Precursor ultrasonic effect on grain structure development of AA6061-T6 friction stir weld

Highlights: • A novel misorientation analysis reveals effect of leading ultrasonic vibrations on horizontal weld nugget microstructure. • Ultrasonic vibrations increase the completely recrystallized and under-recrystallized grains in horizontal nugget center. • Nugget locations with higher grain refinement are accompanied with more ultrasonic induced complete recrystallization. • Anisotropy and B-texture are intensified with ultrasonic vibrations, signaling enhanced deformation and material flow. Specific locations in horizontal cross-sections of AA6061-T6 weld nuggets, prepared with and without precursor ultrasonic vibrations in friction stir welding, were orientation imaged using electron back scattered diffraction and compared. With ultrasonic vibrations, grains exhibited higher orientation propensity along (101) and (111) and higher grain refinement at middle of weld nugget. Misorientation analysis was conducted using a novel approach where grains with misorientation > 45°, $C+B/\stackrel{-}{B}$ mixture with the C component weakening and B component roughly strengthening down the depth. Microtexture variations are more intensified in the middle locations with ultrasonic vibrations, causing increase in anisotropy, contrary to reported anisotropy loss in transverse locations. Scope of the observed variations in anisotropy and recrystallization towards improvement of process effectiveness and weld mechanical properties are discussed.