This study investigates the microstructural evolution and mechanical behavior of Ti–4Al–0.005B alloy T-joints subjected to friction stir welding (FSW) and annealing at 600 and 700°C. Multi-scale characterization is used to reveal relationships between the annealing temperature, microstructure, and properties. The results show that annealing at 600°C significantly promotes dynamic recrystallization (DRX) within the stir zone (SZ), resulting in a fine equiaxed α-phase microstructure, with a recrystallization fraction of 69.27% and high-angle grain boundaries (HAGBs) content of 74.48% in the bottom regions of the SZ. In contrast, the thermo-mechanically affected zone (TMAZ) and heat-affected zone (HAZ) exhibit incomplete recrystallization, retaining a substantial fraction of low-angle grain boundaries (LAGBs). Annealing at 700°C enhances the DRX driving force but also causes abnormal grain growth, reducing the SZ recrystallization fraction to 52.25% and HAGBs content to 51.90%. Kernel average misorientation (KAM) analysis reveals a decrease in the residual dislocation density after annealing at 700°C, indicating enhanced recovery and corresponding grain coarsening, which contribute to a decrease in peak hardness (from HV 340.7 to HV 332.1). Texture analysis shows a pronounced weakening of the basal texture in the SZ at higher temperature (intensity drop from 30 to 15 mud), corroborating the increased DRX activity. Mechanical testing demonstrates that while annealing at 700°C slightly reduces strength, it significantly enhances ductility (elongation from (6.18 ± 0.35)% to (7.25 ± 0.83)% and impact toughness (from (28.13 ± 2.13) to (47.81 ± 4.77) J/cm2). An annealing temperature of approximately 650°C is thus recommended to balance strength and toughness in FSW-treated Ti–4Al–0.005B alloy T-joints.
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