Metallpulvercharakterisierung und Werkstoffuntersuchungen von Stahlwerkstoffen für den Laser Powder Bed Fusion Prozess

Translated title of the contribution: Metal powder characterization and material testing of steel materials for the laser powder bed fusion process

Gerhard Panzl

Research output: ThesisDoctoral Thesis

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Abstract

Additive manufacturing (AM) processes to produce metallic components like the laser powder bed fusion (L-PBF) process are used, when complex and multifunctional parts made of high-performance materials must be manufactured in a short period of time. Due to process related aspects like high costs, a small selection of materials or limited component size and production quantities, the fields of application are still restricted to high-value parts for aerospace, medical or motor sport components. As feedstock material spherical atomized metal powders within a particle size range of 15 to 45 (63) µm are used. A main criterion for the usability of a steel powder would be the carbon content or rather the maximum hardness level of the material during the process. If the used materials exhibit high hardness values during the build and cooling phase, crack formation within the microstructure can be observed. The first part of this work examines the usability of powder materials with different alloying concepts for the L-PBF process by means of parametric studies. Some of the alloys show a high carbon content and increased hardness values. If the parametric studies led to crack-free and non-porous samples, mechanical and technological characteristic values were evaluated and compared to standard materials. It could be shown that optimised L-PBF process parameters and adjusted heat treatments lead to similar and less scattered material property values in comparison to conventional fabricated materials. The quality of the used powder material is decisive for the properties of additively manufactured parts. For each metal powder, the process parameters (layer thickness, laser power, scan velocity, hatch distance) must be adjusted separately. Besides the chemical composition (melting point, solidification temperature range) or optical material properties (reflection, absorption), powder and particle properties are an important factor for parameter settings and processing properties. In order to achieve undisturbed building processes, a smooth powder bed is a basic prerequisite for the L-PBF process. Therefore the metal powder is applied by a recoater with a blade made from ceramics, high speed steel or plastics. The behaviour of the loosely placed powder particles is significant for the properties of the powder layers. One great advantage of the L-PBF process is the possibility to recycle and reuse the powder which surrounds the printed parts. Due to multiple reuse and recycling steps, changes of the powder characteristics can lead to insufficient part quality and hence must be compensated by adjusting the process parameters. To investigate the impact of multiple reuse on the powder characteristics, various metal powders were tested according to standard (VDI 3405) and rheological examination methods and linked to the mechanical properties of fabricated samples. One steel powder was separated into three particle size fractions, blended at certain mixing ratios and tested. It could be shown that most of the standard methods are inappropriate for the characterization of the used metal powders. While multiple reuse led to different powder characteristics, the mechanical properties remained at the same levels. The applied rheometric methods of investigation provide reproducible test results, which are in good correlation with the results of the standard examination methods (e.g. bulk density). The rheometric methods are suitable to determine also minor differences in the powder characteristics. The studies have shown that powder characteristics like flowability, compressibility, permeability and tensile strength are important factors for the specification of metal AM powders. A combination of standard and rheological investigation methods leads to a better understanding of AM powders and provide a deeper insight into the qualification of a powder for the L-PBF process.
Translated title of the contributionMetal powder characterization and material testing of steel materials for the laser powder bed fusion process
Original languageGerman
QualificationDr.mont.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Buchmayr, Bruno, Assessor A (internal)
  • Mitterer, Christian, Assessor B (internal)
Publication statusPublished - 2020

Bibliographical note

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Keywords

  • Additive Manufacturing
  • Laser Powder Bed Fusion
  • Powder Characterization
  • Powder Rheology

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