Local microstructure evolution in Al 6061-T6 friction stir weld nugget enhanced by ultrasonic vibration

Abstract

Ultrasonic vibration enhanced friction stir welding (UVeFSW) experiments were conducted to examine the effect and role of ultrasonic assistance on weld by comparing local microstructure evolution and microtexture of Al 6061-T6 weld nuggets in UVeFSW with those in conventional friction stir welding. Results showed that ultrasonic enhanced joints have higher deformation characteristics in the nugget zone due to ultrasonic softening effect prior to welding. Electron back scattered diffraction studies showed that application of ultrasonic vibration improves recrystallization process, causes better grain refinement and brings about variations in grain orientation along weld center axis. At the weld nugget extremities, ultrasonic energy contributes only towards changes in grain orientation. Ultrasonic assistance, however, did not induce any significant change to the local microtexture; rather the feature observed is a small angular shift in the shear plane which tilted the texture by an equivalent angle. The resulted microtextures are C {001}〈110〉 or B/B- {112}〈110〉 components of simple shear texture along the weld center axis and a mixture of shear fiber components at the weld nugget extremities. It is found that ultrasonic energy has maximum effect at the center of weld center axis and the effect is reduced towards the weld extremities.