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

Abstract

Inhomogeneity in microstructures along the thickness direction in the stir zone of a duplex stainless steel (SUS329J4L) welded at rotational speeds of 275 rpm, 400 rpm, and 800 rpm were investigated using the electron back scattered diffraction method. The changes in 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 due to the introduction of the additional strain by the shoulder. The KAM values in the stir zone exhibited a higher value in austenite than in ferrite through all the thickness direction, which is inferred to be related to the difference in dynamic recrystallization behavior governed by stacking fault energy; i.e., lower in the austenite phase than in ferrite phase. The layer thickness per unit length of the layered structure became smaller than that of base metal as the rotational speed of FSW was reduced to 275 rpm, which implies that new grains nucleate during FSW. Furthermore, some ferrite grains nucleated at austenite/austenite grain boundaries were confirmed to satisfy the Kurdjumov-Sachs orientation relationship. FSW is assumed to promote the nucleation of new grains with different phases probably owing to the stirring effect of 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 rotational speeds.