Anomalous Notch Rupture Behavior of Nickel-Based Superalloy Inconel 718 Due to Fabrication by Additive Manufacturing
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The precipitation-hardened nickel-based superalloy Inconel 718 is a common alloy used for powder-based metallic additive manufacturing (AM). Although Inconel 718 was designed for high-temperature service, there are limited published data on the elevated temperature properties of Inconel 718 prepared by AM techniques. Several studies of the elevated temperature mechanical properties of AM 718 have tested materials with retained as-produced, unrecrystallized microstructures that were subjected to solution treatment and aging.[1–4] Others succeeded in using electron-beam processing conditions to form controlled equiaxed or columnar and highly oriented microstructure after hot isostatic pressing (HIP) at a temperature (1200 C) sufficient to recrystallize the material prior to heat treatment.[5–8] These results include both laser-based and electron-beam powder-bed processes, showing that powder-bed AM 718 has tensile,[2,5,6,9] low-cycle fatigue[7], and creep properties[1,3,4,8] that are comparable to
DAVID B. WITKIN, TAIT D. MCLOUTH, and GLENN E. BEAN are with The Aerospace Corporation, P.O. Box 92957, Los Angeles CA, 90009-2957. Contact e-mail: [email protected] ROBERT W. HAYES is with Metals Technology, Inc., 19801 Nordhoff Street, Northridge, CA 91324. Manuscript submitted February 1, 2019. Article published online May 30, 2019 3458—VOLUME 50A, AUGUST 2019
wrought properties when tested at 650 C only if it has an equiaxed microstructure. Otherwise, it exhibits a high degree of anisotropy based on the build direction, has inferior properties if left unrecrystallized, or retains the highly textured microstructures common to metallic AM fabrication. Use of Inconel 718 in aerospace grade components requires validation of material quality and capability, including tensile testing at room temperature and 650 C and stress rupture testing at 650 C,[10] which is intended to demonstrate the material’s quality at different strain rates. By inference from the previously discussed results, it is likely that parts made by powder-bed AM techniques will require high-temperature heat treatments to transform the c phase microstructure from its as-built condition in addition to solution treatment and aging as a condition of use in aerospace applications. In the present work, Inconel 718 was fabricated by laser powder-bed fusion (LPBF) and HIP’ed to reduce or eliminate porosity and recrystallize the microstructure. Parts were built in an argon atmosphere using a Concept Laser M2 LPBF apparatus using build parameters as previously described.[11,12] One set of samples (Sample Set 1) consisted of specimens built with gage sections of 6.3 mm diameter in either vertical, horizontal, or 45 deg-inclined orientations with respect to the build chamber axes and were tensile tested at room temperature and 650 C and stress rupture tested at 759 MPa and 650 C. To directly compare the influence of heat treatment and section thickness, a second set of vertical samples (Sample Set 2) was built as either near net-shaped dogbones with a gage dia
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