Effect of initial microstructure on plastic flow and dynamic globularization during hot working of Ti-6Al-4V

Abstract

Plastic flow behavior and globularization kinetics during subtransus hot working were determined for Ti-6Al-4V with three different transformed beta microstructures. These conditions consisted of fine lamellar colonies, a mixture of coarse colonies and acicular alpha, and acicular alpha. Isothermal hot compression tests were performed on cylindrical samples at subtransus temperatures and strain rates typical of ingot breakdown (i.e., T ϳ 815 ЊC to 955 ЊC, ϳ 0.1 s Ϫ1 ). For all three material conditions, true stress-true strain curves exhibited a peak stress followed by noticeable flow softening; the values of peak stress and flow softening rate showed little dependence on starting microstructure. On the other hand, the kinetics of dynamic globularization varied noticeably with microstructure. By and large, the globularization rate under a given set of deformation conditions was most rapid for the fine acicular microstructure and least rapid for the mixed coarse-colony/acicular structure. At temperatures close to the beta transus, however, the difference in globularization rates for the three microstructures was less, an effect attributed to the rapid (continuous) coarsening of the laths in the acicular microstructure during preheating prior to hot working. The absence of a correlation between the globularization kinetics and the observed flow softening at low strains suggested platelet/lath bending and kinking as the primary deformation mechanism that controls the shape of the flow curves.

In situ observation of texture and microstructure evolution during rolling and globularization of Ti–6Al–4V

“…the β → α transformation), some new α orientations are observed to adhere to the Burgers OR. Its is therefore apparent that β-annealing Ti-6Al-4V does not result in texture randomisation, but does yield a microstructure conducive to globularisation of the α-phase [28,29].…”

Microstructural Mechanisms during Dynamic Globularization of Ti-6Al-4V Alloy

“…Tests were carried out at 1073 K up to the strain of 1.4 by varying strain rates from 10 À3 s À1 to 1 s À1 . And, according to previous researches, the volume fraction of alpha phase and thickness of alpha lath is about 0.83 and 0.4 mm, respectively, at 1088 K. 3,6) The additional samples with equiaxed microstructure were prepared via the conventional processing method shown in Fig. 2, i.e., partial dynamic globularization and additional annealing, to provide an insight into the microstructural differences from fully dynamically globularized specimen.…”

“…Shell and Semiatin 3) investigated the effect of the alpha plate thickness on the fraction globularized in the three kinds of transformed microstructures which have the different lamellar thickness. They showed that the fraction globularized increases with decreasing the alpha plate thickness.…”

Unified constitutive modelling for two-phase lamellar titanium alloys at hot forming conditions

“…Semiatin et al [5] investigated the dynamic globularization of Ti-6Al-4V during hot working, and analyzed the strains required for its initiation and completion. Shell and Semiatin [6] studied the effect of initial microstructures with different a plate thicknesses on dynamic globularization behaviour of Ti-6Al-4V alloy. Miller et al [7] studied the flow behaviour and its dependence on the a-platelet thickness during hot working of Ti-6Al-4V alloy with lamellar microstructure.…”