Quantitative simulations of grain nucleation and growth at additively manufactured bimetallic interfaces of SS316L and IN625

This paper provides a quantitative understanding of grain nucleation and growth at the interface of SS316L and IN625 bimetallic structures during directed energy deposition (DED) through multi-physics simulations, including phase-field models and computational fluid dynamics analysis. The main finding is that the flow behaviors would lead to composition redistribution and the change of liquidus temperature in the mixing zone at the interface, which further influences the solidification sequence and the final grain structure that are different from general single-metal counterparts without the mixing zone. The results show that during depositing IN625 on SS316L, a gradual transition in composition distribution and liquidus temperature in the well-mixing zone caused by simple clockwise flow leads to epitaxial grain growth from the SS316L substrate and the prevalence of columnar grains with a low undercooling (<1 K) condition. However, when depositing SS316L on IN625, it turns out that initial solidification can occur due to abrupt compositional change at the interface where the well-mixing breaks are caused by two opposite (clockwise and counterclockwise) flow behaviors. Such an abrupt compositional change results in high liquidus temperatures that may trigger the grain nucleation in the middle melt pool due to a high undercooling (>30 K); thus, a mixed grain microstructure is present.