CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Preheating on TIG Welding Processability of Thick Copper Plate
Literature Overview
This 2005 paper published in the journal Welding, authored by Yan Jiuchun, Cui Xihui, Li Qingfen, Li Xuejun, Kong Qingwei, Yang Guofeng, and Hao Guisheng from Harbin Engineering University and Harbin Jiancheng Group Company, investigates the influence of preheating temperature on the TIG welding processability of thick copper plates. Copper and copper alloys are widely used in electrical applications, heat exchangers, and structural components due to their excellent electrical and thermal conductivity, but their high thermal conductivity also presents significant challenges for welding.
Copper Welding Challenges
Thick copper plates (typically defined as 10 mm and above) present unique welding challenges that distinguish them from steel and aluminum welding:
- Extremely high thermal conductivity: Copper conducts heat approximately 8 times better than carbon steel, causing rapid heat dissipation from the weld zone
- Low melting point: At 1085°C, copper melts at a temperature lower than most steels, but the high thermal conductivity means the heat-affected zone is very wide
- Oxide formation: Copper forms tenacious oxide layers (Cu2O and CuO) that can be trapped in the weld as inclusions
- Hot cracking susceptibility: Due to the wide solidification range and presence of low-melting-point impurities
- Porosity tendency: From hydrogen dissolved in the molten pool and released during solidification
| Property | Pure Copper | Brass (CuZn) | Bronze (CuSn) |
|---|---|---|---|
| Thermal conductivity (W/m·K) | 390 | 120 | 66 |
| Melting point (°C) | 1085 | 900-940 | 950-1000 |
| Preheat temperature (°C) | 400-650 | 200-400 | 150-300 |
| Typical TIG current (A) | 200-400 | 150-300 | 150-300 |
| Travel speed (mm/min) | 20-50 | 30-70 | 30-70 |
Preheating Temperature Effects
The research systematically investigated the effects of preheating temperature on weld quality, processability, and mechanical properties. The study examined preheating temperatures ranging from 0°C (no preheat) to 700°C in increments, evaluating each condition through macroscopic examination, metallographic analysis, mechanical testing, and hardness measurement.
Key Findings on Preheat Effects
No preheat (0°C): Welding thick copper plates without preheating results in poor penetration, incomplete fusion at the root, excessive solidification cracking, and high porosity rates. The rapid heat dissipation causes the molten pool to solidify before achieving full fusion with the base metal.
Low preheat (100-200°C): Some improvement in penetration and fusion is observed, but solidification cracking remains prevalent. The preheat is insufficient to significantly reduce the thermal gradient in the weld zone.
Moderate preheat (300-400°C): Significant improvement in weld quality is achieved. Penetration becomes adequate, fusion improves, and cracking rates decrease substantially. This range represents the practical minimum for thick copper plate welding.
High preheat (500-650°C): Optimal weld quality is achieved with minimal cracking and porosity. The high preheat temperature maintains the base metal near the melting point, facilitating easy fusion and reducing thermal gradients. However, excessive preheat can lead to grain coarsening and potential distortion.
Excessive preheat (>700°C): While fusion is excellent, excessive preheat can cause grain coarsening in the HAZ, reduced mechanical properties, and increased distortion. The thermal efficiency of welding decreases as more energy is lost to heating the already-hot base metal.
Process Parameters and Weld Quality
The optimal welding parameters for thick copper plates depend on the preheating temperature and plate thickness. The research identified the following parameter relationships:
| Plate Thickness (mm) | Preheat (°C) | Current (A) | Travel Speed (mm/min) | Wire Diameter (mm) |
|---|---|---|---|---|
| 12 | 400 | 250 | 35 | 2.4 |
| 15 | 500 | 300 | 40 | 2.4 |
| 20 | 550 | 350 | 45 | 3.2 |
| 25 | 600 | 400 | 50 | 3.2 |
| 30 | 650 | 450 | 55 | 3.2 |
Defect Analysis
The research identified several defect types and their relationship to preheating:
- Solidification cracking: Most prevalent without preheat; significantly reduced at preheat temperatures above 300°C. The cracking occurs at grain boundaries due to the wide solidification range and the presence of low-melting-point impurities.
