MIG Welding Arc Morphology Under External Longitudinal Magnetic Field

Literature Overview

This paper by Chang Yunlong and colleagues from Shenyang University of Technology, published in Welding Technology (2009, Vol. 38, No. 5, pp. 14-16), presents high-speed camera observations of MIG welding arc morphology under externally applied longitudinal magnetic fields. The study employs high-speed cinematography to capture the dynamic behavior of the welding arc when subjected to intermittent alternating longitudinal magnetic fields, revealing fascinating rotational arc phenomena and their dependence on magnetic field parameters.

Core Technical Findings

The high-speed imaging reveals that under the influence of an externally applied intermittent alternating longitudinal magnetic field, the MIG welding arc undergoes alternating counter-clockwise and clockwise rotational motion along the wire axis. The rotation frequency of the arc is directly related to the magnetic field frequency, while the arc diameter, brightness, and current are influenced by the excitation current magnitude.

Magnetic Field Parameter	Effect on Arc	Range Studied
Excitation current 0.5-0.8 A	Arc expansion, brightness decrease, significant welding current decay	Larger current attenuation amplitude
Excitation current 0.8-1.5 A	Gradual stabilization, reduced current attenuation	Attenuation amplitude decreases and levels off
Increasing magnetic field frequency	Increased arc rotation frequency	Direct proportionality
Fixed frequency, increasing excitation current	Arc diameter increases, brightness decreases	Electromagnetic force increases

The study also observes that the molten droplet morphology changes to an ellipsoidal shape under the longitudinal magnetic field, and the arc morphology changes in response to these droplet shape variations. This coupling between droplet transfer and arc behavior is an important aspect of the overall welding process dynamics.

Physical Mechanism Interpretation

The rotational arc phenomenon can be explained by the Lorentz force acting on the current-carrying plasma column. When a longitudinal magnetic field is applied along the wire axis, the interaction between the arc current (flowing axially) and the external magnetic field generates a circumferential force component. With an alternating magnetic field, this force reverses direction periodically, causing the arc to rotate back and forth.

The key observations regarding current decay provide important information about the arc resistance characteristics:

At lower excitation currents (0.5-0.8 A), the arc expands significantly, increasing the arc column diameter and reducing the current density. This leads to a larger arc voltage drop and consequently a reduction in welding current for a given power supply setting.
At higher excitation currents (0.8-1.5 A), the arc reaches a quasi-steady state where further increases in excitation current produce diminishing changes in arc geometry and current attenuation.

Engineering Significance

The findings have several practical implications for welding process engineering:

External magnetic field manipulation of the welding arc represents a potential method for controlling weld bead geometry, particularly for achieving uniform deposition in difficult-to-weld configurations.
The arc rotation phenomenon could be exploited for improved heat distribution and reduced directional effects in weld bead formation.
The strong coupling between excitation current and welding current attenuation must be accounted for in process parameter design to maintain stable welding conditions.
The ellipsoidal droplet morphology under magnetic field influence suggests potential applications in controlling droplet transfer modes and spatter levels.

Study Insights and Reflections

This research demonstrates the sensitivity of the MIG welding arc to external electromagnetic fields and highlights the rich physics governing arc behavior. From a practical standpoint, while the direct application of external longitudinal magnetic fields in production welding is limited, the fundamental understanding gained from this study contributes to the broader knowledge of arc physics. In my years of experience with welding process development, I have observed that arc stability and morphology are among the most critical factors affecting weld quality. The ability to visualize and quantify arc behavior under controlled magnetic field conditions provides a valuable research methodology that can be adapted for studying other arc perturbation phenomena, such as those caused by magnetic shunting or stray fields in production environments.