Texture and strain-induced abnormal grain growth in cryogenic friction stir processing of severely deformed aluminum alloy

Abstract

Abnormal grain growth (AGG) phenomenon was observed during cryogenic friction stir processing (FSP) of a severely deformed 1050-aluminum. An ultrahigh cooling rate and a lower peak temperature ahead of the rotating tool along with the preliminarily stored strain within the base metal resulted in the AGG. This event was pronounced not only ahead of the FSP tool, but also in heat affected zone (HAZ). The abnormally grown grains were strain-free and they proceeded to grow beyond the high-angle grain boundaries evolved in the base metal. Also, the incident of AGG was simultaneous with the development of Cube and Goss texture components. The combined effects associated with the development of a dominant shear texture and the existence of strain-free grains in the base metal was found to be linked to the AGG. The base metal encompassed mainly deformed grains with the orientation close to the shear texture component B/B¯. However, other orientations such as θ-fiber texture were evident too. The observation of abnormally grown grains with orientations different from the dominant shear texture suggested the activation of texture-induced AGG. On the other hand, strain-induced grain boundary migration would be responsible for the AGG of a number of strain-free grains with orientations close to the dominant shear texture.