Enhanced long-term thermal stability and mechanical properties of twin-roll cast Al–Mg–Si alloys with Mn and Cu additions

The long-term thermal stability of Al–Mg–Si alloy is of great significance for its engineering application. In this work, the effects of Mn and Cu on the microstructure and mechanical properties of the twin-roll cast (TRC) AA6005 alloy after peak aging and long-term thermal exposure were studied. The results show that the addition of Mn introduces Al(FeMn)Si dispersoids, which improves the mechanical properties of the alloy under the peak aging condition by dispersion strengthening. After thermal exposure at 150 °C for 1000 h, the Al(FeMn)Si dispersoids have good thermal stability while do not significantly improve the mechanical properties. The reason may be related to the decreased number density of the β″ phase after alloying with Mn. The addition of Cu changes the aging precipitation sequence, and the needle-like pre-Q′ phase is formed during artificial aging. Moreover, due to the low diffusion rate of Cu in the Al matrix, Cu atoms can better stabilize the structure and then improve the coarsening resistance of the precipitates, and therefore the Cu-containing precipitates exhibit better thermal stability than the β″ phase. After thermal exposure at 150 °C for 1000 h, the yield strength of the 0.5Cu alloy decreases by only ∼12 MPa (from ∼295 to ∼283 MPa), which is significantly lower than that of the AA6005 alloy (∼41 MPa, from ∼274 to ∼233 MPa). The yield strength of the 0.25Mn0.25Cu alloy decreases by ∼27 MPa (from ∼294 to ∼267 MPa) and is more suitable for industrial production as its composition is within the range of commercial 6xxx aluminum alloys. The results would provide a strategy for the preparation of Al–Mg–Si alloys with high thermal stability.