Development of refined surface layers and correlated microstructure–damping behavior in friction stir processed AA2024 alloy

The present study investigates the microstructural evolution and damping behavior of AA2024 aluminum alloy subjected to friction stir processing (FSP), with emphasis on establishing a quantitative structure-property relationship across multiple length scales. Optical microscopy and ImageJ analysis showed a considerable reduction in average grain size, from 58 ± 11 µm in the base alloy to 5.4 ± 1.3 µm in the stir zone, indicating extensive refinement and homogeneity. Scanning electron microscopy revealed fragmentation and redistribution of coarse second-phase particles. The average particle size decreased from 2.8 ± 0.9 to 0.9 ± 0.3 µm post-processing. Transmission electron microscopy and selected-area diffraction investigations revealed nanoscale particle refinement (from ~ 62 ± 15 to ~ 22 ± 7 nm), increased defect density, and lattice distortion in the treatment region. Dynamic mechanical investigation revealed a consistent increase in damping capacity of the FSPed alloy across the entire frequency range. The improved damping response is due to a combination of increased grain-boundary density, refined particle-matrix interfaces, and defect-mediated microplastic accommodation caused by friction stir processing. Overall, the findings support FSP as an efficient surface modification technique for altering the microstructure and enhancing the functional damping performance of AA2024 aluminum alloy.