Microstructure and Properties of Copper Steel TIG Weld Joints Using High-Entropy Alloy Interlayer

Literature Overview

This study by Jiang Shuying and colleagues from China University of Petroleum (East China), published in Journal of China University of Petroleum (Natural Science Edition) (2023, Vol. 47, No. 6, pp. 154-161), addresses the challenging problem of joining dissimilar metals—specifically T2 pure copper and Q235 carbon steel—using TIG welding with a high-entropy alloy (HEA) interlayer. Copper-steel composite structures are valuable in engineering applications where the combination of copper's electrical conductivity and corrosion resistance with steel's mechanical strength is required. However, the extreme immiscibility of copper and iron leads to liquid phase separation and cracking during conventional welding.

The Dissimilar Metal Welding Challenge

Copper and steel present one of the most challenging dissimilar metal welding combinations due to:

• Large difference in thermal conductivity (copper: 390 W/m·K vs. steel: 50 W/m·K)
• Large difference in thermal expansion coefficients
• Very limited mutual solubility at high temperatures
• Tendency for liquid phase separation during solidification
• Formation of brittle intermetallic compounds (Fe-Cu)

Conventional approaches include using bronze filler metals, nickel-based interlayers, or explosive bonding. Each has limitations in terms of joint strength, electrical conductivity, or process complexity.

High-Entropy Alloy Interlayer Design

The authors designed a Fe₅Co₃₀Cr₃₀Ni₃₀Cu₅ high-entropy alloy as the interlayer material, based on the solid solution-high entropy concept. The composition was carefully selected to:

1. Provide good wettability with both copper and steel
2. Maintain a single-phase FCC structure for ductility
3. Act as a diffusion barrier to prevent direct Fe-Cu intermetallic formation
4. Provide sufficient strength and toughness

Microstructural Analysis

The HEA interlayer itself exhibited a single-phase face-centered cubic (FCC) solid solution structure, confirming that the composition achieved the desired high-entropy stabilization. This is significant because multi-phase HEA structures with intermetallic phases would be more brittle and less suitable for interlayer applications.

The weld zone also showed a single-phase FCC solid solution structure with pronounced high-entropy characteristics. The absence of second phases in the weld metal indicates that the cooling rates during TIG welding were sufficient to maintain the solid solution structure without phase separation or precipitation.

Mechanical Performance

The tensile strength of 225 MPa represents 91% of the copper base metal strength, which is an excellent result for a dissimilar metal joint. The high elongation of 44% indicates that the joint maintains significant ductility, which is critical for applications subject to thermal cycling or mechanical vibration.

The fracture occurring in the copper HAZ rather than at the interface confirms that the bond strength exceeds the cohesive strength of the surrounding material. The dimpled fracture morphology further confirms ductile failure behavior, indicating that the joint can absorb significant energy before failure.

Engineering Applications and Implications

For pipe and fitting applications, copper-steel dissimilar joints are common in:

• Heat exchangers and condensers
• Electrical busbar connections
• Corrosion-resistant pipe systems
• Marine and chemical processing equipment

The HEA interlayer approach offers several advantages over conventional methods:

• Higher joint strength compared to bronze or nickel interlayers
• Better electrical conductivity retention
• Reduced risk of liquid phase separation
• Potential for automated welding processes

However, the availability and cost of HEA materials for interlayer applications may be a practical limitation. The Fe₅Co₃₀Cr₃₀Ni₃₀Cu₅ composition contains expensive elements (Co, Ni, Cr) that may limit widespread adoption in cost-sensitive applications.

Critical Reflections

The success of this approach hinges on the ability to produce and maintain the HEA interlayer material with consistent composition and microstructure. Any variation in the HEA composition could lead to phase separation or precipitation during welding, potentially compromising joint integrity. Quality control of the interlayer material is therefore critical.

From a metallurgical perspective, the single-phase FCC structure in both the interlayer and weld zone is ideal for ductility but may be susceptible to grain coarsening under prolonged thermal exposure. For applications involving elevated temperature service, post-weld heat treatment or selection of a slightly different HEA composition with improved high-temperature stability may be necessary.

The 91% strength retention is impressive, but it should be noted that this represents the strength of the weakest component in the joint (copper). For applications where the steel component governs the design, the joint strength may be more than adequate.

Summary

This research demonstrates that a carefully designed high-entropy alloy interlayer (Fe₅Co₃₀Cr₃₀Ni₃₀Cu₅) enables reliable TIG welding of T2 copper to Q235 steel, achieving 91% of copper base metal tensile strength with excellent ductility. The single-phase FCC structure in both the interlayer and weld zone provides superior mechanical performance compared to conventional interlayer approaches. While the technology shows great promise for dissimilar metal joining in heat exchangers, electrical connections, and corrosion-resistant systems, practical implementation requires consideration of HEA material availability, cost, and long-term thermal stability.