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Predicting the Microstructural Evolution of Electron Beam Melting of Alloy 718 with Phase-Field Modeling
University West, Department of Engineering Science, Division of Subtractive and Additive Manufacturing. (PTW)ORCID iD: 0000-0002-4087-6467
Linköping University, Division of Engineering Materials, Department of Management and Engineering, Linköping, 58183, Sweden.
Chalmers University of Technology, Department of Industrial and Materials Science, Göteborg, 412 96, Sweden.
NTNU, Department of Materials Science and Engineering, IMA, Alfred Getz vei 2, Trondheim, 7491, Norway.
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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).

Place, publisher, year, edition, pages
2019. Vol. 50A, no 5, p. 2527-2537
Keywords [en]
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: urn:nbn:se:hv:diva-13756DOI: 10.1007/s11661-019-05163-7ISI: 000463991300038Scopus ID: 2-s2.0-85062604965OAI: oai:DiVA.org:hv-13756DiVA, id: diva2:1315092
Funder
Knowledge FoundationEuropean Regional Development Fund (ERDF)Available from: 2019-05-10 Created: 2019-05-10 Last updated: 2020-12-15Bibliographically approved
In thesis
1. Microstructure Modelling of Additive Manufacturing of Alloy 718
Open this publication in new window or tab >>Microstructure Modelling of Additive Manufacturing of Alloy 718
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
Phase-Field Modelling; Additive Manufacturing; Phase Transformation; Solidification; Heat Treatment; Superalloy
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
urn:nbn:se:hv:diva-16118 (URN)978-91-88847-83-6 (ISBN)978-91-88847-82-9 (ISBN)
Public defence
2020-12-16, 10:00 (English)
Opponent
Supervisors
Available from: 2020-12-15 Created: 2020-12-15

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Kumara, ChamaraMoverare, JohanNylén, Per

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