Effective Solutions for measuring refractory metal powders

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Refractory metals, particularly tungsten and molybdenum, are often produced in powder form by chemical reduction from ores and oxides, using high-temperature furnaces in a reducing atmosphere, usually hydrogen. Because of their high melting points, these materials do not lend themselves well to conventional melt-and-cast metallurgical processes. Powder metallurgical techniques – consolidation by mechanical pressing and sintering to reduce or eliminate porosity – have been usedto produce solid bodies and parts for high-strength, high-temperature applications. In recent years, metal injection molding (MIM) and additive manufacturing (MAM) processes are being increasingly used to consolidate the powders and produce complex metal shapes, with very little machining necessary.

In all of these consolidation and shaping processes, the particle size of the powders is critical. Because of its simplicity and short analysis time, an air permeability technique for estimating the particle size has been used to control powder production processes and assess the quality of the powders. The technique measures the pressure of air transmitted through a packed bed of powder under a specified input pressure; the transmitted pressure is then related to the specific surface area of the powder, from which an average particle size is calculated.

The air permeability technique is quick and simple, requiring only an amount of powder numerically equal to its density in g/cm3, thus using one cubic centimeter of solid material. The sample can be weighed, packed, and analyzed in a matter of minutes, so the method is often used to control process conditions: Samples are taken periodically from the process furnaces, their particle size measured, and process conditions, such as temperature, atmosphere , and speed through the furnace, are adjusted to produce the desired final particle size, according to the particle size specifications set by the concentration supplier and powder purchaser.

For many years, the refractory metals industry had depended on an instrument called the Fisher SubSieve Sizer (FSSS) for this particle size estimate, using ASTM Standard Test Method B330. The new AutoFSSS from FSSS INSTRUMENT CO., LTD is a mature instrument that makes operators directly and easily Read, Save and Print Out measuring result (average particle size, specific surface area, poroisty).