Conclusions

While elevated-temperature direct extrusion is now a mature metalworking technique, its utility has not diminished with time. Loose powders or precompacted slugs may be processed this way as well as cast billets, and the canned extrusion and filled billet approaches have extended the application of extrusion to powder metals for specific purposes. Examples of the benefits obtained from canned metal powder extrusions include the formation of composites and reinforced materials, near-net sections, sometimes complex in shape, that may be released by chemical removal of a coextruded matrix, sound rods and sections in materials of limited ductility, and the safe containment of materials that may be toxic or reactive.

The mechanisms for powder extrusion have been studied extensively and reported by Sheppard and coworkers using upper-bound analyses and hodographic depictions of the process as material traverses the extrusion deformation zone. They demonstrated that powder is compacted to near-full density before the onset of extrusion, although at this stage the particles remain poorly bonded. As the densified material passes through the extrusion deformation zone, redundant work is expended in welding, breaking welds, and reforming them between particles while they are deforming; the emergent material has suffered considerable shear deformation, and the formation of metallurgically sound interparticle bonds produces full density and wrought properties. The redundant work factor in the process is a function of particle size so that fine powder with a large surface-area-per-unit volume requires a higher extrusion pressure than coarse powder. However, extrusion of powders generally requires lower pressures than cast billets of the same material because the final properties characteristic of the wrought material are developed only after it has passed through the deformation zone.

When powder mixtures or powder-solid combinations are to be extruded, successful coreduction of section depends on the metallurgical and thermomechanical compatibility of the various components. When flow stresses are well matched and the operating temperature does not cause the formation of liquid phases, uniform and proportionate coextrusion will be achieved. Within the broad confines of these simple criteria, it is possible to construct billet assemblies involving multiple components to make products that might otherwise be obtained with great difficulty, if at all.

The extrusion of metal powders is a useful industrial technique with a broad variety of applications that have been extended in part by inventive process and billet design. The obtainable material properties and product characteristics together with the relatively short process cycle involving melt-powder-extruded compact ensure that it will continue to be used well into the future.

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