## 4 Discussion  
The main aim of this study was to predict the position of the tunnel defect based on the analogue model to elucidate the flow-based formation mechanism. Based on the analogue flow visualization at the cross-section, a simplified physical model can summarize the general flow behavior of the process during the interaction between the tool and the substrate. The tool features in bobbin-FSW process generate a specific inherent flow interaction around the tool, compared to the conventional-FSW. Although different defect geometries (open tunnel)void) were observed in flow visualization of the analogue model, the flow mechanism is proposed to be similar. By comparing the weld cross-sections in plasticine analogue and aluminum weld, it was found that the tunnel defect is commonly formed at the advancing side of the tool near the bottom surface. This can be attributed to the sequence of the material transportation relevant to the tool features and the welding process setup. Figure 10 presents the details of the flow lines at the proximity of the tool components (shoulder and pin). The plain view of the plastic flow underneath the shoulder is shown in Figure 10a. It is shown that by entering the pin to the substrate, the materials undergo a local discontinuity caused by the shearing effect of the rotating tool. After circulating of the plastic mass around the pin, the stirred materials are being deposited at the trailing edge of the tool. In this situation, the flow inconsistency can cause a mass deficit between the deposited layers, resulting in void occurrence. It seems that the rotating pin has more responsibility regarding the internal flow features within the stirring zone. Hence, for a better explanation, the mass flow pattern around the pin is drawn in Figure 10b. The side view of the tool in Figure 10b shows that the clockwise rotation in the presence of threads on pin pulls the mass flow upwards. Therefore, the upward trend pumps material vertically from the advancing side to the leading edge of the tool, and simultaneously the fresh material of the retreating side are swept to the trailing edge of the tool. This helical flow path for transportation of the stirred mass cause a refilling inconsistency, in the form of the escape of the material at the backside of the tool, which increases the possibility of the material deficit at the bottom of the advancing side where the material is expected to be deposited after the circulation around the pin. This is where the incomplete filling deficit and eventually a tunnel void emerge as the discontinuity defect between the deposited mass layers. The pitch threads have a positive influence on stirring performance by the increase in velocity of transportation and a helix angle and thus better plastic flow condition at the leading edge of the tool. However, this helical flow path affects the efficiency of the adhesion of the material to the pin during the transportation at the trailing edge of the tool, between AS-to-RS. This is consistent with the cross-section observations which clearly shows that the turbulent-like flow features at the advancing side towards the bottom surface form an open tunnel void defect. The schematic of the dynamic material flow can be shown around the rotating pin in plain view to elucidate the pin-material interaction with a focus on void formation mechanism. Figure 11 demonstrates the sequence of material flow around the pin, based on the flow behavior identified in this work. Referring to the flow visualization developed in this work, by the clockwise rotation of the tool, the material at the AS is stirred and transported to the RS. By entering the tool into the substrate, the pin grabs the material from the leading edge of the tool, see Figure 11a. By further advancement of the tool trough the butt-interface locus, the sharp edges of the pin (threads-flats) dig into the workpiece material (Figure 11b). This yields more plasticized mass at the trailing edge of the pin.