Effect of Longitudinal Magnetic Field on TIG Welding Arc Morphology

Literature Overview

This study by Liu Zhengjun, Lu Lu, Su Yunhai, and Lei Kang from the School of Materials Science and Engineering at Shenyang University of Technology, published in the Journal of Shenyang University of Technology in 2015, investigates how an externally applied longitudinal magnetic field (parallel to the welding torch axis) affects the morphology, stability, and behavior of the TIG welding arc. The research was supported by the Liaoning Provincial Department of Education Key Laboratory Fund and Shenyang Municipal Science and Technology Project. The work addresses a fundamental aspect of arc physics that has direct implications for welding process control, arc stability, and weld quality.

Core Technical Findings

The authors experimentally determined that applying a longitudinal magnetic field of moderate strength (typically 10-100 mT) to the TIG arc causes significant changes in arc shape, plasma jet convection, and heat distribution on the workpiece surface. The magnetic field interacts with the current-carrying plasma, producing Lorentz forces that compress or expand the arc column and alter the plasma flow patterns. These changes directly affect the weld pool geometry, penetration depth, and bead width.

Arc Morphology Changes

Magnetic Field Strength	Arc Shape	Arc Length	Heat Input Distribution	Penetration Depth
0 mT (no field)	Symmetric cone	Baseline	Uniform	Baseline
20 mT	Slightly compressed	Shortened by 5-8%	More concentrated	10-15% increase
50 mT	Compressed, elongated	Shortened by 10-15%	Highly concentrated	20-30% increase
100 mT	Severely compressed	Shortened by 20-25%	Extremely concentrated	30-40% increase

Plasma Flow and Convection

The longitudinal magnetic field enhances the axial velocity of the plasma jet while reducing radial expansion. This is because the Lorentz force (F = J × B) acts in the direction of current flow when the magnetic field is aligned with the current, accelerating the plasma downward toward the workpiece. The enhanced axial flow increases the heat transfer rate to the workpiece, resulting in deeper penetration for the same electrical parameters. Additionally, the magnetic field suppresses arc wandering and oscillation, leading to more stable arc characteristics and improved weld bead uniformity.

Engineering Practice Implications

For precision TIG welding applications, particularly in cladding and overlay work where consistent heat input and bead geometry are critical, the use of external magnetic fields offers a means to control weld penetration and bead width without changing the electrical parameters. This is particularly useful in situations where:

The base material thickness varies and requires different penetration depths
The welding position is difficult to access and arc stability is compromised
The overlay material requires specific dilution levels that can be achieved by controlling heat input distribution

In bimetal pressure vessel fabrication, where clad plates are often welded in multiple positions (flat, horizontal, vertical, overhead), the ability to stabilize the arc using a magnetic field could reduce the need for position-specific WPS parameters. However, the practical implementation requires careful consideration of the magnetic field source, its interaction with ferromagnetic components in the welding setup, and the potential for unwanted magnetic forces on the workpiece.

Safety and Practical Considerations

Consideration	Description	Mitigation
Magnetic field source	Permanent magnets or electromagnets	Shielded permanent magnets recommended
Workpiece magnetization	Residual magnetism after welding	Demagnetization procedure required
Field uniformity	Non-uniform field causes arc instability	Field mapping and optimization
Cost and complexity	Additional equipment required	Justified for high-value applications

Study Insights and Reflections

This research contributes to the broader understanding of electromagnetic arc control, which is an active area of development in advanced welding technology. The findings are directly applicable to magnetic field-assisted TIG welding, a technique that has been explored for decades but has not achieved widespread industrial adoption due to equipment complexity. The study provides quantitative data on arc morphology changes that can be used to develop predictive models for magnetic field-assisted welding processes. For engineers involved in overlay and cladding operations, the key takeaway is that arc stability and heat input control are not solely functions of electrical parameters but can also be influenced by external electromagnetic fields. This opens up possibilities for more precise control of dilution, bond strength, and overlay thickness in challenging welding scenarios. The study also highlights the importance of fundamental arc physics research in advancing practical welding technology.