Powder Characterisation
MPP uses a combination of internal and independent 3rd party accredited test houses to characterise gas atomised powders.
Chemical composition and determination of interstitial elements is usually determined by X‐ray fluorescence spectrometry, wavelength‐dispersive X‐ray fluorescence spectrometry, direct current plasma, or inductively coupled plasma atomic
emission spectrometry (ICP-OES). Determination of interstitial elements is very important especially for reactive metals and alloys, it is often important to control oxygen content. It is very important to pay attention to O, H, and N content in titanium, tantalum, aluminum, and their alloys; on O, C in refractory and reactive metals and their alloys, and steels and nickel alloys.
Porosity can be measured using a combination of XRD, EDX and SEM analysis.
Other key characteristics include particle size distribution, sieve analysis, flowability, apparent density, skeletal density. The shape (morphology) of particles is very important for many downstream processes.
Particle size distribution is one of the most important properties of metal powder s for many powder metallurgy processes. For example in the application additive manufacturing (AM) the particle size distribution of powders is critical to the particular AM process and manufacturer. At MPP we can use a combination of different atomisation parameters with sieving to optimise the size distribution to exact requirements. Particle size distribution is measured by laser diffraction methods (Malvern 3000), and a typical report of measurement has a graph and table with values of particle sizes and their volume. The general characteristics are D10, D50, and D90, which mean that a volume of 10{d3983698f6bbfcfc9ce20a58bcd34d9e7e79e593f810ce5bf49abd797b6c13c4}, 50{d3983698f6bbfcfc9ce20a58bcd34d9e7e79e593f810ce5bf49abd797b6c13c4}, and 90{d3983698f6bbfcfc9ce20a58bcd34d9e7e79e593f810ce5bf49abd797b6c13c4} of particles has a size smaller than the stated value.
Sieve analysis involves using a nested column of sieves with wire mesh cloth (screen). This gives an accurate distribution profile of the powder batch.
Flowability of powder effects on smooth coating and equable feeding of powder in AM systems. The main parameters that have an influence on flowability are particle size distribution, density of metal or alloy, shape of particles, and morphology of their surfaces and humidity. Very fine powder (smaller than 10 μm) typically have a poor flowability or do not flow at all. For laser powder bed manufacturing for example these fines need to be removed (“dedusted”). MPP is able to do this operation using its multi-frequency sieve station.
The density of metal or an alloy powder has an effect because the general principle of flowability is to measure the time of flow through funnel (Hall flowmeter) with 2.5 mm diameter orifice 50 g of powder under itself weight, so if the metal or alloy has high density, powder of this metal or alloy will flow faster. Spherical powder flows better than powder with irregular form, because particles do not “cling” to each other. Humidity of powder makes effect of sticking particles together and leads to getting worse results of flowability measurement, so if this is observed the powders can be dried before using.
Apparent density is the method for the measurement of density of powder compact in a density cup (25 cc) which was received by free flow of powder through a funnel in the density cup. Particle size distribution and shape of particles have influence on apparent density.
Skeletal density shows true solid state density of alloy or powder material. Density depends on quantity of alloying elements, their content in the alloy, and phase composition of material. The determination of skeletal density is made by pycnometry methods. The physical principle of pycnometry is volumetric displacement by fluid and calculation of the ratio of the mass to the volume occupied by that mass.
Shape (morphology) of particles is made by optical microscopy, but more representative results may be obtained by scanning electron microscopy (SEM). MPP uses PSI designed close-coupled die technology which produces spherical powders with minimal to no satellites. The powders are therefore free flowing and ideal for many downstream processes that require this property.
