Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Electron beam-powder bed fusion of Alloy 718: Effect of process parameters on microstructure evolution
University West, Department of Engineering Science, Division of Subtractive and Additive Manufacturing. (PTW)ORCID iD: 0000-0001-6610-1486
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Additive manufacturing (AM) is the technology of building 3D parts through layer-by-layer addition of material. Of the different types of AM techniques, electron beam-powder bed fusion (EB-PBF) has been used in this study. EB-PBF can build parts by melting metallic powders using an electron beam as the energy source. Compared to conventional manufacturing processes, EB-PBF offers a convenient approach and enhanced efficiency in producing customized and specific parts in the aerospace, space, automotive, and medical fields. In addition, the EB-PBF process is used to produce complex parts with less residual stress due to the high-temperature environment within the process.

This thesis has been divided into four stages. In the first stage, the behavior of Alloy 718 during the EB-PBF process as a function of different geometry-related parameters is examined by building single tracks adjacent to each other (track-by track) and single tracks on top of each other (single-wall samples). In this stage,the focus is on understanding the effect of successive thermal cycling on microstructural evolution. In the second stage, the effect of the position-related parameters–including the distance or gap between samples, height from the build plate (in the Z direction), and sample location on the build plate (in the X–Y plane) –on the microstructural characteristics, are revealed. These three position related parameters can have significant effects on the defect content and niobium rich phase fraction. In the third stage, the correlations between the main machinerelated parameters, geometric (melt pool width, track height, remelted depth, and contact angle), and microstructural (grain structure, niobium-rich phase fraction,and primary dendrite arm spacing) characteristics of a single track are delineated.

The results obtained in stages one to three were used as a guideline for the reduction of the internal–external defects and columnar-to-equiaxed transition(CET) in the grain structure of a typical cubic part. The final stage reveals two different strategies that were developed using machine-related parameters (scanning speed, beam current, focus offset, line offset, and line order number) to tailor the grain structures. All investigated parameters with respect to the proper selection of the processing window played a critical role in the solidification parameters (thermal gradient, growth rate, and cooling rate) on the solidification front, which could induce formation of more fine equiaxed grains.

Place, publisher, year, edition, pages
Trollhättan: University West , 2020. , p. 75
Series
PhD Thesis: University West ; 2020:37
Keywords [en]
Additive manufacturing; Electron beam-powder bed fusion; Microstructure evolution; Microstructure tailoring; Process understanding; Alloy 718
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
URN: urn:nbn:se:hv:diva-16013ISBN: 978-91-88847-65-2 (print)ISBN: 978-91-88847-64-5 (electronic)OAI: oai:DiVA.org:hv-16013DiVA, id: diva2:1499956
Public defence
2020-12-01, F131, Trollhättan, 10:00 (English)
Opponent
Supervisors
Available from: 2020-11-10 Created: 2020-11-10 Last updated: 2020-11-10Bibliographically approved
List of papers
1. Microstructure Development in Track-by-Track Melting of EBM-Manufactured Alloy 718
Open this publication in new window or tab >>Microstructure Development in Track-by-Track Melting of EBM-Manufactured Alloy 718
Show others...
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. 643-654Conference paper, Published paper (Refereed)
Abstract [en]

Electron beam melting (EBM) is a powder-bed fusion process within the group of additive manufacturing (AM) technology that is used to fabricate high performance metallic parts. Nickel-Iron base superalloys, such as Alloy 718, are subjected to successive heating and cooling at temperatures in excess of 800 °C during the EBM process. Characterization of the dendritic structure, carbides, Laves and δ-phase were of particular interest in this study. These successive thermal cycles influence the microstructure of the material resulting in a heterogeneous structure, especially in the building direction. Hence, the aim of this study was to gain increased fundamental understanding of the relationship between the processing history and the microstructure formed within a single layer. Different numbers of tracks with equal heights were for this purpose produced, varying from one to ten tracks. All tracks used the same process parameters regardless of number and/or position. Microstructure characteristics (sub-grain structure, grain structure and phases) were analyzed by optical microscopy, scanning electron microscopy equipped with energy disperse spectroscopy and electron backscatter diffraction. The direction of dendrites changed in the overlap zones within the tracks due to re-melting of material in the overlap zone. The primary dendrite arm spacings slightly increased along multi-tracks owing to a slight decrease in cooling rate by addition of the next tracks. Epitaxial growth of grains were observed in all samples due to partial re-melting of grains in previous layers and surface nucleation was also found to occur in all tracks.

