Atomistic Insights on Effect of Thermal Gradient on Solid-Melt Interfacial Properties of Al-4.5 at.% Cu

The solid–liquid interfacial energy and its anisotropy are key parameters controlling the final microstructure of cast metals. While interfacial energy influences nucleation kinetics, crystalline anisotropy governs the preferred growth direction of dendrites. Although solute effects on these properties are well studied, the influence of steep thermal gradients - typical of rapid solidification processes such as metal additive manufacturing and welding, remains poorly understood. This work employs molecular dynamics simulations to investigate the solid–liquid interface of an Al–4.5 at.% Cu alloy under thermal gradients ranging from 0 to 30 K/nm. The results show that increasing the thermal gradient strongly suppresses capillary-wave fluctuations, leading to a linear increase in both interfacial stiffness and average interfacial energy. This behavior is similar to that observed in pure aluminum, indicating that under non-equilibrium conditions, thermal effects dominate over compositional effects. The enhanced interfacial energy anisotropy at higher thermal gradients also helps in explaining a strong preference for <001> grain morphology at higher thermal gradients. These insights can help tune mesoscale solidification models for accurate prediction under rapid solidification conditions.