SOME ASPECTS OF POWDER METALLURGY 435 



resulting from chemical action, may interfere with sintering and the general 

 rule is to avoid them if possible in attempting to produce solid metal. 



Following sintering, there is sometimes a treatment for impregnating 

 a porous structure with some material designed to confer special properties 

 on the compact. Pressed and sintered bearings may, for example, be 

 impregnated with oil, and a strong, porous network of tungsten may be 

 impregnated with copper by suitable means to produce spot and line welding 

 electrode material having high compressive strength associated with good 

 heat and electrical conductivity. 



5. Coining or Sizing 



Although the dimensional tolerances of sintered metal parts can be rather 

 closely controlled, it may be advantageous to control final size and improve 

 surface structure by a coining operation consisting in re- pressing the compact 

 in a die of suitable size. 



The Modern Field of Powder Metallurgy 



Most of the de\'elopments and uses of metal powders described thus far, 

 it should be noted, have been concerned with products which could not 

 be produced in any other way than by powder metallurgy processes. This, 

 in fact, has been the principal field of powder metallurgy. Porous bearings 

 with uniformly distributed porosity could not possibly be fabricated by 

 any of the standard melting and casting techniques, nor could the carbide 

 cutting tools be likewise manufactured. 



In general, the powder metallurgy process has been applied under condi- 

 tions as outlined below^''-^^'^^: 



1. Production of refractory metals such as tungsten, tantalum, colum- 

 bium, and molybdenum. 



2. Development of structures not practical by other methods. These 

 include telephone and radio cores, and articles requiring uniform or 

 controlled porosity such as porous bearings and metallic filters. 



3. Preparation of metals to include uniformly distributed non-metals. 



4. Preparation of samples comprising a metal with another metal or 

 metals which would be immiscible in the molten state, or which 

 do not form alloys. 



5. Preparation of samples of two or more metals where one component 

 has a low boiling point. 



6. Fabrication of products that can be made more economically by the 

 powder process than by other methods'^. 



Considerable work has been done by the automotive industry and others 



