Zhuojin Pipe Fitting Co., LtdNumerical Simulation of TIG Full-Penetrated Weld Pool Liquid Surface Shape
Literature Overview
This paper by He Jingshan and colleagues from the State Key Laboratory of Advanced Welding Production Technology at Harbin Institute of Technology, published in "Rare Metals" (Vol. 26, No. S1, 2007, pp. 73-76), presents a numerical simulation study of the three-dimensional liquid surface behavior of TIG full-penetrated weld pools. The research employs the Surface Evolver finite element analysis software to model the weld pool geometry under different arc conditions, with particular focus on the back fusion zone radius (RB) and the influence of the electric arc on weld pool morphology.
Core Technical Approach
The study establishes three-dimensional shape models of the TIG penetrated weld pool by varying the back fusion zone radius (RB), which serves as a key geometric parameter characterizing the extent of back-side fusion. The numerical methodology combines thermodynamic principles with finite element analysis to predict the equilibrium shape of the liquid metal surface under the competing influences of surface tension, gravity, and electromagnetic forces from the electric arc.
| Parameter | Description | Typical Range |
|---|---|---|
| Back fusion zone radius (RB) | Radius of the fusion zone on the back side | 1-5 mm (depending on thickness and parameters) |
| Weld pool depth | Maximum penetration depth | Related to current and plate thickness |
| Arc voltage | Voltage across the arc gap | 10-20 V (typical TIG range) |
| Welding current | Current intensity | 100-300 A (typical for pipe welding) |
The Surface Evolver software is particularly suited for this type of analysis because it minimizes the total surface energy of the liquid metal, naturally accounting for the balance between surface tension, gravitational forces, and external electromagnetic forces. This energy minimization approach provides physically realistic predictions of weld pool shape without the need for solving complex fluid dynamics equations.
Key Simulation Results
Arc Influence on Weld Pool Geometry
The simulation results demonstrate that the size and shape of the back weld pool are not significantly affected by the arc, while the top surface of the weld pool is lowered by the arc force. This finding is counterintuitive to some degree, as one might expect the electromagnetic forces from the arc to have a more pronounced effect on the entire weld pool geometry. The explanation lies in the relative magnitudes of the forces involved: the electromagnetic force from the arc primarily acts on the top surface of the weld pool where the current density is highest, while the back side of the weld pool is shielded from direct arc force by the thickness of the plate.
Pulse Parameter Effects
The study identifies two contributing factors to the increase in arc voltage from pulse background to pulse peak:
- Increase in welding current — The higher current intensity during the pulse peak directly increases the arc voltage according to the arc characteristic.
- Lowering of the top weld pool surface — The increased electromagnetic force during the pulse peak depresses the top surface of the weld pool, effectively increasing the arc length and consequently the arc voltage.
This dual-mechanism explanation has practical implications for pulse TIG welding parameter optimization. The depression of the weld pool surface during the pulse peak creates a dynamic geometry that must be accounted for in weld pool modeling and process control.
Engineering Practice Implications
Weld Pool Geometry and Defect Prevention
Understanding the three-dimensional shape of the weld pool is critical for predicting and preventing several common welding defects:
| Defect Type | Relationship to Weld Pool Shape | Prevention Strategy |
|---|---|---|
| Back spatter / undercut | Excessive back fusion zone radius | Optimize RB through current and travel speed control |
| Hot cracking | Excessive weld pool depth and width | Reduce heat input and optimize filler metal composition |
| Incomplete fusion | Insufficient penetration depth | Increase current or reduce travel speed |
| Excessive convexity | Excessive top surface depression | Reduce current or increase travel speed |
Application to Pipe Girth Welding
For pipe manufacturing, the simulation results have direct relevance to girth welding operations:
- Full-penetration root pass — The back fusion zone radius (RB) is a critical parameter for ensuring complete root fusion without excessive burn-through. The simulation provides a tool for predicting RB as a function of welding parameters.
- Multi-pass welding — The weld pool shape from the root pass affects the geometry and quality of subsequent fill and cap passes. Understanding the three-dimensional weld pool shape enables better planning of the welding sequence.
- Positional welding — The effect of gravity on weld pool shape varies with welding position. The numerical model can be adapted to predict weld pool behavior in vertical, overhead, and horizontal positions.
Process Parameter Optimization
The simulation results support the following practical recommendations for TIG welding of pipe:
- Current selection — For full-penetration welding of plates up to 6 mm thickness, currents in the range of 150-250 A typically produce optimal weld pool geometry with adequate penetration and controlled back fusion.
- Travel speed optimization — The travel speed must be matched to the current to maintain the desired weld pool geometry. Excessive travel speed leads to incomplete penetration, while insufficient travel speed causes excessive weld pool depression and potential burn-through.
- Pulse parameter tuning — The pulse peak current should be selected to achieve adequate penetration during the peak while the background current maintains the weld pool in a liquid state without excessive heat input.
Key Questions and Reflections
The simulation study raises several important questions for further investigation:
- How accurately does the Surface Evolver model predict weld pool shape compared to experimental measurements using high-speed imaging or neutron radiography?
- Can the model be extended to incorporate the effect of filler metal deposition on weld pool shape evolution during multi-pass welding?
- What is the effect of welding position on the predicted weld pool geometry, and how does this relate to the observed differences in weld quality between flat and overhead positions?
The finding that the back weld pool is relatively insensitive to arc force is particularly important for pipe welding, as it suggests that back-side quality is primarily determined by thermal factors (current, travel speed, plate thickness) rather than electromagnetic factors. This insight simplifies the process control strategy for ensuring consistent root weld quality.
Summary
This numerical simulation study provides valuable insights into the three-dimensional geometry of TIG full-penetrated weld pools, with particular emphasis on the back fusion zone radius and the differential effect of the electric arc on the top and back surfaces of the weld pool. The key engineering takeaway is that back-side weld pool geometry is primarily governed by thermal parameters rather than electromagnetic forces, which simplifies the process control strategy for achieving consistent root weld quality in pipe manufacturing. The dual-mechanism explanation of arc voltage increase during pulse peaks provides a useful framework for pulse parameter optimization. These simulation results should be validated against experimental measurements and integrated into welding procedure qualification programs to improve the predictive capability of process design.