Hot Cracking Characteristics During Single-Mode Fiber and Green Laser Welding Processes in Lithium-Ion Battery Pack Manufacturing

Abstract

Solidification cracking during lithium-ion battery packaging was metallurgically investigated, specifically for Cu-steel dissimilar materials. To this end, single-mode fiber and green lasers were employed under heat input conditions ranging from 1.3 to 8.0 J/mm. For both laser welds, solidification cracking was concentrated in the steel region of the fusion zone, particularly in the locally Cu-depleted region, regardless of the welding condition. Modified self-restraint tests were performed for overlapping dissimilar material combinations to elucidate the mechanism of solidification cracking. Analysis of the solidification cracking surface revealed that approximately 15?30 mass% Cu existed on the surface. Cu was highly enriched with a droplet shape, formed during solidification within the miscibility gap. By calculating the non-equilibrium weld mushy zone range based on the diffusion-controlled Scheil’s model, the solidification cracking in the Cu-depleted region was estimated at 453 K. It was strongly affected by the severe segregation of Cu (95.7 mass%) in the residual liquid at the terminal stage of the solidification path. Therefore, from a welding metallurgical perspective, homogeneous Cu distribution and minimization of Cu segregation within the fusion zone are essential for suppressing or minimizing the solidification cracking susceptibility of Cu?steel dissimilar laser welding.