On the forming mechanism of banded structures in aluminium alloy friction stir welds

Friction stir welding experiments were conducted using an aluminium alloy and Pin–workpiece couples were prepared to more directly determine the formation mechanism of banded structures. It was revealed that shear material in a thread space was spread into a layer, primarily as a result of the equivalent downward motion of the thread flank. An explanation is given for the spacing of the bands being equal to the tool advance per revolution, which is due to a geometrical restriction.