Machinery's Handbook, 31st Edition
Characteristics of Metal Powders
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Characteristics of Metal Powders A powder is defined as a finely divided solid, smaller than 0.039 in. (1 mm) in its max imum dimension. In most cases the powders will be metal, although in many instances they are combined with other materials such as ceramics or polymers. Powders exhibit behavior that is intermediate between that of a solid and a liquid. Powders will flow under gravity to fill containers or die cavities, so in this sense they behave like liquids. They are compressible like a gas. But the compression of a metal powder is essentially irreversible, like the plastic deformation of a metal. Thus, a metal powder is easily shaped, but it has the desirable behavior of a solid after it is processed. When one deal with powders, the properties of both the individual particles and the collective (bulk) properties of the powder must be considered. The properties of single particles include size, shape, and microstructure, which can be determined by optical or scanning electron microscopic observations. In order to characterize a bulk powder, it is necessary to be able to determine at least the following properties: Basic Chemical Composition : The minimum percentage of the base metal or the percent ages of main elements in case of metal alloy powders. Apparent Density: The weight per unit volume of a simply poured metal powder, which is always less than the density of the metal itself. It is measured by letting the powder drop freely through a funnel to fill a 1.52 in 3 (25 cm 3 ) cylindrical container. The ratio between mass and volume, that is, the apparent density, is provided through leveling and weighing and is expressed in kg/m 3 . The apparent density depends on a series of factors, the more important of which are the metal’s true density, powder shape and structure, particle size distribution, corrosion resistance, etc.). Impurities: The percentage of impurities. Particle Size Distribution: (see next section). Flowability: To assess the speed, standardized funnels with varying calibrated openings are used. A certain amount of powder is poured in the funnel and the flow time is recorded. Particle Size Measurement and Distribution.— The size, distribution, and shape of the powder metal particles influence the physical properties of the parts created. A particle is defined as the smallest unit of a powder. The particles of many metal powders are 0.001 to 0.0078 in. (0.025 to 0.2 mm) in size. Describing a three-dimensional particle is often a more complex matter than it first appears. For simplicity, it is convenient to describe particle size in terms of one single number. The sphere is the only shape that can be described by one dimension, its diameter ( D ). However, a particle is rarely a perfect sphere. Over the years, particle size analyses of metal powders have been performed using various techniques, including microscope, sieves, aerodynamic time of flight, and laser diffraction. At present, the most popular technique for measuring the size distribution of metal powders is screening and laser diffraction, typically measuring powders in their natural dry state. Screening Method: The most common method used for measured particle size is the use of screens (sieves) of different mesh sizes. The term mesh count is used to refer to the num ber of openings per linear inch of screen. The basic principle of sieving techniques is as follows. A representative sample of a known weight of particles is passed through a set of sieves of known mesh sizes. The sieves are arranged in downwardly decreasing mesh diameters. The higher the mesh size number, the smaller is the opening in the screen. For example, a mesh size No. 200 has an opening of 0.003 in. (0.074 mm), size No. 100 has an opening of 0.006 in. (0.149 mm), size No. 10 has an opening of 0.078 in. (2 mm). The sieves are mechanically vibrated for a fixed period of time. The weight of particles retained on each sieve is measured and converted into a percentage of the total sample. This method is quick and sufficiently accurate for most purposes. Laser Diffraction Method: Laser diffraction, alternatively referred to as low-angle laser light scattering (LALLS), can be used for the nondestructive analysis of wet or dry
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