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Zeyu, L. (2023). Electron beam powder bed fusion of Nitinol: A development from production process window towards delicate structures. (Doctoral dissertation). KTH Royal Institute of Technology
Open this publication in new window or tab >>Electron beam powder bed fusion of Nitinol: A development from production process window towards delicate structures
2023 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Electron beam powder bed fusion (PBF-EB) is increasingly attracting attention for manufacturing the near-net shape parts due to its incomparable merits, such as free residual stress and superior mechanical performance. Nickel Titanium (NiTi) as the most widely used functional alloy, has not been systematically explored for manufacturing using PBF-EB despite the perfect vacuum and high temperature manufacturing environment. Therefore, this research explores the various aspects of PBF-EB for enabling the manufacturing of NiTi parts.

The first section, the critical role of powder pre-heating in PBF-EB and its relation to smoking and sintering issues when using highly susceptible-to-smoke NiTi powder is studied. The research includes assessments of the electron beam spot size and its impact on smoking. In addition, this study investigates the influence of defocused electron beams on smoking, with negative defocusing mitigating smoke compared to positive defocusing that may increase the smoking phenomenon. Processing windows for pre-heating NiTi powder are developed based on smoke tests and sintering levels, showing three modes: smoke-heating, melting-heating, and healthy-heating. Accordingly, the healthy-heating processing window is chosen to manufacture the dense NiTi parts.

Further, to produce high density and healthy components, the research focuses on investigating the effects of different PBF-EB parameter sets when manufacturing dense NiTi parts, including beam current, scan speed, and cooling conditions. After manufacturing, densest parts with different parameter sets are divided into three groups: i) high power with high scan speed and vacuum slow cooling, ii) low power with low scan speed and vacuum slow cooling and iii) low power with low scan speed and medium cooling rate in helium gas. A combination of low power and low scan speed leads to denser parts. This is attributed to lower electrostatic repulsive forces from lower number density of the impacting electrons. Different cooling conditions are proven to significantly affect phase transformation temperatures. The slower cooling rate leads to a higher Af and Ms temperatures and a wider phase transformation window than those from the parts with the medium cooling rate due to the formation of Ni4Ti3 precipitates. Afterwards, the pseudoelasticity of all the as-built parts is evaluated and illustrated, which shows that correct control of the process can produce components with recoverable strains as high as 8%.

The final part of this thesis the quality and accuracy of manufacturing delicate NiTi parts using PBF-EB is studied. Thin cylinders, thin walls, and lattice structures with various designs are manufactured using different scan strategies. The research reveals that both continuous melting and spot melting modes achieve a dense part in delicate structures. As-built lattice structures exhibit excellent spring-back, with the channel structure displaying the most deformation recoverability. The compressive strength and ultimate compressive strength increase with higher volume fractions. Spot melting is demonstrated as a valuable engineering tool for customizing delicate beam-shaped structures with superior pseudoelasticity.

Abstract [sv]

Elektronstrålepulverbäddfusion (PBF-EB) drar till sig alltmer uppmärksamhet för tillverkning av nära slutprodukt ämnen på grund av dess stora fördelar, såsom mindre restspänningar och överlägsna mekaniska egenskaper. Nickel Titan (NiTi) som den mest använda funktionella legeringen har inte systematiskt undersökts för tillverkning med PBF-EB trots den kontrollerade tillverkningsmiljön avseende vakuum och högtemperatur. Detta forskningsprojekt undersöker därför de olika aspekterna av PBF-EB för att möjliggöra tillverkning av NiTi-delar.

I det första avsnittet undersöks pulverförvärmningens kritiska roll i PBF-EB och dess relation till rökutveckling och sintringsproblem vid användning av NiTi-pulver som är mycket känsligt för rök. Forskningen inkluderar bedömningar av elektronstrålens fläckstorlek och dess inverkan på rökutvecklingen. Dessutom undersöker denna studie inverkan av ofokuserade elektronstrålar på rökutveckling, med negativ defokusering vilket ger dämpande rök jämfört med positiv defokusering som kan öka rökutvecklingen. Processfönster för förvärmning av NiTi-pulver är utvecklade baserat på röktester och sintringsnivåer, som visar tre lägen: rökutvecklingsuppvärmning, smältningsuppvärmning och optimerad uppvärmning. Följaktligen väljs processfönstret för optimerad uppvärmning för att tillverka de täta NiTi-delarna.

