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Proof-of-concept of an absorbance determination of a powder bed by high resolution coaxial multispectral imaging in laser material processing
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling. JENOPTIK Optical Systems GmbH, System Development Advanced Manufacturing, Göschwitzerstraße 25, 07745 Jena, Germany.ORCID-id: 0000-0001-8601-2923
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling. Fraunhofer Institute for Material and Beam Technology IWS, Winterbergstraße 28, 01277 Dresden, Germany.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.ORCID-id: 0000-0002-3569-6795
(engelsk)Inngår i: Additive Manufacturing, ISSN 2214-8604, E-ISSN 2214-7810Artikkel i tidsskrift (Annet vitenskapelig) Submitted
Abstract [en]

Imaging techniques are very popular for process monitoring in laser material processing due to their high information content. At the same time, coaxial systems focused by passive laser optics still present a major challenge, because most laser optics cause imaging errors for the monitoring channel. In this paper, the design, methodology and procedure are shown to be able to acquire coaxial image data by standard laser components, which is demonstrated by components for a laser powder bed fusion system and their use on a powder bed. The focus is on the correction of the image data to produce a high-resolution, geometrically accurate and gap-free overview image of the entire processing area. For this purpose, optical simulations of the system are performed to detect aberrations, distortions and chromatic errors and to correct them by hardware elements or in software post-processing. Over the entire 114 mm by 114 mm working area, objects can be captured geometrically accurate with a maximum deviation of 22 μm - 49 μm, depending on the detection wavelength. By capturing images atPaper C: Coaxial multispectral imaging Benedikt Brandau148wavelengths of 405 nm, 450 nm, 520 nm, 580 nm, 625 nm and 850 nm, multispectral information is gained over the entire working area. In addition, an absorbance of the powder bed is derived from the images. To qualify this methodology, tests are performed on 20 different powders. These include different particle sizes, aged and oxidized powders of different metals. The ability to determine absorbance is simulated by ray tracing powder surfaces. This allows the determination of in-line absorbances from the powder bed with a maximum deviation of 2.5 % compared to absorbance spectra of established methods. The origins of component defects such as foreign particles, powder oxidation, spatter and uncoated areas were able to be identified down to a diameter of 20 μm.

Emneord [en]
Spectroscopic Measurement, Correction Method, Laser Powder Bed Fusion, Scanning Method, Metal Powder, Multispectral Imaging, Laser Material Processing
HSV kategori
Forskningsprogram
Produktionsutveckling
Identifikatorer
URN: urn:nbn:se:ltu:diva-90581OAI: oai:DiVA.org:ltu-90581DiVA, id: diva2:1656941
Tilgjengelig fra: 2022-05-09 Laget: 2022-05-09 Sist oppdatert: 2022-05-10
Inngår i avhandling
1. Spectral analysis in laser powder bed fusion
Åpne denne publikasjonen i ny fane eller vindu >>Spectral analysis in laser powder bed fusion
2022 (engelsk)Licentiatavhandling, med artikler (Annet vitenskapelig)
Alternativ tittel[sv]
Spektralanalys vid laser powder bed fusion
Abstract [en]

This thesis is about the investigation of the spectral interaction of electromagnetic radiation with metal powders. For this purpose, spectral data of powders for laser powder bed fusion processes are investigated in three papers using different techniques. In paper A the spectral radiation behavior of the laser interaction zone is considered, in paper B and C the absorbance behavior of different metal powders depending on their state and measurement method.  

Paper A investigates the spectral signal of the process light generated by laser material interaction in laser powder bed fusion. The detection is performed by a coaxially guided measuring beam and a quasi-coaxial measuring beam simultaneously guided by another scanning optics. The signal characteristics depend on the angle of incidence of the measuring beam to the laser material interaction zone. Using high-speed recordings and optical simulations, a model for describing the signal behavior could be determined. The measured spectral intensity distribution representing the degree for energy coupling can be corrected with a correction factor over the whole field for solid materials. This correction includes a function describing the numerical aperture of the measuring channel and the laser intensity on the working field. For the investigated powder, the measurement signal fluctuated strongly and no transferable model could be formed. The reason for this was the different absorbance behavior of the powders investigated. Paper B therefore deals in detail with the spectral absorbance behavior of metal powders for additive manufacturing. Using a high-precision spectrometer, 39 powders were measured reflectively over a wide spectral range and the absorbance determined. By varying the degree of use, aging, grain size and impurities, various influence parameters are determined experimentally and discussed theoretically. Based on 20 derived laser wavelengths, technically usable wavelengths with better process efficiency and stability are proposed. From the obtained absorbance, the efficiency of energy coupling can be estimated and form a broad data base for the optimization of laser parameters. In order to perform the absorbance determinations also in situ in a laser powder bed fusion system paper C describes a possibility of an inline absorbance determination by high resolution coaxial imaging. A method is discussed for geometrically correct and gapless imaging of the processing plane, recorded through the laser optics. By imaging at six different wavelengths, metal powders can be distinguished by their absorbance spectrum and impurities can be detected. In an experimental implementation the functionality of the method is proven. The results are validated by optical simulations, ray tracing and comparative measurements with a high-precision spectrometer.

sted, utgiver, år, opplag, sider
Luleå: Luleå tekniska universitet, 2022. s. 200
Serie
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
Emneord
Spectral monitoring, Laser powder bed fusion, Laser optics, Absorbance
HSV kategori
Forskningsprogram
Produktionsutveckling
Identifikatorer
urn:nbn:se:ltu:diva-90525 (URN)978-91-8048-098-7 (ISBN)978-91-8048-099-4 (ISBN)
Presentation
2022-06-16, A109, Luleå university of technology, Luleå, 09:00 (engelsk)
Opponent
Veileder
Tilgjengelig fra: 2022-05-10 Laget: 2022-05-09 Sist oppdatert: 2022-05-26bibliografisk kontrollert

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Brandau, BenediktBrückner, FrankKaplan, Alexander

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