Sucking-extruding Theory for the Material Flow in Friction Stir Welds

By using column pin with thread or without thread on the pin surface, friction stir welding is conducted on the lamella structure plates piled up with several thin copper foils, acting as a marker material, and aluminum sheets.The information about the flow field on the weld cross section is collected and analyzed.It is shown that there is a serious material translation in the thickness of the weld, if using a pin with screw thread surface.The thread on the pin is the main cause to drive the weld metal flow in the thickness of the weld.On the basis of the experimental results, a sucking-extruding theory is suggested to explain the metal flow in the thickness of the weld.That is, when the plastic material around the pin flows along the axis direction of the pin with the rotation of the stir tool, a transient cavity is formed at one end of the threaded pin and the plastic metal around the end will be pulled to the cavity, producing a sucking effect.At another end of the pin, the plastic metal will change its flowing direction and extrude the plastic metal around this end, producing an extruding effect.It is due to these effects, i.e.the sucking effect of the cavity on the plastic metal around it and the extruding effect of the flowing metal on the pin on the surrounding metal, that drive the thermal plastic metal translate circularly in the thickness direction.By using the sucking-extruding theory, the onion ring formation is explained and the center location of the onion ring, the location of the channel defects in the weld can be predicted and experimentally confirmed.The effect of the welding parameters on the size of the onion ring can be predicted by applying this theory and is confirmed by this and other author's experiments.