TIG Arc Assisted Friction Stir Welding of Dissimilar Aluminum Alloys AA6016 and AA2519

Literature Overview

The 2025 paper by Tie Yi, Sheng-dan Liu, Chen Fang, Geng-duo Jiang, and Xuan-xuan Dai, published in the Transactions of Nonferrous Metals Society of China (Volume 35, Issue 2, pages 405–417), investigates the effects of TIG arc-assisted friction stir welding (TIG-FSW) on the microstructure, tensile properties, and corrosion resistance of AA6016 and AA2519 dissimilar aluminum alloy lap joints. The research was conducted at Central South University and South China University of Technology, leveraging facilities at the National Key Laboratory of Science and Technology on High-strength Structural Materials. The study addresses a critical challenge in aluminum alloy joining: the formation of intermetallic compounds and microstructural defects at the interface between dissimilar alloys.

Background and Motivation

Friction stir welding (FSW) is a solid-state joining process that has gained widespread adoption in aluminum alloy fabrication due to its ability to produce high-quality joints without melting. However, conventional FSW of dissimilar aluminum alloys often results in tunnel hole defects, uneven material flow, and the formation of brittle intermetallic compounds at the interface. The AA6016 and AA2519 combination is particularly challenging because AA6016 is a 6xxx series alloy with Mg2Si as the primary strengthening phase, while AA2519 is a 2xxx series alloy strengthened by CuAl2 precipitates. The significant difference in composition and mechanical properties leads to asymmetric material flow and potential defects.

Process Description and Experimental Setup

The TIG-FSW hybrid process combines the mechanical deformation of FSW with the thermal input of a TIG arc. The TIG arc is applied to the workpiece during the FSW process, providing additional heat that promotes material flow and diffusion without melting. The experimental setup involved a TIG torch positioned near the friction stir tool, with the arc directed at the workpiece surface ahead of the tool. The following parameters were studied:

Microstructural Analysis

The introduction of the TIG arc during FSW resulted in a more uniform microstructure throughout the joint. The key finding was the complete elimination of tunnel hole defects, which are a common failure mode in dissimilar aluminum FSW joints. The TIG arc promotes better material flow, contact, and diffusion between the two alloys, leading to a more homogeneous joint structure.

The secondary phase particles, which are responsible for precipitation hardening in both alloys, were also affected by the TIG arc. The size and number of secondary phase particles were reduced, which has implications for both mechanical properties and corrosion resistance. In AA6016, the Mg2Si particles were refined, while in AA2519, the CuAl2 particles showed similar refinement. The reduction in particle size and number density suggests that the additional heat input promoted dissolution and re-precipitation of these phases during the welding and cooling process.

Mechanical Properties

The tensile properties showed significant improvement with the TIG arc assistance. The tensile strength increased by 11.5% and the elongation increased by 50.0% compared to conventional FSW joints. These improvements can be attributed to several factors:

• The elimination of tunnel hole defects, which act as stress concentrators and crack initiation sites.
• The more uniform microstructure, which provides more consistent load transfer across the joint.
• The refinement of secondary phase particles, which enhances precipitation hardening without sacrificing ductility.
• The improved bonding at the interface, which reduces the likelihood of interfacial failure.

Corrosion Resistance

The corrosion performance was evaluated through immersion tests and electrochemical measurements. The TIG-FSW joints showed a 78.2% reduction in corrosion current density and a 45.7% reduction in the largest corrosion depth compared to conventional FSW joints. These improvements are significant for applications in corrosive environments, such as marine, automotive, and aerospace structures.

The enhanced corrosion resistance can be attributed to the more uniform microstructure and the reduction in secondary phase particles. In aluminum alloys, corrosion often initiates at the interface between the matrix and secondary phase particles due to galvanic coupling. By reducing the size and number of these particles, the TIG arc minimizes the potential for localized corrosion. Additionally, the elimination of tunnel hole defects removes a potential pathway for corrosive media to penetrate the joint interior.

Engineering Practice Implications

For manufacturers of aluminum alloy structures, the TIG-FSW hybrid process offers a compelling solution to the challenges of dissimilar alloy joining. The lap joint configuration is common in structural applications where overlapping plates are required, such as in automotive body panels, aerospace skin structures, and marine hulls. The improved mechanical properties and corrosion resistance of TIG-FSW joints make them suitable for demanding applications where joint reliability is critical.

However, the implementation of TIG-FSW requires additional equipment and process control compared to conventional FSW. The integration of a TIG torch with the FSW machine requires careful alignment and synchronization. The additional heat input from the TIG arc must be carefully controlled to avoid excessive thermal distortion or the formation of unwanted phases. Process development for TIG-FSW should include thorough parameter optimization for each specific alloy combination and joint configuration.

Key Questions and Reflections

The study raises several important questions for further investigation. First, the long-term mechanical and corrosion properties of TIG-FSW joints under cyclic loading and elevated temperature conditions remain to be established. Second, the scalability of the process to thicker sections and larger joints requires demonstration. Third, the economic viability of TIG-FSW compared to conventional FSW or other joining methods needs to be evaluated in terms of equipment costs, cycle time, and consumable costs.

The reduction in secondary phase particles is a double-edged sword. While it improves corrosion resistance, it may also reduce the precipitation hardening response during aging treatment. For applications requiring post-weld aging, the interaction between the TIG arc and the subsequent heat treatment needs to be carefully considered.

Study Insights and Implications

This study demonstrates the power of hybrid welding processes in overcoming the limitations of individual processes. By combining the solid-state benefits of FSW with the thermal activation of TIG arc, the authors have achieved a synergistic effect that neither process could achieve alone. The elimination of tunnel hole defects is particularly significant, as these defects are a primary cause of joint failure in dissimilar aluminum FSW. The improvements in both mechanical properties and corrosion resistance suggest that TIG-FSW has the potential to expand the range of applications for FSW in dissimilar alloy joining.

For the engineering community, this research underscores the importance of understanding the fundamental mechanisms that govern joint quality. The microstructural uniformity, particle refinement, and defect elimination are not independent phenomena but are interconnected through the material flow and thermal history of the joint. Process development that addresses these fundamental mechanisms, rather than simply optimizing surface parameters, is more likely to yield robust and reliable results. The TIG-FSW approach exemplifies this philosophy, demonstrating that a deep understanding of welding metallurgy can lead to significant improvements in joint quality and performance.