In friction stir welding (FSW), a rotating threaded pin tool is inserted into a weld seam and literally stirs the edges of the seam together. The dynamically recrystallized zone (DXZ) of a polished and etched FSW cross section exhibits contrasting bands (the onion-ring structure), the origins of which are unclear. An orientation image mapping (OIM) study suggests that the bands may correspond, respectively, to a straight-through current of metal bypassing the pin tool in a single rotation or less and a maelstrom current rotating a number of times around the pin tool.