Investigation of the Phosphorus Effect on Solidification Cracking in Cu–Steel Single-Mode Fiber-Laser Welds for Reliable Li-Ion Battery Busbar Assembly

Solidification cracking is a critical defect in Cu–steel dissimilar laser welding for cylindrical lithium-ion battery busbar assembly, yet the metallurgical role of phosphorus (P) in crack formation has not been quantitatively established. In this study, the influence of phosphorus in the coating layer on weld solidification behavior was clarified by preparing Cu substrates with four different coating conditions—Ni–P-coated Cu (10 and 50 μm) and pure Ni-coated Cu (10 and 50 μm)—and performing high-speed single-mode fiber-laser welding under identical heat-input conditions. Shear-tensile testing, EPMA-based microstructural analysis, and Thermo-Calc solidification calculations were combined to correlate P segregation with solidification cracking susceptibility. The Ni–P 10 μm coating generated severe solidification cracking compared with the pure Ni 50 μm coating, which was attributed to excessive P enrichment in the terminal liquid phase (up to 8.8 mass%). This enrichment significantly expanded the mushy-zone width to approximately 869 K, yielding a highly solidification crack-susceptible fusion zone. In contrast, 50 μm pure Ni coatings produced narrow mushy-zone widths (200–400 K) and extremely low residual P levels (~0.1 mass%), resulting in fully crack-free microstructures. The 50 μm Ni coating exhibited the highest shear-tensile strength and largest rupture displacement among all conditions, confirming that suppression of P segregation directly improves both structural integrity and mechanical performance. Overall, this study demonstrates that phosphorus enrichment critically governs the solidification-cracking susceptibility of Cu–steel dissimilar welds by widening the solidification temperature range. Eliminating P from the coating layer and applying an adequately thick pure Ni coating constitute highly effective strategies for achieving crack-free, mechanically robust welds in lithium-ion battery busbar manufacturing.