Phase-field simulation of the formation of new grains by fragmentation during melting of an ABD900 superalloy

Abstract

Laser powder bed fusion (L-PBF) is an additive manufacturing method which involves local laser melting of powder particles, a partial remelting of previously deposited layers, and subsequent re-solidification under high thermal gradients and cooling rates. The transition between melting and re-solidification becomes visible as melt pool boundaries in optical micrographs and plays a crucial role: Apart from creating a strong segregation zone, the transition determines whether the microstructure is inherited and carried over to the next layer, or whether new grains with new orientations are formed. While heterogeneous nucleation is suppressed due to the lack of seeding particles at the small length scales inherent to L-PBF, alternatively, new grains can form via dendrite fragmentation, as demonstrated in this paper by phase-field simulations using the software MICRESS®. By strong coupling between the phase-field equation and a thermal 1D-cylinder approach for the long-range temperature field, consistency between latent heat and microstructure is ensured. To allow for a systematic variation of the orientation relationship between the dendrite growth direction and the respective temperature gradient, a two-step simulation procedure for two overlapping tracks with variable gradient directions is developed. Growth conditions which promote fragmentation and formation of new grains are analyzed and discussed.