Vidare, för att producera högdensitets och optimala komponenter, fokuserar forskningen på att undersöka effekterna av olika PBF-EB-parameteruppsättningar vid tillverkning av täta NiTi-delar, inklusive strålström, skanningshastighet och kylningsförhållanden. Efter tillverkning delas de tätaste delarna med olika parameteruppsättningar in i tre grupper: i) högeffekt med hög skanningshastighet och vakuum med långsam kylning, ii) lågeffekt med låg skanningshastighet och vakuum med långsam kylning och iii) låg effekt med låg skanningshastighet och medelhög kylningshastighet i heliumgas. En kombination av låg effekt och låg skanningshastighet leder till tätare delar. Detta tillskrivs lägre elektrostatiska repulsiva krafter från lägre densitet hos de aktiva elektronerna. Olika kylförhållanden har visat sig påverka fasomvandlingstemperaturerna avsevärt. Den långsammare kylningshastigheten leder till högre Af- och Ms-temperaturer och ett bredare fasomvandlingsfönster än de från delarna med medelhög kylningshastighet på grund av bildandet av Ni4Ti3-fällningar. Efteråt utvärderas och illustreras pseudoelasticiteten hos alla byggdelarna, vilket visar att korrekt kontroll av processen kan ge komponenter med återhämtningsbara spänningar på upp till 8 %.

I den sista delen av denna avhandling studeras kvaliteten och noggrannheten vid tillverkning av ömtåliga NiTi-delar med PBF-EB. Tunna cylindrar, tunna väggar och gitterstrukturer med olika design tillverkas med olika skanningsstrategier. Forskningen visar att både kontinuerlig smältning och punktsmältning uppnår tät materialstruktur i känsliga detaljer. Strukturer byggda som gitter uppvisar utmärkt fjädring, med kanalstrukturer vilka uppvisar mest deformationsåtervinningsbarhet. Tryckhållfastheten och den slutliga tryckhållfastheten ökar med högre volymfraktioner. Punktsmältning demonstreras som ett värdefullt ingenjörsverktyg för att anpassa känsliga strålformade strukturer med överlägsen pseudoelasticitet.​

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2023. p. 66
Series
TRITA-ITM-AVL ; 2023:35
National Category
Production Engineering, Human Work Science and Ergonomics
Research subject
Production Engineering
Identifiers
urn:nbn:se:kth:diva-340161 (URN)978-91-8040-781-6 (ISBN)
Public defence
2023-12-18, M311, Brinellvagen 68, Stockholm, 09:00 (English)
Opponent
Supervisors
Available from: 2023-11-30 Created: 2023-11-29 Last updated: 2023-12-18Bibliographically approved
Zeyu, L., Dadbakhsh, S. & Rashid, A. (2023). Increasing precision towards NiTi lattice structure using PBF-EB. In: : . Paper presented at 34th Annual Symposium: Solid Freeform Fabrication, August 14-16, 2023, Austin, Texas, United States of America.
Open this publication in new window or tab >>Increasing precision towards NiTi lattice structure using PBF-EB
2023 (English)Conference paper, Poster (with or without abstract) (Refereed)
Abstract [en]

The electron beam powder bed fusion (PBF-EB) is limitedly used to manufacture complex structures such as delicate lattices. Nickel titanium (NiTi) has been chosen for fabricating the lattice structure due to its widely utilization in the biomedical sector. However, issues may arise when manufacturing angled trusses while the dimensional inaccuracy increased with the increasing of the angle between the truss member and the vertical build direction. Therefore, two different scan strategies: spot melting and linear melting were used to manufacture the lattice structures respectively to compare the dimensional accuracy of different structures. This investigation highlights that linear melting is prone to maintain the geometrical accuracy of line-based structure with a limited influence from the scan speed while the spot melting is more capable of manufacturing the point-based structure with a higher geometrical resolution.  

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-336584 (URN)
Conference
34th Annual Symposium: Solid Freeform Fabrication, August 14-16, 2023, Austin, Texas, United States of America
Note

QC 20230915

Available from: 2023-09-13 Created: 2023-09-13 Last updated: 2023-09-15Bibliographically approved
Lin, Z., Surreddi, K. B., Hulme-Smith, C., Dadbakhsh, S. & Rashid, A. (2023). Influence of Electron Beam Powder Bed Fusion Process Parameters on Transformation Temperatures and Pseudoelasticity of Shape Memory Nickel Titanium. Advanced Engineering Materials
Open this publication in new window or tab >>Influence of Electron Beam Powder Bed Fusion Process Parameters on Transformation Temperatures and Pseudoelasticity of Shape Memory Nickel Titanium
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2023 (English)In: Advanced Engineering Materials, ISSN 1438-1656, E-ISSN 1527-2648Article in journal, Editorial material (Refereed) Published
Abstract [en]

