Microstructural evolution during hot working of Ti-6Al-4V at high strain rates

摘要

Microstructural conversion from lamellar to equiaxed is an important step in the thermomechanical processing sequence of Ti-6Al-4V and is conventionally achieved by cogging in the alpha-beta phase field. Since hot working at higher strain rates (>0.1 s{sup}(-1)) in the two phase field produces microstructural defects such as adiabatic shear bands, cracking, and lamellae kinking, cogging is performed at slow speeds. Also, the occurrence of strain-induced porosity at lower strain rates and lower temperatures (<850°C) demands higher temperature control during cogging to obtain defect-free products. In view of these difficulties, an effort has been made to find an alternative process for conversion. Isothermal compression tests conducted at high strain rates (1-100 s{sup}(-1)) close to the beta transus revealed the evolution of an equiaxed microstructure consisting of alpha grains surrounded by a thin beta case. The new microstructure is found to be thermally stable and exhibited better mechanical properties over the conventional globularized structure. The equiaxed microstructure has been successfully reproduced under industrial manufacturing conditions using extrusion and sub-scale turbine-engine disk forging experiments at high speeds. The local variations in process parameters are estimated using finite element simulations of these operations and are correlated with microstructural observations.