Numerical simulation and experimental verification of void evolution inside large forgings during hot working

Abstract

Large forgings are the essential parts of some nuclear, electrical power generation, rolling mill equipments. Generally, they are directly obtained by forging the large ingots containing some void defects. In this study, the evolution mechanisms for the spherical or spheroidal voids during hot working are investigated by the numerical simulations and experiments. The effects of the initial void size, aspect ratio and positions on the void evolution were discussed. The results show that the closure process of voids can be divided into two stages. i.e., when the deformation degree is relatively small, the void retains spheroidal. However, the void will not be spheroidal when the deformation degree is relatively large. The changes of void aspect ratio are slightly affected by the void size, but greatly by the initial aspect ratio and position of voids. It also suggests that the strain and stress fields around voids are the key factors influencing the evolution of void aspect ratio. The increase of effective strain contributes to the changes of void aspect ratio. Considering the effects of stress and strain fields on the void evolution, a void aspect ratio evaluation index, which is defined as a function of the stress deviator, effective strain and effective stress, is proposed to describe the changes of void aspect ratio. Based on the results from finite element simulation, a theoretical model is established to predict the changes of void aspect ratio in large forgings during hot working. A good agreement between experimental and simulated results indicates that the proposed void aspect ratio evaluation index and theoretical model can give an accurate description of the void evolution.