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 applied 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 plastic flow mechanism was visualized both, at the surface and the cross-section of the weld-seam. The cross-section of the weld shows the details of the formation of tunnel voids, caused by the failure of the 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.