Abstract
In contrast with traditional manufacturing methods, additive manufacturing (AM) contributes material one layer at a time, until the component is complete. Despite the recognition powder bed fusion (PBF) has gained as a powerful tool for manufacturing, broad acceptance and large-scale production have been inhibited by limited predictability of the quality of the end product. This is highly dependent on the morphology that emerges during solidification. Phase field simulations are able to address some of the limitations of traditional systems for modeling solidification by incorporating the driving forces, determining the phase of the material from its lowest energy state, and providing a visual representation of the results.
The nickel-based superalloy IN718 has been a prime candidate for a wide range of AM applications because of the desirable properties it possesses at high temperatures. As the alloy’s thermophysical properties and solidification behavior are well-documented, it is also a prime candidate for simulation development. The similar properties exhibited at high temperatures give reason to believe that the solidification behavior of refractory metals, for which thermophysical property data is not readily available, may be akin to that of other high-temperature alloys during additive manufacturing. Thus, it is proposed that a phase field model proven to be a valid approximation of the solidification morphology for IN718 could later be adapted for other refractory alloys, such as the niobium-based alloy C103.
The objective of this research was to identify relationships that connect the process to the microstructural development and, ultimately, the behavior of high-temperature alloys during additive manufacturing through the development and use of a phase field model for the solidification of IN718. The study also sought to identify the processing parameters and thermophysical properties which have the greatest influence and ascertain the extent of their impact. Using the Multiphysics Object-Oriented Simulation Environment (MOOSE), an open-source, parallel finite element framework developed by Idaho National Laboratory, a phase field model was constructed and validated with physical experiments. A sensitivity analysis was then conducted with parameters and properties of interest.
Based on the cellular morphology displayed at appropriate parameters, it was determined that this phase field model could serve as an approximation of solidification for IN718 during PBF. The model was also able to provide a concentration field that suggests solute partitioning of niobium within IN718, which is consistent with recently published literature. No single property was directly responsible for a transition between morphology types; however, the growth seemed to be determined by the combination of four factors: thermal diffusivity, interface mobility, gradient energy coefficient, and mode of anisotropy. Further testing would be required to ascertain whether or not this model can serve as an accurate depiction of solidification for C103 or other materials.
