Abstract
In this paper, single-layer and bulk 316 L selective laser melting (SLM) experiments were conducted, fine submicron-scale geometric symmetrical cellular (hexagonal, pentagonal and square), elongated cellular and bands solidification morphologies were found in the laser-melt top surface. Meanwhile, morphological developed sub-grain patterns with quasi-hexagonal cellular, elongated cellular and bands structures (size ~1 μm) coexisting inside one single macro-solidified grain were also identified. This demonstrated the transitions from quasi-hexagonal-cells to elongated cells/bands, and transitions reverse, occurred in the whole bulk under some circumstances during SLM. Based on the experimental realities, these morphologies are formed by the local convection and Bénard instabilities in front of the solid/liquid interface (so-called mushy zones) affected by intricate temperature and surface tension gradients. Quasi-hexagonal cellular convective fields are then superimposed on macro-grain solidification to form the sub-grain patterns and micro-segregations. This explanation seems reasonable and is unifying as it can be expanded to other eutectic alloys with face center cubic (FCC) prevenient phase prepared by SLM, e.g., the Al-Si and Co-Cr-Mo systems.
Highlights
Selective laser melting (SLM) is a member of the additive manufacturing family of technologies whereby a three-dimensional (3D) part is built layer by layer by laser scanning of a precursor powder bed [1,2]
The typical characteristics of SLM alloys microstructures are the unique sub-grain patterns. These sub-grain (0.5~1 μm) cellular/bands structures were found inside each individual large grain in SLM stainless steel [6,7,8,9,10,11], molybdenum was found to be enriched at the 316 L austenite sub-grain boundaries [6]
These explanations neglect the convection and Marangoni effect, another researcher believed the source of unique patterns is assumed to be a convective or diffusive transport of impurities or one of the constituents of the material [50], fluid flow along a solid–liquid interface induces element segregation and morphological instabilities, the singularity of the highest solute concentration is the cause of instability [51,52]
Summary
Selective laser melting (SLM) is a member of the additive manufacturing family of technologies whereby a three-dimensional (3D) part is built layer by layer by laser scanning of a precursor powder bed [1,2]. Fe-Cr-Ni alloys, the different phase (austenite or ferrite) and different microstructure (eutectic, dendrite, band, plane front) are controlled by Cr/Ni ratio and growth velocity, theoretical predictions are compared with experimental results These explanations neglect the convection and Marangoni effect, another researcher believed the source of unique patterns is assumed to be a convective or diffusive transport of impurities or one of the constituents of the material [50], fluid flow along a solid–liquid interface induces element segregation and morphological instabilities, the singularity of the highest solute concentration is the cause of instability [51,52]. 3 ofand the sub-grain cellular/bands microstructures are established
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