TIG Arc Plasma Dual-Temperature Numerical Simulation

Literature Overview

The paper by Huang Yong, Liu Lin, Wang Xinxin, and Lu Suzhong, published in the Transactions of the China Welding Institution in 2018 (Vol. 39, No. 10, pp. 6-10), presents a two-dimensional axisymmetric dual-temperature mathematical model of the TIG welding arc plasma. The work was funded by the National Natural Science Foundation of China (Grant No. 51265029) and conducted at the State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou University of Technology. The study is grounded in fundamental conservation equations — continuity, momentum, electron energy, heavy-particle energy, and Maxwell's equations — and employs FLUENT software for numerical solution.

Core Technical Approach

The authors constructed a coupled system of governing equations that treats the electron temperature (Te) and heavy-particle temperature (Th) as independent thermodynamic variables rather than assuming local thermodynamic equilibrium (LTE). This dual-temperature formulation is critical because, as the authors demonstrate, the assumption of Te = Th throughout the arc column is physically invalid at the arc boundaries and near the anode surface.

The thermodynamic properties and transport coefficients (electrical conductivity, thermal conductivity, viscosity) are all expressed as functions of both Te and Th, introducing significant nonlinearity into the system. The FLUENT solver was configured to handle this coupled, non-equilibrium system iteratively.

Key Findings and Interpretation

The most significant result is the confirmation of non-equilibrium phenomena in the TIG arc. The arc maintains a typical bell-shaped geometry, and at the arc center, Te and Th converge to nearly identical values. However, at the arc periphery and on the anode surface, substantial divergence occurs between the two temperature fields.

This finding has direct implications for welding process optimization. In engineering practice, the heat input distribution at the workpiece surface is governed by the local plasma temperature and velocity at the arc-anode interface. If the dual-temperature effect is ignored, the predicted thermal profile at the anode spot will be inaccurate, leading to errors in predicting weld penetration, bead geometry, and dilution.

Engineering Relevance for Pipe and Fitting Welding

For pipe welding applications, particularly for thin-walled stainless steel pipe (e.g., ASTM A312 TP304/316) and titanium alloy pipe (ASTM B381/B382), the accuracy of arc heat input prediction is paramount. The non-equilibrium zone near the anode surface directly affects:

• Penetration depth prediction for single-pass welding of thin pipe
• Dilution ratio control in dissimilar metal welds (e.g., carbon steel to stainless steel transition)
• Heat-affected zone (HAZ) width estimation for creep-resistant alloys

Methodological Reflections

The dual-temperature model represents a significant advancement over the conventional single-temperature LTE assumption. However, from a practical engineering standpoint, several limitations deserve attention. The model is two-dimensional and axisymmetric, which means it cannot capture the dynamic oscillation behavior of the arc under magnetic field disturbances — a phenomenon well-known in pipe welding where the joint geometry breaks axial symmetry. Furthermore, the model does not incorporate the cathode (tungsten electrode) surface physics, which is essential for predicting electrode wear and arc stability during long-duration production welding.

The coupling of Maxwell's equations with the fluid dynamics equations creates a stiff numerical problem. The authors do not elaborate on the convergence strategy or the sensitivity of results to mesh density and time-stepping parameters, which would be critical information for engineers attempting to replicate or extend this model for specific pipe welding scenarios.

Integration with Process Control

In modern pipe welding production, arc monitoring systems measure voltage, current, and sometimes arc radiation to infer arc characteristics in real time. The dual-temperature model provides a theoretical basis for understanding why simple voltage-current relationships cannot fully characterize the thermal state of the arc. For example, two arcs operating at the same current and voltage may have different Te/Th ratios at the anode boundary, resulting in different penetration profiles. This insight supports the development of more sophisticated arc sensing algorithms for automated pipe welding systems.

Summary

This study provides a rigorous theoretical foundation for understanding the thermal non-equilibrium behavior of the TIG welding arc. The dual-temperature framework reveals that the LTE assumption breaks down precisely at the anode surface — the region that governs weld penetration and bead geometry. For engineers engaged in pipe and fitting welding, this work underscores the need for more physically realistic arc models when optimizing welding parameters for critical applications such as nuclear-grade stainless steel piping or aerospace titanium components. The practical limitation of the axisymmetric assumption and the absence of dynamic arc behavior modeling indicate that further research incorporating three-dimensional, time-dependent formulations with cathode physics would better serve the demands of modern automated pipe welding systems.