Improving the grain size in the forging of large copper lids

Forging of large copper lids for cylindrical canisters meant for nuclear waste is treated. Based on recent full-scale experiments, an alternative way of forming is proposed. In the experiments, a press offering just a low force, 20 MN, was available. Two-step hot forging was used: upsetting of an ingot between flat and parallel tools followed by cogging for filling a die cavity. The initial ingot measured Ø 350 mm and 1400 mm in height buckled and made repeated straightening during the upsetting necessary. This forming procedure showed a poor material yield and resulted in a coarse-grained microstructure in the lids close to the final midpoint contact surfaces. From a 2D FE-analysis it became clear that this structure is associated with the formation of dead metal regions. Coarse grains are not acceptable from mechanical and corrosion point of view and make ultrasonic testing unreliable. Thus, the present work is mainly focussed on improving the structure by presenting an alternative way of manufacturing, which at the same time is faster. The flat dies used in upsetting were furnished with a central protruding part of spherical shape and the cavity filling by cogging was replaced by ordinary closed-die forging. In this way large strains were obtained in the critical, low-strained regions. The modified two-step forging was simulated for different radii of the spheres, keeping their penetration depth constant. The tools designed for the closed-die forging should, according to the FE-analysis, fill the cavity with a moderate force and an acceptable material yield. According to an earlier investigation by the authors the here proposed way of forming should ensure a fine microstructure throughout the whole volume of the lids.