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Thermal post-treatment of Alloy 718 produced by electron beam melting
University West, Department of Engineering Science, Division of Subtractive and Additive Manufacturing. (PTW)ORCID iD: 0000-0001-5676-7903
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 [en]
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: urn:nbn:se:hv:diva-16003ISBN: 978-91-88847-77-5 (print)ISBN: 978-91-88847-76-8 (electronic)OAI: oai:DiVA.org:hv-16003DiVA, id: diva2:1497154
Public defence
2020-11-26, F104 Albertsalen, Högskolan Väst, Trollhättan, 13:15 (English)
Opponent
Supervisors
Funder
Knowledge FoundationAvailable from: 2020-11-05 Created: 2020-11-04 Last updated: 2021-02-03
List of papers
1. The Effect of Location and Post-treatment on the Microstructure of EBM-Built Alloy 718
Open this publication in new window or tab >>The Effect of Location and Post-treatment on the Microstructure of EBM-Built Alloy 718
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2018 (English)In: Proceedings of the 9th International Symposium on Superalloy 718 & Derivatives: Energy, Aerospace, and Industrial Applications / [ed] Ott, E., Liu, X., Andersson, J., Bi, Z., Bockenstedt, K., Dempster, I., Groh, J., Heck, K., Jablonski, P., Kaplan, M., Nagahama, D. and Sudbrack, C., Springer, 2018, p. 115-129Conference paper, Published paper (Refereed)
Abstract [en]

Additive manufacturing (AM) of Ni-based superalloys such as Alloy 718 may obviate the need for difficult machining and welding operations associated with geometrically intricate parts, thus potentially expanding design possibilities and facilitating cost-effective manufacture of complex components. However, processing AM builds completely free from defects, which may impair mechanical properties such as fatigue and ductility, is challenging. Anisotropic properties, microstructural heterogeneities and local formation of undesired phases are additional concerns that have motivated post-treatment of AM builds. This work investigates the microstructural changes associated with post-treatment of Alloy 718 specimens produced by Electron Beam Melting (EBM) for as-built microstructures at 3 build heights: near base plate, in the middle of build and near the top of the build. Two different post-treatment conditions, hot isostatic pressing (HIP) alone and a combined HIP with solutionising and two-step aging were examined and compared to the results for the as-built condition. The influence of various post-treatments on minor phase distributions (δ, γ″, carbides), overall porosity, longitudinal grain widths and Vickers microhardness was considered. The HIP treatment led to significant reduction in overall porosity and dissolution of δ phase, which led to appreciable grain growth for both post-treatment conditions. The variation in hardness noted as a function of build height for the as-built specimens was eliminated after post-treatment. Overall, the hardness was found to decrease after HIP and increase after the full HIP, solutionising and aging treatment, which was attributed to dissolution of γ″ during HIP and its re-precipitation in subsequent heat treatment steps.

Place, publisher, year, edition, pages
Springer, 2018
Series
The Minerals, Metals & Materials Series, ISSN 2367-1181, E-ISSN 2367-1696
Keywords
Additive manufacturing, Electron beam melting, Alloy 718, HIP, Heat treatment, Microstructure, XRD, Hardness, δ, γ″, Carbide
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering; Production Technology
Identifiers
urn:nbn:se:hv:diva-12346 (URN)10.1007/978-3-319-89480-5_6 (DOI)000445800500006 ()2-s2.0-85055988720 (Scopus ID)978-3-319-89479-9 (ISBN)978-3-319-89480-5 (ISBN)
Conference
9th International Symposium on Superalloy 718 & Derivatives, Energy, Aerospace, and Industrial Applications, Pittsburgh, Pennsylvania, USA, 3-6 June, 2018
Funder
Knowledge Foundation, 20160281
Available from: 2018-10-26 Created: 2018-10-26 Last updated: 2020-11-04Bibliographically approved
2. Effect of Different Post-treatments on the Microstructure of EBM-Built Alloy 718
Open this publication in new window or tab >>Effect of Different Post-treatments on the Microstructure of EBM-Built Alloy 718
2019 (English)In: Journal of materials engineering and performance (Print), ISSN 1059-9495, E-ISSN 1544-1024, Vol. 28, no 2, p. 673-680Article in journal (Refereed) Published
Abstract [en]

