Microstructure and Properties of Copper-Steel TIG Welded Joints with High-Entropy Alloy Interlayer

Literature Overview

This 2023 research from China University of Petroleum (East China), conducted by Jiang Shuying, Zhang Junli, and Zhao Ming under the Shandong Provincial Natural Science Foundation (ZR2020ME013), investigates the microstructure and mechanical properties of copper-steel TIG welded joints using high-entropy alloy (HEA) interlayers. Published in the Journal of China University of Petroleum (Natural Science Edition), this work addresses a long-standing challenge in dissimilar metal welding: the formation of brittle intermetallic compounds at the Cu-Fe interface.

The study is particularly significant for heat exchanger manufacturing, electrical equipment fabrication, and cryogenic applications where copper-to-steel joints are required. The use of high-entropy alloy interlayers represents an innovative approach to mitigating the metallurgical incompatibility between copper and steel.

Fundamental Challenges in Copper-Steel Welding

Copper and steel are metallurgically incompatible due to the formation of brittle iron-copper intermetallic compounds (ICs) during welding. The primary intermetallic phases that form include FeCu, Fe2Cu, Fe3Cu, and Cu2Fe, which are extremely brittle and prone to cracking. These phases form readily at welding temperatures and can lead to joint failure at relatively low temperatures.

The following table presents the characteristics of common iron-copper intermetallic compounds:

These intermetallic compounds form in the heat-affected zone adjacent to the weld, creating a continuous or semi-continuous layer that severely compromises joint integrity. The thickness of the IC layer increases with increasing heat input and holding time at elevated temperatures.

High-Entropy Alloy Interlayer Design and Selection

The high-entropy alloy interlayer serves as a diffusion barrier between copper and steel, preventing or reducing the formation of brittle intermetallic compounds. The selection of the HEA interlayer is critical to the success of the joint.

Common HEA compositions investigated for this application include:

The HEA interlayer must satisfy several requirements: it should be weldable to both copper and steel using TIG welding; it should act as an effective diffusion barrier against iron and copper interdiffusion; it should maintain adequate ductility and toughness at service temperatures; and it should be compatible with the base metals in terms of thermal expansion.

Welding Process Parameters and Procedure

The TIG welding of the copper-steel joint with HEA interlayer requires careful parameter selection to minimize heat input and prevent excessive intermetallic formation. The following parameters are typically employed:

The welding sequence typically involves: first welding the HEA interlayer to the steel side, followed by welding the HEA interlayer to the copper side. This sequence ensures that the HEA layer remains intact and functions as an effective diffusion barrier.

Microstructural Evolution and Diffusion Behavior

The microstructure of the welded joint reveals several distinct zones: the weld metal, the HEA interlayer, and the heat-affected zones on both the steel and copper sides. The HEA interlayer undergoes partial melting and solidification during welding, resulting in a mixed structure of the original HEA microstructure and weld metal.

The critical finding of this study is the significant reduction in intermetallic compound formation compared to direct copper-steel welding. The HEA interlayer effectively slows down the diffusion of iron into copper and vice versa, resulting in a thinner and more discontinuous IC layer. The IC layer thickness is typically reduced from 20-50 μm (direct welding) to 5-15 μm (with HEA interlayer).

The microstructure of the HEA interlayer after welding shows evidence of elemental redistribution. The high-entropy alloy structure is partially disrupted by the welding thermal cycle, but the multi-principal-element composition maintains a degree of lattice distortion and sluggish diffusion kinetics that inhibit intermetallic formation.

Mechanical Properties and Joint Performance

The mechanical properties of the copper-steel joint with HEA interlayer demonstrate significant improvement compared to direct welding:

The improved ductility and toughness are attributed to the suppression of brittle intermetallic compound formation. The fracture analysis typically shows ductile fracture characteristics in the HEA interlayer region, indicating that the joint can absorb significant plastic deformation before failure.

Engineering Applications and Practical Considerations

This technology has direct applications in several industrial sectors. In heat exchanger manufacturing, copper-to-steel joints are used in shell-and-tube heat exchangers, condensers, and evaporators where copper tubes are attached to steel headers or channel plates. The HEA interlayer approach provides a viable solution for improving joint reliability in these critical components.

In electrical equipment fabrication, copper-to-steel joints are used in busbar connections, grounding systems, and high-current conductors. The improved mechanical properties and reduced intermetallic formation enhance the long-term reliability of these joints.

For cryogenic applications, such as LNG storage and transportation, copper-to-steel joints must maintain ductility at low temperatures. The HEA interlayer approach may offer advantages in this regard, as the multi-principal-element composition provides inherent cryogenic toughness.

However, several practical challenges remain. The HEA interlayer adds material cost and fabrication complexity. The interlayer must be machined to precise dimensions and fit-up tolerances. Welding qualification procedures must be developed to address the unique metallurgy of the HEA interlayer. Additionally, the long-term performance of the joint under thermal cycling and corrosion conditions requires further investigation.

Key Insights and Future Directions

This study demonstrates that high-entropy alloy interlayers are an effective approach to improving the weldability of copper-steel joints. The multi-principal-element composition of HEAs provides inherent advantages in terms of diffusion barrier properties and mechanical performance.

Future research should focus on several areas. First, optimizing the HEA composition and thickness for specific applications and service conditions. Second, developing automated welding procedures for large-scale production. Third, investigating the long-term performance of the joint under thermal cycling, corrosion, and fatigue loading. Fourth, extending the approach to other dissimilar metal combinations, such as nickel-based alloy to steel, or titanium to steel.

The use of high-entropy alloy interlayers represents a paradigm shift in dissimilar metal welding technology. By leveraging the unique properties of HEAs, engineers can overcome traditional metallurgical incompatibilities and achieve reliable joints in previously challenging applications. This approach opens new possibilities for material selection in pressure vessel fabrication, heat exchanger manufacturing, and other critical industrial applications.