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  • 1.
    Hulme-Smith, Christopher
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.
    Flow behavior of magnetic steel powder2022In: Particulate Science and Technology, ISSN 0272-6351, E-ISSN 1548-0046, Vol. 40, no 5, p. 576-588Article in journal (Refereed)
    Abstract [en]

    Flow occurs in most powder-based processes, opposed by various cohesive forces. Magnetism is often overlooked for metal powders. Here, flowability and magnetization were measured for a dual-phase steel powder in size fractions from (Formula presented.) to > 200 µm. The finest fraction did not flow through a Hall flowmeter, then flow time increased continuously with particle size from 12 ± 1 s for the next fraction ((Formula presented.)) to > 28 ± 0.5 s for > 200 µm. Drying had little effect. Key metrics derived from shear tests gave no overall relationship between flow behavior and particle size. Magnetism was considered the most likely reason for this behavior. Magnetometry showed a remanent magnetization of (Formula presented.) which causes ∼ 5 µN cohesion between 200 µm diameter particles. X-ray diffractometry showed that the powder contained 77 wt%-80 wt% of (magnetic) martensite. Liquid bridging, van der Waals forces and friction (in the Hall flowmeter geometry) contribute 50 µN, 0.08 µN and 4 µN, respectively, to cohesion in 200 µm particles. These results can be used to help explain flow behavior in other magnetic powders and allow optimization of powders and/or powder-based processes. 

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  • 2.
    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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  • 3.
    Skrinjar, Olle
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Size Ratio Effects on Particle Contact Evolution at Uniaxial Powder Compaction2012In: Particulate Science and Technology, ISSN 0272-6351, E-ISSN 1548-0046, Vol. 30, no 4, p. 364-377Article in journal (Refereed)
    Abstract [en]

    Cold compaction of composite powders has been analyzed using a discrete element method (DEM). Powder aggregates consisting of up to approximately 10,000 particles and formed by two powder populations with known material strength and size ratios have been compacted both isostatically and uniaxially (die compaction). The particles were assumed constitutively to be perfectly plastic or rigid and as a result, local contacts between the particles were described by a linear force-displacement relation given by previous in-depth analyses of spherical indentation problems. Particular emphasis has been placed on investigating the particle contact evolution at die compaction and to compare the results with previous ones pertinent to the isostatic case. Consequently, the predictive capability of the fundamental assumptions frequently used in theoretical analyses of compaction problems is determined for a uniaxial situation. The main conclusion is that size ratio effects are substantial at die compaction and when such features are present, theoretical predictions overestimates the (average) number of contacts per particle. It was also found that the mechanical behaviors at isostatic and die compaction are very similar even though die compaction values are slightly higher at high values on the relative density of powder materials.

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