Fig 2 Stress conditions for a forging and b hot pressing

Because of the differences in speed and deformation mode between forging and hot pressing, the metallurgical mechanisms of densification are different. Forging relies primarily on plastic deformation to close the voids during the few milliseconds that the material is under pressure. Hot pressing involves plastic deformation, creep, and diffusion mechanisms to cause localized flow of material into the pores.

Forging involves only axial compressive stress while the material is undergoing lateral flow, or upsetting. When the forged material reaches the die wall, however, lateral pressure acts on the material much like repressing (Fig. 1a). In hot pressing, the material is subjected to lateral pressure p in addition to axial pressure p, similar to repressing (Fig. 1b and 2b). The numerical difference between p and p diminishes as full density is reached. Closure of pores by plastic deformation becomes more difficult as the lateral and axial pressures approach equality; i.e., a hydrostatic stress state. Therefore, it is important in forging that most of the densification occurs by shear before the material reaches the die walls.

While both powder forging and hot pressing can consolidate most powder materials to full density, forging is applied primarily to high production parts in ferrous alloys, such as automobile connecting rods, transmission parts, and hand tool components. Hot pressing is used primarily for specialty materials such as tungsten carbide, beryllium, and ceramics. Hot pressing is also used as a consolidation process to produce billets for further processing by conventional forging or extrusion of materials such as high strength P/M aluminum alloys, tool steels, and superalloys.

Successful application of forging and hot pressing involves careful consideration of powder preparation (compaction, heating, and atmosphere protection) and forming process parameters (preform design, heating, tooling, and equipment). Computer modeling of these processes has been introduced recently to accelerate the development of the proper process parameters and tooling for new applications and the solution of problems in existing processes. The important process features are described here for both powder forging and hot pressing, along with specific applications and materials.

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