Abstract: The two dimensional flow behavior of the plasticized materials in friction stir welded joints of aluminum LF6 was investigated with the copper sheet as an insert tracer material.The distribution of the copper on the plane parallel to the surface was observed by optical microscopy.The results indicate that the material flow was asymmetrical on the plane parallel to surface during friction stir welding.On the advancing side,the material in the parent metal flowed along the welding direction and the material in the weld flowed opposite to the welding direction;on the retreating side,the material only flowed opposite to the welding direction and some flowed into advancing side behind the pin.To analyze the material flow pattern on the plane parallel to the surface,a tentative two dimensional extrusion mode was presented based on the assumption that there was a thermal plasticized zone close to the pin.During welding the shape of the zone was steady if the center of the pin was taken as the original point of the coordinate.The effect of the pin's rotational and linear movement on the plasticized material within the thermal plasticized zone was analyzed separately and the flow pattern could be given according to these movements,which could explain the distribution of the tracer material observed.