Influences of the beam offset on microstructures and performance of fiber laser welded Nb521/Ti dissimilar joints

The influences of different laser beam offsets (LBOs) on the microstructures and mechanical performance of the welded joints were studied. In joints with LBO = +0.3, LBO = 0, and LBO = −0.3, the widths of fusion zones (FZs) are 0.98, 1.03, and 1.21 mm, respectively. Additionally, the equivalent grain diameters in the FZs are measured to be 45.3, 44.1, and 19.6 μm, respectively. The FZ of the joint with LBO = 0 contains β-Ti and Nb solid solution, as well as a small amount of α' phases. The FZ of the joint with LBO = +0.3 mainly consists of Nb solid solution and β-Ti phase. In contrast, the FZ of the joint with LBO = −0.3 is mainly composed of Nb solid solution as well as ω, α′, and α″ phases. The FZ of the joint with LBO = 0 shows the lowest Vickers microhardness (149.2 ± 10.4 HV on average), which decreases by 18.9% and 40.0% when composed with those of joints with LBO = +0.3 and LBO = −0.3. The joint with LBO = 0 is found to have the smoothest transition of the microhardness from the FZ to the base metal, which favors the performance of the joints. The average tensile strength values of joints with LBO = +0.3, LBO = 0, and LBO = −0.3 are separately 318.8 ± 20.6, 315.4 ± 19.7, and 317.9 ± 23.9 MPa. All tensile specimens are fractured on the side of Ti, which suggests the superior tensile performance of the welded joints.