On the Formation of Spherical Metastable BCC Single Crystal Spatter Particles During Selective Laser Melting
42 Pages Posted: 10 Oct 2019 First Look: Under Review
Spatter particles, created during laser powder-bed-fusion (L-PBF) additive manufacturing process of 316L stainless steel, solidified as single-crystal, non-equilibrium, body-centered cubic (BCC) ferrite, which has not been reported before. This phenomenon is unusual considering that the composition of stainless steel 316L typically ensures primarily austenitic (face-centered-cubic, FCC) solidification. These particles were analyzed with multi-length scale microscopy and diffraction methods. Mechanisms for the competition between BCC and FCC phases were evaluated with computational thermodynamic and interface response function theories, as a function of thermal boundary conditions. These results indicate that the particles solidify at rapid rates and/or that conditions exist during solidification that allow for the nucleation and growth of the BCC phase that outcompete the FCC phase. The novelties of the work pertain to three aspects (i) discovery of fully single crystal BCC particles residing within spatter, (ii) rationalization of the mechanisms for this phenomenon with suites of characterization and modeling tools, as well as, (iii) the suggestion that L-PBF and associated spattering processes can be used as a synthesis route to produce metastable, single-crystal structures.
Keywords: L-PBF, Additive Manufacturing, solidification, steel, ferrite, oxide
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