Close-die forging of large Cu-lids by a method of low force requirement

Abstract

This paper presents a FE-analysis and experimental results of a hot forging method for fabrication of large copper canister lids with a low forging load. The forging sequence is carried out in two parts, namely axisymmetric and cogging. The cogging tool is used to bite off small areas of the workpiece periphery surface at the same time as the outer material is forced into the deep die cavity of the bottom die. During this process, the bottom tool is rotated about 45° after each pressing before a new bite is taken. As a result the main part of the cavity is filled at the same time as a protrusion develops at the central part of the workpiece. The cogging tool works on it later. Due to the small contact surface between the upper tool and the workpiece, the forging load needed to complete die filling is small and does not exceed the maximum load of the press. The process was earlier optimised so that no flash was needed for complete die filling. Process parameters affecting the evolution of the microstructure are analysed. Material flow and forging load in different stages of the process are studied. Microstructures obtained in the final product are presented for different locations along the radius of a vertical symmetry plane of the circular lid. Grain size evaluations are roughly co-related to the accumulated effective strain distribution obtained during the different manufacturing operations and to the temperature distribution obtained at the end of the forming process. It is concluded from the simulations that a 4600-t press is sufficient to achieve complete die filling and from experiments that an acceptably small grain size is obtained throughout the lid. From the simulation results the whole volume of the lid has experienced high strains after the manufacturing steps. No regions of low strain were found which according to experience is coincidental with a coarse-grained microstructure.