**3. Methods of characterization of metal powders properties**

In this part of the chapter, standardized methods for the characterization of metal powders will be briefly presented and given that an information about the methods is not standardized, but allow to receive an additional information about properties of metal powders. First, it should be noted that there exists a Technical Committee 119 at the International Standards Organization, since 1967 it has developed and published numerous powder metallurgy standards; most of them have adopted national versions.

All standards of powder metallurgy can be divided in two groups: standards that are similar to material characterization (they are quite typical for non‐powder materials) and standards for the characterization of properties of powder.

The first group includes general standards for the determination of common properties of material that relates to typical methods applicable to compact (non‐powdered) materials: determination of chemical composition and determination of interstitial elements. Chemical composition is usually determined by X‐ray fluorescence spectrometry, wavelength‐disper‐ sive X‐ray fluorescence spectrometry, direct current plasma, or inductively coupled plasma atomic emission spectrometry. Determination of interstitial elements is very important especially for reactive metals and alloys, and it is also important to control oxygen content, because the oxygen content may change after several reusing of metal powder due to heat affecting in additive manufacturing. It is very important to pay attention on 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.

*2.2.2. Chemical processes*

220 New Trends in 3D Printing

lowering pressure to metal powder.

*2.2.3. Plasma spheroidization*

plasma chamber.

several uses in additive manufacturing [13].

standards; most of them have adopted national versions.

for the characterization of properties of powder.

Other chemical conversion processes include the following:

– Chemical precipitation of metals from solutions of soluble salts. [9]

The leading of chemical process is a carbonyl process, which allows to produce nickel and iron powder. The crude metal reacts with gaseous carbon oxide under pressure and temperature that lead to the formation of carbonyl, which is decomposed under raising temperature and

– The manufacture of powders from sponges by thermally decomposing chlorides.

– The manufacture of powders by hydrogen reduction of salts solution under pressure.

One of the new technics for powder production for additive manufacturing is plasma spher‐ oidization. In fact, this is not the method for the production but method of additional treatment of non‐spherical powder, which allows to change the shape of particles to ideal spheres.

The world leader in production of plasma spheroidization equipment is a company Tekna. The company's line of products consists of four systems: from laboratory‐scale to industrial‐ scale serial production. Depending on parameters of initial powder, it is possible to make controllable process of full melting and get spherical form of particles during the flight through

The process benefits do not limited by changing of the shape of particles; it also decreases internal porosity of powder, improves powder flowability, increases apparent density, and enhances powder purity. The last one is quite strong benefit for posttreatment powders after

In this part of the chapter, standardized methods for the characterization of metal powders will be briefly presented and given that an information about the methods is not standardized, but allow to receive an additional information about properties of metal powders. First, it should be noted that there exists a Technical Committee 119 at the International Standards Organization, since 1967 it has developed and published numerous powder metallurgy

All standards of powder metallurgy can be divided in two groups: standards that are similar to material characterization (they are quite typical for non‐powder materials) and standards

The first group includes general standards for the determination of common properties of material that relates to typical methods applicable to compact (non‐powdered) materials:

**3. Methods of characterization of metal powders properties**

The second group of standards includes methods for the determination of next properties of powders: particle size distribution; sieve analysis; flowability; apparent density; skeletal density and determination of porosity; and shape of particles.

*Particle size distribution* is one of the most important properties of metal powder for the application of additive manufacturing. All AM‐system producers recommend to use powders prepared and supplied by manufacturer of AM system, and also manufacturer gives recom‐ mendations for particle size distribution of powders applicable to their systems. Particle size distribution is usually measured by laser diffraction methods, and a typical report of meas‐ urement has a graph and table with values of particle sizes and their volume. The general characteristics are D10, D50, and D90, which mean that volume of 10, 50, and 90% particles has a size smaller than respective values.

*Sieve analysis* is commonly used to change particle size distribution, for example, to separate huge particles. A typical sieve analysis involves a nested column of sieves with wire mesh cloth (screen). It is possible to make particle size distribution suitable to requirements and recommendation of AM‐system manufacturer by sieving.

*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 has a poor flowability or do not flow at all, but powder compositions that content fine or big particles have a good flowability. Density of metal or alloy makes 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 each other. Humidity of powder makes effect of sticking particles together and leads to getting worse results of flowability measurement, so it is strongly recommended to dry powder 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 the ratio of the mass to the volume occupied by that mass.

**Figure 1.** Images of cross section of X22CrMoV powder particles received by gas atomization with internal porosity (a) and Ti–6Al–4V powder received by plasma atomization (b).

