Effect of solidification pathway during additive manufacturing on grain boundary fractality

Austenitic stainless steels 304 L (SS304) and 316 L (SS316) are additive manufactured under the same processing conditions to reveal two distinct microstructures. Particularly, the resulting grain morphology for SS304 is singular – there are subgrains dispersed across the sample; there is a wide range of grain size spanning nearly two orders of magnitude; and grain boundaries are convoluted, resembling a fractal object. The materials solidification pathway governed by chemical composition is responsible for the grain boundary fractality (ferrite-to-austenite solidification for SS304 and direct transformation to austenite for SS316). Operando X-ray diffraction studies at Cornell High Energy Synchrotron Source substantiate the solidification pathway of the materials. The findings from the study open up a new avenue for grain boundary engineering using additive manufacturing.