**4. Conclusions**

In this study, high-temperature titanium alloy TC31 was joined by the laser oscillating welding method and good mechanical properties were achieved. The appearance, microstructure and mechanical properties of the welded joints were investigated. High-temperature titanium alloy TC31 formed sound joints by the laser oscillating welding method under the optimized process parameters with the weaving frequency of 200 Hz, weaving amplitude of 0.3 mm, laser power of 2000 W and welding speed of 1500 mm/min. The joints exhibit sound bonding without lack of fusion, slag or hot cracks. The welded region was composed of acicular α' phase resulting from the high degree of supercooling during the laser welding process. The welded joints exhibit the highest tensile strength of 1200 ± 10 MPa at room temperature, which is approaching that of the base metal. The tensile strength of the joints at the high temperature of 923 K reaches 638 ± 6 MPa. The welded joints exhibit ductile fracture with dimples at both room temperature and high temperature. Considering the mechanical properties of TC31 laser-weld joints at different temperatures and the potential usage environment of this high-temperature titanium alloy, W1 joint with 200 Hz weaving frequency, 0.1 mm weaving amplitude, 1900 W laser power and 1200 mm/min welding speed, exhibit outstanding mechanical properties in this study. Furthermore, laser oscillating welding is beneficial to the repression of porosity for welding high temperature titanium alloy.

**Author Contributions:** Conceptualization, data curation, investigation and resources, Z.W.; writing—original draft preparation, L.S.; methodology, W.K.; validation, Z.Z.; supervision, W.Y.; funding acquisition, writing—review and editing, C.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Key R&D Program of China, grant number 2017YFB1301603.

**Conflicts of Interest:** The authors declare no conflict of interest.
