SOME ASPECTS OF POWDER METALLURGY 445 



Another method to minimize or eliminate oxidation in sintering oper- 

 ations utilizes, in addition to the 50 aluminum-50 iron powder, approx- 

 imately 2 per cent of titanium hydride incorporated in a powder mixture 

 of aluminum-iron-nickel^^. Decomposition of the hydride commences at 

 about 450° C. with release of nascent hydrogen so that during the sintering 

 operation, oxidation is prevented and part or all of any oxide already present 

 may be reduced.* 



8. Metal filters and screens 



Related to the porous metal type of bearing and prepared in much the 

 same way are the metal filters and screens made by powder methods^-'*^. 

 Bronze, copper-nickel alloys, or pure nickel may be utilized, and porosities 

 up to 80% by volume may be obtained. These filters have been used to 

 advantage in the chemical industry for filtering strong alkahne solutions 

 and other hquids of many kinds. One reported application is as a fuel 

 filter 5 inches long and 2 inches in diameter for a Diesel engine^. 



Generally, the filter part can be bonded to steel or copper and made an 

 integral part of the apparatus in which it is to function. 



In the manufacture of the filters, the prosity can be accurately controlled. 

 In addition to the methods of producing porous parts as previously described, 

 a highly porous metalUc mass can be prepared by sintering the component 

 metal powders (sometimes with volatile additions) in the uncompacted 

 condition using a protective atmosphere and a temperature determined 

 by the type of powders used^". 



9. Alloys having special properties dependent on close control of 

 composition 



There are some alloys for special purposes where accurate control of 

 composition and reproducibility of composition are of primary importance. 

 Two such materials are: low-expansion alloys for metal to glass seals, and 

 thermocouple wire for temperature measurement. 



An alloy of 54% iron-28% nickel-18% cobalt having approximately 

 the same coefi6cient of expansion as certain grades of glass is normally 

 prepared by melting and casting procedures. This alloy can be prepared 

 by sintering methods, however, with the same physical characteristics, 

 but with closer composition control and less contamination'*'*. 



Alloys of nickel-molybdenum and nickel-tungsten have been prepared 



* The need for titanium hydride in the preparation of allo\-s of this type, and the 

 effect of the hydride in controUing oxidation has been the subject of some discussion''^. 

 Its use is mentioned here only as a variation of the method described above and apart 

 from any effects it may have on the magnetic properties of the alloys to which it is added. 



