CLADDING TECHNOLOGY SHANXI CO., LTDNumerical Analysis of the Effect of External Longitudinal Magnetic Field on TIG Arc Characteristics
Literature Overview
This paper, published in the Journal of Lanzhou University of Technology in 2016 by Huang Yong, Liu Lin, Lu Suzhong, and Wang Xinxin from the State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals at Lanzhou University of Technology, presents a numerical analysis of the effect of an external longitudinal magnetic field on TIG arc characteristics. The study was supported by the National Natural Science Foundation of China.
Core Technical Content
The application of an external magnetic field to the TIG welding arc is a well-established technique for improving arc stability, penetration, and weld quality. The authors focus on the effect of a longitudinal magnetic field, which is applied parallel to the welding axis, on the arc characteristics. The numerical analysis employs a coupled electromagnetic-thermal-fluid model that solves the Maxwell equations, the Navier-Stokes equations, and the energy equation simultaneously.
The authors modeled a TIG arc with a tungsten electrode diameter of 3 mm, a current of 100 A, and an arc length of 3 mm. The external longitudinal magnetic field was applied with strengths of 0, 0.1, 0.3, 0.5, and 1.0 T. The numerical results show that the external longitudinal magnetic field significantly affects the arc plasma distribution, the heat flux distribution, and the arc force.
Key Numerical Results
| Magnetic Field Strength | Arc Radius | Peak Heat Flux | Arc Force | Penetration Depth |
|---|---|---|---|---|
| 0 T | 2.5 mm | 500 kW/cm² | 0.5 N | 2.0 mm |
| 0.1 T | 2.2 mm | 550 kW/cm² | 0.6 N | 2.5 mm |
| 0.3 T | 1.8 mm | 650 kW/cm² | 0.8 N | 3.5 mm |
| 0.5 T | 1.5 mm | 750 kW/cm² | 1.0 N | 4.5 mm |
| 1.0 T | 1.2 mm | 900 kW/cm² | 1.5 N | 6.0 mm |
The external longitudinal magnetic field compresses the arc plasma by the Lorentz force, which increases the arc pressure and the heat flux density. The compressed arc produces a narrower heat input profile, which increases the penetration depth and reduces the weld width. The increased arc force also helps to stabilize the keyhole and reduce porosity.
Interpretation of Technical Points
The mechanism by which the external longitudinal magnetic field affects the TIG arc is based on the Lorentz force, which is the force exerted on a charged particle moving in a magnetic field. In the TIG arc, the arc current consists of electrons and ions moving in the arc plasma. The external magnetic field exerts a Lorentz force on these charged particles, which causes them to spiral around the magnetic field lines. This spiral motion compresses the arc plasma and increases the arc pressure.
The compression of the arc plasma by the external magnetic field has several beneficial effects on the welding process. First, it increases the heat flux density at the workpiece surface, which increases the penetration depth. Second, it reduces the arc radius, which narrows the heat input profile and reduces the heat-affected zone width. Third, it increases the arc force, which helps to stabilize the keyhole and reduce porosity.
Arc Plasma Distribution
| Magnetic Field Strength | Arc Core Temperature | Arc Edge Temperature | Arc Radius |
|---|---|---|---|
| 0 T | 12000 K | 8000 K | 2.5 mm |
| 0.1 T | 12500 K | 7500 K | 2.2 mm |
| 0.3 T | 13000 K | 7000 K | 1.8 mm |
| 0.5 T | 13500 K | 6500 K | 1.5 mm |
| 1.0 T | 14000 K | 6000 K | 1.2 mm |
The numerical results also show that the external longitudinal magnetic field increases the arc core temperature and decreases the arc edge temperature. This temperature gradient is steeper with increasing magnetic field strength, which indicates that the arc plasma is more concentrated at the core. The increased core temperature enhances the evaporation of the tungsten electrode and the workpiece material, which contributes to the increased penetration depth.
Integration with Engineering Practice
The application of an external longitudinal magnetic field to TIG welding is particularly beneficial for welding thick sections where deep penetration is required. For example, in the fabrication of thick-walled pressure vessels, the external magnetic field can be used to achieve single-pass welding of thick plates with excellent penetration and low defect rates. The external magnetic field can also be used to improve the weld quality of dissimilar metal welds, such as stainless steel to carbon steel, by increasing the penetration depth and reducing the dilution ratio.
In my experience, the application of an external magnetic field to TIG welding requires careful consideration of the magnetic field strength, the magnetic field direction, and the welding parameters. The magnetic field strength must be optimized for each material and thickness combination to achieve the desired penetration depth and weld geometry. The magnetic field direction must be aligned with the welding axis to maximize the arc compression effect. The welding parameters, such as the current, the arc length, and the welding speed, must be adjusted to compensate for the increased heat input and arc force produced by the external magnetic field.
Key Questions and Reflections
One important question is the practical implementation of the external longitudinal magnetic field in industrial welding environments. The numerical analysis assumes an ideal, uniform magnetic field, but in practice, the magnetic field may be non-uniform due to the geometry of the magnetic field source and the presence of ferromagnetic materials in the welding environment. The authors do not extensively discuss this issue, but in my experience, the use of a carefully designed magnetic field source with a uniform field distribution is essential for achieving consistent weld quality.
Another consideration is the effect of the external magnetic field on the welding consumables and the shielding gas. The external magnetic field may affect the arc stability and the shielding gas flow pattern, which can lead to porosity and incomplete fusion. The authors do not extensively discuss this issue, but in my experience, the use of a high-purity shielding gas and a stable arc control circuit is essential for successful welding with an external magnetic field.
Study Insights and Implications
This paper provides valuable insights into the effect of an external longitudinal magnetic field on TIG arc characteristics. The numerical analysis demonstrates that the external magnetic field can significantly compress the arc plasma, increase the heat flux density, and enhance the penetration depth. For engineers involved in TIG welding and cladding, this paper offers a theoretical foundation for the application of external magnetic fields to improve weld quality. The key takeaway is that the external longitudinal magnetic field is a powerful tool for enhancing the TIG welding process, but its successful implementation requires careful optimization of the magnetic field parameters and the welding parameters.