On the formation of onion rings in friction stir welds

Onion rings are the most prominent features of most friction stir welds. The origin and the effect of these on properties are not clearly understood. In this paper, an attempt has been made to explain the formation of onion rings. The formation of onion ring is found to be a geometric effect due to the fact that cylindrical sheets of material are extruded during each rotation of the tool and the cutting through the section of the material...