## 3 Results and Discussion
Instead of one cross-section perpendicular to the welding direction, samples were cut at the AS and RS of the stirring zone at 45\({}^{\circ}\) angle on the welding direction. Figure 7a,b shows the cross-sections of the entry zone at the AS and RS, respectively. Figure 7a,e reveal the flock-shape patterns on the AS of the stirring zone, representative of the pin profile at the entry and exit zones, respectively. By the interaction between the threaded pin and the substrate, the material forms a vortex area near the bottom surface and a lamellar structure underneath the top shoulder. The RS cross-section shows an arc flow for the stirring zone with a curved wave mixing pattern at the position of the pin. Similar flow patterns are also visible in the exit zone, observed in Figure 7e,f, for the AS and RS of the weld-line, respectively. In both entry and exit zones, the birds-flock swarm pattern at the AS of the weld is representative of direct interaction between the threaded pin and the workpiece material which is inscribed around the hole defect because of the shortage of the refilling material to provide the integrity of the mass circulation. The literature reports the middle of the stirring zone as the onion ring pattern. By generating a continuous refilling flow behind the tool, the plastic layers from the onion ring flow pattern at the contact region of the pin and the stirred mass. The flow trend for the onion rings can be conducted by the angle of the pin threads which result in a curved lamellar structure. Figure 7c,d, shows the flow patterns formed in the form of onion rings for the AS and RS cross-sections. Although both patterns demonstrate a similar curved lamellar pattern, the RS onion rings have better integrity between the flow layers. It implies the flow consistency at the leading edge of the pin, where the plasticized mass is transferred from AS to RS. The simultaneous advancement and rotation of the tool cause a flow complexity at the trailing edge of the pin (AS-to-RS transportation) whereby form a turbulent vortex flow deposited toward the AS. This is visible in all sections of Figure 7 that the flow lines in RS (Figure 7b,d,f) possess a more steady shape compared to the AS (Figure 7a,c,e). The discontinuity pattern at the AS region of the entry and exit zones represent the formation of the open tunnel void as the most common defect emerging at the BFSW weld structure. To validate the internal flow visualization of the BFSW weld obtained by the analogue plasticine, these flow patterns need to be compared with the actual weld samples. Figure 8 shows the etched weld  
Figure 7: The cross-sections of the stirring zone showing the internal flow details for the plasticine analogue sample; (**a,b**) cross-sections of the entry zone; the advancing side (AS) and retreating side (RS) transverse sections, respectively; (**c,d**) cross-sections of the mid-veld; the AS and RS transverse sections, respectively; (**e,f**) cross-sections of the exit zone; the AS and RS transverse sections, respectively. samples selected from the AS and RS cross-sections of the entry zone, similar regions in Figure 7a,b, respectively. In the case of the AS, Figure 8a demonstrates an open void surrounded by the birds-flock swarm patterns, similar to Figure 7a. as the tool passes from the entry zone, the void deficiency emerged at the AS interface. However, the forging effect of the tool at the RS generates a refilling effect whereby it forms the onion ring pattern in the stirring zone, see Figure 8b. The flow lines at the RS interface shows a lamellar structure, moving with a horizontal trend from the top surface underneath the top shoulder to the center of the stir zone. This flow detail in the RS (Figure 8b) indicates an optimized refilling at the trailing edge of the tool without void occurrence. It confirms that for integrity in the refilling mechanism, the simultaneous performance of the pin and shoulder should generate an overlapped mass flow which can cover the material deficit after the tool leaves the stirring position.