Powder Metallurgy - Processing

Materials

The majority of the structural components produced by fixed die pressing are iron based. The powders are elemental, pre-alloyed, or partially alloyed. Elemental powders, such as iron and copper, are easy to compress to relatively high densities, produce pressed compacts with adequate strength for handling during sintering, but do not produce very high strength sintered parts.

Pre-alloyed powders are harder, less compressible and hence require higher pressing loads to produce high density compacts. However, they are capable of producing high strength sintered materials. Pre-alloying is also used when the production of a homogeneous material from elemental powders requires very high temperatures and long sintering times. The best examples are the stainless steels, whose chromium and nickel contents have to be pre-alloyed to allow economic production by powder metallurgy.

Partially alloyed powders are a compromise approach. Elemental powders, e.g. Iron with 2 wt.% Copper, are mixed to produce an homogeneous blend which is then partially sintered to attach the copper particles to the iron particles without producing a fully diffused powder but retaining the powder form. In this way the compressibilities of the separate powders in the blend are maintained and the blend will not segregate during transportation and use.

A similar technique is to ‘glue’ the small percentage of alloying element onto the iron powder. This ‘glueing’ technique is successfully used to introduce carbon into the blends, a technique which prevents carbon segregation and dusting, producing so-called ‘clean’ powders.

Powder Consolidation

Components or articles are produced by forming a mass of powder into a shape, then consolidating to form inter-particle metallurgical bonds. An elevated temperature diffusion process referred to as sintering, sometimes assisted by external pressure, accomplishes this. The material is never fully molten, although there might be a small volume fraction of liquid present during the sintering process. Sintering can be regarded as welding the particles present in the initial useful shape.

As a general rule both mechanical and physical properties improve with increasing density. Therefore the method selected for the fabrication of a component by powder metallurgy will depend on the level of performance required from the part. Many components are adequate when produced at 85-90% of theoretical full density (T.D.) whilst others require full density for satisfactory performance.

Some components, in particular bush type bearings often made from copper and its alloys, are produced with significant and controlled levels of porosity, the porosity being subsequently filled with a lubricant.

Fortunately there is a wide choice of consolidation techniques available.

Cold Uniaxial Pressing

Elemental metal, or an atomised prealloyed, powder is mixed with a lubricant, typically lithium stearate (0.75 wt.%), and pressed at pressures of say, 600 MPa (87,000 lb/in2) in metal dies. Cold compaction ensures that the as-compacted, or ‘green’, component is dimensionally very accurate, as it is moulded precisely to the size and shape of the die.

Irregularly shaped particles are required to ensure that the as-pressed component has a high green strength from the interlocking and plastic deformation of individual particles with their neighbours.

One disadvantage of this technique is the differences in pressed density that can occur in different parts of the component due to particle/particle and die wall/particle frictional effects. Typical as-pressed densities for soft iron components would be 7.0 g/cc, i.e. about 90% of theoretical density. Compaction pressure rises significantly if higher as-pressed densities are required, and this practice becomes uneconomic due to higher costs for the larger presses and stronger tools to withstand the higher pressures.

Cold Isostatic Pressing

Metal powders are contained in an enclosure e.g. a rubber membrane or a metallic can that is subjected to isostatic, that is uniform in all directions, external pressure. As the pressure is isostatic the as-pressed component is of uniform density. Irregularly shaped powder particles must be used to provide adequate green strength in the as-pressed component. This will then be sintered in a suitable atmosphere to yield the required product.

Normally this technique is only used for semi-fabricated products such as bars, billets, sheet, and roughly shaped components, all of which require considerable secondary operations to produce the final, accurately dimensioned component. Again, at economical working pressures, products are not fully dense and usually need additional working such as hot extrusion, hot rolling or forging to fully density the material.

Sintering

Sintering is the process whereby powder compacts are heated so that adjacent particles fuse together, thus resulting in a solid article with improved mechanical strength compared to the powder compact. This “fusing” of particles results in an increase in the density of the part and hence the process is sometimes called densification. There are some processes such as hot isostatic pressing which combine the compaction and sintering processes into a single step.

After compaction the components pass through a sintering furnace. This typically has two heating zones, the first removes the lubricant, and the second higher temperature zone allows diffusion and bonding between powder particles. A range of atmospheres, including vacuum, are used to sinter different materials depending on their chemical compositions. As an example, precise atmosphere control allows iron/carbon materials to be produced with specific carbon compositions and mechanical properties.

The density of the component can also change during sintering, depending on the materials and the sintering temperature. These dimensional changes can be controlled by an understanding and control of the pressing and sintering parameters, and components can be produced with dimensions that need little or no rectification to meet the dimensional tolerances. Note that in many cases all of the powder used is present in the finished product, scrap losses will only occur when secondary machining operations are necessary.

Hot Isostatic Pressing

Powders are usually encapsulated in a metallic container but sometimes in glass. The container is evacuated, the powder out-gassed to avoid contamination of the materials by any residual gas during the consolidation stage and sealed-off. It is then heated and subjected to isostatic pressure sufficient to plastically deform both the container and the powder.

The rate of densification of the powder depends upon the yield strength of the powder at the temperatures and pressures chosen. At moderate temperature the yield strength of the powder can still be high and require high pressure to produce densification in an economic time. Typical values might be 1120°C and 100 MPa for ferrous alloys. By pressing at very much higher temperatures lower pressures are required as the yield strength of the material is lower. Using a glass enclosure atmospheric pressure (15 psi) is used to consolidate bars and larger billets.

The technique requires considerable financial investment as the pressure vessel has to withstand the internal gas pressure and allow the powder to be heated to high temperatures.

As with cold isostatic pressing only semifinished products are produced, either for subsequent working to smaller sizes, or for machining to finished dimensions.

Hot Forging (Powder Forging)

Cold pressed and sintered components have the great advantage of being close to final shape (near-nett shape), but are not fully dense. Where densification is essential to provide adequate mechanical properties, the technique of hot forging, or powder forging, can be used.

In powder forging an as-pressed component is usually heated to a forging temperature significantly below the usual sintering temperature of the material and then forged in a closed die. This produces a fully dense component with the shape of the forging die and appropriate mechanical properties.

Powder forged parts generally are not as close to final size or shape as cold pressed and sintered parts. This results from the allowances made for thermal expansion effects and the need for draft angles on the forging tools. Further, minimal, machining is required but when all things are considered this route is often very cost effective.

Metal Injection Moulding (MIM)

Injection moulding is very widely used to produce precisely shaped plastic components in complex dies. As injection pressures are low it is possible to manufacture complex components, even some with internal screw threads, by the use of side cores and split tools.

By mixing fine, typically less than 20 μm diameter, spherical metal powders with thermoplastic binders, metal filled plastic components can be produced with many of the features available in injection moulded plastics. After injection moulding, the plastic binder material is removed to leave a metal skeleton which is then sintered at high temperature.

Dimensional control can be exercised on the as-sintered component as the injected density is sensibly uniform so shrinkage on sintering is also uniform.

Shrinkage can be large, due to both the fine particle size of the powders and the substantial proportion of polymer binder used.

Primary author: G. Greetham

Abstracted from Materials information Service, edited by Stephen Harmer.

For further details on the Materials Information Service visit The Institute of Materials

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