On the Origin of Onion-ring Structure in Friction Stir Welding

Onion-ring structure is a prominent feature in many of the friction stir welds but the origin of this structure has not been firmly established yet. In this paper, it is shown that the weld has lamellar structure and the weld layers have the same recesses and dimples. Furthermore, the onion-ring structure formation in the weld cross-sections is due to cutting these curved layers similar to the layers in a cut onion.

Keywords:

Friction stir welding , Butt joint , Onion-ring , Copper