Finite Element Analysis of TIG Welding Temperature Field

Literature Overview

This paper, published in the Journal of Mechanical Engineering (1996, Vol. 32, No. 2, pp. 34-39), presents a three-dimensional finite element analysis (FEA) model of the temperature field during TIG welding under a moving arc. The authors from Xi'an Jiaotong University developed a computational model incorporating the concept of thermal enthalpy and a surface double-elliptical heat source distribution. The model was used to predict weld pool shape, dimensions, and HAZ temperature distribution, with results validated against experimental data.

Core Technical Findings

The developed FEA model successfully predicted the geometry and dimensions of the TIG weld pool, including its depth of penetration, width, and surface depression. The HAZ temperature distribution was calculated and showed good agreement with experimental thermocouple measurements. The double-elliptical heat source model, which distributes the heat flux over an elliptical area on the weld surface with different heat flux intensities in the front and back halves of the ellipse, provided more realistic temperature field predictions than simpler heat source models.

Interpretation of Technical Points

The double-elliptical heat source model is a refinement of the original Goldak-Florian-Wright (GFW) model, which was originally developed for GTA welding. In the double-elliptical model, the heat flux is distributed over two elliptical regions on the weld surface: a front ellipse (in the direction of welding travel) and a rear ellipse. The front ellipse typically has a smaller area and higher heat flux intensity, simulating the compressive effect of the arc on the leading edge of the weld pool, while the rear ellipse has a larger area and lower intensity, simulating the trailing depression of the pool. This asymmetry is critical for accurately predicting the weld pool shape, particularly the depth of penetration and the tail geometry of the pool.

The incorporation of the thermal enthalpy concept addresses the challenge of modeling phase change during solidification. During welding, the material transitions from solid to liquid and back to solid, and the latent heat of fusion must be accounted for. The thermal enthalpy method effectively "smeares" the latent heat over a temperature range around the melting point, avoiding the computational difficulties associated with sharp phase boundaries. This approach is particularly important for predicting the solidification front position and the cooling rate in the fusion zone, which directly determine the grain structure and phase composition of the weld metal.

Engineering Practice Implications

Finite element analysis of welding temperature fields is a powerful tool for process optimization and quality prediction, but its practical utility depends on the accuracy of material property data, heat source model calibration, and boundary condition specification. For TIG welding specifically, the model predictions can be used to estimate the cooling rate at various locations in the weld, which is directly related to the resulting microstructure and hardness distribution. Engineers can use FEA results to predict whether a given welding parameter set will produce a martensitic or ferritic microstructure in steels, or whether the cooling rate will be sufficient to avoid excessive grain growth in austenitic stainless steels. The model can also be used to predict distortion by coupling the thermal analysis with a mechanical analysis, providing estimates of residual stresses and angular or longitudinal distortion that can be used to design fixture and clamping strategies.

Study Insights and Reflections

This 1996 study represents an important milestone in the development of welding simulation capabilities, at a time when computational resources were significantly more limited than today. The fact that the authors achieved good agreement between calculated and experimental results using the computational tools available at that time speaks to the robustness of the methodology. From a modern perspective, the double-elliptical heat source model remains one of the most widely used heat source models in welding FEA, though more sophisticated models incorporating convection within the weld pool, arc pressure effects, and keyhole formation have been developed since then. For practical engineering applications, the key lesson is that FEA models must be calibrated against experimental data before they can be used for process optimization, and the quality of predictions is always limited by the accuracy of the input material properties, which can vary significantly between heat lots and production batches.