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As-Built and Post-treated Microstructures of an Electron Beam Melting (EBM) Produced Nickel-Based Superalloy
University West, Department of Engineering Science, Division of Subtractive and Additive Manufacturing. (PTW)ORCID iD: 0000-0001-5676-7903
Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, India (IND).
Xi’an Jiaotong University, School of Materials Science and Engineering, Xi’an, 710049, China (CHN).
Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai, 400076, India (IND).
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2020 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 51, no 12, p. 6546-6559Article in journal (Refereed) Published
Abstract [en]

The microstructures of an electron beam melted (EBM) nickel-based superalloy (Alloy 718) were comprehensively investigated in as-built and post-treated conditions, with particular focus individually on the contour (outer periphery) and hatch (core) regions of the build. The hatch region exhibited columnar grains with strong texture in the build direction, while the contour region had a mix of columnar and equiaxed grains, with no preferred crystallographic texture. Both regions exhibited nearly identical hardness and carbide content. However, the contour region showed a higher number density of fine carbides compared to the hatch. The as-built material was subjected to two distinct post-treatments: (1) hot isostatic pressing (HIP) and (2) HIP plus heat treatment (HIP + HT), with the latter carried out as a single cycle inside the HIP vessel. Both post-treatments resulted in nearly an order of magnitude decrease in defect content in hatch and contour regions. HIP + HT led to grain coarsening in the contour, but did not alter the microstructure in the hatch region. Different factors that may be responsible for grain growth, such as grain size, grain orientation, grain boundary curvature and secondary phase particles, are discussed. The differences in carbide sizes in the hatch and contour regions appeared to decrease after post-treatment. After HIP + HT, similar higher hardness was observed in both the hatch and contour regions compared to the as-built material.

Place, publisher, year, edition, pages
2020. Vol. 51, no 12, p. 6546-6559
Keywords [en]
Microstructures, Electron Beam Melting
National Category
Metallurgy and Metallic Materials
Research subject
Production Technology
Identifiers
URN: urn:nbn:se:hv:diva-16004DOI: 10.1007/s11661-020-06037-zISI: 000579330600002Scopus ID: 2-s2.0-85092712120OAI: oai:DiVA.org:hv-16004DiVA, id: diva2:1497146
Funder
Knowledge Foundation, 20160281Available from: 2020-11-04 Created: 2020-11-04 Last updated: 2021-04-29Bibliographically approved
In thesis
1. Thermal post-treatment of Alloy 718 produced by electron beam melting
Open this publication in new window or tab >>Thermal post-treatment of Alloy 718 produced by electron beam melting
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Additive manufacturing (AM) has emerged as a disruptive technology and it is a vital part in the present era of fourth industrial revolution, Industry 4.0.Electron beam melting (EBM), a metal AM process, has received considerable industrial attention for near net shape manufacture of complex geometries with traditionally difficult-to-machine materials. EBM production of Alloy 718, a nickel-iron based superalloy possessing good mechanical and corrosion properties at elevated temperatures, is particularly promising for aerospace and energy sectors. However, EBM Alloy 718 builds are typically characterized by presence of inevitable defects and anisotropy, warranting post-processing thermal-treatments (post-treatments) to ensure that the components eventually meet the critical servicerequirements. The existing post-treatment standards include hot isostatic pressing (HIPing) over the temperature range of 1120°C-1185°C, followed by solution treatment (ST) and a two-step (‘8+8’ hours) aging under conditions conventionally adopted for cast and wrought Alloy 718, and no effort has yet been invested in optimizing post-treatment schedules specifically for EBM Alloy 718. Consequently, the objective of this work was to systematically investigate the response of EBM-built material to eachof the post-treatment steps to develop an improved understanding of howthe microstructure evolves with time during each step, since such knowledge can lay the foundation for optimizing the post-treatment protocol.Through study of microstructure and mechanical property assessment it was found that the temperature during HIPing can be reduced to 1120°C compared to the common practice employing higher temperatures. In addition, HIPing also caused complete dissolution of δ and γ"/γ' phases, promoted homogenization and resulted in drop in hardness but had no evident effect on the carbides and inclusions such as TiN and Al2O3 present in the as-built material. Subjecting EBM Alloy 718 to ST and two-step agingled to precipitation of δ phase and γ"/γ' phases, respectively.

The evolution of microstructure during ST and two-step aging was also systematically investigated. Progressive precipitation and growth of grain boundary δ phase precipitates was observed during the entire 1 hour duration of ST, with samples not subjected to prior-HIPing exhibiting higher amount of the δ phase precipitation during ST. During the two-stepaging, detailed investigation of microstructure evolution and hardness changes showed that, particularly the conventional ‘8+8’ hour long two-stepaging treatment can be shortened to a ‘4+1’ hours treatment. Such shortened treatment was observed to be robust when applied to various kinds of EBM builds. Another approach for shortening post-treatment by integrating HIPing and HT inside the HIP vessel was also successfully implemented. These approaches with shortened post-treatment were also found to not compromise the mechanical response of EBM Alloy 718. Further shortening of the typical long thermal post-treatment cycle, through reduction in HIPing time from 4 hours to 1 hour and possible elimination of ST, also appears promising.

Place, publisher, year, edition, pages
Trollhättan: University West, 2020. p. 95
Series
PhD Thesis: University West ; 41
Keywords
Additive Manufacturing, Electron Beam Melting; Alloy 718; Hot Isostatic Pressing; Heat Treatment; Microstructure Evolution; Mechanical Properties
National Category
Metallurgy and Metallic Materials
Research subject
Production Technology
Identifiers
urn:nbn:se:hv:diva-16003 (URN)978-91-88847-77-5 (ISBN)978-91-88847-76-8 (ISBN)
Public defence
2020-11-26, F104 Albertsalen, Högskolan Väst, Trollhättan, 13:15 (English)
Opponent
Supervisors
Funder
Knowledge Foundation
Available from: 2020-11-05 Created: 2020-11-04 Last updated: 2021-02-03

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Goel, SnehaJoshi, Shrikant V.

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