Early Stages of Precipitation in a Naturally Aged and Pre-Aged Al-Cu-Mg Alloy (AA2024)

AA2024 aluminum alloy is widely used in aerospace applications, including aircraft skin, due to its high strength, fatigue resistance, and formability. This study investigates the early-stage precipitation behavior of AA2024 under solution-annealed, naturally aged, and pre-aged conditions, linking microstructural evolution to mechanical properties and differential scanning calorimetry and thermal scanning resistometry measurements. Tensile testing revealed differences in strength and the Portevin–Le Chatelier effect. Atom probe tomography identified copper segregation along dislocations and nanoscale precipitates in naturally aged specimens, whereas pre-aged specimens exhibited needle-shaped S phase precipitates. Transmission electron microscopy confirmed these observations and provided direct imaging of Guinier–Preston–Bagaryatsky (GPBn) zones and S phase. Natural aging promotes numerous GPB1 zones and sporadic S phase formation, while pre-aging favors higher-order GPBn zones and more pronounced S phase precipitation, including type I and II. Differential scanning calorimetry revealed distinct exothermic and endothermic features for the various heat treatment conditions, while thermal scanning resistometry enabled differentiation between GPBn and S phase formation and dissolution. Combined with microstructural analysis, this approach facilitated the identification of overlapping peaks and sub-peaks in differential scanning calorimetry. The integrated methodology demonstrates how nanoscale, thermal, electrical, and mechanical analyses together provide a comprehensive understanding of early precipitation pathways in AA2024 and the modifying role of pre-aging on subsequent natural aging.