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

Abstract

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°, <5° and between 15 and 45° were considered as completely recrystallized, under-recrystallized and moderately recrystallized, respectively. The analysis quantitatively showed that higher fractions of completely recrystallized and under-recrystallized grains coexisted in the middle locations where the ultrasonic effect on grain refinement was maximum. A normalized factor revealed that trends of ultrasonic effect on complete recrystallization and grain refinement of nugget are similar, suggesting that ultrasonic induced complete recrystallization is the reason for higher grain refinement. Microtexture was C+B/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.