Molecular dynamics simulations of nanoscale solidification in the context of Ni additive manufacturing

Solidification is a key step in additive manufacturing technology because it determines the microstructure and performance of the final product. Our work provides the nanoscale description of relevant solidification pro-cesses for polycrystalline Ni by means of molecular dynamics simulations. In particular, we focus on the thermal effects associated with the characteristic non-stationary conditions of additive manufacturing. Directional so-lidification and homogeneous nucleation are investigated as a function of operating parameters that control the temperature gradient and the cooling rate. We show that a planar solid/liquid interface propagates at a constant speed, in the presence of a temperature gradient between the melt pool and the solidified region. By adding cooling to the melt pool, a columnar-to-equiaxed transition can be captured at the nanoscale. If the undercooling is not sufficient to promote nucleation, a strong instability of the interface develops, forming protusions. The behaviors observed at the nanoscale are interpreted in terms of the classical theories of solidification and nucleation.