The influence of natural aging on microstructure and hardening behavior during final aging in Al-Mg-Si alloys

This study systematically investigated the influence of natural aging (NA) on precipitation behavior and age-hardening responses in Al-Mg-Si alloys with different Mg/Si ratios. By combining hardness measurements, differential scanning calorimetry (DSC), and High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM), the influence of the Mg/Si ratio on NA-induced clustering and precipitation pathways was systematically clarified. The results demonstrate that, regardless of Mg/Si ratio, short-term NA at room temperature rapidly increases hardness due to vacancy-assisted solute clustering, with the excess-Si alloy exhibiting the highest early-stage cluster density. However, prior NA significantly reduces the peak-aged hardness of all alloys, accompanied by a decrease in precipitate number density and an increase in average precipitate length. Atomic-scale observations reveal that short-term NA promotes Disordered Frank-Kasper, while prolonged NA drives composition-dependent cluster stabilization into square and layered GP zones structures. These clusters act as precursors that modify subsequent precipitation pathways, promoting the development of complex β’’ type precipitates containing mixed β’’ and β’’ structural features. The compositional and structural differences of these precipitates help explain the reduced peak-hardening response after NA. Overall, these findings provide guidance for tailoring clustering and precipitation in 6xxx alloys to better control precipitation hardening and resulting mechanical properties.