Effect of Deformation Resistance of Steel on Material Flow in Friction Stir Welding – In-situ Observation by X-ray Radiography –

Abstract

Material flow of steels with different composition in friction stir welding (FSW) was directly investigated by x-ray radiography. The three-dimensional flow patterns in various FSW conditions were obtained using two pairs of x-ray transmission real-time imaging systems. It revealed that the material flow pattern and the flow rate were tightly connected to the deformation resistance of the steel at process temperature. The material flow zone of the steel with higher carbon content, which has higher deformation resistance, was narrower, and its flow rate was lower than those of the steel with lower carbon content. The shape of the material flow zone around the rotating tool was changed from concentric to ellipse due to the stagnation of the material flow on advancing side (AS) in stir zone (SZ) under the insufficient heat-input FSW condition. Further decrement of the heat-input in FSW led to the defect on AS in SZ. The obtained relationship between the FSW conditions and the flow pattern showed that the lower travel speed of the tool is effective to keep the concentric shape so as not to form the defect for the steel with higher carbon content.