Q. What is the purpose of :

         (i) Sizing,                  (ii) Coining operations in powder metallurgy,

        (iii) Infiltration,           (iv) Impregnation 

Ans. (i) Sizing : When dimensionally accurate parts are required, sintering is followed by a sizing operation. In this, the sintered part is placed in sizing dies which are specially made and can give dimensions within 0.0125 mm or less of exact size. The part is repressed after placing in the sizing die. This operation is normally hand fed but when large quantities are involved, the process can be automated.

         (ii) Coining : The coining operation is similar to sizing operation where in the P/M parts repressed after sintering in the same die or sometimes in a separately made die. The purpose of coining is to reduce the void space and increase the density of the part. Coining is also used to emboss in signature marking, lettering and to produce small notches on P/M parts.

          Coining changes the size of the part in the direction of pressing. Thus, the original tooling must provide for this changes, usually after the coining operation the part is resintered for stress relief. Coining increase the density, hardness and tensile strength of the part.

        (iii) Infiltration : It is a process in which molten metal of a lower melting point than the major constituent is forced under pressure to fill the pores. A proper size piece of copper or brass is placed either on the top or bottom of the part and when this secondary metal melts, during sintering of iron, due to capillary action the infiltrant soaks through the porous part.

       The effect of infiltration is to give the P/M parts higher tensile strength, fatigue strength and hardness. The density is increased from 70% to 100%. In addition to these it seals the surface porosity so that secondary operations such as plating can be carried out if required.

        (iv) Impregnation : It is a process of introducing oil, grease, wax or other lubricating materials when self lubricating properties are desired. The process consists of immersing the parts in a bath of lubricant which is heated to a temperature of about 95ºC. The porous parts are kept for a period of about 10 to 20 minutes in the bath. The impregnation can also be carried out by drawing oil through the part by vacuum. The finished bearing may be saturated by a considerable quantity of lubricant 20% by volume. The lubricant is retained in the part by capillary action and is withdrawn by heat or pressure as required in service.

        The impregnation is also done by low melting metals or preferably by plastic for the purpose of eliminating surface pores for plating the P/M parts.