Numerical Simulation of Temperature and Residual Stress Fields in IN738 Superalloy TIG Butt Welding

Literature Overview and Research Context

The paper by Nan Qing and colleagues from Xi'an Thermal Power Research Institute, published in Hot Working Technology (2017, Vol. 46, No. 17), presents a finite element analysis of the temperature field and residual stress field during TIG butt welding of IN738 nickel-based superalloy. The study was supported by the National Natural Science Foundation of China (Grant No. 51501151) and a Huaneng Group technology project. IN738 is a widely used casting superalloy in aerospace and power generation industries, known for its excellent creep resistance and hot corrosion resistance at elevated temperatures. However, its susceptibility to hot cracking and its narrow solidification temperature range make welding a challenging process. Understanding the thermal and mechanical behavior during welding is critical for developing sound welding procedures.

Core Technical Approach and Methodology

The researchers employed MSC.Marc finite element analysis software to model the welding process. A sequential thermomechanical analysis approach was adopted, where the temperature field was calculated first using a coupled heat transfer model, and then the residual stress field was derived from the temperature distribution through a plasticity-based mechanical analysis. The moving heat source was modeled following a Gaussian distribution, with the welding current varied as the primary parameter to study its influence on weld geometry and stress distribution.

The simulation considered the thermophysical properties of IN738 across a wide temperature range, including temperature-dependent thermal conductivity, specific heat, and density. The material's high thermal conductivity at elevated temperatures and relatively low thermal diffusivity were captured in the model. The residual stress analysis incorporated the elastic-plastic constitutive behavior of the alloy, accounting for thermal strain, plastic strain, and elastic recovery during cooling.

Key Findings and Technical Interpretation

The study produced several important observations regarding the welding behavior of IN738:

• The welding current exhibits a direct positive correlation with the maximum temperature achieved during the welding process, the molten pool dimensions, and the magnitude of residual stress values. Higher currents lead to deeper penetration and wider fusion zones, which in turn generate more significant thermal gradients and therefore larger residual stresses.
• On the surface at the weld centerline cross-section, both transverse and longitudinal residual stresses are tensile in nature. This is consistent with the general behavior of arc welding processes, where the weld metal contracts upon cooling, pulling the surrounding material into a state of residual tension.
• On the mid-plane surface of the weldment, the longitudinal residual stress is tensile in the weld zone, reaches its maximum value in the heat-affected zone (HAZ), and transitions to compressive stress in regions far from the weld. This pattern is characteristic of through-thickness constraint conditions in butt welds.
• The transverse residual stress on the mid-plane is relatively small and remains tensile throughout the observed region.

Engineering Practice Implications

For engineers working with IN738 and similar nickel-based superalloys, these findings carry several practical implications. First, the direct correlation between welding current and residual stress magnitude suggests that minimizing welding current within the range required for full penetration is a viable strategy to reduce residual stress. Second, the observation that the maximum longitudinal residual stress occurs in the HAZ rather than in the weld metal itself is particularly important for fatigue assessment, as the HAZ in superalloys is often the weakest region due to sensitization and grain boundary carbide precipitation. Third, the tensile nature of residual stresses in the weld and HAZ regions increases the susceptibility to stress corrosion cracking and creep-fatigue damage in high-temperature service environments.

In practice, post-weld heat treatment (PWHT) such as solution treatment and aging is typically applied to IN738 weldments to relieve residual stresses and restore the microstructure. However, the finite element results provide quantitative data that can inform the design of PWHT schedules and the prediction of residual stress distribution before and after heat treatment. For critical applications such as turbine blades and hot section components, understanding the exact location and magnitude of peak residual stresses is essential for ensuring long-term structural integrity.

Study Insights and Reflections

This paper demonstrates the value of numerical simulation as a complementary tool to experimental welding studies, particularly for expensive superalloys where extensive trial welding is costly. The sequential thermomechanical analysis approach, while not capturing the full coupling between thermal and mechanical fields, provides sufficiently accurate results for engineering purposes. The finding that residual stress peaks in the HAZ rather than in the weld centerline is a reminder that the HAZ, not the weld metal itself, is often the critical region for failure in superalloy weldments. Engineers should pay particular attention to HAZ microstructure control through careful selection of welding parameters, interpass temperature, and post-weld heat treatment schedules.