Study of pore closure in the manufacturing of heavy rings

The manufacturing of two types of heavy rings has been analysed with respect to pore closure. The forming routes differ significantly from a principal point of view. One ring is associated with problems regarding remaining pores whilst the other is not. The objective of the present work is to explain this feature by studying the effective strain and hydrostatic pressure distribution in the workpieces throughout the processes using a commercial FEM-code, FORM2D. It is assumed that large strains and high pressures are favourable. The forming route resulting in a ring, which is free from pores, starts with a heavy reduction in the upsetting of a cylindrical billet, followed by piercing in closed dies. After that a thin circular bottom disc is cut away in another closed die at the same time as an outer flange is formed. The other route starts with a light reduction in upsetting, followed by open die piercing and shearing away a thick bottom plate. Finally the upper part of the ring is flattened by a small reduction in height between plane and parallel dies. The theoretically determined shapes of the workpieces, after different forming steps, are compared with the results of full scale experiments, good agreement being obtained. The final ring rolling of the two ring types is not included in the analysis. Regarding the ring that is formed in closed dies, it is concluded that the final accumulated effective strains are found at high levels for large areas of the axial symmetry plane compared to the other ring. For both rings the strains decrease from the inner towards the outer surface. Characteristic for the ring of a pore closure problem, is that the effective strain decreases along the inner surface, from the cutting region towards the flattened part of the ring. As expected the main part of the forming operation takes place under a higher hydrostatic pressure for the ring formed in closed dies. Predictions regarding pore closure are compared with ultrasonic tests and are found to be in good agreement.