Electron beam melting (EBM) of Alloy 718 is of rapidly growing interest as it allows cost-effective production of complex components. However, the inherent flaws in the component in as-built state are of concern in view of the severe working conditions in which Alloy 718 components typically operate. The present work entails an investigation of changes in microstructure that accompany some post-treatments that are being widely considered to address defects in EBM processed Alloy 718. The effect of two different post-treatments, namely hot isostatic pressing (HIP) and a combined HIP + heat treatment (HT) carried out inside the HIP vessel, have been studied and results from as-built and post-treated specimens were compared in terms of porosity/lack-of-fusion, microstructure, phase constitution (NbC content, ÎŽ-phase) and micro-hardness. Post-treatment resulted in reduction in defect content by more than an order of magnitude. HIPing led to complete dissolution of ÎŽ phase. In comparison to as-built material, HIPed specimens exhibited significant drop in hardness. However, a sharp ‘recovery’ of hardness to yield values higher than in as-built condition was observed after HIP + HT and can be attributed to precipitation of γ′′ phase. © 2018, The Author(s).

Keywords
3D printers, Cost effectiveness, Defects, Electron beam melting, Electron beams, Hardness, Heat treatment, Hot isostatic pressing, Microhardness, Microstructure, Alloy 718, Complete dissolution, Complex components, Cost-effective production, Defect contents, Phase constitution, Post treatment, Yield value, Niobium compounds
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-13133 (URN)10.1007/s11665-018-3712-0 (DOI)000458782100012 ()2-s2.0-85055983075 (Scopus ID)
Funder
Knowledge Foundation, 20160281
Note

First Online: 29 October 2018

Available from: 2018-11-19 Created: 2018-11-19 Last updated: 2020-11-04Bibliographically approved
3. Effect of post-treatments under hot isostatic pressure on microstructural characteristics of EBM-built Alloy 718
Open this publication in new window or tab >>Effect of post-treatments under hot isostatic pressure on microstructural characteristics of EBM-built Alloy 718
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2019 (English)In: Additive Manufacturing, ISSN 2214-8604, E-ISSN 2214-7810, Vol. 28, p. 727-737Article in journal (Refereed) Published
Abstract [en]

Electron beam melting (EBM) has emerged as an important additive manufacturing technique. In this study, Alloy 718 produced by EBM was investigated in as-built and post-treated conditions for microstructural characteristics and hardness. The post-treatments investigated were hot isostatic pressing (HIP) and combined HIP + heat treatment (HIP + HT) carried out as a single cycle inside the HIP vessel. Both the post-treatments resulted in significant decrease in defects inevitably present in the as-built material. The columnar grain structure of the as-built material was found to be maintained after post-treatment, with some sporadic localized grain coarsening noted. Although HIP led to complete dissolution of δ and γ′′ phase, stable NbC and TiN (occasionally present) particles were observed in the post-treated specimens. Significant precipitation of γ′′ phase was observed after HIP + HT, which was attributed to the two-step aging heat treatment carried out during HIP + HT. The presence of γ′′ phase or otherwise was correlated to the hardness of the material. While the HIP treatment resulted in drop in hardness, HIP + HT led to 'recovery' of the hardness to values exceeding those exhibited by the as-built material. © 2019 Elsevier B.V.

Keywords
3D printers; Additives; Coarsening; Electron beam melting; Electron beams; Hardness; Heat treatment; Hot isostatic pressing; Sintering; Titanium nitride, Alloy 718; Columnar grain structure; Complete dissolution; Grain coarsening; Hot isostatic pressure; Manufacturing techniques; Micro-structural characteristics; Post treatment, Niobium compounds
National Category
Manufacturing, Surface and Joining Technology Metallurgy and Metallic Materials
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-14457 (URN)10.1016/j.addma.2019.06.002 (DOI)000492672300070 ()2-s2.0-85067846675 (Scopus ID)
Funder
Knowledge Foundation, 20160281
Available from: 2019-10-02 Created: 2019-10-02 Last updated: 2021-06-11Bibliographically approved
4. Can Appropriate Thermal Post-Treatment Make Defect Content in as-Built Electron Beam Additively Manufactured Alloy 718 Irrelevant?
Open this publication in new window or tab >>Can Appropriate Thermal Post-Treatment Make Defect Content in as-Built Electron Beam Additively Manufactured Alloy 718 Irrelevant?
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2020 (English)In: Materials, E-ISSN 1996-1944, Vol. 13, no 3, article id 536Article in journal (Refereed) Published
Abstract [en]

