Microstructure and Properties of Aluminum-Copper Dissimilar Butt Joints by TIG Welding with Filler Wire

Literature Overview

This study, published in the Chinese Journal of Nonferrous Metals in 2015 (Volume 25, Issue 4, pages 975-981), was conducted by researchers at the School of Aeronautical Manufacturing and Engineering, Nanchang Hangkong University. The work investigates the microstructure and mechanical properties of butt joints produced by gas tungsten arc welding (TIG/GTAW) of T2 pure copper to LY16 aluminum alloy using ER8515 flux-cored zinc-aluminum filler wire on a Lincoln TIG-355 AC/DC TIG welding machine. The study provides valuable insights into the metallurgical behavior and mechanical performance of aluminum-copper dissimilar joints, which are of interest in electrical engineering, heat exchanger manufacturing, and lightweight structural applications.

Technical Background

The direct welding of aluminum to copper is notoriously difficult due to the formation of brittle intermetallic compounds at the interface, the large difference in thermal conductivity and thermal expansion between the two metals, and the tendency of aluminum to form a tenacious oxide layer. These factors typically result in joints with poor mechanical properties and limited reliability. The use of a filler wire containing zinc and aluminum, as employed in this study, is a strategy to modify the weld metal composition and potentially reduce the formation of detrimental intermetallics.

The ER8515 filler wire is a flux-cored wire with a zinc-aluminum composition. The flux core serves to clean the weld pool, remove oxides, and potentially modify the solidification behavior of the weld metal. The zinc content in the filler wire is intended to promote the formation of Cu-Zn phases, which may be less brittle than the Al-Cu intermetallics that would form in a direct aluminum-copper joint.

Welding Parameters and Process

The welding was performed using a Lincoln TIG-355 AC/DC TIG welding machine with the following parameters:

The selection of 100 A and 62 mm/min represents a relatively low heat input, which is appropriate for the thin sections typical of aluminum-copper applications. The low heat input helps to minimize the extent of intermetallic formation and reduces the risk of cracking in the weld metal. The shielding gas flow rate of 15 L/min is sufficient to protect the weld pool from atmospheric contamination, which is particularly important for aluminum due to its high reactivity with oxygen.

Microstructural Analysis

The microstructural examination using SEM and EDS revealed several important features. The weld metal is characterized by white blocky CuZn₄ compounds uniformly distributed within an (α+η) eutectic matrix composed of Zn-based and Al-based solid solutions. This microstructure is consistent with the solidification behavior of the Zn-Al-Cu system and indicates that the filler wire composition has been effectively incorporated into the weld metal.

The copper side of the joint exhibits a flat and clear interface, with a relatively high density of intermetallic compound phases near the interface. This is expected, as the copper is in direct contact with the molten weld metal and undergoes interfacial reactions. The formation of Cu-Zn intermetallics is thermodynamically favorable and is driven by the diffusion of zinc from the weld metal into the copper base metal.

The aluminum side of the joint presents a different morphology: a chain-like distribution of gray-black spherical α-phase aluminum-based solid solution particles along the interface. This morphology suggests that the aluminum side underwent limited melting and mixing with the weld metal, resulting in a partially fused interface with a distinct solidification pattern. The presence of aluminum-based solid solution particles indicates that the aluminum did not fully dissolve into the weld metal, which is consistent with the lower melting point of aluminum compared to copper.

Mechanical Performance

The tensile strength of the joint at the optimal welding parameters (100 A, 62 mm/min, 15 L/min) reached 240 MPa, with fracture occurring in the heat-affected zone (HAZ) on the copper side. This is a significant result, as it demonstrates that the joint strength is limited by the copper HAZ rather than by the weld metal or the interface. The fracture location in the copper HAZ suggests that the interface and weld metal are stronger than the softened copper HAZ, which is a positive indicator of joint integrity.

The 240 MPa tensile strength is comparable to or exceeds the tensile strength of many aluminum-copper joints produced by other methods, such as friction stir welding or explosion welding. This suggests that the TIG welding with ER8515 filler wire is a viable process for producing aluminum-copper joints with acceptable mechanical properties.

Engineering Practice and Application Considerations

The successful production of aluminum-copper joints with TIG welding and ER8515 filler wire has several practical implications. First, the process is relatively simple and does not require specialized equipment beyond a standard TIG welding machine and the appropriate filler wire. This makes it accessible to workshops and fabrication shops that may not have access to more advanced joining technologies. Second, the process is suitable for joining thin sections, which is common in electrical and heat exchanger applications. Third, the joint quality can be controlled through careful selection of welding parameters and filler wire composition.

However, several factors must be considered when implementing this process in production:

• The joint strength of 240 MPa is limited by the copper HAZ, which may be a concern for applications requiring higher strength.
• The interface morphology and intermetallic formation may vary with welding parameters, and a systematic study of parameter effects is recommended.
• The long-term reliability of the joint under thermal cycling, vibration, and corrosion conditions must be evaluated.
• The process must be qualified in accordance with applicable codes and standards for the intended application.

Key Questions and Reflections

The study does not address the electrical conductivity of the joint, which is a critical property for electrical applications. The formation of intermetallic compounds at the interface may reduce the electrical conductivity, and the extent of this reduction must be quantified for applications where electrical performance is important. Additionally, the study does not evaluate the joint under fatigue or impact loading, which are common failure modes in structural applications.

The use of ER8515 filler wire is a promising approach to aluminum-copper joining, but the optimal filler wire composition and welding parameters may vary with the specific application. A systematic study of filler wire composition, welding parameters, and joint properties would be valuable for developing a comprehensive process window for this technology.

Summary and Conclusions

This study demonstrates that TIG welding with ER8515 flux-cored zinc-aluminum filler wire can produce aluminum-copper butt joints with good weld quality and acceptable mechanical properties. The tensile strength of 240 MPa and the fracture location in the copper HAZ indicate that the joint is limited by the base metal rather than by the weld metal or interface, which is a positive indicator of joint integrity. The microstructural analysis reveals a complex but well-defined weld metal and interface morphology, with CuZn₄ compounds in the weld metal and intermetallic phases at the copper interface. Engineers considering this process for aluminum-copper joining should conduct additional testing to evaluate the joint under realistic service conditions, including thermal cycling, corrosion, and fatigue loading, and should qualify the process in accordance with applicable codes and standards.