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

Literature Overview

This paper by Shi Xiaoyue, Tian Wei, and Chen Linling, published in Hot Working Technology in 2023 (Vol. 52, No. 7, pp. 117-121), investigates the effect of preheat temperature on the weld formation, microstructure, and mechanical properties of pure copper (red copper) MIG welding joints. The authors are affiliated with Nantong Runbang Heavy Machinery Co., Ltd., a company involved in heavy equipment manufacturing. Pure copper is widely used in electrical equipment, heat exchangers, and heavy machinery due to its excellent electrical and thermal conductivity, but its high thermal conductivity and low melting point present significant challenges for welding.

Core Technical Findings

The study systematically investigates the effect of preheat temperature ranging from 100 degrees Celsius to 400 degrees Celsius on pure copper MIG welding joints. The key findings are presented below.

Interpretation of Technical Points

Pure copper (Cu) has a thermal conductivity of approximately 401 W/(m-K), which is about 7-8 times higher than that of carbon steel. This extremely high thermal conductivity results in rapid heat dissipation from the weld pool, making it difficult to achieve adequate penetration with conventional welding processes. The use of preheat is therefore essential for copper welding, as it reduces the thermal gradient and slows the cooling rate, allowing the weld pool to remain molten long enough to achieve proper fusion.

The finding that weld spatter decreases and weld formation quality improves with increasing preheat temperature is consistent with the understanding that preheat reduces the thermal gradient and stabilizes the arc. At low preheat temperatures, the rapid heat dissipation causes the weld pool to solidify quickly, leading to an unstable arc and increased spatter. As preheat temperature increases, the weld pool becomes more stable, the arc is better maintained, and the weld formation improves.

The microstructural findings are also significant. The weld metal consists of copper-based solid solution columnar grains, which is expected for pure copper welding where no alloying elements are introduced. The HAZ consists of equiaxed grains, indicating that the grain structure in the HAZ is refined by the thermal cycling of welding. The hardness decreases from base metal to weld metal, which is typical for pure metal welding where the weld metal is fully annealed during welding.

The non-monotonic relationship between preheat temperature and tensile strength is a key finding. At low preheat temperatures, the tensile strength is lower because the weld pool solidifies quickly, potentially leading to incomplete fusion or porosity. As preheat temperature increases, the weld pool becomes more stable and penetration improves, leading to higher tensile strength. However, at excessively high preheat temperatures (above 300 degrees C), the tensile strength decreases, possibly due to excessive grain growth in the HAZ or changes in the microstructure that reduce strength.

Process and Standards Analysis

Pure copper welding is governed by standards such as AWS C3.1 for welding copper and copper alloys, ISO 13919 for welding of copper and copper alloys, and GB/T 3190 for copper and copper alloy products. The MIG welding of copper typically requires high current densities, short arc lengths, and high preheat temperatures to compensate for the high thermal conductivity of the base metal.

The preheat temperature range of 100-400 degrees C investigated in this study is consistent with the recommendations in welding handbooks for copper welding. AWS C3.1 recommends preheat temperatures of 200-500 degrees C for copper welding, depending on the thickness and configuration of the joint. The optimal preheat temperature of 300 degrees C found in this study falls within this recommended range.

The weld metal microstructure of copper-based solid solution columnar grains is consistent with the solidification behavior of pure copper. The absence of alloying elements means that the weld metal has essentially the same composition as the base metal, and the columnar grain structure is a result of the directional solidification from the fusion boundary toward the weld centerline. The equiaxed grain structure in the HAZ indicates that the thermal cycling during welding has caused recrystallization and grain growth, resulting in a more uniform grain structure.

Integration with Engineering Practice

In heavy machinery manufacturing, pure copper components are used in electrical contacts, busbars, and heat exchangers. The welding of these components requires careful process control to ensure joint strength and electrical continuity. The study's findings provide practical guidance for welding procedure specification development.

The optimal preheat temperature of 300 degrees C is a valuable finding for engineering practice. In a manufacturing environment, this means that the preheat process must be carefully controlled to achieve this temperature range. In practice, this can be accomplished using:

• Induction heating for localized preheat of the joint area.
• Torch preheat for smaller components.
• Resistance heating for specific configurations.
• Temperature monitoring using thermocouples to ensure the preheat temperature is within the target range.

The improvement in weld formation quality with increasing preheat temperature is particularly important for automated welding processes. In automated MIG welding of copper, stable weld formation is critical for consistent joint quality, and the reduction in spatter with preheat improves the process reliability and reduces cleaning requirements.

However, the slight decrease in hardness with increasing preheat temperature is a consideration for applications where hardness is a critical property. While pure copper is typically used for its conductivity rather than its mechanical strength, some applications may require a minimum hardness level, and the preheat temperature must be selected to balance weld formation quality with mechanical properties.

Key Questions and Reflections

The study provides useful insights into the effect of preheat temperature on pure copper MIG welding, but several questions remain. First, the study does not specify the thickness of the copper plates used, which is a critical parameter for determining the required preheat temperature. Thicker sections generally require higher preheat temperatures to achieve adequate penetration.

Second, the study does not report on the electrical conductivity of the weld joints, which is a critical property for many copper applications. The welding process can affect the electrical conductivity of the joint due to oxide formation, porosity, and microstructural changes, and this should be evaluated for electrical applications.

Third, the study does not address the effect of welding parameters (current, voltage, speed) on the joint properties in conjunction with preheat temperature. The interaction between preheat temperature and welding parameters is complex, and a comprehensive parametric study would provide more complete guidance for process optimization.

Study Insights and Implications

This paper provides practical guidance for the MIG welding of pure copper, demonstrating that a preheat temperature of 300 degrees C offers the best balance of weld formation quality and tensile strength. The findings on the reduction of spatter and improvement in weld formation with preheat are directly applicable to manufacturing environments where consistent joint quality is required. The microstructural analysis confirms that the weld metal and HAZ have expected microstructures for pure copper welding, and the hardness and tensile strength trends provide a useful basis for process parameter selection.