- Porosity: Hydrogen porosity is common in copper welds. Preheating above 400°C reduces porosity by allowing dissolved gases to escape before solidification. However, excessive preheat can increase porosity if the base metal becomes too hot and absorbs more hydrogen from the atmosphere.
- Oxide inclusions: Copper oxide inclusions are difficult to prevent completely. Preheating helps by maintaining a molten pool that can float oxide inclusions to the surface. However, interpass cleaning is still required to remove surface oxides between passes.
- Incomplete fusion: Primarily occurs at the root of multi-pass welds. Adequate preheat ensures that the base metal is hot enough to achieve full fusion with the weld metal.
- Undercut: More common at higher travel speeds and lower currents. Preheating allows for higher travel speeds while maintaining adequate penetration, reducing the likelihood of undercut.
Engineering Applications and Practical Recommendations
The findings of this research have direct implications for the welding of copper components in various industries. In electrical applications, copper busbars and conductors require high-quality welds to ensure low electrical resistance and reliable current carrying capacity. In heat exchanger manufacturing, copper tubes and plates must be welded with minimal distortion to maintain dimensional accuracy and heat transfer efficiency. In marine applications, copper alloys such as bronze and brass are used for propellers, shafts, and fasteners that must withstand harsh environments and cyclic loading.
Practical Implementation Guidelines
Based on the research findings, the following practical guidelines are recommended for TIG welding of thick copper plates:
- Preheat temperature selection: Use a preheat temperature of 400-600°C depending on plate thickness. For plates up to 15 mm, 400-500°C is sufficient. For plates 15-30 mm, 500-600°C is recommended.
- Preheat method: Use induction heating, gas heating, or resistance heating to achieve uniform preheat across the weld area. Avoid localized heating that can create thermal gradients and distortion.
- Interpass temperature control: Maintain interpass temperatures between 200-400°C to prevent excessive grain growth while maintaining adequate fusion.
- Shielding gas selection: Use high-purity argon (99.99% or higher) with flow rates of 15-25 L/min. Helium or argon-helium mixtures can improve penetration but increase cost.
- Filler wire selection: Use filler wire with composition matching or slightly alloyed to the base metal. ERNiCu-7 (nickel-silver) is often used for brass and bronze welding to improve weldability.
- Weld sequence planning: For multi-pass welds, plan the sequence to minimize distortion and thermal stress. Use symmetric welding patterns and skip welding techniques where applicable.
Study Insights and Industry Impact
This research provides valuable practical guidance for the welding of thick copper plates, an application that is common in electrical, marine, and heat exchanger industries but often receives less attention than steel or aluminum welding. The systematic investigation of preheat effects demonstrates the importance of preheating as a process variable that can dramatically improve weld quality and productivity.
The research also highlights the unique challenges of copper welding compared to more common metals. The extremely high thermal conductivity of copper means that conventional welding parameters developed for steel or aluminum are often inadequate. Engineers must develop copper-specific procedures that account for the material's unique thermal and metallurgical characteristics.
From a quality assurance perspective, the research emphasizes the importance of preheat temperature monitoring and control. Without proper preheat, weld quality is unpredictable and defect rates are high. With controlled preheat, consistent weld quality can be achieved with minimal defects. This underscores the need for reliable temperature measurement and control systems in copper welding operations.
The findings of this research remain highly relevant for contemporary copper welding applications. While modern welding equipment offers more precise control over process parameters, the fundamental principles identified in this study continue to guide practical copper welding practice. Engineers involved in copper welding should ensure that their procedures incorporate adequate preheating and that their quality control systems include temperature monitoring to verify compliance with specified preheat requirements.