Inhomogeneity of Microstructure along the Thickness Direction in Stir Zone of Friction Stir Welded Duplex Stainless Steel

Inhomogeneity in microstructures along the thickness direction in the stir zone of duplex stainless steel (SUS329J4L) welded at rotational rates of 275, 400, and 800 rpm was investigated using the electron backscattered diffraction method. Changes in the volume fractions and average grain sizes of ferrite and austenite along the thickness direction may reflect the temperature gradient along the thickness direction. However, near the top surface, significant grain refinement occurred, presumably owing to the introduction of additional strain by the shoulder. The kernel average misorientation (KAM) values in the stir zone were higher in austenite than in ferrite along all thickness directions, which is inferred to be related to the difference in dynamic recrystallization behavior governed by stacking fault energy, which is lower in the austenite phase than in the ferrite phase. The layer thickness per unit length of the layered structure became smaller than that of the base metal as the rotational rate of friction stir welding (FSW) was reduced to 275 rpm, which implies that new grains nucleated during FSW. Furthermore, some ferrite grains nucleated at the austenite/austenite grain boundaries, satisfying the Kurdjumov-Sachs orientation relationship. FSW is assumed to promote the nucleation of new grains with different phases, probably because of the stirring effect of the elements by FSW. In a duplex structure formed in the stir zone of FSW, a linear relationship between the ferrite and austenite grain sizes was found to hold irrespective of the rotational rate.