Experimental evaluation of strain and strain rate during rapid cooling friction stir welding of pure copper

Abstract

The strain and strain rate during friction stir welding was evaluated by measuring the distortion of the marker material. A thin Cu-40Zn foil as marker was inserted into the butting surface of two pure copper workpieces and the tool ‘stop action’ was employed. The results show that the strain in the shoulder-affected zone increases in a two stair-step shape as the material flows from the front to the rear of the tool, corresponding to the first accelerated and then decelerated flow stages. However, the strain at the two stages has opposite directions. In other words, a strain reversal occurs. Accordingly, the strain rate in the shoulder-affected zone varies in a sinusoidal shape. In the probe-affected zone, there is no obvious strain reverse occurring due to the formation of banded structures. The average strain rate during the band formation is significantly higher than the maximum strain rate in the shoulder-affected zone.