Microstructural and mechanical properties of laser-welded molybdenum joints with the addition of nano-sized ZrC in the weld pool

Effects of adding nano-sized ZrC in the fusion zone (FZ) on the microstructures and properties of laser-welded joints of pure molybdenum (Mo) and underlying mechanisms were explored. Compared with the joints without ZrC, the average microhardness of the FZ increases from 180 HV to 330 HV, the tensile strength of joints grows from 32 MPa to 386 MPa, and the fracture mode of joints switches from intergranular fractures to that dominated by cleavage fractures, after adding ZrC. ZrC, ZrO₂, and Mo₂C are detected in the FZ after adding ZrC. ZrC and ZrO₂ is mainly found on grain boundaries (GBs). ZrC is able to deplete O in the FZ, thus decreasing the amount of harmful molybdenum (Mo) oxides on GBs, so it plays a role in purifying and strengthening GBs. ZrC, ZrO₂, and Mo₂C particles not only hinder dislocation movement and GB movement, but also serve as effective heterogeneous nucleation cores and therefore can refine grains in the FZ. The average grain size in the FZ of the joint without ZrC is 47.6 μm, while it is 28.4 μm after adding ZrC, which reduces by 40.3 % compared with the former. The significant improvement of the joint strength after adding ZrC is a result of joint action of dispersion strengthening, fine-grain strengthening, and GB purification.