16 - Forging

Summary

The classification system for metal forming processes shown in Fig. 2.1 partitions the forging processes into three different groups: open-die forging, closed-die forging, and impact extrusion. In reality, open-die forging consists of a number of different subprocesses, in each of which considerable parts of the workpiece are formed freely, without full contact with the dies; see, for instance, cogging, as shown in Fig. 2.2(c). Closed-die forging, on the other hand, refers to forging between dies, where at the end of forging, either the entire workpiece is confined in a closed space between the dies, or an amount of excess material is used, which flows out of the flash gap; see Fig. 2.3(b). When flash is formed, the process is most commonly called closed-die forging. When there is no flash, the process is called precision forging, flashless forging, or zero-loss forging. Impact extrusion, such as forward and backward cup extrusion, is also described in Ch. 2; see Fig. 2.4. Both are commonly classified as forging processes. In addition, many more exotic forging processes have been developed over the years.

In this chapter, some practical aspects of importance in closed-die forging processes will be considered. First, the concept of forgeability will be discussed. Then the implications of this concept for forging practice will be dealt with, such as selection of forging temperature and possible shapes of forgings. A common classification system for forged components will also be presented. After this, commonly used design principles in multistep forging processes will be discussed.