Prediction of Void Closure in Metal Forming: One Cylindrical Through-hole

Abstract

Since voids in a billet or an ingot are detrimental under tensile stress, they are closed by forging or rolling through a combination of collapse and contraction. In plane-strain forging or rolling, a cylindrical void or through-hole was found experimentally as well as analytically to be closed as the effective strain at the center of the void reached a certain value. In the present investigation, this finding was further examined for transverse forging, axial rolling and axial forging, in which the void was elongated or shortened in length. Since strain as well as stress components in length were irrelevant to a description of cross sectional changes, a concept of the planar effective strain and the planar hydrostatic stress was introduced. As a result, void closure on cross section was able to be predicted by the planar effective strain and the normalized planar hydrostatic stress at the center of the cross section, which were obtained from an analysis of non-void model. However, there were two exceptions; one was the case in which the planar hydrostatic stress was positive in sign and the other was the axial forging in which the void never collapsed on cross section.