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J. Manuf. Mater. Process. 2018, 2(2), 30; https://doi.org/10.3390/jmmp2020030

Microstructural and Microhardness Evolution from Homogenization and Hot Isostatic Pressing on Selective Laser Melted Inconel 718: Structure, Texture, and Phases

1
Chemical Engineering Department, Khalifa University of Science and Technology, Masdar Institute, Masdar City, Abu Dhabi P.O. Box 54224, UAE
2
Department of Mechanical Engineering, Tafila Technical University, Tafila 66110, Jordan
3
Department of Mechanical Engineering, École de Technologie Supérieure, 1100 Notre-Dame Street West, Montreal, QC H3C 1K3, Canada
4
Mechanical Engineering Department, Concordia University, 15151 rue Sainte Catherine Ouest, Montreal, QC H3G 2W1, Canada
*
Author to whom correspondence should be addressed.
Received: 11 April 2018 / Revised: 8 May 2018 / Accepted: 9 May 2018 / Published: 16 May 2018
(This article belongs to the Special Issue Additive Manufacturing)
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Abstract

In this work, the microstructure, texture, phases, and microhardness of 45° printed (with respect to the build direction) homogenized, and hot isostatically pressed (HIP) cylindrical IN718 specimens are investigated. Phase morphology, grain size, microhardness, and crystallographic texture at the bottom of each specimen differ from those of the top due to changes in cooling rate. High cooling rates during the printing process generated a columnar grain structure parallel to the building direction in the as-printed condition with a texture transition from (001) orientation at the bottom of the specimen to (111) orientation towards the specimen top based on EBSD analysis. A mixed columnar and equiaxed grain structure associated with about a 15% reduction in texture is achieved after homogenization treatment. HIP treatment caused significant grain coarsening, and engendered equiaxed grains with an average diameter of 154.8 µm. These treatments promoted the growth of δ-phase (Ni3Nb) and MC-type brittle (Ti, Nb)C carbides at grain boundaries. Laves phase (Fe2Nb) was also observed in the as-printed and homogenized specimens. Ostwald ripening of (Ti, Nb)C carbides caused excessive grain growth at the bottom of the HIPed IN718 specimens, while smaller grains were observed at their top. Microhardness in the as-fabricated specimens was 236.9 HV and increased in the homogenized specimens by 19.3% to 282.6 HV due to more even distribution of secondary precipitates, and the nucleation of smaller grains. A 36.1% reduction in microhardness to 180.5 HV was found in the HIPed condition due to   γ phase dissolution and differences in grain morphology. View Full-Text
Keywords: Inconel 718; additive manufacturing; 3D printing; selective laser melting (SLM); hot isostatic pressing (HIP); homogenization; hardness; precipitation; microstructure; texture Inconel 718; additive manufacturing; 3D printing; selective laser melting (SLM); hot isostatic pressing (HIP); homogenization; hardness; precipitation; microstructure; texture
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).
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Seede, R.; Mostafa, A.; Brailovski, V.; Jahazi, M.; Medraj, M. Microstructural and Microhardness Evolution from Homogenization and Hot Isostatic Pressing on Selective Laser Melted Inconel 718: Structure, Texture, and Phases. J. Manuf. Mater. Process. 2018, 2, 30.

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