Q. Describe the continuous production of metal strip by roll compacting of powder. Differentiate between impregnation and infiltration.
Ans. Roll-compacting : It is the process of converting loose powder into a green compact of accurate shape and size.
During operation, the powder is compressed to nearly one-third of its original volume. A large number of presses generally used for this purpose are either mechanical or hydraulic or a combination of both. Mechanical presses are suitable for low pressures. But their production rate is high. Hydraulic presses are best suited where higher pressures are employed. They also facilitate production of parts of uniform density, which is not possible with mechanical presses.
The metal powder compacted into the form of metal strips by passing a discrete stream of powder through a pair of rolls rotating in opposite directions. It is known as roll-compacting or roll-pressing as shown in fig.
Impregnation : It is a process of introducing oil, grease wax or other lubricating materials when self lubricating properties are desired. The process consist of immersing the parts in a both 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 finish 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 The lubricant is retained in the part by capillary action and is withdrawn by heat or pressure as required in service.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 plasting the pm/parts.
Infiltration : It is a process in which molten metal of a lower melting point than the major constitutent is forced under pressure to fill the pores. A proper size piece of copper 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 infiltrate 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.