Subgrain microstructural features in directed energy deposited stainless steel 316L: The influence of morphology on mechanical properties

Abstract

Recent studies have established that the exceptional combination of strength and ductility of directed energy deposited stainless steel 316 L is primarily due to the barrier effect against mobile dislocations introduced by cellular subgrain feature boundaries. Interestingly, the morphology of the feature, and therefore its boundaries, exhibits a significant variation with process parameters. This article presents an experimental study to determine the mechanism responsible for the formation of cellular subgrain feature with different morphologies and the influence of morphology on mechanical properties. Two builds with cellular subgrain features of different morphologies, tubular and vermicular, are manufactured using different process parameters and scanning strategies. The microstructural analysis of the specimens shows very different element segregation and secondary phase distribution. The mechanism responsible for the formation of different morphologies is the differences in the solidification modes. The barrier effect has two contributions; a hard barrier effect introduced by delta ferrite secondary phases and a soft barrier effect introduced by austenite cell wall regions. Irrespective of different morphologies, the two builds exhibited similar macroscopic mechanical behavior. This indicates that the total barrier effect exerted by the cellular subgrain feature determines the mechanical properties of directed energy deposited stainless steel 316 L, not necessarily its morphology.