Dynamic Image Analysis for Particle Characterisation of Metal Powders

Production capacities for specialized metal powders are increasing all over the world. Improvements in production technologies create a demand for customized and reliably controlled powders with distinctive properties in terms of particle size distribution, chemical composition, and particle morphology, as these strongly influence the subsequent processing. This applies to standard powder metallurgy processes (mixing-compacting-sintering) as well as to more sophisticated techniques like additive manufacturing (AM) or metal injection molding (MIM). For MIM, metal powder is mixed with a binder and then injected into a mold, followed by chemical or thermal removal of the binder and final sintering (Figure 1). A rapidly growing technology for the production of metal parts is additive manufacturing. The term comprises various techniques such as selective laser melting, selective laser sintering, or electron beam melting. The product is built layer-by-layer in all these processes. Each layer is sintered from a smooth surface of metal powder by a laser beam, followed by adding a new layer of powder (Figure 1).

Metal Injection Molding (MIM) (Source: www.epma.com)

Additive Manufacturing with selective laser melting. The workpiece is designed digitally and built layerby-layer. The sintering is achieved with a laser, followed by lowering the platform and adding another layer of powder

Figure 1. Top: Metal Injection Molding (MIM) (Source: epma.com) Bottom: Additive Manufacturing with selective laser melting. The workpiece is designed digitally and built layerby-layer. The sintering is achieved with a laser, followed by lowering the platform and adding another layer of powder

This article presents several examples of how the shape and size of typical metal powders such as Ti64, Al, Ni, W, Cr, as well as of alloys can be characterized by Dynamic Image Analysis using the CAMSIZER X2. The benefits of this method are high resolution, short analysis times and exceptional repeatability. Additionally, a wealth of material data is provided, offering the user an in-depth understanding of the powder quality.

Image Analysis: What You See is What You Get

Imaging techniques offer a direct approach to particle size analysis. The fundamental idea is simple: “What you see is what you get”. Automatic software algorithms define morphology and size based on pictures of individual particles. Particle width and length information is directly available as shown in Figure 2. DIA offers considerable versatility by simultaneously measuring particle shape and size. A selection of shape parameters is explained in Figure 3.

Selection of basic size parameters used in image analysis. The size distributions are based on width (red), length (blue) or equal area diameter (green).

Figure 2. Selection of basic size parameters used in image analysis. The size distributions are based on width (red), length (blue) or equal area diameter (green).

Selection of basic shape parameters used in image analysis.

Figure 3. Selection of basic shape parameters used in image analysis.

Two imaging methods are available, Static and Dynamic Image Analysis (SIA and DIA, ISO 13322-1 and 2). The static optical microscopy (SIA) has frequently been used to obtain a qualitative impression of the shape of the particles. However, the small amount of material and the insufficient dispersion of the particles on the microscope slide prevent reliable quantitative analysis. The same disadvantages are associated with Scanning Electron Microscopy, plus this method is a lot more difficult, time consuming and expensive.

In the measurement set-up of Dynamic Image Analysis, particles, typically in a size range from 0.8 µm to several millimeters, move in front of a camera system, either transported in liquid or by air flow. Thus, it is possible to obtain data from hundreds of thousands up to several millions of particles within a few minutes. The results are based on a representative amount of sample material and are thus statistically sound.

Figure 4 shows the key set-up of the optics for Dynamic Image Analysis. As the particles travel through the field of view a light source illuminates the particles from one direction while a camera system captures pictures from the opposite side. A software assesses the shadow projections of the particles to establish the size distribution of the sample with a high acquisition rate. An innovative feature of Retsch Technology’s CAMSIZER X2 is the dual camera technology: Two cameras with diverse magnifications spanning a broad measuring range. One camera with high magnification is designed for the analysis of small particles, a second camera with a lower magnification but wide field of view allows simultaneous analysis of the larger particles with high detection efficiency. The CAMSIZER X2 records over 300 pictures per second with one single image easily comprising hundreds of particles.

Patented measuring principle of CAMSIZER X2

Figure 4. Patented measuring principle of CAMSIZER X2

DIA allows to measure quantitative particle shape (percentage of round versus irregular-shaped particles, agglomerates, satellites etc.) and particle size distribution. Smallest quantities of undersized, oversized, or irregular-shaped particles can be detected, even with a percentage as low as 0.01%. DIA enables users to gain a full and thorough understanding of size-and morphology-related sample properties. DIA is the suitable technique for both R&D applications and quality control as it offers accuracy and sensitivity as well as easy handling and robustness.

