Numerical Simulation of Temperature Field in Non-Preheated Pure Copper TIG Welding

Literature Overview

This paper by Lei Yucheng, Yu Wenxia, and Li Caihui from Jiangsu University, published in the Journal of Jiangsu University (Natural Science Edition) (2005, Vol. 26, No. 6, pp. 506-509), addresses a challenging welding problem: achieving satisfactory weld quality in pure copper without preheating, using tungsten inert gas (TIG) welding with Ar+N2 mixed shielding gas. Funded by the National "863" Program (Grant No. 2003AA305970), this research develops a three-dimensional transient numerical model for TIG welding of pure copper, incorporating thermal enthalpy concepts and a double-elliptical heat source distribution model.

Core Technical Findings

The study demonstrates that non-preheated TIG welding of thick-wall pure copper is feasible when Ar+N2 mixed shielding gas is employed, which significantly enhances the thermal effect of the arc. The numerical model accurately predicts the temperature field, with calculated values showing good agreement with experimental measurements. The research establishes relationships between different process parameters and weld penetration depth and width.

Key Process Parameters and Their Effects

Technical Analysis of Pure Copper Weldability Challenges

Fundamental Weldability Issues

Pure copper presents exceptional challenges for arc welding due to several inherent material characteristics. The extremely high thermal conductivity (approximately 390 W/m·K at room temperature) causes rapid heat dissipation from the weld zone, requiring substantial energy input to achieve proper fusion. Additionally, copper's high thermal expansion coefficient leads to significant thermal stresses and distortion, while its tendency to absorb hydrogen during welding creates a risk of porosity in the solidified weld metal.

The absence of preheating in this study represents a significant departure from conventional practice, where preheating to 400-600°C is typically recommended for copper welding. The challenge is to compensate for the lack of preheating through optimization of the welding process parameters and shielding gas composition.

Role of Ar+N2 Mixed Shielding Gas

The introduction of nitrogen into the argon shielding gas serves multiple purposes. First, nitrogen increases the thermal conductivity of the shielding gas mixture, enhancing heat transfer from the arc to the workpiece. Second, the presence of nitrogen modifies the arc characteristics, potentially increasing the arc voltage and energy density. Third, nitrogen can improve arc stability and penetration characteristics.

However, the use of nitrogen in copper welding requires careful control. Excessive nitrogen can lead to nitrogen porosity in the weld metal, particularly in copper alloys. For pure copper, the risk is lower but still present. The optimal nitrogen percentage must be determined through systematic experimentation, balancing the enhanced thermal effect against the risk of gas porosity.

Numerical Model Development

The three-dimensional transient numerical model developed in this study incorporates several advanced features:

• Energy conservation principles governing heat transfer through conduction, convection, and radiation
• Thermal enthalpy concept to account for phase change during melting and solidification
• Double-elliptical heat source distribution to accurately represent the TIG arc heat input pattern
• Moving arc simulation to capture the transient nature of the welding process

The double-elliptical heat source model is particularly important for TIG welding, as it distinguishes between the front and rear halves of the arc, accounting for the asymmetric heat distribution caused by arc drag and electrode geometry. This level of modeling sophistication is essential for accurately predicting the weld pool geometry and temperature distribution.

Engineering Practice Applications

Process Parameter Optimization

The relationships established between process parameters and weld characteristics provide practical guidance for welding engineers:

• Higher welding currents increase penetration depth but also increase distortion and potential for hydrogen porosity.
• Optimal welding speed balances penetration requirements against productivity and distortion control.
• Arc voltage adjustment provides a means to control weld width without significantly affecting penetration.
• The Ar+N2 gas mixture ratio should be optimized for each specific welding configuration and thickness range.

Quality Control Considerations

For non-preheated TIG welding of pure copper, the following quality control measures are essential:

• Pre-weld cleaning to remove oxide layers and contaminants that could promote porosity
• Post-weld heat treatment to relieve residual stresses and improve weld metal properties
• Non-destructive testing (UT or RT) to detect internal porosity and lack of fusion
• Visual inspection of weld surface to assess profile and detect surface defects

Study Insights and Reflections

This research addresses a practical welding challenge that is of considerable importance in electrical engineering applications, where pure copper components require welding without the thermal budget for preheating. The demonstration that non-preheated welding is feasible through process optimization represents a significant advance in welding technology.

The numerical model developed in this study provides a powerful tool for process development and optimization. By enabling virtual experiments, the model reduces the need for costly physical trials while providing insights into the fundamental heat transfer mechanisms governing the welding process.

One limitation of this study is the focus on temperature field analysis without detailed consideration of fluid flow within the weld pool. The fluid dynamics of the molten pool significantly influence the final weld geometry and microstructure, and a coupled thermal-fluid analysis would provide more comprehensive predictions of weld quality.

The practical implications of this research extend to various industrial applications including electrical busbar fabrication, copper cable termination, and electrical equipment manufacturing. The ability to achieve quality welds without preheating reduces production costs, simplifies the welding process, and enables welding of components where preheating is impractical or undesirable.

This work demonstrates the power of numerical simulation in welding research and provides a foundation for further process optimization. The combination of theoretical modeling with experimental validation establishes confidence in the results and provides engineers with reliable guidance for practical implementation.