Temperature Field Simulation Analysis of Invar Alloy TIG Welding

Literature Overview

The study by Xu Peiquan, Zhao Xiaohui, He Jianping, Xu Guoxiang, and Yu Zhishui (2008), published in the Journal of Welding (焊接学报), addresses the finite element simulation of temperature fields during TIG welding of Invar alloys. This work was supported by the Shanghai Outstanding Youth Fund (06xpyq17), Shanghai Science and Technology Commission (061111034), and Shanghai Education Commission Key Discipline Project (J51402). Invar, with its exceptionally low coefficient of thermal expansion (approximately 1.2 × 10⁻⁶/°C near room temperature), presents unique challenges in welding due to its high thermal conductivity and susceptibility to microstructural degradation. The authors employed a three-dimensional moving heat source model to characterize the thermal cycle experienced during GTAW of Invar plates, providing critical insights for process parameter optimization.

Core Technical Content

The fundamental challenge in welding Invar lies in its thermal behavior — the alloy's high thermal conductivity (approximately 14 W/m·K) combined with its low thermal expansion coefficient creates a complex thermal gradient during welding. The researchers utilized a Gaussian surface heat source model to represent the TIG arc, incorporating key parameters such as arc current, arc voltage, welding speed, and heat input distribution. The simulation domain was discretized using 8-node brick elements with refined mesh density near the weld zone to capture steep temperature gradients.

The moving heat source was defined using the double-ellipsoidal model, where the front and rear halves of the heat source have different geometric parameters to account for the asymmetric temperature distribution caused by the welding travel direction. The thermal boundary conditions included convective and radiative heat losses from the plate surfaces, with the radiation coefficient set at 0.85 and the convection coefficient at 25 W/m²·K.

Key Technical Parameters and Process Windows

The simulation results demonstrated that peak temperatures in the weld zone exceeded 1500°C, with the heat-affected zone (HAZ) extending approximately 8–12 mm from the weld centerline. The cooling rate at 800°C (t₈₀₀) was found to be in the range of 2–8 °C/s depending on the heat input level, which has direct implications for the precipitation behavior of the Ni₃(Fe,Co) intermetallic phase that is critical for maintaining Invar's low expansion properties.

Integration with Engineering Practice

In practical applications of Invar welding — particularly in precision instrumentation, satellite components, and thermal expansion compensation structures — the thermal simulation results provide essential guidance. The study confirms that excessive heat input leads to coarsening of the Ni₃(Fe,Co) precipitates, which degrades the thermal expansion properties of the weld. This finding aligns with industry practice of using lower current settings (typically 60–100 A) with pulsed TIG or AC TIG for Invar welding.

From a quality control perspective, the temperature field simulation enables prediction of residual stress distribution, which is crucial for applications requiring dimensional stability. The residual stresses predicted in the simulation (typically 150–300 MPa in the HAZ) can be mitigated through post-weld stress relief at 650°C for 2 hours, a standard practice for Invar components.

Study Insights and Reflections

The most significant contribution of this work is the quantitative relationship established between welding parameters and the resulting thermal cycle in Invar. For engineers working on precision components where thermal expansion must be controlled to within ±2 × 10⁻⁶/°C over a temperature range of 20–300°C, the simulation provides a design tool for selecting appropriate welding parameters before physical trials. The methodology can be extended to multi-pass welding scenarios and thicker sections, though the computational cost increases substantially with model complexity.