Zhuojin Pipe Fitting Co., LtdNumerical Simulation of Arc Characteristics of Hollow Tungsten Electrode TIG Welding
Literature Overview
This paper by Lei Zheng and colleagues from Southwest Jiaotong University, published in the Journal of Welding in 2021, presents a comprehensive numerical simulation study of the arc characteristics of a hollow tungsten electrode TIG welding process. The authors established a numerical model for a hollow tungsten electrode with an inner diameter of 2 mm and employed Fluent software with User Defined Functions (UDF) to load the argon electrical conductivity, momentum equations, and energy equations. The steady-state welding current was set at 60 A, and the results were compared with conventional solid tungsten electrode TIG welding under identical conditions. This work was supported by the National Key R&D Program of China (2016YFB1102103-3) and the Sichuan Provincial Key R&D Program (2020YFG0096).
Core Technical Findings
The simulation revealed several distinctive features of the hollow tungsten electrode TIG arc that differ markedly from conventional solid electrode configurations. The arc exhibited a bell-shaped morphology, and the annular discharge at the hollow tungsten electrode combined with the cooling effect of the central gas flow caused the temperature distribution contour at the top to be concave downward. The arc plasma moved at a relatively high velocity beneath the tungsten electrode, while the arc pressure at the anode surface showed a columnar distribution. The spatial pressure distribution in the arc column region was relatively uniform.
| Parameter | Hollow Tungsten Electrode TIG | Solid Tungsten Electrode TIG | Difference |
|---|---|---|---|
| Peak arc temperature | Reduced | Baseline | -17.3% |
| Peak temperature at 2 mm below electrode | Reduced | Baseline | -27% |
| Maximum plasma velocity | Reduced | Baseline | -40% |
| Peak arc pressure | Reduced | Baseline | -57% |
| Weld bead width (surfacing) | Increased | Baseline | +30% |
| Weld penetration depth | Reduced | Baseline | -27.9% |
Technical Analysis of Arc Physics
The reduction in peak arc temperature by 17.3% is a critical finding that has direct implications for weld pool dynamics and microstructural evolution. The cooling effect of the central gas flow within the hollow electrode serves as a natural heat sink, redistributing the thermal energy away from the arc core and toward the periphery. This explains the observed increase in weld bead width by 30% coupled with a 27.9% reduction in penetration depth. The arc pressure reduction of 57% is particularly significant because arc pressure is a primary driver of mechanical stirring and penetration in TIG welding. With substantially lower arc pressure, the mechanical stirring effect on the weld pool is diminished, which further accounts for the shallower penetration.
The bell-shaped arc morphology is noteworthy from a process design perspective. In conventional TIG welding, the arc tends to be more conical or cylindrical. The bell shape of the hollow electrode arc suggests that the energy delivery is more distributed laterally rather than concentrated axially. This characteristic makes the hollow tungsten electrode TIG process particularly suitable for applications where wide, shallow welds are desirable, such as surfacing operations, cladding, and thermal spraying-like applications where dilution control is paramount.
The plasma velocity reduction of 40% beneath the electrode indicates that the internal gas flow partially counteracts the outward plasma jet, creating a more confined and controlled arc environment. This confinement effect can be advantageous for maintaining arc stability, particularly in positions where arc drift is a concern, such as vertical or overhead welding positions.
Engineering Practice Implications
From a practical standpoint, the hollow tungsten electrode TIG process offers several advantages that merit attention in industrial applications. The reduced heat input per unit volume, evidenced by the lower peak temperatures, can help minimize distortion in thin-walled components and reduce the risk of excessive grain growth in the heat-affected zone. For welding of stainless steel pipes and fittings, where distortion control is critical to maintaining dimensional tolerances and avoiding post-weld machining, this process could offer meaningful benefits.
The wider, shallower weld profile is particularly relevant for overlay welding of corrosion-resistant alloys on carbon steel substrates. In the fabrication of pipe fittings for sour service, where CRA cladding layers are applied to base materials, controlling dilution is essential. The hollow electrode's ability to produce wider beads with reduced penetration could minimize dilution of the overlay layer, thereby preserving the corrosion resistance of the cladding material.
However, the significantly reduced penetration depth of 27.9% must be carefully considered. For full-penetration welds in structural applications, such as pipe butt welds, the hollow electrode process may require multiple passes or process modifications to achieve adequate fusion. This limitation confines the primary application domain of the hollow electrode TIG process to surfacing, cladding, and thin-section welding where deep penetration is not required.
Reflections and Study Insights
The numerical modeling approach adopted by the authors is rigorous and well-suited to capturing the complex multiphysics interactions within the arc. The use of UDFs in Fluent to incorporate the electrical conductivity of argon, momentum source terms, and energy source terms demonstrates a sophisticated understanding of arc physics modeling. The comparison between hollow and solid electrode configurations under identical current conditions provides a clear baseline for evaluating the process advantages and limitations.
One aspect that warrants further investigation is the transient behavior of the hollow electrode arc, particularly during start-up and shutdown phases. The steady-state assumption, while computationally convenient, may not fully capture the dynamic arc characteristics that occur during actual welding operations. Additionally, the effect of electrode geometry parameters, such as inner diameter, outer diameter, and electrode protrusion length, on arc characteristics would be valuable to explore systematically.
This work contributes meaningfully to the understanding of modified TIG welding processes and opens avenues for developing specialized welding consumables and techniques tailored to specific industrial requirements.