CLADDING TECHNOLOGY SHANXI CO., LTDTIG Arc Behavior Analysis Under Magnetic Field
Literature Overview
This paper, published in Aerospace Materials and Technology in 2021 by researchers from Sichuan Aerospace Chang Zheng Equipment Manufacturing Co., Ltd. and Chengdu Aeronautical Vocational and Technical College, provides a comprehensive analysis of TIG arc behavior under the influence of external magnetic fields. The work was supported by the Sichuan Provincial Science and Technology Plan Key R&D Project (2020YFG0197) and Chengdu Aeronautical Vocational and Technical College Natural Science Research Project (062029). The research is particularly relevant to aerospace manufacturing, where magnetic field effects from residual magnetism, electromagnetic forming processes, and magnetic particle inspection equipment can significantly influence welding quality.
Core Technical Viewpoints
The paper systematically investigates how external magnetic fields of varying intensity and orientation affect TIG arc geometry, heat distribution, weld pool dynamics, and the resulting weld metal properties. The authors examine magnetic field strengths ranging from 0 to 50 mT, which covers the range of residual magnetism found in aerospace components and the fields generated by magnetic particle inspection (MPI) equipment used in welding quality assurance.
The key findings reveal that magnetic fields above 5 mT produce measurable effects on arc behavior, with significant modifications occurring above 20 mT. The Lorentz force generated by the interaction between the arc current and the external magnetic field deflects the arc column, redistributes the heat input, and modifies the weld pool flow patterns. These effects can be either detrimental or beneficial depending on the magnetic field orientation relative to the welding direction and the specific welding application.
Magnetic Field Effect Thresholds
| Magnetic Field Strength (mT) | Arc Deflection Angle (°) | Heat Distribution Change (%) | Weld Pool Flow Modification |
|---|---|---|---|
| 0–5 | < 1 | < 2 | Negligible |
| 5–15 | 1–5 | 2–8 | Slight directional bias |
| 15–30 | 5–15 | 8–20 | Moderate flow redirection |
| 30–50 | 15–30 | 20–40 | Significant flow modification |
| > 50 | > 30 | > 40 | Severe arc instability |
Arc Physics Under Magnetic Field Influence
The interaction between the TIG arc and an external magnetic field is governed by the Lorentz force equation, where the force density is proportional to the cross product of the current density vector and the magnetic field vector. In a TIG arc, the current density is not uniform but follows a complex distribution that varies with arc length, electrode geometry, and shielding gas composition.
The magnetic field effect on the arc can be categorized into three distinct regimes:
- Arc deflection regime (B < 20 mT): The magnetic field produces a lateral deflection of the arc column without fundamentally altering the arc structure. The arc remains stable but is displaced from its nominal position, resulting in asymmetric heat distribution on the workpiece.
- Arc oscillation regime (20–50 mT): The magnetic force is sufficient to cause the arc to oscillate between equilibrium positions. The arc root moves back and forth across the workpiece surface, creating a dynamic heat input pattern that can either improve or degrade weld quality depending on the oscillation frequency and amplitude.
- Arc instability regime (B > 50 mT): The magnetic force overwhelms the surface tension and electromagnetic pinch forces that maintain arc stability. The arc becomes erratic, with unpredictable deflection and potential arc extinction.
Weld Pool Flow Patterns Under Magnetic Field
| Magnetic Field Orientation | Weld Pool Flow Pattern | Effect on Weld Geometry |
|---|---|---|
| Parallel to welding direction (forward) | Accelerated flow ahead of arc | Narrower, deeper weld with reduced cap |
| Parallel to welding direction (backward) | Decelerated flow ahead of arc | Wider, shallower weld with increased cap |
| Perpendicular to welding direction | Lateral flow deflection | Asymmetric weld with one-sided reinforcement |
| Vertical (into workpiece) | Enhanced downward flow | Increased penetration depth |
| Vertical (from workpiece) | Reduced downward flow | Decreased penetration, increased cap |
| Rotating field | Complex swirling flow | Uniform heat distribution, refined grains |
Engineering Practice Implications
For engineers in the aerospace and pressure vessel industries, understanding magnetic field effects on TIG arc behavior is critical for several practical scenarios:
- Welding on magnetized components: Aerospace components that have undergone magnetic particle inspection retain residual magnetism that can affect subsequent welding operations. The residual field strength typically ranges from 1–10 mT, which is within the range where measurable arc deflection occurs.
- Welding near magnetic equipment: TIG welding operations conducted near MRI scanners, magnetic separators, or electromagnetic forming equipment may encounter significant external magnetic fields that compromise weld quality.
- Deliberate magnetic field application: The research demonstrates that controlled magnetic field application can be used to modify weld pool dynamics and improve weld quality, similar to the concept of magnetic arc oscillation used in some advanced welding systems.
Quality Control Considerations
| Inspection Method | Magnetic Field Sensitivity | Mitigation Strategy |
|---|---|---|
| Visual inspection (VT) | Low | No special measures required |
| Magnetic particle inspection (MT) | High | Demagnetize before welding; verify residual field < 3 mT |
| Ultrasonic testing (UT) | Moderate | Account for magnetic field-induced grain structure changes |
| Radiographic testing (RT) | Low | No special measures required |
| Dye penetrant testing (PT) | None | No special measures required |
Study Insights and Implications
The research provides a fundamental understanding of how magnetic fields interact with TIG arcs, but the practical implications extend well beyond basic physics. For engineers responsible for welding procedure development in environments where magnetic fields are present, the key insight is that magnetic field assessment should be an integral part of the pre-welding evaluation process.
The research also suggests opportunities for process improvement. By deliberately applying controlled magnetic fields to the welding zone, engineers can modify weld pool dynamics to achieve specific metallurgical outcomes. For example, a rotating magnetic field can promote equiaxed grain formation in the weld metal, improving transverse mechanical properties. A forward-directed magnetic field can enhance penetration depth without increasing welding current, reducing heat input and minimizing dilution in cladding applications.
However, the technology also presents challenges for process control and qualification. Magnetic field parameters must be precisely controlled and documented to ensure reproducibility of weld quality. The interaction between magnetic field effects and other process parameters (current, travel speed, electrode geometry) is complex and requires systematic investigation for each specific application.
For pressure vessel fabrication, where welding procedures must be qualified under NB/T 47014 or ASME IX, the introduction of magnetic field control as a process variable requires careful consideration of the qualification framework. The magnetic field parameters must be defined as essential variables in the welding procedure specification, and the effect of magnetic field variations on weld quality must be demonstrated through comprehensive mechanical property testing and non-destructive examination.