SOME ASPECTS OF POWDER METALLURGY 427 



in the field, but a somewhat unwise course for industry as a whole to pursue. 

 That there are limitations to powder metallurgy and many serious problems 

 unsolved, is generally now recognized, and there is a tendency toward more 

 conservative evaluation of the potentialities of the process. 



It is the purpose of the remainder of this article to describe some of the 

 common methods of preparing metal powders, to explain the fundamental 

 principles invoh-ed in powder metallurgy, to describe the advantages and 

 limitations of the process, and to indicate the t}'pe of product that may be 

 expected. 



Manufacture of Metal Powders 



Metal powders are made in a variety of ways, each method of preparation 

 being suited to the metal being treated or to the end product desired. 

 Experience has shown that no one type of metal powder can serve all the 

 projected uses in industry, so it is not surprising that there have been 

 developed numerous methods for the preparation of metal powders, each 

 of which has advantages for certain types of work, and which may or may 

 not be suited for other uses"'^^. Listed below are some of the common 

 methods which have been developed for producing metals and alloys in 

 powder form. No attempt is made here to discuss these methods in detail 

 or to point out the relative hazards-*^ involved in the various processes. It is 

 worthy of note, however, that many metal powders in a finely divided state 

 have such a large surface area in proportion to their bulk that they are usually 

 subject to rapid oxidation, so rapid in many instances that they constitute an 

 explosion hazard. Care must therefore be exercised throughout in the 

 preparation of these powders, and many must be prepared and stored in 

 inert atmospheres. 



1. Machining 



Machining of metals to produce powder has been mentioned above in 

 connection with the process of Bessemer. A relatively coarse powder is 

 produced. The cost of production is usually high, and the powder use 

 is hmited to a few special applications such as dental alloys where no fines 

 or dust are allowable, and where the high cost of the alloy itself justifies 

 the extra cost of this method. 



2. Milling 



By the use of various types of mills such as stamp mills, jaw crushers, 

 gyratory crushers, impact, and ball mills, both brittle and malleable metals 

 can be reduced to powder. The friable metals tend to produce angular, 

 jagged, particles of irregular shape while the malleable metals usually 

 produce flakes. Because of the lubricant necessary with malleable metals 



