Microstructure and Properties of Friction Stir Welded 2024 Aluminum Alloy Wire-Fed TIG Weld Joints

Literature Overview

This 2024 study published in Chinese Journal of Mechanical Engineering by Wu Jie, Shen Yifu, and Huang Guoqiang from Nanjing University of Posts and Telecommunications and Nanjing University of Aeronautics and Astronautics investigates the microstructure and mechanical properties of 2024 aluminum alloy wire-fed TIG weld joints after friction stir welding (FSW) treatment. The research, supported by the National Natural Science Foundation of China (52205436), addresses a significant challenge in aluminum alloy welding: the poor weldability of 2024 alloy due to its susceptibility to hot cracking, loss of strength in the weld zone, and poor fatigue resistance.

Technical Background

2024 aluminum alloy (Al-Cu-Mg, with approximately 4.4% Cu and 1.5% Mg) is a precipitation-hardened alloy with excellent strength-to-weight ratio, widely used in aerospace applications. However, welding 2024 alloy is notoriously difficult because:

• The Cu content promotes hot cracking during welding.
• The weld zone loses the precipitation hardening, resulting in strength reduction of 40–60% compared to the base metal.
• The HAZ experiences overaging of the T6 precipitates, further reducing strength.

Wire-fed TIG welding (also known as pulsed TIG with consumable electrode or TIG-MIG hybrid) offers higher deposition rates and better penetration than conventional TIG, but the fundamental metallurgical challenges remain. The application of FSW to TIG weld joints is an innovative approach to restore the microstructure and mechanical properties of the weld zone.

Process Parameters

Microstructural Analysis

TIG Weld Joint

The TIG weld joint of 2024 alloy exhibits the following microstructural features:

• Weld Zone: Equiaxed grains with a size of 20–40 μm, containing coarse CuAl₂ and Al₂Cu precipitates that formed during solidification. No β'' (Al₃Cu) or S-phase (Al₂CuMg) precipitates are present, which are responsible for the T6 strengthening.
• HAZ: The original T6 precipitates dissolve and partially re-precipitate as coarse equilibrium phases. The grain size increases from 15 μm (base metal) to 45 μm.
• Base Metal: Fine equiaxed grains (15 μm) with a full complement of β'', S, and θ' precipitates.

Post-FSW Microstructure

After FSW treatment of the TIG weld joint, the microstructure transforms significantly:

• Stir Zone: Ultrafine equiaxed grains (2–5 μm) with a high density of fine precipitates. The dynamic recrystallization during FSW refines the grain structure and re-distributes the precipitates.
• Thermo-Mechanically Affected Zone (TMAZ): Elongated grains with partially dissolved precipitates. The grain size is intermediate (10–20 μm).
• Heat-Affected Zone (HAZ): Similar to the original TIG HAZ, with some coarsening of precipitates.

The key finding is that the FSW stir zone exhibits a microstructure that is not only finer than the base metal but also contains a higher density of fine precipitates due to the dynamic recrystallization and re-precipitation during the thermal cycle of FSW.

Mechanical Properties

The FSW treatment restores the tensile strength to 89% of the base metal value (420 MPa vs. 470 MPa), compared to only 59% for the as-welded TIG joint (280 MPa). The hardness in the stir zone reaches 115 HV, compared to 75 HV in the as-welded state.

Hardness Profile

The hardness profile across the joint shows:

The FSW treatment eliminates the hardness valley in the weld zone, creating a uniform hardness profile across the joint.

Engineering Practice Implications

For aerospace applications where 2024 alloy is widely used, the FSW treatment of TIG weld joints offers significant benefits:

• Fatigue resistance: The ultrafine grain structure in the FSW stir zone improves fatigue resistance by providing more grain boundaries to arrest crack propagation.
• Corrosion resistance: The uniform precipitate distribution reduces susceptibility to intergranular corrosion and stress corrosion cracking.
• Weld repair: The approach provides a viable method for repairing defective TIG welds in 2024 alloy components, extending the service life of aerospace structures.

However, the approach has limitations:

• The FSW tool must be compatible with the joint geometry, which can be challenging for complex structures.
• The process is relatively slow (50–100 mm/min) compared to TIG welding (300–400 mm/min).
• The residual stresses introduced by FSW must be carefully managed to avoid distortion.

Key Questions and Reflections

The study raises an important question about the long-term stability of the fine precipitates in the FSW stir zone. The fine precipitates formed during FSW may coarsen over time at service temperatures, particularly for aerospace applications where temperatures can reach 150–200°C. The authors did not conduct aging studies, which is a gap that should be addressed.

Another reflection concerns the scalability of the approach. While FSW treatment of individual TIG weld joints is demonstrated, the application to large structures with multiple welds requires careful planning of the FSW sequence to avoid interaction effects between adjacent treated zones.

Study Insights and Conclusions

This study demonstrates that FSW treatment of 2024 aluminum alloy TIG weld joints is an effective method for restoring mechanical properties to near-base-metal levels. The ultrafine grain structure and fine precipitate distribution in the FSW stir zone provide a significant improvement in strength and hardness, making this approach viable for aerospace applications where weld strength is critical. For cladding and overlay applications involving 2024 alloy, the FSW treatment can be applied to overlay welds to improve the bond strength and fatigue resistance of the cladding layer. The research opens new possibilities for the repair and rehabilitation of aluminum alloy structures, providing a valuable tool for extending the service life of aerospace components.