Abstract: Friction stir welding was conducted by using four types of pin and copper foils, piled up with aluminum plate alternately, as a tracer material.The distribution feature of the tracer materials was observed after welding.The results showed that, the flow of the plasticized metal in the weld is influenced by the pin shopes, which results in the variety of the morphology of the weld.If the screw thread on the pin is counter-clockwise, the metal around the pin will move downwards, which drives the metal around the pin tip to move around and upwards.So the center of the nugget is located at the lower part of the tansverse weld section.If the screw thread on the pin is clockwise, the metal around the pin will move upwards to the root of the pin and moves outwards along the shoulder surface, which causes that the center of the nugget will located at the upper part of the weld section.By using the pin with cylindrical shape, the transverse weld section appears onion ring pattern;but the onion ring pattern is not clear in the weld transverse section for the pin with taper shape or cam shape.If the length of the pin and shape at the tip are changed, the flow state of the plasticized metal below the pin and around the tip of the pin will be changed, therefore, the morphology of the weld at the bottom will change and the depth that the aluminum layer on the back surface of the plate enteed into the weld will be larger.