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Spectral analysis in laser powder bed fusion
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.ORCID iD: 0000-0001-8601-2923
2022 (English)Licentiate thesis, comprehensive summary (Other academic)Alternative title
Spektralanalys vid laser powder bed fusion (Swedish)
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.

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 2022. , p. 200
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
Keywords [en]
Spectral monitoring, Laser powder bed fusion, Laser optics, Absorbance
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-90525ISBN: 978-91-8048-098-7 (print)ISBN: 978-91-8048-099-4 (electronic)OAI: oai:DiVA.org:ltu-90525DiVA, id: diva2:1656943
Presentation
2022-06-16, A109, Luleå university of technology, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2022-05-10 Created: 2022-05-09 Last updated: 2022-05-26Bibliographically approved
List of papers
1. Absorbance study of powder conditions for laser additive manufacturing
Open this publication in new window or tab >>Absorbance study of powder conditions for laser additive manufacturing
Show others...
2022 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 216, article id 110591Article in journal (Refereed) Published
Abstract [en]

Absorbance is often used for simulations or validation of process parameters for powder-based laser materials processing. In this work, the absorbance of 39 metal powders for additive manufacturing is determined at 20 laser wavelengths. Different grain sizes and aging states for: steels, aluminum alloys, titanium alloys, Nitinol, high entropy alloy, chromium, copper, brass and iron ore were analyzed. For this purpose, the absorbance spectrum of the powders was determined via a dual-beam spectrometer in the range of λ = 330 - 1560 nm. At the laser wavelengths of λ = 450 nm, 633 nm and 650 nm, the absorbance averaged over all materials was found to increase by a factor of 2.4 up to 3.3 compared to the usual wavelength of λ = 1070 nm, with minimal variations in absorbance between materials. In the investigation of the aged or used powders, a loss of absorbance was detectable. Almost no changes from the point of view of processing aged and new AlSi10Mg powders, is expected for laser sources with λ = 450 nm. The resulting measurements provide a good basis for process parameters for a variety of laser wavelengths and materials, as well as a data set for improved absorbance simulations.

Place, publisher, year, edition, pages
Elsevier, 2022
Keywords
Spectroscopic measurement, Absorption spectra, Laser powder bed fusion, Metal powder, Multiple reflections, Laser absorption
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-90043 (URN)10.1016/j.matdes.2022.110591 (DOI)000793277800004 ()2-s2.0-85128304808 (Scopus ID)
Note

Validerad;2022;Nivå 2;2022-04-01 (joosat);

 Funder: EIT Raw Materials, European SAMOA project (18079)

Available from: 2022-04-01 Created: 2022-04-01 Last updated: 2023-09-04Bibliographically approved
2. Angular dependence of coaxial and quasi-coaxial monitoring systems for process radiation analysis in laser materials processing
Open this publication in new window or tab >>Angular dependence of coaxial and quasi-coaxial monitoring systems for process radiation analysis in laser materials processing
2022 (English)In: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 155, article id 107050Article in journal (Refereed) Published
Abstract [en]

Process monitoring is becoming increasingly important in laser-based manufacturing and is of particular importance in the field of additive manufacturing [e.g. Laser Powder Bed Fusion (LPBF)]. Process monitoring enables a reduction of production costs and a lower time-to-market. Furthermore, the data can be used to create a digital twin of the workpiece. There are already many established processing head-integrated monitoring systems for such applications as the multispectral analysis of process radiation. However, the monitoring of complex signals in systems with F-Theta scanner lenses is very challenging and requires specially adapted optics or measuring sensors.

In this paper a potential arrangement for spectroscopy-based process monitoring in pre-objective scanning is presented. The process radiation was monitored using a coaxial and a quasi-coaxial observation system. The measurements were carried out on both a solid and a powder coated sample of 2.4668 (Inconel 718) to show the potential use of these systems in laser-based additive manufacturing. In order to obtain comprehensive data about the process signal, the process zone was analyzed at different angles of incidence (AOI) of the laser using a high-speed camera (HSI) and a spectrometer. The connection between the HSI and the spectral measurements is discussed. The ionization of the material and the formation of a plasma was observed and found to lose intensity as the angle of incidence increases. A model of the system that demonstrates the intensity of the emitted radiation of the plasma was created. It enables the measured values to be corrected. The corrected measurement data can be used to detect impurities or a non-ideal energy input across the entire processing field, which is a move towards robust process monitoring.

Place, publisher, year, edition, pages
Elsevier, 2022
Keywords
Laser scanner, Process control, Laser powder bed fusion, Laser welding, Spectroscopic measurements, Monitoring techniques
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-90062 (URN)10.1016/j.optlaseng.2022.107050 (DOI)000819874000003 ()2-s2.0-85127174513 (Scopus ID)
Note

Validerad;2022;Nivå 2;2022-04-04 (hanlid)

Available from: 2022-04-04 Created: 2022-04-04 Last updated: 2022-07-15Bibliographically approved
3. Proof-of-concept of an absorbance determination of a powder bed by high resolution coaxial multispectral imaging in laser material processing
Open this publication in new window or tab >>Proof-of-concept of an absorbance determination of a powder bed by high resolution coaxial multispectral imaging in laser material processing
(English)In: Additive Manufacturing, ISSN 2214-8604, E-ISSN 2214-7810Article in journal (Other academic) 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.

Keywords
Spectroscopic Measurement, Correction Method, Laser Powder Bed Fusion, Scanning Method, Metal Powder, Multispectral Imaging, Laser Material Processing
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-90581 (URN)
Available from: 2022-05-09 Created: 2022-05-09 Last updated: 2022-05-10

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