Electron beam powder bed fusion (PBF-EB) is used to manufacture dense nickel titanium parts using various parameter sets, including the beam current, scan speed and post cooling condition. The density of manufactured NiTi parts are investigated with relation to the linear energy input. The results implies the part density increases with increasing linear energy density to over 98% of the bulk density. With a constant energy input, a combination of low power and low scan speed leads to denser parts. This is attributed to lower electrostatic repulsive forces from lower number density of the impacting electrons. After manufacturing, densest parts with distinct parameter sets are categorized into three groups: i) high power with high scan speed and vacuum slow cooling, ii) low power with low scan speed and vacuum slow cooling and iii) low power with low scan speed and medium cooling rate in helium gas. Among these, a faster cooling rate suppresses phase transformation temperatures, while vacuum cooling combinations do not affect the phase transformation temperatures significantly. All the printed parts in this study exhibit almost 8% pseudoelasticity regardless of the process parameters, while the parts cooled in helium have a higher energy dissipation efficiency ( ), which implies faster damping of oscillations. 

Place, publisher, year, edition, pages
John Wiley & Sons, 2023
Keywords
PBF-EB, cooling rate, NiTi, AM, process paremeters
National Category
Materials Engineering
Research subject
Industrial Engineering and Management
Identifiers
urn:nbn:se:kth:diva-326103 (URN)10.1002/adem.202201818 (DOI)000975548500001 ()2-s2.0-85154049056 (Scopus ID)
Note

QC 20230426

Available from: 2023-04-24 Created: 2023-04-24 Last updated: 2023-11-30Bibliographically approved
Zeyu, L. & Dadbakhsh, S. (2023). The processing windows for NiTi alloy manufactured by PBF-EBessing windows for NiTi alloy manufactured by PBF-EB. In: The processing windows for NiTi alloy manufactured by PBF-EB: . Paper presented at Swedish Arena for additive manufacturing of metals 2023, Stockholm, Sweden, Januari 11–12, 2023.
Open this publication in new window or tab >>The processing windows for NiTi alloy manufactured by PBF-EBessing windows for NiTi alloy manufactured by PBF-EB
2023 (English)In: The processing windows for NiTi alloy manufactured by PBF-EB, 2023Conference paper, Oral presentation with published abstract (Other academic)
Abstract [en]

Nickel titanium (NiTi) as one of the most utilized shape memory alloy has drawn significant interest due to its unique characteristics. However, NiTi is also considered a susceptible material to smoke during electron beam powder bed fusion (PBF-EB) process, which restricts the manufacturing possibility of the components. This work investigates processing windows for pre-heating and melting of NiTi powder to allow fabricating healthy parts. The smoke tests were carried out at different focus offsets and beam currents in relation to beam speeds. It is noted that a smaller EB spot can effectively prevents smoking while it may cause the strong powder bonding which can affect powder recycling negatively. Thus, a less focused beam (or larger EB spot) was selected to reach medium but efficient sintering. Moreover, it was observed that a negative defocused EB mitigates the smoke phenomenon compared to the positive defocused EB with a similar spot size. After that, parts having a relative density over 99% were successfully manufactured with PBF-EB. It is also found that with the same level of energy input, a set of low power with low scan speed leads to denser parts compared to a set of high power with high scan speed. This is attributed to less complexities in the melt dynamic which is related to lower density of impacting electrons. Besides, the combination of low power with low scan speed also improves geometrical accuracy of the parts attributed to the smaller spot size and smaller melt pool sizes. 

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-326942 (URN)
Conference
Swedish Arena for additive manufacturing of metals 2023, Stockholm, Sweden, Januari 11–12, 2023
Note

QC 20230807

Available from: 2023-05-15 Created: 2023-05-15 Last updated: 2023-08-07Bibliographically approved
Yang, M., Bao, X. & Zeyu, L. (2022). A STUDY ON THE BASIC PRINCIPLES OF METAPHORICAL THINKING IN MYTHOLOGICAL LANGUAGE AND THE CHANGES OF EMOTIONAL BEHAVIOR. International Journal of Neuropsychopharmacology, 25(SUPPL 1), A94-A94
Open this publication in new window or tab >>A STUDY ON THE BASIC PRINCIPLES OF METAPHORICAL THINKING IN MYTHOLOGICAL LANGUAGE AND THE CHANGES OF EMOTIONAL BEHAVIOR
2022 (English)In: International Journal of Neuropsychopharmacology, ISSN 1461-1457, E-ISSN 1469-5111, Vol. 25, no SUPPL 1, p. A94-A94Article in journal, Meeting abstract (Other academic) Published
Place, publisher, year, edition, pages
Oxford University Press, 2022
National Category
Social Sciences Interdisciplinary
Identifiers
urn:nbn:se:kth:diva-316039 (URN)000827854900127 ()
Note

