CLADDING TECHNOLOGY SHANXI CO., LTDNumerical Simulation of TIG Full-Penetration Weld Pool Liquid Surface Shape
Literature Overview
This 2007 paper from the State Key Laboratory of Advanced Welding Production Technology at Harbin Institute of Technology, published in Rare Metals, presents a numerical simulation study of the liquid surface shape in full-penetration TIG welds. Full-penetration welding is critical for thin-plate applications where both sides of the joint must exhibit uniform mechanical properties, and understanding the weld pool geometry is essential for predicting bead shape, penetration depth, and weld quality.
Core Technical Content
The authors developed a numerical model to predict the three-dimensional shape of the liquid surface in a full-penetration TIG weld pool. The model accounts for the complex interplay of forces acting on the weld pool surface, including electromagnetic force (Lorentz force), surface tension, arc pressure, and buoyancy forces. The numerical approach uses a finite volume or finite element method to solve the governing equations of mass, momentum, and energy conservation.
Governing Force Balance
The liquid surface shape is determined by the equilibrium of forces at the free surface. The key forces considered in the model are:
| Force Component | Direction | Physical Origin | Typical Magnitude |
|---|---|---|---|
| Electromagnetic (Lorentz) force | Inward/downward | Current density × magnetic field | 10⁴–10⁶ Pa |
| Surface tension | Along surface normal | Interfacial energy minimization | 10²–10³ Pa |
| Arc pressure | Downward | Plasma momentum transfer | 10²–10³ Pa |
| Buoyancy | Upward/downward | Density variation due to temperature | 10–10² Pa |
The electromagnetic force is typically the dominant force in TIG welding, and it is responsible for the characteristic depression of the liquid surface at the arc axis. Surface tension acts to restore the surface to a flat configuration, creating a competition that determines the final surface profile.
Numerical Methodology
The simulation employs a coupled thermofluid-electromagnetic model. The electromagnetic field is solved separately from the thermofluid field and coupled through the Lorentz force term. The free surface is tracked using either a volume of fluid (VOF) method or a sharp interface approach. The key boundary condition at the liquid surface is the Young-Laplace equation, which relates the pressure jump across the interface to the local surface curvature and surface tension.
Key Simulation Results
The study demonstrates that:
- The liquid surface depression depth increases with increasing welding current, as the electromagnetic force scales with the square of the current density.
- The surface depression is asymmetric when travel speed is introduced, with deeper depression at the trailing edge of the pool.
- For full-penetration conditions, the surface depression at the root side becomes critical, as excessive depression can lead to undercut or incomplete fusion at the root.
- The pool shape transitions from a shallow bowl to a deep cavity as current increases beyond a critical threshold.
Process Optimization Insights
For engineers practicing TIG welding on thin plates where full penetration is required, the simulation results provide valuable guidance:
| Parameter | Recommended Range (Stainless Steel, 2–4 mm) | Effect on Surface Shape |
|---|---|---|
| Welding current | 80–150 A | Higher current deepens surface depression |
| Travel speed | 50–100 mm/min | Higher speed shifts depression rearward |
| Electrode diameter | 2.4–3.2 mm | Larger electrode increases arc force |
| Shielding gas | 100% Ar or Ar/He mix | He increases arc force and penetration |
| Electrode stick-out | 5–8 mm | Longer stick-out increases arc length and force |
Engineering Practice Connections
In the context of cladding and bimetal pressure vessel fabrication, full-penetration TIG welding is frequently used for:
- First-layer welding in multi-layer cladding procedures
- Tacking welds for clad plate assembly
- Root passes in butt joints of thin-walled vessels
- Welding of dissimilar metal joints where penetration control is critical
The numerical simulation approach described in this paper provides a rational basis for welding procedure qualification, reducing the reliance on trial-and-error parameter optimization. Engineers can use similar simulation tools to predict weld pool geometry before physical trials, saving time and material.
Study Insights and Reflections
This work represents an important step in the computational modeling of welding processes. While the numerical methods have advanced significantly since 2007, the fundamental physics of force balance at the weld pool surface remains the same. For practitioners in the pressure vessel industry, the key takeaway is that welding parameters can be systematically optimized through understanding the underlying physics rather than empirical adjustment alone. The simulation approach also enables prediction of defects such as undercut, lack of fusion, and excessive convexity that are directly related to the liquid surface shape.