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Microstructure Modelling of Additive Manufacturing of Alloy 718
University West, Department of Engineering Science, Division of Subtractive and Additive Manufacturing. (PTW)ORCID iD: 0000-0002-4087-6467
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [sv]

In recent years, additive manufacturing (AM) of Alloy 718 has received increasing interest in the field of manufacturing engineering because of its attractive features compared with those of conventional manufacturing methods. Nevertheless, owing to the inherent nature of the process, the build material is exposed to complex thermal conditions that affect the microstructure. In addition, the post heattreatments applied to the built component further cause microstructural changes. Thus, obtaining the desired microstructure that gives the desired properties is still a challenging task. Therefore, understanding the microstructure formation during the build and subsequent post-heat treatment is important and is the objective of this thesis work.

To this end, a computational modelling approach was used that combines multiphase-field modelling with transformation kinetics modelling. Two different AM processes, laser metal powder directed energy deposition (LM-PDED) and electron beam powder bed fusion (EB-PBF), were considered in this study.Based on the modelling work, it was observed that solidification conditions (thermal gradients and cooling rates) that occur during the AM process have an impact on the as-solidified microstructure in Alloy 718 and the resultant Laves phase formation. With an increase in cooling rate, the Laves phase volume fraction becomes lower and the morphology tends to become discrete particles,which is important for resisting the formation of liquation cracks in Alloy 718. It was also found that the precipitates formed during the solidification process did not undergo any significant change during subsequent thermal cycles associated with the deposition of subsequent layers, given that the deposition of the subsequent layer does not increase the global temperature of the build to> 600 °C. If the global temperature increases above 600 °C, then phase changes are expected, depending on the temperature value. In the case of the EB-PBF process, the high build temperature maintained in the build chamber resulted in an ‘‘in situ’’ heat treatment, which had a homogenisation effect on the as-solidified microstructure because of the smaller dendrite spacing and relatively low Lavesphase size. In the case of the LM-PDED, the microsegregation of composition observed in the as-built microstructure was shown to change the equilibrium conditions and precipitation kinetics of Alloy 718. As a result, excess precipitationof γ'/γ″ and δ was observed in the interdendritic region compared with the dendrite core, depending on the type of heat treatment used.

In addition, modelling was performed to evaluate the elastic properties of EB-PBF Alloy 718. To this end, crystallographic orientation data gathered from EBSD data and single-crystal elastic constants were used. The prediction showed good agreement with published literature data. The hatch (bulk) region of the EB-PBF samples showed significant anisotropic elastic properties because of the strong crystallographic texture observed in the microstructure. The lowest Young’s modulus was observed along the build direction. Normal to the build direction, the elastic properties were shown to be isotropic. Overall, the elastic behaviour of the hatch region was similar to that of a transversely isotropic case

Place, publisher, year, edition, pages
Trollhättan: University West , 2020. , p. 89
Series
PhD Thesis: University West ; 43
Keywords [en]
Phase-Field Modelling; Additive Manufacturing; Phase Transformation; Solidification; Heat Treatment; Superalloy
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
URN: urn:nbn:se:hv:diva-16118ISBN: 978-91-88847-83-6 (print)ISBN: 978-91-88847-82-9 (electronic)OAI: oai:DiVA.org:hv-16118DiVA, id: diva2:1509951
Public defence
2020-12-16, 10:00 (English)
Opponent
Supervisors
Available from: 2020-12-15 Created: 2020-12-15
List of papers
1. Microstructure modelling of laser metal powder directed energy deposition of alloy 718
Open this publication in new window or tab >>Microstructure modelling of laser metal powder directed energy deposition of alloy 718
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2019 (English)In: Additive Manufacturing, ISSN 2214-8604, E-ISSN 2214-7810, Vol. 25, p. 357-364Article in journal (Refereed) Published
Abstract [en]

A multi-component and multi-phase-field modelling approach, combined with transformation kinetics modelling, was used to model microstructure evolution during laser metal powder directed energy deposition of Alloy 718 and subsequent heat treatments. Experimental temperature measurements were utilised to predict microstructural evolution during successive addition of layers. Segregation of alloying elements as well as formation of Laves and δ phase was specifically modelled. The predicted elemental concentrations were then used in transformation kinetics to estimate changes in Continuous Cooling Transformation (CCT) and Time Temperature Transformation (TTT) diagrams for Alloy 718. Modelling results showed good agreement with experimentally observed phase evolution within the microstructure. The results indicate that the approach can be a valuable tool, both for improving process understanding and for process development including subsequent heat treatment.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Phase-field, DED, Heat treatment, Thermal cycle, Modelling
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology; ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-13195 (URN)10.1016/j.addma.2018.11.024 (DOI)000456378800034 ()2-s2.0-85057193791 (Scopus ID)
Funder
Knowledge Foundation
Note

Funders: European Regional Development Fund for project 3Dprint

Available from: 2018-12-12 Created: 2018-12-12 Last updated: 2021-06-11Bibliographically approved
2. Modelling of anisotropic elastic properties in alloy 718 built by electron beam melting
Open this publication in new window or tab >>Modelling of anisotropic elastic properties in alloy 718 built by electron beam melting
2018 (English)In: Materials Science and Technology, ISSN 0267-0836, E-ISSN 1743-2847, Vol. 34, no 5, p. 529-537Article in journal (Refereed) Published
Abstract [en]

Owing to the inherent nature of the process, typically material produced via electron beam melting (EBM) has a columnar microstructure. As a result of that, the material will have anisotropic mechanical properties. In this work, anisotropic elastic properties of EBM built Alloy 718 samples at room temperature were investigated by using experiments and modelling work. Electron backscatter diffraction data from the sample microstructure was used to predict the Young’s modulus. The results showed that the model developed in the finite element software OOF2 was able to capture the anisotropy in the Young’s modulus. The samples showed transversely isotropic elastic properties having lowest Young’s modulus along build direction. In addition to that, complete transversely isotropic stiffness tensor of the sample was also calculated. © 2018 Institute of Materials, Minerals and Mining.

