Defect formation mechanisms and control strategies for high-performance welding of medium-thick components

Abstract

High-power laser-arc hybrid welding is a critical technology for achieving single-pass double-sided welding of medium-thick components, while the mechanisms of defect formation and suppression, as well as the microstructural effects on mechanical performance under full penetration, remain unclear. This study integrates extensive welding experiments with an advanced ray-tracing based computational fluid dynamics model to systematically reveal the formation mechanisms and suppression strategies of incomplete penetration, root humping, and upper surface collapse. In addition, electron backscatter diffraction analysis clarifies the microstructural strengthening mechanisms governing weld performance. On this basis, both a wide process window for stable weld formation and a refined window for high-performance welding are established. Experimental results show that laser power and welding velocity mainly affect the morphology of the lower weld surface, whereas wire feeding rate predominantly controls the upper surface. Simulations demonstrate that in the incomplete penetration state, the keyhole–molten pool system exhibits quasi-periodic oscillations, driven by the cyclic expansion and contraction of the keyhole bottom opening, resulting in periodic fluctuations of penetration depth. Root humping and upper surface collapse are primarily caused by the violent keyhole fluctuations at the keyhole bottom. Both experiments and simulations confirm that matching high laser power with high welding velocity and wire feeding rate effectively suppresses these fluctuations, reducing the standard deviation of keyhole area variation from 0.094 mm² to 0.065 mm². Under fully penetrated conditions, a moderate heat input intensifies molten pool convection, which leads to dendrite fragmentation and the formation of new intragranular nucleation sites. This process intensifies the lateral competition growth between grains, promotes grain refinement, increases dislocation density, and elevates the fraction of high-angle grain boundaries. Meanwhile, the enlarged mushy zone and extended solidification time facilitate the δ to γ transformation, collectively improving tensile strength. Accordingly, an optimized and wide process window for well-formed welds is defined by laser power of 10–18 kW, welding velocity of 20–36 mm/s, and wire feeding rate of 233–333 mm/s. Within this window, the high-quality and high-strength process window, defined by a laser power of 15–18 kW, welding velocity of 24–36 mm/s, and wire feeding rate of 233–290 mm/s, enables stable full penetration and defect-free morphology on both sides, achieving single-pass welding of 10 mm-scale medium-thick components.