CLADDING TECHNOLOGY SHANXI CO., LTDNumerical Simulation of Dual-Temperature in TIG Arc Plasma
Literature Overview
This research, published in the Welding Journal in 2018 by Huang Yong, Liu Lin, Wang Xinxin, and Lu Suzhong from the State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals at Lanzhou University of Technology, presents a numerical simulation of the dual-temperature model in TIG arc plasma. The study was funded by the National Natural Science Foundation of China (Project No. 51265029). The dual-temperature model is a fundamental approach in arc plasma physics that accounts for the non-equilibrium between electrons and heavy particles (ions and neutrals) in the arc column.
Core Technical Concept
The dual-temperature (or two-temperature) model assumes that the electron temperature (Tₑ) and the heavy particle temperature (Tₕ) are different in the arc plasma, which is a valid approximation for low-pressure or high-current-density arcs where the electron-heavy particle collision frequency is insufficient to maintain thermal equilibrium. This is in contrast to the single-temperature (equilibrium) model, where Tₑ = Tₕ = T, which is valid only for high-pressure arcs with frequent collisions.
The governing equations for the dual-temperature model include:
- Continuity equation for the electron and heavy particle densities.
- Momentum equation for the arc plasma, accounting for the different temperatures.
- Energy equations for both the electron and heavy particle subsystems, coupled through the electron-heavy particle energy exchange term.
- Equation of state relating pressure, density, and temperatures.
- Transport property equations for electrical conductivity, thermal conductivity, and viscosity, which are functions of both Tₑ and Tₕ.
The key coupling term is the electron-heavy particle energy exchange rate, which depends on the collision frequency and the temperature difference between the two subsystems.
| Model Parameter | Typical Value / Range | Description |
|---|---|---|
| Electron temperature (Tₑ) | 10000–20000 K | Higher than heavy particle temperature |
| Heavy particle temperature (Tₕ) | 5000–15000 K | Lower than electron temperature |
| Arc current | 50–300 A | Determines arc power and temperature distribution |
| Arc pressure | 1–10 atm | Affects collision frequency and equilibrium state |
| Arc length | 1–5 mm | Determines arc geometry and heat transfer |
| Tungsten electrode diameter | 2–6 mm | Affects current density and arc stability |
| Shielding gas | Ar, He, or mixture | Determines arc properties and temperature distribution |
Numerical Simulation Methodology
The numerical simulation typically employs a finite element method (FEM) or finite volume method (FVM) to solve the coupled partial differential equations governing the arc plasma. The simulation domain includes the tungsten electrode, the arc column, and the workpiece surface. The boundary conditions account for the electrode surface temperature, the arc column pressure, and the workpiece heat transfer.
The key challenges in the numerical simulation include:
| Challenge | Description | Solution Approach |
|---|---|---|
| Nonlinear coupling | Equations are highly nonlinear and coupled | Iterative solution with under-relaxation |
| Moving boundary | Arc shape changes with current and geometry | Dynamic mesh or fixed mesh with remapping |
| Transport properties | Strongly dependent on temperature and composition | Empirical correlations or database interpolation |
| Convection | Arc plasma is highly convective | Coupled flow-heat transfer solution |
| Radiation | Significant radiation loss from arc | Discrete ordinate or P₁ approximation |
The simulation results typically show that the electron temperature is significantly higher than the heavy particle temperature in the arc column, with the difference increasing as the arc current decreases or the arc pressure decreases. The temperature difference is most pronounced in the low-pressure regions of the arc, where the collision frequency is insufficient to maintain thermal equilibrium.
Results and Interpretation
The numerical simulation results provide valuable insights into the arc plasma behavior:
| Observation | Physical Interpretation | Engineering Implication |
|---|---|---|
| Tₑ > Tₕ in arc column | Non-equilibrium between electrons and heavy particles | Single-temperature model is inaccurate for low-pressure arcs |
| Temperature gradient is steeper near electrode | High current density and heat generation | Electrode erosion and arc stability are critical |
| Arc shape is affected by Tₑ and Tₕ | Different thermal conductivities | Arc force and heat input depend on temperature distribution |
| Radiation loss is significant | Arc emits thermal radiation | Shielding gas must account for radiation cooling |
| Convection dominates heat transfer | Arc plasma is highly convective | Shielding gas flow affects arc stability and heat input |
The simulation results demonstrate that the dual-temperature model is essential for accurately predicting the arc plasma behavior, particularly in low-pressure environments or high-current-density applications. The temperature difference between electrons and heavy particles affects the transport properties, arc force, and heat transfer, which in turn influence the weld quality and process stability.
Engineering Applications
The numerical simulation of the dual-temperature model has direct applications in:
- Welding process optimization: Understanding the arc temperature distribution allows for optimization of welding parameters to achieve desired weld geometry and properties.
- Arc stability analysis: The temperature difference affects arc stability, which is critical for consistent welding quality.
- Electrode design: The electrode temperature distribution affects electrode erosion and arc stability, guiding electrode design and selection.
- Shielding gas selection: The gas composition affects the arc temperature distribution and heat transfer, guiding gas selection for different applications.
- Process monitoring: The simulation results can be used to develop process monitoring algorithms that detect deviations from the expected arc behavior.
Key Insights and Reflections
The study highlights the importance of accurately modeling the arc plasma physics for welding process development and optimization. The dual-temperature model provides a more realistic representation of the arc plasma behavior than the single-temperature model, particularly for low-pressure or high-current-density arcs. This has direct implications for welding procedure development, as the arc temperature distribution affects the heat input, weld geometry, and microstructure.
A critical insight is that the temperature difference between electrons and heavy particles is not merely a theoretical curiosity but has practical consequences for welding process stability and quality. The non-equilibrium between the two subsystems affects the transport properties, arc force, and heat transfer, which in turn influence the weld quality. Engineers must be aware of these effects when developing welding procedures for critical applications.
Summary
This study presents a rigorous numerical simulation of the dual-temperature model in TIG arc plasma, demonstrating the importance of accounting for the non-equilibrium between electrons and heavy particles in arc plasma modeling. The findings have direct applications in welding process optimization, arc stability analysis, and process monitoring, providing a valuable tool for improving welding quality and efficiency.