Influence of solidification cell structure on the martensitic transformation in additively manufactured steels

Comparative study of the microstructure between a laser beam melted 17-4PH stainless steel and its conventional counterpart

“…All the specific microstructural features previously cited for the AM-H900 samples could contribute to explain these observations. It could be assumed that finer martensite laths were associated with a higher level of internal by grinding or polishing [36,37]. Therefore, surface preparation (i.e.…”

“…In the current study, the austenite amount for as-built AM samples was only 12.5%, which corresponded to a mainly martensitic microstructure, which could be explained by referring to the gas used during the building, in good agreement with Murr et al work [6] who observed mostly martensitic microstructures for the built parts when the AM process was performed under Ar, independently of the powder. Furthermore, to explain the level of retained austenite in as-built AM samples, Freeman et al [36] showed that they were able to stabilise a fully austenitic microstructure in an AM 17-4PH MSS at room temperature, because as-built AM 17-4PH MSS contained small solidification cells of 0.2-2 μm that suppressed the thermally-induced martensitic transformation, whereas former austenitic grain size controlled the transformation for wrought MSS [38][39][40][41][42][43]. Finally, Yadollahi et al showed that the formation of smaller grains and interdendritic spacing, as well as the presence of strain at high-angle grain boundaries, associated with high dislocation density in AM processes should explain the level of retained austenite in as-built AM samples [12].…”

Heterogeneous Aspects of Additive Manufactured Metallic Parts: A Review

“…Also, material composition and crystal structure might play roles as they influence the solidification phenomena [101]. In Ti-alloys, although high dislocation densities are reported, they did not show cellular structure [102]; however, Al alloys [40] and steels [103] showed cellular structures. Theoretical models considering all the aspects of solidification, crystal structure, residual stresses, and sample location would give more insights.…”

“…The presence of cellular structure affects the solid-state phase transformation. In the SLM processed precipitationhardened martensitic stainless steel (17-4PH), Freeman et al [103] showed complete suppression of thermal martensite formation. The laser PBF samples showed a hierarchical microstructure with coarse columnar grains (10-100 μm) containing sub-grain cellular structures (0.2-2 μm) with dense dislocation walls.…”

“…Although the cellular walls are low angle boundaries ~ 1°-2° (Fig. 11) [89,103,104], the dense dislocation and the segregation of solute elements around the walls make them sufficiently robust to contain the dislocation motion and strengthen the material in a manner generally attributed to grain boundaries. At a relatively low strain (~ 3%), dislocation slip was dominant, the cellular size, shape, and local misorientation across the walls remain unchanged (Fig.…”