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Laser beam-material interaction in Powder Bed Fusion
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.ORCID iD: 0000-0003-4443-3097
2021 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The acceptance of additive manufacturing (AM) depends on the quality of final parts and the process repeatability. Recently, many studies have been dedicated to the establishment of the relationship between the process behavior and material performance. Phenomena such as laser-material interaction, melt pool dynamics, ejecta formation and particle movement behavior on a powder bed are of a particular interest for the AM community as these events directly influence the outcome of the process. Another aspect, which hinders the adoption of AM, is the need for cost-efficient powder materials and their sustainable processing and subsequent recycling. 

The research work presented in this thesis, to a certain degree, covers the above mentioned scientific aspects and focuses on the behavior of gas and water atomized steel powders in laser powder bed fusion (LPBF). 

Paper I demonstrates a comparative study of dissimilarly-shaped gas and water atomized low alloy steel powders regarding their processability, packing capacities, particle movement behavior and powder performance in LPBF. The impact of chemical composition and morphology of the powders on the process behavior was revealed. Powder spattering and melt pool instabilities were discussed in detail. 

Paper II explains the role of ejecta in the recycled powder and the changing behavior of the material due to ejecta pick-up. The impact of multiple powder recycling on the degradation of low alloy steel powder in laser powder bed fusion was studied. Oxygen content, particle size and ejecta occurrence gradually increased after each recycling step and were identified as the main contributors to the property alterations observed in the powder during recycling. In addition, a direct correlation between the increase in oxygen with repeated recycling and a more frequent spatter ejection after each recycle was established. 

Paper III is a successor of Paper I and contains a research on the particle movement and denudation behavior on a powder bed when using near-spherical and non-spherical steel powders. The influence of particle morphology on the dynamics of arbitrary-shaped powder particles was studied by applying an analytical correlation formula to calculate the drag force exerted on powder particles of various shape. Particle entrainment of gas and water atomized powders in front of the laser beam was measured, revealing a significant difference in the powder transfer towards the melt pool.

Place, publisher, year, edition, pages
Luleå University of Technology, 2021.
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
National Category
Metallurgy and Metallic Materials Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-84303ISBN: 978-91-7790-858-6 (print)ISBN: 978-91-7790-859-3 (electronic)OAI: oai:DiVA.org:ltu-84303DiVA, id: diva2:1554751
Presentation
2021-09-01, E632, 09:00 (English)
Opponent
Available from: 2021-05-18 Created: 2021-05-17 Last updated: 2022-03-02Bibliographically approved
List of papers
1. A comparative study of water and gas atomized low alloy steel powders for additive manufacturing
Open this publication in new window or tab >>A comparative study of water and gas atomized low alloy steel powders for additive manufacturing
2020 (English)In: Additive Manufacturing, ISSN 2214-8604, E-ISSN 2214-7810, Vol. 36, article id 101675Article in journal (Refereed) Published
Abstract [en]

This work reports a study of the differences between laser processing of water and gas atomized low alloy steel powders with a focus on powder behavior and performance in additive manufacturing. Material packing densities were measured to establish a relationship between powder packing and track formation. The results showed that the track height when using water atomized powder was 15% lower than the value achieved for the gas atomized powder. High-speed imaging was utilized to observe the material behavior and analyze the powder particle movement under laser irradiation. It was found that water atomized powder has less particle entrainment due to its tendency towards mechanical interlocking. The occurrence of powder spattering and melt pool instabilities was also studied. More frequent spatter ejection is believed to be due to the higher amount of oxygen in the water atomized powder.

Place, publisher, year, edition, pages
Elsevier, 2020
Keywords
Additive manufacturing, Laser powder bed fusion, Water atomized powder, Packing density, Low alloy steel, High-speed imaging
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-81523 (URN)10.1016/j.addma.2020.101675 (DOI)000600807800194 ()2-s2.0-85095580581 (Scopus ID)
Funder
Interreg Nord, 304–7463-2018
Note

Validerad;2021;Nivå 2;2021-01-04 (alebob);

Finansiär: EIT Raw Materials Knowledge and Innovation Community, Europe (17070)

