Influence of pressure on the closure of voids during plastic deformation

Abstract

Metals in the as-cast condition usually contain voids, which defects are formed during solidification due to material shrinkage and gas entrapment. It is important that the void fraction is reduced or preferably eliminated in the subsequent process of hot working. It is well known that void closure is more easily obtained if plastic deformation takes place under the influence of hydrostatic pressure. Consequently it is of the utmost importance that this fact is taken into consideration when a rolling or forging schedule is planned. The influence of pressure on the closure of voids is analysed. The theoretical analysis is carried out by means of plane-strain upper-bound solutions, assuming a rigid-perfectly-plastic material. It is often observed by people in industry that it is in the last few passes of a deformation schedule that hydrostatic pressure is of greatest importance. From the present work, however, it is concluded that the pressure at the beginning of a schedule is of great importance as well. If the pressure is sufficiently high it will result in a mechanism of favourable void closure, that is, a rapid decrease in void fraction at the beginning of the deformation comprising no extension of the voids perpendicular to the compression direction. After some deformation in the favourable manner, void closure will still take place according to the same mechanism even if the pressure is changed into a moderate tensile stress.