End forming behaviour of friction stir processed Al 6063-T6 tubes at different tool rotational speeds

Abstract

The main aim of this work is to study the end forming behaviour of friction stir processed Al 6063-T6 tubes fabricated at different tool rotational speeds. Tube expansion, tube reduction and tube beading are performed on the friction stir processed and parent tubes. The load, thinning and hardness evolution during the end forming operations are evaluated. A rotational speed yielding moderate combination of initial hardness and hardening index of the friction stir processed zone governs the maximum load required during end forming. Tube reduction and tube beading need larger load because of tube thickening, as compared to tube expansion, in which tube thinning is witnessed. The friction stir processed zone of the processed tube undergoes larger thinning and thickening as compared to base material in tube expansion and reduction, respectively, while base material thickens more in tube beading. The hardness and strain hardening exponent difference govern such thickness evolution. Mechanical instabilities like failure of friction stir processed zone, wrinkling and overlapping of friction stir processed zone, and fine cracks development are seen during the end forming operations. The effect of rotational speed on the hardness and grain size distribution of friction stir processed zone is negligible. The hardness of processed zone is found to increase after end forming operations.