Metal Powders particle characterization for additive manufacturing processes

Additive manufacturing technologies are increasingly used in the construction of machines, means of transportation and many other products. In aircraft construction, for example Metallic 3-D printing opens entirely new possibilities for weight reduction, and consequently the reduction of kerosene consumption.

Parts that previously had to be assembled from dozens of individual components can now be manufactured directly in one piece. Advances in the development of additive manufacturing allow more and more parts to be produced in large quantities by 3D printing.

Metal Powders used for additive manufacturing must meet the highest quality standards: The particle size distribution should be narrow and must be known as precisely as possible in order to control the behavior of the material during the sintering process.

MICROTRAC particle analyzers are ideally suited to determine the particle size distribution of metal powders used for additive manufacturing processes. The following provides an introduction to the suitable measuring technologies, general considerations as well as different examples of metal powder particle characterization.

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Metal Powders & Additive Manufacturing Particle characterization methods

In additive manufacturing, the particle size range of the powder used usually lies between 20 and 80 μm. Dust, non-spherical particles or large, fused grains disturb the manufacturing process and can cause defects in the component.

Since only a small portion of the powder is incorporated in the component, there is inevitably a lot of powder left over which is reused for the next process. Whether the recycled powder still meets the high quality requirements is one of the most important questions in the analysis of metal powders.

Microtrac offers two different technologies for the particle size characterization of metal powders: Laser Diffraction and Dynamic Image Analysis. Both methods provide a size distribution, but only imaging methods also detect the particle shape which is crucial for the suitability of a powder for additive manufacturing. Whereas Microtrac's CAMSIZER series is a range of dedicated image analysis devices, the SYNC combines Laser Diffraction and Dynamic Image Analysis in a unique way.

Another powder metallurgical process that is particularly suitable to produce small components with complex geometry in large quantities is Metal Injection Molding (MIM). With a particle size of typically 1-10 μm, the powders used for this process are even finer than those used for additive manufacturing. With Microtrac technology and equipment, however, even these fine powders can be analyzed without any problems.

Metal Powders & Additive Manufacturing - Figure 1

Figure 1:
With additive manufacturing techniques such as selective laser sintering, complex components can be manufactured in one piece. Only a small portion of the powder used becomes part of the product and may have to be prepared and tested before reuse. Image: Premium Aerotec

Metal Powders & Additive Manufacturing Dynamic Image Analysis (DIA)

With Dynamic Image Analysis, a particle stream is generated which is guided past a camera system. The resulting particle images are transferred directly to a PC and are evaluated in real time. The sample moves either in an air stream or in liquid.

The CAMSIZER X2 with a measuring range from 0.8 μm to 8 mm and an image acquisition rate of over 300 frames / second is particularly suitable for fine metal powders, as required in additive manufacturing.

Metal Powders & Additive Manufacturing - Figure 2

Figure 2:
CAMSIZER X2 operating principle. By using two cameras with different magnifications, a wide measuring range is covered. Large and small particles can be analyzed simultaneously under optimum measuring conditions.

CAMSIZER X2

Metal Powders & Additive Manufacturing Laser Diffraction combined with Image Analysis

Laser diffraction is the standard method for determining particle size distributions in many industries. This technique can also analyze particles in an air stream or as a suspension in a liquid.

The measuring method is based on the principle that laser light is diffracted or scattered at different angles from particles of different sizes. The calculation of the size distribution is based on the analysis of the scattered light patterns.

The strength of the measuring method lies in its high flexibility, easy handling and the extremely wide measuring range of 10 nm to 4 mm. However, laser diffraction is not suitable to determine the particle shape.

For this reason, Microtrac has equipped its powerful laser diffraction analyzer SYNC with an additional camera module based on the principle of dynamic image analysis. This uses the same measuring cell and the same dispersion system as for scattered light analysis.

The SYNC is a high-end laser diffraction analyzer with integrated imaging module.

SYNC

Example Metal Powders characterized by Laser Diffraction and Image Analysis

Four metal powders were analyzed with both measuring instruments, CAMSIZER X2 and SYNC. The size distributions show the same trend: Sample 1 and 2 are relatively fine powders with a median of about 30 μm, whereas sample 1 contains particles < 20 μm which are missing in sample 2. It is noticeable that in the CAMSIZER analysis the fine fraction of sample 1 is measured in a clearly separated way (bimodal), whereas the laser result shows a gradual transition. Samples 3 and 4 are coarser, but similar to each other. Fig. 4 and 5 show the size results of image analysis and laser diffraction.

Metal Powders & Additive Manufacturing - Figure 4

Metal Powders & Additive Manufacturing - Figure 5

Figure 4:
Size distribution of four metal powders, analyzed with CAMSIZER X2 (size definition x_area).

