Abstract The flow of the shear layer around the tool pin and how it deposits into the cavity at the trailing end of the pin to form the weld Nugget zone during friction stir welding (FSW) had not been fully understood. In this study, FSW experiments were conducted to result in pins broken and embedded in the workpiece of a work-hardened aluminium alloy during FSW, followed by metallography analysis of the weld samples. Evidence has suggested that a portion of the shear zone formed a layer and detached in a trailing–retreating location of the pin, one in each revolution. The shear layer detached from the pin leaving little material to rotate with the pin. The detached layers continued to flow, pushed by the incoming material in the shear zone, filling the stable and forward moving cavity afterward. The velocity of the freshly deposited shear layer was very low in comparison to the pin periphery velocity.