Ultrastrong heterojunctions without bubble defect in laser joining metal to polymer via two-step strategy

A two-step joining approach was proposed to minimize or eliminate decomposition bubbles in laser joining metal to polymer. Finite element simulation was utilized to analyze the temperature distribution and incorporated the bubble shrinkage mechanism and thermal capillary effect, providing the theoretical basis for explaining the reduction in bubbles and their movement. Two experimental setups were employed: with and without laser beam offset. In experiments without offset, the bubbles after the second joining process underwent significant shrinkage, resulting in a notable reduction in volume. In offset experiments, the bubbles not only reduced in size but also exhibited movement and elimination. This was induced by the laser beam offset, which created a temperature gradient perpendicular to the joining direction, triggering thermal capillary effects that caused the bubbles to migrate towards the hotter side and eventually escape from the joint edges. Consequently, the joints obtained through double welding exhibited higher fracture stress as compared to those obtained through single welding. The theoretical analysis of the bubble reduction mechanism aligned with experimental observations, further validated by camera images.