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

The phenomenon of strain-induced porosity (SIP) in extra-low interstitial ( ELI) grade Ti-6Al-4V with a transformed beta starting microstructure is inv estigated to understand its origin during alpha-beta cogging. For this purp ose, the constitutive behavior of the material is coupled with finite-eleme nt method (FEM) simulations of the cogging process. Two distinct types of S IP relevant to cogging speeds and temperatures, viz., shear cracking and vo id nucleation, are identified, While the former occurs at the prior beta gr ain boundaries below 825 degreesC, the latter occurs at the prior colony bo undaries when the deformation temperature is close to the beta transus. The FEM simulations have shown that deformation conditions existing in the mid region of the billet are favorable for void nucleation. The mechanism of vo id 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 larg e numbers. To avoid both types of defects in any region of the billet, a pr actical solution has been developed by introducing a differential temperatu re profile from the surface to the center, and the validity of the proposed scheme is verified with FEM heat-transfer simulations.