Influence of grain structure evolution and microtexture development on hardness, residual stresses, and wear behavior of AA6061 alloy processed via cryogenic friction stir processing

This study investigates the impact of cryogenic friction stir processing (FSP) on the microstructure, hardness, residual stresses, and wear behavior of AA6061 alloy. Electron backscattered diffraction analysis revealed a transformation from coarse, elongated grains (∼90.5 µm, 38% high-angle boundaries) in the base material to fine, equiaxed grains (∼3.6 µm, 51% high-angle boundaries) after FSP, driven by dynamic recrystallization (DRX), severe plastic deformation, and rapid cooling, which suppress recovery and grain growth. Texture evolution shifted from a weak S-{123}<634 > component in the base material to strong Goss-{110}(100) and A-{111}(110) components, indicating enhanced grain alignment. Microhardness testing showed reduced surface hardness due to grain refinement and inhibited dislocation recovery, while deeper regions exhibited increased hardness due to recrystallization and the dominant A-{111}(110) texture. Residual stress analysis revealed compressive stresses near the surface and tensile stresses at greater depths, consistent with the hardness gradient. Cryogenic FSP enhanced wear resistance by refining the microstructure, inducing compressive stresses, and reducing both the wear rate and coefficient of friction, particularly under higher wear loads.