In the following sections, a range of application examples shows the suitability of DIA to fully characterize metal powders.

Wide Range of Materials, Particle Sizes and Particle Shapes

The results of the size analysis of ten different metal powders which are commonly used for powder metallurgical applications are shown in Figure 5. Regardless of the difference in density, size, chemistry, and shape, all samples can be measured using the CAMSIZER X2, with one instrument setup. An automatic feeding chute moves the sample to the analyzer where the particles are captured by an air flow. The air pressure may be set between 5 kPa and 460 kPa. In this case, 50 kPa have been found adequate to accomplish full dispersion, i.e. separation of individual particles.

Particle size analysis of ten different metal powders with the CAMSIZER X2. The direct measurement ensures accurate results.

Figure 5. Particle size analysis of ten different metal powders with the CAMSIZER X2. The direct measurement ensures accurate results.

The samples display a range of mean particle sizes between 10 and 50 µm, with varying widths of distribution (refer Figure 5). In this example, the iron powder (Fe) is the coarsest, while the steel powder (316) is the finest. The titanium powder is characterized by an extremely narrow size distribution.

The shape diagram, as shown in Figure 6, illustrates that the iron powder has the least aspect ratio (breadth/length), while the titanium powder has the largest share of spherical particles.

Analysis of particle shape of 10 different metal powders with Dynamic Image Analysis (CAMSIZER X2). Beside the quantitative results, the recorded images allow an intuitive understanding of morphology and size differences. More spherical particles with higher aspect ratio plot on the right side of the diagram. Detecting smallest amounts of irregular particles in a large quantity of predominantly spherical particles is a great advantage of DIA.

Figure 6. Analysis of particle shape of 10 different metal powders with Dynamic Image Analysis (CAMSIZER X2). Beside the quantitative results, the recorded images allow an intuitive understanding of morphology and size differences. More spherical particles with higher aspect ratio plot on the right side of the diagram. Detecting smallest amounts of irregular particles in a large quantity of predominantly spherical particles is a great advantage of DIA.

Powder metallurgical processes typically require an extensive size distribution to enable packing the powder into the die easier by filling the spaces between large particles with smaller ones. An irregular shape is mostly advantageous for the sintering process as it increases the contact between particles. However, the particles should not be very irregular as this will make compaction harder.

For additive manufacturing, a narrow, uniform particle size distribution and a spherical shape are needed to form a smooth, homogenous layer of powder to guarantee accurate sintering. The average particle size is typically between 10-50 µm, therefore the titanium powder in the above example is ideal for additive manufacturing. Oversized particles or those with a very irregular shape need to be detected with utmost accuracy as these are likely to cause defects in the finished workpiece. DIA reliably detects even small quantities of these undesired particles. Figure 6 reveals clearly how easily DIA can identify faulty particles.

Fine Metal Powders for Metal Injection Molding

For MIM applications, metal powders with very fine spherical particles are essential, typically with a median size below 10 µm. The example in Figure 7 illustrates the measurement results of two different types of metal powder as they are used for MIM. The analyses have been conducted with the CAMSIZER X2 in dry mode at 50 kPa dispersion pressure. Keep in mind that the CAMSIZR X2 is able to detect even minutest differences between the two materials and accurately characterizes the distribution width.

Two measurements of two different metal powders with a median (d50) size of 4.5 µm and 5.2 µm. The CAMSIZER X2 detects particles as small as 0.8 µm.

Figure 7. Two measurements of two different metal powders with a median (d50) size of 4.5 µm and 5.2 µm. The CAMSIZER X2 detects particles as small as 0.8 µm.

Reproducibility Study with Solder Powders

Powder metallurgy is a key application area for metal powders but there are others such as solder powder for circuit boards. Various types of solder powder are available which have to be characterized accurately in relation to size and shape because of tight product specifications (Figure 8).

Measurement results of 6 different solder powders obtained from different manufacturers. Displayed are the cumulative distributions (Q3, left y-axis) and the corresponding frequency density distributions (q3, right y-axis)

Figure 8. Measurement results of 6 different solder powders obtained from different manufacturers. Displayed are the cumulative distributions (Q3, left y-axis) and the corresponding frequency density distributions (q3, right y-axis)

Reproducibility is a main criterion to assess the reliability of any measuring device. One of Retsch Technology’s customers, who is a producer of solder powder, has performed a reproducibility test by measuring the same sample of solder powder with four different CAMSIZER units in two different plants. The test included 180 measurements in total and the results are provided in Figure 9. The median size of the test material was established to be 27.3 µm with a standard deviation below 0.1 µm!

