Q. What is sintering ? Explain the stages of sintering.

Ans. The operation of heating a green compact to an elevated temperature is known as sintering. It is a process by which solid bodies are bonded by atomic forces. It is carried out below the melting temperature of the major constituent of the blended power. Most metals are sintered at 70% to 80% of the melting temperature.

         Following changes occur during the process of sintering :

         (i) The density of the part is increased.

         (ii) Grain boundaries are formed.

 (iii) Plasticity is increased.

 (iv) Better mechanical interlocking is produced.

 (v) Volatile materials that are present are driven off, thus producing

porous product.

        The sintering atmosphere is controlled to prevent oxidation of the work. The most frequently used atmosphere are dissolved ammonia and hydrogen.

                    Table : Typical Sintering Temperature and Times

       Metal                         (ºF)                       (ºC)                        Time (min)

       Brass                        1600                      850                                25

       Bronze                      1500                      820                                15

       Copper                      1600                      850                                25

       Iron                            2000                    1100                                30

       Stainless steel           2200                    1200                                45

       Tungsten                   4200                     2300                               480

       There is always some dimensional change of parts in sintering. It may be growth or a shrinkage.

       Recent advancement in sintering process is the spark discharge sintering, where in high energy electrical spark is produced during compaction. Due to the high energy of the spark the diffusion bonding is instantaneous.