Experimental analysis and thermodynamic calculations of an additively manufactured functionally graded material of V to
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Richard Otis, John Paul Borgonia, Robert Peter Dillon, Andrew A. Shapiro, and Bryan McEnerney Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA
Zi-Kui Liu and Allison M. Beesea) Department of Materials Science & Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, USA (Received 19 December 2017; accepted 26 March 2018)
Functionally graded materials (FGMs) in which the elemental composition intentionally varies with position can be fabricated using directed energy deposition additive manufacturing (AM). This work examines an FGM that is linearly graded from V to Invar 36 (64 wt% Fe, 36 wt% Ni). This FGM cracked during fabrication, indicating the formation of detrimental phases. The microstructure, composition, phases, and microhardness of the gradient zone were analyzed experimentally. The phase composition as a function of chemistry was predicted through thermodynamic calculations. It was determined that a significant amount of the intermetallic r-FeV phase formed within the gradient zone. When the r phase constituted the majority phase, catastrophic cracking occurred. The approach presented illustrates the suitability of using equilibrium thermodynamic calculations for the prediction of phase formation in FGMs made by AM despite the nonequilibrium conditions in AM, providing a route for the computationally informed design of FGMs.
I. INTRODUCTION
Functionally graded materials (FGMs) are a class of materials in which the chemistry or microstructure varies as a function of position within a component, resulting in location-specific material properties that are difficult to obtain using traditional manufacturing processes. Methods for producing metallic FGMs reported in the literature include ultrasonic welding,1 fusion welding,2 layer/disk remelting,3 and powder metallurgy.4 Directed energy deposition (DED) additive manufacturing (AM) may also be used to fabricate metallic FGMs.5–7 DED AM uses a heat source to create a melt pool in a substrate and deposited layers and delivers either powder or wire feedstock into the melt pool. In powder-based DED AM, multiple powder hoppers, filled with different powder feedstock, can be used, and the relative fractions of these powders delivered to the melt pool can be changed by a few volume percent per layer. DED is ideal for fabricating FGMs as these incremental changes can be used to tailor the chemistry of the component as a function of position. The present study investigates an FGM that grades from V to Invar 36 (henceforth referred to as Invar). a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.92
V strengthens ferronickel alloys8; thus, it could be used to produce an FGM with graded strength. Additionally, a successful FGM between V and Invar could be incorporated as an intermediate gradient within a Ti– 6Al–4V to Invar FGM since a linear gradient between these latter two alloys results in extensive cracking.7 Grading from Ti–6Al–4V to Invar would be ideal f
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