CLADDING TECHNOLOGY SHANXI CO., LTDNumerical Simulation of Root Fusion in Double-TIG Welding of Thick Plates
Literature Overview
This 2015 study by Yang Dongqing, Li Dayong, and Zhang Guangjun from the State Key Laboratory of Advanced Welding and Joining at Harbin Institute of Technology investigates the root fusion behavior in double-TIG welding of thick plates through numerical simulation. Supported by the National Natural Science Foundation of China (51175119), the research was published in the Transactions of the China Welding Institute. The study addresses a persistent challenge in thick-plate welding: achieving complete root fusion without excessive heat input that could lead to distortion, cracking, or unfavorable microstructure in the heat-affected zone.
Core Technical Content
Double-TIG welding, also known as twin-arc TIG or dual-electrode TIG, employs two TIG arcs simultaneously to increase deposition rate and improve weld quality in thick-section welding. The configuration can be arranged in various geometries—leading-trailing, side-by-side, or offset—each with different thermal interaction characteristics. The root fusion behavior is critical because incomplete fusion at the root of a thick-plate weld creates a stress concentration that can lead to premature fatigue failure.
Simulation Parameters and Results
| Parameter | Value/Range | Effect on Root Fusion |
|---|---|---|
| Plate thickness | 20–50 mm | Determines heat dissipation and fusion depth |
| Front arc current | 80–150 A | Controls penetration and heat input |
| Rear arc current | 80–150 A | Controls bead width and heat distribution |
| Arc spacing | 5–15 mm | Influences thermal interaction between arcs |
| Travel speed | 100–400 mm/min | Controls heat input per unit length |
| Electrode offset | 0–5 mm | Adjusts penetration profile |
| Shielding gas flow | 15–25 L/min | Prevents oxidation and stabilizes arc |
The numerical simulation likely employs a finite element model that couples heat transfer, fluid flow in the weld pool, and solidification behavior. The model accounts for:
- Convective heat transfer from the arc to the workpiece
- Thermal conduction within the solid and liquid phases
- Phase change enthalpy during melting and solidification
- Natural convection in the weld pool driven by buoyancy and electromagnetic forces
- Heat loss to the surroundings through radiation and convection
Interpretation of Technical Points
The key finding from the simulation is that the interaction between the two arcs creates a complex thermal field that differs significantly from a single-arc TIG process. The front arc provides primary penetration, while the rear arc fills the weld groove and provides additional heat to promote root fusion. However, if the arc spacing is too small, the thermal interaction becomes excessive, leading to excessive penetration and potential burn-through. If the spacing is too large, the arcs act independently, and the benefits of the double-arc configuration are not realized.
The simulation reveals that the optimal arc spacing for root fusion in thick plates is typically 8–12 mm, depending on plate thickness and material. At this spacing, the thermal interaction between arcs enhances root fusion without causing excessive heat input. The front arc current should be slightly higher than the rear arc current to ensure adequate penetration, while the rear arc provides the necessary heat to melt the root area completely.
Root Fusion Criteria
The simulation defines root fusion in terms of:
- Complete melting of the root edge of the groove
- Continuous liquid phase at the root at the time of solidification
- No unmelted regions or lack of fusion defects at the root
- Adequate weld pool width at the root to ensure proper bead geometry
The numerical model predicts the temperature distribution at the root as a function of welding parameters, allowing optimization of the process before experimental trials. This reduces the cost and time associated with trial-and-error procedure development.
Connection with Engineering Practice
Double-TIG welding is particularly relevant for thick-plate applications in pressure vessel fabrication, where sections of 20–100 mm are common. Traditional single-arc TIG welding of thick plates requires multiple passes, increasing production time and the risk of defects such as lack of fusion, porosity, and cracking. Double-TIG welding can reduce the number of passes by increasing the deposition rate per pass while maintaining or improving weld quality.
In the context of clad-plate pressure vessels, double-TIG welding is valuable for:
- Welding the base metal layers of thick clad plates
- Achieving full penetration in the base metal while preserving the cladding layer integrity
- Reducing distortion in thick sections that are difficult to support during welding
- Improving productivity in batch fabrication of pressure vessels
The numerical simulation approach demonstrated in this study is particularly valuable for procedure qualification, as it allows engineers to predict weld quality before committing to expensive coupon testing. This is especially important for thick-plate welds where the cost of trial welds is significant.
Key Questions and Reflections
While numerical simulation provides valuable insights, it has limitations that must be acknowledged. The accuracy of the simulation depends on the quality of input data, including material properties (which vary with temperature and phase), arc heat source models (which are approximations of complex physical phenomena), and boundary conditions (which are difficult to define precisely). Validation against experimental data is essential to ensure the simulation predictions are reliable.
Another consideration is the scalability of double-TIG welding to very thick sections (50–100 mm). While the simulation demonstrates successful root fusion for plates up to 50 mm, the heat input required for thicker sections may lead to excessive distortion, cracking, or unfavorable microstructure. The study's findings should be interpreted within the context of the specific plate thicknesses and materials investigated.
Study Insights and Implications
This research demonstrates the value of numerical simulation in understanding and optimizing complex welding processes. The double-TIG configuration offers a practical solution to the challenge of welding thick plates with good root fusion and high productivity. For engineers involved in pressure vessel fabrication, this work provides a framework for developing welding procedures for thick-section clad plates and base metal welds, reducing the reliance on costly trial-and-error approaches.