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, Microstructure, Track-by-Track
National Category
Metallurgy and Metallic Materials Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering; Production Technology
Identifiers
urn:nbn:se:hv:diva-12345 (URN)10.1007/978-3-319-89480-5_42 (DOI)000445800500042 ()2-s2.0-85053834715 (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
Note

First Online: 13 May 2018

Available from: 2018-10-26 Created: 2018-10-26 Last updated: 2020-11-10Bibliographically approved
2. Influence of successive thermal cycling on microstructure evolution of EBM-manufactured alloy 718 in track-by-track and layer-by-layer design
Open this publication in new window or tab >>Influence of successive thermal cycling on microstructure evolution of EBM-manufactured alloy 718 in track-by-track and layer-by-layer design
Show others...
2018 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 160, p. 427-441Article in journal (Refereed) Published
Abstract [en]

Successive thermal cycling (STC) during multi-track and multi-layer manufacturing of Alloy 718 using electron beam melting (EBM) process leads to a microstructure with a high degree of complexity. In the present study, a detailed microstructural study of EBM-manufactured Alloy 718 was conducted by producing samples in shapes from one single track and single wall to 3D samples with maximum 10 longitudinal tracks and 50 vertical layers. The relationship between STC, solidification microstructure, interdendritic segregation, phase precipitation (MC, δ-phase), and hardness was investigated. Cooling rates (liquid-to-solid and solid-to-solid state) was estimated by measuring primary dendrite arm spacing (PDAS) and showed an increased cooling rate at the bottom compared to the top of the multi-layer samples. Thus, microstructure gradient was identified along the build direction. Moreover, extensive formation of solidification micro-constituents including MC-type carbides, induced by micro-segregation, was observed in all the samples. The electron backscatter diffraction (EBSD) technique showed a high textured structure in 〈001〉 direction with a few grains misoriented at the surface of all samples. Finer microstructure and possibility of more γ″ phase precipitation at the bottom of the samples resulted in slightly higher (~11%) hardness values compared to top of the samples. © 2018 Elsevier Ltd

Keywords
3D printers; Carbides; Cooling; Electron beam melting; Electron beams; Hardness; Segregation (metallography); Solidification; Thermal cycling, Alloy 718; Electron backscatter diffraction technique; Interdendritic segregation; Layer by layer; Micro-structure evolutions; Primary dendrite arm spacings; Solidification microstructures; Track by track, Microstructure
National Category
Manufacturing, Surface and Joining Technology Metallurgy and Metallic Materials
Research subject
ENGINEERING, Manufacturing and materials engineering; Production Technology
Identifiers
urn:nbn:se:hv:diva-13042 (URN)10.1016/j.matdes.2018.09.038 (DOI)000453008100040 ()2-s2.0-85053828514 (Scopus ID)
Funder
Knowledge Foundation
Note

Funders: Simulation and Control of Material affecting Processes (SiCoMap); "SUMAN-Next"

Available from: 2018-10-29 Created: 2018-10-29 Last updated: 2020-11-10Bibliographically approved
3. Influence of build layout and orientation on microstructural characteristics of electron beam melted Alloy 718
Open this publication in new window or tab >>Influence of build layout and orientation on microstructural characteristics of electron beam melted Alloy 718
Show others...
2018 (English)In: The International Journal of Advanced Manufacturing Technology, ISSN 0268-3768, E-ISSN 1433-3015, Vol. 99, no S1, p. 2903-2913Article in journal (Refereed) Published
Abstract [en]

Effects of build layout and orientation consisting of (a) height from the build plate (Z-axis), (b) distance between samples, and (c) location in the build plate (X-Y plane) on porosity, NbC fraction, and hardness in electron beam melted (EBM) Alloy 718 were studied. The as-built samples predominantly showed columnar structure with strong ˂001˃ crystallographic orientation parallel to the build direction, as well as NbC and ÎŽ-phase in inter-dendrites and grain boundaries. These microstructural characteristics were correlated with the thermal history, specifically cooling rate, resulted from the build layout and orientation parameters. The hardness and NbC fraction of the samples increased around 6% and 116%, respectively, as the height increased from 2 to 45 mm. Moreover, by increasing the height, formation of ÎŽ-phase was also enhanced associated with lower cooling rate in the samples built with a greater distance from the build plate. However, the porosity fraction was unaffected. Increasing the sample gap from 2 to 10 mm did not change the NbC fraction and hardness; however, the porosity fraction increased by 94%. The sample location in the build chamber influenced the porosity fraction, particularly in interior and exterior areas of the build plate. The hardness and NbC fraction were not dependent on the sample location in the build chamber. © 2018, The Author(s).

