MIG Welding Arc Behavior Under Alternating Longitudinal Magnetic Field

Literature Overview

Published in Journal of Thermal Processing and Materials (Vol. 32, No. 11, 2011, pp. 23–27), this study by Zhu Sheng and colleagues from the State Key Laboratory of Remanufacturing Technology at the Academy of Armored Force Engineering investigates the physical behavior of the MIG welding arc under the influence of an alternating longitudinal magnetic field. The research was supported by multiple national and defense-related funding programs.

Technical Background

The welding arc is the energy delivery mechanism in arc welding processes, and its stability directly determines weld quality. Arc instability manifests as arc wandering, excessive spatter, irregular bead profile, and porosity formation. External magnetic fields can be deliberately applied to manipulate arc behavior, a technique known as magnetic arc blowing. While transverse magnetic fields have been extensively studied, the effect of longitudinal magnetic fields—applied parallel to the electrode axis—has received less attention.

The alternating longitudinal magnetic field creates a time-varying Lorentz force on the charged particles in the arc plasma. The force on a charged particle is given by F = q(v × B), where q is the charge, v is the velocity, and B is the magnetic field vector. When the magnetic field is applied longitudinally and alternates in polarity, the Lorentz force on radially moving charged particles causes them to rotate around the arc axis, producing a helical motion that manifests macroscopically as arc rotation.

Experimental Methodology

The authors used high-speed camera imaging to capture the arc morphology under various magnetic field conditions. The experimental setup included a standard MIG welding configuration with an external electromagnet positioned to generate an alternating longitudinal magnetic field along the electrode axis. The excitation current was varied to produce different magnetic field strengths.

The key experimental variable was the excitation current of the electromagnet, which directly controls the magnetic field strength. The arc behavior was characterized by observing the arc shape, rotation pattern, deflection angle, and stability using high-speed video recording.

Key Technical Findings

The study established a clear relationship between the alternating longitudinal magnetic field strength and arc behavior:

Without an external magnetic field, the free arc burns stably with the arc axis coinciding with the electrode axis. When an alternating longitudinal magnetic field is applied, the arc undergoes alternating counter-clockwise and clockwise rotational motion around the electrode axis, causing the arc axis to deviate from the electrode axis. As the excitation current increases, the rotation radius increases, the deflection angle increases, and the arc bright area decreases. At an excitation current of 30 A, the maximum deflection angle reaches 45°, at which point the arc becomes unstable and may extinguish, rendering the welding process unreliable.

The physical mechanism is as follows: the alternating magnetic field exerts a Lorentz force on the radially moving electrons and ions in the arc plasma. Because the field alternates in direction, the force direction also alternates, causing the charged particles to rotate around the arc axis. The rotation rate and amplitude are proportional to the magnetic field strength. At high field strengths, the rotational velocity of the plasma becomes large enough to disrupt the thermal equilibrium of the arc, leading to instability and potential extinction.

Engineering Practice Implications

This study has direct relevance to welding processes in the presence of external magnetic fields, which can occur in several industrial scenarios:

• Welding in the vicinity of electromagnetic equipment: Industrial environments with large motors, transformers, or electromagnetic forming equipment can produce stray magnetic fields that affect arc stability.
• Magnetic arc blowing correction: Understanding the threshold field strength for arc instability (approximately 30 A excitation current in this study) helps in designing magnetic field compensation systems for arc welding in magnetically contaminated environments.
• Deliberate arc manipulation: Controlled application of longitudinal magnetic fields could be used to achieve specific welding effects, such as arc rotation for improved bead uniformity or arc deflection for root pass penetration enhancement.

The 45° deflection angle at 30 A excitation current provides a quantitative benchmark for arc stability limits. Engineers should note that the actual threshold depends on the specific welding parameters (current, voltage, shielding gas, electrode diameter) and the geometry of the magnetic field source. The study's results should be used as a starting point for process-specific qualification rather than as absolute limits.

Study Insights and Reflections

The use of high-speed camera imaging to capture arc morphology is a straightforward but powerful technique that provides direct visual evidence of arc behavior. The clear progression from stable to unstable arc as the magnetic field increases demonstrates a well-defined physical mechanism that can be predicted from basic plasma physics principles.

The finding that the arc bright area decreases with increasing magnetic field strength is particularly interesting. The bright area of the arc corresponds to the region of highest current density and temperature. A reduction in bright area suggests that the current density is being redistributed, potentially leading to non-uniform energy deposition on the workpiece. This could result in inconsistent weld penetration and bead width, which are critical quality parameters in structural welding.

The alternating nature of the magnetic field is significant because it produces a time-averaged zero net deflection but a non-zero rotational effect. This is fundamentally different from a DC longitudinal field, which would produce a steady deflection in one direction. The alternating field's ability to cause rotation without net displacement makes it potentially useful for applications where arc stirring is desired without directional bias.

Reference Value and Outlook

This paper provides valuable quantitative data on the interaction between longitudinal magnetic fields and MIG welding arcs. The results are applicable to any welding process where external magnetic fields are present or where deliberate magnetic arc manipulation is employed.

Future research should extend this work to include the effect of magnetic field frequency on arc behavior, as the study focuses on a specific alternating frequency. Additionally, the effect of different shielding gases (Ar, CO2, Ar/CO2 mixtures) on the magnetic field sensitivity of the arc should be investigated, as the plasma composition and conductivity vary significantly between gas types. The study also warrants extension to other arc welding processes, including GTAW, GMAW, and FCAW, to establish a comprehensive understanding of magnetic field effects across different welding configurations.