Q.6 Define powder-metallurgy. What are various important techniques for compacting of metal powders ?

Ans. Powder metallurgy is a technique of producing metal powders and making serviceable articles from them. This technique was first used by the egyptains us 3000 B.C., to make iron tools. By the end of 15^th century. It was rediscovered metallurgy method. It was used again after a long lapse of time in 1916 to produce the first commercial tungsten wire. However, its importance as a manufacturing technique was realized when Germans used this technique for producing tungsten carbide cutting tool tips after the first World War. The good forming qualities of this material soon led to other applications. The past three decades have seen major advances in the use of powder metal parts in consumer, industrial and military applications.

          Compaction or Briquetting

          (i) Die assembly

          (ii) Filling the measured amount of appropriate powder into the die cavity.

          (iii) Pressing the metal powder by one or more plungers into a coherent mass and,

          (iv) Removing the compact from the die.

          The compacting pressure varies over a wide range as shown in Table. Mechanically and hydraulically operated presses are used for compacting. Mechanical presses are used when pressures are not very high and the volume of production is high while hydraulic presses are used when higher pressures are required.

                               Table : Compacting Pressure

        Metal Pressure                                             (N/mm²)

      Aluminium, Brass, Bronze                              100-350

      Aluminium, Brass, Bronze                              100-350

      Iron, Steel, Nickel Alloys                                 400-700

      Tungsten Carbide                                             900

   The workpart after pressing is called as green compact, the work green meaning not yet fully processed. As a result of pressing, the density of the part, called the green density, is much greater than the starting bulk density. The green strength of the part when pressed is adequate form handling, but far less than that achieved after sintering.

    The applied pressure in compaction results initially in repacking of the powders into a more efficient arrangement, eliminating bridges formed during filling, reducing pore space and increasing the number of contacting points between particles. As pressure increases, the particles are plastically deformed, causing interparticle contact area to increase and additional particles to make contact. This is accompanied by a further reduction in pore volume. The progression is illustrated in three view as shown in figure for starting particles of spherical shape. Also shown is the associated density represented by the three use as a function of applied pressure.

       Other methods used for compaction include :

       (i) Centrifugal compacting,                    (ii) Slip casting,

      (iii) Extruding,                                          (iv) Gravity sintering,

      (v) Rolling and                                         (vi) Isostatic moulding

        In centrifugal compacting the powder metal is filled into the die cavity and then centrifuged to pressure of about 3 N/mm². Uniformally dense structure is obtained in this method due to the fact that centrifugal force acts on each particle. This method is suitable for limited parts made from heavy metal powders only. In extruding process billets are first made of composite powder metal and then it is extruded in the usual way through dies way the application of pressure.

         Gravity sintering is a process in which the blended powder is spread on ceramic trays to desired thickness then sintered. This method is used to manufacture porous sheets from which filters for various uses can be made.

         In rolling the prepared powder is fed from hopper through two rollers which compress the powder into sheets which are conveyed through sintering furnace. This method is used to prepare sheets of copper, brass, bronze and stainless steel.

         Isostatic Pressing : This is a process of obtaining uniform density of the compact by the application of equiaxial pressure. This is attained by surrounding the metal powder in an elastic container immersed in liquid which is subjected to hydraulic pressure. This process requires no lubricant, the tooling is inexpensive and the equipment is simple to use. However in this process the production rate is slow and the accuracy is not as high can be obtained with regular pressing. Subsequent machining is therefore required on the outer surface to provide correct shape and accuracy.