## 3 Results and Discussion  
Figure 2 shows the physical appearance at the top plan view of the analogue plasticine sample after welding with the 3D printed plastic tool. The relevant dynamic force changes measured in longitudinal and traverse directions of the weld-line also are presented beside the weld analogue. The welded samples show two main defects at the entry and exit zones, as the inherent of the butt-joint bobbin weld. The welding process by the bobbin tool, there are three stages; entry, welding zone, and exit. The  
related areas for these three parts can be identified in the analogue model by the material movement patterns. The weld stage is located between the entry and exit. The entry can be characterized as a sudden high deformation which decreases over time before becoming stable as the tool travels completely into the plate. The exit phase is when the leading edge of the tool first starts to leave the plate, and material deformation decreases steadily with a sudden last drop of plastic deformation and disintegration of the material. The physical size of defects is corresponding with the dynamic changes of the load during the process. The surface appearance of the weld-seam shows a regular circular ripple pattern which is related to the shoulder performance during the advancement of the tool through the weld-line interface, see Figure 3. The comparison between these surface pitch ripples in analogue plasticine and aluminum welds shows high similarity in shape and size. Therefore, the formation mechanism of these circular surface patterns is relevant to the tool features and flow interaction underneath the scrolled shoulder. As a general interpretation, these pitch ripples are produced by the final ejection of the depositing mass layers at the trailing circumferential edge of the shoulder, during the longitudinal travelling of the tool. Although the pitch patterns have similar shapes in both analogue and metal samples, the  
Figure 2: Photograph of the bobbin friction stir welding (BFSW) section sample (top surface): plasticine analogue model. (**a**) Entry zone of weld. (**b**) Exit zone of weld. (**c**) Weld-line body, through the weld-line. actual size of the ripples can be defined by the speed ratio of the tool (relative feeding rate per rotation; distance per revolution). However, the overlapping of the combined successive revolutions makes it hard to provide an accurate measurement of the distance between the pitch layers to determine the optimized condition for the curved patterns of the weld surface. The 3D printed plastic tool has a suitable mechanical consistency in its interaction with the substrate, regarding the observation of the physical changes in flow mechanism as the tool travels forward. The sample is suspected to contain more internal defects within the weld structure. Therefore, several extra samples were tested with the same tool features in different color patterns and under similar welding parameters to reveal more details of the internal flow mechanism during the bobbin friction stir welding. Figure 4 shows the cross-section of the plasticine weld in a multi-layered slab. Further details of the flow behavior at the cross-section of the analogue plasticine are observed in Figure 4b. The analogue model of the weld suggests a possible position for the tunnel defect between the entry zone  
Figure 3: In situ model of the tear drop pattern through the weld-line; (**a**) plasticine analogue, (**b**) aluminum weld sample. and the exit zone of the weld (Figure 4a). Also, the nucleation of the defect in correspondence with the material flow interaction in the middle of the stirring zone is visible (Figure 4b). The previous research confirmed that the tunnel void originally forms at the entry zone. By including the flow visualization from the actual aluminum sample, compared with the analogue model, can construct a better understanding of the history of the flow interaction during the stirring process. Figures 5 and 6 show some magnified views of the entry zone in aluminum weld sample.