Effect of Shielding Gas And Spot Size on Humping Suppression in High-speed Laser Welding

01 Paper Introduction
High-speed laser welding is a key process for improving production efficiency in the manufacturing of fuel cell bipolar plates. In practical production, it is required to efficiently complete long and narrow weld seams, where increasing welding speed is the core approach to achieving large-scale manufacturing. However, when the welding speed exceeds a critical value, defects such as humping and root sagging are prone to occur. Both defects deteriorate the weld surface quality and significantly reduce mechanical performance, becoming major bottlenecks restricting the development of high-speed laser welding. Existing studies mainly focus on explaining the formation mechanism of humping based on volume conservation theory and Rayleigh instability models, or on observing molten pool and keyhole dynamics via in-situ imaging. However, there is still a lack of systematic research on the mechanisms of defect suppression through shielding gas control and laser spot size adjustment, the quantitative relationship between defects and mechanical properties, and the incorporation of process parameters into defect prediction models. This study investigates ferritic stainless steel foil, exploring the suppression effects of nitrogen shielding gas and reduced laser spot size on humping and root sagging, revealing the molten pool and keyhole dynamic mechanisms of defect suppression, and analyzing the correlation between defect size and T-peel strength.

02 Overview
This study systematically investigates the control of humping and root sagging in high-speed laser welding using SS439L ferritic stainless steel foil. Fiber laser welding experiments were conducted with two laser spot diameters (43 μm and 26 μm) and five different welding speeds as variables, combined with nitrogen shielding gas for comparative analysis. Weld surface morphology was characterized, cross-sectional metallography was analyzed, and T-peel strength was measured to evaluate weld quality. In-situ high-speed synchrotron X-ray imaging was employed to observe keyhole and molten pool dynamic behavior in real time, and a dimensionless humping index model was optimized. The results show that nitrogen shielding suppresses humping to the initial formation stage at a welding speed of 1.00 m/s and slightly improves peel strength. When the spot size is reduced to 26 μm combined with nitrogen shielding, humping is completely eliminated at 1.00 m/s, and root sagging is eliminated at 1.50 m/s, with peel strength stabilized at 28 N/mm. It is also confirmed that humping occurs prior to root sagging, and that root sagging has a more significant negative impact on mechanical performance.

03 Results and Discussion
Figure 1 illustrates the effects of welding speed, spot diameter, and shielding gas on weld defect indicators (hump density, hump height, root sagging depth) and mechanical performance (T-peel strength). As welding speed increases, hump height and root sagging depth increase significantly, while T-peel strength continuously decreases. Nitrogen shielding slightly reduces defect size and improves joint strength. The combination of a 26 μm small spot and nitrogen shielding significantly reduces hump height and completely eliminates root sagging, with T-peel strength stabilized at 28 N/mm.

Figure 2 presents the dynamic evolution of the molten pool and keyhole captured by in-situ high-speed synchrotron X-ray imaging. Under identical welding parameters, comparisons between conditions with and without nitrogen shielding show that nitrogen has minimal effect on keyhole geometry but significantly alters molten pool dynamics. It suppresses periodic violent fluctuations and bulging phenomena observed without shielding gas, stabilizing the molten pool and maintaining it at the initial stage of humping formation. This confirms that nitrogen suppresses humping primarily by stabilizing molten pool behavior.

Figure 3 shows time-dependent curves of instantaneous molten pool length and corresponding sequential micro-images under both conditions. Without nitrogen shielding, the molten pool exhibits periodic abrupt changes in length, triggering humping formation. With nitrogen shielding, these fluctuations are significantly reduced, and the average minimum molten pool length decreases from 672.71 μm to 619.91 μm. A shorter molten pool length effectively reduces the risk of Rayleigh instability.

Figure 4 reveals the physical and chemical mechanisms of humping suppression. It presents the variation of surface tension with temperature and oxygen content in the molten pool, as well as differences in flow direction and morphology evolution under different conditions. Without shielding gas, higher oxygen content leads to abnormal surface tension gradients, causing reverse molten flow collision at the tail of the pool and forming humps. Nitrogen shielding reduces oxygen activity, optimizes the surface tension gradient, and maintains stable unidirectional flow, preventing flow collision and hump formation.

Figure 5 presents the visualization of the optimized dimensionless humping index model. Contour plots show distributions under different laser power and welding speed conditions, dividing process regions into defect-free, initial humping, and fully developed humping zones. A full penetration depth contour of 170 μm is overlaid to validate the model. Both nitrogen shielding and reduced spot size lower the index value, shifting the process window toward the defect-free region. The combination of a 26 μm spot and nitrogen shielding effectively suppresses humping at 1.00 m/s, confirming the model’s accuracy in predicting humping tendency and guiding process optimization.

04 Conclusion
This study systematically investigates the suppression effects of nitrogen shielding and reduced laser spot size on humping and root sagging in high-speed laser welding of stainless steel. The combined strategy significantly improves weld formation and mechanical performance: humping is completely eliminated at 1.00 m/s, root sagging is eliminated at 1.50 m/s, and T-peel strength is stabilized at 28 N/mm. Humping is identified as the initial defect, while root sagging develops as humping intensifies, with the latter having a more severe impact on mechanical properties. Nitrogen suppresses defects by enhancing thermal convection, shortening molten pool length, reducing oxygen content, and optimizing surface tension gradients, while smaller spot size reduces heat input and molten pool flow velocity. Together, these mechanisms suppress defects at the molten pool dynamics level. The optimized dimensionless humping index model incorporates shielding gas and spot size effects, accurately defining process thresholds and enabling quantitative prediction of defect formation, providing a strong theoretical foundation for process optimization and precise defect control in high-speed laser welding.

References: https://doi.org/10.1016/j.jmapro.2025.12.022

**--Cite the article published by 高能束加工技术 on April 19th, 2026, in the WeChat public account "High-Energy Beam Processing Technology and Applications."