Molecular Dynamics Simulations of Heterogeneous Nucleation in Aluminium Alloys Under Rapid Heating and Cooling in Powder-Bed Fusion Type Additive Manufacturing

Additive manufacturing (AM) has garnered substantial attention owing to its capability to fabricate parts with complex shapes. In addition, the powder-bed fusion (PBF) type AM process can also control internal microstructures by controlling the rapid cooling conditions unique to the PBF process [1]. On the other hand, we previously reported that the rapid heating condition during the melting process also affects the microstructure formation in Al-Si eutectic alloy, in which the solid-Si particles with high melting temperature remained after the rapid melting and act as heterogeneous nucleation sites during subsequent solidification [2]. The remaining Si particles in the remelting process are caused not only by the fast cooling but also by the significantly large heating rates in the PBF process. However, the heterogeneous nucleation cannot be explained by classical nucleation theory. It is supposed that the results and the theory do not agree because the crystallization occurs from a non-equilibrium heterogeneous liquid containing remaining Si particles due to the rapid heating and cooling conditions of the PBF process. In this study, we conducted molecular dynamics (MD) simulations of heterogeneous nucleation of Al from the Si solid/liquid interface to clarify the heterogeneous nucleation behavior during the PBF process. For comparison, we also performed the MD simulations on L1₂-type Al₃Zr crystal substrates, which cause the frequently heterogeneous nucleation in the PBF process [3].<br/>The MD simulations were performed using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). The Modified Embedded-Atomic Method (MEAM) potential [4,5] was used as the interatomic potential. First, the Si diamond crystal, L1₂ type Al₃Zr crystal, and Al liquid models were equilibrated at 973 K, which is the Al melting point of the used MEAM potential. Si solid/Al liquid phase models and Al3Zr solid/Al liquid phase models with 10 nm × 10 nm × 10 nm were prepared from these equilibrated models. The solid-liquid models were held isothermally in the temperature range from 300 K to 850 K, and time-temperature-transformation (<i>TTT</i>) diagrams were obtained.<br/>All solid/liquid models crystalized the shortest time under isothermal treatment, around 600 K. On the other hand, the time and undercooling required for the crystallizations largely depend on the substrate and its orientation: crystallization occurred in the shortest time and wider temperature range in the model with the [111] orientated Si and [100] oriented Al₃Zr substrates and the time and the temperature range are the almost the same. We found that Al liquid forms an ordered structure near the solid-liquid interface in the model with the [111] oriented Si substrate, and the development of Al crystals started from the ordered region. These results suggest that the ordering of Al liquid near the solid-liquid interface is a key to frequently heterogeneous nucleation observed in the PBF process of the Al-Si eutectic alloy.<br/>[1] K. Hagihara, T. Nakano, JOM. 74 (2021) 1760–1773.<br/>[2] M. Okugawa et al., J. Alloys Compd., 919, 165812 (2022).<br/>[3] J. H. Martin et al., Nature, 549, 365 (2017).<br/>[4] R. Fereidonnejad et al., Comput. Mater. Sci., 213, 111685 (2022).<br/>[5] X. Huang et al., J. Non. Cryst. Solids. 503–504 (2019), 182.