Effect of Preheating on TIG Welding Performance of Thick Copper Plates

Literature Overview

This paper by Yan Jiuchun et al., published in 2005 in the Welding journal, systematically investigates the influence of preheating on the TIG welding of thick pure copper plates. The authors conducted welding experiments at various preheat temperatures and evaluated the microstructural morphology, mechanical properties, and associated issues such as surface oxidation, deformation, and grain growth. The study was conducted jointly by Harbin Engineering University and Harbin Jiancheng Group, reflecting a strong industry-academia collaboration in addressing practical welding challenges for copper components used in electrical and thermal applications.

Core Technical Points

Pure copper (Cu-ETP or Cu-OF grades) is known for its excellent electrical and thermal conductivity but presents significant welding challenges due to its high thermal diffusivity, tendency for hot cracking, and susceptibility to oxidation at elevated temperatures. The research addresses how preheating modifies these fundamental challenges.

Preheating Effects on Microstructure

The study reveals several key microstructural observations:

• Without preheating, the weld and heat-affected zone (HAZ) exhibit fine grain structures but are prone to hot cracking due to high cooling rates and the formation of low-melting-point copper oxide films at grain boundaries.
• Moderate preheating (150-250°C) reduces the thermal gradient, lowers the cooling rate, and decreases the susceptibility to hot cracking by allowing more time for liquid film healing at grain boundaries.
• High preheating (above 350°C) causes severe grain coarsening in both the base metal and HAZ, with grain sizes increasing from approximately 20-30 μm to over 100 μm.
• The weld metal microstructure transitions from columnar dendritic to more equiaxed as preheat temperature increases, but excessive preheat leads to coarse equiaxed grains that degrade mechanical properties.

Mechanical Property Changes

Additional Issues with High Preheating

The authors identify three critical problems associated with high preheat temperatures:

1. Surface oxidation: At temperatures above 300°C, copper rapidly forms a thick oxide layer (Cu₂O and CuO), which contaminates the weld pool, causes porosity, and degrades electrical conductivity.
2. Excessive deformation: The larger heated zone at high preheat temperatures leads to greater thermal distortion, requiring additional fixture rigidity or post-weld straightening.
3. Grain coarsening: Severe grain growth in the HAZ reduces ductility and may create soft zones susceptible to stress corrosion cracking in corrosive environments.

Process Optimization Recommendations

Based on the findings, the following process window is recommended for thick copper plate TIG welding:

• Preheat temperature: 150-250°C for plates 20-50 mm thick.
• Welding current: 250-350 A for single-pass welding of 20 mm plates, increasing proportionally for thicker sections.
• Travel speed: 100-200 mm/min, adjusted to maintain a stable arc and minimize overheating.
• Shielding gas: 99.99% argon at 20-25 L/min, with backside helium or argon backing for root protection.
• Filler wire: ER Cu-2 or ER Cu-3 (AWS classification), with low sulfur and phosphorus content to minimize hot cracking.
• Post-weld treatment: Controlled cooling in still air or light oil quench for very thick sections to minimize residual stress.

Engineering Practice Cases

In the manufacturing of copper busbars for electrical switchgear and copper heat exchanger tubes, the lessons from this paper have direct application. For example, in welding 30 mm thick copper busbars, a preheat temperature of 200°C combined with a two-pass welding sequence (root pass at lower current, fill pass at higher current) has proven effective in eliminating hot cracks while maintaining acceptable grain size. The surface oxide layer formed during preheating is removed by mechanical grinding before welding, and a fresh oxide layer formed during welding is broken by the arc to ensure proper wetting.

For copper tube-to-tube-sheet welding in heat exchangers, the challenge is even greater due to the thin wall thickness and the need for high electrical conductivity. In such cases, preheating is generally avoided, and instead, advanced techniques such as high-frequency induction heating of the tube end or the use of flux-core wire with built-in deoxidizers are employed.

Key Questions and Reflections

A critical question from this study is: what is the optimal preheat temperature that balances hot crack prevention against grain coarsening and oxidation? The paper demonstrates that there is a trade-off, and the answer depends on the specific application requirements. For structural copper components where mechanical strength is paramount, moderate preheating is beneficial. For electrical components where conductivity is critical, preheating must be minimized or eliminated, and alternative crack prevention methods must be employed.

Another reflection concerns the role of filler metal composition. The study uses standard copper filler wires, but the addition of trace elements such as selenium or phosphorus to the filler metal could potentially reduce hot cracking susceptibility without requiring high preheat temperatures. This represents an area for further research.

Study Insights and Implications

The work by Yan et al. provides a comprehensive understanding of the preheating-copper welding interaction. The key insight is that preheating is a double-edged sword: it effectively reduces hot cracking but introduces new problems at excessive temperatures. Engineers working with thick copper weldments should adopt a systematic approach to preheat temperature selection, considering the material thickness, joint geometry, and performance requirements. The study also reinforces the importance of metallographic examination in weld quality assessment, as grain size and microstructural uniformity are critical indicators of weld quality that cannot be detected by volumetric NDT alone. This paper remains a valuable reference for copper welding process development in electrical, thermal, and structural applications.