Melt flow and grain refining in laser oscillating welding of β-21S titanium alloy

• Average grain sizes were reduced from 236 μm to 168 μm by oscillating laser beam. • Reasons of grain refining are remelting and lower temperature gradient. • Brittle ω induced Portevin-Le Chatelier effect first found in β-21S titanium alloy. Properties and serviceability of laser welded joints depend on the defect, weld geometry, and microstructures, which in turns are affected by molten pool behaviors. In this work, beam oscillation was used to improve microstructures and mechanical properties of welded joints by changing molten pool behaviors. The 2 mm-thick β-21S titanium alloy plates were welded using three beam oscillating modes (non-oscillation, transverse oscillation, and vertical oscillation). The molten pool behaviors for three oscillation modes were observed using high-speed cameras, and the weld morphology, microstructures and mechanical properties of the corresponding joints were examined. The results showed that defect-free joint with sound weld appearances could be achieved using beam oscillation that affected molten pool behaviors. The higher probability of equiaxed grain formation and significant grain refinement were found in the joints made using beam oscillation. The average grain sizes of welded joints for non-oscillating, transverse and vertical oscillating modes are 236, 184 and 168 μm, respectively. The welded joint made using beam oscillation also showed a higher tensile strength of 900 MPa at ambient temperature and 534 MPa at 450 °C. Furthermore, the Portevin-Le Chatelier (PLC) effect was observed in the stress-strain curves of welded joint at 450 °C, which was attributed to the precipitation of brittle ω phase.