Zhuojin Pipe Fitting Co., LtdNumerical Simulation of TIG Welding Pool Surface Deformation Under Arc Pressure
Literature Overview
This paper by Zhang Jianbao and Wang Hong from the School of Materials Science and Engineering, Taiyuan University of Science and Technology, published in Hot Working Technology in 2016 (Volume 45, Issue 11, pages 223–225), presents a numerical simulation study of TIG welding pool surface deformation caused by arc pressure. The authors developed a finite element model that accounts for the free surface of the molten pool, employed a moving double-ellipsoidal heat source to represent the arc heat input, and used FLUENT software to simulate the stress field on the pool surface under different welding currents. The study provides valuable insights into the complex interaction between arc forces, fluid dynamics, and pool geometry in TIG welding.
Core Technical Content
Free Surface Modeling Approach
Modeling the free surface of a welding pool is inherently challenging because the pool boundary is not fixed — it deforms continuously under the combined influence of surface tension, arc pressure, buoyancy forces, and electromagnetic forces. The authors employed a free surface model that tracks the pool boundary as it deforms during welding. This is a significant advancement over earlier models that assumed a fixed pool shape or used simplified geometric approximations.
The governing equations for the pool fluid flow and heat transfer include:
- Continuity equation — Conservation of mass for the incompressible molten metal
- Momentum equation — Navier-Stokes equations with body force terms for arc pressure, buoyancy, and electromagnetic forces
- Energy equation — Heat conduction with convective heat transfer and heat source terms
- Free surface boundary conditions — Stress balance at the pool surface accounting for surface tension, arc pressure, and atmospheric pressure
Moving Double-Ellipsoidal Heat Source
The heat source model used in this study is a double-ellipsoidal geometry that moves with the welding torch. The front half of the ellipsoid (in the direction of travel) represents the keyhole region with concentrated heat input, while the rear half represents the backflow region with more distributed heat input.
| Heat Source Parameter | Symbol | Typical Value | Description |
|---|---|---|---|
| Front ellipsoid length | a | 1.5–3.0 mm | Half-length in travel direction |
| Front ellipsoid width | b | 1.5–3.0 mm | Half-width perpendicular to travel |
| Front ellipsoid depth | c | 2.0–5.0 mm | Half-depth into the workpiece |
| Rear ellipsoid length | d | 2.0–4.0 mm | Half-length in travel direction |
| Rear ellipsoid width | e | 2.0–4.0 mm | Half-width perpendicular to travel |
| Rear ellipsoid depth | f | 2.5–6.0 mm | Half-depth into the workpiece |
| Front heat fraction | q₁ | 0.4–0.6 | Fraction of total heat in front ellipsoid |
| Rear heat fraction | q₂ | 0.6–0.4 | Fraction of total heat in rear ellipsoid |
| Total heat input | Q | 4–15 kW | Depends on current and voltage |
The double-ellipsoidal model is widely used in welding simulation because it captures the asymmetric heat distribution observed experimentally — the heat input is concentrated in the front half of the pool (in the direction of travel) due to the keyhole effect, while the rear half has a more distributed heat profile.
Arc Pressure and Pool Deformation Results
The simulation results reveal several important characteristics of the pool surface stress distribution:
- Lateral symmetry — The stress distribution on the pool surface is symmetric in the direction perpendicular to the weld travel direction (left-right symmetry). This is expected because the welding process is symmetric about the centerline of the weld.
- Longitudinal asymmetry — The stress distribution is asymmetric in the direction of travel (front-back asymmetry). The front portion of the pool (ahead of the arc) has a more concentrated stress distribution, while the rear portion (behind the arc) has a more dispersed distribution. This asymmetry is caused by the directional nature of the arc force and the fluid flow within the pool.
- Parabolic lateral distribution — The stress values along the lateral direction (perpendicular to travel) follow a parabolic distribution, with the maximum stress at the center of the weld and decreasing values toward the pool edges. This is consistent with the pressure distribution of a cylindrical arc column.
- Trailing effect — The stress distribution exhibits a "trailing" phenomenon, where the high-stress region is shifted toward the rear of the pool. This is attributed to the fluid dynamics within the pool — the arc force drives molten metal outward and backward, creating a wake-like pattern in the stress distribution.
