Co-precipitation strengthening behavior on paint-baked Al–Mg–Si–Cu aluminium alloys under combined pre- and natural ageing

Optimizing the interplay between pre-ageing (PA), natural ageing (NA), and paint-baking (PB) is critical for enhancing the precipitation hardening of Al-Mg-Si-Cu alloys. This study investigates the multiphase precipitation kinetics in an Al-0.7Si-0.9Mg-0.2Cu (wt%) alloy subjected to PA (80 °C for 1 h), varying NA durations (1–30 days), and PB (180 °C for 30 min). By combining synchrotron small-angle X-ray scattering (SAXS) with high-resolution transmission/scanning transmission electron microscopy (TEM/STEM) characterization, we reveal two governing mechanisms responsible for the superior mechanical performance. First, prolonged NA (30 days) promotes the atomic-scale ordering of GP zones. Initially disordered clusters evolve into ordered configurations containing Si/Cu-rich layers, effectively reducing the thermodynamic barrier for β''/β' precipitation during subsequent baking. Second, we elucidate a strain-induced selective nucleation mechanism for θ'/Q' phases along dislocations. Continuous solute segregations transition into segmented precursors, where only variants satisfying crystallographic matching conditions undergo growth into discrete precipitates. This hierarchical microstructure, comprising dense β''/β' in the matrix and discrete θ'/Q' along dislocations, enhances both strength and ductility. The optimized processing yields an ultimate tensile strength of ∼249 MPa with excellent ductility (∼19.6%), demonstrating that the strategic application of prolonged NA can effectively tailor precipitate architecture in Al-Mg-Si-Cu alloys.