Numerical Analysis of Oxygen Mass Transfer in Dual TIG Arc Welding

Literature Overview

The paper by Wang Xinxin, Chi Loxin, Xu Huibin, and Fan Ding, published in the Journal of Mechanical Engineering in 2021 (Vol. 57, No. 4, pp. 53–62), presents a three-dimensional steady-state mathematical model for oxygen mass transfer in dual tungsten inert gas (TIG) arcs. The study investigates how different current distribution ratios between the two tungsten electrodes affect the distribution of oxygen mass fraction within the dual arc zone. Funded by the National Natural Science Foundation of China (51705054) and the Chongqing Municipal Commission of Education (KJ1600903), this research contributes to the fundamental understanding of oxygen behavior in multi-arc welding processes, which is essential for controlling weld metal composition and quality.

Core Technical Findings

Oxygen Distribution Patterns

The numerical analysis reveals that oxygen in the dual TIG arc exhibits a distinctly non-uniform distribution. Two primary scenarios were examined: cases where the two tungsten electrodes carry different currents, and cases where both electrodes carry equal currents.

When the two tungsten electrodes operate at different current levels, the oxygen introduced from the lower-current electrode side preferentially diffuses toward the outer region of that electrode's arc zone. This asymmetric diffusion pattern is attributed to the differences in arc constriction and plasma flow velocity between the two electrode sides. The higher-current electrode produces a more constricted and higher-velocity arc, which entrains oxygen more effectively toward the arc center.

When both electrodes carry equal currents, oxygen diffuses more prominently toward the central region of the arc, resulting in higher oxygen concentration at the arc center. This symmetric diffusion pattern creates a more uniform oxygen distribution across the arc width but at a higher overall concentration.

Current Distribution and Oxygen Pickup

A critical finding is that the oxygen mass fraction and particle number flux at a position 0.1 mm above the anode surface are lower when the two electrodes carry different currents compared to when they carry equal currents. This implies that asymmetric current distribution between the two electrodes results in less oxygen being transferred into the molten weld pool. From a metallurgical perspective, reduced oxygen pickup in the weld metal is generally beneficial, as it minimizes oxide inclusion formation and improves weld metal cleanliness.

Numerical Model Considerations

The three-dimensional steady-state model accounts for the coupled interactions between electromagnetic fields, plasma fluid dynamics, and species transport within the dual arc. The model solves the Navier-Stokes equations for plasma flow, the energy equation for temperature distribution, and the species conservation equations for oxygen transport. Boundary conditions include the tungsten electrode surface temperature, the anode (workpiece) surface temperature, and the inflow oxygen concentration. The steady-state assumption is appropriate for continuous welding processes where the arc parameters remain constant over time.

Process Analysis and Metallurgical Implications

The findings of this study have direct implications for the metallurgical quality of dual TIG welds, particularly in applications where oxygen sensitivity is a concern. In austenitic stainless steels, for example, oxygen pickup can lead to the formation of MnS and other sulfide inclusions, which reduce both mechanical properties and corrosion resistance. In nickel-based superalloys, oxygen can promote the formation of delta phase (Ni3Nb) in the HAZ, which adversely affects creep strength and fracture toughness.

The recommendation to use asymmetric current distribution between the two tungsten electrodes to minimize oxygen pickup is a practical and actionable finding. Engineers designing dual TIG welding procedures can intentionally set different current levels for the two electrodes to achieve lower oxygen transfer into the weld pool while maintaining adequate arc stability and penetration.

Comparison with AA-TIG and Conventional TIG

The dual TIG process with oxygen introduction can be viewed as a variant of AA-TIG welding, but with the added complexity of two independent arc zones. Compared to single-torch AA-TIG, the dual TIG configuration offers greater flexibility in controlling arc geometry and energy distribution. However, it also introduces additional variables — namely, the current ratio between the two electrodes — that must be optimized. The numerical results provide a theoretical basis for this optimization, demonstrating that asymmetric current distribution is the preferred configuration for minimizing oxygen pickup.

Integration with Engineering Practice

In industrial applications of dual TIG welding, such as the welding of thick-section austenitic stainless steel or nickel-based alloys, the current distribution ratio between the two electrodes is a critical parameter that is often overlooked. Many practitioners simply set both electrodes to carry equal currents for simplicity, without considering the metallurgical consequences. This study provides the quantitative justification for adopting asymmetric current distribution.

For pipeline applications involving duplex stainless steels or super duplex grades, where the ferrite-austenite balance is critical for corrosion resistance, controlling oxygen pickup is essential. Excessive oxygen can shift the microstructure toward more ferrite, potentially promoting intergranular corrosion in chloride-containing environments. The numerical model developed in this study could be extended to predict oxygen pickup under various welding conditions, providing a tool for process optimization.

Key Questions and Reflections

Several limitations and open questions arise from this study. First, the model assumes steady-state conditions, which may not accurately capture the transient oxygen dynamics during weld pool initiation and termination. Second, the model considers oxygen transport in the arc plasma but does not account for oxygen absorption from the atmosphere or from the workpiece surface. Third, the numerical results are validated against experimental measurements, but the extent of validation is not detailed in the abstract. Engineers applying these findings should conduct their own experimental verification under their specific welding conditions.

The study also does not address the interaction between oxygen transport and other species, such as nitrogen, which can be simultaneously introduced into the weld pool. In practice, both oxygen and nitrogen can affect weld metal properties, and their combined effects may not be simply additive. Future research should consider multi-species transport models that account for the simultaneous behavior of oxygen, nitrogen, and other reactive species.

Study Insights and Implications

The numerical analysis of oxygen mass transfer in dual TIG arcs provides valuable theoretical insights for optimizing weld metal quality in multi-arc welding processes. The finding that asymmetric current distribution between the two electrodes reduces oxygen pickup into the weld pool is a practically significant result that can be readily implemented in production welding. For engineers working with oxygen-sensitive materials such as austenitic stainless steels, nickel-based alloys, and titanium alloys, this knowledge enables more informed decisions about welding parameter selection. The three-dimensional numerical model developed in this study serves as a foundation for further research into multi-species transport and for the development of process simulation tools that can predict weld metal composition under various welding conditions.