Bulging and folding-over in plane-strain upset forging

In many industrial metal-forming processes, bulging of free workpiece surfaces occures. At the same time these surfaces may fold over and come into contact with the dies. These kind of phenomena are often neglected in metal-forming analyses. The present work deals with the problem of bulging and folding over in plane-strain upset forging. It is a basic work which is meant for a deeper understanding of the problem, and for future utilisation in more complicated simulations such as that of filling of die cavities in the closed-die forging of long components. A computer program has been developed by means of which it is possible to study the influence of workpiece geometry and friction. The analysis is carried out with the upper-bound method for a rigid-perfectly plastic material. A flexible velocity field is utilised and a rigid region of variable shape is supposed to be in contact with the dies. By minimizing the total rate of energy dissipation the best velocity field is obtained. The shape and extension of the dead region is found to be influenced heavily by friction. Theoretically and experimentally obtained fold-overs, free surface shapes and grid network distortions are in satisfactory agreement.