Numerical Simulation of Dual-Temperature Fields in TIG Arc Plasma

Literature Overview

This study by Huang Yong, Liu Lin, Wang Xinxin, and Lu Suzhong from the State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals at Lanzhou University of Technology, published in the Transactions of the China Welding Institute in 2018, addresses a fundamental challenge in tungsten inert gas welding arc modeling: the coexistence of two distinct temperature regimes within the plasma column. The work was supported by the National Natural Science Foundation of China (Grant No. 51265029). The authors propose a dual-temperature numerical model that decouples the electron temperature from the heavy-particle temperature, recognizing that thermal equilibrium between electrons and heavy species is not always maintained in high-current TIG arcs, particularly in the high-temperature core region.

Core Technical Approach

The fundamental premise of this research is that conventional single-temperature plasma models fail to accurately predict arc behavior in high-current TIG welding because they assume local thermodynamic equilibrium (LTE) throughout the entire plasma volume. In reality, the electron temperature (T_e) and heavy-particle temperature (T_h) diverge significantly in the arc core where current densities exceed 10^6 A/m². The dual-temperature model introduces separate energy equations for electrons and heavy particles, coupled through energy exchange terms.

The governing equations include the electron energy equation with terms for thermal conduction, Joule heating, and electron-heavy particle energy exchange, alongside the heavy-particle energy equation accounting for convective heat transfer, radiation, and the reciprocal exchange term. The model also incorporates the Sheppard model for electron thermal conductivity and the radiation loss function derived from spectroscopic data.

Key Technical Parameters and Findings

The numerical results demonstrate that at welding currents of 300 A, the electron temperature in the arc core reaches approximately 25,000-28,000 K, while the heavy-particle temperature remains at 18,000-22,000 K. This temperature difference of 5,000-8,000 K is substantial and directly impacts the arc's electromagnetic properties, particularly the electrical conductivity and radiation characteristics.

The arc constriction effect becomes pronounced at currents above 200 A, where the arc diameter at the workpiece surface decreases from approximately 12 mm at 100 A to less than 6 mm at 400 A. This constriction is attributed to the Lorentz force (J × B) acting on the current-carrying plasma, which intensifies as current density increases in the core region.

Engineering Practice Implications

For engineers involved in clad plate fabrication and bimetal pressure vessel welding, understanding the dual-temperature behavior of TIG arcs is critical when selecting welding parameters for overlay applications. The concentrated heat flux at high currents directly affects dilution rates in clad welding operations. When applying a nickel-based alloy overlay (such as Inconel 625) onto carbon steel substrates, the arc constriction at high currents can lead to deeper base metal penetration and higher dilution, potentially compromising the corrosion resistance of the overlay layer.

In practice, when performing TIG overlay welding for clad pressure vessels per GB/T 150 or ASME VIII Div. 1 requirements, the understanding that arc energy becomes more concentrated at higher currents helps explain why multi-pass overlay strategies with lower per-pass currents often produce superior metallurgical interfaces compared to single-pass high-current approaches. The dilution control becomes particularly important for critical applications such as hydrogenation reactor cladding where the overlay composition must maintain specific chromium and nickel contents to resist hydrogen attack.

Key Questions and Reflections

One critical question this research raises is the boundary condition treatment at the electrode-plasma interface. The authors adopt the constant current density boundary condition at the tungsten cathode, which simplifies the model but may introduce errors in predicting the initial arc attachment behavior. For engineering applications, this means that while the model accurately captures the bulk plasma behavior, it may not fully represent the transient phenomena occurring during arc initiation and electrode wear.

Another significant insight is the radiation loss function's dependence on temperature. The model shows that radiation losses become dominant above 20,000 K, acting as a self-limiting mechanism on the maximum achievable arc temperature. This has direct implications for understanding why increasing welding current beyond certain thresholds does not proportionally increase the heat input to the workpiece, as more energy is radiated away from the arc column.

The study also highlights the importance of the electron-heavy particle energy exchange coefficient, which governs how rapidly thermal equilibrium is established. In high-speed welding applications, such as those encountered in titanium tube welding for heat exchanger fabrication, the rapid plasma motion may prevent full thermalization, making the dual-temperature model particularly relevant.

Study Insights and Implications

This research provides a more physically accurate foundation for predicting TIG arc behavior, which translates directly to improved process control in cladding and overlay welding operations. For the fabrication of bimetal pressure vessels, where overlay weld quality directly impacts the vessel's corrosion resistance and service life, accurate thermal modeling enables better prediction of dilution, microstructural evolution, and residual stress distribution.

The dual-temperature approach also has implications for understanding defects in overlay welds. Cracking in the overlay layer, particularly in nickel-based alloys welded to carbon steel, is often attributed to thermal stresses arising from temperature gradients. A more accurate temperature field prediction allows for better assessment of cracking susceptibility and informed selection of welding sequences and interpass temperatures.

The practical takeaway for engineers is that at welding currents above 200 A, the arc energy distribution is significantly more concentrated than predicted by single-temperature models, which has direct consequences for penetration depth, dilution, and microstructural formation in overlay welds. This understanding should inform parameter selection for critical cladding applications in pressure vessel manufacturing.