The stir zone microstructure and its formation mechanism in Ti–6Al–4V friction stir welds

The two-phase α + β titanium alloy, Ti–6Al–4V, was friction stir welded using a W–Re pin tool, and defect-free welds were produced with proper welding parameters. A bimodal microstructure was formed in the stir zone due to dynamic recrystallization and phase transformation. Dynamic recrystallization was the main mechanism for microstructural refinement in the stir zone.