QC 20220809

Available from: 2022-08-09 Created: 2022-08-09 Last updated: 2022-08-09Bibliographically approved
Zeyu, L., Dadbakhsh, S. & Rashid, A. (2022). Developing processing windows for powder pre-heating in electron beam melting. Journal of Manufacturing Processes, 83, 180-191
Open this publication in new window or tab >>Developing processing windows for powder pre-heating in electron beam melting
2022 (English)In: Journal of Manufacturing Processes, ISSN 1526-6125, Vol. 83, p. 180-191Article in journal (Refereed) Published
Abstract [en]

Powder pre-heating is a critical step in electron beam melting (EBM), while there has been no systematic work tostudy the corresponding processing windows so far. Accordingly, this work investigates the relation between thesintering and the issues appearing during pre-heating (e.g., smoking or excessive sintering) in EBM of highlysusceptible-to-smoke Nickel-Titanium (NiTi) powder. First, the EB spot size was assessed depending on differentfocus offsets and beam currents from beam tracking experiments on a ceramic-coated stainless steel plate. Af-terwards, the smoke tests were carried out at different focus offsets and beam currents in terms of beam speeds. Itis shown that a smaller EB spot can effectively prevents smoking by enhancing the sintering degree. However,since this high sintering degree can cause strong powder bonding preventing the powder recycling, less focusedbeam (or larger EB spot) was selected to reach medium but efficient sintering in the level of around 30 %.Moreover, due to the influence of the diverging angle on the EB-material interaction, it is found that the negativedefocused EB mitigates the smoke phenomenon compared to the positive defocused EB with a similar spot size.Based on the smoke test results, linked to the sintering degree, the processing windows for pre-heating NiTipowder are developed demonstrating three different modes: smoke-heating, melting-heating and healthy-heating. 

Place, publisher, year, edition, pages
Elsevier BV, 2022
National Category
Engineering and Technology Production Engineering, Human Work Science and Ergonomics
Research subject
Production Engineering
Identifiers
urn:nbn:se:kth:diva-317459 (URN)10.1016/j.jmapro.2022.08.063 (DOI)000870827000003 ()2-s2.0-85137722020 (Scopus ID)
Note

QC 20220930

Available from: 2022-09-12 Created: 2022-09-12 Last updated: 2023-11-30Bibliographically approved
Dadbakhsh, S., Zhao, X., Chinnappan, P. K., Shanmugam, V., Zeyu, L. & Hulme-Smith, C. (2022). Process and geometrical integrity optimization of electron beam melting for copper. CIRP annals, 71, 201-204
Open this publication in new window or tab >>Process and geometrical integrity optimization of electron beam melting for copper
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2022 (English)In: CIRP annals, ISSN 0007-8506, E-ISSN 1726-0604, Vol. 71, p. 201-204Article in journal (Refereed) Published
Abstract [en]

This work systematically analyzes and optimizes the process of electron beam melting for pure copper. It is shown that, for reliable manufacturing, the preheating temperature should be optimized to avoid porosity as well as part deformation. The electron beam should be fully focused to prevent shrinkage voids (correlated to negative defocusing) and material spattering (linked to positive defocusing). Smoother surfaces from lower hatch spacing (e.g., 100µm) can improve the density reliability, while longer overhangs are reached by a higher hatch spacing. A suitable starting contour strategy is also applied to mitigate border porosities, reduce side roughness and increase geometric precision.

Place, publisher, year, edition, pages
Elsevier BV, 2022
Keywords
Additive Manufacturing, Optimization, Electron Beam Melting (EBM)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-312467 (URN)10.1016/j.cirp.2022.03.041 (DOI)000886363500002 ()2-s2.0-85129085733 (Scopus ID)
Note

QC 20220530

Available from: 2022-05-18 Created: 2022-05-18 Last updated: 2024-01-17Bibliographically approved
Zeyu, L. (2021). Evaluating the electron beam spot size in electron beam melting for additive manufacturing. In: : . Paper presented at Virtual Lamdamap 14th International Conference & Exhibition 10th -11th March 2021.
Open this publication in new window or tab >>Evaluating the electron beam spot size in electron beam melting for additive manufacturing
2021 (English)Conference paper, Oral presentation with published abstract (Refereed)
National Category
Production Engineering, Human Work Science and Ergonomics Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:kth:diva-292009 (URN)
Conference
Virtual Lamdamap 14th International Conference & Exhibition 10th -11th March 2021
Note

