Investigation on the void closure efficiency in cogging processes of the large ingot by using a 3-D void evolution model

The internal void defects often exist in the large ingot inevitably due to the non-uniform solidification of the materials. To guarantee the mechanical performance of the products, these voids should be closed and eliminated in the subsequent forging process. The main purpose of this study is to investigate the void closure efficiency in different cogging processes of large ingot by using a 3-D void evolution model. In order to obtain a more reasonable description of the actual engineering condition, the voids were considered as prolate ellipsoidal, and the influences of the instantaneous void shape changing, stress state and deformation history were taken into account for the void evolution. According to the results, alternate compression in different directions makes the changing of the void shape counteracted and decreases the void closure efficiency. The initial void shape impacts the void closure significantly, as the non-spherical void shape causes the anisotropy of the void closing behavior. It can be found that the compression perpendicular to the longer principal axis of the prolate void provides the higher void closure efficiency than the compression aligned with this direction. Therefore, using the extend-forging as the first step in cogging process is more efficient to close the voids, considering the morphology of the real voids in the ingot. Moreover, appropriate processing parameters were determined to enhance the void closure efficiency in extend-forging. Besides, the surface bonding experiments show that the high pressure and temperature, as well as the long holding time, are favorable to eliminate the void defects after the void closure. It implies that the processing sequence, which can make the void closed completely at high pressure and high temperature and keep the sufficient interval time between forming stages, should be emphasized in the process planning.