1. ELECTRIC COMPONENTS

Composite structures attainable only by powder metallurgy methods have been used extensively in the manufacture of electrical contacts and current collector brushes .

It is possible to combine the desirable conducting properties and low contact resistance of silver or copper with the strength, heat-resistance, and resistance to arc erosion of tungsten, molybdenum, nickel, etc or with the lubricating qualities of graphite.

Sintered electrical contact components

Controlled porosity is also employed in the manufacture of metal filters and diaphragms .They have the advantage over their ceramic counterparts

• of higher strength
  
  
• and resistance to mechanical and thermal shock.

The close control over the pore size and permeability is achieved by the use of powders having a narrow range of particle size.

• Perhaps the most commonly used filter elements are made of bronze (89/11 Cu/Sn), and spherical powder are used.

The filter profile is formed by a loose packing of the powder in the mould and the inherently poor compressibility of spheres is no disadvantage. Where products are required to have limited or localised porosity, conventional pressing is necessary and irregularly shaped particles are more suitable.

• Metal filters are available in a wide range of materials including copper,nickel,bronze,stainless steel and 'Monel',
  
  
• and are widely used for the filtration of fuel oils, chemical solutions and emulsions.

They are also efficient in separating liquids of varying surface tension, and have been successfully applied to jet engine fuels, where water is removed at the same time as the fuel is filtered.

Similar devices are widely used for sound damping on air compressors and the likes.

3. FRICTION MATERIALS

Sintered metal friction components are particularly useful for heavy-duty applications , e.g. aircraft brakes, heavy machinery clutch and brake linings etc.

They consist essentially of a continuous metal matrix, into which varying amounts of non-metallic friction generators, such as silica and emery are bonded.

• Compositions tend to be complex in view of the characteristics required, and may include copper, tin, iron, lead, graphite, carborundum, silica, alumina, emery and asbestos substitutes.

The sintered material has a high thermal conductivity , and may be used over a wide range of temperature.

• Satisfactory performance figures have been reported for copper-based materials, operating at surface temperatures up to 800°C and from iron-based materials up to 1000°C.
  
  
• The resistance to wear is superior to resin-bonded materials, and therefore, permits the use of components of thinner section.
  

Because of the large surface area, and this thinness of section, the components are relatively weak.

• Mechanical strength is imparted by bonding the friction element to a steel backing-plate, either by brazing or welding, or by sintering the two components together under pressure.
  
  
• Compared with solid phosphor bronze or aluminium bronze friction elements, the sintered material offers many advantages.
  

The most important is probably the much wider range of friction characteristics which can be obtained from variations in the dispersion of non-metallic particles.

4. METAL MATRIX

Rapid Solidification - Metals having a dispersion of a finely divided non-metallic phase have been known for many years the idea being to provide the strengthening that is produced by precipitation hardening without the drawback that the second phase goes into solution as the temperature rises thus limiting the operating temperature.

Metal matrix composites (MMC) as such materials are now called, represent a major step forward in the search for improved materials i.e. with better mechanical properties especially at elevated temperatures. Powder metallurgy is the most important route by which such composites are produced. In the majority of cases so far developed the strengthening phase is a stable oxide usually of another metal and the term ODS - oxide dispersion strengthening is in everyday use. A number of different processes are available for producing the very fine dispersions required:

• In one process an alloy of the matrix metal with the metal that forms the stable oxide is heated in an atmosphere that is reducing to the matrix metal but oxidising to the second metal.
  
  
• The latter is converted to oxide uniformly distributed throughout the matrix.
  
  

In the case of precious metals - Ag, Pt etc heating in air can be used and a range of electrical contact materials consisting of silver with a dispersion of e.g. Cd oxide, Sn oxide, and/or In oxide are now widely used. The internal oxidation as the process is called occurs as a result of the diffusion of oxygen through the silver lattice and with large sections, this is a lengthy process.

