Effect of static annealing on superplastic behavior of a friction stir welded Ti-6Al-4V alloy joint and microstructural evolution during deformation

•A new process of FSW + static annealing + superplastic deformation was proposed to eliminate strain localization.•Similar superplastic abilities were obtained in the BM and NZ with totally different structures.•An optimized view on the classical Langdon theory was put forward in the field of superplasticity.•The mechanisms of the spheroidization of lamellae and the fragmentation of equiaxed grains were discussed.•This study provides a new way to fabricate large-scale complex Ti alloy components.