Microstructure and Mechanical Properties of Cu-Al Dissimilar Metal Joints by TIG Brazing with Zn-Al Filler Wire

Literature Overview

The research by Sun Jianxin, Sun Zhen, Xu Wenkai, Zhao Hongli, Hu Ruiling, and Li Zhiqiang from Hubei University of Automotive Technology and Dongfeng Motor Corporation investigates the TIG brazing of pure copper and pure aluminum using a Zn-5wt%Al flux-cored wire. Supported by Hubei Provincial Department of Education Science and Technology Research Project (Q20171801), this work was published in Hot Working Technology (Vol. 47, No. 21, 2018, pp. 66-70). Dissimilar metal joining of copper and aluminum is a critical engineering challenge in electrical engineering, automotive powertrain systems, and thermal management applications.

Technical Challenge of Cu-Al Dissimilar Metal Joining

The direct fusion welding of copper and aluminum is extremely challenging due to:

• Large difference in melting points (Cu: 1085°C, Al: 660°C)
• Formation of brittle intermetallic compounds (Cu₄Al₃, CuAl₂, CuAl)
• Large difference in thermal expansion coefficients (Cu: 16.5×10⁻⁶/K, Al: 23.1×10⁻⁶/K)
• Formation of continuous brittle intermetallic layers at the interface during fusion welding

TIG brazing with a Zn-Al filler alloy offers an alternative approach by avoiding the high temperatures required for fusion welding, thereby limiting intermetallic compound formation and maintaining ductile joints.

Experimental Design and Parameters

The study employed a lap joint configuration with pure copper and pure aluminum plates. The Zn-5wt%Al flux-cored wire was used as the filler metal, and the welding was performed using conventional TIG (GTAW) equipment with varying welding currents while other parameters remained constant.

Key Results: Wetting Behavior and Interface Microstructure

The study demonstrates that increasing TIG current reduces the wetting angle of the filler metal on both the copper and aluminum base metal surfaces. This indicates improved capillary action and spreading of the molten filler metal, which is essential for achieving a sound brazed joint. The improved wetting at higher currents is attributed to:

• Higher heat input increasing the temperature at the joint interface
• Enhanced flux activity at elevated temperatures
• Better removal of surface oxides from both base metals

At the optimal current of 70 A, the joint achieves the best mechanical performance. The brazed seam microstructure is predominantly Zn-Al eutectic, without significant intermetallic compound formation. A diffusion layer enriched in Zn, approximately 3-4 μm thick, forms at both the Cu and Al interfaces.

Mechanical Performance Analysis

The tensile-shear strength exceeding 90 MPa represents approximately 75% of the industrial pure aluminum tensile strength, which is considered a satisfactory joint strength for many engineering applications. The absence of significant intermetallic compounds is particularly important because Cu-Al intermetallics are extremely brittle and would severely compromise joint toughness and fatigue resistance.

Interface Diffusion and Reaction Analysis

The formation of a Zn-enriched diffusion layer at both interfaces is a critical microstructural feature. This diffusion layer:

• Acts as a buffer zone between the base metals and the brazed seam
• Limits the direct contact between Cu and Al, preventing Cu-Al intermetallic formation
• Provides a graded transition in composition and mechanical properties
• Is thin enough (3-4 μm) to not significantly reduce joint strength

The Zn-Al eutectic structure in the brazed seam is advantageous because:

• It has a relatively low melting point, enabling low-temperature joining
• It maintains ductility at room temperature
• It does not undergo brittle phase transformations during service

Engineering Applications and Practical Considerations

This technology has direct relevance to several engineering applications:

1. Automotive electrical systems: Battery connections, motor windings, and current collectors require reliable Cu-Al joints. The 90 MPa shear strength is adequate for most automotive electrical applications.
2. Power electronics: Heat sinks and thermal interfaces often require Cu-Al bonding. The low-temperature process protects temperature-sensitive components.
3. Heat exchangers: Cu-Al joints are used in compact heat exchangers for thermal management systems.
4. Aerospace applications: Lightweight structures requiring dissimilar metal joints benefit from the low-temperature joining process.

For production implementation, the following factors must be addressed:

• Surface preparation of both base metals to ensure oxide removal
• Precise control of welding current to maintain optimal heat input
• Flux-cored wire consistency and supply reliability
• Joint geometry design to ensure proper capillary action
• Post-weld inspection methods for joint quality verification

Study Insights and Limitations

The research demonstrates that TIG brazing with Zn-Al filler wire is a viable method for Cu-Al dissimilar metal joining, achieving acceptable mechanical properties without forming detrimental intermetallic compounds. The optimal current of 70 A represents a process window that balances adequate heat input for wetting against excessive thermal damage. The 75% strength retention relative to the weaker base metal (aluminum) is competitive with other low-temperature joining methods. However, the study does not address fatigue performance, thermal cycling resistance, or long-term aging behavior, which are critical for many engineering applications. The diffusion layer thickness of 3-4 μm is favorable but may grow with time at elevated temperatures, potentially degrading joint properties during prolonged service. Further research should focus on these durability aspects and the scalability of the process for industrial production.