Compaction of Metal Powders

After blending, compaction is the next step in which blended powders are pressed into the die to form the product with desired density. The dies used are made of high-carbon steel. high chromium-vanadium steel and tongsten carbide; the latter gives a long life to the dies which are used for long runs of mass production. The benefits of compaction can be summarized as follows.

(i) Cold welding (or solid-state welding) of metal powder particles among themselves which results in imparting strength to the green compact.
(ii) Reduction of voids between powder particles.
(iii) Increasing the density of compact.
(iv) Compaction also results in plastic deformation of powder particles which subsequently allow better recrystallization during sintering, giving a stronger product with higher density.

Compaction of powder consists of filling the die with metal powders and pressing them there with the help of one or more plungers or punches. Various steps involved in making the green compact by compacting metal powders in the die are shown in (Fig.). A predecided measured amount (weight) of powder is poured in the die. High pressures (275 to 800 MPa) are used for compaction of powders, soft metal powders as that of aluminium, brass need lower compaction pressures (275 to 400 MPa) whereas iron powder needs higher compaction

Compacting in Powder

Steps involved in forming a green compact of a bush: (a) Metal powder being filled in the die cavity by a powder feeder, (b) Compacting the metal powder by pressing it from both the top and bottom sides with the help of upper punch and lower punch respectively. (c) Ejection of the green compact from the die in progress, and (d) Green compact (bush) fully ejected from the die and pushed away from the die top by power feeder for onward shifting to sintering furnace.

pressures (up to 800 MPa), Cemented carbide powders need compaction pressures between 140 and 400 MPa. Lubrication of die walls is essential, for which various lubricants (metallic stearates, graphite) are either mixed with metal powders during their blending or separately coated on the die walls.

Presses of different capacity are used for compacting powders. Smaller presses with capacity below 100 tonnes are crank or eccentric type. Medium capacity presses, 200 to 300 tonnes, are of toggle type. And higher capacity presses, 5000 tonnes or more, are hydraulic presses.

Density of green compact largely depends on the compacting pressure, higher compacting pressures give higher density of compacts. The tensile strength, elastic modulus, electrical conductivity and elongation (of sintered products) are increased with the increase in the density of green compact. Further, if the particles (of powder) are of same size, they will give a porous green compact with lower density but by introducing smaller particles (of powder) in the die mix, a denser compact is obtained (as smaller particles fill up the gaps between larger particles).

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