Pressure Shaping

a group of industrial processes that produce changes in the shape of metal billets (without disrupting their continuity) by relative displacement of certain parts of the billets—that is, by plastic deformation. The main forms of pressure shaping are rolling, pressing, drawing, forging, and stamping. Pressure shaping may also be used to improve surface quality.

The adoption of industrial processes based on pressure shaping is steadily increasing in comparison to other types of metal-working (casting, machining). This results from the decrease in losses of metal and from the possibility of achieving a high level of mechanization and automation of industrial processes.

Pressure shaping is capable of producing articles with constant or periodically changing cross section (by rolling, drawing, or pressing), and also piece goods of various shapes (by forging, or stamping). The products may be identical to the finished articles in shape and dimensions or may differ only insignificantly from them. Piece goods are usually machined. The quantity of metal removed in machining depends on the extent to which the shape and dimensions of the forging or stamping approach the shape and dimensions of the finished article. In many cases, pressure shaping produces articles that do not require machining (bolts, screws, and most sheet-stamping products).

Pressure shaping may be used not only for the production of billets and parts but also as a finishing operation after machining (broaching, peening by rollers and balls, and other finishing operations) to reduce the surface roughness, strengthen the surface layers of the articles, and produce the desired distribution of residual stresses, which leads to improved performance properties of the article (such as increased fatigue strength).

Pressure shaping is performed by the action of external forces on the billet. The deformation force may be produced by the muscular energy of man (in manual forging and knockout) or by the energy produced in special machines, such as rolling and drawing mills, presses, and hammers. Deformation forces may also be produced by the action of a shock wave on the billet (in explosive forming) or by powerful magnetic fields (in electromagnetic stamping). The deformation forces are imparted to the billet by a tool, which is usually rigid and experiences small elastic deformations during plastic deformation of the billet. Some techniques use elastic media (for example, rubber or polyurethane in stamping) or liquids (in hydrostatic pressing).

A distinction is made between hot and cold pressure shaping. Hot pressure shaping is characterized by the phenomena of recovery and recrystallization and the absence of work hardening; the mechanical and physicochemical properties of the metal remain relatively unchanged. Plastic deformation does not produce banding (nonuniformity) of the microstructure, but it leads to the formation of banded macrostructure in cast semifinished products (castings) or to changes in the direction of the strands of the macrostructure (strands of nonmetallic inclusions) during pressure shaping of billets produced by rolling, pressing, and drawing. Banding of the macrostructure generates anisotropy of mechanical properties, in which case the properties of the material are usually better along the strands than across them. In cold pressure shaping, plastic deformation is accompanied by work hardening, which changes the mechanical and physico-chemical characteristics of the metal and produces a banded microstructure, and also changes the direction of the strands of the macrostructure. Cold pressure shaping generates texture, which leads to anisotropy of not only the mechanical but also the physicochemical properties of the metal. By taking advantage of the effect of pressure shaping on the properties of metals, parts with optimum properties and minimum weight may be produced.

During pressure shaping, changes in the stress diagram of the billet being deformed make it possible to influence the changes in the shape of the billet. Under nonuniform compression from all sides, the plasticity of the metal increases in proportion to the compressive stresses applied. Proper selection of the pressure-shaping operations and the conditions of deformation (hydrostatic pressing, extrusion with back pressure, and planetary rolling) not only makes possible an increase in the permissible change in shape but also permits the use of pressure shaping for the production of articles from high-strength alloys that are difficult to deform.

The theory of pressure shaping, which is a scientific discipline that synthesizes various branches of metal physics, and the theory of plasticity are the scientific basis for the design and control of industrial processes for pressure shaping. The main problems of the theory of pressure shaping are (1) development of methods for the determination of the forces and work expended on deformation, (2) calculation of the dimensions and shape of the billet and the nature of changes in its shape, (3) development of methods for determining the maximum permissible changes in the shape of the billet (without failure or the appearance of other defects), (4) estimation of changes in the mechanical and physicochemical properties of the metal during deformation, and (5) determination of the optimum deformation conditions.