Improving long-term thermal stability in twin-roll cast Al-Mg-Si-Cu alloys by optimizing Mg/Si ratios

Achieving high thermal stability in the 6xxx series alloys remains a challenging task, which limits their engineering application. Herein, Al-Mg-Si-Cu alloys with various Mg/Si ratios (0.5, 1, 2, and 4) were fabricated by twin-roll casting (TRC), and the microstructure evolution and mechanical properties during long-term thermal exposure of 150°C/1000 h were studied. The results disclosed that alloys with a high Mg/Si ratio exhibited better thermal stability. The alloys with the Mg/Si ratio of 2 (Mg/Si∼2) achieved a stable high yield strength of ∼330 MPa and meanwhile maintained a satisfactory fracture elongation (> 10%) throughout the thermal exposure process. This excellent thermal stability can be attributed to the microstructure consisting of high-density L phases and fine α-AlFeSi phases, which was related to the optimized Mg/Si ratio. Specifically, L phases were dominated in peak-aged Mg/Si∼2 alloys, while the counterparts in alloys with the Mg/Si ratio of 1 (Mg/Si∼1) were β'' and Q' phases. During the thermal exposure process, the L phases remained stable without coarsening, which was mainly due to the high coherence and low interfacial energy of the L-matrix interface. Meanwhile, the main Fe-containing phases in Mg/Si∼2 and Mg/Si∼1 alloys were fine near-spheroidal α-AlFeSi and large-size needle-like β-AlFeSi, respectively, which lead to a better ductility of Mg/Si∼2 alloys. This work may provide a strategy for the preparation of 6xxx series alloys with high thermal stability.