CLADDING TECHNOLOGY SHANXI CO., LTDNumerical Simulation of Molten Pool Surface Deformation in TIG Welding
Literature Overview
The study by Zhang Jianbao and Wang Hong from Taiyuan University of Science and Technology (2016) presents a numerical simulation of the molten pool surface deformation during gas tungsten arc (TIG) welding. Molten pool surface morphology is a critical indicator of welding quality, as it directly influences the weld bead geometry, the solidification structure, and the formation of surface defects such as ripples, grooves, and porosity. The numerical approach combines electromagnetic force modeling, fluid flow simulation, and heat transfer analysis to predict the pool surface shape under various welding conditions.
Core Technical Concepts
Governing Physical Phenomena
The molten pool surface deformation in TIG welding results from the combined action of several forces:
- Electromagnetic (Lorentz) force: Generated by the interaction of the welding current with its self-induced magnetic field, this force acts radially inward and downward, depressing the pool surface and creating a crater.
- Surface tension gradient (Marangoni) force: Driven by temperature-dependent surface tension variations, this force creates surface flow patterns that shape the pool boundary.
- Buoyancy force: Arising from density differences due to temperature gradients, this force drives natural convection within the pool.
- Arc pressure: The momentum transfer from the plasma jet to the pool surface creates a localized depression.
| Force Component | Magnitude Order | Direction | Effect on Pool |
|---|---|---|---|
| Lorentz force | 10³–10⁴ Pa | Radial inward, downward | Pool depression |
| Marangoni force | 10¹–10² Pa | Along surface gradient | Surface flow |
| Buoyancy force | 10⁰–10¹ Pa | Upward (hot) | Natural convection |
| Arc pressure | 10²–10³ Pa | Downward (axial) | Pool crater |
Numerical Model Formulation
The simulation solves the coupled Navier-Stokes equations for fluid flow, the energy equation for heat transfer, and the electromagnetic force equation. The pool surface is modeled as a free surface using a volume-of-fluid (VOF) method or a level-set method, which tracks the liquid-gas interface as it deforms under the combined action of all forces.
The boundary conditions include:
- Thermal: Arc heat flux distribution on the pool surface (typically Gaussian or double-Gaussian)
- Mechanical: Surface tension, Lorentz force, and arc pressure on the free surface
- Inlet: Solute diffusion and thermal boundary conditions at the solid-liquid interface
Process Analysis and Simulation Results
Pool Surface Deformation Patterns
The numerical results reveal that the pool surface deformation is highly sensitive to the welding current, travel speed, and electrode configuration. At low currents, the pool surface is relatively flat with a slight depression at the arc center. As current increases, the depression deepens and the pool surface develops a characteristic shape with a raised rim ahead of the arc and a deep crater beneath it.
| Current (A) | Pool Depth (mm) | Pool Width (mm) | Surface Depression (mm) |
|---|---|---|---|
| 80 | 1.2 | 6.0 | 0.3 |
| 120 | 2.0 | 9.0 | 0.8 |
| 160 | 3.0 | 12.0 | 1.5 |
| 200 | 4.2 | 15.0 | 2.4 |
Effect of Travel Speed
The travel speed affects the pool surface deformation by changing the residence time of the arc on any given point. At low travel speeds, the pool has sufficient time to reach a quasi-steady state, and the surface deformation is symmetric about the arc axis. At high travel speeds, the pool becomes elongated in the travel direction, and the surface deformation becomes asymmetric with a raised lip at the trailing edge.
The critical travel speed at which the pool surface transitions from symmetric to asymmetric deformation depends on the material's thermal diffusivity and the pool's solidification rate. For carbon steel, this transition typically occurs at travel speeds of 30–50 cm/min for currents of 100–150 A.
Engineering Practice Integration
Connection to Weld Bead Geometry
The pool surface deformation directly determines the weld bead geometry after solidification. A deep pool depression results in an undercut at the weld toe, while an elongated pool produces a bead with a raised trailing edge. These geometric features affect the stress concentration at the weld toe and can initiate fatigue cracks under cyclic loading.
For clad pressure vessel fabrication, the weld bead geometry of the cladding welds must be carefully controlled to ensure uniform clad thickness and to avoid undercut at the clad-to-base metal interface. The numerical simulation provides a predictive tool for optimizing welding parameters to achieve the desired bead profile.
Application to Weld Overlay Quality
In weld overlay applications, the pool surface deformation affects the dilution ratio between the overlay material and the base metal. A deeper pool depression increases the volume of base metal melted, leading to higher dilution and reduced corrosion resistance of the overlay layer. The simulation can predict the dilution ratio as a function of welding parameters, enabling engineers to select parameters that minimize dilution while maintaining adequate bond strength.
| Overlay Material | Target Dilution | Pool Depth Limit | Parameter Adjustment |
|---|---|---|---|
| Inconel 625 | <15% | <2.5 mm | Reduce current, increase speed |
| 316L | <20% | <3.0 mm | Moderate current, moderate speed |
| Monel 400 | <10% | <2.0 mm | Low current, high speed |
Key Questions and Reflections
The numerical simulation provides valuable insights into the pool surface deformation mechanism, but several simplifying assumptions limit its direct applicability to production welding. The model typically assumes a stationary arc and does not account for arc oscillation, which is common in production TIG welding to improve bead uniformity. Additionally, the surface tension model used in most simulations assumes a pure metal surface, while in practice, surface-active elements (such as sulfur and phosphorus) in the weld metal can significantly alter the surface tension gradient.
Another important consideration is the validation of the simulation against experimental data. Pool surface measurements are challenging to obtain due to the high temperatures and rapid solidification, and most available data come from high-speed photography or thermography. The agreement between simulation and experiment is typically within 10–15% for pool dimensions, which is adequate for process optimization but insufficient for precise defect prediction.
Study Insights and Implications
The numerical simulation of molten pool surface deformation provides a powerful tool for understanding the fundamental physics of TIG welding and for predicting weld bead geometry. For engineers involved in bimetallic cladding and pressure vessel fabrication, the key takeaway is that pool surface morphology is not merely an aesthetic concern but a critical quality parameter that affects dilution, defect formation, and mechanical performance. The simulation approach enables virtual process optimization before physical trials, reducing the cost and time of procedure development. However, the engineer must exercise judgment in interpreting simulation results, recognizing the limitations of the model and validating predictions with physical experiments before applying them to production welding.