Numerical Analysis of Dual TIG Welding Arc

Literature Overview

This paper, published in the journal "Hot Working Technology" in 2020 by Wang Xinxin, Luo Yi, Li Chuntian, and Chi Luxin from Chongqing University of Technology, presents a comprehensive numerical study of the dual-TIG welding arc. The research establishes a mathematical model of the dual-TIG welding arc that includes the tungsten electrodes, and numerically solves for the temperature field, flow field, and electromagnetic field under different current distribution conditions. The study analyzes the plasma jet shear force and arc pressure on the workpiece surface. The work was supported by the National Natural Science Foundation of China (Grant 51705054), the Chongqing Municipal Education Commission Project (KJ1600903), and the Chongqing University of Technology Spark Program (2015XH20).

Mathematical Model and Numerical Approach

Model Configuration

The dual-TIG arc model incorporates both tungsten electrodes as part of the computational domain, which is a significant advancement over models that treat the electrode as a boundary condition. The model accounts for:

Thermal transport: Energy equation including Joule heating, convection, and radiation losses.
Fluid dynamics: Navier-Stokes equations for the plasma flow, including electromagnetic body forces.
Electromagnetic field: Maxwell's equations for the electric and magnetic fields, including the Lorentz force that drives plasma convection.
Species transport: Conservation equations for electron density and ion species, which determine the electrical conductivity and arc properties.

Boundary Conditions

The model applies appropriate boundary conditions at the tungsten electrode surfaces, including the thermionic and field emission electron flux at the cathode, and the heat flux and electron emission at the anode (workpiece surface). The arc column is treated as a quasi-neutral plasma with local thermodynamic equilibrium (LTE) assumptions, which is reasonable for the high-temperature core of the arc but may introduce errors near the electrode boundaries.

Key Numerical Results

Symmetric Current Distribution

When both tungsten electrodes carry equal current (symmetric case), the numerical results show that:

The temperature field is symmetric about the midpoint between the two electrodes.
The flow field exhibits a mirror-symmetric pattern with plasma jets converging from both electrodes.
The arc pressure distribution on the workpiece surface is symmetric and uniform.

This symmetric case serves as a baseline for understanding the fundamental behavior of the dual-TIG arc and provides a reference against which asymmetric cases can be compared.

Asymmetric Current Distribution

When the current is distributed unequally between the two electrodes, several important phenomena emerge:

Parameter	Symmetric Distribution	Asymmetric Distribution
Temperature field symmetry	Symmetric	High-temperature region shifts toward higher-current electrode
Arc position	Centered	Arc tilts toward lower-current electrode
Arc pressure peak	Centered	Shifted toward lower-current electrode
Plasma jet shear force	Symmetric	Peak biased toward lower-current electrode

The observation that the arc tilts toward the lower-current electrode is physically intuitive: the higher-current electrode produces a stronger plasma jet, which pushes the arc column toward the opposite side. This behavior has direct implications for weld geometry, as the asymmetric heat input distribution will produce a weld bead that is not symmetric about the torch centerline.

Arc Pressure and Plasma Jet Force Comparison with Single TIG

A particularly important finding is the comparison of arc pressure and plasma jet shear force between the dual-TIG configuration and a single-TIG arc at the same total current:

Equal current distribution: Both arc pressure and plasma jet shear force on the workpiece surface are significantly lower than those of a single-TIG arc at the same total current. This suggests that the dual-TIG configuration, when operated with equal current sharing, does not provide the same level of arc force enhancement as a single electrode carrying the full current. The energy is distributed over a larger area, reducing the peak pressure and force.
Unequal current distribution: The arc pressure and plasma jet shear force peaks shift toward the lower-current electrode side. This is consistent with the arc tilting behavior observed in the temperature and flow field analyses. The practical implication is that the weld pool will be preferentially influenced by the arc force on the side of the lower-current electrode, which could be exploited to control weld geometry.

Physical Mechanisms

Arc Tilting Mechanism

The tilting of the dual-TIG arc toward the lower-current electrode can be explained by the interaction of two plasma jets. Each electrode produces a directed plasma flow that impinges on the workpiece surface. When the currents are unequal, the plasma jet from the higher-current electrode is stronger and exerts a greater lateral force on the arc column. This lateral force displaces the arc column toward the lower-current electrode, creating an asymmetric configuration.

The numerical results confirm that this tilting is accompanied by a redistribution of the arc pressure and shear force. The peak pressure occurs near the lower-current electrode because the arc column is compressed against that side by the stronger plasma jet from the higher-current electrode.