Keywords
Elastic constants; Elasticity; Electron beam melting; Electron beams; Finite element method; Melting; Microstructure; Models, Alloy 718; Anisotropic elastic properties; Anisotropic mechanical properties; Columnar microstructures; EBSD; Electron back scatter diffraction; Finite element software; Transversely isotropic, Anisotropy
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-12134 (URN)10.1080/02670836.2018.1426258 (DOI)000428303200005 ()2-s2.0-85041234684 (Scopus ID)
Funder
Knowledge Foundation, SUMAN-Next
Note

Published online: 28 Jan 2018

Funders:  European Regional Development Fund, 3Dprint

Available from: 2018-02-20 Created: 2018-02-20 Last updated: 2020-12-15Bibliographically approved
3. Predicting the Microstructural Evolution of Electron Beam Melting of Alloy 718 with Phase-Field Modeling
Open this publication in new window or tab >>Predicting the Microstructural Evolution of Electron Beam Melting of Alloy 718 with Phase-Field Modeling
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2019 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 50A, no 5, p. 2527-2537Article in journal (Refereed) Published
Abstract [en]

Electron beam melting (EBM) is a powder bed additive manufacturing process where a powder material is melted selectively in a layer-by-layer approach using an electron beam. EBM has some unique features during the manufacture of components with high-performance superalloys that are commonly used in gas turbines such as Alloy 718. EBM has a high deposition rate due to its high beam energy and speed, comparatively low residual stresses, and limited problems with oxidation. However, due to the layer-by-layer melting approach and high powder bed temperature, the as-built EBM Alloy 718 exhibits a microstructural gradient starting from the top of the sample. In this study, we conducted modeling to obtain a deeper understanding of microstructural development during EBM and the homogenization that occurs during manufacturing with Alloy 718. A multicomponent phase-field modeling approach was combined with transformation kinetic modeling to predict the microstructural gradient and the results were compared with experimental observations. In particular, we investigated the segregation of elements during solidification and the subsequent "in situ" homogenization heat treatment at the elevated powder bed temperature. The predicted elemental composition was then used for thermodynamic modeling to predict the changes in the continuous cooling transformation and time-temperature transformation diagrams for Alloy 718, which helped to explain the observed phase evolution within the microstructure. The results indicate that the proposed approach can be employed as a valuable tool for understanding processes and for process development, including post-heat treatments. © 2019, The Author(s).

Keywords
3D printers; Deposition rates; Electron beam melting; Electron beams; Forecasting; Gas turbines; Microstructural evolution; Solid solutions; Temperature, Additive manufacturing process; Continuous cooling transformation; Elemental compositions; Layer-by-layer approaches; Microstructural development; Microstructural gradients; Transformation diagrams; Transformation kinetics, Heat treatment
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-13756 (URN)10.1007/s11661-019-05163-7 (DOI)000463991300038 ()2-s2.0-85062604965 (Scopus ID)
Funder
Knowledge FoundationEuropean Regional Development Fund (ERDF)
Available from: 2019-05-10 Created: 2019-05-10 Last updated: 2020-12-15Bibliographically approved
4. Toward a better understanding of phase transformations in additive manufacturing of Alloy 718
Open this publication in new window or tab >>Toward a better understanding of phase transformations in additive manufacturing of Alloy 718
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2020 (English)In: Materialia, E-ISSN 2589-1529, Vol. 13, article id 100862Article in journal (Refereed) Published
Abstract [en]

This paper presents a discussion on the phase-transformation aspects of additively manufactured Alloy 718 during the additive manufacturing (AM) process and subsequent commonly used post-heat treatments. To this end, fundamental theoretical principles, thermodynamic and kinetics modeling, and existing literature data are employed. Two different AM processes, namely, laser-directed energy deposition and electron-beam powder-bed fusion are considered. The general aspects of phase formation during solidification and solid state in Alloy 718 are first examined, followed by a detailed discussion on phase transformations during the two processes and subsequent standard post heat-treatments. The effect of cooling rates, thermal gradients, and thermal cycling on the phase transformation in Alloy 718 during the AM processes are considered. Special attention is given to illustrate how the segregated composition during the solidification could affect the phase transformations in the Alloy 718. The information provided in this study will contribute to a better understanding of the overall process–structure–property relationship in the AM of Alloy 718 718. © 2020

Keywords
3D printers; Additives; Solidification, Cooling rates; Directed energy; General aspects; Kinetics modeling; Literature data; Overall process; Phase formations; Post heat-treatment, Heat treatment
National Category
Metallurgy and Metallic Materials Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
urn:nbn:se:hv:diva-15772 (URN)10.1016/j.mtla.2020.100862 (DOI)000568771200009 ()2-s2.0-85089507104 (Scopus ID)
Funder
Knowledge FoundationEuropean Regional Development Fund (ERDF)
Available from: 2020-09-04 Created: 2020-09-04 Last updated: 2023-03-28Bibliographically approved

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Kumara, Chamara

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