Surface microstructuring strategy for suppressing hump defects in ultra-high-power laser penetration welding of thick plates: Experimental and numerical insights

Ultra-high-power laser enables single-pass welding of thick-section components with high efficiency and low cost. However, its application is hindered by the frequent occurrence of hump defects. In this work, a method based on surface microstructuring of the butt joint cross section is proposed to suppress hump defects, achieving well-formed single-pass welding of 20 mm thick stainless steel. A computational fluid dynamics model incorporating surface microstructural features is developed and shows good agreement with experimental results. Surface microstructuring introduces two direct effects: it facilitates molten pool expansion and enhances laser energy absorption. These effects promote the transition of the keyhole from a non-penetrating to a penetrating state. The melt near the keyhole wall exhibits a higher tangential velocity while the downward flow of molten material is suppressed. No low-speed zone is observed at the bottom of the molten pool, enabling rear-side melt contraction driven by surface tension, which contributes to hump suppression. The resulting weld exhibits finer grains and a higher fraction of low-angle grain boundaries. The tensile strength and elongation reach 96 % and 65 % of those of the base metal, respectively. The changes in weld microstructure are attributed to the higher temperature thermal history experienced by the molten pool. The improved mechanical properties are primarily due to the enhanced weld formation quality. The proposed method requires no additional auxiliary equipment during welding and provides a promising solution for the industrial application of ultra-high-power laser single-pass welding in thick-section component manufacturing.