Hardness recovery mechanism in the heat-affected zone during long-term natural aging and its influence on the mechanical properties and fracture behavior of friction stir welded 2024Al–T351 joints

The microstructural evolution in the heat-affected zone (HAZ) of 5 mm thick friction stir welded (FSW) 2024Al–T351 joints during long-term natural aging and its effect on mechanical properties were investigated by a combination of transmission electron microscopy, atom probe tomography, differential scanning calorimetry and mechanical property tests. FSW thermal cycle resulted in two low hardness zones (LHZs) in the HAZ: LHZ I, near the nugget zone (NZ), with grain coarsening and the dissolution of Guinier–Preston–Bagaryatsky (GPB) zones and solute clusters, as well as the formation and coarsening of S (Al2CuMg) phases; and LHZ II, far from the NZ, with the dissolution of GPB zones and solute clusters. After 4–12 months of natural aging, the hardness recovered in the LHZ II due to the increase in number densities of Cu–Mg, Cu and Mg clusters, while there was no obvious change in the microstructure and hardness in LHZ I. The tensile strength of FSW 2024Al–T351 joints increased as the welding speed increased from 100 to 400 mm min−1 and was weakly enhanced by the long-term natural aging, but was independent of the rotation rates from 400 to 1200 rpm. The FSW joints fractured along LHZ I under a low welding speed of 100 mm min−1. With the increase of the welding speed and the prolongation of natural aging time, the joints fractured at LHZ I, LHZ II or the interface of the NZ/thermo-mechanically affected zone. The variation in the fracture locations was rationalized based on the microstructural evolution.