Determining Particle Size for Quality Control of Metal Powders

Often, refractory metals, such as tungsten and molybdenum, undergo production in powder form. This process usually involves chemical reduction from ores and oxides (such as hydrogen), with the help of high-temperature furnaces in a reducing atmosphere. Owing to the high melting points of these materials, they are ill-equipped for conventional melt-and-cast metallurgical processes.

In the past, powder metallurgical techniques – consolidation by mechanical pressing and sintering which reduces or eliminates porosity – have been utilized to produce solid bodies and parts, especially in the case of high-strength, high-temperature applications. However, in the last few years, processes such as metal injection molding (MIM) and additive manufacturing (MAM) are being used more frequently to consolidate the powders and produce complex metal shapes. The advantage here is the requirement for very little machining.

Air Permeability Technique

An important point to note is the importance of the particle size of the powders, which is critical in all these consolidation and shaping processes. Due to its simplicity and short analysis time, the use of an air permeability technique for estimating the particle size has been adopted. This was done to control powder production processes and also help in the assessment of the quality of the powders.

Air permeability determines particle size by monitoring an air pressure drop across a packed powder bed.  The smaller the particles, the greater the pressure difference. In addition to particle size data, air permeability also yields specific surface area and powder bed porosity data.

The air permeability technique is rapid and simple. The analysis only requires enough material to fill one cubic centimeter of volume in the sample tube. This is a simple calculation if the density of the material in known. Another advantage is that the sample can be weighed, packed, and analyzed in a few short minutes, giving real time results and allowing for modifications to the manufacturing process. Here, samples are taken periodically from the process furnaces, and their particle size is measured. Further, process conditions, such as temperature, atmosphere concentration, and speed through the furnace, are adjusted to create the final particle size, which follow the specifications set by the supplier and powder purchaser.

Fisher SubSieve Sizer

The refractory metals industry had relied on an instrument called the Fisher SubSieve Sizer (FSSS) to estimate the above particle size, with the use of ASTM Standard Test Method B330. However, the instrument suffers from lack of precision. This is because it estimates pressures and packing factors using “eyeball” estimates of packed sample volumes and standpipe manometer pressures. Moreover, the Fisher SubSieve Sizer is no longer available or supported today.

Subsieve AutoSizer from Micromeritics

The Micromeritics Subsieve AutoSizer II (SAS II) has been developed as an automated direct replacement of the Fisher Subsieve Sizer.  The Micromeritics SAS II improves upon the precision of the technology by utilizing traceable, calibrated pressure transducers and precise sample height measurement (see Figures 1 and 2).

Subsieve AutoSizer (SAS)

Figure 1. Subsieve AutoSizer II (SAS II)

Subsieve Auto Sizer Schematic

Figure 2. Subsieve Auto Sizer Schematic

Figure 3 shows results on several tungsten powders using both the Fisher SubSieve Sizer and the Subsieve AutoSizer as an example of the SAS II's enhanced precision. This figure also shows the variability of the measurements that are based on a modicum of repeatability, which is defined as the range of variability of repeated measurements of the same material, on the same instrument, by the same operator. Moreover, over the entire range of measurement, the variability of the SAS II measurement is significantly lower.

Repeatability Comparison

Figure 3. Repeatability Comparison

Figure 4 shows an even more dramatic example of the SAS II’s increased precision. Here, the result of measurements on the same powders are the same as in Figure 3, but the process is performed using different instruments. Further, the reproducibility can be defined as the variability of measurements performed on different instruments, often by different operators in different laboratories.

The goal for defining specifications for these materials has always depended upon the supplier and purchaser of powder materials agreeing on factors such as particle size measurements, within narrow limits. Figure 4 shows more reasonable limits for agreement using the SAS II, considering that lower limits of the SAS II can help avoid serious conflicts. Meanwhile, the larger variability of the FSSS often can exceed the specification limits, leading to confusion and disagreement among the parties mentioned.

Reproducibility Comparison

Figure 4. Reproducibility Comparison

As compared to other particle size measurement techniques, the use of the SAS II’s air permeability method of particle size determination results in significant analysis time savings. This is because many other particle size measurement techniques require more elaborate and time-consuming methods of sample preparation and dispersion. With the SAS II, samples can now be analyzed quickly and efficiently, minus the hold up of production processes.

As a result, metal powder production processes are markedly more tightly controlled, which leads to improved quality control and fewer false rejections of in-spec material. Most importantly, the SAS II is very easy to use, since it requires a minimal amount of operator training.

Conclusion

Refractory metals producers are now in a better position to control the particle size of their powders using the Micromeritics SAS II. This results in significantly less rework, and increased confidence from powder users in their specifications. Further, the SAS II causes a decrease in rejection of good material, and also lowers the incidence of using out-of-spec powders. Finally, the SAS II’s ease and speed of analysis creates higher material throughput, with very little delay caused by analysis time.

 

This information has been sourced, reviewed and adapted from materials provided by Micromeritics Instrument Corporation.

For more information on this source, please visit Micromeritics Instrument Corporation.

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