SOME ASPECTS OF POWDER METALLURGY 431 



dioxide to tungsten as later described in the section on types of metal powder 

 products. 



The physical properties of the metal powders are also determining factors 

 in their selection. These include particle shape, size, hardness, particle 

 size distribution, flow characteristics, apparent density of loose powder, 

 and particle grain structure. 



Particle shape and size are governed largely by the method of production 

 of the powder as has been suggested previously. The carbon}^ process 

 yields spherical particles, for example, while other methods produce particles 

 that are angular, acicular, spongy, flat, rounded, granular, dendritic or 

 otherwise irregular. 



The hardness depends largely upon the metal itself, its purity, and the 

 method of preparation. Hardness, in addition to shape of the particle, 

 will be reflected in the amount of pressure required to obtain a given density 

 in a finished part, and is a factor in the economics of die cost because of 

 its influence on die life. 



Particle size distribution in a metal powder is of great importance al- 

 though no particular specification can be set up at present. The problem 

 of size distribution and shape has been treated in some detail by W. D. 

 Jones-^ and others, especially as concerned with interstitial volume or 

 porosity. If all particles were cubes of the same size and could be placed 

 in perfect order with the cube faces matching identically, there would be 

 a minimum of porosity in the powder and in the pressed part. This is 

 obviously impossible of attainment. In practice, packing is not systematic, 

 but random, and even if identically sized cubes could be obtained, the voids 

 between particles would be appreciable. In addition to the porosity re- 

 sulting from the random packing, there are cavities which are due to bridging 

 action of the particles themselves. This bridging is not due to irregular 

 or angular particle shape, but can occur quite easily with spherical particles. 

 Shaking or compressing the powder tends to destroy the bridges or arches 

 and allow denser packing. As the powder is shaken down there is rotation 

 of particles until corresponding surfaces come in contact and relatively 

 dense packing is obtained. Such a rotation may not be present, however, 

 during the rapid stroke in a die, and the particles cannot seek corresponding 

 surfaces. In this case, there is a deformation of the particles pressed against 

 one another so that there may be an actual keying, and the smaller particles 

 may be pressed into the voids to produce the same result of denser packing. 

 With a distribution of particle size, the voids between larger particles can 

 be filled with smaller particles and, in practice, that is what is sought. 

 The problem of setting up specified sizes or particle size distribution for 

 powder metallurgy methods is not easy, however, because of practical 

 complications arising in the pressing and sintering operations. Pore size 



