Abstract: Cu additions to Al-Mg-Si alloys enable a desirable combination of high strength and thermal stability. This study investigates the microstructure and mechanical response of Al-1.0Mg-1.2Si (wt.%) with 0, 0.8, and 1.6 wt.% Cu during artificial aging at 175 °C for up to 400 h using Vickers hardness testing, tensile testing, and transmission electron microscope techniques, with quantitative statistics of precipitate morphology, number density, and volume fraction. The Cu-containing alloys reach peak yield strengths of ∼366 MPa (0.8Cu) and ∼373 MPa (1.6Cu), maintaining ∼350 MPa with ∼13 % elongation after 200 h aging, demonstrating an excellent strength-ductility balance. The enhanced tensile properties arise from stronger initial solid-solution strengthening and fine, dense precipitates. Prolonged aging of the 1.6Cu alloy produces plate-like θ′-Al₂Cu phase, whereas the 0.8Cu alloy shows no θ′ under the same conditions, thereby indicating a composition-time window for θ′ precipitation at 175 °C. Precipitation-kinetics analyses based on classical nucleation theory support the thermodynamic/kinetic feasibility of the θ′ phase in the higher-Cu system. Overall, these results clarify how Cu content governs competing precipitation pathways and the resulting mechanical properties of Al-Mg-Si-Cu alloys.

Key words: Al-Mg-Si-Cu alloy, Mechanical properties, Precipitation strengthening, Aging precipitation kinetics