Directional solidification of Cu with dispersed W nanoparticles: A molecular dynamics study in the context of additive manufacturing

Directional solidification, resulting in the development of columnar grains, is prevalent in laser-additive manufacturing of metal parts. The addition of grain refiners or inoculants enables the control of the nucleation process and columnar-to-equiaxed transition. In this study, we applied molecular dynamics simulations to investigate the directional solidification in liquid copper inoculated with tungsten nanoparticles. The interaction of the solidification front with inert nanoparticles resulted in either their engulfment in the Cu matrix or their pinning by grain boundaries. Analysis of the forces acting on the nanoparticles and the temperature field provides conditions for engulfment, determining the feasibility of copper matrix composites. By global cooling reflecting the thermal dynamics characteristics of additive manufacturing, a columnar-to-equiaxed transition can be captured at the nanoscale, with heterogeneous nucleation occurring at the nanoparticle surfaces. The conditions for the onset of heterogeneous nucleation were assessed on the basis of nanoparticle size and interpreted in terms of classical nucleation theory. Our results highlight the critical role of W-nanoparticle wetting by Cu solid grains in increasing grain refinement efficiency and suppression of columnar growth in additive manufacturing.