*3.1. Macrostructure of the Cross-Section*

Figure 2a,b reveal the macrostructural examinations of the AA6082-T6 BFSW joints (etched by Reagent A) for two different sets of welding speeds (longitudinal or rotating), refer to Table 1. Both joints exhibit a distinctive zone in the middle, processed by thermo-mechanical plastic deformation induced by the bobbin-shaped tool. This hourglass-shaped region—different from the basin-shaped SZ in CFSW [13]—is discernible in the middle of the weld from the base metal (BM) by the symmetrically curved borders at the both advancing side (AS) and retreating side (RS).

It is observed that the SZ, at the middle of the weld, is larger than the diameter of the pin. This is attributed to: (a) the pin recruiting a wider volume of matter through frictional contact with the substrate, and (b) recruitment of the substrate material via frictional heating from the shoulders. Both the pin and shoulders create frictional heat.

Basically, in BFSW the fully contained pin has eliminated the incomplete root penetration of CFSW, but in Figure 2a, a macro-size tunnel void has emerged at the bottom surface towards the AS, while Figure 2b shows a defect-free weld with an integrated structure. The origins of tunnel void emergence are not fully understood in the literature; however, it is clear that the formation of this macro-size defect in a solid state process does not seem to be more definite to have a metallurgical explanation. Existing theory attributes the tunnel voids to incomplete backfilling [8,31], that arises from incompatible welding speed parameters (ω*, V*), or dynamic interaction between the tool and workpiece [31,32]. It appears that there is insufficient material. As the metallography samples were cross sectioned from the middle of the weldment, the amount of the material loss can be considered similar to the size of the sprayed tail defect at the entry zone. The continuous tunnel void shows that the plates were not fully butted.

**Figure 2.** Macrostructure of the cross-section of the BFSW joint for two different welding speeds (etched by reagent A), (**a**) (w = 400 rpm, 350 mm/min), and (**b**) (w = 650 rpm, 400 mm/min). Arrows show proposed internal flows. Dashed white lines show the SZ borders. (1) weld crown, (2) tunnel void, (3) AS border, and (3) RS border.

From observation of the macrostructure, we infer the existence of the internal flow directions. This can affect the weld quality, when the failure of the flow regimes leads to the emerging of the defects (e.g., tunnel voids or hair-line micro-cracks). To provide a better explanation for the origins of the defects based on a flow-based observation, the metallographic measurements needed to be conducted in more depth with a focus on microstructural and flow feature integrity.