QC 20210710

Available from: 2021-03-23 Created: 2021-03-23 Last updated: 2022-06-25Bibliographically approved
Lin, Z., Zhao, X., Dadbakhsh, S. & Rashid, A. (2021). Evaluating the electron beam spot size in electron beam melting machines. In: : . Paper presented at Virtual Lamdamap 14th International Conference & Exhibition, 10th-11th March 2021.
Open this publication in new window or tab >>Evaluating the electron beam spot size in electron beam melting machines
2021 (English)Conference paper, Oral presentation with published abstract (Refereed)
Abstract [en]

Since electron beam (EB) is the main additive manufacturing (AM) tool in electron beam melting (EBM), EB spot size plays a significant role in the parts quality, surface roughness as well as the microstructure and corresponding properties. So far, the research on the relationship between the machine parameters and the EB spot size has been mainly based on the single track and powderless single track printing on a metal plate such as stainless steel. However, this method, due to material thermal properties as well as the melting phenomena, cannot reveal the actual value for the EB spot size. This research is carried out to establish a simple methodology on measuring the EB spot size in a more accurate way with a low cost. To do so, a ceramic surface coating was applied to the surface of a metal copper starting plate and stainless steel plate. Afterwards, the EB applied the tracks onto the coatings and regular metal plate. The analysis showed that the EB tracks on ceramic coated stainless steel plates could be the best replica for the electron beam among those materials tested in this work.  

National Category
Production Engineering, Human Work Science and Ergonomics
Identifiers
urn:nbn:se:kth:diva-292324 (URN)
Conference
Virtual Lamdamap 14th International Conference & Exhibition, 10th-11th March 2021
Note

QC 20210401

Available from: 2021-03-30 Created: 2021-03-30 Last updated: 2024-03-18Bibliographically approved
Zeyu, L., Zhao, X., Dadbakhsh, S. & Rashid, A. (2021). Evaluation of the electron beam spot size in electron beam melting for additive manufacturing. In: Laser Metrology and Machine Performance XIV - 14th International Conference and Exhibition on Laser Metrology, Machine Tool, CMM and Robotic Performance, LAMDAMAP 2021: . Paper presented at 14th International Conference on Laser Metrology, Coordinate Measuring Machine and Machine Tool Performance, LAMDAMAP 2021, 10 March 2021 through 11 March 2021, Virtual, Online. (pp. 89-92). euspen
Open this publication in new window or tab >>Evaluation of the electron beam spot size in electron beam melting for additive manufacturing
2021 (English)In: Laser Metrology and Machine Performance XIV - 14th International Conference and Exhibition on Laser Metrology, Machine Tool, CMM and Robotic Performance, LAMDAMAP 2021, euspen , 2021, p. 89-92Conference paper, Published paper (Refereed)
Abstract [en]

Since electron beam (EB) is the main additive manufacturing (AM) tool in electron beam melting (EBM), EB spot size plays a significant role in the parts quality, surface roughness as well as the microstructure and corresponding properties. So far, the research on measuring EB spot size has been mainly based on printing with/without powder single tracks on a metal plate such as stainless steel. However, this method, due to material thermal properties as well as the melting phenomena, cannot reveal the actual value for the EB spot size. This research is carried out to establish a simple methodology on measuring the EB spot size in a more accurate way at a low cost. To do so, a ceramic surface coating was applied to the surface of a copper starting plate and a stainless steel starting plate respectively. Afterwards, the EB applied the tracks onto the coated starting plate and regular metal starting plate. The analysis showed that the EB tracks on ceramic coated stainless steel plates could be the best replica for the electron beam among those materials tested in this work.

Place, publisher, year, edition, pages
euspen, 2021
Keywords
3D printers, Additives, Electrons, Machine tools, Plate metal, Stainless steel, Surface roughness, Beam spot size, Electron-beam, Low-costs, Manufacturing tools, Metal plates, Part quality, Property, Quality surfaces, Simple++, Single-tracks, Electron beams
National Category
Building Technologies
Identifiers
urn:nbn:se:kth:diva-313274 (URN)2-s2.0-85119597131 (Scopus ID)
Conference
14th International Conference on Laser Metrology, Coordinate Measuring Machine and Machine Tool Performance, LAMDAMAP 2021, 10 March 2021 through 11 March 2021, Virtual, Online.
Note

Part of proceedings: ISBN 978-099577518-3

QC 20220601

Available from: 2022-06-01 Created: 2022-06-01 Last updated: 2024-01-17Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0003-4364-0844

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