Zhuojin Pipe Fitting Co., LtdFriction Stir Processing Enhancement of TIG Welded Dissimilar Aluminum Alloy Joints
Literature Overview
Published in Defence Technology in 2021 by Husain Mehdi and R.S. Mishra from Delhi Technological University, this paper addresses a critical challenge in aerospace and automotive engineering: the reliable joining of dissimilar aluminum alloys AA6061 and AA7075. These alloys are widely used in structural applications due to their high strength-to-weight ratios, yet their dissimilar combination creates inherent metallurgical incompatibilities during fusion welding. The authors propose a hybrid approach combining TIG welding with post-weld friction stir processing (FSP) to mitigate defects such as micro-cracking, coarse grain structure, and porosity inherent to conventional TIG welds of dissimilar aluminum alloys.
Technical Analysis of the Hybrid Process
The experimental design involved TIG welding of AA6061 and AA7075 plates using two different filler metals: ER4043 (Al-Si alloy) and ER5356 (Al-Mg alloy). The TIG welds were subsequently subjected to FSP at varying tool rotation speeds and feed rates. Finite element analysis using ANSYS Fluent software was employed to model the heat transfer and residual stress distribution during the FSP operation.
| Parameter | TIG Only (ER4043) | TIG + FSP (ER5356) | TIG + FSP (ER4043) |
|---|---|---|---|
| Max Compressive Residual Stress (MPa) | 73 (FZ) | 37 (SZ) | Between 37-73 |
| Max Heat Flux (W/m²) | N/A | 5.33 × 10⁶ | N/A |
| Peak Temperature (°C) | N/A | 515 | N/A |
| Tool Rotation Speed (rpm) | N/A | 1600 | 1600 |
| Feed Rate (mm/min) | N/A | 63 | 63 |
The residual stress analysis reveals a fundamental metallurgical insight: the maximum compressive residual stress of 73 MPa at the fusion zone of the TIG-only weld with ER4043 filler is beneficial for fatigue resistance but insufficient to counteract the tensile residual stresses that develop at the interface of dissimilar alloys. After FSP treatment, the compressive residual stress distribution becomes more uniform, with the stir zone showing 37 MPa compressive stress. The peak heat flux of 5.33 × 10⁶ W/m² and temperature of 515 °C during FSP are carefully controlled to remain below the melting point of aluminum alloys, ensuring solid-state processing without remelting.
Metallurgical Considerations for Dissimilar Aluminum Welding
The choice of filler metal is critical in dissimilar aluminum welding. ER4043 (Al-5% Si) produces a eutectic weld with lower melting point, reducing hot cracking susceptibility but potentially introducing brittle intermetallic phases at the AA7075 interface. ER5356 (Al-5% Mg) provides better mechanical properties in the weld but carries higher hot cracking risk due to the wider solidification range. The FSP post-processing addresses these concerns by:
- Refining the grain structure in the fusion zone through severe plastic deformation
- Breaking up coarse dendritic structures that form during TIG solidification
- Homogenizing the chemical composition at the dissimilar interface
- Introducing beneficial compressive residual stresses that improve fatigue life
Engineering Practice Integration
For aerospace structural applications where AA6061 and AA7075 components must be joined, this hybrid TIG+FSP approach offers a practical solution that avoids the complexity and cost of friction stir welding (FSW) while achieving comparable metallurgical quality. The FSP process requires only a simple shoulder-pin tool and can be performed on existing TIG welds without disassembly. Engineers should note that the FSP parameters must be optimized for each specific joint configuration, as excessive tool rotation speed or insufficient feed rate can cause material smearing, tool wear, or even local melting. The finite element simulation results provide a validated baseline for process parameter selection, reducing the need for extensive trial-and-error experimentation.
This work demonstrates that post-weld thermomechanical processing can effectively remediate the inherent limitations of fusion welding for dissimilar aluminum alloys. The combination of computational modeling and experimental validation provides a rigorous methodology that can be adapted to other alloy combinations and joint configurations in aerospace and automotive manufacturing. The approach represents a practical bridge between traditional welding practices and advanced solid-state joining technologies.