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Secondary Operations

 

Infiltration

The 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.

Oil Impregnation

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.

We 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.

Steam treatment

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.

We use the steam treatment for a considerable improvement in corrosion resistance, increased hardness, increased resistance to compressive strength and wear resistance.

Re-pressing

We 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.

By 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.

Machining

Although a major attraction of producing sintered components is the ability to produce complex shapes and close tolerances, limitations do exist.

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 optimum results.

To 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 affected.

De-burring

We 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.

Joining

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.

Heat treatment

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.

Surface plating

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

 
 
 
 
 
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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