Material flow during friction stir welding is very complex and not fully understood. Most of studies in literature used threaded pins. However, analyzing material flow using smooth pins is of great interest for the validation of a numerical model in which threads are difficult to represent. In this study, FSW experiments were performed using smooth pin based on plasticine analog. Longitudinal, horizontal and transverse sections of welds were observed with the use of plasticine of identical type but different color as material marker to investigate the material flow. A horizontal ‘vortex-like’ action has been occurred. In the agitating-extruding region (inside the pin diameter), a clockwise (identical to the rotating tool) layer appears around the tool, as a result the material both the advancing side and retreating side is mixed. In the deformation region (outside the pin diameter), the material is driven by the clockwise viscous force of the rotating pin, and the effect is reduced with the increase of the distance to the weld center.