Material Flow During Friction Stir Welds

Abstract

The flow of metal during Friction Stir Welding is clarified using a faying surface tracer and a nib frozen in place during welding. It is shown that material is transported in two distinct streams or currents. One stream is a wiping of material from the advancing front side of the nib onto a plug of material that rotates and advances with the nib. The material undergoes a helical motion within the plug that both rotates and advances with the plug and descends in the wash of the threads on the nib and rises on the outer part of the plug. After one or more rotations, this material is sloughed off the plug in the wake of the tool primarily on the advancing side. The second stream of material is an entrainment of material from the retreating side of the nib that fills in between the sloughed off pieces from the advancing side. These two processes produce material with different mechanical properties and the strength of a weld should depend on the relative importance of the processes.