Effect of Tungsten Electrode Spacing on Arc Characteristics of Dual-Tungsten TIG Welding

Literature Overview

This paper, published in Ordnance Materials and Science (2025, Vol. 48, No. 1, pp. 58-63) by Guo Chaobo and colleagues from Henan University of Engineering, investigates the arc characteristics of dual-tungsten electrode TIG welding using a three-dimensional steady-state mathematical model. The study systematically examines how the spacing between the two tungsten electrodes affects arc temperature, plasma velocity, arc pressure, electromagnetic force, and arc morphology. This research is significant for understanding the fundamental physics of coupled arc behavior, which has implications for welding process design, heat input distribution, and weld quality optimization.

Core Technical Findings

Mathematical Model and Assumptions

The study establishes a three-dimensional steady-state mathematical model for the dual-tungsten TIG welding arc. The model solves the coupled control equations for:

Mass conservation
Momentum conservation
Energy conservation
Maxwell's equations for electromagnetic fields
Ohm's law for current distribution

The boundary conditions and assumptions are carefully defined to represent the physical reality of the dual-electrode configuration. The model is validated by comparing results with single-tungsten TIG welding arc characteristics, ensuring that the dual-electrode model reduces to the single-electrode case when the spacing approaches infinity.

Arc Temperature Distribution

Condition	Arc Temperature	Comparison
Single tungsten TIG	Baseline	Reference
Dual tungsten TIG (coupled)	Significantly reduced	Lower than single
Effect of spacing	Decreases then increases	Non-monotonic

The coupled dual-tungsten arc exhibits significantly lower temperatures compared to single-tungsten TIG welding. This reduction is attributed to the mutual interaction between the two arcs, which redistributes energy and creates a more uniform temperature field. As the electrode spacing increases, the interaction weakens, and the coupled arc gradually separates into two independent arcs.

Plasma Velocity and Arc Pressure

Parameter	Single Tungsten	Dual Tungsten (Coupled)	Trend with Spacing
Plasma velocity	Baseline	Significantly reduced	Decreases with spacing
Arc pressure	Single peak distribution	Double peak distribution	Transitions with spacing

The plasma velocity in the coupled dual-tungsten arc is substantially lower than in single-tungsten TIG welding. This reduction in velocity affects:

Heat transfer to the workpiece
Arc stability and oscillation characteristics
Penetration profile and weld geometry
Metal transfer in consumable electrode variants

The arc pressure distribution transitions from a single peak (single tungsten) to a double peak (dual tungsten) as the electrode spacing decreases. The maximum arc pressure first decreases and then increases with increasing spacing, reflecting the complex interaction between the two arc columns.

Electromagnetic Force Distribution

Spacing	Electromagnetic Force Direction	Magnitude
Small spacing	Toward arc center	Higher
Medium spacing	Transitioning	Moderate
Large spacing	Toward both sides	Lower

As the electrode spacing increases, the electromagnetic force direction gradually shifts from toward the arc center to toward both sides. The magnitude of the electromagnetic force decreases with increasing spacing as the interaction between the two arcs weakens. This behavior has implications for arc stability and the tendency toward arc oscillation or detachment.

Arc Morphology

The coupled dual-tungsten arc exhibits a non-axisymmetric distribution, unlike the axisymmetric single-tungsten arc. The morphology changes with spacing:

Small spacing: Highly coupled, merged arc column with complex shape
Medium spacing: Partially coupled, two distinct but interacting arc columns
Large spacing: Essentially two independent single-tungsten arcs

The non-axisymmetric morphology affects:

Heat input distribution on the workpiece
Weld bead geometry and profile
Residual stress patterns
Microstructural evolution in the weld and HAZ

Engineering Practice Implications

Heat Input and Penetration

The reduced arc temperature and plasma velocity in dual-tungsten TIG welding suggest:

Lower peak temperatures at the workpiece surface
Reduced heat input per electrode compared to single-electrode operation
Potentially deeper penetration due to the focused interaction zone
More uniform heat distribution across the weld width

These characteristics may be beneficial for:

Welding of thin sections where heat input must be controlled
Applications requiring uniform weld profiles
Situations where reduced distortion is critical

Process Design Considerations

Parameter	Recommendation	Rationale
Electrode spacing	Optimize based on workpiece thickness	Balance coupling strength and arc stability
Welding current	Reduce per electrode compared to single	Compensate for reduced arc temperature
Travel speed	May increase	Lower heat input allows faster welding
Shielding gas	Ensure adequate coverage for both arcs	Prevent oxidation in coupled region
Electrode geometry	Consider non-conical tips	May enhance coupling and stability

Weld Quality Implications

The dual-tungsten configuration may produce:

More uniform weld bead profiles due to symmetric heat input
Reduced solidification cracking susceptibility due to lower peak temperatures
Modified grain structure in the weld metal
Different residual stress patterns compared to single-electrode welding

However, the non-axisymmetric arc morphology introduces challenges:

Asymmetric heat input if electrodes are not perfectly symmetric
Potential for uneven penetration
Complexity in parameter optimization
Need for careful electrode alignment

Key Questions and Reflections

The numerical simulation approach provides valuable insights into the fundamental physics of dual-tungsten TIG welding, but several questions remain:

Experimental validation: How well do the simulated results match experimental measurements of arc temperature, pressure, and morphology?
Dynamic effects: The steady-state model does not capture arc oscillation, fluctuation, or transient behavior that may occur in practice.
Workpiece interaction: The model focuses on arc characteristics but does not fully account for the interaction with the molten pool and solidifying weld metal.
Parameter sensitivity: How sensitive are the results to variations in gas flow rate, electrode material, and workpiece properties?

The finding that arc pressure transitions from single peak to double peak with decreasing spacing is particularly interesting. This suggests that the electromagnetic forces acting on the molten pool will also change character, potentially affecting weld pool shape and flow patterns. Understanding these interactions is critical for predicting weld geometry and quality.

Study Insights and Outlook

This research provides a comprehensive understanding of the arc characteristics in dual-tungsten TIG welding as a function of electrode spacing. The three-dimensional steady-state model reveals that the coupled arc exhibits significantly different temperature, velocity, pressure, and electromagnetic force distributions compared to single-tungsten TIG welding. The non-monotonic behavior of maximum arc pressure with spacing and the transition from single-peak to double-peak pressure distribution highlight the complex physics of coupled arc interaction. For engineering applications, these findings suggest that dual-tungsten TIG welding offers a pathway to reduced heat input and more uniform weld profiles, but careful optimization of electrode spacing and welding parameters is essential. Future work should focus on experimental validation, dynamic modeling, and investigation of the workpiece-arc interaction to fully exploit the potential of this welding configuration for improved weld quality and productivity.