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.
The above technique measures the transmission of air pressure through a packed bed of powder under a given rate of input pressure. Further, this transmitted pressure then relates to the specific surface area of the powder. It is from this the average particle size is calculated.
Moreover, the technique of air permeability is rapid and simple, since it requires only the amount of powder numerically equal to its density in g/cm3, thereby using one cubic centimeter of solid material. Another advantage is that the sample can be weighed, packed, and analyzed in a few short minutes, thus enabling its use in the control process conditions. Here, samples are taken periodically from the process furnaces, following which 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 (SAS) has been developed to take the place of the Fisher Subsieve Sizer, since it exhibits far superior precision owing to its use of traceable, calibrated pressure transducers and precise sample height measurement (see Figures 1 and 2). Further, the ASTM Standards B330 (for refractory metals), C721 (for certain ceramic powders), and E2980 (a general standard applying to nearly all classes of powder materials) now include procedures for the use of the SAS.
Figure 1. Subsieve AutoSizer (SAS)
Figure 2. Subsieve Auto Sizer Schematic
Figure 3 depicts an example of the SAS’s enhanced precision, which is the product of measurements on several tungsten powders using both the Fisher SubSieve Sizer and the Subsieve AutoSizer. 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 measurement is significantly lower.
Figure 3. Repeatability Comparison
Figure 4 shows an even more dramatic example of the SAS’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, considering that lower limits of the SAS 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.
Figure 4. Reproducibility Comparison
As compared to other particle size measurement techniques, the use of the SAS’s air permeability method of particle size estimation 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, 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 is very easy to use, since it requires a minimal amount of operator training and education.
Due to the SAS’s much better precision, refractory metals producers are now in a better position to control the particle size of their powders. Thus, this results in significantly less rework, with increased confidence from powder users in their specifications. Further, the SAS causes a decrease in rejection of good material, and also lowers the incidence of using out-of-spec powders. Finally, the SAS’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 Particulate Systems.
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