On the formation of large grain structure after friction stir processing of ultrafine-grained aluminium alloy

A tendency for formation of large grain structure after friction stir processing (FSP) of ultrafine-grained aluminium produced by severe plastic deformation was thoroughly discussed. To do so, microstructural assessments using electron backscattered diffraction (EBSD) analysis at various regions, including ahead of and beneath the FSP tool were carried out. It was found that ultrafine-grained microstructure of the base metal played an important role in dynamic recrystallisation (DRX) occurring in the stir zone. There are some enlarged grains just ahead of the FSP tool prior to being stirred by it. Partial static recrystallisation followed by grain growth as well as strain-induced grain boundary migration was responsible for the formation of such grains. The large grains could postpone grain subdivision process through continuous and geometrical DRX mechanisms. Moreover, the remained part of the stored strain provided driving force for grain growth after grain subdivision in the stir zone.