Flow behavior and globularization kinetics during hot working of Ti-6Al-4V with a colony alpha microstructure

The effect of process variables on how response and microstructure evolution during hot working of Ti-6Al-4V with a colony alpha preform microstructure was established using isothermal hot compression tests. Testing was conducted on material with prior-beta grain sizes of 100 mu m or 400 mu m at strain rates of 0.001-10 s(-1) test temperatures between 815 and 955 degrees C, and height reductions of 40-80%. All of the flow curves exhibited a peak stress followed by moderate flow softening. The absence of a grain/colony size dependence of flow behavior, coupled with relatively low values of the strain rate sensitivity of the flow stress ( similar to 0.05-0.30). led to the conclusion that deformation was controlled by dislocation glide/climb processes. Flow softening was interpreted in terms of deformation heating and substructure/texture evolution. The dependence on strain rate and temperature of the kinetics of dynamic globularization of the colony microstructure was complex and appeared to be of second-order importance compared to the effects of strain per se, thus suggesting the dominance of dislocation-type processes for the control of globularization as well. (C) 1999 Elsevier Science S.A. All rights reserved.