Hot working of commercial Ti–6Al–4V with an equiaxed α–β microstructure: materials modeling considerations

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Abstract

The hot deformation behavior of Ti–6Al–4V with an equiaxed α–β preform microstructure is modeled in the temperature range 750–1100°C and strain rate range 0.0003–100 s−1, for obtaining processing windows and achieving microstructural control during hot working. For this purpose, a processing map has been developed on the basis of flow stress data as a function of temperature, strain rate and strain. The map exhibited two domains: (i) the domain in the α–β phase field is identified to represent fine-grained superplasticity and the peak efficiency of power dissipation occurred at about 825°C/0.0003 s−1. At this temperature, the hot ductility exhibited a sharp peak indicating that the superplasticity process is very sensitive to temperature. The α grain size increased exponentially with increase in temperature in this domain and the variation is similar to the increase in the β volume fraction in this alloy. At the temperature of peak ductility, the volume fraction of β is about 20%, suggesting that sliding of α–α interfaces is primarily responsible for superplasticity while the β phase present at the grain boundary triple junctions restricts grain growth. The apparent activation energy estimated in the α–β superplasticity domain is about 330 kJ mol−1, which is much higher than that for self diffusion in α-titanium. (ii) In the β phase field, the alloy exhibits dynamic recrystallization and the variation of grain size with temperature and strain rate could be correlated with the Zener–Hollomon parameter. The apparent activation energy in this domain is estimated to be 210 kJ mol−1, which is close to that for self diffusion in β. At temperatures around the transus, a ductility peak with unusually high ductility has been observed, which has been attributed to the occurrence of transient superplasticity of β in view of its fine grain size. The material exhibited flow instabilities at strain rates higher than about 1 s−1 and these are manifested as adiabatic shear bands in the α–β regime.

Introduction

Ti–6Al–4V (Ti–6–4) alloy is a two-phase alloy, which has low density and attractive mechanical and corrosion resistant properties that make it an ideal choice for many aerospace applications [1]. It is commercially available in two grades — low oxygen (ELI) grade and high oxygen (commercial) grade. It is the high oxygen grade that is widely used in gas turbine engine parts, chemical reactors and bioengineering applications. The alloy may be heat treated [2] to obtain a variety of microstructures ranging from β-transformed (martensitic/lamellar) to equiaxed α–β. Bulk mechanical processing of this material to manufacture semi-products includes ingot breakdown at temperatures above the β transus and ‘conversion’ of transformed β lamellar structure into equiaxed α–β by subtransus deformation [3], [4], [5]. The finishing operations for component manufacture are generally conducted [6] on the equiaxed structure in the α–β phase field during which only minor changes in the microstructure take place. One of the very common industrial processes used for this purpose is superplastic forming, which requires very fine grain sizes and slow speeds of deformation [7], [8]. The characteristics of superplastic deformation in Ti–6–4 have been studied extensively in the literature [9], [10], [11], [12], [13], [14]. Although superplastic elongations have been recorded by several investigators, there are reports of cavity formation at higher strains and lower temperatures (<850°C) [12], [15]. The constitutive behavior of commercial grade Ti–6–4 during hot deformation in the α–β range has been characterized by Sheppard and Norley in torsion [16] and Seetharaman et al. in compression [17] and microstructural mechanisms have been identified to be dynamic recrystallization (DRX) and spheroidization respectively.

The objective of this study is to model the microstructural mechanisms of hot deformation of commercial Ti–6–4 with an equiaxed α–β microstructure over a wide range of temperature and strain rate such that windows for industrial processing may be identified for optimizing workability and controlling microstructure. To reach this objective, the characteristics of the alloy have been studied in the temperature ranges covering not only α–β and β phase fields but also the transus. Likewise, a wide strain rate range that encompasses the speeds of commonly used machines in the industry like hydraulic presses (slow), mechanical/friction screw presses and hammers (medium), continuous rolling and ring rolling (fast) mills, is investigated.

Several approaches of materials modeling are in vogue and these include analysis of shapes of stress–strain curves [5], kinetic analysis [18], and processing maps [19] and these have been reviewed recently [20]. Although all the approaches have been followed in this paper, the emphasis has been on processing maps since this approach has been found to be consistent in predicting the behavior of a wide range of materials [21]. In brief, the processing maps consist of a superimposition of power dissipation maps and instability maps developed in a frame of temperature and strain rate. The power dissipation maps represent the pattern in which the power is dissipated by the material through microstructural changes. The rate of this change is given by a dimensionless parameter called the efficiency of power dissipation:η=2mm+1where m is strain rate sensitivity of flow stress. Over this frame is superimposed a continuum instability criterion for identifying the regimes of flow instabilities, developed on the basis of extremum principles of irreversible thermodynamics as applied to large plastic flow [13] and given by:ξε̇=lnm/m+1lnε̇+m<0where ξε̇ is a dimensionless instability parameter and ε̇ is the applied strain rate. Flow instabilities are predicted to occur when ξε̇ becomes negative. The processing maps exhibit domains in which specific microstructural mechanisms operate as well as regimes where there will be flow instabilities like adiabatic shear bands or flow localization.

Section snippets

Material

Commercial grade Ti–6–4 having the following composition (wt.%) was used in this study: 6.28 Al, 3.97 V, 0.18 O, 0.052 Fe, 0.0062 N, 0.008 C, 0.0049 H, and balance Ti. The β transus for this material is about 1010°C. As received bar stocks of 20 mm diameter in the mill annealed condition were used for testing and the starting microstructure is shown in Fig. 1. It consisted of equiaxed α grains of about 8 μm average diameter with a small amount of intergranular β.

Hot compression testing

Compression specimens of 15 mm

Stress–strain behavior

The shapes of stress–strain curves indicate some features that help in identifying the mechanisms of hot deformation, although not in a conclusive fashion. Commercial Ti–6–4 with equiaxed α+β microstructure exhibited three different generic shapes of stress–strain curves in the ranges of temperature and strain rate covered in this investigation. Curves representing these features are given Fig. 2a and b, which reveal the following features:

  • 1.

    At strain rates slower than 0.1 s−1 and at all

Conclusions

Hot deformation behavior of a commercial grade Ti–6–4 with an equiaxed α–β starting microstructure is characterized with the help of isothermal compression tests in the temperature range 750–1100°C and strain rate range 0.0003–100 s−1. The data are analyzed with the help of available materials models and the following conclusions are drawn from this study:

(1) The material exhibits fine-grained superplasticity in the temperature range 750–950°C and strain rates slower than 0.002 s−1. The apparent

Acknowledgements

The authors would like to thank Dr J.C. Malas of WPAFB for many stimulating discussions. One of the authors (YVRKP) is thankful to the National Research Council, USA, for awarding him an associateship and to the Director of the Indian Institute of Science, Bangalore, for granting him a sabbatical leave. The assistance rendered by S. Sasidhara and R. Ravi of Department of Metallurgy, Indian Institute of Science is gratefully acknowledged.

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