interconnected porosity is filled with an alloy having a melting point
lower than the sintering temperature of the metal of which the component
is made, e.g. ferrous parts are infiltrated by copper-based alloys,
usually during the sintering phase.
Infiltration makes the components impermeable and there is some increase
in mechanical properties, but at expense of dimensional accuracy.
Infiltration simplifies some heat treatments. For instance, it is easier
to obtain a defined case depth without interconnected porosity.
Sintered parts achieve greater protection against corrosion by being
impregnated by oil or other non-metallic materials. As described
previously, self-lubricating bearings are manufactured by impregnating
porous sintered bearings with lubricants. Self-lubricating bearings can
only be produced by powder metallurgy.
Sizing and Coining
Sizing and Coining are additional press operation after sintering. The
main objective is to improve the dimensional accuracy, but we also improve
the surface finish. Quite moderate pressures are normally required for
sizing, since only a slight plastic deformation is necessary.
have two purposes by coining: Not only is dimensional accuracy improved,
but the use of higher pressures also increases the density of the part.
Normally, a press tool specific to the task of sizing or coining is used.
Applicable only to ferrous parts. By heating the parts to a temperature of
550ºC and exposing them to water vapour, a thin layer of Fe3O4
is formed both on the outer surface and along the interconnected porosity.
use the steam treatment for a considerable improvement in corrosion
resistance, increased hardness, increased resistance to compressive
strength and wear resistance.
consider re-pressing as an operation that serves to decrease porosity for
application for where density is crucial to achieve the required
mechanical or magnetic properties.
pr-sintering the pressed parts at the temperature of 700-800ºC, the
admixed lubricant is burned off and recrystallisation takes place. Once
the work-hardening and internal stresses are removed, the material
reacquires its ductility and therefore its capacity for further
densification. After re-pressing, the parts are sintered.
Although a major attraction of producing sintered components is the
ability to produce complex shapes and close tolerances, limitations do
Therefore, we use machining operations such as milling, drilling (e.g.
holes perpendicular to the pressing direction), threading and machining to
achieve features not possible to obtain by pressing in rigid dies.
Sintered metals are generally less easy to machine than wrought alloy of
similar composition, so we adjust cutting speed and cutting tools for
increase tool life, machinability-enhancing additives such as Mns or MnX
can be admixed with the powder. After sintering, they remain evenly
distributed in the structure and mechanical properties are only marginally
utilize this operation to remove burrs resulting from the compacting
operations or machining step. The most common method is tumbling, and
sometimes a liquid medium with an abrasive powder is employed.
Larger parts and very complex shapes can be obtained by joining. We use
several techniques for joining, such as diffusion bonding, sinter brazing
and laser welding.
Phase transformation depends on the composition and homogeneity of the
alloy, not on its porosity. So all heat treatments applicable to the given
alloy are applicable for sintered materials as well.
Hardening operations with quenching and tempering substantially increase
strength, and improve wear-resistance; but at the expense of ductility. We
also utilize carburizing and carbonitriding for surface hardening.
When needed, corrosion protection can be obtained by plating. However, low
density parts must be impregnated before plating, to prevent electrolyte
from entering the pores and causing subsequent corrosion