Other Shaping Methods

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.

Procedures for getting very much finer dispersions of the non-metallic phase have been developed, and 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.
• A relatively new 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.
• The ‘alloyed’ powder is subsequently compacted, sintered, and normally wrought by extrusion or hot rolling.
• Superalloys made in this way are now in service, and mechanically alloyed aluminium alloys are under trial.
• 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.

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.

Powder forging produces fully dense PM steel parts , such as the automotive connecting rod used in BMW V8 engines.

The production of traditional PM parts has been expanding at a significantly faster rate than the general growth of engineering production and when it was originally developed in the 1970s powder forging or sinter forging was expected to alter fundamentally the scale of the PM industry.

PROCESS In this process, a powder blank is pressed to a simple shape halfway between that of a forging billet and the required finished part.

This compact, referred to as a preform, is sintered and then hot forged to finished size and shape in a closed die.

The amount of deformation involved is sufficient to give a final density very closely approaching that of the solid metal , and consequently, the mechanical properties are comparable with those of material forged from wrought bar.

ADVANTAGES Indeed they may be superior in some respects because of the freedom of the sinter forged part from directionality, the greater homogeneity as regards composition, and a finer microstructure, as well as the absence of internal discontinuities resulting from ingot defects that are possible in forgings made from cast metal.

An additional advantage is the dimensional consistency achievable in consequence of the accurate metering of the quantity of powder used.

LIMITATIONS There are limitations to the steel compositions that can be successfully produced on a commercial scale.

• Steels containing readily oxidisable elements such as chromium and manganese – which happens to be also the cheaper strengthening elements – cannot easily be used, but special compositions , generally containing as alloying elements, nickel and molybdenum, the oxides of which are reduced in sintering atmospheres, have been developed .
• Powder forged steel parts can be heat treated in the same manner as wrought steels.

Production costs in powder forging are generally higher than in conventional casting or forging due mainly to the high price of the starting material and tooling. However, the high precision achieved in powder forging results in considerable savings on machining costs and hence savings on investments in machining operations.

This has particularly proved to be the case for powder forged connecting rods which are gaining in popularity all over the world due to their improved dimensional accuracy, higher dynamic properties, smoother running in the engine, and significant cost savings.

Many companies in North America, Japan and Europe now have large powder forging installations mainly to produce parts for the automotive industry . Such parts can have inside and outside spline forms, cam forms, and other forms that require extensive machining. In addition to the well known connecting rod other applications include bearing races, torque convertor hubs, and gears.

This term is applied to the process, now established on an industrial scale, wherein a metal powder is fed continuously into a rolling mill which may be heated, and compacted between the rolls into strip .

This strip is then passed through a sintering furnace and rerolled to finished size.

In general the product does not have any advantage over strip produced by rolling cast billets , although in some cases superior homogeneity can be demonstrated as well as freedom from laminations that can arise from ingot defects.

The main advantage is economic , and depends on the following features:

• (a) The yield of finished strip from castings is low.
• (b) The cost of fettling the ingot, of the extensive rolling, annealing and pickling, is considerable.

Powder rolling is economical, therefore especially in cases where the metal is produced cheaply as a powder directly during the extraction process , e.g. nickel, and in the case of a material that work-hardens rapidly and, therefore, requires many intermediate annealing and pickling operations during reduction of a rolling slab, e.g. stainless steel.

The production of small quantities of special materials by powder rolling is increasing for applications such

• as cobalt- or nickel-base alloy strip for welding,
• nickel-iron strip for controlled expansion properties,
• special Cu-Ni-Sn alloys for electronics,
• porous nickel strip for alkaline batteries and fuel cell electrodes, composite bearings, etc

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

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 – 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 material 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.

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.

• The ‘alloyed’ powder is subsequently compacted, sintered, and normally wrought by extrusion or hot rolling. Superalloys made in this way are now in service, and mechanically alloyed aluminium alloys are under trial.
• 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.

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.