Effect of Welding Current
| Current (A) | Maximum Arc Pressure (MPa) | Pool Surface Deformation (mm) | Stress Gradient |
|---|---|---|---|
| 100 | 0.08 | 0.15 | Moderate |
| 150 | 0.15 | 0.30 | Higher |
| 200 | 0.25 | 0.45 | High |
| 250 | 0.38 | 0.60 | Very high |
| 300 | 0.52 | 0.75 | Extremely high |
The results clearly demonstrate that both arc pressure and pool surface deformation increase with welding current. The relationship is nonlinear — the rate of increase accelerates at higher currents. This has practical implications for weld geometry: higher currents not only increase heat input but also increase the mechanical force exerted on the pool surface, which can lead to deeper penetration and a wider weld bead.
Engineering Practice Integration
Weld Geometry Prediction
The numerical simulation results can be used to predict weld geometry as a function of welding parameters. By correlating pool surface deformation with weld bead dimensions, engineers can optimize welding parameters to achieve desired weld profiles. For example:
| Desired Weld Profile | Recommended Current Range | Expected Pool Deformation | Key Consideration |
|---|---|---|---|
| Narrow, deep penetration | 150–200 A | 0.30–0.45 mm | High arc pressure drives deep penetration |
| Wide, shallow bead | 100–150 A | 0.15–0.30 mm | Lower arc pressure allows wider spread |
| Balanced profile | 150–200 A | 0.30–0.45 mm | Optimal balance of width and depth |
Distortion Control
Understanding pool surface deformation is also critical for controlling welding distortion. The asymmetric stress distribution causes asymmetric thermal expansion and contraction, which contributes to angular and longitudinal distortion. By simulating the stress field, engineers can predict distortion patterns and design fixture strategies to minimize them.
Process Optimization
The simulation can be used to optimize welding parameters for specific applications. For example, in the fabrication of thin-walled stainless steel pipes, excessive pool deformation can lead to burn-through. By simulating the pool deformation at different current levels, the maximum allowable current can be determined before burn-through occurs.
Study Insights and Reflections
This paper makes a valuable contribution to the understanding of TIG welding pool dynamics by explicitly modeling the free surface and its deformation under arc pressure. The use of a moving double-ellipsoidal heat source is appropriate and well-validated in the welding simulation community. The simulation results are consistent with experimental observations, which validates the model and gives confidence in its predictive capability.
One particularly insightful finding is the parabolic lateral stress distribution. This suggests that the arc pressure acts as a distributed load on the pool surface, similar to a fluid pressure on a submerged surface. The trailing effect in the longitudinal direction is also physically intuitive — the arc force pushes molten metal backward and outward, creating a wake pattern that persists in the stress distribution.
The nonlinear relationship between current and pool deformation has practical significance. It means that small increases in current at high current levels can cause disproportionately large changes in pool geometry, which can lead to sudden weld quality degradation. This insight should be incorporated into welding procedure design guidelines, with particular attention to the upper limits of current for thin-section applications.
A limitation of the current model is that it focuses on the stress field and pool deformation but does not fully account for the dynamic evolution of the pool shape during the welding process. Future work could extend this model to include a full fluid-structure interaction (FSI) analysis that captures the time-dependent pool shape evolution, including the formation and collapse of the keyhole.
Reference Value and Outlook
This study provides a solid foundation for understanding the mechanics of TIG welding pool deformation. The numerical model and simulation results are directly applicable to welding procedure optimization and distortion prediction. The approach demonstrated here — combining finite element analysis with physically motivated heat source models — is a powerful tool for welding process development and can be extended to other welding processes such as plasma arc welding, laser welding, and electron beam welding. Future research should focus on multi-physics coupling that includes electromagnetic forces, fluid dynamics, and solidification dynamics in a unified framework, enabling more accurate predictions of weld geometry and quality. The insights gained from this study — particularly the nonlinear current-deformation relationship and the asymmetric stress distribution — should be incorporated into welding engineering practice to improve process control and quality assurance.