In bobbin friction stir welding, achieving the defect-free joints needs a comprehensive control of the process parameters. Material flow transportation around the rotating tool and the mass deposition at the backside of the tool are critical characteristics of friction stir welding. To achieve an optimized weld structure, the history of the plastic deformation needs to be identified with a flow- based elucidation. In this study, an analogue model was developed to evaluate the formation of a banded structure using the bobbin tool, with a focus on the interaction between the tool-workpiece. The flow visualization in plasticine analogue was validated in comparison with the aluminium welds. The cross-section of the weld demonstrated the details of the formation of tunnel voids, caused by the failure of flow regimes. A physical model of the material flow was proposed to explain the formation mechanism of the tunnel void as a discontinuity during the mass refilling at the rear of the tool. The microscopic analysis was used to determine further microstructural evolution in BFSW aluminium welds. The metallurgic measurements of the weld texture showed the formation of low-angle and high- angle grain boundaries (LAGBs, HAGBs) in different regions of the weld microstructure, as a consequence of thermomechanical nature of the process. The stirring zone (SZ) underwent severe grain fragmentation and a uniform dynamic recrystallization (DRX). The transition regions of the weld experienced stored strain which changed the grain size and morphology via grain misorientation transformations. Other observations were of micro-cracks, the presence of oxidization, and the presence of strain hardening associated with precipitates. Flow arms in welds are caused by DRX processes including shearing deformation, which affects microstructural integrity of the internal plastic flow. The work evaluated the internal flow features of the aluminium butt-joints in bobbin friction stir welding, using the metallographic observations. A model of discontinuous flow within the weld was proposed the key findings are that the packets of material ('flow patches') being transported around the pin during the stirring action. The intermixing of the flow patches from both the advancing and retreating sides of the weld experience high localized shearing at the stirring zone, evidenced in the boundary layers of the deposited mass within the weld structure. The flow failure of the plasticized mass during the deposition and refilling of the material at the trailing edge of the tool causes a discontinuity within the weld seam, leading in formation of the flow-based defects, e.g. tunnel void.