J. Schneider, Mississippi State University, Mississippi State, MS; A. C. Nunes, NASA-MSFC, Marshall Space Flight Center, AL
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 corresponding 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.
The texture observed in OIM mapping correlate with the flow path expected for a retained current in the secondary induced rotational flow around the pin tool. It is this residue current that is carried for multiple rotations of the tool and swept down by vertical currents induced by the threads on the tool.
