Fiber Laser Fillet Welding of Nb1Zr Thin Tube and Molybdenum End Plug in Ultra-high-Temperature Heat Pipe

Fillet welding methodology of molybdenum end plug and Nb1Zr thin tube was explored by presetting niobium filler material with square cross section and inserting titanium foil between the filler material and the end plug. It was found that the incidence angle of laser beam greatly affected the energy density on workpiece surface and the intensity of metal evaporation. The weld quality was poor under thermal conduction welding mode, due to the geometric asymmetry of the joint and the difference of physical properties between Mo and Nb1Zr. Under deep penetration mode, the melting behavior of the filling material was dominated by recoil pressure induced by intense evaporation and fillet joint free of defects was obtained. Analysis of microstructures and performance revealed that various elements were distributed uniformly in the fusion zone (FZ) of the fillet joints, without brittle intermetallic compounds (IMCs). Mixture of Mo and Nb in the FZ presented a significant solid-solution strengthening effect, while the TiO₂ and Nb₂O₅ phases generated during welding were dispersed in the FZs, causing dispersion strengthening and grain boundary strengthening. The larger number of high-angle grain boundaries (HAGBs) in the heat-affected zone (HAZ) was responsible for the softening of HAZ. The average microhardness of FZ was much higher than that of base metal (BM) in both sides, reaching 399.8 HV. The joints were fractured in the HAZ in the Nb1Zr side in the tensile tests, where the tensile strength (395.6 MPa) declined slightly compared with that of the BM (408.2 MPa), showing ductile fractures.