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Evaluation of the electron beam spot size in electron beam melting for additive manufacturing
KTH, School of Industrial Engineering and Management (ITM), Production Engineering, Manufacturing and Metrology Systems.ORCID iD: 0000-0003-4364-0844
KTH, School of Industrial Engineering and Management (ITM), Production Engineering, Manufacturing and Metrology Systems.ORCID iD: 0000-0002-2582-9910
KTH, School of Industrial Engineering and Management (ITM), Production Engineering, Manufacturing and Metrology Systems.ORCID iD: 0000-0003-4120-4790
KTH, School of Industrial Engineering and Management (ITM), Production Engineering, Manufacturing and Metrology Systems.ORCID iD: 0000-0002-5960-2159
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. p. 89-92
Keywords [en]
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: urn:nbn:se:kth:diva-313274Scopus ID: 2-s2.0-85119597131OAI: oai:DiVA.org:kth-313274DiVA, id: diva2:1662890
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
In thesis
1. Electron beam powder bed fusion of Nitinol: A development from production process window towards delicate structures
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

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Zeyu, LinZhao, XiaoyuDadbakhsh, SasanRashid, Amir

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