Engineering Chemistry S Datta

METALLURGY 321
(i) Pulverisation of metallic powders. For pulverisation of metals techniques adopted
are:
(a) Mechanical: This method may be adopted in the case of Mg, Al, Zn, and Pb. For
pulverisation ball mills can be employed.
(b) Atomisation: This method involves forcing molten metal through a nozzle or
spinnerette in a stream of water or compressed air, when fine powder of metals
are obtained. This method requires control of pressure and temperature and the
oxidation of metal into an atmosphere of inert gas. This method is commonly
employed for metals like Mg, Cd, Zn, Pb, Sn etc.
(c) Chemical:
1. By reduction,
WO 2
+ 2H 2 ⎯⎯→
∆
2. By decomposition,
W + 2H 2
O
Ni(CO) 4
⎯⎯→
∆ Ni + 4CO↑
The metals are obtained in a state of fine powders in both cases.
(ii) Mixing and blending. This operation is required when a combination of metallic
powders are used. They are uniformly mixed in right proportions. Lubricants may be
added if necessary.
(iii) Compacting. The metal powder in right proportions is then taken in a die into the
cavity (female joint) of accurate dimensions and then compressed (100–1000 N/m 2 ) by
the male joint of the die. The solid in this stage is known as ‘green compact’.
(iv) Pre-Sintering. It involves heating the green compact to a temperature below the
sintering temperature. This step increases the strength of green compact as well as
removes the lubricant if used. This step is essential for tungsten.
High
pressure
Guide pins
Top moulding part
(Male joint)
Bottom moulding part
(Female joint)
Alloy ingredients
Extraction pin
Fig. 14.14 Technique of powder metallurgy adopted in a die.
(v) Sintering. Compacted pieces are heated to a temperature below and very close to the
melting point of the metal in a furnace, preferably in an atmosphere of hydrogen,
whereby metal particles sinter to a coherent mass. Time and temperature of sintering
depend upon the nature of the metal.
(vi) Finshing operations. The sintered articles require operations like machinery, plating
etc., to get accurate shape of the work pieces.