Figure 5:
The same four metal powders analyzed with laser diffraction

With image analysis using the CAMSIZER X2, three size distributions can be determined for each sample, based on the width, length and diameter of the equal area circle (xarea) of each particle projection. If the particles are approximately spherical, like samples 1 and 2, these three distribution curves are almost congruent. If the sample contains non-spherical particles, as in material 3 and 4, the distributions for length, width and xarea are different. The more irregular the particle shape, the further apart the curves lie. Laser diffraction does not distinguish between length and width, all measurement signals are related to the diameter of the equivalent sphere. The size distribution consequently lies between the length and width distribution of the image analysis results (Fig. 6 below).

Metal Powders & Additive Manufacturing - Figure 6a

Metal Powders & Additive Manufacturing - Figure 6b

Metal Powders & Additive Manufacturing - Figure 6c

Metal Powders & Additive Manufacturing - Figure 6d

Figure 6:
Comparison of CAMSIZER X2 image analysis and SYNC Laser Diffraction for all four samples. CAMSIZER particle width (red), CAMSIZER particle length (blue), CAMSIZER x_area (green), SYNC Laser Diffraction (black).

Sample 2 was screened at 50 μm, so no particles above this size should be present. In the CAMSIZER analysis the distribution follows the expected behavior: the curves reach 100% at 50 μm. Only in the case of the length measurement some % larger than 50 μm are detected. Since the particles pass through the apertures of a sievewith their smallest projection area, the width of these particles is less than 50 μm, but they can still be longer!

Here, the laser measurement even shows about 5 % particles larger than 50 μm. If, however, the image evaluation function is used on the SYNC analyzer, the sharp separation at 50 μm also evident here. This shows that by using the image evaluation function with the SYNC, the upper limit of the distribution can be detected with similar accuracy as with the CAMSIZER. A laser analyzer without integrated image evaluation does not have this possibility!

Metal Powders & Additive Manufacturing - Figure 7

Figure 7:
Comparison of CAMSIZER X2 and SYNC Image Analysis for sample 2. CAMSIZER particle width (red), CAMSIZER particle length (blue), CAMSIZER x _area (green), SYNC Image Analysis (black).

Example Oversize Particles

Many production processes, including additive manufacturing, are sensitive to small quantities of large particles (oversize). In metal powders, for example, these large particles can lead to cavities or weak points in the end product.

Simply determining the average or mean particle size is not enough to predict manufacturing performance. The volume of particles larger than a certain limit size must be carefully monitored. It is possible to define a specification that no more than a small fraction of the particles can be larger than a critical size.

For example, you could require that no more than 0.01% by volume of the particles are larger than 200 microns.In this measurement example, a sample of metal powder with different amounts of impurities (oversize particles) was gravimetrically prepared and the resulting size distributions were measured to illustrate how the high-speed dual camera system of the CAMSIZER X2 can be used to find small amounts of impurities with large particles

Metal Powders & Additive Manufacturing - Figure 8a

Metal Powders & Additive Manufacturing - Figure 8b

Metal Powders & Additive Manufacturing - Figure 8c

Figure 8:
Detecting oversize with the CAMSIZER X2. Left: weighing of the powder; Middle: adding a defined amount of oversize; Right: CAMSIZER X2 image acquired during analysis showing many small metal powder particles and one oversize piece

A metal powder sample was first sieved through a 200 μm test sieve to ensure the removal of large contaminants. This screened powder was then weighed and a small amount of large particles was added in a controlled manner. This resulted in a series of samples with known amounts of impurities. Concentrations were 0.005%, 0.01%, 0.02%, 0.05%, 0.1%, 0.2% and 1% (mass % each). The sample quantities for analysis were approximately 35-40 grams. Fig. 9, Fig. 10, and the table show how accurately the oversize grain can be detected with the CAMSIZER.

Metal Powders & Additive Manufacturing - Figure 9

Metal Powders & Additive Manufacturing - Figure 10

Figure 9:
CAMSIZER X2 result for metal powder with 1% oversize added: the distribution is between 50 μm and 200 μm. The oversize is represented as a step in the cumulative Q3 distribution at 99% (red). It is also visible in the q3 frequency distribution (blue).

Figure 10:
Q3-distribution of metal powder with different amount of added oversize: 0.2 % (green), 0.1 % (blue), 0,05 % (purple), 0.02 %(orange), 0.01 % (brown) and 0.05 % (red)

% oversize> 200 μm added	% oversize>200 μm detected by CAMSIZER X2	Difference
0.005 %	0.005 %	0.000 %
0.010 %	0.013 %	0.003 %
0.020 %	0.019 %	0.001 %
0.050 %	0.054 %	0.004 %
0.100 %	0.107 %	0.007 %
0.200 %	0.201 %	0.001 %
1.000 %	0.936 %	0.064 %

In Laser Diffraction, it is assumed that under favorable conditions oversized particles can be detected if the percentage is >2 % by volume. Laser diffraction evaluates a signal generated by all particles simultaneously. This is therefore referred to as a collective measurement method, as opposed to an individual particle measurement method such as image analysis in which each particle detected generates a measurement value. In laser diffraction, if the proportion of a certain fraction is too small, the contribution of these particles to the total scattered light signal is also too small to be distinguishable from background noise. This situation cannot be compensated for by measuring larger sample quantities.

