Selective laser melting of pure iron: Multiscale characterization of hierarchical microstructure

Abstract A complex multiscale study of the microstructure produced by selective laser melting (SLM) in metals exhibiting diffusionless solid-state phase transformations was performed on iron as a model material. This study shows that even in such a simple material as pure iron is, SLM produces a hierarchical microstructure. This hierarchy consists of (i) large, nearly equiaxed grains separated from each other by high-angle grain boundaries; (ii) micro- meter-sized subgrains with low-angle grain boundaries which are located inside the majority of these grains; and (iii) nanometer-sized cells appearing inside the subgrains that are mutually separated by dislocation walls. Based on theoretical analysis of the orientations relationships of the grains, which occur during repetitious α → γ → α transformations in SLM iron, we formulated a model describing the resulting microstructure. This model also explains the occurrence of nearly equiaxed grains instead of columnar ones, which would be expected to appear during the SLM process. The equiaxed grains occur when the laser is repeatedly scanned over the studied location, temperature changes steeply up and down and induces quick α → γ → α transformations resulting in the breakdown of the initially formed columnar grains.