Periodical plastic flow pattern in friction stir processed Al–Mg alloy

Onion rings are a prominent feature of the stir zone in many friction stir welded/processed samples. In this paper, a set of equations is proposed to describe the horizontal onion ring patterns that are believed to result from a geometric effect. The calculation indicates that the spacing of the adjacent rings is equal to the advance per revolution. The origin of the onion ring patterns in a longitudinal section consisting of wavy flow patterns is discussed.