Zhuojin Pipe Fitting Co., LtdNumerical Simulation of Temperature Field in Deep Penetration TIG Water-Cooled Welding of Low Alloy Steel
Literature Overview
This research by Huang Yifei, Luo Zhen, Wang Zhengming, and Feng Yueqiao from the School of Materials Science and Engineering at Tianjin University, published in the Transactions of the China Welding Institute in 2017 (Vol. 38, No. 11, pp. 103-107), presents a finite element numerical simulation of the temperature field during deep penetration TIG welding with back-side water cooling of Q345 low alloy steel. The work was supported by the National Natural Science Foundation of China (Grant Nos. 51275342, 51405334). The study employs ANSYS finite element software with a combined heat source model consisting of a double-ellipsoidal heat source and a cylindrical heat source, and demonstrates excellent agreement between simulated and experimental temperature measurements.
Core Technical Principles
Deep penetration TIG welding (also known as plasma arc welding or high-performance TIG welding) produces a deep, narrow weld with a high aspect ratio by concentrating the arc energy through electromagnetic compression and plasma jet effects. The addition of back-side water cooling is a process innovation that further enhances penetration by removing heat from the root side of the weld, effectively increasing the net heat input to the weld zone.
Combined Heat Source Model
The heat source model used in this study combines two components:
| Heat Source Component | Geometry | Purpose |
|---|---|---|
| Double-ellipsoidal | Ellipsoidal volume | Models the distributed heat input from the main arc |
| Cylindrical | Cylindrical volume | Models the concentrated energy deposition at the weld root |
This combined model is more physically representative than a single-ellipsoidal model because deep penetration TIG welding produces a heat distribution that is both volumetrically distributed (from the arc) and locally concentrated (at the keyhole root). The double-ellipsoidal component captures the broader thermal influence of the arc, while the cylindrical component models the intense energy concentration at the penetration front.
Effect of Back-Side Water Cooling
The simulation results clearly demonstrate that back-side water cooling significantly reduces the weld pool volume. This reduction occurs because the water cooling removes heat from the bottom of the weld pool, effectively lowering the temperature at the root side and causing the weld pool to contract. The rapid cooling of the root weld bead liquid metal promotes solidification at the root, which helps maintain the mechanical balance of forces acting on the weld pool.
The force balance in the weld pool during deep penetration welding involves several competing forces:
| Force | Direction | Effect |
|---|---|---|
| Arc pressure (electromagnetic + plasma jet) | Downward | Drives keyhole formation and deep penetration |
| Surface tension | Upward and inward | Resists keyhole collapse, shapes weld pool surface |
| Buoyancy | Upward | Drives natural convection in weld pool |
| Marangoni force | Complex (surface tension gradient) | Drives surface flow, affects weld pool shape |
| Hydrostatic pressure | Upward | Resists penetration depth |
| Water cooling effect | Reduces temperature at root | Promotes solidification, stabilizes keyhole |
The water cooling effect indirectly influences this force balance by reducing the temperature at the root, which increases the solidification rate and promotes the formation of a solidified root that acts as a mechanical support for the keyhole. This mechanical support helps maintain the keyhole stability, which is essential for achieving consistent deep penetration.
Engineering Practice Implications
For engineers implementing deep penetration TIG welding with water cooling on low alloy steel structures, this study provides several practical insights:
- Heat source modeling: The combined double-ellipsoidal and cylindrical heat source model provides a more accurate representation of the actual heat input distribution than simpler models. Engineers using finite element analysis for welding process simulation should adopt this approach for deep penetration processes.
- Water cooling benefits: The significant reduction in weld pool volume due to water cooling means that the thermal distortion of the workpiece is reduced. This is particularly beneficial for thin-wall structures and precision components where distortion control is critical.
- Keyhole stability: The rapid cooling of the root weld bead promotes solidification that helps maintain keyhole stability. This is a critical factor in achieving consistent deep penetration, as keyhole collapse leads to lack of penetration and weld defects.
- Process parameter optimization: The simulation provides a tool for optimizing welding parameters (current, voltage, travel speed, water flow rate) without extensive trial-and-error experimentation. Engineers can use the validated finite element model to predict weld geometry and temperature field for different parameter combinations.
Process Parameter Windows
Based on the simulation methodology and results, the following parameter considerations are important for deep penetration TIG water-cooled welding of Q345 steel:
| Parameter | Typical Range | Effect on Weld Quality |
|---|---|---|
| Arc current | 200-400 A | Higher current = deeper penetration, but risk of keyhole instability |
| Arc voltage | 18-25 V | Higher voltage = wider weld, more heat input |
| Travel speed | 200-600 mm/min | Higher speed = narrower weld, less heat input |
| Water flow rate | 5-15 L/min | Higher flow = more cooling, smaller weld pool, better keyhole stability |
| Gas flow rate | 10-20 L/min | Adequate shielding to prevent oxidation |
Methodology Assessment and Validation
The use of ANSYS finite element software with a validated combined heat source model represents a robust numerical methodology for welding process analysis. The excellent agreement between simulated and experimental temperature measurements validates the model's predictive capability and provides confidence in using it for process optimization.
The study's approach of first validating the model against experimental data and then using it to investigate the effects of water cooling is methodologically sound. This validation-first approach ensures that subsequent simulation results are reliable and can be used for engineering decision-making.
Study Insights and Implications
This study contributes valuable insights into the thermal behavior of deep penetration TIG welding with water cooling. The demonstration that back-side water cooling significantly reduces weld pool volume while promoting keyhole stability through rapid root solidification provides a clear mechanistic understanding of the water cooling effect.
For engineering practice, the key insight is that water cooling is not merely a heat removal technique but a process control tool that influences the fundamental force balance in the weld pool. By promoting root solidification, water cooling creates a mechanical support that stabilizes the keyhole, enabling consistent deep penetration at higher welding speeds. This understanding should guide the design of water cooling systems, including nozzle geometry, water flow rate, and cooling zone positioning.
The validated finite element model provides a powerful tool for process development and optimization. Engineers can use this model to predict weld geometry, temperature field, and cooling rate for different parameter combinations, reducing development time and cost. The combined heat source model should be considered a best practice for deep penetration welding simulation, as it captures the essential physics of the process more accurately than simpler models.