Melt flow and grain refining in laser oscillating welding of 13-21S 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 13-21S 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 highspeed 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 mu m, 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 degrees C. Furthermore, the Portevin-Le Chatelier (PLC) effect was observed in the stress-strain curves of welded joint at 450 degrees C, which was attributed to the precipitation of brittle co phase.

Automated summary

This study demonstrated that beam oscillation can improve the microstructures and mechanical properties of welded joints by affecting molten pool behaviors. The use of beam oscillation resulted in defect-free joints with sound weld appearances, higher probability of equiaxed grain formation, and significant grain refinement. Additionally, welded joints made using beam oscillation showed higher tensile strength and the occurrence of the Portevin-Le Chatelier effect due to the precipitation of brittle co phase at elevated temperatures.