CLADDING TECHNOLOGY SHANXI CO., LTDTIG Welding Arc Behavior Under External Longitudinal Magnetic Field
Literature Overview
Published in the Welding Journal (Hanzhuang Xuebao) in 2010 by researchers from the Provincial Key Laboratory of Advanced Welding Technology and Automation at Shenyang University of Technology, this study examines the behavior of the gas tungsten arc welding arc when subjected to an externally applied longitudinal magnetic field. The work was supported by the Shenyang Key Fund Project (1071201-1-00) and addresses a fundamental question in arc physics: how does an axial magnetic field alter arc stability, energy density, and penetration characteristics?
Core Technical Content
The researchers constructed an experimental apparatus capable of applying a longitudinal magnetic field of controlled intensity along the welding direction while simultaneously recording arc voltage, current waveforms, arc photographs, and weld bead geometry. The study systematically varied magnetic field intensity from 0 to 0.5 T and documented the progressive changes in arc morphology and welding performance.
Arc Morphology Under Longitudinal Magnetic Field
Without an external magnetic field, the TIG arc exhibits a characteristic conical shape with the arc root concentrated near the tungsten tip. Under a longitudinal magnetic field, the arc column is stretched and elongated along the field direction, producing a more collimated plasma jet. The researchers observed that at magnetic field intensities above 0.1 T, the arc becomes noticeably narrower at the workpiece surface while maintaining its full current-carrying capacity.
| Magnetic Field Intensity | Arc Length | Arc Width at Workpiece | Penetration Depth | Bead Width |
|---|---|---|---|---|
| 0 T (baseline) | 5 mm | 4.2 mm | 2.8 mm | 8.5 mm |
| 0.1 T | 5 mm | 3.8 mm | 3.2 mm | 7.8 mm |
| 0.2 T | 5 mm | 3.3 mm | 3.8 mm | 7.0 mm |
| 0.3 T | 5 mm | 2.9 mm | 4.3 mm | 6.4 mm |
| 0.5 T | 5 mm | 2.5 mm | 5.1 mm | 5.6 mm |
Penetration Enhancement Mechanism
The key finding is that the longitudinal magnetic field produces a significant increase in weld penetration without proportional increase in bead width, resulting in a higher depth-to-width ratio. The mechanism involves two coupled effects: first, the Lorentz force acting on the current-carrying plasma column compresses the arc toward the centerline, increasing energy density at the weld pool center; second, the magnetic pressure generated at the arc root accelerates plasma flow toward the workpiece, enhancing convective heat transfer and deepening the weld pool.
Arc Stability and Deflecting Behavior
The researchers also examined arc stability under various field conditions. At moderate field strengths (0.1-0.2 T), the arc becomes more stable because the magnetic field suppresses electromagnetic instabilities and arc oscillation. However, at higher field strengths (>0.3 T), a new instability mode emerges where the arc root begins to drift laterally, creating non-uniform heat input and potential weld defects.
Engineering Practice Implications
This research has direct applications in narrow-gap welding of thick-section materials where deep penetration with narrow bead width is desired. For bimetal pressure vessel fabrication, the magnetic field technique could be applied during overlay welding of corrosion-resistant alloys onto thick carbon steel or low-alloy steel substrates, where controlling dilution and penetration profile is critical.
The technique is particularly relevant for weld overlay applications where minimizing dilution of the base metal into the cladding layer is essential. By concentrating the arc energy into a narrower, deeper profile, the magnetic field method reduces lateral heat spread and thus reduces the volume of base metal melted and incorporated into the overlay.
Key Insights and Reflections
The study demonstrates that magnetic field control of the welding arc represents a promising approach to process optimization without requiring modifications to electrode geometry, filler metal composition, or shielding gas. For engineers working on cladding and overlay welding where dilution control determines the corrosion resistance of the final product, this technique offers a novel lever for process refinement. The practical challenge lies in generating a sufficiently strong and uniform longitudinal field in a production environment, which requires compact superconducting or permanent magnet assemblies positioned around the workpiece. Future development should focus on integrating magnetic field coils into robotic welding systems for automated narrow-gap and overlay operations.