Al/Cu Dissimilar Metal Pulsed Bypass Coupled Arc MIG Brazing Joints

Literature Overview

The paper by Shi Yu, Zhou Xianglong, Zhu Ming, Li Guang, and Fan Ding (2017), published in The Chinese Journal of Nonferrous Metals, investigates the pulsed bypass coupled arc MIG brazing of 5052 aluminum alloy to T2 pure copper using ER4047 aluminum-silicon filler wire. This research was supported by the National Natural Science Foundation of China (grant 51675256), the State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals open fund (SKLAB 020114208), the Lanzhou University of Technology Hongliu Outstanding Talent Cultivation Program (J201201), and the Gansu Provincial Basic Research Innovation Group Program (17JR5RA107). The study addresses the challenging problem of joining aluminum to copper, a dissimilar metal combination that is common in electrical connections, heat exchangers, and electronic packaging applications.

Core Technical Approach

The pulsed bypass coupled arc MIG brazing process is a specialized variant of MIG welding that uses a bypass current path to create a coupled arc between the filler wire and the base metals. This configuration allows for controlled heat input and improved wetting of the filler metal on both base metals, enabling brazing rather than fusion welding. The process is particularly suitable for aluminum-copper joints because it minimizes the formation of thick, brittle intermetallic compounds that would otherwise compromise joint strength.

The key process variable investigated was welding heat input, which was controlled by adjusting the welding current and travel speed. The authors systematically varied these parameters to determine the optimal conditions for achieving good joint formation and maximum tensile strength.

Microstructural Analysis and Intermetallic Compound Formation

The brazed joint consists of three distinct regions: the aluminum-side fusion zone, the weld zone, and the copper-side pseudo-brazing zone. The copper-side pseudo-brazing zone can be further divided into an intermetallic compound layer region and an Al-Cu eutectic region. The weld zone microstructure consists of coral-like Al-Cu eutectic crystals uniformly distributed in the α(Al) solid solution. The copper-side intermetallic compound layer is primarily composed of strip-shaped Al2Cu.

The formation of Al2Cu at the copper interface is thermodynamically favored because it is the most stable intermetallic compound in the Al-Cu system. However, the thickness of this layer must be controlled because excessive thickness leads to brittleness and reduced joint strength. The authors found that as welding heat input increased, the intermetallic compound layer thickness increased, while the joint tensile strength first increased and then decreased.

Mechanical Performance and Optimal Parameters

The maximum tensile strength of 167.7 MPa was achieved when the filler metal and aluminum base metal wetted the copper base metal well and the intermetallic compound layer thickness between the weld and copper base metal was small. This result indicates that there is an optimal heat input window for achieving maximum joint strength. Below this window, incomplete wetting results in poor joint formation. Above this window, excessive intermetallic compound formation reduces joint strength.

The non-monotonic relationship between heat input and tensile strength is a critical finding for process optimization. Engineers must identify the optimal heat input for their specific application, considering factors such as joint geometry, base metal thickness, and filler wire composition. The pulsed bypass coupled arc configuration provides the necessary control to operate within this optimal window.

Engineering Practice and Application Considerations

Aluminum-copper joints are common in electrical and thermal applications, where the combination of high electrical/thermal conductivity (copper) and lightweight (aluminum) is desired. However, the formation of brittle intermetallic compounds at the interface has historically limited the strength and durability of these joints. The pulsed bypass coupled arc MIG brazing process offers a promising solution by enabling controlled heat input and minimized intermetallic compound formation.

For industrial implementation, several factors must be considered. The pulsed bypass coupled arc equipment is more complex than conventional MIG welding systems and requires careful calibration to maintain the bypass current balance. The ER4047 filler wire composition, with its silicon content, influences the wetting behavior and intermetallic compound formation, and may need to be optimized for specific applications. Additionally, surface preparation of both base metals is critical for achieving good wetting and joint formation.

Study Insights and Reflections

This research demonstrates that the pulsed bypass coupled arc MIG brazing process can produce Al/Cu joints with tensile strengths approaching 168 MPa, which is a significant achievement for a dissimilar metal brazed joint. The key to success lies in controlling the intermetallic compound layer thickness through careful management of welding heat input. The non-monotonic relationship between heat input and joint strength highlights the importance of process optimization and quality control in achieving consistent joint performance.

For engineers working on aluminum-copper joint design, this study provides valuable guidance on process selection and parameter optimization. The pulsed bypass coupled arc configuration offers a technically superior alternative to conventional fusion welding for Al/Cu joints, and should be considered for applications where joint strength and durability are critical requirements. The findings of this research should inform both process development and quality assurance practices in industries relying on aluminum-copper dissimilar metal joints.