## 3 Results and Discussion
The flow patterns at the entry demonstrate the plastic deformation patterns caused at the substrate in interaction with the rotating tool. According to Figure 5, the initial deformation process at the entry zone contains highly plastic deformation in an unstable mode. As the unstable rotating tool passes through the weld-line, the plasticized mass underneath the shoulder escapes outwards the workpiece, whereby forms the spray zone at the entrance position of the tool. Because of the instability of the deformation, the plasticized mass curves from the advancing side (AS) and stretched to the retreating side (RS) of the clockwise rotating tool. The large discontinuity at the AS of the weld-line is the plausible position of the tunnel void as a flow-based defect. Figure 4: Photograph of the BFSW section sample; plasticine analogue. **(a)** A model for the possible position of tunnel void, **(b)** weld cross-section, representative of the flow patterns. The surface weld pattern typically shows a mixing pitched patterns as a result of the intercalation of the plasticized mass layers from both of the interface (AS and RS) come together to form the joint-line. The curved flow layers at the surface area of the entry zone can be the starting of these pitched surface patterns. Figure 5 reveals the twisting flow pattern of the plastic deformation in the entry spray of the aluminum weld. The striation lines of the plastic flow consolidated at the surface of the entry spray suggests that during the tool-substrate interaction, the simultaneous rotation and movement of the tool along with the edge of the workpiece caused a driving force for the strain planes underneath the shoulder. Because of the stirring action, the strain planes rotate with a twisting flow pattern. In this twisting flow mechanism, each plane after travelling a specified distance, can transform its driving force to the surface. The rotation of the surface is a rotation of the surface and the rotation of the surface. The rotation of the surface is a rotation of the surface and the rotation of the surface is a rotation of the surface. force to the neighboring planes and become settled. Spatial location for each plane in the settlement position is in a different geometry in comparison with the original position. The consequence of these series of movements and settlements of the strain planes is a stirred zone in the body of the weld region. Since this twisting pattern caused a movement, there is a gap between the primary and new location of the planes. As it is illustrated in Figure 6, by pushing the stirring front to the entry zone (region c), a channelled flow in the neighborhood regions of the advancing side forms which are representative of the original position of materials before taking part in a stirred zone. In general, the surface flow lines are on a curvature direction from the advancing side toward the retreating side, according to the clockwise rotation of the bobbin tool. However, because of the scroll patterns inscribed on the shoulder surface, there is a bunching of the segmented contours which are formed as a continuous succession of the deposited mass. All of these successive segments are formed the spray region of entry zone. This continuous twisting movement can finally form a stirred zone for the bonding of the interface whereas there remains a tunnel defect as the footprint of unstable plastic deformation during the twisting pattern of plastic flow mechanism at the beginning of the stirring action. For further analyzing the stirring action inside the workpiece, the material mixing phenomenon at the cross-sections of the plasticine analogue is shown in Figure 7.  
Figure 6: Twisting model for the plastic deformation in the stirred zone of the entry spray. (**a**) Edge view of the spray region. (**b,c**) Surface patterns in higher magnification. Figure 7 shows the material flow in three regions of entry zone, the body of weld and exit zone.