Microstructure and Mechanical Properties of Aluminum Copper Dissimilar Metal Pulsed Bypass Coupled Arc MIG Brazed Joints

Literature Overview

Published in 2017 in the Chinese Journal of Nonferrous Metals, this study by Shi Yu, Zhou Xianglong, Zhu Ming, Li Guang, and Fan Ding from the State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals at Lanzhou University of Technology investigates the microstructure and mechanical properties of aluminum-to-copper dissimilar metal joints fabricated using a pulsed bypass coupled arc MIG brazing process. This work is supported by multiple national and provincial research grants, reflecting the significant scientific interest in aluminum-copper joining for electrical, thermal management, and structural applications. The pulsed bypass coupled arc technique represents an advanced variant of conventional pulsed MIG welding, incorporating a bypass current path that enables finer control over droplet transfer and heat input.

Core Technical Points

The aluminum-copper dissimilar metal joint presents unique challenges due to the formation of brittle intermetallic compounds, primarily Al₂Cu, Al₄Cu₉, and AlCu, at the interface. Unlike aluminum-steel joints where brazing is universally employed, aluminum-copper joints can sometimes be achieved through fusion welding with appropriate filler metals, but the brittleness of the copper-rich intermetallics severely limits mechanical performance. The pulsed bypass coupled arc process addresses this by operating in a semi-solid brazing regime where the aluminum side is heated above its melting point but the copper side remains solid, allowing the molten aluminum (with added filler) to wet and bond to the copper surface through capillary action and diffusion.

The bypass coupling mechanism works by introducing an auxiliary current path through the workpiece that modulates the arc current waveform. This creates a dual-pulse characteristic where the primary pulse provides the main heat input and droplet detachment, while the bypass pulse fine-tunes the arc stability and heat distribution. The result is a more stable arc with reduced spatter and more uniform thermal input compared to conventional pulsed MIG.

Microstructural Analysis

The joint microstructure reveals four distinct regions: the aluminum base metal with a recrystallized grain structure in the HAZ; the solidified filler zone showing a dendritic Al-Si eutectic microstructure; the intermetallic reaction zone at the Al/Cu interface; and the copper base metal with minimal microstructural change due to its high melting point.

The IMC layer at the aluminum-copper interface is the critical feature governing joint strength. The study identifies that the Al₂Cu phase forms preferentially at the copper interface, with Al₄Cu₉ appearing at the aluminum side of the IMC layer. The total IMC thickness in optimized conditions ranges from 8-15 μm, with a maximum acceptable thickness of approximately 20 μm before significant strength degradation occurs. Exceeding this threshold leads to a sharp decline in tensile strength from approximately 80-100 MPa to below 40 MPa.

The silicon content in the filler alloy plays a crucial role in controlling IMC growth. AlSi12 filler produces thinner and more uniform IMC layers compared to AlSi5 because silicon acts as a diffusion barrier and promotes the formation of more stable Al-Si eutectic structures that reduce the driving force for aluminum-copper interdiffusion. However, excessive silicon content can lead to increased brittleness of the filler zone itself.

Mechanical Performance and Fracture Behavior

Tensile testing of lap and butt joints reveals that the optimized pulsed bypass coupled arc MIG brazed joints achieve tensile strengths of 80-110 MPa, with fracture consistently occurring in the IMC layer or at the Al/IMC interface. The fracture surfaces exhibit a mix of cleavage and intergranular features, confirming the brittle nature of the failure mode. Shear strength values of 55-75 MPa are reported for optimized parameters.

The study demonstrates that the pulsed bypass coupled arc process produces joints with approximately 15-20% higher strength compared to conventional pulsed MIG under equivalent average heat input conditions. This improvement is attributed to the more stable droplet transition and reduced thermal cycling, which results in thinner and more uniform IMC layers. The bypass current effectively reduces the peak temperature at the interface while maintaining sufficient heat for wetting, creating a narrower thermal gradient that limits diffusion-driven IMC growth.

Engineering Applications and Quality Control

For engineering applications, aluminum-copper joints are most commonly encountered in electrical busbar connections, heat exchanger tube-to-header joints, and thermal management systems in electronics. The pulsed bypass coupled arc MIG brazing process offers a cost-effective alternative to traditional brazing methods (such as vacuum brazing or flux brazing) for medium-scale production.

Quality control considerations include:

From the perspective of bimetal pressure vessel engineering, the aluminum-copper joining technology has relevance to heat exchanger fabrication where copper tube bundles are brazed to aluminum headers in aerospace and marine applications. The IMC control principles established in this study directly apply to the quality assurance of such critical components.

Study Insights and Reflections

This research contributes valuable process-structure-property relationships for aluminum-copper dissimilar metal joining that are directly applicable to engineering design. The pulsed bypass coupled arc concept represents an elegant solution to the fundamental challenge of balancing heat input for adequate wetting against excessive heat that promotes brittle IMC growth. The key insight is that process innovation—specifically the modulation of arc current through bypass coupling—can achieve better results than simply optimizing conventional parameters. For practitioners in the bimetal products field, this reinforces the principle that interface engineering is paramount, and that advanced process control techniques can significantly improve joint reliability without requiring exotic filler materials or costly post-weld treatments. The findings provide a solid foundation for developing welding procedure specifications (WPS) for aluminum-copper joints in heat exchanger and electrical busbar applications.