POWDER FORGING AND HOT PRESSING processes are similar in terms of their external physical characteristics. Each process involves the simultaneous application of pressure and heat to consolidate powders to full, or nearly full, density. Both processes occur within a die that gives shape to the final densified component.

At a more detailed level, forging and hot pressing are significantly different. Forging involves the deformation of a powder preform in which considerable lateral flow occurs (Fig. 1a). Forging is carried out on high-speed presses that are automated for mass production. Hot pressing, on the other hand, involves compression of loose powder or a powder compact fitted tightly in the die cavity, similar to repressing, resulting in very little lateral deformation, as shown in Fig. 1(b). Hot pressing generally is carried out at very low deformation rates in hydraulic presses.

Fig. 1 Deformation mode for (a) forging and (b) hot pressing

At an even finer scale, viewing the localized deformation of a representative unit of material containing a pore (or void space) between particles, the lateral flow of forging leads to considerable shear, which breaks up any oxide surfaces on the particles and enhances bonding (Fig. 2a). In hot pressing, pores and voids simply collapse under axial deformation; see Fig. 2(b) (Ref 1). Hot pressing is similar in this respect to cold die compaction.

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