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

Literature Overview

This study by Peng Chi and colleagues from the School of Aeronautical Manufacturing and Engineering at Nanchang Hangkong University, published in the Chinese Journal of Nonferrous Metals in 2015 (Vol. 25, No. 4, pp. 975-981), investigates the microstructural evolution and mechanical properties of butt joints produced by TIG welding between T2 pure copper and LY16 aluminum alloy using Lincoln TIG-355 AC/DC TIG welding equipment and 8515 zinc-aluminum core flux wire. The research employs scanning electron microscopy and energy-dispersive X-ray spectroscopy for microstructural and compositional analysis, supplemented by tensile testing for mechanical property evaluation.

The aluminum-copper dissimilar joint is a technically challenging connection due to the significant differences in physical properties, thermal expansion coefficients, and metallurgical compatibility between the two materials. The formation of brittle intermetallic compounds at the interface is a well-known challenge, and this study provides valuable insights into the joint characteristics under specific welding conditions.

Microstructural Characteristics of the Joint

The joint was found to be free of porosity and cracking defects, with good weld bead formation, which is a positive finding for a dissimilar metal weld. The weld metal microstructure consists of white strip-shaped CuZn₄ compounds uniformly distributed within a (α+η) eutectic matrix composed of Zn-based and Al-based solid solutions. This microstructure indicates that the filler wire composition significantly influences the weld metal composition, with the zinc content from the 8515 wire creating a ternary Al-Zn-Cu system in the weld.

The copper side of the interface is described as flat and clear, with a relatively well-defined boundary between the weld metal and the copper base metal. Near this interface, a higher density of intermetallic compound phases is observed, indicating that interfacial reactions occurred during welding and subsequent cooling. The aluminum side interface shows a chain-like distribution of gray-black spherical α-phase aluminum-based solid solution particles along the interface, suggesting a different reaction mechanism on the aluminum side compared to the copper side.

The asymmetric interface morphology is notable. The copper side shows a flat interface with concentrated IMCs, while the aluminum side shows dispersed spherical particles in a chain-like arrangement. This asymmetry likely results from the different diffusion rates of copper and aluminum into the liquid filler metal, as well as the different solidification behaviors at each interface.

Mechanical Properties and Fracture Analysis

Under the optimized welding parameters of 100 A current, 62 mm/min travel speed, and 15 L/min shielding gas flow rate, the joint achieved a tensile strength of 240 MPa. The fracture occurred in the heat-affected zone on the copper side, which is an important finding for engineering design. Fracture in the copper HAZ rather than at the interface suggests that the interface itself has adequate bonding strength, but the copper HAZ has been weakened by thermal effects.

The 240 MPa tensile strength should be evaluated in context. LY16 aluminum alloy has a typical tensile strength of approximately 350-400 MPa, while T2 pure copper has a tensile strength of approximately 200-250 MPa. The joint strength of 240 MPa is close to the strength of the pure copper base metal, indicating that the joint strength is governed by the weaker copper side. This is a reasonable outcome for a dissimilar joint where one base metal is inherently weaker than the other.

The fracture in the copper HAZ rather than at the interface is significant. It suggests that the interfacial bonding is stronger than the HAZ material, which is a desirable characteristic. However, it also means that the copper HAZ is the critical region for joint integrity, and any further weakening of this region due to corrosion, fatigue, or thermal cycling could lead to joint failure.

Welding Parameter Optimization and Process Considerations

The selected welding parameters represent an optimization balance between penetration, fusion, and heat input. At 100 A and 62 mm/min, the heat input is moderate, which is appropriate for the relatively thin plates typically used in such applications. The AC/DC capability of the Lincoln TIG-355 machine allows for AC welding on the aluminum side (for oxide removal) and DC welding on the copper side (for deeper penetration), which is advantageous for dissimilar metal welding.

The use of 8515 zinc-aluminum core flux wire is an interesting choice. The zinc content in the filler creates a ternary Al-Zn-Cu system in the weld, which may help to control the formation of brittle Al-Cu intermetallic compounds by introducing zinc as a third element that modifies the phase diagram. The CuZn₄ compound observed in the weld metal is less brittle than some Al-Cu intermetallics such as CuAl₂, which may contribute to the acceptable joint ductility.

For engineering applications, the following considerations are important. First, the joint design should account for the asymmetric mechanical properties, with the copper HAZ being the critical region. Second, corrosion protection should be considered, as the dissimilar metal joint is susceptible to galvanic corrosion in aggressive environments. Third, fatigue performance should be evaluated, as the interface and HAZ regions may be susceptible to fatigue crack initiation under cyclic loading.

Study Insights and Reflections

This paper provides valuable data on the microstructural and mechanical characteristics of Al/Cu dissimilar joints produced by TIG welding with a specific filler wire composition. The finding that the joint is free of porosity and cracking defects is encouraging, as these are common defects in dissimilar metal welding. The fracture in the copper HAZ rather than at the interface is also a positive finding, indicating that the interfacial bonding is adequate.

The use of zinc-containing filler wire to modify the weld metal composition and reduce brittle intermetallic formation is an innovative approach. The introduction of zinc into the Al-Cu system creates a more complex phase diagram with potentially less brittle compounds, which is a useful strategy for dissimilar metal joining. Future work should investigate the long-term stability of these modified phases under thermal cycling and corrosion conditions.

One limitation of this study is the absence of fatigue and corrosion testing. For engineering applications, particularly in aerospace and marine environments, the fatigue and corrosion resistance of the joint are critical. The CuZn₄ compound in the weld metal may be susceptible to dezincification corrosion in certain environments, which could lead to premature joint failure. Additionally, the fatigue performance of the copper HAZ, which is the fracture-initiating region, should be evaluated to determine the joint's suitability for cyclic loading applications.

Overall, this research demonstrates that Al/Cu dissimilar joints can be produced with acceptable quality using conventional TIG welding equipment and appropriate filler wire selection. The key to success lies in careful parameter optimization and filler wire selection to control the weld metal composition and minimize brittle intermetallic formation.