When dealing with Additive Manufacturing technology a strong preference goes to powders obtained through gas-atomizing processes, because they allow to obtain highly spherical particles.
Particles morphology can be observed using a Scanning Electron Microscope (SEM); this high-magnification laboratory microscope allows to investigate the geometric properties of the powder such as quantify its roundness and observe the possible presence of defects (particles clusters, satellites, empty particles, etc.).
In case of highly spherical powders, every single powder particle can be assimilated to a perfect sphere of which we can measure the diameter. The particles size distribution are usually measure by means of a Laser Diffraction Analyzer that uses a laser beam and advanced mathematical algorithm in order to calculate the particles size distribution, the mean particles diameter and other geometrical parameters useful for characterized the powder. The typical powders size distribution for Additive Manufacturing application is unimodal with a grain size between 10 and 45 micron but other particles size range could be adopted in function of the specific process, machine, material and final application.
Powders morphology and size distribution are very important technological parameters because they contribute to define the powder flowability, therefore its layering capability (e.g. the easiness of being layered). Determination of particle size profile of the powders can be useful as quality control for new materials or for monitoring material feedstock after powders recovery.
The particles shape influences the flowability and is therefore an important properties during the deposition of a layer. Particles with a spherical shape flow better that irregular particles, such as, course particles with a narrow size distribution flow better than fine particles with wide size range. Fine particles has also the tendency to create agglomerates reducing in this way the ability to flow. Anyway, flowability cannot be considered a direct property of the material since it results both from the material’s physical properties but also from some external factors as the system adopted to measure it, material density, friction, surface area, moisture, etc.
Flowability is usually measure using a special funnel having a calibrate orifice called Hall Flowmeter and expressed as sec/50g; other method keep in account the compressibility (ratio between Tap Density and Apparent Density) of the powder as an indicator of its tendency to flow (Hausner ratio and Carr index). Determination of flowability of powders can be useful for quality control in case of new material or for monitoring material feedstock after powders recovery.
The apparent density is strongly affected by particles size but other powders characteristics such as metal density, particles morphology, surface area and so on, must be considered. Apparent density generally decrease when present an high fraction of fine particles such as when the particles shape becomes less spherical and more irregular.
For Additive Manufacturing powders, with unimodal distribution and high spherical shape, the free-flow apparent density is about 45-60% of the bulk material density.
Determination of apparent density of powders can be useful as quality control for new materials bathes or for monitoring materials feedstock after powders recovery.
Vibrational movements of loose powder induces lower friction between particles increasing the powder packaging; tap density is therefore higher in comparison to apparent density. Tap density is mainly correlated to the shape and size distribution of the particles.
Tap density is measured using a graduate glass cylinder of appropriate capacity and accuracy; the cylinder is tapped by means of mechanical device at the rate of 200-300 taps per minute until the maximum powders packaging is achieved.
Determination of tap density of powders can be useful as quality control incase of new materials batches or for monitoring feedstock powders after their recovery.
Determination of the chemical composition via wet method using emission spectroscopy with plasma torch (ICP-EOS). Analysis of both the macro-elements and impurities.
Determination by combustion analysis of chemical elements such as oxygen and nitrogen (data not obtainable by means of ICP-EOS).
Determination by combustion analysis of chemical elements such as carbon and sulfur (data not obtainable by means of ICP-EOS).
Determination of the density of the sample using Archimedes' method.
Determination of the porosity distribution area of metallographic section of the specimen by processing images obtained by optical microscopy (LOM).
Determination of the melting range of the material by means of differential thermal analysis (DTA).
Determining spectrometric CIELab color coordinates of metallographic section of the specimen.
Determination of roughness (eg value of Ra[m] Rz [m]) on the specimen. Possibility of testing in different conditions of surface finishing (eg: after blasting and / or shot peening).
Determination of the main mechanical characteristics of the material via pull test (eg: load at break, yield stress, elongation). Ability to run the test in different states metallurgical (eg after heat treatment).
Tensile test machine
Facilities for metallographic preparation
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