Effect of multi-stage aging on the precipitation strengthening and mechanical properties for an Al-Mg-Si-Ag alloy

Multi-step aging and microalloying are the methods that can effectively facilitate the precipitation strengthening and enhance the performance of Al-Mg-Si alloys. The effect of multi-step aging on the precipitation and properties for an Ag-microalloying Al-Mg-Si alloy is systematically investigated in this article. Controlling temperatures of the primary and secondary aging steps are essential for promoting the effectiveness of the multi-stage aging. The optimal multi-stage aging treatment is identified as: 100 °C × 2 h + 180 °C × 2 h, which gives a peak hardness of 142.4 HV (13.7 HV higher than that of the single-stage aging) for this alloy. It is demonstrated that the primary aging at 100 °C can promote the formation of high density of Cluster(2), and the secondary aging at 180 °C accelerates the transformation of Cluster(2) to β″ caused by the good combination of thermodynamics and kinetics of the transformation. The incorporation of Ag atoms can further promote the nucleation of the β” phase.