SOME ASPECTS OF POWDER METALLURGY 451 



below in somewhat more detail are these considerations and others which 

 enter into an evaluation of the process as a whole. In those instances 

 where a product cannot be made in any way except by powder methods, 

 evaluation is easy. But for those products which must compete with stand- 

 ard methods of fabrication, the problem is more complex, and generalizations 

 cannot always be applied. 



The following are some of the advantages of the powder metallurgy 

 process: 



1. High purity of the metal content of the finished product can be 

 maintained. Control of the manufacture of the powders enables 

 producers to supply metals that generally run well above 99 per 

 cent purity, and often as high as 99.99 per cent for some metals such 

 as tungsten, tantalum, and zirconium^*. Opportunity for addi- 

 tional impurity pickup is slight under the conditions prevaihng in 

 the pressing and sintering operations, so that the original metal 

 purity is retained, and may even be improved by oxide reduction 

 or removal of volatile impurities. 



2. Composition of the product can be accurately controlled and repro- 

 duced^" ■"**. There are no losses due to oxidation or slagging as in 

 melting processes so that the metal content can be quite readily 

 fixed. 



3. Structures, alloys, or materials not possible of fabrication by any 

 other method can be produced by powder methods-^ ■^"•^^■^^. These 

 have been adequately described and include porous bearings, sintered 

 carbides, refractory metals such as tungsten and tantalum, and 

 combinations of metals, and of metals and non-metals that do not 

 alloy. 



4. High production rates^^'^^ ■*'^, especially on small parts, can be attained 

 by use of automatic presses of the pill tabletting type and of con- 

 tinuous type sintering furnaces. One order of forty million, small 

 parts required by the Navy was produced at the rate of 520 pieces 

 per minute by powder methods^^. 



Larger size articles cannot be produced at any such rate because 

 of press limitations which may necessitate hand operation, but with 

 pressed iron parts, high rate of production is one of the factors that 

 allows the process to compete with other standard methods of manu- 

 facture. 



5. A wide range of certain physical properties can be obtained for any 

 particular material being fabricated^^-^-. Control can be exercised 

 over such properties as density, porosity, grain size, and strength 

 by variation of the type and size of powder particles, die pressure, 

 and sintering time and temperature. 



In some instances such as small Alnico magnets, structures devel- 



