No abstract

Numerical simulations are used in this work to investigate aspects of microstructure and microseg-regation during rapid solidification of a Ni-based superalloy in a laser powder bed fusion additive manufacturing process. Thermal modeling by finite element analysis simulates the laser melt pool, with surface temperatures in agreement with in situ thermographic measurements on Inconel 625. Geometric and thermal features of the simulated melt pools are extracted and used in subsequent mesoscale simulations. Solidification in the melt pool is simulated on two length scales. For the multicomponent alloy Inconel 625, microsegregation between dendrite arms is calculated using the Scheil-Gulliver solidification model and DICTRA software. Phase-field simulations, using Ni–Nb as a binary analogue to Inconel 625, produced microstructures with primary cellular/dendritic arm spacings in agreement with measured spacings in experimentally observed microstructures and a lesser extent of microsegregation than predicted by DICTRA simulations. The composition profiles are used to compare thermodynamic driving forces for nucleation against experimentally observed precipitates identified by electron and X-ray diffraction analyses. Our analysis lists the precipitates that may form from FCC phase of enriched interdendritic compositions and compares these against experimentally observed phases from 1 h heat treatments at two temperatures: stress relief at 1143 K (870 °C) or homogenization at 1423 K (1150 °C).

Elemental segregation is a ubiquitous phenomenon in additive-manufactured (AM) parts due to solute rejection and redistribution during the solidification process. Using electron microscopy, in situ synchrotron X-ray scattering and diffraction, and thermodynamic modeling, we reveal that in an AM nickel-based superalloy, Inconel 625, stress-relief heat treatment leads to the growth of unwanted δ-phase precipitates on a time scale much faster than that in wrought alloys (minutes versus tens to hundreds of hours). The root cause for this behavior is the elemental segregation that results in local compositions of AM alloys outside the bounds of the allowable range set for wrought alloys. In situ small angle scattering experiments reveal that platelet-shaped δ phase precipitates grow continuously and preferentially along their lateral dimensions during stress-relief heat treatment, while the thickness dimension reaches a plateau very quickly. In situ XRD experiments reveal that nucleation and growth of δ-phase precipitates occur within 5 min during stress-relief heat treatment, indicating a low nucleation barrier and a short incubation time. An activation energy for the growth of δ phase was found to be (131.04 ± 0.69) kJ mol−1. We further demonstrate that a subsequent homogenization heat treatment can effectively homogenize the AM alloy and remove the deleterious δ phase. The combined experimental and modeling methodology in this work can be extended to elucidate the phase evolution during heat treatments in a broad range of AM materials.