Multi-layered metal flow and formation of onion rings in friction stir welds

The friction stir welding (FSW) is achieved with an external tool consisting of a shoulder and pin. The shoulder and pin transfer the metal from the front side to the rear side in two distinct modes; the first and second modes of metal transfer, respectively. In the present study, the pin in the form of a cylindrical tool is used to analyse the metal flow during FSW in the second mode of metal transfer. Movement of the cylindrical tool transfers the metal from the front side to the rear side layer by layer. The longitudinal and transverse forces during the metal transfer are measured and the layered metal flow phenomenon has been proposed, due to stick and slip conditions. Based on the results obtained, the two modes of metal transfer as well as the formation of onion rings in friction stir welds have been explained. The present work can be used to model the process for improving tool and fixture design.

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