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 7 third alloy composition may be select- ed for nonlinear compositional gradient setup, similar to the use of strike coat- ings during metal plating operations. Performance benefit: The goal is to eliminate abrupt property transitions at dissimilar interfaces along with allevi- ating residual stress across large struc- tures. Apart from alleviating coefficient of thermal expansion (CTE)-driven ther- mal mismatch between different alloy systems, smooth compositional transi- tions can also exhibit performance ben- efits for static, dynamic, and high-speed impact property response. In short, the graded concept can help eliminate the weak link in dissimilar welded parts, which is often the weld zone itself. Gradient design adapted for AM: While monolithic alloy use has been employed successfully, this is primarily due to the high strength and high tem- perature capabilities of nickel-base su- peralloys and refractory alloys. Further benefit for structural components may be realized when site-specific chemis- try—tailored to desired performance requirements—is employed. Additional functionalities enabled by AM for topo- logical optimization and manufacturing unitized components with complex ge- ometries may be enhanced by coupling advanced design tools with fabrication of multi-material parts [6] . Compared to conventional cast- ing, the directed energy deposition (DED) additive modality reduces ini- tial investment by not requiring casting molds. In addition, DED offers near-net shape structures, which significantly reduce machining and material waste compared to forging. High-speed print- ing via DED can also enable printing of large-scale structures at a fast produc- tion rate. The goal of this effort is to fabricate unitized parts with the pro- posed technology, with a reduction in part count to decrease supply chain costs. The unique challenges of addi- tively manufacturing high tempera- ture alloys involve crack sensitivity with regard to as-built process param- eters and post-processing treatments. While the thermal gradients and cool- ing rates in and around the melt pool control aspects of the liquid-to-solid phase transformation, spatially var- ied thermal cycles may result in resid- ual stresses and distortion in solidified components (e.g., due to subsequent solid-state transformations) [7,8] . Material properties and deforma- tion characteristics also depend on the actual alloy class being studied. For example, low gamma prime Ni-base superalloys like IN718 and IN625 are readily weldable, and AM build param- eters dictate thermal gradients and cooling rate dependencies on micro- structure. However, the thermal gradi- ent effect and cooling rate-dependent transformation kinetics are quite dif- ferent for Nb-base refractory alloys like C103 [9,10] . Figure 2 is an illustration of varied operating temperatures among different alloy classes [9] . Note that for monolithic builds, the alloy of choice re- stricts us, either by cost or performance, to operate within a specific tempera- ture range. The proposed composition- al transition from a high-temperature Ni-base superalloy (e.g., IN718) to an extreme temperature-capable Nb-base refractory alloy (e.g., C103) will allow fine-tuning of microstructures for zone- based performance over a wider tem- perature range. In addition, this concept helps to simultaneously investigate the complex phase transformations, deformation mechanisms, and interrelationships across graded alloy chemistries. This effort is relevant to a wide range of ap- plications in aviation (structural en- gine components), space (access, high velocity) and energy fields (marine, nuclear, and renewables). For exam- ple, NASA and Boeing have adopted zone-based deployment of lightweight and high-temperature materials for the rocket nozzles and hypersonic air breathing structures (X-15, X-43A, and X-51A), respectively [11,12] . APPROACH AND RESULTS The proposed study of developing a graded structure from IN718 to C103 alloy may be broken down into the fol- lowing categories: (a) find thermody- namically stable, kinetically feasible gradient pathways between terminal alloys from two different alloy class- es—Ni-base superalloy (IN718) used for high temperature applications and Nb-base refractory alloy (C103) capable of operating at extreme temperatures; (b) adopt optimal build strategies to ad- ditively manufacture crack-free graded structures with hybrid microstructures; (c) understand phase transformation, microstructural evolution, and varia- tion in residual stress as a function of thermal history (as-built and post-pro- cessed states); and (d) correlate these with the mechanical property response of graded specimens. Keep in mind that this is awork in progress and the current manuscript primarily deals with the first task, which is CALPHAD-based analyses of IN718 to C103 graded structures. Design of the gradient pathway between two terminal alloys is a crit- ical step as it ensures build compat- ibility, both thermodynamically and Fig. 2 — Tensile strengths of various alloy classes as a function of temperature [9] .