Visualisation of material flow in autogenous friction stir welds

Abstract

Material flow in friction stir welds has been visualised using embedded marker materials. The fidelity of the visualisation technique has also been demonstrated. Results from the flow visualisation show that the friction stir welding process can be roughly described as an in situ extrusion process wherein the tool shoulder, the weld backing plate, and the cold base metal outside the weld zone form an ‘extrusion chamber’ which moves relative to the workpiece. Deviation from this description occurs primarily at the top surface of the weld where significant material transport occurs owing to the action of the rotating tool shoulder. The transport caused by the shoulder leads to a small amount of circulation about the longitudinal axis of the weld with material moving primarily from trailing to leading sides at the top of the weld and primarily from leading to trailing near the middle and bottom of the weld.