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.