Insight into the mechanisms of columnar to equiaxed grain transition during metallic additive manufacturing

The columnar to equiaxed transition (CET) of grain structures associated with processing conditions has been observed during metallic additive manufacturing (AM). However, the formation mechanisms of these grain structures have not been well understood under rapid solidification conditions, especially for AM of superalloys. This paper aims to uncover the underlying mechanisms that govern the CET of AM metals, using a well-tested multiscale phase-field model where heterogeneous nucleation, grain selection and grain epitaxial growth are considered. Using In718 as an example, the simulated results show that the CET is critically controlled by the undercooling, involving constitutional supercooling, thermal and curvature undercoolings in the melt pool, which dictates the extent of heterogeneous nucleation with respect to the grain epitaxial growth during rapid solidification.