Development of Refined Surface Layers and Correlated Microstructure–Damping Behavior in Friction Stir Processed AA2024 Alloy

The present work investigates the microstructural development and damping behavior of AA2024 alloy processed via friction stir processing (FSP), emphasizing the correlation between grain refinement, precipitation phenomena, nanoscale structural features, and energy-dissipation mechanisms. Optical microscopy showed that the original coarse, elongated grains of the base metal were transformed into fine, equiaxed grains within the stir regions as a result of complete dynamic recrystallization (DRX). Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS) results showed fragmentation, dissolution, and fine reprecipitation of Cu-, Mg-, Fe-, and Mn-rich intermetallics, signifying extensive redistribution of alloying constituents. Transmission Electron Microscope (TEM) findings confirmed the refinement of nanoscale precipitates and a notable rearrangement of dislocations, while Selected Area Diffraction (SAD) patterns exhibited diffuse and streaked reflections characteristic of lattice distortion, subgrain structures, and ultrafine crystallites. Dynamic mechanical analysis revealed a substantial improvement in the damping capacity of AA2024 after FSP, extending across the entire frequency spectrum. This enhancement is owing to enhanced grain-boundary sliding, intensified dislocation–precipitate interactions, and microstrain accommodation resulting from nanoscale refinement. By combining OM, SEM-EDS, TEM-SAD, and damping results, this study develops a comprehensive structure–property framework that elucidates the synergistic mechanisms responsible for the enhanced damping behavior of FSP-processed AA2024 alloy. Overall, the results confirm FSP as an efficient surface modification technique for tailoring and enhancing the functional damping performance of high-strength aluminum alloys.