01 Introduction
In the manufacturing of large components such as high-speed trains, shipbuilding, and energy equipment, thick plate welding is one of the key processes. However, due to limitations in machining accuracy, assembly errors, and thermal deformation during the welding process, the weld gap often changes. When the gap between plates is small, incomplete penetration or root ripples are likely to occur, whereas large gaps tend to result in weld collapse. Current research is mostly based on constant gap conditions, and studies on welding with variable gaps are relatively lacking. In particular, in laser–arc hybrid welding, achieving both ripple suppression under small gaps and good bridging capability under large gaps remains a challenge in engineering applications. This study focuses on 12 mm-thick weathering steel, aiming to clarify the weld formation and defect suppression mechanisms during oscillating laser–arc hybrid welding under variable gap conditions, providing theoretical and process support for thick plate welding with variable gaps, and promoting the further industrial application and adoption of oscillating laser–arc hybrid welding technology.
02 Full Text Overview
This study addresses the challenges of root humps and insufficient bridging capability in thick steel plate variable-gap laser-arc hybrid welding and systematically investigates the mechanism by which oscillating lasers affect the welding process. The experimental base material was 12 mm thick S355J2W weathering steel. A hybrid welding system was constructed using a TruDisk-10002 fiber laser (maximum power 10 kW, wavelength 1070 nm) in combination with arc welding equipment, with a continuously varying assembly gap (0 - 3 mm) set along the entire weld seam to simulate the variable-gap conditions commonly encountered in actual production. During the study, laser power (6.5 kW), welding speed (16 mm/s), and wire feed speed (10 m/min) were kept constant, with laser oscillation parameters (amplitude, frequency) as the core controlled variables in the experiments. High-speed photography was used to synchronously record the molten pool behavior and arc morphology on the front and back sides of the weld. Additionally, the PIVlab toolbox in MATLAB was employed to perform cross-correlation analysis on the high-speed imagery of the molten pool, quantitatively extracting the liquid metal velocity field and vorticity field during the formation of humps. This method converts flow visualization data into quantifiable physical parameters (velocity, vorticity), providing solid data support for revealing the mechanism of hump formation. Regarding the analysis of arc morphology, the researchers precisely assessed the effect of the oscillating laser on arc behavior by calculating the standard deviation of the arc deflection angle. Ultimately, under oscillation parameters of 1.5 mm amplitude and 200 Hz frequency, good weld formation without humps or collapse was achieved across a variable gap range of 0-2.5 mm. Comprehensive analysis indicated that the closure of the keyhole leads to root hump formation, whereas the oscillating laser effectively suppresses hump formation by stabilizing the keyhole, improving molten pool fluidity, and increasing surface tension at the tail of the molten pool.
Figure 03 illustrates a direct comparison of the decisive impact of different oscillation parameters on the formation of variable-gap welds. Without laser oscillation, a root hump occurs at a small gap (1 mm), and as the gap increases, surface collapse appears, indicating poor gap adaptability. Changing the laser oscillation parameters improves the front-side formation, but the backside still has humps or the weld becomes narrower. The final parameters are an amplitude of 1.5 mm and a frequency of 200 Hz. Within the entire variable-gap range, excellent welds without humps or collapse are achieved on both sides, demonstrating the key role of optimizing oscillation parameters.
Figure 1. Weld formation under different welding parameters. The weld width varies from 0 mm to 3 mm along the welding direction: (a) No oscillation; (b) Oscillation amplitude 1 mm, frequency 100 Hz; (c) Oscillation amplitude 1.5 mm, frequency 100 Hz; (d) Oscillation amplitude 1.5 mm, frequency 200 Hz.
Figure 2 shows that within one cycle, without oscillation, the arc deflects irregularly to the left and right, whereas with an oscillating laser, the arc remains stably centered, with a full and stable shape, showing no significant lateral deflection. This demonstrates that under conditions without an oscillating laser, the large gap itself is the fundamental cause of arc shape instability. The arc tends to seek the nearest conductive path (i.e., the sidewall of the groove), resulting in uneven heating. The introduction of an oscillating laser, regardless of whether the parameters are optimal, can greatly suppress the lateral deflection of the arc and keep it stable in the center of the weld.
Figure 2. Weld morphology at different welding speeds: (a) 1.5 m/min (b) 1.8 m/min (c) 2.1 m/min.
Figure 3 quantifies the℃of arc deflection. Without laser oscillation, the standard deviation of the deflection angle is 23.6℃, indicating severe arc fluctuation; after using oscillating laser, the standard deviation drops to 3.5℃, with stability improving by 85.2%. This provides data evidence that 'oscillating laser can significantly stabilize the arc.'
Figure 3. Measurement of arc deflection angles six times under a 2.5 mm gap: (a) Schematic diagram of arc deflection angles; (b) Degree of arc deflection under different parameters. The difference between 1 and 2 represents the℃of arc deflection.
Figure 4 illustrates that during the welding process, molten metal flows toward the keyhole in the form of waves, causing the keyhole to fluctuate violently and collapse. Laser oscillation can enhance thermal convection in the molten pool, forming vortices near the keyhole. Molten metal flows from around the keyhole to its tail, cushioning the impact of droplets and keeping the keyhole stably open. This indicates that oscillating lasers can stabilize the welding process by altering the flow field of the molten pool.
Figure 4. Melt pool flow from time T0 to T0 + 2.7 ms under zero gap conditions: (a) No laser oscillation; (b) Amplitude 1 mm, frequency 100 Hz; (c) Amplitude 1.5 mm, frequency 200 Hz. Yellow and green arrows indicate the vortices generated by the oscillating laser and the flow direction of the molten metal, respectively; white and orange lines indicate the keyhole and molten droplets, respectively.
Figure 5 illustrates the dynamic behavior of the molten metal in the weld pool under non-optimized oscillation parameters (amplitude 1 mm, frequency 100 Hz) as the root hump is forming, advancing the study of welding defects from macroscopic morphological observation to a new level of quantitative fluid dynamics analysis. The velocity vector distribution shows the direction and magnitude of the molten metal flow within the weld pool, while the velocity field more intuitively displays the spatial distribution of flow speed. At the same time, high vorticity values exist in the hump formation region, indicating strong rotational or shear flow of the liquid there. This rotational flow pattern promotes the accumulation and unstable growth of molten metal, which is a typical flow field characteristic of hump formation.
Figure 5. Particle image velocimetry results at different moments during root hump formation: (a) velocity vector distribution; (b) velocity field distribution; (c) vorticity field distribution. Yellow and white dashed lines indicate the contour of the hump.
04 Summary: This study addresses the industry challenges of root humps and insufficient gap-bridging capability in thick plate variable-gap laser-arc hybrid welding. Through systematic experiments combined with advanced diagnostic techniques such as high-speed imaging and particle image velocimetry, the defect suppression mechanism of oscillating laser was revealed. The results indicate that under optimized oscillation parameters, the laser, by enlarging and stabilizing the keyhole, significantly enhances the arc conductive channel, reducing the℃of arc deflection by 85.2%, thereby stabilizing arc behavior. At the same time, the oscillating laser alters the melt pool flow field, forming a stable vortex and maintaining keyhole openness, ultimately achieving high-quality welds free of humps and collapse in a variable gap range of 0-2.5 mm. This study not only deepens the theoretical understanding of welding defect formation and suppression mechanisms from a fluid dynamics perspective but also provides a reliable process scheme and theoretical basis for solving the variable-gap welding challenges in large component manufacturing, which is of significant value for promoting the application of laser-arc hybrid welding technology in major engineering projects.
