Effect of Mo Content on Microstructures and Mechanical Properties of TZM/CoCrFeNiMo_x/Q235 Joints by Electron Beam Welding

The electron beam welding joints of TZM molybdenum alloy and Q235 steel are achieved using CoCrFeNiMo_x as the interlayer material. The effects of Mo content in the interlayers on the microstructures and mechanical properties of electron beam-welded TZM/Q235 joints are systematically investigated. There is an obvious Fe–Mo reaction layer between the TZM and the weld. The phase compositions of the fusion zone are face-centered-cubic (FCC) solid solutions, and a minor fraction of intermetallic compounds (ICs) distributes between the grains. The hard and brittle ICs in the reaction layer lead to a substantial elevation in microhardness, and the microhardness of the fusion zone increases with the increase of Mo content of the interlayer. All specimens undergo fracture exclusively at the reaction layer adjacent to the TZM side, exhibiting a brittle fracture mode. The reaction layer of TZM/CoCrFeNiMo_0.2/Q235 joint presents a fine eutectic structure composed of FCC solid solutions and ICs. This unique microstructural configuration contributes favorably to enhancing the overall mechanical properties of the joint, and the tensile strength of the joint is 267.8 MPa. Compared with direct welding, the tensile strength of the TZM/CoCrFeNiMo_0.2/Q235 joint is increased by 57%.