Influences of Si-Ti combined alloying on microstructures and properties of laser welded socket joints of molybdenum

This study selected silicon (Si) and titanium (Ti) as alloying elements to explore influences of the Si:Ti mass ratio on performance of molybdenum bead on plate laser welded joints. When the Si:Ti ratio was 1:1, the silicides exhibited a uniform continuous distribution, and significantly increased room temperature tensile strength of molybdenum bead on plate from 48 MPa to 231.6 MPa. In addition, Si-Ti combined alloying was applied to Mo tube-end plug socket joints with the optimal Si:Ti ratio. Results showed that Si and Ti were uniformly distributed in the weld of laser welded socket joints. Addition of Si and Ti in the weld increased the fraction of low-angle grain boundaries (LAGBs), which was conducive to improving the bonding strength of grain boundaries. The second-phase particles Mo₃Si and Ti₅Si₃ were observed in the weld after adding Si and Ti, which played a role in second-phase strengthening. The room-temperature tensile strength of laser welded socket joints of Mo with the Si:Ti mass ratio of 1:1 reached 301 MPa, and the joint showed mixed fractures. The fractures in the FZ exhibited morphologies of cleavage fractures, while those in the heat affected zone (HAZ) showed mixed morphologies of intergranular and cleavage fractures. Finally, the results of high-temperature nanoindentation creep testing showed that after applying a load of 8 mN for 600 s at 400℃, the creep displacement of the LW-socket joint was measured at 20.28 nm, whereas the LW-SiTi socket joint exhibited a creep displacement of only 6.63 nm. The research findings of this paper provide references for alloying methods in laser welding of molybdenum and molybdenum alloys.