The combination of image analysis and laser diffraction improves the detection probability of impurities, but the performance here does not come close to that of a specialized dynamic image analyzer like the CAMSIZER X2. This is mainly due to the image acquisition rate of the CAMSIZER X2 which is 14 times higher. The dispersing system, sample feed and instrument setup of the SYNC are optimized to generate high quality scattered light data in a short time with the additional possibility of image acquisition. The entire hardware of the CAMSIZER X2, i.e. dispersion, sample feed, light sources and cameras, is optimized to acquire and evaluate many images in a short time. The number of particles evaluated, as well as the total amount of sample material used is considerably larger with the CAMSIZER X2.

Nevertheless, the SYNC is clearly superior to other laser analyzers with regard to the detection of oversized particles thanks to the advanced image evaluation.

Example Satellites

Due to production conditions, particles can be fused together in gas-atomized metal powders. Aggregates of several spherical particles are considerably larger and can be removed by sieving. More problematic are so-called satellites. These are small particles that adhere to larger ones. Figure 11 shows some of the images taken by the CAMSIZER X2 of particles with satellites. Since these have a negative influence on the flow and sintering behavior of the metal powder during additive manufacturing, the metal powder must not contain too many satellites.

Metal Powders & Additive Manufacturing - Figure 11

Figure 11:
CAMSIZER X2 images of almost perfectly round metal particle (left) and particles with satellites (right).Size and shape data are displayed next to every particle. By selecting appropriate shape parameters and threshold values, the amount oof defevtive particles in a sample can be measured.

The measurement example shows the comparison of the particle shape of samples 2 and 4 from Fig. 6. Sample 4 contains significantly more non-spherical particles or satellites. This is shown by the Q3 distribution of the shape parameters aspect ratio and symmetry. The further the curve in the diagram lies to the right (values closer to 1), the more symmetrical or round the particles are.

Metal Powders & Additive Manufacturing - Figure 12a

Metal Powders & Additive Manufacturing - Figure 12b

Figure 12:
CAMSIZER X2 shape analysis. Aspect Ratio (width divided by length, left side) and Symmetry (right).
Sample 2 (red) and sample 4 (blue).

The image evaluation of the SYNC can also be used to describe the particle shape and to make a statement about the content of satellites and non-spherical particles. Fig. 13 shows scattergrams of sample 2 and sample 4, where each point represents a measured particle. Fig. 14 shows examples of some spherical and non-spherical particles as recorded by the SYNC camera.

Metal Powders & Additive Manufacturing - Figure 13a

Metal Powders & Additive Manufacturing - Figure 13b

Figure 13:
SYNC image analysis – scattergram of size and sphericity for sample 2 (left) and sample 4 (right). Almost no particles with sphericity < 0.95 are present in sample 2. A perfect sphere will have sphericity of 1.

Metal Powders & Additive Manufacturing - Figure 14

Figure 14:
Sync image evaluation of non-spherical metal powder particles (left) and round particles (right).

Both instruments can detect differences in particle shape and clearly distinguish a sample with many satellites from a sample with few satellites. Which shape parameter is most suitable depends on the application and the resolution of the measuring instrument.

The user has to define suitable parameters and threshold values in the course of application development: Which symmetry and sphericity characterize a particle as "faulty", how many "faulty" particles may the material contain so that the production process still functions acceptably? Experience is required.

The easiest way is to analyze and compare samples of different quality levels, e.g. "excellently suitable", "well suitable", "just about suitable" and "unsuitable". This gives an overall picture when comparing and interpreting the data. Then, any new unknown samples can be immediately assessed with regard to their suitability for additive manufacturing.

Metal Powders & Additive Manufacturing Method Comparison and Summary

The measurement examples show that laser diffraction is suitable for fast and reliable determination of the particle size distribution of metal powders in additive manufacturing applications. But this is not enough for many requirements. The particle shape can only be described with imaging techniques. The recorded particle images immediately provide the user with qualitatively and quantitatively valuable additional information about the sample material.

This is possible with a combined device like the SYNC. Nevertheless, the dispersion system and the measuring procedure is optimized for laser analysis, so that only a 100% image analysis instrument, like the CAMSIZER X2, can fully utilize the advantages of the method.

The CAMSIZER X2 evaluates larger sample quantities and analyzes more images per second which leads to higher statistical certainty and significance of the results. However, if also finer particles are to be measured, the flexibility of the diffraction method with the capability of measuring particles < 1 μm could make the SYNC the more suitable device.

Both methods can analyze the samples either dry in an air stream or wet in a suspension. With the CAMSIZER X2, dry measurement would be preferable, since the advantages of the large sample quantity are particularly evident here. With SYNC, wet measurement would tend to be the method of choice.

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