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  • 1.
    Hulme-Smith, Christopher
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.
    Mellin, P.
    Marchetti, Lorenzo
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.
    Hari, Vignesh
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Uhlirsch, M.
    Strandh, E.
    Saeidi, K.
    Dubiez-Le Goff, S.
    Saleem, S.
    Pettersson, V.
    Memarpour, A.
    Jakobsson, K.
    Meurling, F.
    A practicable and reliable test for metal powder spreadability: development of test and analysis technique2023In: Progress in Additive Manufacturing, ISSN 2363-9512, Vol. 8, no 3, p. 505-517Article in journal (Refereed)
    Abstract [en]

    A crucial step in the powder bed metal additive manufacturing process is the formation of a thin layer of powder on top of the existing material. The propensity of the powder to form thin layers under the conditions used in additive manufacturing is critically important, but no test method has yet been established to measure this characteristic, which is sometimes referred to as spreadability. The current work spreads a single layer of powder using commercial equipment from the paint and food industries and derives the density of a layer of powder, which is of a similar thickness to that in additive manufacturing. Twenty-four powders from eight suppliers have been tested and the density of the layers has been measured as a function of various parameters. Twenty-two of the powders successfully form thin layers, with a density of at least 40% of each powder’s apparent density. Hall flow time did not correlate with the spread layer density, although the two powders that did not spread did not pass through the Hall funnel. The roughness of the plate onto which the powder was spread, the recoater speed, the layer thickness, particle size and aspect ratio all affect the measured layer density. Results of the new test are repeatable and reproducible. These findings can be used to develop a test for spreadability for metal powders that can be used for additive manufacturing, which will help to improve the quality of printed components. 

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  • 2.
    Marchetti, Lorenzo
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.
    Hulme-Smith, Christopher
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.
    Comparative study of metallic powder flowability testing methods2019In: Euro PM 2019 Congress and Exhibition, European Powder Metallurgy Association (EPMA) , 2019Conference paper (Refereed)
    Abstract [en]

    The flowability of a powder is an important characteristic that depends on both the physical properties of the particle ensemble and the testing method, equipment and conditions. In additive manufacturing processes, flowability can be an indicator of the quality of the deposited powder layer and, therefore, the final component. In this study, we carried out a comparison of the flowability of different steel powders and the data generated by a range of testing methods. Each sample had a unique combination of composition and particle size range. Flowability testing equipment and methods were selected according to standards (angle of repose, Hausner Ratio, Hall flow) or published literature (FT4 Powder Rheometer). We measured the flowability of different samples for each testing method, in order to obtain a first dataset. Secondly, we evaluated the correlations between different flowability testing methods.

  • 3.
    Marchetti, Lorenzo
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.
    Hulme-Smith, Christopher
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.
    Flowability of steel and tool steel powders: A comparison between testing methods2021In: Powder Technology, ISSN 0032-5910, E-ISSN 1873-328X, Vol. 384, p. 402-413Article in journal (Refereed)
    Abstract [en]

    Theflow behaviour of a powder is critical to its performance in many industrial applications and manufacturing processes. Operations such as powder transfer, die filling and powder spreading all rely on powder flowability. Multiple testing methods can help in assessing flowability, but it is not always clear which may better represent specific flow conditions or how different metrics correlate. This study compares 8 different flowability testing methods using 11 steel powders varying in chemistries and size fractions. Regression analysis was used to test the relationship between each flowability metric obtained. Some metrics, such as the conditioned bulk density,relate to many flowability indicators. Others, such as the basic flowability energy, show poor correlations to othervariables, likely describing different aspects of the powder flow behaviour. When two metrics show a strong correlation, as between conditioned bulk density and Hausner ratio, a numerical relationship is derived: CBD =−(5.65 ± 0.86)HR g cm−3.

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  • 4.
    Marchetti, Lorenzo
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.
    Mellin, P.
    Hulme-Smith, Christopher
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.
    Effects of Humidity on the Flowability of Steel Powders2020In: Proceedings - Euro PM2020 Congress and Exhibition, European Powder Metallurgy Association (EPMA) , 2020Conference paper (Refereed)
    Abstract [en]

    Humidity and temperature can vary across the world. For this reason, during storage and processing, batches of the same powder may interact with diverse environmental conditions and eventually vary in their properties. For example, a higher atmospheric water vapour content could increase the water adsorbed on the surface of individual powder particles. This adsorbed water can alter the surface interactions between particles, affecting its flow behaviour. In this study, we measured the flow behaviour of various sieve fractions and steel powders in different environmental conditions. We set the environmental conditions in a C1000-40 climate chamber to different levels of relative humidity and temperature. The powder samples and a Freeman FT4 powder rheometer were placed in the climate chamber and equilibrated for 72h. Subsequently, we tested the flow behaviour using two different programs of the powder rheometer. Ultimately, we found that powder flowability is adversely affected by increased humidity.

