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
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.