Microstructure and Properties of TIG Thermal Conduction Welding Joints of 6061 Aluminum Alloy and H60 Copper Alloy

Literature Overview and Industrial Context

This 2017 paper published in Welding, authored by researchers from China Petroleum and Natural Gas Pipeline Engineering Co., Ltd., examines the microstructure and mechanical properties of dissimilar joints between 6061 aluminum alloy and H60 copper alloy produced by TIG thermal conduction welding. Dissimilar metal welding between aluminum and copper is a notoriously challenging operation due to the formation of brittle intermetallic compounds (IMCs) at the interface, the significant difference in thermal conductivity between the two metals, and the lack of solid solubility between aluminum and copper. The industrial motivation for such joints is evident in applications such as electrical busbar connections, heat exchanger tube-to-tubesheet joints, and cryogenic system components where aluminum and copper must be joined directly.

Microstructural Evolution and Intermetallic Phase Formation

The fundamental metallurgical challenge in aluminum-copper dissimilar welding is the formation of Al-Cu intermetallic phases, primarily Al2Cu (eta phase) and AlCu (theta phase), at the weld interface. These intermetallics are hard, brittle, and susceptible to cracking under thermal and mechanical loading. The TIG thermal conduction welding process, as described in this study, employs a specific technique to control the heat input and minimize IMC formation.

The TIG thermal conduction welding technique involves welding through a thermal barrier or using a controlled heat input approach that allows the copper side to melt preferentially while limiting the aluminum side temperature to below its melting point. This asymmetric melting approach reduces the extent of interdiffusion and, consequently, the thickness of the intermetallic layer at the interface.

The microstructural analysis reveals that the intermetallic layer thickness is the critical factor governing joint strength. Layers thinner than approximately 20 micrometers can be tolerated if they are continuous and free of cracks, while thicker layers (>50 micrometers) lead to premature brittle fracture. The morphology of the intermetallics is also significant: continuous, plate-like intermetallics are more detrimental than discontinuous, particle-like precipitates.

Mechanical Properties and Fracture Behavior

The tensile testing results demonstrate that the joint strength is governed by the weaker of the two base metals or the interface region, depending on the IMC layer thickness. When the IMC layer is thin and well-controlled, the joint strength approaches that of the base metals, with fracture occurring in the aluminum weld zone or the aluminum heat-affected zone. When the IMC layer is thick, fracture initiates at the interface and propagates through the intermetallic layer, resulting in brittle fracture with minimal plastic deformation.

The welding parameters that influence IMC layer thickness include current level, travel speed, arc length, and the direction of welding relative to the copper and aluminum sides. Welding from the aluminum side toward the copper side, with the arc positioned slightly toward the aluminum, tends to produce thinner intermetallic layers because the copper side receives less direct heat input.

Engineering Practice and Application Considerations

For pressure vessel and heat exchanger applications involving aluminum-copper dissimilar joints, the engineering approach must address several critical issues. First, the joint design should incorporate stress-relieving features such as fillet welds or mechanical interlocks that reduce the stress concentration at the interface. Second, the welding procedure must be qualified specifically for the aluminum-copper combination, with rigorous microstructural examination of the interface region. Third, the service environment must be considered: aluminum-copper joints are susceptible to galvanic corrosion in the presence of electrolytes, and the selection of appropriate coatings or cathodic protection is essential for long-term durability.

The study's findings on thermal conduction welding provide a practical pathway for joining aluminum and copper components where traditional fusion welding produces unacceptable intermetallic layers. For engineers working on cryogenic pressure vessels or heat exchangers that combine aluminum and copper components, the TIG thermal conduction welding technique offers a viable alternative to mechanical fastening or brazing, provided that the welding parameters are carefully controlled and the resulting intermetallic layer is verified to be within acceptable thickness limits.