Strain-induced porosity during cogging of extra-low interstitial grade Ti-6Al-4V

Abstract

The phenomenon of strain-induced porosity (SIP) in extra-low interstitial (ELI) grade Ti-6Al-4V with a transformed β starting microstructure is investigated to understand its origin during α-β cogging. For this purpose, the constitutive behavior of the material is coupled with finite-element method (FEM) simulations of the cogging process. Two distinct types of SIP relevant to cogging speeds and temperatures, viz., shear cracking and void nucleation, are identified. While the former occurs at the prior β grain boundaries below 825 °C, the latter occurs at the prior colony boundaries when the deformation temperature is close to the β transus. The FEM simulations have shown that deformation conditions existing in the midregion of the billet are favorable for void nucleation. The mechanism of void growth in the presence of tensile residual stress and temperature during resoaking is modeled using the Cocks-Ashby coupled growth model. Repeated cogging and resoaking steps cause multiplication of void population in large numbers. To avoid both types of defects in any region of the billet, a practical solution has been developed by introducing a differential temperature profile from the surface to the center, and the validity of the proposed scheme is verified with FEM heat-transfer simulations.