Effect of beam oscillating and nitrogen alloying upon microstructure and mechanical properties in laser welding of molybdenum alloy

The problems of coarse grains and intergranular segregation in molybdenum alloy welding lead to poor mechanical properties of the joints. The experimental study was carried out by using laser beam oscillation and nitrogen alloying. The results show that when using beam oscillating only, the average grain size in the weld zone was reduced by about 28 %, the microhardness of the weld center was increased from 190 HV to 200 HV, and the tensile strength of molybdenum alloy joint was increased from 29.83 MPa to 130.03 MPa. When using nitrogen alloying only (shielding gas 10 % N₂ + 90 % Ar), the microhardness of weld center was increased from 190 HV to 240 HV, and the tensile strength of molybdenum alloy joint was increased from 29.83 MPa to 350.94 MPa. Furthermore, the tensile strength of the joint reached 439.43 MPa which was 67.8% of the tensile strength of the base metal when laser oscillating and nitrogen alloying were used simultaneously, and the fracture mode changes from intergranular fracture to intergranular fracture and transgranular cleavage fracture. The analysis shows that the strengthening effect of nitrogen alloying on the properties of the joint benefits from the formation of Mo₂N phase in the grain and at the grain boundary.