The formation mechanism and process regulation of collapse defects in high-power laser penetration welding of 20mm thick 316L stainless steel plates

Abstract

Laser penetration welding provides significant efficiency advantages for single-pass joining of thick stainless steel plates. However, as plate thickness increases, the challenge of "full penetration leads to collapse" persists. This study successfully achieves single-pass welding formation of 20 mm thick steel plates, identifying the process bottleneck in high-power thick-plate laser full-penetration welding as the excessively large output laser spot size and addressing the welding challenges associated with thicker plates. A novel numerical model was employed to explore the mechanisms behind collapse defects. To quantitatively assess weld quality, the degrees of "underfill" and "sagging" were evaluated based on national standards, and various weld formations were categorized accordingly. The results indicated that welding with a large spot size of 937.5 μm rarely produced a well-formed weld. In contrast, using a smaller spot size of 600 μm created a partial process window, where the laser power threshold for "full penetration leads to collapse" decreased from 25,000 W to 20,000 W. High-speed imaging and numerical simulations further revealed two critical factors necessary for achieving high-quality weld formation. First, the mass of molten material lost as spatter and droplets must remain minimal. Second, a well-defined backflow channel must be established, ensuring the upward movement of molten material under the influence of the Marangoni effect. This study highlighted the crucial role of spot size in the laser penetration welding of thick plates and provides guidance for selecting optimal laser parameters. Additionally, it elucidated the underlying mechanisms of weld formation, offering theoretical insights for process regulation.