Orientation microscopy study of cryogenic-temperature friction stir processed AA6xxx sheet: microstructure and second phase particles evolution

Cryogenic-temperature friction stir processing (CT-FSP) was performed on AA6xxx sheets at 600 RPM and 500 mm/min using in-process liquid N₂ cooling to investigate microstructure, precipitate evolution, and the resulting mechanical response. Multiscale characterization EBSD, SEM/EDS and microhardness tests were conducted. CT-FSP produced a highly refined predominantly equiaxed stir zone (SZ) with average grain sizes of approximately 1.6–1.8 µm at the surface and mid-thickness, and slightly coarser grains (~ 4.4 µm) near the bottom and within the thermomechanically affected zone (TMAZ). Dynamic recrystallization, evidenced by a high fraction of high-angle grain boundaries and low KAM, dominates the SZ. The recrystallization fraction varies laterally and through the thickness due to heterogeneous strain and thermal fields. Second phase particles experience mechanical fragmentation, partial solvus dissolution, and rapid reprecipitation during CT-FSP, yielding a finer and more uniformly dispersed distribution in the upper SZ, while coarser Al–Fe–Si and Mg₂Si particles persist near the bottom/TMAZ. Deep-learning-based image segmentation reveals that the mean equivalent-circle diameters of Al–Fe–Si and Mg₂Si phases decrease from ~ 0.97 µm and 0.80 µm in the as-received sheet to ~ 0.6–0.8 µm locally after CT-FSP, accompanied by reduced nearest-neighbor spacing in the SZ. This localized particles refinement and redistribution, together with extensive DRX, accounts for the combined SZ softening and TMAZ hardening, establishing a clear microstructure-property linkage in cryogenic FSPed AA6xxx sheet.