Bulk Deformation Processes

Powder Forging. In powder forging, a preform shape is cold pressed to between 75 and 85% of full density, sintered, heated to a forging temperature, and then forged in trapped dies using one blow to produce a fully dense net or near-net shape. The sintering step is optional, but it is normally included as a particle surface cleaning step (deoxidation) and to improve the workability of the porous preform. Powder forging fundamentals and general applications are presented in the article "Powder Forging and Hot Pressing" in this Volume. More detailed information is also contained in the article "Powder Forged Steel."

Normally, powder forging is performed hot (1000 to 1200 °C, or 1800 to 2200 °F, for steel powders), but it can also be conducted at warm or cold forging temperatures. Typical steel powder forging pressures are 550 to 950 MPa (40 to 70 tsi). Because the workability of a porous preform is poor, the design of the preform is critical to the process in terms of avoiding defects. Local surface tensile stresses and internal hydrostatic tension must be avoided. Nonetheless, with a correct preform shape and a well controlled process, powder forged parts have sound microstructures, good hardenability, and performance that meets or exceeds cast-wrought part performance.

There are two classes of forging practice. In repressing, the preform shape is nearly identical to the forged shape. In true forging, considerable shear deformation is involved since the preform is different in shape from the forging. The dynamic properties of toughness and fatigue resistance are higher for forged parts than for repressed parts.

Powder Extrusion. Loose powder can be containerized and extruded to full density, either with or without heat. Extrusion ratios of at least 9:1 have been shown to produce full density, and many materials are extruded to full density using much higher extrusion ratios. Extrusion at such high reductions subjects the powder particles to high levels of shear deformation and compression as they pass through the die. The result is sound particle bonding. As in hot isostatic pressing, the powder is usually hermetically sealed in a container prior to extrusion. In some alloy systems, residual air provides oxygen for selection oxidation of alloy ingredients prior to extrusion, and an oxide dispersion strengthened material is produced. In some other cases, particulates or chopped fibers may be added to the matrix powder, and extrusion produces a composite material. In still other cases, combinations of powders or powder plus wrought pieces are extruded to produce multimetal parts, e.g., bimetallic tubing. These and other uses of extrusion are discussed in the article "Extrusion of Metal Powders" in this Volume.

Powder rolling or roll compaction of powders consolidates loose powder(s) into a porous strip as it passes through a roll gap. It is possible to produce a monolithic strip or a multilayer strip. Further processing may include sintering of the strip and additional rolling to densify the strip. Applications of this type of processing include such diverse products as clad metal for coin stock, automotive sleeve bearings, and electrode stock.

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