Abstract: This paper established a model about the flow process of softened material around rotational tool during friction stir welding.According to the flow pattern of different parts of softened material,the flow process was divided into three parts:flow process near the shoulder,near the pin top and near the pin end.The softened material near the shoulder firstly flew into the cavum behind the pin,and the residual material rotated around the shoulder side slowly,forming the arc crest of arc corrugation on friction-stir weld.The softened material near the pin top flew to retreating side from advancing side in a shear way.The softened material near the pin end flew to the retreating side from the advancing side in an extrusion way.