Abstract: The plastic flow behavior of the weld metal during friction stir welding was analyzed by using the multi-plate piled up with several thin copper foils and aluminum sheets and the copper foils serves as a marker material.The results show that there is a serious material translation in the thickness of the weld along the screw thread if using a pin with screw threaded surface.These flowing materials will slough off from the thread at the tip or root of the pin and squeeze into the surrounding area, forming a solid ring with the center being the circle of tip or root of the pin.There is a distinct boundary between the solid ring and its surrounding parent metal. The onion ring is actually the projecting picture of the solid ring cut at the cross section of the weld.The driving force of the metal flow in the thickness of the weld is the pressure of the skew threads on the pin surface to its surrounding plastic metal.When the stir tool rotates clockwisely, the center of the onion rings deflect to the bottom of the weld in the case of the pin with left-hand skew thread on its surface, or deflect to the up surface of the weld in the case of the pin with right-hand skew thread on its surface.