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Research Papers

Correlation Between Microstructure and Mechanical Properties in an Inconel 718 Deposit Produced Via Electron Beam Freeform Fabrication

[+] Author and Article Information
Wesley A. Tayon

NASA Langley Research Center,
Hampton, VA 23681
e-mail: wesley.a.tayon@nasa.gov

Ravi N. Shenoy

Northrop Grumman,
Technical Services
Hampton, VA 23681

MacKenzie R. Redding

Engineering Physics Department,
University of Virginia,
Charlottesville, VA 22904

R. Keith Bird, Robert A. Hafley

NASA Langley Research Center,
Hampton, VA 23681

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received April 8, 2014; final manuscript received September 2, 2014; published online October 24, 2014. Assoc. Editor: Joseph Beaman.

This material is declared a work of the US Government and is not subject to copyright protection in the United States. Approved for public release; distribution is unlimited.

J. Manuf. Sci. Eng 136(6), 061005 (Oct 24, 2014) (7 pages) Paper No: MANU-14-1152; doi: 10.1115/1.4028509 History: Received April 08, 2014; Revised September 02, 2014

Electron beam freeform fabrication (EBF3), a metallic layer-additive manufacturing process, uses a high-power electron beam in conjunction with a metal feed wire to create a molten pool on a substrate, which on solidification produces a component of the desired configuration made of sequentially deposited layers. During the build-up of each solidified layer, the substrate is translated with respect to the electron beam and the feed wire. EBF3 products are similar to conventional cast products with regard to the as-deposited (AD) microstructure and typical mechanical properties. Inconel 718 (IN 718), a high-temperature superalloy with attractive mechanical and oxidation properties well suited for aerospace applications, is typically used in the wrought form. The present study examines the evolution of microstructure, crystallographic texture, and mechanical properties of a block of IN 718 fabricated via the EBF3 process. Specimens extracted out of this block, both in the AD and in a subsequently heat treated (HT) condition, were subjected to (1) microstructural characterization using scanning electron microscopy (SEM); (2) in-plane elastic modulus, tensile strength, and microhardness evaluations; and (3) crystallographic texture characterization using electron backscatter diffraction (EBSD). Salient conclusions stemming from this study are: (1) mechanical properties of the EBF3-processed IN 718 block are strongly affected by texture as evidenced by their dependence on orientation relative to the EBF3 fabrication direction, with the AD EBF3 properties generally being significantly reduced compared to wrought IN 718; (2) significant improvement in both strength and modulus of the EBF3 product to levels nearly equal to those for wrought IN 718 may be achieved through heat treatment.

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Figures

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Fig. 1

The IN 718 EBF3 block (AD condition) showing tensile specimen orientations L, T, and 45 deg in the L–T plane

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Fig. 2

Back-scattered SEM image of the dendritic microstructure in the S–T plane of the IN 718 EBF3 block (AD condition)

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Fig. 3

Microhardness (Knoop hardness number) evaluation of crystal anisotropy in the L–T plane in various conditions of IN 718

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Fig. 4

L-axis inverse pole figure microstructural map for IN 718 base plate with colors referenced to stereographic triangle. Thin black lines denote high-angle grain boundaries.

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Fig. 5

Inverse pole figure maps for IN 718 EBF3 block (AD condition), referenced to direction L (top) and direction T (bottom). Thin black lines denote high-angle grain boundaries. The legends apply to both maps.

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Fig. 6

001, 011, and 111 pole figures for the IN 718 EBF3 block in the AD condition

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Fig. 7

Elastic modulus maps for the L (top) and T (bottom) directions for IN 718 EBF3 block in the AD condition

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Fig. 8

In-plane (L–T) variations in EBSD-computed elastic modulus for the AD IN 718 EBF3 block

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Fig. 9

In-plane (L–T) variations in average Taylor factor in the AD IN 718 EBF3 block

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Fig. 10

L-axis inverse pole figure map in IN 718 EBF3 block in the HT condition

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Fig. 11

001, 011, and 111 pole figures for IN 718 EBF3 block (HT condition). The contours are scaled to match with the pole figures for the AD EBF3 condition in Fig. 6.

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Fig. 12

Variation in EBSD-computed elastic modulus for the IN 718 EBF3 block (HT condition)

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Fig. 13

Variations in average Taylor factor for the IN 718 EBF3 block (HT condition)

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