Finite Element Analysis of Three-Dimensional Temperature Field in TIG Welding of Stainless Steel Thin Plate Using SYSWELD

Literature Overview

This paper by Li Ruiying and colleagues from China University of Petroleum, published in Hot Working Technology in 2007, presents a finite element analysis (FEA) of the three-dimensional temperature field during TIG welding of stainless steel thin plate, using the commercial welding simulation software SYSWELD. The study was supported by the China University of Petroleum Doctoral Research Fund (Grant Y070305). The work employs a double-ellipsoid heat source model to represent the moving arc heat input and simulates the dynamic evolution of the temperature field and weld pool morphology from arc ignition to quasi-steady state.

Modeling Approach and Heat Source Representation

The SYSWELD software platform was used to establish a finite element model of the TIG welding thermal process on stainless steel thin plate. The double-ellipsoid heat source model was selected to represent the three-dimensional heat input distribution of the moving arc. This model divides the heat input into two ellipsoidal regions: a front ellipsoid representing the pre-heat zone ahead of the arc and a rear ellipsoid representing the re-melt zone behind the arc, with different power fractions assigned to each. This approach captures the asymmetric heat distribution characteristic of moving arc welding processes, where the front region experiences a different thermal history than the rear region.

The simulation was performed from arc ignition through to the quasi-steady state, capturing the transient thermal evolution that is critical for understanding the formation of the weld pool and the development of residual stresses. The model incorporated temperature-dependent material properties, including thermal conductivity, specific heat, and density, which are essential for accurate prediction of the thermal field in stainless steel, a material with relatively low thermal conductivity and high specific heat.

Key Results and Validation

The simulation predicted the dynamic evolution of the welding temperature field and weld pool morphology from arc ignition to quasi-steady state. The critical validation metric was the comparison between the computed pool length and width at the quasi-steady state and the experimentally measured values. The authors reported that the simulated pool dimensions were in good agreement with the experimental results, confirming the accuracy of the finite element model and the double-ellipsoid heat source representation.

The following table summarizes the key aspects of the finite element modeling approach:

Engineering Practice Implications

For engineers in pipe and fitting manufacturing, the finite element analysis of welding temperature fields is a powerful tool for predicting and controlling welding quality. The ability to simulate the temperature field evolution from arc ignition to quasi-steady state provides insights into the thermal history experienced by the weld metal and heat-affected zone, which directly influences the microstructure, mechanical properties, and residual stress distribution of the weld joint.

The use of the double-ellipsoid heat source model is particularly important because it captures the asymmetric thermal distribution of moving arc welding, which is a fundamental characteristic of all TIG welding processes. In pipe welding applications, where the arc moves along a curved path and the heat input distribution is affected by the pipe geometry, the ability to model the asymmetric heat source is essential for accurate prediction of weld pool geometry and penetration.

The validation of the model against experimental pool dimensions provides confidence in using the simulation for process optimization and procedure development. For thin-walled stainless steel pipe components, where the margin between full penetration and burn-through is narrow, the ability to predict pool dimensions under different welding parameter combinations can significantly reduce the number of trial welds required for procedure qualification.

Reflections and Study Insights

The choice of the double-ellipsoid heat source model is a significant methodological decision that warrants discussion. While more sophisticated heat source models exist, including the conical, hyperbolic, and Gaussian models, the double-ellipsoid model offers a good balance between accuracy and computational efficiency. For the simulation of TIG welding, where the arc is relatively stable and the heat input is primarily axial, the double-ellipsoid model is well-suited. However, for processes with more complex arc behavior, such as plasma arc welding or electron beam welding, more advanced heat source models may be necessary.

The simulation from arc ignition to quasi-steady state is an important aspect of this study because it captures the transient thermal effects that are often neglected in simplified models. In practice, the initial thermal conditions during arc ignition can significantly influence the weld pool formation, particularly for thin plate welding where the thermal mass of the workpiece is small and the temperature rises rapidly. The ability to model this transient phase provides a more realistic representation of the welding process and enables the prediction of weld defects that may be related to the initial thermal transient, such as incomplete fusion at the start of the weld or excessive spatter during arc initiation.

The good agreement between the simulated and experimental pool dimensions at the quasi-steady state is encouraging, but it is important to recognize that pool geometry is only one aspect of weld quality. The temperature field simulation can also be used to predict the cooling rate, which directly influences the microstructure and mechanical properties of the weld metal and HAZ. For stainless steel welding, the cooling rate determines the amount of delta ferrite formed, the degree of carbide precipitation, and the susceptibility to solidification cracking and hot cracking. These microstructural and mechanical aspects, while not directly addressed in this paper, are critical extensions of the thermal analysis for engineering applications.

In summary, this paper demonstrates that finite element analysis using the SYSWELD software and a double-ellipsoid heat source model provides accurate predictions of the temperature field and weld pool geometry during TIG welding of stainless steel thin plate. The validated model can be used as a tool for welding procedure development, process optimization, and defect prediction in pipe and fitting fabrication, particularly for thin-walled stainless steel components where precise control of the thermal input is essential for achieving full penetration without burn-through.