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

Literature Overview

This 2017 study by Shi Yu, Zhou Xianglong, Zhu Ming, Li Guang, and Fan Ding from Lanzhou University of Technology, published in the Chinese Journal of Nonferrous Metals (中国有色金属学报), investigates the microstructure and mechanical properties of aluminum-copper dissimilar metal joints produced by a pulsed bypass coupled arc MIG brazing process. Supported by multiple funding sources including the National Natural Science Foundation (Grant No. 51675256) and the State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, this work addresses a critical challenge in electrical and thermal management applications where aluminum and copper must be joined with high electrical conductivity and mechanical integrity.

Core Technical Points

Technical Challenges of Aluminum-Copper Joining

Aluminum and copper are thermodynamically incompatible metals that form multiple brittle intermetallic compounds (Al₂Cu, AlCu, Al₂Cu₃) when subjected to elevated temperatures. The formation of these IMCs at the interface severely degrades mechanical properties and electrical conductivity. Additionally, the significant difference in thermal expansion coefficients (aluminum: ~23×10⁻⁶/K; copper: ~17×10⁻⁶/K) creates residual stresses during cooling. The pulsed bypass coupled arc MIG brazing process is designed to address these challenges by using pulsed current to control heat input precisely and a bypass electrode configuration to improve arc stability and droplet transfer characteristics.

Microstructural Analysis

The pulsed bypass coupled arc process produces a joint with a layered interfacial structure. The aluminum side exhibits a partially melted zone with fine dendritic structures, while the copper side shows minimal heat-affected zone modification due to the process's ability to direct heat preferentially toward the aluminum side. The interfacial region contains a thin intermetallic compound layer (typically 5-30 μm) composed of Al₂Cu and AlCu phases, depending on the local temperature and cooling rate.

The pulsed current operation creates a periodic cycle of droplet detachment and arc current reduction, which controls the heat input per unit length and minimizes excessive melting of either base metal. The bypass electrode configuration provides an additional current path that stabilizes the arc and improves the transfer of molten metal across the joint, promoting uniform joint formation.

Mechanical and Electrical Properties

The joint strength is directly related to the IMC layer thickness and the quality of the bonding interface. Thinner IMC layers (5-15 μm) generally correlate with higher joint strength, while thicker layers (>25 μm) lead to brittle interfacial failure. Typical joint tensile strengths range from 60 to 120 MPa, with failure occurring at or near the interface.

Process Parameters and Their Effects

The pulsed bypass coupled arc MIG process involves several interrelated parameters that must be optimized simultaneously:

Pulse Parameters

• Peak current determines the droplet detachment force and penetration depth
• Background current maintains arc stability between pulses
• Pulse frequency controls the heat input rate and droplet transfer frequency
• Pulse duration ratio (peak/background time ratio) affects the thermal cycle

Arc Configuration

• Bypass electrode offset and angle influence arc force direction and metal transfer pattern
• Gas shielding composition (Ar + CO₂ mixture) affects arc stability and penetration
• Wire feed speed must be synchronized with pulse frequency for consistent droplet transfer

Joint Design

• Gap size between aluminum and copper components (typically 0.5-2.0 mm)
• Fit-up tolerance and surface preparation
• Joint geometry (butt, lap, T-joint)

Defect Analysis and Countermeasures

Engineering Practice Integration

This technology finds direct application in:

• Electrical busbar connections in power distribution systems
• Heat exchanger tube-to-plate joints in HVAC and automotive applications
• Battery module connections in electric vehicles
• Thermal management systems in electronics
• Power electronics cooling systems

The pulsed bypass coupled arc process offers particular advantages for joining thin aluminum to thick copper components, a common configuration in electrical and thermal applications where the aluminum provides lightweight structural support and the copper ensures high electrical or thermal conductivity.

Key Questions and Reflections

A significant challenge remains in achieving consistent joint quality over long production runs, as the process is sensitive to parameter drift and consumable variations. The long-term reliability of the joint under thermal cycling (as experienced in electrical service) needs further investigation, particularly regarding IMC growth and interface degradation over time. Additionally, the economic viability of this specialized process compared to simpler joining methods (such as mechanical connections with solder or brazing alloys) must be evaluated for each specific application.

Study Insights and Implications

This research demonstrates that the pulsed bypass coupled arc MIG brazing process provides effective control over the interfacial reaction zone in aluminum-copper joints, enabling the production of joints with acceptable mechanical and electrical properties. The key insight is that pulse parameter optimization, combined with the unique bypass electrode configuration, allows for precise heat input control that minimizes detrimental IMC formation while maintaining adequate bonding. This approach represents a significant advancement over conventional MIG welding for dissimilar metal joining, where excessive heat input invariably leads to poor joint quality. The findings have direct implications for the design and manufacturing of high-reliability electrical and thermal connections in power systems and electronic applications.