  During the laser powder bed fusion (L-PBF) process, the built part undergoes
multiple rapid heating-cooling cycles, leading to complex microstructures with
nonuniform properties. In the present work, a computational framework, which
weakly couples a finite element thermal model to a non-equilibrium PF model was
developed to investigate the rapid solidification microstructure of a Ni-Nb
alloy during L-PBF. The framework is utilized to predict the spatial variation
of the morphology and size of cellular segregation structure as well as the
microsegregation in single-track melt pool microstructures obtained under
different process conditions. A solidification map demonstrating the variation
of microstructural features as a function of the temperature gradient and
growth rate is presented. A planar to cellular transition is predicted in the
majority of keyhole mode melt pools, while a planar interface is predominant in
conduction mode melt pools. The predicted morphology and size of the cellular
segregation structure agrees well with experimental measurements.
