Heterogeneous connection of N6 nickel and Zr-2Hf annular lap joint by vacuum electron beam welding: Processing, microstructure characterization, and crack formation mechanism

Crack formation poses a major challenge in the vacuum electron beam welding of N6/Zr-2Hf annular lap joint. To investigate the crack formation mechanism, a comparative study was conducted using both microstructural characterization and finite element simulations on tube/rod joint, with direct welding and offset welding methods. The results demonstrated that the key factors contributing to joint cracking include the presence of large size brittle Ni₅Zr phases, the unstable phase interface between γ-Ni/Ni₅Zr, and high residual stresses within the weld. Compared with the direct welding, Ni0.3-offset welding could effectively control the melting proportion of the base metals in the weld. The size of Ni₅Zr in the weld decreased, while the proportion of γ-Ni + Ni₅Zr ultra-fine eutectic phase increased, and the microstructure of the weld was optimized. When the beam was offset 0.3 mm to Ni side for welding, the peak stress of the weld decreased from 413 MPa to 320 MPa, which reduced the possibility of cracks in the weld. The tensile strength of the joint was increased from 82 MPa to 138 MPa, and the reliability of the joint was improved. This study provided some guidance for the subsequent welding of Zr/Ni dissimilar materials.