TIG Arc Behavior Under Longitudinal Magnetic Field: MHD Modeling and Experimental Analysis

Literature Overview

This paper by Zhang Xiaohong, Si Zhongqi, Fan Yifei, Wen Yuanhua, and Zhang Wanchun (2021), published in Aerospace Materials and Technology (Vol. 51, No. 3, pp. 44-48), investigates the behavior of TIG arcs under the influence of an external longitudinal magnetic field. The research was conducted at Sichuan Aerospace Long March Equipment Manufacturing Co., Ltd., and Chengdu Aviation Vocational Technical College, and was supported by the Sichuan Provincial Science and Technology Program. The study employs magnetohydrodynamic (MHD) modeling to analyze the temperature and flow field distributions in the arc.

Core Technical Findings

The researchers established a mathematical model of TIG arc behavior under longitudinal magnetic field in a rotating cylindrical coordinate system, based on MHD theory. Using Fluent with user-defined functions (UDF), they performed coupled solving of the flow field and electromagnetic field to obtain the temperature and flow field distributions.

Key Findings on Arc Behavior

Technical Interpretation

MHD Model Formulation

The MHD model for TIG arc under longitudinal magnetic field involves solving the following coupled equations:

1. Continuity equation: Conservation of mass in the arc plasma.
2. Momentum equation: Includes Lorentz force term (J × B) due to the magnetic field.
3. Energy equation: Includes Joule heating, radiation, and convection terms.
4. Maxwell's equations: Relate the current density to the electric field and account for the magnetic field effects.
5. Ohm's law: Relates current density to electric field and plasma conductivity.

The rotating cylindrical coordinate system is chosen because the TIG arc exhibits rotational symmetry, and the longitudinal magnetic field aligns with the arc axis.

Temperature Field Behavior

The most striking finding is the transformation of the temperature field from a solid bell shape (conventional TIG) to a hollow bell shape with double peaks under longitudinal magnetic field. This occurs because the Lorentz force (J × B) acts radially inward on the current-carrying plasma, compressing the arc column and creating a central region of lower temperature. The "low-temperature cavity" near the anode surface is a direct consequence of this radial compression.

The double-peak temperature distribution indicates that the maximum temperature occurs at two symmetric locations around the arc axis, rather than at the center. This has significant implications for weld pool dynamics, as the heat input is concentrated in two distinct zones rather than uniformly distributed.

Pressure Field Behavior

The anode surface pressure distribution also exhibits a double-peak pattern under longitudinal magnetic field, with peak pressures reduced by approximately 70% compared to the no-field case. The Lorentz force creates a radial inward pressure that compresses the arc, reducing the axial pressure component. The higher the current density, the more pronounced this effect, as the Lorentz force scales with the product of current density and magnetic field strength.

Engineering Practice Implications

For engineers working on advanced welding processes, particularly in aerospace and precision manufacturing:

1. Weld pool control: The hollow temperature distribution under longitudinal magnetic field can be used to control weld pool shape and penetration. The two hot zones can be positioned to achieve specific weld geometry requirements.
2. Distortion reduction: The reduced axial pressure on the anode surface may help reduce weld pool turbulence and associated distortion, which is beneficial for thin-walled structures and precision components.
3. Penetration modification: The concentrated heat input in the double-peak zones can enhance penetration in specific regions, potentially allowing for deeper welds with lower total energy input.
4. Process stability: The magnetic field provides additional stabilization of the arc, which can improve welding consistency and reduce defects.

Key Questions and Reflections

The paper provides valuable insights into arc behavior under longitudinal magnetic field but raises several practical questions. How does the magnetic field strength interact with the welding current to optimize weld quality? What is the minimum magnetic field strength required to observe significant effects on arc behavior?

For pipe welding applications, where joint geometry varies (butt joints, fillet welds, pipe-to-flange connections), the magnetic field orientation and strength must be carefully optimized for each configuration. The longitudinal field analyzed in this study may not be optimal for all joint types, and further research is needed to determine the best magnetic field configuration for specific welding scenarios.

Additionally, the paper focuses on the arc behavior in the gas phase but does not extensively address the effects on the weld pool and solidification process. For practical welding applications, it is essential to understand how the modified arc behavior translates to weld pool dynamics, solidification patterns, and final weld properties.

Study Insights

This research contributes fundamental knowledge to the understanding of TIG arc behavior under magnetic field influence, which is essential for developing advanced welding processes with improved controllability. The MHD modeling approach provides a powerful tool for predicting and optimizing arc behavior under various magnetic field configurations. For engineers working on precision welding and advanced manufacturing processes, these findings offer a pathway to achieving more precise control over weld geometry and quality through magnetic field manipulation. The technology has particular relevance for aerospace and defense applications where weld quality and consistency are paramount, and where the additional complexity of magnetic field application can be justified by the improved performance.