In-situ formation of B2-MoNi and its strengthening mechanisms in vacuum laser welded joints of Mo alloy

The intrinsic brittleness and ultra-high melting point of molybdenum alloys result in poor weldability, with the tensile strength of welded joints typically falling below 150 MPa. In this work, fine and dispersed B2-type MoNi phases were successfully induced during vacuum laser welding of Mo–5Re tubes by incorporating a nickel foil (0.1 mm thickness), utilizing rapid solidification and in-situ reaction mechanisms. The results showed that the B2-type MoNi phases were distributed along grain boundaries and interdendritic regions, exerting pronounced Zener pinning effects that suppressed grain growth. Additionally, semi-coherent MoNi–Mo interfaces featuring nanoscale transition layers were formed, effectively reducing lattice mismatch. Consequently, the fusion-zone grain size was refined from 125.2 μm to 42.4 μm, and the dislocation density increased by 41%. The finely dispersed MoNi particles acted as secondary-phase strengtheners, cooperatively enhancing joint performance. Therefore, the tensile strength of the welded joint increased from 149 MPa in the Ni-free condition to 373 MPa of the Mo–0.1Ni joint, although excessive Ni caused segregation-assisted coarsening and strength degradation. Quantitative analysis indicated that grain refinement, dislocation strengthening, and MoNi-induced precipitation strengthening contributed comparably to the strength increment, with precipitation strengthening providing the largest share. This study demonstrates that in-situ MoNi formation under vacuum pressure constitutes an effective strategy to overcome weldability constraints in refractory Mo-alloy joints.