Cu content has been successfully optimized in a thixo-cast and directly aged (T5-treatment) Al–7Si–0.5Mg (wt.%) alloy. Even after the 0.5% Cu addition, plate-like Al2Cu (θ’) phases, which contribute to high strengths and large plasticity, are precipitated. The formation of a large fraction of Cu-containing brittle phases could be suppressed as well. Then, the Al–7Si–0.5Mg–0.5Cu alloy exhibits high ultimate tensile strength, 0.2% proof stress, and elongation to failure of 296 MPa, 209 MPa, and 8.8%, respectively. This results in a high quality index similar to that of solution-treated and subsequently aged (T6-treated) Al–7Si–Mg alloys. A Cu addition over 1.0% further improves the strengths; however, this leads to poor ductility because of the high fraction of Cu-containing brittle phases.