Synergistic precipitation and strengthening of Al-Cu-Mg-Si alloy via an optimized two-step solution treatment and aging

In this study, the Al-2.2Cu-0.75Mg-0.87Si alloy was systematically investigated to compare the influence mechanisms of single-step and two-step solution treatments on microstructural evolution, aging precipitation behavior, and mechanical properties. The results demonstrated that the dissolution of intermetallic phases during the single-step solution process followed a progressive sequence of θ-Al₂Cu → β-Mg₂Si → Q-AlCuMgSi. Full dissolution of all the intermetallic phases was difficult with the single-step solution treatment, which limited supersaturation of solute atoms. After peak aging at 180 °C, the alloy precipitated only a single type of needle-like θ'' precipitate or lath-like Q′ precipitate, or formed a combination of plate-like θ' precipitates and rod-like Q′ precipitates with low number density, resulting in limited strengthening. The optimized two-step solution treatment (500 °C × 8 h + 520 °C × 2 h) achieved complete dissolution of the high-melting-point intermetallic phases while preventing the incipient melting of low-melting-point phases, maximizing the uniform solid solution of solute atoms. After peak aging at 180 °C, a synergistic combination of high-density plate-like θ' precipitates and rod-like Q′ precipitates formed, resulting in a remarkable improvement in mechanical properties. The tensile strength of the alloy reached 457 MPa, with an elongation of 9.88%, attaining the optimization of strength-ductility synergy.