Selective laser melting involves melting and solidification of metal powder particles in a track-by-track and layer-by-layer method to fabricate 3D parts. The present investigation focuses on understanding the effect of laser power and scan speed on the evolution of melt pool, porosity and multiple thermal cycling effects on the microstructure in parts fabricated using selective laser melting. In this study, Ti-6Al-4V pre-alloyed powder was used to produce single-track deposits and bulk parts. Using different combinations of laser power and scan speeds, single-track deposits and bulk parts were produced. The cross-sections of the single-track deposits and bulk samples were prepared for metallographic observations and the melt pool shape and size and porosity were evaluated. When a low energy density was applied the un-melted powder particles produced irregularly shaped porosity, and a high energy density resulted in rounded porosity, which was due to keyhole effects. The samples produced with a proper combination of power and speeds were fully dense. Further, microstructural development under the influence of process condition was highlighted. Overall, the study demonstrates a good correlation between the single-track melt pool geometries, porosity in bulk parts and also demonstrates the microstructural inhomogeneity during deposition.
Criegee
intermediates are important atmospheric oxidants, and quantitative
kinetics for stabilized Criegee intermediates are key parameters for
atmospheric modeling but are still limited. Here we report barriers
and rate constants for unimolecular reactions of s-cis-syn-acrolein oxide (scsAO), in which the vinyl group makes it a prototype
for Criegee intermediates produced in the ozonolysis of isoprene.
We find that the MN15-L and M06-2X density functionals have CCSD(T)/CBS
accuracy for the unimolecular cyclization and stereoisomerization
of scsAO. We calculated high-pressure-limit rate constants by the
dual-level strategy that combines (a) high-level wave function-based
conventional transition-state theory (which includes coupled-cluster
calculations with quasiperturbative inclusion of quadruple excitations
because of the strongly multiconfigurational character of the electronic
wave function) and (b) canonical variational transition-state theory
with small-curvature tunneling based on a validated density functional.
We calculated pressure-dependent rate constants both by system-specific
quantum Rice–Ramsperger–Kassel theory and by solving
the master equation. We report rate constants for unimolecular reactions
of scsAO over the full range of atmospheric temperature and pressure.
We found that the unimolecular reaction rates of this larger-than-previously
studied Criegee intermediate depend significantly on pressure. Particularly,
we found that falloff effects decrease the effective unimolecular
cyclization rate constant of scsAO by about a factor of 3, but the
unimolecular reaction is still the dominant atmospheric sink for scsAO
at low altitudes. The large falloff caused by the inclusion of the
stereoisomerization channel in the master equation calculations has
broad implications for mechanistic analysis of reactions with competitive
internal rotations that can produce stable rotamers.
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