Zhuojin Pipe Fitting Co., LtdFree Surface Evolution Behavior of Molten Pool in Stationary TIG Welding
Literature Overview
This paper by Fan Ding, Huang Lin, Huang Jiankang, and Shi Yu, published in 2014 in the Journal of Lanzhou University of Technology, presents a three-dimensional numerical simulation of the molten pool free surface evolution during stationary TIG welding. Funded by the National Natural Science Foundation of China (Project 51205179), the study employs the Volume of Fluid (VOF) method combined with a Gaussian heat source model to track the free surface deformation of the molten pool over time.
Core Technical Points
Traditional numerical models of welding molten pools often assume a flat or parabolic free surface, which simplifies the mathematical formulation but fails to capture the true physical behavior. This study takes a more rigorous approach by solving the full Navier-Stokes equations with the VOF method to explicitly track the liquid-gas interface.
The forces acting on the molten pool are:
- Electrode arc pressure: The force exerted by the arc plasma on the molten pool surface, typically 0.5–2.0 kPa.
- Electromagnetic force (Lorentz force): Generated by the interaction of arc current with induced magnetic fields, driving fluid flow toward the pool center.
- Buoyancy force: Driven by temperature-dependent density variations, promoting upward flow of hotter, lighter metal.
- Marangoni shear force: Surface tension gradient force driven by temperature variations along the free surface, creating inward or outward surface flow depending on the surface tension gradient.
Numerical Model and Boundary Conditions
The simulation model incorporates:
| Model Component | Description |
|---|---|
| Heat source | Double-ellipsoidal Gaussian distribution (Goldak model) |
| Free surface tracking | VOF method with sharp interface capture |
| Flow solver | FLOW3D with user-defined subroutines |
| Phase change | Enthalpy method for latent heat treatment |
| Surface tension | Temperature-dependent σ(T) = σ₀(1 - β(T-Tm)) |
| Mesh | Adaptive mesh refinement near free surface |
The governing equations include:
- Mass conservation: ∂ρ/∂t + ∇·(ρu) = 0
- Momentum conservation: ρ(∂u/∂t + u·∇u) = -∇p + ∇·(μ∇u) + F_arc + F_Lorentz + F_buoyancy + F_Marangoni
- Energy conservation: ρCp(∂T/∂t + u·∇T) = ∇·(k∇T) + Q_arc + Q_latent
Results and Analysis
The study reveals several important findings regarding free surface evolution:
- Initial stage (0–1 s): The molten pool forms rapidly under arc pressure, creating a depressed surface profile. The pool width increases faster than the pool depth.
- Intermediate stage (1–5 s): Marangoni convection develops as surface tension gradients establish. The free surface begins to exhibit undulations due to the interplay between inward Marangoni flow and outward arc pressure.
- Steady-state stage (> 5 s): The pool reaches a quasi-steady configuration with a wide and shallow morphology. The free surface displays pronounced convex-concave deformations that stabilize over time.
The following table summarizes typical pool dimensions at steady state for common TIG welding parameters:
| Current (A) | Pool Width (mm) | Pool Depth (mm) | Aspect Ratio (W/D) |
|---|---|---|---|
| 100 | 8.0–10.0 | 2.0–3.0 | 3.0–4.0 |
| 150 | 12.0–15.0 | 3.0–4.5 | 3.0–4.0 |
| 200 | 16.0–20.0 | 4.0–6.0 | 3.0–4.0 |
| 250 | 20.0–25.0 | 5.0–7.5 | 3.0–4.0 |
Engineering Practice Integration
Understanding the free surface morphology is critical for predicting weld bead geometry and defect formation:
- Weld bead convexity: The free surface profile directly determines the final weld bead shape. A highly convex pool surface produces a convex weld bead, which may have lower fatigue strength.
- Crater defects: The final depression of the pool surface during solidification can lead to crater porosity and hot cracking.
- Undercut formation: The interaction between the pool surface and the base metal edge determines whether undercut will form at the weld toe.
For process optimization, the following strategies emerge:
- Current pulsing: Reducing the mean current while maintaining peak current can flatten the pool surface, reducing convexity.
- Electrode angle control: Tilting the electrode changes the arc pressure distribution, allowing asymmetric pool shaping.
- Magnetic field application: External magnetic fields can be used to manipulate pool flow and surface morphology.
Key Questions and Reflections
The study demonstrates that the free surface is far from flat during TIG welding, which challenges the common simplification used in many analytical models. However, the stationary weld simulation does not account for the transient effects of electrode movement, which creates additional surface waves and pool asymmetry in the travel direction.
A practical question for engineers is: How does the free surface deformation translate to solidified weld bead geometry? The relationship is not straightforward because solidification occurs from the pool boundary inward, and the final bead shape depends on the thermal history as well as the instantaneous pool shape.
Study Insights and Implications
This research advances the fundamental understanding of TIG welding pool physics by demonstrating the importance of free surface dynamics. The VOF-based approach provides a framework that can be extended to moving weld simulations, multi-pass welding, and other arc welding processes. For engineers involved in welding process development, these findings emphasize that pool surface morphology is a key intermediate variable linking process parameters to final weld quality. Computational models incorporating free surface tracking can serve as powerful tools for virtual process optimization, reducing the need for extensive physical trial-and-error experimentation. The work also highlights the continued importance of fundamental physics research in supporting practical welding applications, bridging the gap between theoretical understanding and industrial implementation.