Effect of strain reversal on the dynamic spheroidization of Ti-6Al-4V during hot deformation

Abstract

The effect of strain-path reversal on the kinetics of dynamic spheroidization during subtransus hot working was determined for Ti-6Al-4V with a colony ␣ structure. Isothermal torsion tests were conducted at a temperature of 815°C and a strain rate of 0.001 s Ϫ1 ; strain-path reversals were achieved by applying forward and reverse torsion sequentially. The kinetics of spheroidization were measured as a function of the local (macroscopic) strain for monotonic-deformation, reversed-torsion, and double-reversed-torsion tests. Strain-path reversal led to a reduction in the spheroidization kinetics compared with monotonic deformation for a given total strain. The slower rate of dynamic spheroidization associated with strainpath reversals was ascribed to a reduced rate of sub-boundary formation/lower sub-boundary energies, which drive the boundary splitting process, and less sharp ␣/␤ interface curvatures, which control the coarsening process that also contributes to spheroidization.

The Impact of Strain Reversal on Microstructure Evolution and Orientation Relationships in Ti-6Al-4V with an Initial Alpha Colony Microstructure

“…A summary of the effect of total strain on the extent of dynamic and static spheroidization from the present study and previous works [4,13] is shown in Figure 9. Dynamically spheroidized fractions of alpha grains are compared for different deformation modes and temperatures (Figure 9(a)).…”

“…Furthermore, the results obtained in previous work by other researchers suggested, that there is a monotonic strain that is needed to be exceeded for the rapid completion of the dynamic spheroidization process. [4] It can be seen on Figure 9(a) that after exceeding the monotonic strain of 1, the rate of spheroidization significantly increases and is completed by a strain of 2, however, this behavior is only noticeable for the sample that was monotonically strained. In the case where the strain was reversed, the progress of dynamic spheroidization is lower and the rate of spheroidization is constant and similar to the one during prestraining, leading to incomplete spheroidization.…”

“…Typical cylindrical torsion samples with strain gage diameter of 10 mm and strain gage length of 18 mm were machined out of the provided bars. In order to produce a colony a microstructure similar to that generated in an industrially sized beta recrystallized billet, [4] a multiple-stage heat treatment was carried out on these samples prior to mechanical testing- Figure 1.…”

“…[5] It is therefore not surprising that non-monotonic types of deformation (e.g., reversed torsion, tension/compression), which change the intensity of shear bands within the alpha lamella, have also been shown to significantly reduce the rate of spheroidization compared to equivalent monotonic strains. [4] Thus understanding strain path effects on spheroidization is critical for optimization of the forging process, which is highly non-linear and involves continuous strain path changes, [3] which, in turn, could lead to a substantial cost reduction in the widely used alpha + beta alloys. However, most of the recent strain path research has been performed at very low strain rates (that are far from the real industrial processes) and focused only on quantification the extent of spheroidization.…”

Microstructure and Texture Evolution during Friction Stir Processing of Fully Lamellar Ti-6Al-4V

“…Poths et al [27] described the evolution of a fully lamellar microstructure in terms of local continuum strain during hot torsion at 815°C at a strain rate of 10 À3 s À1 . In general, kinking of lamellae was first observed at strains around 0.4 in certain colonies, presumably those in crystallographically soft orientations, but there was no significant change in microstructure.…”