Effect of Oscillating Laser on The Molten Pool Dynamics in Narrow-gap Welding of 5A06 Aluminum Alloy.

01 Introduction
5A06 aluminum alloy, with its high specific strength and excellent thermal conductivity, has become a core material for lightweight thick-plate structures in aerospace and new energy vehicles. Laser welding technology, due to its high efficiency and low heat input, is considered an ideal process for manufacturing such thick-section components. However, conventional laser welding faces significant bottlenecks in narrow-gap thick plate welding: the energy distribution is overly concentrated, making it difficult to achieve uniform fusion between the filler wire and groove sidewalls, which easily leads to lack-of-fusion defects. These defects can cause stress concentration, significantly reducing joint fatigue life and severely affecting structural integrity. To overcome these inherent limitations, laser beam oscillation has emerged as an effective strategy. By spatially modulating the beam, oscillation significantly enlarges the effective heat source area, promotes sidewall fusion, and improves weld quality.

02 Overview
This study focuses on the process optimization of narrow-gap laser welding (NGLW) for 5A06 aluminum alloy thick plates. By combining in-situ monitoring with numerical simulation, it investigates the influence mechanism of laser oscillation modes on molten pool dynamics and weld formation quality. To address the lack-of-fusion defects commonly observed in conventional non-oscillating laser welding, a circular laser oscillation method is introduced. A thermo-fluid coupling model considering plasma plume energy attenuation is established to capture molten pool flow behavior and weld morphology evolution. Both experimental and simulation results show that, compared with non-oscillating laser welding, circular oscillation reduces plume height and laser energy attenuation rate. It also alters the molten pool flow direction—from sidewall-to-center convergence to center-to-sidewall diffusion—thereby enhancing the interfacial bonding between filler metal and groove sidewalls and effectively eliminating lack-of-fusion defects.

03 Results and Discussion
Figure 1 compares molten pool surface characteristics under different oscillation modes. It is observed that in non-oscillating laser welding, the molten pool length fluctuates significantly with poor stability. In contrast, circular oscillation expands the effective heat source area, resulting in a more uniform temperature distribution, a longer and more stable molten pool, and effective suppression of irregular fluctuations at the leading edge.

Figure 2 presents a time-sequence comparison between experimentally measured and numerically simulated molten pool lengths for both non-oscillating and circular oscillating laser welding. The simulation results show good agreement with experimental data. Particularly, the maximum deviation for circular oscillation is only 8.9%, confirming the reliability of the numerical model. It also clearly demonstrates that molten pool length fluctuations are larger in non-oscillating welding, while circular oscillation improves stability.

Figure 3 illustrates the fluid flow behavior in molten pool cross-sections. In non-oscillating laser welding, Marangoni convection dominates, causing molten metal to converge from both sides toward the center due to sidewall constraints. This leads to insufficient filling at the groove bottom and results in lack-of-fusion defects. In contrast, circular oscillation introduces periodic stirring and recoil pressure, driving molten metal flow from the center toward the sidewalls. Combined with a more uniform temperature field, this enhances interfacial bonding between filler metal and sidewalls, forming a concave weld profile that effectively suppresses defects.

Figure 4 analyzes the relationship between groove width and maximum weld width under a fixed laser oscillation amplitude of 2.4 mm. As the groove width increases from 3 mm to 6 mm, the maximum weld width shows a slight decreasing trend but remains larger than the groove width, ensuring good weld formation without lack-of-fusion defects. This indicates that under a 2.4 mm oscillation amplitude, the applicable groove width range for circular oscillation laser welding is up to 6 mm.

04 Conclusion
This study systematically investigates the influence of circular laser oscillation on molten pool dynamics in narrow-gap welding of 5A06 aluminum alloy thick plates through combined in-situ monitoring and numerical simulation. It reveals the intrinsic mechanism by which laser oscillation suppresses lack-of-fusion defects and identifies the optimal groove width range of 3–6 mm at a 2.4 mm oscillation amplitude. The results confirm that circular oscillation reduces plasma plume energy attenuation, enhances interfacial bonding between filler metal and sidewalls, and improves weld formation quality. A thermo-fluid coupling model considering plume energy attenuation is established, improving prediction accuracy of molten pool behavior. This work provides both theoretical guidance and technical support for process optimization in narrow-gap laser welding of aluminum alloy thick plates.

References:https://doi.org/10.1016/j.icheatmasstransfer.2025.110479

**--Cite the article published by 高能束加工技术 on April 18th, 2026, in the WeChat public account "High-Energy Beam Processing Technology and Applications."