Effect of Friction Stir Processing on Mechanical Properties and Heat Transfer of TIG Welded Joint of AA6061 and AA7075

Literature Overview

This research by Husain Mehdi and R.S. Mishra from the Department of Mechanical Engineering, Delhi Technological University, published in Defence Technology in 2021 (Volume 17, Issue 3, pages 715-727), investigates the combined TIG welding and friction stir processing (FSP) approach for joining dissimilar aluminum alloys AA6061 and AA7075. The study addresses the well-known challenges of aluminum alloy welding—porosity, coarse grain structure, micro-cracking, and residual stress—by applying post-weld FSP as a mechanical working and heat treatment step. The work includes both experimental characterization and finite element simulation using ANSYS Fluent.

Core Technical Findings

The study reports the following principal results:

• Maximum compressive residual stress of 73 MPa was achieved at the fusion zone of the TIG weldment with ER4043 filler.
• Minimum compressive residual stress of 37 MPa was obtained at the stir zone of the TIG+FSP joint with ER5356 filler.
• Maximum heat flux of 5.33 × 10^6 W/m² and temperature of 515°C were observed at tool rotation of 1600 rpm with feed rate of 63 mm/min.
• FSP effectively reduces porosity, refines grain structure, and improves mechanical properties compared to as-welded TIG joints.

Process Parameters and Heat Transfer Analysis

Mechanical Properties Comparison

Microstructural Evolution Through FSP

The FSP process fundamentally alters the weld microstructure through severe plastic deformation (SPD):

1. Grain refinement: The intense shear deformation during FSP breaks down the coarse equiaxed grains formed during TIG solidification into fine, equiaxed grains through dynamic recrystallization.
2. Porosity elimination: The plastic flow of material during FSP closes internal porosity by forcing material into void spaces.
3. Stress relief: The plastic deformation during FSP relaxes residual stresses, particularly converting tensile residual stresses to compressive states.
4. Precipitate modification: In AA7075 (containing Mg, Zn, and Cu), the FSP thermal cycle can partially dissolve and redistribute strengthening precipitates, affecting the final mechanical properties.

Engineering Practice Implications

The TIG+FSP combined approach has significant implications for aluminum alloy welding in aerospace and automotive applications:

• Dissimilar joint quality: The combination addresses the fundamental challenge of joining AA6061 (6xxx series) and AA7075 (7xxx series), which have different thermal expansion coefficients and solidification behaviors.
• Residual stress management: The compressive residual stress achieved through FSP is beneficial for fatigue life, as compressive stresses inhibit crack initiation and propagation.
• Porosity control: For applications requiring leak-tight joints—such as aerospace fuel tanks or hydraulic systems—the porosity elimination capability of FSP is critical.
• Post-weld processing: FSP serves as an in-situ heat treatment and mechanical working step, potentially replacing or reducing the need for conventional post-weld stress relief treatments.

Finite Element Simulation Validation

The ANSYS Fluent simulation provides quantitative heat transfer modeling of the FSP process. The agreement between simulated and experimental results gives confidence in using the simulation for parameter optimization before actual processing. Key simulation parameters include:

• Friction coefficient between tool shoulder and workpiece
• Thermal conductivity of aluminum alloys (temperature-dependent)
• Tool geometry and surface roughness effects
• Material flow pattern during plastic deformation

Key Questions and Reflections

A critical question for industrial adoption is the scalability of FSP. The process is inherently slow (63 mm/min in this study) and requires high axial force, making it suitable for relatively thin sections and limited geometries. For thick-section aluminum structures or large production volumes, alternative post-weld treatments such as cryogenic treatment or low-temperature stress relief may be more practical.

Another consideration is the effect of FSP on the thermomechanical history of the base metal. The FSP process heats the surrounding material to temperatures approaching 500°C, which could affect the precipitate distribution in the AA7075 base metal if the processing parameters are not carefully controlled. Engineers must ensure that the FSP thermal cycle does not cause over-aging or precipitate coarsening in the adjacent base metal.

Study Insights and Implications

This research demonstrates that combining TIG welding with post-weld friction stir processing is an effective strategy for producing high-quality dissimilar aluminum alloy joints with improved mechanical properties, reduced porosity, and beneficial compressive residual stress states. The finite element modeling capability provides a valuable tool for process parameter optimization. For engineers working with aluminum alloy piping, aerospace structures, or automotive components, the TIG+FSP approach represents a viable path to overcoming the inherent limitations of conventional aluminum welding. The study reinforces the principle that post-weld mechanical working can fundamentally alter the metallurgical quality of a weld joint, transforming it from a potentially weak link into a reliable structural connection. The integration of experimental characterization with computational modeling in this work exemplifies the modern approach to welding process development, where simulation-guided optimization reduces trial-and-error and accelerates the path to production-ready processes.