Available from: 2020-11-23 Created: 2020-11-23 Last updated: 2023-09-06Bibliographically approved
2. Spattering and oxidation phenomena during recycling of low alloy steel powder in Laser Powder Bed Fusion
Open this publication in new window or tab >>Spattering and oxidation phenomena during recycling of low alloy steel powder in Laser Powder Bed Fusion
2021 (English)In: Materials Today Communications, ISSN 2352-4928, Vol. 27, article id 102241Article in journal (Refereed) Published
Abstract [en]

This study reports on the impact of repeated powder recycling on the degradation of low alloy steel powder in Laser Powder Bed Fusion. The average powder particle size increased slightly upon recycling due to powder agglomeration and the presence of spatters and other ejecta from the process zone. The oxygen content showed a continuous growth after each recycle, while the other chemical elements of the recycled powder remained largely unchanged. A map of ejecta classification is presented, featuring various ejecta types formed during laser processing. Ejecta of increased diameter and different shapes were observed in the recycled powder, using high-speed imaging and Scanning Electron Microscopy. The ejecta were collected after each powder recycle to enable the calculation of the ejecta mass generated during the process. The result showed a direct correlation between oxygen content in the powder and spatter/ejecta formation with the number of recycling events. It is likely that the increase in oxygen contributes to powder spattering.

Place, publisher, year, edition, pages
Elsevier, 2021
Keywords
Low alloy steel, Spattering, Powder degradation, Recycling, Laser Powder Bed Fusion, Additive manufacturing
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-83247 (URN)10.1016/j.mtcomm.2021.102241 (DOI)000683034600006 ()2-s2.0-85102510106 (Scopus ID)
Funder
Interreg Nord, 304-7463-2018
Note

Validerad;2021;Nivå 2;2021-03-23 (johcin);

Finansiär: EIT Raw Materials Knowledge and Innovation Community (17070)

Available from: 2021-03-12 Created: 2021-03-12 Last updated: 2023-03-02Bibliographically approved
3. The role of powder morphology in particle movement behavior in laser powder bed fusion with an emphasis on fluid drag
Open this publication in new window or tab >>The role of powder morphology in particle movement behavior in laser powder bed fusion with an emphasis on fluid drag
2022 (English)In: Powder Technology, ISSN 0032-5910, E-ISSN 1873-328X, Vol. 395, p. 720-731Article in journal (Refereed) Published
Abstract [en]

This study investigates the movement behavior of particles of dissimilar morphology in the powder bed in Laser Powder Bed Fusion. Gas atomized (GA) and water atomized (WA) low alloy steel powders were employed to study their motion around the laser scan path. Particle velocities, entrainment distances and denudation zones were measured for both powders using high-speed imaging. The entrainment of GA powder particles in front of the laser beam towards the process area was initiated 1.6 mm from the edge of the melt pool, whereas the distance was 0.6–0.8 mm for the WA powder. The differences in observed behavior were related to the variations in particle shape of the two types of powder. The processing of WA powder resulted in a 16% narrower denudation zone (for a low volumetric energy density) compared to GA powder. However, the denudation width difference decreased with increasing volumetric energy density, most likely due to a steeper pressure gradient in the process area which diminishes the impact of powder shape. X-ray computed microtomography was utilized to estimate the drag force acting on the powder particles of various morphologies. The results showed that the radial drag force exerted on GA powder was 64% greater than when using WA powder. Moreover, if the WA powder particles were of elongated shape the drag force decreased by almost an order of magnitude, demonstrating the importance of the particle's morphology in the process dynamics.

Place, publisher, year, edition, pages
Elsevier, 2022
Keywords
Water atomized powder, Particle movement, Dissimilar morphology, Drag force, Laser powder bed fusion, Additive manufacturing
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-87583 (URN)10.1016/j.powtec.2021.10.020 (DOI)000718001100004 ()2-s2.0-85117723002 (Scopus ID)
Funder
Interreg Nord, 304-7463-2018
Note

Validerad;2021;Nivå 2;2021-10-25 (beamah);

Funder: EIT Raw Materials (No.17070)

2022-03-02: Dubblettpost PID: 1554744 har raderats. (sofila)

Available from: 2021-10-21 Created: 2021-10-21 Last updated: 2023-03-02Bibliographically approved

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