Keywords
3D printers, Cooling, Electron beam melting, Electron beams, Grain boundaries, Hardness, Location, Niobium compounds, Porosity, Alloy 718, Build direction, Columnar structures, Crystallographic orientations, Micro-structural characteristics, Micro-structural characterization, Orientation parameter, Sample location, Porous plates
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering; Production Technology
Identifiers
urn:nbn:se:hv:diva-13043 (URN)10.1007/s00170-018-2621-6 (DOI)000452076900065 ()2-s2.0-85053670925 (Scopus ID)
Funder
European Regional Development Fund (ERDF)Knowledge Foundation
Note

First Online: 17 September 2018

Available from: 2018-10-29 Created: 2018-10-29 Last updated: 2020-11-10Bibliographically approved
4. Effect of build location on microstructural characteristics and corrosion behavior of EB-PBF built Alloy 718
Open this publication in new window or tab >>Effect of build location on microstructural characteristics and corrosion behavior of EB-PBF built Alloy 718
Show others...
2020 (English)In: The International Journal of Advanced Manufacturing Technology, ISSN 0268-3768, E-ISSN 1433-3015, Vol. 106, no 7-8, p. 3597-3607Article in journal (Refereed) Published
Abstract [en]

Electron beam-powder bed fusion (EB-PBF), a high-temperature additive manufacturing (AM) technique, shows great promise in the production of high-quality metallic parts in different applications such as the aerospace industry. To achieve a higher build efficiency, it is ideal to build multiple parts together with as low spacing as possible between the respective parts. In the EB-PBF technique, there are many unknown variations in microstructural characteristics and functional performance that could be induced as a result of the location of the parts on the build plate, gaps between the parts and part geometry, etc. In the present study, the variations in the microstructure and corrosion performance as a function of the parts location on the build plate in the EB-PBF process were investigated. The microstructural features were correlated with the thermal history of the samples built in different locations on the build plate, including exterior (the outermost), middle (between the outermost and innermost), and interior (the innermost) regions. The cubic coupons located in the exterior regions showed increased level (~ 20 %) of defects (mainly in the form of shrinkage pores) and lower level (~ 30-35 %) of Nb-rich phase fraction due to their higher cooling rates compared to the interior and middle samples. Electrochemical investigations showed that the location indirectly had a substantial influence on the corrosion behavior, verified by a significant increase in polarization resistance (Rp) from the exterior (2.1 ± 0.3 kΩ.cm2) to interior regions (39.2 ± 4.1 kΩ.cm2). © 2020, The Author(s).

Keywords
3D printers; Additives; Aerospace industry; Corrosion; Corrosive effects; Electron beams; Hardness; High temperature applications, Alloy 718; Electrochemical investigations; Functional performance; Micro-structural characteristics; Micro-structural characterization; Microstructural features; Polarization resistances; Powder bed, Location
National Category
Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:hv:diva-15004 (URN)10.1007/s00170-019-04859-9 (DOI)000511506500069 ()2-s2.0-85077549789 (Scopus ID)
Funder
Knowledge FoundationEuropean Regional Development Fund (ERDF)
Available from: 2020-02-24 Created: 2020-02-24 Last updated: 2020-11-10
5. EBM-manufactured single tracks of Alloy 718: Influence of energy input and focus offset on geometrical and microstructural characteristics
Open this publication in new window or tab >>EBM-manufactured single tracks of Alloy 718: Influence of energy input and focus offset on geometrical and microstructural characteristics
2019 (English)In: Materials Characterization, ISSN 1044-5803, E-ISSN 1873-4189, Vol. 148, p. 88-99Article in journal (Refereed) Published
Abstract [en]

Electron beam melting-powder bed fusion (EBM-PBF) is an additive manufacturing process, which is able to produce parts in layer-by-layer fashion from a 3D model data. Currently application of this technology in parts manufacturing with high geometrical complexity has acquired growing interest in industry. To recommend the EBM process into industry for manufacturing parts, improved mechanical properties of final part must be obtained. Such properties highly depend on individual single melted track and single layer. In EBM, interactions between the electron beam, powder, and solid underlying layer affect the geometrical (e.g., re-melt depth, track width, contact angle, and track height) and microstructural (e.g., grain structure, and primary dendrite arm spacing) characteristics of the melted tracks. The core of the present research was to explore the influence of linear energy input parameters in terms of beam scanning speed, beam current as well as focus offset and their interactions on the geometry and microstructure of EBM-manufactured single tracks of Alloy 718. Increased scanning speed led to lower linear energy input values (<0.9 J/mm) in an specific range of the focus offset (0–10 mA) which resulted in instability, and discontinuity of the single tracks as well as balling effect. Decreasing the scanning speed and increasing the beam current resulted in higher melt pool depth and width. By statistical evaluations, the most influencing parameters on the geometrical features were primarily the scanning speed, and secondly the beam current. Primary dendrite arm spacing (PDAS) slightly decreased by increasing the scanning speed using lower beam current values as the linear energy input decreased. By increasing the linear energy input, the chance of more equiaxed grain formation was high, however, at lower linear energy input, mainly columnar grains were observed. The lower focus offset values resulted in more uniform grains along the 〈001〉 crystallographic direction. © 2018 Elsevier Inc. 

