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

Literature Overview

This study by Shi Xiaoyue, Tian Wei, and Chen Linling from Nantong Runbang Heavy Machinery Co., Ltd. investigates the influence of preheating temperature on the weld bead appearance, microstructure, and mechanical properties of pure copper (red copper) MIG welded joints. Published in Thermal Processing Technology in 2023 (Volume 52, Issue 7, pages 117-121), the work addresses a practical welding challenge encountered in the fabrication of copper components for electrical equipment, heat exchangers, and heavy machinery. Pure copper welding is notoriously difficult due to its extremely high thermal conductivity, which causes rapid heat dissipation from the weld zone, resulting in poor penetration and incomplete fusion if not properly managed.

Key Findings and Technical Parameters

The preheating temperature was varied from 100°C to 400°C, and the resulting effects on weld appearance, microstructure, and mechanical properties were systematically evaluated.

The optimal preheating temperature of 300°C yielded the highest tensile strength and good weld bead quality. The weld metal microstructure consists primarily of copper-based solid solution columnar grains, while the heat-affected zone (HAZ) exhibits equiaxed grain morphology. The hardness decreases progressively from the base metal toward the weld metal across all preheat conditions.

Metallurgical Interpretation

The progressive improvement in weld penetration with increasing preheat temperature is directly attributable to the reduction in heat loss from the weld zone. Copper's thermal conductivity of approximately 401 W/(m·K) at room temperature is among the highest of all engineering metals, and this extreme heat dissipation capability makes it challenging to maintain a stable molten pool. Preheating reduces the temperature gradient between the weld zone and the surrounding base metal, effectively reducing the heat flux away from the arc and allowing the molten pool to achieve greater depth.

The microstructural observations reveal important aspects of copper weld metal solidification. The columnar grain structure in the weld metal is characteristic of directional solidification from the fusion boundary toward the weld center, driven by the steep thermal gradient at the solidification front. The equiaxed grain structure in the HAZ results from the higher nucleation rate at the grain boundaries of the original base metal, which provides abundant heterogeneous nucleation sites during the austenite-to-grain-boundary-recrystallization transition.

The hardness decrease from base metal to weld metal is attributed to the difference in grain size and microstructure. The base metal typically has a finer grain structure resulting from prior cold working or controlled annealing, while the weld metal has a coarser grain structure due to the high solidification temperature and rapid solidification rate. The slight decrease in hardness with increasing preheat temperature is consistent with the increased HAZ and weld width, which allows for more complete recrystallization and grain growth at elevated temperatures.

Process Optimization and Practical Recommendations

The identification of 300°C as the optimal preheat temperature provides a clear process parameter for production welding of pure copper. However, several practical considerations must be addressed when implementing this preheat strategy:

1. Preheat method: Induction heating or resistance heating is preferred over flame preheating for copper, as flame preheating can cause surface oxidation and contamination that leads to porosity and reduced joint quality.
2. Interpass temperature control: For multi-pass welding, the interpass temperature should be maintained between 200-300°C to prevent excessive grain growth while ensuring adequate penetration.
3. Shielding gas selection: Pure argon or argon-helium mixtures (75% Ar + 25% He) should be used, with helium providing higher arc energy to compensate for copper's heat dissipation.
4. Welding position: Flat and horizontal positions are preferred for pure copper MIG welding to ensure stable molten pool control. Vertical and overhead positions require significantly higher heat input and are not recommended for critical applications.

From a 5W2H perspective, the process parameters for pure copper MIG welding at 300°C preheat can be summarized as follows:

Study Insights and Implications

The finding that tensile strength increases and then decreases with preheat temperature reflects a fundamental trade-off. At low preheat temperatures, incomplete fusion and poor penetration limit joint strength. At excessively high preheat temperatures, the prolonged time at elevated temperatures promotes grain growth in both the weld metal and HAZ, reducing the yield strength and hardness of the joint. The 300°C optimum represents the balance point where penetration is adequate and grain coarsening is minimal.

For engineering applications involving copper-copper or copper-aluminum dissimilar joints, the preheat strategy must be adapted to account for the different thermal conductivities of the base materials. When welding copper to steel or copper to aluminum, the preheat temperature should be set based on the material with the lower thermal conductivity to ensure adequate heat input to the copper side while avoiding excessive heating of the other material.

This study provides practical guidance for fabricators working with pure copper components and highlights the critical role of preheating in overcoming the inherent welding difficulties of high-thermal-conductivity metals. The 300°C optimum should be incorporated into welding procedure specifications (WPS) for pure copper MIG welding operations.