General principle of forging

Forging is a massive forming process; the temperature of the workpiece is increased to such an extent that the deformation forces required are considerably less than would be needed to cold work it. The two most important forging processes are open-die forging (in which forming of the workpiece takes place locally and mostly using simple dies) and closed-die forging (where the workpiece is fully enclosed in a die whose form determines the shape of the forging) (cf. illustrations).

For large-scale production, closed-die forging is usually used because it is regarded as being a very reliable process. Thanks to the superior mechanical properties obtained, the process can compete with the most advanced casting processes. Compared with casting, however, the range of possible shapes that can be produced is more limited. In particular, it is difficult to produce sharp corners, undercuts and cavities by use of forging.

The Institut für Integrierte Produktion Hannover (Institute for Integrated Production) has developed the hydroforging process to produce hollow parts; this involves forging aluminium components using an active fluid medium. In order to maintain a constant forging temperature, molten tin solder, for example, is used as the active medium. By using a sealed piston to increase the internal pressure, whilst at the same time closing both open ends of the hollow profile, the process can be used to produce hollow aluminium bodies having a uniform wall thickness. The forging process usually consists of the following steps:

  • sawing the extruded or continuously cast feedstock,
  • heating the blank,
  • upsetting or bending,
  • forging (rough and final forging),
  • deburring and, if necessary, punching,
  • heat treatment ,
  • pickling or blasting and
  • final inspection.

Unlike sheet forming, in forging there is always a change in the cross section of the feedstock. Generally speaking, changes to the cross-section are achieved either by material displacement (forging, extrusion, rolling, and cold impact extrusion) or by material accumulation (upsetting) (c.f. figure).
At the same, the processes mentioned enable a change in the direction of flow of the material to occur.

Cavities are an added design possibility of massive forming.

Cutting processes are also always a part of the forging process. They are used to cut the feedstock, produce openings in forgings or produce blanks.