The plastic deformation of a metal between opposed dies while it is held above its crystallization temperature thus avoiding work hardening of the workpiece.
Theory and Application
Hot forging is one of the oldest metal forming processes which is still used extensively for the mass production of parts typically for the automotive and aerospace industries.
It appears in many different forms. The simplest being open-die-forging, illustrated in Fig. 1, where the tools have flat or slightly curved profiles.
Another common form is upsetting where again flat die are used as illustrated in Fig. 2. This process is often in conjunction with closed-die-forging.
Heading is another form of open-die-forging which is also used in conjunction with closed-die-forging. This is illustrated in Fig. 3.
In closed-die-forging, illustrated in Fig. 4, the tools envelope the workpiece and have a profile corresponding to the shape being produced. An important feature of the closed-die-forging process is the use of a flash. The flash enables control of the vertical dimensions while accommodating the inevitable slight variations in the volume of the original billet. It further acts as a trap to the lateral flow of material thus facilitating the flow on the workpiece material into the cavities of the die impression. The variations in excess volume are held in the flash gutter which is subsequently removed by a trimming process Fig. 5.
A typical sequence of operations in closed-die-forging involves performing, first impression, finishing die, and trimming as illustrated in Fig. 6. The compression force for forging can be supplied by hydraulic presses (up to 75,000 tonnes) or from mechanical presses using flywheels and linkages (up to 12,000 tonnes). Mechanical systems are generally less powerful but much faster than hydraulic systems and are therefore more suitable for mass production and not for forging operations where heat loss of the workpiece might be critical. The compression force can also be supplied by the energy stored in a falling hammer. The mass can be up to 22,500 kg and may fall by gravity or be accelerated by steam, air, or hydraulics. This method is generally the cheapest and is often referred to as drop forging. The different types of metal forming machinery are illustrated schematically in. Altan et al. (2004), Doege and Behrens (2010) and Lange (2002).
Lange K (ed) (2002) Umformtechnik-handbuch für industrie und wissenschaft: grundlagen [Metal forming-handbook for research and industry: fundamentals], 2nd edn. Berlin/Heidelberg, Springer (in German)Google Scholar