A Survey of the Possibilities of this Comparatively New Technique
(from Journal of the Institute of Production Engineering)
by W.D. Jones, M.Eng., Ph.D.
|This article first appeared in the Volume 6, Number 67 (May, 1944) issue of Aircraft Production magazine, and is presented here through the kind permission of Flight International. Thanks also to Bruce Vander Mark for furnishing volumes of Aircraft Production for scanning.|
Powder metallurgy can be considered simply as an art of making metallic articles or masses by moulding powdered metals. It bears very close resemblance to the plastics trade and follows in many respects much of the technique used in plastics.
Broadly, the technique consists in commencing with powdered metal, or alloy, or mixture, of a fineness generally less than 100 mesh. The powder, or mixture of powders, is inserted into a die and pressed under anything from 10,000 to 200,000 psi pressure, forming a moulding, or pressing, or ''compact", which is the word generally used in this connection, which is then ejected from the die. With all suitable commercial powders this pressing stage produces a solid coherent compact at least sufficiently strong to permit moderately rough handling, although without any real physical strength. The porosity of such a compact is generally more than 10% and may be as much as 50%. Its strength is due to the mechanical interlocking of each particle and to the cold welding that has taken place between each particle.
The compact is now heat-treated or sintered by passing through a furnace at a temperature generally of the order of two-thirds of the melting point of the metal or alloy, but this temperature is frequently higher than this figure although always below the melting point. Normally the compact is protected from oxidation by maintaining in the furnace a suitable reducing atmosphere, although in some cases good results are obtained by sintering in air or in a neutral atmosphere. During the sintering stage actual welding of the various particles of metal takes place and the aggregate adheres into a strong mass that may, under favorable circumstances, have physical properties approaching those of cast or forged metal.
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