To address the insufficient strength of friction-stir-welded (FSW) ultra-high-strength Al–Cu– Li alloy joints, the effects of post-weld heat treatment (PWHT) on microstructural evolution and mechanical properties were systematically investigated. The as-welded joint showed a “W”-shaped microhardness profile, with the minimum value located in the thermomechanically affected zone (TMAZ), mainly caused by the dissolution of T1 phases and precipitation of coarse AlCu, AlCuMg, and AlCuMn phases during welding. Direct artificial aging at 155 ◦C for 24 h failed to improve joint strength due to solute depletion induced by pre-existing coarse secondary phases. Solution treatment re-dissolved coarse precipitates into the matrix, and subsequent aging led to uniform precipitation dominated by T1 and θ′ phases, with a consistent microhardness of ~155 HV across all zones. By introducing pre-stretching deformation after solution treatment, T1 became the dominant strengthening phase in all regions, accompanied by a remarkable increase in both microhardness and tensile strength. With 3% pre-stretching, the microhardness reached 185 HV, and the ultimate tensile strength of the joint reached 600 MPa, corresponding to a joint efficiency as high as 95%, which is superior to most reported values for Al–Li alloy FSW joints. This study clarifies the precipitation evolution mechanism under tailored PWHT and provides an effective strategy for property regulation of high-performance Al–Cu–Li alloy FSW structures in aerospace applications.
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