Effect of Aging Conditions on Precipitation Hardening in Al–Si–Mg and Al–Si–Cu–Mg Alloys

The present study was carried out on a series of heat-treatable aluminum-based alloys, the objective being to determine the effect of aging time and temperature on the precipitation of Mg2Si and Al2Cu in these alloys. Tensile test bars (dendrite arm spacing ~24 µm) were solutionized for either 5 or 12 h at 540 °C (Al–Si–Mg alloys) or 495 °C (Al–Si–Mg–Cu alloys), followed by quenching in warm water (60 °C). Subsequently, the quenched samples were aged at 160 and 220 °C for times up to 200 h. Microstructural assessment was performed using thermal analysis technique, image analysis and field emission scanning electron microscopy. All heat-treated samples were pulled to fracture at room temperature. The results show that the presence of Sr delays the precipitation of Mg2Si or Al2Cu during the aging process. The precipitation of Mg2Si occurs in the form of spherical particles—rather than cuboid particles, thin platelets or short rods—regardless of the aging time or aging temperature. Particle coarsening takes place by the dissolution of some small particles and re-deposition onto larger ones. The shape and size of the precipitated Al2Cu particles are found to be strongly dependent on the aging conditions. The tensile behavior of Sr-modified 356 alloy revealed a wavy-type nature with respect to aging time instead of a single peak, due to variation in the size and distribution of the precipitates over several stages. The ultimate tensile strength–ductility relationships have been presented in the form of Q-charts reflecting the effect of aging conditions and melt treatment, i.e., modification with Sr on the alloy performance.