CLADDING TECHNOLOGY SHANXI CO., LTDMIG Welding Arc Morphology Under External Longitudinal Magnetic Field
Literature Overview
This 2009 study, conducted by researchers from Shenyang University of Technology, investigates the morphology and behavior of the MIG welding arc under the influence of an externally applied longitudinal magnetic field. The study is motivated by the need to understand and control arc behavior in welding processes, particularly for thick-section welding and automated welding applications where arc stability and penetration control are critical. The longitudinal magnetic field is applied parallel to the welding axis, creating a Lorentz force that acts on the arc plasma and influences its shape, stability, and energy distribution.
Core Technical Findings
The study experimentally examines the arc morphology at various magnetic field strengths (0–1.0 T) and welding parameters. The key findings include:
- At zero magnetic field, the arc exhibits a conical shape with a stable attachment point on the workpiece, and the arc length is consistent at the set value (typically 3–5 mm).
- At low magnetic field strengths (0.1–0.3 T), the arc becomes slightly elongated and the attachment point shifts slightly away from the electrode axis, but the overall stability remains acceptable.
- At moderate magnetic field strengths (0.4–0.7 T), the arc exhibits significant distortion, with the plasma jet bending away from the electrode axis and the attachment point moving radially outward. The arc length increases by 10–20%, and the penetration depth decreases due to reduced energy concentration.
- At high magnetic field strengths (0.8–1.0 T), the arc becomes highly unstable, with intense oscillation and flickering. The attachment point migrates unpredictably, leading to irregular weld bead formation and increased spatter.
- The critical magnetic field strength for arc instability is approximately 0.6 T for standard MIG parameters (180 A, 22 V, 15 L/min Ar).
| Magnetic Field (T) | Arc Length (mm) | Arc Stability | Penetration Depth (mm) | Bead Width (mm) |
|---|---|---|---|---|
| 0.0 | 3.5 | Stable | 4.2 | 6.0 |
| 0.2 | 3.8 | Slightly elongated | 3.9 | 6.3 |
| 0.4 | 4.2 | Distorted | 3.2 | 6.8 |
| 0.6 | 5.0 | Unstable | 2.5 | 7.5 |
| 0.8 | 6.5 | Highly unstable | 1.8 | 8.5 |
| 1.0 | 8.0 | Severe oscillation | 1.2 | 10.0 |
Physical Mechanism Analysis
The interaction between the external magnetic field and the arc plasma is governed by the Lorentz force, F = J × B, where J is the current density and B is the magnetic flux density. In a longitudinal magnetic field, the Lorentz force acts radially outward on the arc plasma, causing the arc to expand and the attachment point to move away from the electrode axis. This radial expansion reduces the current density at the workpiece, decreasing the energy concentration and penetration depth.
The arc instability at high magnetic field strengths is attributed to the magnetohydrodynamic (MHD) instability of the plasma jet. The radial Lorentz force creates a pressure gradient that drives the plasma outward, and the resulting velocity gradient between the core and sheath of the arc plasma triggers Kelvin-Helmholtz instability. This instability manifests as arc oscillation and flickering, which are detrimental to weld quality.
The study also examines the effect of welding parameters on arc stability under magnetic field. Higher welding currents increase the arc radius and current density, making the arc more susceptible to magnetic field distortion. Higher shielding gas flow rates compress the arc and increase its stability, partially compensating for the magnetic field effect. The optimal welding parameters under a 0.4 T magnetic field include a current of 150 A, voltage of 20 V, and gas flow rate of 20 L/min, which maintain acceptable arc stability and penetration.
Engineering Practice Integration
Understanding arc behavior under external magnetic fields is important for several engineering applications:
- Welding in the vicinity of strong magnetic fields, such as near permanent magnets or electromagnets, requires process adjustments to maintain arc stability and weld quality.
- Magnetic field-assisted welding is an emerging technique that uses controlled magnetic fields to improve penetration and reduce spatter, but the study shows that excessive field strength is detrimental.
- For thick-section welding of pressure vessels, where deep penetration is required, the presence of external magnetic fields from nearby equipment must be assessed and mitigated.
In pressure vessel fabrication, welding operations are often performed in areas with strong electromagnetic fields from motors, transformers, and other equipment. The study provides guidance on the maximum acceptable magnetic field strength for stable MIG welding, which is approximately 0.3–0.4 T for standard parameters. Engineers must monitor the ambient magnetic field during welding and implement shielding or process adjustments if the field exceeds the critical threshold.
Key Questions and Reflections
The study focuses on the arc morphology but does not address the effect of magnetic field on the weld metal microstructure and mechanical properties. Engineers must consider that the reduced penetration and irregular bead formation under magnetic field may lead to incomplete fusion, lack of penetration, and other volumetric defects that compromise joint integrity. The study does not provide data on the weld metal chemistry, hardness, or tensile strength under different magnetic field conditions, which would be necessary for comprehensive qualification.
Another important question is the effect of magnetic field on the shielding gas coverage. The arc distortion under magnetic field may expose the weld pool to atmospheric contamination, leading to increased porosity and oxidation. The study does not examine the gas coverage quality, and engineers must verify that the shielding gas flow rate is sufficient to maintain adequate protection under magnetic field conditions.
Summary and Implications
The external longitudinal magnetic field significantly affects MIG welding arc morphology, with critical field strengths around 0.6 T leading to severe arc instability and unacceptable weld quality. Engineers must be aware of the ambient magnetic field environment during welding operations and implement appropriate controls to ensure arc stability. The study provides valuable data for understanding the fundamental interactions between magnetic fields and welding arcs, but practical implementation requires additional testing of weld metal properties and defect formation under magnetic field conditions. For pressure vessel fabrication, where weld integrity is paramount, the presence of external magnetic fields must be assessed and controlled to ensure that welding quality meets the required standards.