  Additive manufacturing of a single crystalline metallic column is studied
using molecular dynamics simulations. In the model, a melt pool is
incrementally added and cooled to a target temperature under isobaric
conditions to build a metallic column from bottom up. Common neighbor analysis
(CNA) is used to observe the evolution of atomic scale defects during this
process. The solidification is seen to proceed along two directions for an
added molten layer. The molten layer in contact with the cooler lattice has a
fast solidification front that competes with the slower solidification front
starting from the top layer. Defect structure formed strongly depends on the
speeds of the two competing solidification fronts. Up to a critical layer
thickness, defect free single crystals are obtained as the faster
solidification front reaches the top of the melt pool before the initiation of
the slower front from the top. Slower cooling rates lead to reduction in
defects, however, the benefits diminish below a critical rate. Defect content
can be significantly reduced by raising the temperature of the powder bed to a
critical temperature. This temperature is determined by two competing
mechanisms: slower cooling rates at higher temperatures competing against
increase in amorphousness as one gets closer to the melting point. Finally,
effect of added soft inclusion (SiS2) and a hard inclusion (SiC) on the defect
structure is studied. Hard inclusions lead to retained defect structure while
soft inclusions reduce defective content compared to a pure metal.