• However, if powder is used a relatively short oxidising cycle is required so that the pressing and sintering of internally oxidised powder is the best procedure.

In this case the object is not to improve the strength but the electrical properties, i.e. the resistance to contact welding.

• In other cases the matrix metal sometimes in the form of salt is mixed with a solution of a salt of the metal with the more stable oxide and mixture is heated in an atmosphere that is reducing to the matrix metal but oxidising to the second metal.

  ODS platinum and tungsten are made in this way.

• Other composites use fibres or whiskers as the strengthening agent.

5. OTHER TECHNIQUES

As-deposited/machined IN625 tubes (400mm O.D.) (Sandvik Steel)

a) Spray deposition: This is not strictly a powder metallurgical process. It involves the atomisation of molten metal, but instead of being allowed to solidify as powder, the spray is collected on a substrate to form billets for subsequent forging. Spray deposition is not a powder metallurgical process within the strict definition of that term since metal in actual powder form is not involved.

Molten metal is gas atomised in the normal way and the spray is caused to impinge while still in the liquid or semi-solid state on a solid former where a layer of dense solid metal of a pre-determined shape is produced.

The solid thus produced has a structure similar to that of powder-based material with all the attendant advantages of fine grain, freedom from macro-segregation, etc.

In common with the PM process, spray deposition facilitates the production of alloy compositions that are difficult if not impossible to produce conventionally, and in certain cases the benefits that rapid solidification offers can be obtained also.

Properties even superior to those of powder-based wrought products have been reported; for example superalloy having a much lower inclusion count than that of its powder-based equivalent.

• The range of materials that are being processed in this way is extremely wide and includes Al alloys, Cu alloys, stainless steels, high Cr alloy steels, and superalloys. The range of shapes is extensive also; - round billets, tubes, strip and sheet, and near-net shape pre-forms.

  Clad materials are also being produced, for example low alloy steel rolls clad with high speed steel.

• The sizes that can be produced are, naturally, a function of the available plant and they are continually rising.

  A recent installation will produce tube blanks weighing up to 4.5t.

The commercial viability of the process is markedly influenced by the yield of usable product - i.e. the proportion of the metal atomised that is deposited on the substrate. This in turn is dependent on the design of the equipment, the spray pattern, and the co-ordinated movements of the substrate. The amount of 'over-spray' has been progressively reduced and yields as high as 90% are being claimed.

• With conventional products such as, for example, stainless steel tubing, the benefit of spray deposition is mainly cost saving, in other cases there are significant property improvements. Rolls for metal rolling mills spray-deposited and HIPped have been found to have 2 or 3 times the life of cast rolls of similar composition.
  
  
• Among the materials that cannot be made conventionally, but can be made by spray deposition, are rapidly solidified Al-Li alloys, Al-Sn alloys with high Zn content (11%), highly alloyed Cu-Ni-Sn and Cu-Cr, as well as the composites referred to in a previous section. In this last case, the re-inforcing particles are injected into the metal stream during the atomisation process. Spray deposition seems destined to have a very interesting future.

b) Mechanical alloying: One recent process that represents a major step forward in materials for very high temperature applications, gas turbines for jet engines in particular, is mechanical alloying.

This process involves milling, usually in an attritor, a mixture of a metal powder and a refractory material for long periods during which the refractory particles are broken up and incorporated in the metal.

In the case of aluminium another mechanical alloy is made by a similar milling process starting with a mixture of aluminium powder and graphite which during the milling process is incorporated in the metal as aluminium carbide. Al4C3.

c) Meta Stable Wrought Sintered Material: 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 payloady.

In addition to these breakthrough developments, steady progress is being maintained in the traditional areas of powder metallurgy. The quality of commercially available powders has been improved, die materials and die designs are better, and presses have become more efficient as well as more powerful.

These developments have resulted both in quality, range of product, and cost competitiveness, and there is little doubt that, in addition to the many exciting developments in products that can be made only by PM , the sintering process will continue to take an increasing share of the market for traditional engineering components