Numerical Simulation of K-TIG Weld Pool Behavior

Literature Overview

The study by He Hongyun, Wang Xinxin and Chen Dawei, published in The Welding Journal (2026, Vol. 47, No. 4, pp. 105-117), presents a numerical simulation of weld pool behavior during keyhole tungsten inert gas (K-TIG) welding. Funded by the National Natural Science Foundation of China (510705054) and Chongqing municipal research programs, this work employs the Level-Set method to track the free surface evolution of the molten pool and establishes a two-dimensional axisymmetric transient numerical model for stationary welding. The study systematically investigates the individual contributions of buoyancy, electromagnetic force, arc pressure, plasma shear force, and surface tension to weld pool dynamics.

Core Technical Content

K-TIG welding is distinguished from conventional TIG welding by its ability to achieve deep penetration through keyhole formation, enabling single-sided welding with full penetration on thick plates. Understanding the weld pool behavior is essential for process optimization, but direct experimental measurement of internal flow patterns is extremely challenging. Numerical simulation provides a powerful tool for revealing the complex physics governing weld pool dynamics.

Governing Forces and Their Effects on Molten Pool Flow

The study decomposed the complex weld pool behavior into individual force contributions, providing clear insight into the relative importance of each mechanism.

Flow Velocity Analysis

A key finding is that plasma shear force and surface tension (with negative temperature coefficient) produce molten pool liquid metal flow velocities of the same order of magnitude, with surface tension generating the larger velocity. In contrast, buoyancy, electromagnetic force, and arc pressure contribute relatively minor effects on pool convection, with arc pressure having the smallest influence.

This hierarchy of forces has direct implications for process control. Since surface tension and plasma shear force dominate the flow dynamics, parameters that influence these forces—such as arc power, electrode configuration, and surface contamination—should be prioritized in process optimization.

Surface Tension Temperature Coefficient Effect

The sign of the surface tension temperature coefficient (∂σ/∂T) is critical to weld pool behavior. When ∂σ/∂T < 0 (the typical case for clean metals), the Marangoni flow drives liquid from the hot center toward the cooler periphery, creating an outward flow pattern that influences penetration depth and weld width. When ∂σ/∂T > 0 (which can occur with sulfur or other surface-active elements), the flow reverses, driving liquid inward and potentially affecting keyhole stability.

Surface Deformation Analysis

The weld pool surface deformation under different force conditions reveals important insights for keyhole stability:

• Under buoyancy, electromagnetic force, and positive Marangoni force alone: slight convex deformation.
• Under plasma shear force, negative Marangoni force, and arc pressure alone: concave deformation.
• Arc pressure produces the maximum surface concavity, which is directly relevant to keyhole formation and maintenance.

The arc pressure-induced surface depression is the primary mechanism for keyhole initiation. The deeper the surface depression, the more likely a keyhole forms and sustains, leading to deeper penetration.

Engineering Practice Implications

For engineers developing K-TIG welding processes, this numerical study provides several practical insights:

• Arc pressure is the dominant force for surface deformation and keyhole formation, confirming that arc power density is the primary control parameter for penetration depth.
• Surface tension effects are comparable to or larger than plasma shear effects on flow velocity, suggesting that surface cleanliness and surfactant content are critical process variables.
• Buoyancy and electromagnetic forces are secondary effects that can often be neglected in simplified process models without significant loss of accuracy.
• The Level-Set method provides a robust framework for tracking complex free surface evolution during welding, which is essential for modeling keyhole dynamics.

Process Optimization Strategies

Based on the force hierarchy revealed by the simulation, the following process optimization strategies are recommended:

1. Maximize arc power density through electrode geometry optimization and current concentration to enhance arc pressure and keyhole formation.
2. Maintain strict surface cleanliness to ensure predictable surface tension behavior and avoid surfactant-induced flow reversal.
3. Optimize electrode angle and stand-off distance to control plasma shear force distribution and keyhole geometry.
4. Consider the combined effect of multiple forces in process parameter selection, as the actual weld pool behavior results from the superposition of all contributing mechanisms.
5. Use numerical simulation as a tool for pre-optimization of process parameters before experimental trials, reducing development time and cost.

Key Questions and Reflections

The two-dimensional axisymmetric model represents a significant simplification of the actual three-dimensional welding process. While this simplification is necessary for computational tractability and provides valuable insight into fundamental physics, it cannot capture directional effects such as weld pool asymmetry during travel welding, gravitational effects during vertical or overhead welding, or three-dimensional keyhole geometry. The extension of this modeling approach to three-dimensional transient simulations with travel welding would provide more directly applicable results for production process development.

The study focuses on stationary welding, which is a fundamental but simplified case. In practical welding, the pool dynamics change continuously as the arc travels, and the transient effects of pool evolution ahead of and behind the arc are important for understanding weld bead geometry and solidification microstructure. Future work incorporating travel welding conditions would bridge the gap between fundamental understanding and practical process application.

Additionally, the simulation validates against experimental results, confirming model accuracy. However, the specific experimental validation methodology and the degree of agreement in quantitative terms (not merely qualitative trends) would strengthen the confidence in using the model for predictive process design.

Reference Value and Outlook

This numerical study provides a clear and systematic understanding of the force balance governing K-TIG weld pool behavior. The identification of arc pressure as the dominant force for surface deformation and keyhole formation, and surface tension and plasma shear force as the primary drivers of internal flow, establishes a clear hierarchy of physical mechanisms that can guide process development. For engineers developing K-TIG welding processes for thick plate applications, this work provides a physics-based foundation for parameter optimization that complements empirical approaches. The Level-Set methodology demonstrated here offers a versatile tool for modeling complex free surface phenomena in welding, and its application to three-dimensional transient problems would significantly enhance its practical utility. The clear separation of force contributions also enables targeted experimental verification, where each force can be individually validated through controlled parameter variation, building confidence in the overall model predictive capability.