SOME ASPECTS OF POWDER METALLURGY 455 



ance between die parts and plungers, and the clearances may be 

 cumulative^'*. 

 9. There is a lack of technical information available for engineers and 

 designers. Tests on metal powders and fmished parts have not 

 been standardized, and until such standardization has been achieved 

 the metal powder consumer and the ultimate user of the sintered 

 parts have no check on the respective products. This situation is 

 now being remedied. 



10. There are some thermal limitations that may cause difficulties in 

 the sintering process in certain instances^^. Some oxides can be 

 reduced only at temperatures above the melting point of the metal 

 itself and prevent effective welding of the powder particles. 



11. Metal powders in a fine state of subdivision are readily combustible 

 and must be treated as potential fire and explosion hazards^" ■^2. 

 Zirconium, magnesium, aluminum, and titanium are the most inflam- 

 mable with iron, manganese, zinc, silicon, tin, and antimony, mod- 

 erately inflammable. Precautions must be taken to keep dust out 

 of the air in the mixing and pressing rooms, not only because of the 

 explosion hazard, but also because of possible toxic effect on workers. 



12. Deterioration of metal powders may occur in storage due to oxidation 

 or absorption of moisture with subsequent chemical reaction to 

 change the composition^^. 



Conclusion 



This correlated review of some of the more common aspects of powder 

 metallurgy is presented to provide information on an increasingly important 

 production method. The review makes no pretense of complete coverage 

 of the subject, and many important topics such as hot pressing, press and 

 furnace design and operation, sintering atmospheres, and die design and 

 operation have not been described. These and other more specialized 

 topics that are beyond the scope of this paper may be found in the appended 

 list of references. 



Acknowledgment 



The authors have drawn freely from many of the articles on powder 

 metallurgy published during the past ten years. Wherever possible, ref- 

 erence is made to the original source of the topic material or to associated 

 articles where more complete information may be found. To these sources 

 listed below, the authors are indebted for much of the material presented 

 in this review. 



The authors express appreciation to Mr. Charles Hardy for permission 

 to use his pictures of powder metal parts shown in Figs. 1,2, and 3. 



