Effect of Preheat Temperature on Microstructure and Mechanical Properties of Pure Copper MIG Welded Joints

Literature Overview

This research, published in Hot Working Technology (2023), was conducted by Shi Xiaoyue, Tian Wei, and Chen Linling from Nantong Runbang Heavy Machinery Co., Ltd. The study investigates how preheat temperature influences the microstructure and mechanical properties of pure copper MIG welded joints. Pure copper (T2 or equivalent) is widely used in electrical equipment, heat exchangers, and specialized industrial applications where high electrical and thermal conductivity are essential.

Core Technical Content and Methodology

Pure copper presents unique welding challenges due to its extremely high thermal conductivity (approximately 390 W/m·K at room temperature), which causes rapid heat dissipation from the weld zone. This results in steep thermal gradients, incomplete fusion, and high residual stresses if proper preheating is not applied. The study systematically examines preheat temperatures in the range of 200°C to 600°C, evaluating their effects on weld microstructure, hardness distribution, tensile strength, and electrical conductivity.

Key Technical Parameters

Microstructural Evolution with Preheat Temperature

The microstructure of pure copper welds is primarily governed by the cooling rate, which is directly influenced by preheat temperature. Without preheat, the rapid cooling rate in thin copper plates can produce fine-grained recrystallized structures with high dislocation density, leading to elevated hardness but potentially reduced ductility. As preheat temperature increases, the cooling rate decreases, promoting grain growth and recrystallization, which reduces hardness but improves ductility and electrical conductivity.

At lower preheat temperatures (200°C), the weld metal exhibits fine grains with high dislocation density, resulting in hardness values of 70-90 HV. The tensile strength may reach 250-300 MPa, but elongation can be limited to 20-30%. At higher preheat temperatures (500-600°C), the cooling rate decreases significantly, producing coarser grains with lower dislocation density. Hardness drops to 50-65 HV, tensile strength decreases to 200-250 MPa, but elongation improves to 35-45%.

Microstructural Features at Different Preheat Temperatures

Mechanical Property Analysis

The mechanical properties of pure copper MIG welded joints show a clear trade-off between strength and ductility as preheat temperature increases. Higher preheat temperatures reduce the cooling rate, allowing more time for recrystallization and grain growth, which reduces dislocation density and thus lowers strength but increases ductility. This behavior is consistent with the Hall-Petch relationship and dislocation theory, where fine grains and high dislocation density contribute to higher strength but lower ductility.

The hardness distribution across the weld cross-section is also affected by preheat temperature. Without preheat, significant hardness variations exist between the weld metal, heat-affected zone (HAZ), and base metal, with the HAZ often exhibiting the highest hardness due to work hardening from thermal cycling. Increased preheat temperature reduces these hardness variations by allowing more uniform recrystallization and grain growth throughout the weld zone.

Common Defects and Countermeasures

Integration with Engineering Practice

In industrial applications, pure copper welding is commonly encountered in electrical busbar fabrication, heat exchanger tube-to-tube-sheet joints, and specialized equipment manufacturing. The preheat temperature selection must balance multiple objectives: ensuring complete fusion, controlling residual stress, maintaining electrical conductivity, and meeting mechanical property requirements.

For electrical applications where conductivity is critical, higher preheat temperatures (400-600°C) are preferred to maximize electrical conductivity and minimize resistivity. However, for structural applications where strength is more important, lower preheat temperatures (200-300°C) may be more appropriate. The engineer must carefully evaluate the specific application requirements and select the optimal preheat temperature accordingly.

A practical welding procedure for pure copper typically involves:

1. Thorough surface cleaning to remove oxide layers and contaminants
2. Preheating to the selected temperature using induction heating or gas torch
3. Using pure argon or Ar-He shielding gas with high flow rate
4. Employing a copper welding rod (such as ERCu or equivalent)
5. Maintaining consistent travel speed and torch angle
6. Allowing controlled cooling to avoid thermal shock

The study provides valuable data for welding procedure specification (WPS) development, enabling engineers to select preheat temperatures that achieve the desired balance of mechanical properties, electrical conductivity, and weld quality. This information is particularly useful for thick copper sections where heat dissipation is most severe and preheating is essential for achieving sound welds.

Key Questions and Reflections

One important consideration is the effect of preheat temperature on the long-term performance of copper welds under thermal cycling conditions. Pure copper is often used in applications subject to repeated heating and cooling, such as heat exchangers and electrical connectors. The microstructural stability of the weld under thermal cycling is critical, and higher preheat temperatures may produce more stable microstructures that resist degradation during service.

Another question concerns the interaction between preheat temperature and welding speed. Increasing preheat temperature allows for faster travel speeds while maintaining adequate fusion, which can improve productivity. However, the optimal combination of preheat temperature and travel speed requires careful optimization to balance productivity with weld quality.

The research also raises questions about the scalability of these findings to different copper grades and thickness ranges. Pure copper grades such as T1, T2, and T3 have slightly different compositions and properties, and the preheat temperature requirements may vary. Additionally, thick copper sections may require higher preheat temperatures or even interpass temperature control to prevent cracking and ensure sound welds.

Study Insights and Implications

This research provides systematic data on the effects of preheat temperature on pure copper MIG welded joints, offering practical guidance for welding procedure development. The findings demonstrate that preheat temperature is a critical parameter that significantly influences microstructure, mechanical properties, and electrical conductivity. Engineers must carefully select preheat temperatures based on the specific application requirements, balancing strength, ductility, and conductivity objectives.

The work highlights the importance of understanding the fundamental relationships between welding parameters, microstructure, and properties in pure copper welding. This knowledge enables engineers to make informed decisions about welding procedure design, reducing the risk of weld defects and ensuring reliable performance in service. For industrial applications involving pure copper, this research contributes to improved welding quality, reduced rework costs, and enhanced product reliability.