| 1. |
- Hryha, Eduard, 1980, et al.
(author)
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Surface Oxide State on Metal Powder and its Changes during Additive Manufacturing: an Overview
- 2018
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In: Metallurgia Italiana. - 0026-0843. ; 110:3, s. 34-39
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Journal article (other academic/artistic)abstract
- Quality and usefulness of the powder for additive manufacturing (AM) are strongly determined by the surface composition of the powder. In addition, taking into account harsh conditions during AM process, significant changes in the powder surface chemical composition are taking place, limiting powder recyclability. Hence, knowledge concerning amount of oxides, their composition and spatial distribution on the powder surface determines further powder recycling. This communication summarizes possibilities of qualitative and quantitative analysis of powder surface chemistry by surface-sensitive chemical analyses using XPS and HR SEM coupled with EDX. The effect of alloy composition, AM process applied and powder handling on the surface composition of the powder are addressed. Results indicate significant enrichment in the thermodynamically stable surface oxides in case of high-alloyed powder for both, EBM and LS processes. A generic model for the oxide distribution, depending on the alloy composition and powder surface degradation during AM manufacturing, is proposed.
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| 2. |
- Leicht, Alexander, 1987
(author)
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Aspects of building geometry and powder characteristics in powder bed fusion
- 2018
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Licentiate thesis (other academic/artistic)abstract
- Additive manufacturing (AM) produces near-net-shaped parts directly from a 3D-CAD model in a layer-by-layer manner. One of the most common AM technique for fabricating metallic components is powder bed fusion (PBF). The PBF process has shown great potential in fabricating metallic parts with properties better or comparable to conventional methods. However, there are some challenges in reproducibility, process stability, robustness, etc. This thesis elaborates on several of these challenges and addresses the influences of feedstock material, build orientation and part design on the final outcome. The PBF process uses fine metal powder as feedstock material and in order to have an economically feasible process, powder recycling is a necessity. However, to ensure a robust process and consistent material properties, the feedstock material need to be handled with caution as powder properties will affect the part quality. The obtained results for 316L stainless steel from this study indicate that powder degradation in terms of surface product changes occurs when the powder is recycled. It was revealed that both recycled and virgin powder were covered by a heterogeneous oxide layer, composed by a homogeneous iron oxide layer with the presence of Cr-Mn-rich oxide particulates that were growing during PBF processing. The results showed that the powder degradation was more pronounced when used in the electron beam system compared to a laser based system due to the long exposure at high temperatures. The manufacturing capabilities of the PBF process has enabled the production of lattice structures without extensive tooling. The properties of such lattice will be influenced by the microstructure. Hence, it is of importance to understand how the part geometry would affect the microstructure. This study presents the effect of build geometry, as e.g. wall thickness and build angle on the 316L microstructure. The obtained results indicated that in the center of ribs over 0.6 mm in thickness, large elongated grains with preferential orientation were created. Reducing the part thickness to below 0.6 mm reduced the predominant texture. The increased cooling rate close to the part surface inhibited grain growth and changed the preferential grain orientation. For the process parameters used, the critical part thickness to avoid large elongated grains was found to be about 0.4 mm. The obtained results could be used for further development of design rules and prediction of mechanical properties of AM parts with small wall thicknesses.
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| 3. |
- Leicht, Alexander, 1987, et al.
(author)
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Effect of build geometry on the microstructural development of 316L parts produced by additive manufacturing
- 2018
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In: Materials Characterization. - : Elsevier BV. - 1044-5803. ; 143, s. 137-143
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Journal article (peer-reviewed)abstract
- Additive manufacturing (AM) technology has shown great potential in manufacturing complex products, such as lattice structures without extensive tooling. The focus of this study was to understand the effect of the part geometry on the resulting microstructure. Direct metal laser sintering was used for fabrication of ribs with thicknesses varying between 0.2 and 3.0 mm as well as inclined ribs the with build angles of 30° and 45°. The investigation showed that grains were growing in an epitaxial manner parallel to the building direction. The large elongated grains had a preferential <101> orientation, resulting from the temperature gradient. Additionally, it was found that small grains had formed close to the part surface which were inclined towards the center of the rib. In contrast to the elongated grains, they had a random orientations. The results also indicated that at build angles below 45° the formed microstructure consisted of the large grains elongated in the building direction. For the used process parameters, the critical part thickness to avoid large elongated grains was found to be about 0.4 mm. These findings allowed us to establish the basics for design rules when it comes to thin wall structures.
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| 4. |
- Vedantha Krishna, Amogh, 1990-, et al.
(author)
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Potential approach towards effective topography characterization of 316L stainless steel components produced by selective laser melting process
- 2018
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In: European Society for Precision Engineering and Nanotechnology, Conference Proceedings - 18th International Conference and Exhibition, EUSPEN 2018. - Bedford : euspen. - 9780995775121 - 0995775125 ; , s. 259-260
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Conference paper (peer-reviewed)abstract
- In this paper, an attempt is made to explain the surface texture of Selective Laser Melting (SLM) parts more satisfyingly than the existing methods. Investigations were carried out on the 316L stainless steel SLM samples. To account for most of the surface conditions, a truncheon artefact was employed for the analysis. A Stylus Profilometer was employed as a metrology tool for obtaining the 3D surface measurements. A methodology is proposed to extract and characterize the topographic features of Additive Manufactured (AM) surfaces. Here, the overall roughness of the surface is segregated into the roughness of the powder particles and the waviness due to thermal and the "staircase" effects. Analyzing these features individually results in an increased understanding of the AM process and an opportunity to optimize machine settings.
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