It is often used a gas pycnometry where helium or nitrogen is used as fluid medium, because these gases have small atomic sizes and have possibility to penetrate in defects. Sometimes, it is used a liquid pycnometry where dispersion of liquids with high‐penetration properties is used as fluid medium (ethanol, oils, butanol, acetone etc.). Measuring of skeletal density is important for the estimation of quantity of defective particles with cracks, satellites, opened and closed pores (see **Figure 1**). Pycnometry also may be used for analyzing of compact materials with irregular shape. For the determination of internal porosity of powder particles, preparation of cross sections and investigation by optical of electron microscopy can also be used.

There is no international standard for measuring of *particles shape*, but there exist national standards (for example, American ASTM E20 and Russian GOST 25849) that content ap‐ proaches for the description and classification of metal powders by shape (**Figure 2**).

Determination of shape of particles can be made by optical microscopy, but more representa‐ tive results may be obtained by scanning electron microscopy (SEM). A shape of particle depends on technology, on which a powder has been made. Spherical and spheroidal shape is more specific for the atomization technologies; angular form is typical for mechanical milling and mechanical alloying; dendritic, rod, needle like, and particles with internal void are obtained by electrolysis and chemical processes; plate‐like and flaky powder can be produced by mechanical milling in shear mode.

**Figure 2.** Shapes of powders according to GOST 25849.

The determination of skeletal density is made by pycnometry methods. The physical principle of pycnometry is volumetric displacement by fluid and calculation the ratio of the mass to the

**Figure 1.** Images of cross section of X22CrMoV powder particles received by gas atomization with internal porosity (a)

It is often used a gas pycnometry where helium or nitrogen is used as fluid medium, because these gases have small atomic sizes and have possibility to penetrate in defects. Sometimes, it is used a liquid pycnometry where dispersion of liquids with high‐penetration properties is used as fluid medium (ethanol, oils, butanol, acetone etc.). Measuring of skeletal density is important for the estimation of quantity of defective particles with cracks, satellites, opened and closed pores (see **Figure 1**). Pycnometry also may be used for analyzing of compact materials with irregular shape. For the determination of internal porosity of powder particles, preparation of cross sections and investigation by optical of electron microscopy can also be

There is no international standard for measuring of *particles shape*, but there exist national standards (for example, American ASTM E20 and Russian GOST 25849) that content ap‐

Determination of shape of particles can be made by optical microscopy, but more representa‐ tive results may be obtained by scanning electron microscopy (SEM). A shape of particle depends on technology, on which a powder has been made. Spherical and spheroidal shape is more specific for the atomization technologies; angular form is typical for mechanical milling and mechanical alloying; dendritic, rod, needle like, and particles with internal void are obtained by electrolysis and chemical processes; plate‐like and flaky powder can be produced

proaches for the description and classification of metal powders by shape (**Figure 2**).

volume occupied by that mass.

222 New Trends in 3D Printing

and Ti–6Al–4V powder received by plasma atomization (b).

by mechanical milling in shear mode.

used.

In **Figure 3**, it is shown the scanning electron microscopy images of powders with different shapes.

International standards were developed for traditional powder metallurgy technologies of compacting (hot and cold pressing, hot and cold isostatic pressing, metal injection molding etc.), and additive manufacturing technologies have some particularities, so at this moment, an actual purpose consists in developing of methods of determination of properties of metal powders for AM applications.

**Figure 3.** SEM images of powders obtained by different technologies. (a) gas atomized In718; (b) chemical reduction Fe; (c) gas atomized Ti–6Al–4V; (d) plasma atomized Ti–6Al–4V; and (e) mechanically alloyed Fe–18Cr–8Ni–12Mn–N.

One of such methods is described in work [14]. The method is based on measuring of dynamic properties of powder. For measuring, FT4 powder rheometer was used, which allows the measuring of shear, dynamic, and bulk properties. Dynamically determined powder proper‐ ties are particularly more helpful for defining flowability under the low stress conditions that apply to the most parts of AM process.

Another promising method for testing of powder material was named revolution powder analyzer. The revolution powder analyzer consists of rotating drum covered on the both sides with transparent glass and camera that records pictures of rotating drum (0–200 min-i) before backlight. This method allows the modelling of powder behavior during the coating in powder bed in additive manufacturing systems. As a measuring parameter, the angle of linear regression of the free powder surface measured to a horizontal line is used, just before an avalanche starts [15].