  • 5.
    Marchetti, Lorenzo
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.
    Mellin, P.
    Swerim AB, Kista, Sweden.
    Hulme-Smith, Christopher
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.
    Negative impact of humidity on the flowability of steel powders2022In: Particulate Science and Technology, ISSN 0272-6351, E-ISSN 1548-0046, Vol. 40, no 6, p. 722-736Article in journal (Refereed)
    Abstract [en]

    Atmospheric humidity is introduced into powders during handling, transportation, and storage. High moisture content can increase cohesive forces between particles and make it difficult to spread a powder into thin layers in powder bed processes or to fill a mold in processes such as press-and-sinter. Furthermore, water can cause porosity and uptake of oxygen in the final component, damaging its mechanical properties. In this study, a Freeman FT4 powder rheometer was placed inside a climate chamber. Both flowability and shear tests were performed on four steel powders under a range of humidity and temperatures. Basic flowability energy and specific energy were both found to increase significantly with humidity (typically increase by 50% for 80% of relative humidity compared to dry conditions) and were insensitive to temperature change (10–30 °C). Conversely, the behavior of the powders under shear was neither sensitive to relative humidity nor temperature. Measurements of moisture content revealed that finer powders contained more moisture than coarser ones, but the moisture content was not correlated with humidity, probably due to shortcomings with the measurement method. This knowledge can be used to optimize powder processing conditions.

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  • 6.
    Marchetti, Lorenzo
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.
    Mellin, Pelle
    Swerim AB, Kista, Sweden.
    Neveu, Aurélien
    Granutools, Liege, Belgium.
    Hulme-Smith, Christopher
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.
    On Metal Powder Tribocharging and Humidity Adsorption2022In: World PM 2022 Congress Proceedings, European Powder Metallurgy Association (EPMA) , 2022Conference paper (Refereed)
    Abstract [en]

    In this work, three characterization techniques were applied to a set of alloys in fine powder form (TiAl6V4, AlSi10Mg, AlSi7, 316L, ferritic stainless steel, martensitic stainless steel and WC-Co-mixes). We sought deeper understanding of response to humidity and flowability as a result of the powder alloy. Slight tribo-charging (induced and measured using a GranuCharge setup) against metal surfaces was found to occur for all alloys. Although the accumulated charge was small and dissipated quickly. Greater charging occurs if the environment is humid, and if the powder slides against plastic surfaces. Dynamic Vapor Sorption (DVS) was employed to understand the adsorption capacity of powders. It showed that WC-Co-mixes adsorbed much more humidity than the other materials. Some alloys retained some of the adsorbed mass when humidity returned to normal conditions. RPA was tested on powders during exposure to 20-98% RH, which above 80% RH caused declining flowability.

  • 7.
    Mellin, Pelle
    et al.
    Swerim AB.
    Rashidi, Masoud
    Chalmers University of Technology.
    Fischer, Marie
    Chalmers University of Technology.
    Nyborg, Lars
    Chalmers University of Technology.
    Marchetti, Lorenzo
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.
    Hulme-Smith, Christopher
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.
    Uhlirsch, Markus
    Swerim AB.
    Strondl, Annika
    Swerim AB.
    Moisture in Metal Powder and Its Implication for Processability in L-PBF and Elsewhere2020In: Berg- und Huttenmännische Monatshefte (BHM), ISSN 0005-8912, E-ISSN 1613-7531, Vol. 166, no 1, p. 33-39Article in journal (Refereed)
    Abstract [en]

    The impact of moisture on flowability and spread- ability is discussed. More notably we also present research on the impact of moisture on built nickel-base material. One lot of a newly opened Hastelloy X (HX) L-PBF powder was split into two equal batches. One batch was moisturized using a programmable climate chamber, the other was un- treated. We built bars with both batches for mechanical testing, in an EOS M100, with a cold build plate. The mois- ture content of the two powder batches, before and after the build-jobs, were determined using Karl Fischer titration (KF). Regarding the periodical monitoring of moisture con- tent, it is not needed according to the findings of this paper. More moisture contributes to a slightly higher O-content in the built material, and in turn, a very slight reduction in im- pact toughness. If a newly purchased powder exhibits poor flowability or high oxygen content, the analysis using oven- desorption followed by KF is recommended. If the moisture content is high, a drying of the powder is recommended

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