Clarification of material flow and defect formation during friction stir welding

The defect formation mechanism during friction stir welding (FSW) was investigated by the three-dimensional visualisation of material flow around a tool. The three-dimensional flow patterns under various FSW conditions were obtained using two pairs of X-ray transmission real time imaging systems. The flow patterns revealed that the tilt of horizontal material flow around the tool and the stagnation of material flow on the advancing side (AS) of the tool were significantly correlated to the formation of defects. The material flow velocity during FSW was directly calculated based on the results of three-dimensional visualisation. The material flow velocity on the AS obviously decreased under the condition where defects were formed.