March_2022_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A R C H 2 0 2 2 1 9 temperatures for each unicomposition- al zone in the six-zone linear composi- tionally graded model. The table clearly shows a significant variation in freezing range, which is the difference between liquidus and solidus temperatures. This is also an indication of how quick- ly a liquid solidifies: (i) in a layer-by- layer fashion for materials having small- er freezing ranges (e.g., eutectics), or (ii) for an extended period of time re- sulting in elongated grains and possibly with enhanced hot tearing tendencies in alloys that have relatively large freez- ing ranges. Similar to the CALPHAD results for pure Ni-Nb gradation, these predic- tions also indicate the formation of fcc and bcc phases for two terminal alloy compositions. However, for intermedi- ate compositions, a C14 Laves phase is predicted as the first phase to solidify. Pandat software was also used to pre- dict equilibrium phase fractions at both room (25 o C) and elevated (1000 o C) tem- peratures, as compiled in Tables 2 and 3, respectively. These tables indicate that as the composition of a block is altered from IN718 to C103 in a stepwise man- ner, the primary phase with the highest phase fraction changes from fcc to in- termediate delta and C14 Laves phases, and finally to a bcc crystal structure. It is evident that these equilibrium thermodynamic predictions point to- ward considerable challenges—such as hot cracking and distortion due to resid- ual stress—associated with additively building a IN718 to C103 graded struc- ture. However, a blown powder DED modality provides a unique opportunity to regulate the deposition rates of ter- minal alloys via a multi-hopper powder feed system, along with the potential to regulate the substrate and surround- ing temperature. Note that prior stud- ies have discussed the nuances of laser-processed microstructures and defect distribution in Ni-base super- alloys [13-16] and Nb-base refractory alloys [17-20] . Based on the above calcula- tions, Fig. 4 shows a schematic of IN718 to C103 graded block that will be used for initial build trials. As indicated in the figure, the build height of two terminal compositions will be approximately 40 mm, while each of the intermediate four layers will be 5 mm tall. The overall block size of 100 x 25 x 10 mm may be subsequent- ly machined to extract specimens for characterization and testing of com- posite builds. Based on the literature, at least two or three build trials will likely be required to identify overlapping pro- cess parameters between the terminal alloys, as well as to conduct post-build analyses to ensure defect-free builds. A successful gradient build via one com- bination of gradient step size and path- way would be considered a sufficient criterion to move forward with post- build characterization. During this step, thermodynamic predictions could be validated, and heat treatment and me- chanical property response could also be investigated. CONCLUSION The proposed research effort delves into one of the many unique AM functionalities, by using a combi- national approach to generate hybrid microstructures tailored for site-spe- cific property response. This materi- al-agnostic concept is currently being studied on alloys that are used for high temperature and extreme environment applications—namely Ni-base superal- loys (IN718) and Nb-base refractory al- loys (C103), respectively. Preliminary CALPHAD-based thermodynamic pre- dictions show significant variations in freezing ranges and primary phases to form within the individual unicom- positional zones in a six-zone linear compositionally graded model. The equilibrium phase fractions at both room (25 o C) and elevated (1000 o C) tem- peratures point toward essentially TABLE 3 — PHASE PREDICTION AND CORRESPONDING PHASE FRACTION AT 1000°C FOR SIX-ZONE LINEAR COMPOSITIONALLY GRADED MODEL Compositional gradient Predicted phase and phase % at 1000°C Fraction of IN718 alloy Fraction of C103 alloy Liquid FCC Delta Laves, C14 Mu B2 BCC Other minor phases 1 0 2.2 97.8 0.8 0.2 2.2 30.4 38.7 21.7 7 0.6 0.4 6.8 6.2 85.5 1.5 0.4 0.6 49.3 27.5 6.9 16.3 0.2 0.8 27.5 11.5 8.4 52.6 0 1 100 Fig. 4 — Schematic of a six-zone compositionally graded structure with approximate block dimension.