Copper Steel TIG Welded Joint with High-Entropy Alloy Interlayer

Literature Overview

This paper by Jiang Shuying and colleagues from the School of Materials Science and Engineering at China University of Petroleum (East China), published in the Journal of China University of Petroleum in 2023, presents an innovative approach to joining dissimilar metals: T2 pure copper and Q235 carbon steel using a high-entropy alloy (HEA) interlayer. The research is supported by the Shandong Provincial Natural Science Foundation (ZR2020ME013). The work addresses the fundamental challenge of joining materials with vastly different physical and chemical properties through a novel metallurgical strategy.

Technical Problem and Design Philosophy

Copper-steel composite structures are desirable in many engineering applications because they combine the excellent electrical and thermal conductivity of copper with the structural strength of steel. However, the direct welding of copper and steel is notoriously difficult due to several metallurgical incompatibilities. Copper and iron have extremely poor mutual solubility in the solid state, leading to liquid phase separation during solidification. The large difference in thermal conductivity (copper approximately 400 W/m·K versus steel approximately 50 W/m·K) causes uneven heat distribution during welding. The coefficient of thermal expansion mismatch (copper approximately 17×10⁻⁶/°C versus steel approximately 12×10⁻⁶/°C) generates significant residual stresses. Furthermore, the formation of brittle intermetallic compounds such as CuFe, Cu₂Fe, and Cu₃Fe at the interface leads to poor ductility and crack susceptibility.

The authors' design philosophy is based on the concept of solid solution and high-entropization of the weld metal. By introducing a high-entropy alloy interlayer, they aim to create a transition zone that accommodates the metallurgical incompatibilities between copper and steel. The selected HEA composition is Fe₅Co₃₀Cr₃₀Ni₃₀Cu₅, which was designed to have a single-phase face-centered cubic (FCC) solid solution structure with good strength and ductility.

Experimental Methodology

The TIG welding process was used to join T2 pure copper sheets to Q235 carbon steel sheets using the HEA interlayer. The interlayer was placed between the copper and steel base metals, and the TIG arc was used to melt and fuse all three materials together. The resulting joints were characterized using optical microscopy, SEM, EDS, X-ray diffraction (XRD), tensile testing, and hardness profiling.

The selection of TIG welding is appropriate for this application because it provides precise heat input control, excellent weld bead formation, and minimal dilution of the base metals. The non-consumable tungsten electrode ensures that the composition of the weld metal is determined primarily by the base metals and the interlayer, without additional filler metal dilution.

Key Findings

HEA Interlayer Characterization

The Fe₅Co₃₀Cr₃₀Ni₃₀Cu₅ high-entropy alloy was confirmed to have a single-phase FCC solid solution structure. This is significant because multi-phase HEAs often exhibit brittleness due to the presence of intermetallic compounds. The single-phase FCC structure provides good ductility and toughness, which are essential for accommodating the thermal and mechanical stresses at the dissimilar metal interface.

The HEA interlayer exhibited good strength and ductility properties, making it suitable as a transition material between the copper and steel base metals.

Weld Joint Integrity

The TIG welding with the HEA interlayer achieved reliable joining of T2 copper and Q235 steel with good weld bead formation and no porosity or cracking defects. This is a significant achievement given the inherent difficulties of copper-steel welding. The absence of porosity indicates effective shielding and proper wetting of the joint. The absence of cracking suggests that the residual stresses were adequately managed by the ductile HEA interlayer.

Microstructural Analysis

The weld metal region exhibited a single-phase FCC solid solution structure with pronounced high-entropization characteristics. This means that the five principal elements (Fe, Co, Cr, Ni, Cu) were uniformly distributed throughout the weld metal, creating a high configurational entropy that stabilizes the FCC phase and suppresses the formation of brittle intermetallic compounds. The high-entropization effect is the key mechanism that enables the successful joining of these otherwise incompatible materials.

Mechanical Properties

The average tensile strength of the welded joints was 225 MPa, reaching 91% of the copper base metal strength. The average elongation after fracture was 44%, indicating excellent ductility. The fracture consistently occurred in the copper side HAZ, and the fracture surface exhibited extensive dimple structures characteristic of ductile fracture.

Engineering Practice Implications

This technology has significant potential for applications where copper-steel joints are required, such as electrical connectors, heat exchangers, and composite structural components. The use of a HEA interlayer eliminates the need for complex cladding or brazing processes, simplifying manufacturing. The single-phase FCC weld metal avoids the brittleness associated with copper-iron intermetallics, ensuring long-term reliability.

However, practical implementation requires consideration of several factors. The HEA interlayer material must be manufactured with controlled composition and properties, which may add cost. The TIG welding parameters must be optimized to achieve complete melting and fusion of all materials without excessive dilution. The thermal cycling behavior of the joint during service must be evaluated, as the coefficient of thermal expansion mismatch between copper and steel may generate cyclic stresses that could eventually lead to fatigue failure.

Critical Reflections

The use of a high-entropy alloy as an interlayer for dissimilar metal welding is an innovative and promising approach. The concept of using configurational entropy to stabilize a single-phase structure and suppress brittle phase formation is elegant and well-founded in materials thermodynamics. However, the study would benefit from additional characterization, including detailed EDS line scans across the interface to quantify elemental diffusion, and thermal cycling tests to evaluate long-term stability. The effect of welding heat input on the HEA interlayer thickness and composition should also be investigated, as excessive heat input could lead to excessive interdiffusion and IMC formation.

Summary

This study demonstrates that a Fe₅Co₃₀Cr₃₀Ni₃₀Cu₅ high-entropy alloy interlayer enables reliable TIG welding of T2 copper and Q235 steel, producing joints with 91% of copper base metal strength and 44% elongation. The single-phase FCC weld metal with high-entropization characteristics eliminates the brittle intermetallic compounds that plague conventional copper-steel welds. This approach offers a novel metallurgical solution to the long-standing challenge of dissimilar metal joining and has significant potential for electrical, thermal, and structural applications.