Investigation of Microstructure and Microtexture Development and Its Correlation with Mechanical and Formability Behavior of Dissimilar Friction Stir Welded AA6061 and AA5754 Sheets

The dissimilar friction stir welding (FSW) of 6061 T4 and 5754 aluminum alloys has drawn attention for its ability to join precipitation-hardenable and strain-hardened materials effectively. The study reveals that microstructure evolution, texture development, and precipitate behavior strongly dictate mechanical strength and formability. Grain refinement in the stirred zone improves hardness, while precipitate dissolution near the thermo-mechanically affected zone (TMAZ) leads to localized softening. Texture gradients across the weld interface influence anisotropy in tensile response, yet post-weld heat treatments can partially restore ductility. Controlled process parameters such as tool design, rotation speed, and thermal input are key to achieving balanced microstructural characteristics across both alloys.

Characteristics of 6061 T4 Aluminum Alloy in Friction Stir Welding

The metallurgical characteristics of 6061 T4 play a decisive role in determining weld quality and joint performance. Its precipitation-hardening nature contrasts with the strain-hardened 5754 alloy, producing distinct thermal responses during FSW.

Metallurgical Features of 6061 T4 Condition

The 6061 T4 alloy contains Mg₂Si as its main strengthening phase formed through precipitation hardening. In this temper condition, the alloy is solution-treated and naturally aged, leaving a supersaturated solid solution that responds sensitively to thermal cycles during welding. Thermal-mechanical processing stabilizes grain structures but can trigger partial dissolution of metastable phases under frictional heat. Compared with T6 or O conditions, T4 offers better weldability since it avoids excessive hardening or softening during joining.

Interaction Between 6061 T4 and AA5754 During FSW

When paired with AA5754, differences in composition—particularly magnesium content—alter heat input distribution and material flow behavior. The 6061 T4 side experiences localized softening due to precipitate dissolution, whereas the strain-hardened 5754 retains higher work-hardening resistance. Thermo-mechanical compatibility between these alloys affects interfacial mixing; fine lamellar structures often form at the boundary due to differential plastic flow. This dissimilar pairing results in microstructural gradients where recrystallized zones transition into elongated grains near the interface.

Microstructure Evolution in Dissimilar AA6061-T4/AA5754 FSW Joints

During FSW, intense plastic deformation combined with frictional heating drives dynamic recrystallization processes that refine grains within the stir zone while altering precipitate morphology.

Grain Refinement Mechanisms Induced by FSW

Dynamic recrystallization dominates microstructural evolution within the nugget zone. The severe shear deformation breaks down original grains into ultrafine equiaxed structures. Tool rotation speed significantly influences grain size: higher speeds enhance heat generation leading to coarser grains, while moderate speeds favor finer distributions. Traverse rate also modifies thermal exposure time, impacting recovery kinetics. Zener pinning from dispersed second-phase particles restricts grain boundary migration, maintaining refined structures even at elevated temperatures.

Phase Transformation and Precipitate Behavior

The high frictional heat causes dissolution of Mg₂Si precipitates in the 6061 T4 region followed by reprecipitation during cooling. Redistribution of Mg and Si occurs through diffusion-assisted stirring, forming heterogeneous precipitate distributions across the weld nugget. Near the TMAZ, incomplete reprecipitation leads to precipitate coarsening that reduces local hardness. These transformations create a complex balance between strengthening from fine dispersoids and weakening from overaged regions.

Texture Development and Crystallographic Orientation Changes

Texture evolution during FSW reflects intense shear deformation paths imposed by tool motion. The resulting crystallographic orientations affect mechanical anisotropy across different weld zones.

Formation of Shear Texture Components in Stirred Zone

Within the stirred zone, strong shear textures such as {111