180 different measurements of the same sample type with four different CAMSIZER units at two different locations. The x-axis shows the measurement number, the y-axis the mean particle diameter. The average measured particle size varies by less than +/- 0.1 µm

Figure 9. 180 different measurements of the same sample type with four different CAMSIZER units at two different locations. The x-axis shows the measurement number, the y-axis the mean particle diameter. The average measured particle size varies by less than +/- 0.1 µm

Advantages of DIA over Other Particle Sizing Techniques

For metal powders, mechanical sieve analysis is traditionally the standard technique for particle sizing. Standards ASTM B214 and ISO 4497 provide the most relevant procedures.

The absolute lower size limit for sieve analysis is defined by the smallest practically usable mesh size of 20 µm (air jet sieving), which is well above the average particle size of many samples for MIM or AM. As a consequence, air jet sieving is not suitable for the reliable and precise analysis of the entire size distribution of fine powders. It is frequently used for detecting the quantity of oversized particles with one sieve only, for instance with 63 µm or 45 µm aperture size. Another disadvantage is that sieve analysis does not supply any details on particle morphology.

Laser diffraction is extensively used to measure fine metal powders with particle sizes below 100 µm. This technology employs static light scattering, as mentioned, for example, in ISO 13320. Laser diffraction analyzers are easy to operate and provide quick measuring results; however, this technique calculates the particle size from the scattering angle and light intensity of a laser light beam interacting with the sample and is therefore based on indirect measurement. Advanced software algorithms are required to calculate the particle size distribution based on approximations and assumptions. One general assumption is, for instance, that all particles are spherical. Consequently, no information on particle geometry is given and any deviation of the actual particle shape from the "perfect" shape causes inconsistencies in the calculated particle size distribution. This produces inaccurate results, particularly when it comes to measuring the correct distribution width. Another major disadvantage is the very low sensitivity for detection of small quantities of over- and undersized particles.

Table 1. Comparison sieve analysis and dynamic image analysis

Sieve Analysis Image Analysis
Measuring range 20 µm – 125 mm CAMSIZER® P4: 20 µm – 30 mm
CAMSIZER® X2: 0.8 µm – 8 mm
Particle shape analysis no yes
Detection of oversized grains each particle CAMSIZER® P4: each particle
CAMSIZER® X2: < 0.01% Vol.
Resolution poor very high
Dissolution of multimodalities poor excellent
Repeatability and lab-to-lab comparison limited very good
Comparability of methods identical results possible
Handling simple, time-consuming, error-prone simple, objective, fast

Table 2. Comparison laser diffraction and dynamic image analysis

Laser Diffraction Image Analysis
Measuring range 10 nm – 5 mm > 0.8 µm
Particle shape analysis no yes
Detection of oversized grains > 2% CAMSIZER® P4: each particle
CAMSIZER® X2: < 0.01% Vol.
Resolution good in the micron range, getting worse with increasing particle size excellent over the whole measuring range
Identification of multimodalities limited (starting with factor 3 for mixtures) significantly better size resolution
Comparability to sieve analysis poor identical results possible
Information content equivalent diameter, spherical model, indirect method real particle dimensions, direct length

Conclusion

With metal injection molding and additive manufacturing becoming more and more dominant techniques, there is a growing demand for specifically designed metal powders with very specific features. Both chemical composition and particle size and shape are of high importance for the processability of the powders. Based on the application, the powder must match a range of specifications. Dynamic Image Analysis with the CAMSIZER X2 provides all appropriate data on particle size and shape. In comparison to laser diffraction or (electron or optical) microscopy, the measurement data is based on a large number of analyzed particles and is thus statistically more sound and offers higher reproducibility. A single measurement just takes 1 to 3 minutes which allows for a high sample throughput and nonstop quality control. For both powder manufacturers and producers of metal components, the CAMSIZER X2 is a precise and efficient tool which helps to significantly enhance the quality control process.

This information has been sourced, reviewed and adapted from materials provided by RETSCH Technology GmbH.

For more information on this source, please visit RETSCH Technology GmbH.

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