Enhancement of Weld Penetration via Arc Constriction in 316L Stainless Steel Using High-Frequency Flat-Top Longitudinal Magnetic Field-Assisted TIG Welding

This study proposes a novel high-frequency flat-top longitudinal magnetic field-assisted tungsten inert gas (HF-FTLMF TIG) welding method to improve arc constriction and weld penetration. The effects of magnetic field intensity and frequency on arc morphology, weld appearance, and microstructure were systematically investigated. The results show that, compared with conventional TIG welding, when the frequency of the FTLMF exceeds 1000 Hz, the arc becomes noticeably constricted, the conical angle decreases, and the heat input becomes more concentrated. Under appropriate magnetic field conditions, the arc pressure increases from 251.3 Pa for the free arc to 452.9 Pa, and the weld penetration depth increases from 0.84 mm to 1.09 mm. In addition, HF-FTLMF reduces surface unevenness and improves weld uniformity. Compared with low-frequency FTLMF (<1000 Hz), HF-FTLMF exhibits more stable arc behavior and better weld formation. SEM and EDS results further indicate that the application of HF-FTLMF affects the fusion-zone microstructure and elemental redistribution, and a relatively finer microstructure was observed under the 2000 Hz condition. These findings suggest that HF-FTLMF provides an effective approach for regulating TIG arc behavior and improving weld formation through magnetic field assistance.