Machinery's Handbook, 31st Edition
Characteristics of Metal Powders 1525 Since laser diffraction is such a popular method of measuring the size of metal powders, an ASTM standard was written to guide proper technique. ASTM B822-02, “Standard Test Method for Particle Size Distribution of Metal Powders and Related Compounds by Light Scattering,” provides guidance on the use of laser diffraction for measuring metal powders. The ASTM standard notes the requirement for a representative sample and suggests a micro-sample splitter to minimize sampling errors. When powder in a liquid suspension is measured, the standard points out that it is important to create a stable dispersion. Laser diffraction is a nondestructive, nonintrusive method that can be used for either dry or wet samples. As it derives particle size data using fundamental scientific principles there is no need for external calibration; well-designed instruments are easy to set up and run, and they require very little maintenance. Particle Shape and Internal Structure.— There are various types of metal powder parti cle shapes, several of which are illustrated in Fig. 2. The choice depends on the require ments of the final product. The microstructure, bulk and surface properties, and porosity depend on the particle shape. There will be a variety of particle shapes in a collection of metal powders, just as the particle size will vary.
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(e) (g) Fig. 2. Schematic Illustration of Several Particle Shapes in Metal Powders: a) Acicular; b) Spherical; c) Rounded; d) Spongy; e) Flakey; f) Cubic; g) Angular (f)
The particle shape is widely accepted as important if process operation is to be opti mized. One of the simple and most useful measuring techniques is digital imaging . By this technique, particle shape is quantified in two ways. First, the aspect ratio determination is made. Aspect Ratio: In general, the aspect ratio is the ratio of the maximum dimension to the minimum dimension for a given particle and is defined more specifically in this method as the ratio of the maximum and minimum particle dimension that passes through the geo metric center of the particle. This is illustrated in Fig. 3.
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Fig. 3. Particle Aspect Ratio Definition The aspect ratio for a spherical particle is 1.0, but for another grain, like a needle-shaped grain (acicular), the ratio might be up to 4. Any amount of loose powder contains pores between the particles. These are cooled open pores but in some cases might be voids in the structure of individual particles (closed pores) and their influence on powder characteris tics is minor.
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