Understanding defect structures in nanoscale metal additive manufacturing via molecular dynamics

Abstract

Additive manufacturing of a single crystalline metallic column at nanoscale 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 the bottom up. The polyhedral template matching (PTM) is used to observe the evolution of atomic-scale defects during this process. The solidification is seen to proceed in 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 that starts from the top of the melt layer. The defect structure formed strongly depends on the speed of the two competing solidification fronts. Up to a critical layer thickness, the 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. A slower cooling rate leads to a reduction in defects, however, the benefits diminish below a critical rate. The defect content can be significantly reduced by raising the temperature of the powder bed to a critical temperature. This temperature is governed by two competing mechanisms: the slower cooling rates at higher temperatures and the increase in amorphousness as one gets closer to the melting point of the metal. Finally, the effect of an added soft inclusion (SiS2) and a hard inclusion (SiC) on the defect structures is explored. The hard inclusion leads to a retained defect structure while soft inclusions reduce defective content compared to the pure metal.