Even with the best control that is feasible in practice, there will inevitably be some variation in the dimensions of parts produced from a given material in a given die set.

Typically, it is possible for parts 'as-sintered' to be accurate to a tolerance of

-0.0508mm per mm, in the direction at right angles to the pressing -direction, and 0.1016mm per mm parallel to the pressing direction.

Dimensional accuracy can be greatly improved by re-pressing the part after sintering. This operation is called sizing.

Sizing may be done in the die that was used for compacting the powder in those cases where the dimensional change on sintering is controlled at or very near to zero, but commonly separate sizing tools are used.

Re-pressing is used also to imprint or emboss the face(s) of the component in contact with the punch(es), in which case the process is referred to, for obvious reasons, as coining.

During re-pressing the density of the part is generally increased, especially if the as-sintered density is low.

• In certain cases where strength and other mechanical properties are required to be at maximum, re-pressing is used principally to achieve such densification.
• Further improvement is achieved by re-sintering.

Hot Repressing will give even greater densification, with consequent greater improvement in the mechanical properties, but less accurate control of the final dimensions is to be expected.

HIP is used as a post-sintering operation to eliminate flaws and microporosity in cemented carbides.

It is predicted that as HIP vessels increase in size the economics of the process will be sufficiently attractive to allow its use even on low-alloy steel structural PM parts in order to achieve full-density.

Forging is a comparatively recent technique in which a blank is hot re-pressed in a closed die which significantly changes the shape of the part, and at the same time can give almost complete density and hence mechanical properties approaching or even surpassing those of traditional wrought parts.

Sinter forging is dealt with in more detail in a later section.

An alternative method of improving the strength of inherently porous sintered parts is to fill the surface connected pores with a liquid metal having a lower melting point.

Pressure is not required:

• - capillary action is sufficient, provided that the infiltrant wets the metal concerned. It is desirable, however, that the infiltrant have a limited capacity to dissolve the metal being infiltrated otherwise the surface of the part may be eroded.

The process is used quite extensively with ferrous parts using copper as infiltrant but to avoid erosion, an alloy of copper containing iron and manganese, is often used.

Clearly if the molten copper is already saturated with iron its ability to erode the surface is lost.

Other proprietary infiltrant compositions are also on the market.

However, as we saw in the section on dimensional change during sintering, the diffusion of copper into iron can lead to growth.

Infiltration is also used to make composite electrical contact material such as tungsten/copper and molybdenum/silver; the lower melting point metal being melted in contact with an already sintered skeleton of W or Mo.

This term is used for a process analogous to infiltration except that the pores are filled with an organic as opposed to a metallic material.

• An outstanding example is oil-impregnated bearing materials which are dealt with in detail later; but, increasingly, impregnation with thermo-setting or other plastic materials is being done.
• The benefits to be obtained include some increase in mechanical properties, sealing of the pores which may provide pressure-tightness and will also prevent the entry of potentially corrosive electrolyte during a subsequent plating operation.
• Additionally the machining of sintered parts is improved, a feature that is referred to in more detail later.

Although many, perhaps the bulk of sintered structural parts are used in the as-sintered or sintered and sized condition, large quantities of iron-based parts, correctly steels, are supplied in the hardened and tempered conditions.

Conventional hardening processes are used, but because of the porosity inherent in sintered parts, they should not be immersed in corrosive liquids - salt baths, water, or brine - since it is difficult to remove such materials from the pores.

Heating should be in a gas atmosphere followed by oil-quenching.

These restrictions may not apply to very high density parts 7.2 g/cc nor to parts that have been infiltrated.

Carburizing and carbonitriding of PM parts is extensively used, and again gaseous media are indicated.

Because of the porosity of the 'case' is generally deeper and less sharply defined than with fully dense steels, but this is generally an advantage rather than the revers.

A process peculiar to PM parts is steam-treatment which involves exposing the part at a temperature around 500°C to high pressure steam.

This leads to the formation of a layer of magnetite (iron oxide) on all accessible surfaces and a number of desirable property changes result.

• Firstly the corrosion resistance is increased by the filling of some of the porosity,
• and secondly, this reduction in porosity of the surface layer leads also to improved compressive strength.
• Thirdly, the oxide layer significantly increases the surface hardness and more importantly the wear resistance.

  Steam-treatment is often followed by dipping in oil which enhances the blue/black appearance and still further increases the corrosion resistance.

The treatment is not generally applicable to hardened and tempered parts because the exposure to the high temperature would undo the hardening.

Sintered parts may be plated in much the same way as wrought or cast metals, and copper, nickel, cadmium, zinc, and chromium plating are all used.

• However, it is important to note that low density parts should be sealed - e.g. by resin impregnation, before plating, to prevent the electrolyte from entering the pores and causing corrosion subsequently.
• Parts that have been oil-quenched cannot be plated satisfactorily unless the oil is removed before resin impregnation.

Recent work has shown that it may be possible successfully to plate unimpregnated porous parts with nickel by electroless plating, which process will plate also the surfaces of any of the pores into which the solution penetrates.

A large percentage of hardmetal cutting tool inserts are now coated using chemical vapour deposition (CVD) or physical vapour deposition (PVD).

The lower temperature PVD process also allows steels to be given a wear resistant layer of TiC, TiN, Al2O3 or a combination of these materials and sintered high speed steel tools are also now being coated.

Although a major attraction of PM parts is that they can be produced accurately to the required dimensions, there are limitations to the geometry that can be pressed in rigid dies, and subsequent machining, for example of transverse holes or re-entrants at an angle to the pressing direction is not uncommon.

The existence of porosity alters the machining characteristics and in general tool wear is greater than with the same composition in the fully dense form.

Carbide tools are recommended, and lower cutting speeds may be necessary.

The machinability can be improved by incorporating certain additions in the powder mix - e.g. lead, copper, graphite, sulphur or a metal sulphide such as manganese sulphide, and, as already indicated, by infiltration or resin impregnation.

If these points are borne in mind all the traditional machining processes - turning, milling, drilling, tapping, grinding, etc. - can be done quite readily.

De-burring is done with sintered parts, and is used to remove any 'rag' on edges, resulting from the compacting operation or a machining step.

Tumbling, sometimes in a liquid medium with an abrasive powder, is normally employed. At the same time, brushing and some hardening of the surface layers may occur.

Another class of wrought sintered material that is beginning to make an impact is based on particulate material - powder or chopped ribbon - that has been solidified and cooled at a very high rate such that metastable non-equilibrium microstructures result. They may be microcrystalline or amorphous.

The process is applicable only to certain alloys, and one important feature is that the matrix metal can retain in solid solution a much higher than the equilibrium percentage of the alloying element.

Providing that the densification and mechanical working is carried out at a temperature low enough to avoid destroying the non-equilibrium structure, remarkably enhanced mechanical properties can be achieved.

A major development programme is underway with alloys of aluminium, titanium, and magnesium, the hope being that their use in aircraft structures will significantly reduce the weight and increase the payload.