Q. What are the secondary operations we apply in powder metallurgy methods. Give their use also.

Ans. Secondary Processes of Manufacture : In addition to the basic processes discussed in the preceeding article, there is a good number of secondary processes in powder metallurgy technique. The selection of the secondary process depends upon the requirements of the components, e.g. if more accuracy is desired a secondary operation of sizing process is necessary. The functions of secondary operations are varied. They include densification sizing. Impregnation, infiltration, heat treatment, finishing.     

       → Sizing : When dimensionally accurate parts are required, sintering is followed by a sizing operation. In this, the sintered parts are 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.        

       → 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.

       → 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.

       → 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.

       → Forging : The process was first tried in 1969 and was first used in the manufacture of automobile gears. The process consists of making powder metal performs of measured slug by compaction, the part is then sintered, heated to forging temperature in a furnace and forged to its final shape and size by application of pressure.

       Unlike conventional forging where the parts are made from bar stock etc. and a number of blow are required for forging the P/M performs can be formed to final shape in one forging blow. The process has an advantage of improved material utilization through reduction or elimination of machining, uniformly of structure and improved directionality of properties relative to conventionally forged parts. The density is increased to nearly 100 percent and the production rate is high.

       → Machining : One of the major objectives of powder metallurgy is to produce a dimensionally accurate finished component without doing any further work on it. However, there are certain operations such as threading, drilling grooves, undercuts, etc., which require subsequent machining of the part.

       Machining on sintered P/M parts is usually performed by using carbide tipped tools. However when parts are fabricated from difficult to machine powder the problem is alleviated by employing a presintering operation and sintering follows machining.

        Ordinary coolants are not used in machining operation as they cannot be easily removed from pores. This causes corrosion of the component. However, coolant oils of the type used to impregnate self lubricating bearings are sometimes used. Volatile coolants such as carbon tetrachloride which leaves no residue can be used.

        → Electroplating : Sintered powdered metal components can be electroplated in the usual way except that certain precautions are to be taken to avoid the ingress of the plating media by capillary action into the pores of the component. Penetration can be avoided by using high density components and giving mechanical surface treatment such as burnishing or by impregnation of parts with plastic material. The purpose of both methods is to fill in surface pores of the component.

        Sintered parts may be plated with chromium, nickel, zinc, copper and other metals as required by using appropriate electrolyte.

        Electroplating is mostly used for decorative purposes and frequently to increase the resistance of the component to the surface wear. Sometimes it is used to reduce friction on the rubbing surfaces.