A reactive molecular dynamics study of bi-modal particle size distribution in binder-jetting additive manufacturing using stainless-steel powders

Binder-jetting is a powder-bed-based additive manufacturing (AM) process that is uniquely different from other powder-bed “fusion” metal AM technologies because it is a binder-based consolidation process similar to powder metallurgy “green” part and offers a larger selection of materials and part design capabilities. In order to improve a final part's density and achieve desired mechanical properties, binder-jetting usually requires lengthy post-processing steps such as curing, sintering, and infiltration. The role of particle size distribution in this process has been demonstrated to have a major impact. When comparing different distributions such as mono- and bi-modal sizes, the latter, consisting of a mix between coarser and finer particles, has shown to increase packing density and decrease porosity for a printed part. In this present work, we employ ReaxFF reactive force-field-based molecular dynamics (MD) simulations to study the atomistic level mechanism of binder-jetting using a bi-modal austenitic stainless-steel powder mixture. In addition, we compare the fracture process of the bi-modal powder mixture system with that of a system with mono-modal particle size, aiming to understand how the finer particles in the bi-modal powder mixture contribute to raising rupture strength. The MD simulation results show that the energy barriers after curing and sintering in the bi-modal particle system increase by 42.9% and 40.9%, respectively than in the mono-modal particle system. Moreover, the analysis of chemical composition and microstructure shows that iron is dominantly oxidized by oxygen atoms rather than hydroxyl radicals. Besides, the finer particle is subject to internal oxidation during sintering because its iron core melts. In contrast, the iron core of the coarser particle remains crystalline. Additionally, the statistical analysis of bonding oxygen atoms for each reference iron atom indicates that both particles have a small ratio of iron oxidized to Fe(II) but only slowly oxidizes to Fe(III) in the binder-jetting process. The coarser particle has a lot of non-oxidized iron atoms, while the majority of iron atoms in the finer particle bond with one oxygen atom during the time scale of our MD simulations. Furthermore, de-hydroxylation and oxygen inward diffusion lead to the reduction of chromium cations throughout sintering. The original findings of this study provide a nanoscale explanation for the mechanical property improvement using a bi-modal powder mixture. Moreover, the study of chemical composition and microstructure also contributes to improving the chemical properties of binder-jetting products.