Microstructural evolution and mechanical properties of friction stir spot welded pure Ta

Tantalum (Ta), owing to its extremely high melting point and inherent limitations in fusion welding, requires the use of solid-state joining techniques such as friction stir spot welding (FSSW). In this study, the tool rotation speed and plunge rate are held constant, while the plunge depth is varied to identify the optimal welding parameters. The fundamental welding behavior of Ta is elucidated by examining the microstructural and mechanical differences between the upper and lower regions of the single-metal joint. Dynamic recrystallization in the stir zone (SZ) is confirmed by electron backscatter diffraction (EBSD) analysis, with the average grain size reduced from approximately 20.8 μm in the base material to about 1 μm in the SZ. Vickers hardness testing verifies that this microstructural refinement results in increased hardness within the welded zone. The present study represents an initial systematic investigation of FSSW for Ta, highlighting its feasibility for high-integrity joining applications. • FSSW yields defect-free Ta joints, proving a robust solid-state joining method. • CDRX refines SZ grains to ∼5 % of the base metal grain size. • EBSD shows SZ has recrystallized grains and TMAZ has deformed/substructured ones. • Upper/lower weld regions show distinct microstructures from thermal asymmetry. • Hardness map shows SZ hardening via Hall–Petch from grain refinement.