Laser welding of molybdenum socket joint for ultra-high-temperature heat pipes based on niobium alloying

The influence of Ti and Zr, Nb alloying on the microstructures and performance of laser-welded molybdenum socket joints was investigated. Following Nb alloying, the average microhardness of the fusion zone (FZ) increased from HV 194.7 to HV 283.3. Additionally, Nb can react with O to form dispersed Nb₂O₅ along grain boundaries, impeding grain boundary migration and dislocation movement while reducing the content of volatile Mo oxide along these boundaries. The incorporation of Nb in FZ partially inhibits pore defects and enhances joint load- bearing capacity. In comparison to the laser-welded joints without adding Nb (LW), the tensile strength of the laser- welded joints with Nb alloying (LW-Nb) was significantly improved by approximately 69% from 327.5 to 551.7 MPa. Furthermore, the fracture mechanism of the joints transitioned from intergranular fracture to transgranular fracture.