Formation of Porosity in Laser Welding of Medium-Thick Aluminum Alloys with Different Laser Oscillation Paths

Scholars from Shanghai Jiao Tong University published an article titled "Formation and migration of bubbles under different laser oscillation paths during laser welding process of medium-thick Al alloy" in the International Journal of Heat and Mass Transfer.

01 Paper Introduction

In aluminum alloy laser welding, pore formation is a key challenge affecting weld quality, especially in medium-thickness plates. Existing studies have attempted to suppress porosity by adjusting welding parameters (e.g., reducing heat input, increasing speed) or using dual-beam laser welding, but vacuum-assisted methods are limited by equipment complexity. Oscillating laser welding techniques (e.g., spiral or infinity oscillation) show promise by optimizing energy distribution and molten pool dynamics. However, a systematic comparison of the pore formation mechanisms under different oscillation paths (non-oscillation, spiral, and infinity) remains incomplete. This paper focuses on three oscillation modes—Non-oscillation (N-mode), Circular/spiral oscillation (C-mode), and Infinity oscillation (I-mode)—and deeply analyzes energy distribution, keyhole stability, and bubble escape behavior through combined numerical simulations and experiments, providing new insights for controlling aluminum welding defects.

02 Full Paper Overview

A 3D transient numerical model was developed in this study, incorporating multi-reflection heat source and adiabatic bubble models, to compare the effects of N-mode, C-mode, and I-mode laser paths on porosity in medium-thickness aluminum alloy welding. The model considered energy distribution, keyhole behavior, and molten pool flow: oscillation modes significantly reduced energy density, improved keyhole stability, and decreased bubble generation. Additionally, oscillation paths enhanced molten pool stirring and altered fluid flow regions (e.g., reducing the clockwise bottom flow zone), promoting bubble escape. Experimental results confirmed that the I-mode was most effective in reducing porosity and optimizing weld morphology.

03 Visual Analysis

Figure 1 shows the energy density distribution along the path within a single cycle. N-mode, due to uniform movement, maintained a high and uniform energy density (peak 1466 J/mm²), while C-mode and I-mode significantly reduced peak energy due to higher linear velocity (reduced to 173 J/mm² and 124 J/mm², respectively). This dispersed energy distribution is the fundamental mechanism of oscillation welding in suppressing keyhole instability.

Figure 1. Energy density distribution along the weld path within a single cycle.

Figure 2 illustrates the molten pool contours and weld cross-sections under different oscillation modes. The molten pool length under I-mode reached 11.48 mm (simulation: 10.46 mm), far exceeding N-mode’s 7.94 mm (simulation: 7.28 mm). Meanwhile, I-mode reduced penetration depth from 6.68 mm (N-mode) to 5.12 mm and increased weld width from 2.29 mm to 2.38 mm, proving that dispersed heat input optimized molten pool morphology.

Figure 2. Molten pool profile and weld cross-section for different oscillation modes.

Figure 3 displays the interaction between the keyhole and bubbles during different oscillation welding modes. It can be seen that the oscillating keyhole captures bubbles moving into the molten pool, increasing bubble escape efficiency. The I-mode, with a swing distance twice that of C-mode, effectively captures bubbles and increases the escape rate to 90%.

Figure 3. Interaction between keyhole and bubble during welding with different oscillation modes.

Figure 4 presents the temperature and flow fields in the molten pool under different oscillation modes. In I-mode, the counterclockwise flow region expands to the molten pool's top (Figure 4c, red zone), while the clockwise flow region at the bottom shrinks in depth (Figure 4f, blue box). This reconstructed flow field constrains bubbles near the keyhole rather than allowing diffusion to the pool bottom. Combined with the I-mode path's swing distance being twice that of C-mode, bubbles are more easily recaptured by the oscillating keyhole, effectively improving the bubble escape rate.

Figure 4. Molten pool temperature and flow fields for different oscillation modes.

Figure 5 shows the distribution of pores in the central longitudinal cross-section under different oscillation modes. In N-mode, pores are distributed throughout the weld (simulated porosity rate 4.37%). After oscillation, pores mainly concentrate in the lower-middle region, with I-mode achieving the lowest porosity rate (0.03%).

Figure 5. Distribution of pores in the central longitudinal cross-section for different oscillation modes.

04 Conclusion

Oscillating laser welding significantly reduces the porosity rate in medium-thickness aluminum alloy welds, with I-mode showing the best results. The mechanisms include: lower energy density and higher scanning speed reduce the probability of spontaneous keyhole instability, decreasing bubble generation; the longer oscillation swing of the keyhole (I-mode is twice that of C-mode) enhances bubble capture and escape; and changes in the molten pool’s flow field (e.g., expanded top counterclockwise flow and reduced bottom clockwise region) limit bubble migration in the molten pool, improving the bubble escape rate (90% in I-mode).

**--Cite the article published by 高能束加工技术 on July 28, 2025, in the WeChat public account "High-Energy Beam Processing Technology and Applications."