Energy Distribution and Weld Penetration

The relationship between arc pressure distribution and weld penetration depth can be understood through the following mechanism:

Higher arc pressure at the workpiece surface increases the mechanical force on the molten pool, pushing liquid metal downward and increasing penetration depth.
The plasma jet shear force drags liquid metal in the direction of the arc travel, affecting the weld pool shape and solidification pattern.
In the dual-TIG configuration, the interaction of two plasma jets creates a complex force field that can either enhance or reduce penetration depending on the current distribution.

The finding that equal current distribution produces lower arc pressure than a single TIG arc at the same total current is significant. It implies that for maximizing penetration in a dual-TIG configuration, an asymmetric current distribution is necessary, with one electrode carrying substantially more current than the other.

Engineering Practice Implications

Current Distribution Optimization

Based on the numerical results, the following guidelines for current distribution in dual-TIG welding can be derived:

For maximum penetration: Use an asymmetric current distribution with the rear electrode carrying higher current. The optimal ratio depends on the specific application and base metal thickness.
For uniform heat input: Use a symmetric current distribution, accepting the lower peak arc pressure and potentially shallower penetration.
For wide, shallow welds: Use a symmetric or near-symmetric distribution with higher total current.

Arc Length and Electrode Spacing Effects

The numerical model provides insights into how arc length and electrode spacing affect the arc behavior:

Arc length: Increasing arc length reduces the arc pressure and plasma jet force at the workpiece surface, consistent with the well-known behavior of single-TIG arcs. In the dual-TIG configuration, this effect is compounded by the interaction between the two arcs.
Electrode spacing: Closer spacing enhances the interaction between the two plasma jets, potentially increasing the peak arc pressure at the workpiece. However, very close spacing may lead to arc instability due to the strong interaction between the two arc columns.

Weld Pool Modeling Integration

The numerical results for the dual-TIG arc provide essential boundary conditions for weld pool modeling. The arc pressure distribution and plasma jet shear force calculated in this study can be directly used as boundary conditions in finite element or finite volume models of the weld pool. This coupling between arc modeling and weld pool modeling is essential for accurate prediction of weld geometry and residual stress.

Validation and Comparison with Existing Research

The paper notes that the simulation results are in good agreement with existing research findings. This validation is important for establishing the credibility of the numerical model. The agreement suggests that the model captures the essential physics of the dual-TIG arc and can be used with confidence for process optimization studies.

The numerical approach used in this study represents a significant advancement in the modeling of multi-electrode welding arcs. Previous studies had largely focused on single-TIG arcs, and the extension to dual-electrode configurations requires careful treatment of the interaction between the two arc columns. The inclusion of both tungsten electrodes in the computational domain, rather than treating them as boundary conditions, allows for a more accurate representation of the arc-electrode interaction.

Key Questions and Reflections

The numerical model assumes local thermodynamic equilibrium (LTE) for the plasma, which is a simplification that may introduce errors in the near-electrode regions where the plasma may be non-equilibrium. Future work could investigate the effects of non-LTE conditions on the dual-TIG arc behavior, particularly at lower currents where the plasma may not be fully ionized.

The model also assumes a stationary arc, which is a simplification for welding applications where the arc is continuously moving. The effects of arc travel speed on the dual-TIG arc behavior, including the transient interaction between the two arcs and the weld pool, would require a more complex time-dependent simulation.

The practical implementation of dual-TIG welding requires careful consideration of torch design, power supply configuration, and shielding gas management. The numerical results provide guidance for optimizing these parameters, but experimental validation under actual welding conditions is essential before process deployment.

Study Insights and Implications

This numerical study provides a valuable theoretical foundation for understanding the dual-TIG welding arc. The insights gained from the simulation have direct practical implications for process development and optimization. The understanding of how current distribution affects arc behavior enables engineers to make informed decisions about torch configuration and parameter selection.

The finding that the dual-TIG arc pressure is lower than a single-TIG arc at the same total current (for symmetric distribution) is a critical insight for process design. It indicates that the dual-TIG configuration is not simply a way to increase penetration at the same current level; rather, it requires careful optimization of current distribution to achieve the desired weld geometry.

For pipe welding applications, the numerical results suggest that the dual-TIG process could be advantageous for welding thick-walled stainless steel pipes where deep penetration is required. The ability to control the arc force distribution through current adjustment provides additional flexibility in shaping the weld pool, which is particularly useful for welding in different positions (flat, horizontal, vertical, overhead).

The numerical modeling approach demonstrated in this study can be extended to more complex configurations, such as three-electrode or multi-electrode welding, and can be coupled with weld pool models to provide a complete process simulation. This integrated modeling capability is essential for the rational design of advanced welding processes and for reducing the experimental effort required for process qualification.