Pre-aged and paint-baked strengthening response on the prolonged natural-aged Al-Mg-Si-Cu aluminum alloys

This study investigates the response of pre-aging and paint-baking on the microstructure evolution and precipitation kinetics in prolonged natural aged Al-Mg-Si-Cu aluminum alloys using differential scanning calorimetry (DSC), transmission electron microscopy/scanning transmission electron microscopy (TEM/STEM), small-angle X-ray scattering (SAXS), and mechanical tensile testing. The prolonged natural aged samples for 14 days contained a higher fraction of rod-like GP zones with a size of ∼1.1 nm and ∼5.5 nm in diameter and length, respectively. After pre-ageing treatment at 80°C for 1 h, some of the existing GP zones grew, exhibiting an increased aspect ratio from 5.0 to 7.7, while others dissolved, leading to a decrease in UTS strength from 211.3 ± 2.1 MPa to 193.6 ± 2.4 MPa. In the direct paint-baked samples (at 180°C for 30 mins), the partial dissolution and size reduction of GP zones promoted the formation of rod-like L _p-phase precipitates, which grew on (020) _Al habit plane, and characterized by Cu-rich layered convex units along the [020] _Al direction. In samples subjected to combined pre-aging and paint-baking treatments, the formation of rod-like β" phases within the aluminium matrix as well as disk-like θ` and Q′ phases along the dislocations played an important role in co-precipitation strengthening. Moreover, the presence of co-existing θ`/Q′ along pre-existing dislocations acted as effective barriers, mitigating lattice strain during tensile deformation. This resulted in a higher UTS of 233.0 ± 5.8 MPa and improved elongation of 29.1 ± 3.0 %.