Effect of Ti–N composite alloying on the microstructure and mechanical properties of laser-welded pure molybdenum joints

Molybdenum welded joints suffer from high brittleness and low strength, which seriously restricts their application as heat pipe materials for space nuclear reactors. The microstructure and mechanical properties of three types of laser-welded joints, including non-alloyed (LW), single Ti-alloyed (LW-Ti), and Ti–N composite alloyed (LW-Ti-N₂), were comparatively investigated.The results show that the average microhardness of the fusion zone of the LW joint is 191.4 HV, lower than that of the base metal (∼240 HV); the fusion zone microhardness of the LW-Ti and LW-Ti-N₂ joints increases to 240.8 HV and 299.8 HV, respectively. The tensile strength of the LW joint is only 33.8 MPa; the room-temperature tensile strength of the LW-Ti-N₂ joint reaches 237.7 MPa, approximately 1.7 times that of the LW-Ti joint, and the displacement elongation increases by about 3.5 times. EBSD analysis reveals that the solid solution strengthening of Ti and the grain boundary purification effect of TiO₂ are the main reasons for the performance improvement of the LW-Ti joint. In the LW-Ti-N₂ joint, in-situ formed second-phase particles including grain-boundary-distributed TiN and TiO₂, as well as diffusely distributed Mo₂N, are generated. The combined effect of second-phase strengthening from TiN/Mo₂N and grain boundary purification by TiO₂ remarkably enhances the comprehensive mechanical properties of the joint. Furthermore, the LW-Ti-N₂ joint maintains a high-temperature tensile strength of 133 MPa at 1100°C.