Correlation Between Solidification Cracking and Composition Distribution in Green Laser Welds of Lithium-Ion Battery Busbars

This study investigates the solidification cracking susceptibility of Cu-Steel laser welds used for cylindrical lithium-ion battery busbars. The research utilized green laser technology with three distinct beam patterns: Linear, Spiral + Wobble, and Circular + Wobble. Additionally, Ni-P coating layers of varying thicknesses (10, 50, 100 μm) were applied to the Cu material to examine their effect on weld quality. The high absorption rate of green laser by Cu materials significantly enhances weld stability, reducing common defects such as cracks and porosity, which are often observed in other laser sources. This study employed microstructural analysis and shear tensile testing to evaluate solidification cracking behavior, and found that cracking susceptibility strongly depends on the beam pattern and the thickness of the Ni-P coating. Among the tested beam patterns, Circular + Wobble showed superior performance, effectively suppressing cracking by narrowing the weld mushy zone range and alleviating P segregation. Notably, crack-free welds were achieved with 100 μm Ni-P coatings under this beam pattern. Furthermore, a comparative analysis with a single-mode fiber laser revealed that green lasers were more effective in maintaining stable and suppressed solidification cracking susceptibility. These findings emphasize the potential of green laser welding as a robust and efficient process for dissimilar material joining in lithium-ion battery manufacturing, offering enhanced reliability and mechanical properties.