Effect of laser offset on microstructure and mechanical properties of Nb521/TA15 dissimilar joints

The Nb521/TA15 dissimilar joints were fabricated using laser center welding and laser offset welding techniques. The microstructures and mechanical properties of joints with three laser beam offsets (LBO = −0.2 mm, 0 mm, +0.2 mm) were systematically investigated, and high-temperature tensile tests were conducted on the center-welded joint. The joints with LBO = −0.2 and LBO = 0 are well-formed, but there are lack-of-fusion defects on the Nb521 side of the LBO = +0.2 joint. The fusion zone of the LBO = −0.2 joint primarily comprised Nb solid solution and β-Ti phases, while the LBO = 0 joint exhibited Nb solid solution, β-Ti, α and α′ phases. In contrast, the LBO = +0.2 joint was dominated by α and α′ phases. Solid solution strengthening occurred in the LBO = −0.2 and LBO = 0 joints, with their fusion zone microhardness values intermediate between those of the Nb521 and TA15 base metals. The LBO = +0.2 joint, predominantly composed of melted TA15, exhibited higher fusion zone microhardness than the TA15 base metal due to martensite formation. The tensile strengths of LBO = −0.2, LBO = 0, and LBO = +0.2 joints were 516.4, 540.2, and 75.6 MPa, respectively. Both LBO = −0.2 and LBO = 0 joints demonstrated excellent room-temperature tensile properties, with fractures occurring in the Nb521 base metal. However, the LBO = +0.2 joint fractured near the Nb521 interface due to significant lack-of-fusion defects. The LBO = 0 joint exhibited superior high-temperature tensile properties, with strengths of 383.2 MPa, 322.5 MPa and 100.4 MPa at 300 °C, 600 °C an 900 °C, respectively, and fractures consistently located in the Nb521 side.