Electron beam melting (EBM) is gaining rapid popularity for production of complex customized parts. For strategic applications involving materials like superalloys (e.g., Alloy 718), post-treatments including hot isostatic pressing (HIPing) to eliminate defects, and solutionizing and aging to achieve the desired phase constitution are often practiced. The present study specifically explores the ability of the combination of the above post-treatments to render the as-built defect content in EBM Alloy 718 irrelevant. Results show that HIPing can reduce defect content from as high as 17% in as-built samples (intentionally generated employing increased processing speeds in this illustrative proof-of-concept study) to <0.3%, with the small amount of remnant defects being mainly associated with oxide inclusions. The subsequent solution and aging treatments are also found to yield virtually identical phase distribution and hardness values in samples with vastly varying as-built defect contents. This can have considerable implications in contributing to minimizing elaborate process optimization efforts as well as slightly enhancing production speeds to promote industrialization of EBM for applications that demand the above post-treatments.

Keywords
additive manufacturing, electron beam melting, defects, microstructure, hardness, alloy 718, hot isostatic pressing, post-treatment
National Category
Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:hv:diva-14926 (URN)10.3390/ma13030536 (DOI)000515503100043 ()2-s2.0-85079600396 (Scopus ID)
Funder
Knowledge Foundation, 20160281
Available from: 2020-01-30 Created: 2020-01-30 Last updated: 2024-07-04
5. Response of different electron beam melting produced Alloy 718 microstructures to thermal post-treatments
Open this publication in new window or tab >>Response of different electron beam melting produced Alloy 718 microstructures to thermal post-treatments
2020 (English)In: Materials Characterization, ISSN 1044-5803, E-ISSN 1873-4189, Vol. 167, article id 110498Article in journal (Refereed) Published
Abstract [en]

Electron beam melting (EBM) was used to produce Alloy 718 specimens with different microstructures (columnar, equiaxed and a combination thereof) by varying the process parameters. The present study aimed at assessing the response of such varying as-built microstructures to identical thermal post-treatments, which included hot isostatic pressing (HIPing) followed by heat treatment involving solution treatment and aging. The effect of these treatments on defect content, grain structure, hardness and phase constitution in the specimens was specifically analysed. Despite differences in defect content of as-built specimens with distinct microstructures, HIPing was effective in closing defects leading to samples exhibiting similar density. After HIPing, grains with equiaxed morphology or columnar grains with lower aspect ratio showed higher tendency for grain growth in comparison to the columnar grains with higher aspect ratio. The various factors affecting the stability of grains during HIPing of builds with distinct microstructures were investigated. These factors include texture, grain size, and secondary phase particles. The carbide sizes in the different as-built samples varied but were found to be largely unaffected by the post-treatments. Solution treatment following HIPing led to greater precipitation of grain boundary ÎŽ phase in regions with coarser grains than the smaller ones. After HIPing and heat treatment, all specimens exhibited similar precipitation of γ″ phase regardless of their grain morphology in the as-built condition. © 2020 The Author(s)

Keywords
Carbides; Electron beam melting; Electron beams; Grain boundaries; Grain growth; Grain size and shape; Heat treatment; Hot isostatic pressing; Morphology; Textures, Coarser grains; Defect contents; Grain morphologies; Phase constitution; Process parameters; Secondary phase particles; Solution treatments; Thermal post-treatments, Aspect ratio, Additive manufacturing, Alloy 718, Columnar, Equiaxed, Post-Treatment
National Category
Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:hv:diva-15744 (URN)10.1016/j.matchar.2020.110498 (DOI)000558805700044 ()2-s2.0-85087961064 (Scopus ID)
Funder
Knowledge Foundation, 20160281
Available from: 2020-08-25 Created: 2020-08-25 Last updated: 2020-11-04Bibliographically approved
6. As-Built and Post-treated Microstructures of an Electron Beam Melting (EBM) Produced Nickel-Based Superalloy
Open this publication in new window or tab >>As-Built and Post-treated Microstructures of an Electron Beam Melting (EBM) Produced Nickel-Based Superalloy
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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.

Keywords
Microstructures, Electron Beam Melting
National Category
Metallurgy and Metallic Materials
Research subject
Production Technology
Identifiers
urn:nbn:se:hv:diva-16004 (URN)10.1007/s11661-020-06037-z (DOI)000579330600002 ()2-s2.0-85092712120 (Scopus ID)
Funder
Knowledge Foundation, 20160281
Available from: 2020-11-04 Created: 2020-11-04 Last updated: 2021-04-29Bibliographically approved

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Goel, Sneha

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