Zhuojin Pipe Fitting Co., LtdNumerical Simulation of Keyhole-TIG Welding Arc Using Fluent Software
Literature Overview
This 2023 study published in Heat Treatment Technology, authored by Zhang Lingfeng and collaborators from Shanghai University of Engineering Science and international partners, presents a comprehensive numerical simulation of the free-burning Keyhole-TIG welding arc using ANSYS Fluent software. The research establishes a unified two-dimensional axisymmetric model of the arc-tungsten-electrode-anode system and simulates the steady-state behavior at a welding current of 520 A. The work is supported by multiple funding sources including the National Natural Science Foundation of China (Grant No. 52005320), Jiangsu Provincial Science and Technology Achievement Transformation Special Fund (BA2020068), and Zhejiang Provincial Key R&D Program (2021C01085).
Model Construction and Governing Equations
The model treats the welding arc as a magnetohydrodynamic (MHD) flow problem, solving the coupled Navier-Stokes equations for momentum, the energy equation for thermal transport, the species transport equation for plasma composition, and the Maxwell equations for electromagnetic field interaction. The axisymmetric assumption is justified by the geometric symmetry of a standard TIG torch configuration with a conical tungsten electrode and a flat workpiece.
Key physical phenomena incorporated into the model include:
- Lorentz force generation due to the interaction between current density and magnetic field
- Cathode jet effect, where high-velocity plasma is emitted from the tungsten electrode tip
- Thermal radiation from the arc plasma column
- Joule heating within the arc column
- Buoyancy-driven convection in the arc plasma
The boundary conditions at the tungsten electrode surface account for thermionic electron emission and the Schottky effect, while the workpiece boundary incorporates arc heat flux input with a Gaussian distribution profile.
Key Simulation Results
The simulation reveals several critical physical phenomena that govern the Keyhole-TIG process. The maximum temperature within the arc, located directly beneath the tungsten electrode tip, reaches approximately 36,000 K, which is significantly higher than the boiling point of most structural metals and provides sufficient thermal energy to produce the keyhole effect.
| Physical Quantity | Simulated Value | Engineering Significance |
|---|---|---|
| Maximum arc temperature | 36,000 K | Ensures complete metal vaporization for keyhole formation |
| Welding current | 520 A | High-current regime enabling deep penetration |
| Current entering through keyhole | 19% of total | Majority of current flows through workpiece surface |
| Heat input through keyhole | 17% of total arc heat | Most heat is deposited on workpiece upper surface |
| Cathode jet velocity | High-speed plasma | Primary mechanism for keyhole formation via dynamic pressure |
The simulation demonstrates that the cathode jet effect is the dominant mechanism for keyhole formation. High-velocity plasma emitted from the tungsten electrode generates substantial dynamic pressure on the molten metal surface, creating a depression that deepens into a full keyhole. This finding is significant because it shifts the understanding of keyhole formation from being purely thermal to being predominantly hydrodynamic in nature.
An important observation is that despite the high current density, only 19% of the total welding current enters through the keyhole, with the remaining 81% flowing through the workpiece surface surrounding the keyhole. Similarly, only 17% of the total arc heat flux is transmitted through the keyhole to the deep penetration zone, while the majority is deposited on the upper surface of the workpiece. These findings have direct implications for weld geometry prediction and penetration control.
Engineering Implications for Pipe and Fitting Welding
For heavy-wall pipe welding applications such as LSAW or UOE pipes with wall thicknesses exceeding 25 mm, Keyhole-TIG welding has become an attractive alternative to conventional multi-pass TIG welding due to its superior penetration capability and reduced number of passes. The simulation results provide quantitative data that can be used to calibrate finite element models for weld pool geometry prediction, which is essential for ensuring full root fusion and adequate reinforcement in butt-welded pipe joints.
The finding that most heat is deposited on the workpiece surface rather than through the keyhole has important consequences for heat-affected zone (HAZ) width and residual stress distribution. In thick-walled pipe welding, excessive HAZ width can lead to coarse grain growth and reduced toughness, particularly in low-alloy steels susceptible to temper embrittlement. The simulation data can inform the selection of welding parameters to minimize HAZ extent while maintaining adequate penetration depth.
The cathode jet mechanism also explains why Keyhole-TIG welding is sensitive to torch orientation and workpiece geometry. In pipe welding, where the joint geometry changes continuously around the circumference, variations in the effective angle between the torch and the joint surface can alter the cathode jet impact angle, affecting keyhole stability and penetration uniformity. This understanding supports the development of automated welding systems with real-time torch position feedback for orbital pipe welding.
Study Reflections
The use of commercial CFD software such as Fluent for welding arc simulation represents a practical approach to capturing the complex multiphysics involved in arc phenomena. The axisymmetric assumption simplifies the computation significantly and is reasonable for straight seam welding, but for orbital pipe welding, three-dimensional effects become important due to the varying joint geometry and gravity-induced convection. Future work should extend this model to three dimensions and incorporate workpiece melting and fluid flow within the weld pool to provide a complete simulation of the welding process. The quantitative data on current and heat distribution through the keyhole are particularly valuable for developing empirical relationships that can be used in welding procedure qualification and process optimization.