Numerical Simulation of TIG Weld Pool Internal Forces Using Fluent

Literature Overview

The paper by Shi Yu, Guo Zhaobo, Xu Lesheng, Li Weidong, Huang Jiankang, and Fan Ding, published in the journal Electric Welder (Vol. 41, No. 9, 2011, pp. 21–24), presents a two-dimensional axisymmetric numerical model of a TIG weld pool under a fixed heat source. The authors employed Fluent software with User-Defined Functions (UDF) for secondary development, incorporating phase-change latent heat and temperature-dependent thermophysical properties. The study systematically analyzes the influence of buoyancy, Marangoni force, electromagnetic force, and arc force on the flow field and temperature field within the weld pool, with particular emphasis on the interplay between Marangoni and electromagnetic forces.

Core Technical Points

Force Decomposition in the TIG Weld Pool

The weld pool in TIG welding is subject to multiple driving forces that collectively determine the pool geometry, flow patterns, and ultimately the weld bead profile. The authors identify four primary forces:

Role of Surface Tension Temperature Coefficient

The Marangoni force is the most critical factor governing the surface flow pattern. When the surface tension temperature coefficient is negative (as in most steel alloys without strong surface-active elements), surface tension decreases with increasing temperature, driving molten metal from the hot center toward the cooler edges. This outward surface flow results in a wide, shallow weld pool. Conversely, a positive coefficient (e.g., in oxygen-rich or sulfur-rich environments) causes inward surface flow, producing a narrow, deep penetration profile. The authors demonstrate that small changes in this coefficient can fundamentally alter the weld pool cross-sectional morphology.

Electromagnetic Force and Current Sensitivity

The electromagnetic force follows the Lorentz force law: F_L = J × B, where J is current density and B is magnetic field. As welding current increases, the Lorentz force grows approximately with the square of current, intensifying the downward pinch effect. This directly correlates with deeper penetration at higher currents. The numerical results confirm that welding current is a decisive parameter for pool depth and cross-sectional shape.

Process and Standards Analysis

Numerical Modeling Approach

The model is constructed as a two-dimensional axisymmetric geometry, which is a standard simplification for TIG welding with a stationary or slowly moving heat source. Key modeling considerations include:

• Heat source model: A fixed Gaussian or double-elliptical heat source representing the arc energy input
• Enthalpy-porosity method: Used to handle solid-liquid phase change, with latent heat incorporated into the energy equation
• Temperature-dependent properties: Thermal conductivity, viscosity, density, and surface tension are all defined as functions of temperature via UDF
• Boundary conditions: Free surface with surface tension and shear stress, symmetry axis, and convective/radiative cooling at edges

Engineering Relevance for Pipe Welding

In the context of steel pipe manufacturing—particularly for ERW, HFW, and submerged-arc welded pipes—understanding weld pool dynamics is essential for optimizing penetration profiles and minimizing defects. For TIG root passes in heavy-wall pipe welding, the Marangoni-dominated flow pattern directly influences root reinforcement and potential undercut formation. Engineers should note that the numerical predictions align with empirical observations: increasing current deepens the pool but may increase the risk of burn-through if not compensated by reduced travel speed or increased filler wire diameter.

Key Questions and Reflections

Why Marangoni Force Dominates Over Electromagnetic Force

The study reveals that while both Marangoni and electromagnetic forces are significant, the Marangoni force tends to dominate the surface flow pattern, whereas the electromagnetic force governs the bulk downward convection. This has practical implications: controlling the surface tension temperature coefficient through gas composition (e.g., adding small amounts of oxygen to argon shielding gas) can be more effective in modifying weld geometry than simply adjusting current. However, in high-current applications (above 200 A), the electromagnetic force becomes more competitive and must be accounted for in process design.

Limitations of the Axisymmetric Assumption

The two-dimensional axisymmetric model, while computationally efficient, neglects the three-dimensional effects of travel speed, which introduces asymmetry in the weld pool (wider at the trailing edge, narrower at the leading edge). For engineering applications involving moving sources, a three-dimensional model with a moving heat source would provide more accurate predictions. Nevertheless, the axisymmetric model remains valuable for understanding fundamental force interactions and for calibrating material property inputs.

Study Insights and Implications

This paper provides a rigorous numerical framework for understanding the multi-physics interactions within a TIG weld pool. For pipe manufacturing engineers, the key takeaway is that weld pool geometry—and consequently weld quality—is not solely determined by thermal input but is critically governed by the surface tension temperature coefficient and the electromagnetic pinch effect. In practical welding procedure development for pipeline applications, this insight supports the use of shielding gas modification (e.g., Ar + 2% O₂ for stainless steel pipe welding) as a controlled means to optimize penetration profiles without altering the fundamental thermal cycle. The methodology demonstrated here—combining CFD with UDF-based property modeling—represents a powerful tool for virtual welding procedure qualification, potentially reducing the number of physical trial welds required in production environments.