Keywords
3D printers; Contact angle; Dendrites (metallography); Design of experiments; Electron beam melting; Electron beams; Scanning; Speed, Alloy 718; Geometrical characteristics; Powder bed; Single-tracks; Solidified microstructures, Geometry
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-13365 (URN)10.1016/j.matchar.2018.11.033 (DOI)000458228100011 ()2-s2.0-85058512738 (Scopus ID)
Funder
European Regional Development Fund (ERDF)Knowledge Foundation
Available from: 2019-01-08 Created: 2019-01-08 Last updated: 2020-11-10Bibliographically approved
6. Contour design to improve topographical and microstructural characteristics of Alloy 718 manufactured by electron beam-powder bed fusion technique
Open this publication in new window or tab >>Contour design to improve topographical and microstructural characteristics of Alloy 718 manufactured by electron beam-powder bed fusion technique
2020 (English)In: Additive Manufacturing, ISSN 2214-8604, E-ISSN 2214-7810, Vol. 32, article id 101014Article in journal (Refereed) Published
Abstract [en]

Additive manufacturing (AM) processes are being frequently used in industry as they allow the manufacture ofcomplex parts with reduced lead times. Electron beam-powder bed fusion (EB-PBF) as an AM technology isknown for its near-net-shape production capacity with low residual stress. However, the surface quality andgeometrical accuracy of the manufactured parts are major obstacles for the wider industrial adoption of thistechnology, especially when enhanced mechanical performance is taken into consideration. Identifying theorigins of surface features such as satellite particles and sharp valleys on the parts manufactured by EB-PBF isimportant for a better understanding of the process and its capability. Moreover, understanding the influence ofthe contour melting strategy, by altering process parameters, on the surface roughness of the parts and thenumber of near-surface defects is highly critical. In this study, processing parameters of the EB-PBF techniquesuch as scanning speed, beam current, focus offset, and number of contours (one or two) with the linear meltingstrategy were investigated. A sample manufactured using Arcam-recommended process parameters (threecontours with the spot melting strategy) was used as a reference. For the samples with one contour, the scanningspeed had the greatest effect on the arithmetical mean height (Sa), and for the samples with two contours, thebeam current and focus offset had the greatest effect. For the samples with two contours, a lower focus offset andlower scan speed (at a higher beam current) resulted in a lower Sa; however, increasing the scan speed for thesamples with one contour decreased Sa. In general, the samples with two contours provided a lower Sa (∼22 %)but with slightly higher porosity (∼8 %) compared to the samples with one contour. Fewer defects were detected with a lower scanning speed and higher beam current. The number of defects and the Sa value for thesamples with two contours manufactured using the linear melting strategy were ∼85 % and 16 %, respectively,lower than those of the reference samples manufactured using the spot melting strategy.

Keywords
3D printers; Additives; Electron beams; Scanning; Speed; Surface defects; Surface properties; Surface roughness, Alloy 718; Geometrical accuracy; Linear and spot melting strategies; Mechanical performance; Micro-structural characteristics; Near-surface defects; Powder bed; Processing parameters, Melting
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology; ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-14993 (URN)10.1016/j.addma.2019.101014 (DOI)000522928600020 ()2-s2.0-85078915522 (Scopus ID)
Available from: 2020-02-24 Created: 2020-02-24 Last updated: 2021-06-11Bibliographically approved

Open Access in DiVA

Kappa(3451 kB)583 downloads
File information
File name FULLTEXT01.pdfFile size 3451 kBChecksum SHA-512
159c3a23945d5d9cf70df729985673cef23f0c6cb99747b6ba73ff6b91251346c1436681c076a0aaf3dd0e37c3db6132e19f4fe9850af26875bb4151f6e71738
Type fulltextMimetype application/pdf
Spikblad(139 kB)69 downloads
File information
File name FULLTEXT02.pdfFile size 139 kBChecksum SHA-512
063803438da1fecd331c73f65b7c137e5f3230bfc24b2e12cb40d09fda2a7592370e894a40cd21e95b0f07a4556f7f90d04b0a2b42a931d3e409518b630bfffa
Type fulltextMimetype application/pdf

Authority records

Karimi Neghlani, Paria

Search in DiVA

By author/editor
Karimi Neghlani, Paria
By organisation
Division of Subtractive and Additive Manufacturing
Manufacturing, Surface and Joining Technology

Search outside of DiVA

GoogleGoogle Scholar
Total: 655 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

isbn
urn-nbn

Altmetric score

isbn
urn-nbn
Total: 869 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf