Effect of preform microstructure on the hot working mechanisms in ELI grade Ti–6Al–4V: transformed β v. equiaxed (α + β)

Abstract

Processing of Ti–6Al–4V includes hot working above and below the β transus and the various stages of manufacture involve preforms with either transformed β (Widmanstätten colony type) or equiaxed (α + β ) microstructure. For achieving defect free products during manufacture, it is important to understand the response of these two preform microstructures to the imposed hot working conditions. In this paper, the influence of the preform microstructure on the hot working mechanisms of extra low interstitial (ELI) grade Ti–6Al–4V has been studied with the help of hot compression experiments conducted in the temperature range 750–1100°C and strain rate range 0·001–100 s-1. The data have been analysed using the standard kinetic approach as well as the more recent approach of processing maps. In the α–β range, the stress–strain behaviour of transformed β preform is marked by a higher flow stress and a continuous flow softening while the equiaxed (α +β ) preform exhibits steady state flow at lower strain rates. By deforming in the α–β range, the transformed β microstructure is converted into an equiaxed one by a process of globularisation. On the other hand, the equiaxed (α + β ) preform deforms superplastically with an associated minor change in its microstructure. In the β range, the transformed β deforms by a process of large grained superplasticity involving the sliding of prior colony (Widmanstätten) boundaries with an associated diffusion accommodated flow. However, dynamic recrystallisation of β occurs in the equiaxed preform. Deformation near the transus for both the preforms is associated with local minima in the tensile ductility indicating the possibility of void nucleation. At strain rates higher than about 0·1 s-1, both the preforms exhibit flow instabilities manifested in the form of flow localisation due to adiabatic shear band formation which is severe in the case of transformed β preform.