Effect of niobium on the mechanical strength of the laser beam welding joints of molybdenum

The low strength and plasticity of molybdenum (Mo) fusion welding joints are the key problem hindering the application in the engineering structures field. The main reason for the low strength and plasticity of Mo joints is the low grain boundary bonding strength, which is also known as grain boundary brittleness. The segregation of oxygen (O) at the grain boundaries and the distribution of the lamellar MoO2 phases are the intrinsic reasons for brittleness. In this study, the Mo fusion welding joints were processed by laser beam welding. The maximum tensile strength of the unalloyed joint could only reach 169.8 MPa. Meanwhile, the maximum tensile strength of joints with Nb alloyed could reach 426.7 MPa. During the welding, the niobium (Nb) element preferred to reduce the O content and realize the second-phase strengthening at the grain boundaries. The results of XPS and TEM confirmed that Nb mainly existed in the form of Nb2O5 and NbC in the fusion zone (FZ) of joints with Nb alloyed. The SEM and EBSD showed that a certain amount of NbC is precipitated at the grain boundaries, and the amount of MoO2 is significantly reduced. The lamellar MoO2 could be clearly observed on the grain boundary surface of the FZ of the joints without alloying.