Experimental validation of Scheil-Gulliver simulations for gradient path planning in additively manufactured functionally graded materials

Additive manufacturing (AM), through directed energy deposition, supports planned composition changes between locations within a single component, allowing for functionally graded materials (FGMs) to be developed and fabricated. The formation of deleterious phases along a particular composition path can cause significant cracking during the AM build process that makes the composition path unviable to produce structurally stable FGMs, but it is challenging to predict which phases will be present in as-built additively manufactured parts by analyzing only equilibrium phase relations. Solute segregation during solidification can lead to the formation of non-equilibrium phases that are stable at compositions far from the nominal composition of the melt, leading to crack formation. In this work, we developed and validated a Scheil-Gulliver simulation tool based on pycalphad. We used this tool to compare the non-equilibrium phases predicted to form during the AM build process using the Scheil-Gulliver model with experimentally measured phases at several locations with different composition in a Ti-6Al-4V to Invar-36 FGM and a commercially pure Ti to Invar-36 FGM. We showed that the phases predicted to form by the Scheil-Gulliver model agree better with the experimental results than the predictions made by assuming equilibrium solidification, proving that the Scheil-Gulliver model can be applied to FGMs. Further, we demonstrated the use of our Scheil-Gulliver simulation tool as a method of designing FGMs through screening potential FGM pathways by calculating the solidification phase fractions along the experimental gradient path in composition space.