Optimizing Annealing Heat Treatment for Improved Ductility in Recycled High-Pressure Die Cast AlSi7MnMg Alloy

The increasing use of recycled aluminium in automotive applications requires the development of energy-efficient heat treatments capable of optimizing mechanical performance while preserving microstructural integrity. In this study, the effect of annealing on the microstructure and mechanical behaviour of a recycled AlSi7MnMg alloy produced by high-pressure die casting was systematically investigated. Annealing treatments were carried out in the temperature range 340–400 °C for holding times between 0.5 and 2 h. The results showed a progressive reduction in hardness, yield strength, and ultimate tensile strength with increasing annealing temperatures and times, accompanied by a significant increase in ductility. The overall mechanical performance was further assessed through a toughness-based quality index, enabling a comprehensive comparison between the different annealing conditions. The microstructural evolution was dominated by spheroidization and coarsening of eutectic silicon, while Fe-based intermetallic compounds exhibited only a mild coarsening. Fractographic observations confirmed a transition towards a ductile mechanism at elevated annealing temperatures. The optimal compromise between strength and ductility was achieved at 340 °C × 2 h (YS = 101 MPa, UTS = 199 MPa, ef = 8.0%), which also exhibited a limited environmental burden when evaluated in terms of specific CO2-eq emissions.