Materials Letters 58 (2004) 3622 – 3629 www.elsevier.com/locate/matlet
Workability of Ti–6Al–4V alloy at high temperatures and strain rates S. Bruschia, S. Poggiob, F. Quadrinib, M.E. Tatab,*
Universitu di Padova, Padova, Italy ˆ Universita di Roma " Tor Vergata ", Dip. Ing. Meccanica, Via di Tor Vergata, 00133, Rome, Italy ` Received 21 June 2004; accepted 30 June 2004 Available online 8 August 2004
Abstract Hot workability of Ti–6Al–4V has been investigated by means of hot compression tests carried out in the 880–950 8C temperature range and 1–50 sÀ1 strain rate range. The effect of microstructural characteristics of the deformed specimens have been studied and correlated with the test temperature, total strain and strain rate. A constitutive equation for the flow stress has been defined and the test conditions for a homogeneous deformation evaluated. The machine employed for testing allowed to reach very high strain rates by means of a uniform compression for long strains (until 0.9), whereas data extracted from the scientific literature are significantly limited in comparison. In this way, a higher accuracy could be obtained in material behaviour modelling for forging process simulation. D 2004 Elsevier B.V. All rights reserved. Keywords: Titanium alloy; Forging; Microstructure; Hot compression test; X-ray diffraction
1. Introduction The Ti–6Al–4V alloy has an attractive combination of characteristics in terms of high mechanical properties, elevated corrosion resistance and low density. The importance of this material is testified from the fact that it covers more than 50% of the titanium alloy industrial production. Particularly for forging, Ti–6Al–4V is an important material for the near net shape forming of medium weight components with high mechanical performances (such as turbine blades). For this reason, great attention is given in the scientific literature to the study of the behaviour of this alloy under deformation. In fact, due to the high cost and working complexity of titanium alloys, material experimental testing plays a central role for process simulation and optimisation. Particularly the actual trend is to define test conditions which are as similar as possible to the real forging conditions. Isostatic hot compression tests have recently been developed to correctly characterise material plastic flow. A lot of interesting microstructural and mechanical aspects are then correlated to the test conditions. * Corresponding author. Tel.: +39 0672597181; fax: +39 062021351. E-mail address: firstname.lastname@example.org (M.E. Tata). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.06.058
Seetharaman et al.  studied initially the plastic deformation behaviour of Ti–6Al–4V in the a+b and b phase fields, developing a unified constitutive equation until the definition of a complete processing map. Semiatin et al.  successively evaluated the response and microstructure evolution during hot working of Ti–6Al–4V with a a preform microstructure. In an analogous manner, Seshacharyulu et al.  evaluated the effect of an equiaxed a+b preform microstructure and of a lamellar one , and Semiantin and Bieler  deepened the effect of alpha platelet thickness. Moreover Nemat-Nasser et al.  studied the dynamic thermomechanical response of Ti–6Al–4V alloys with three different microstructures, obtaining that initial microstructure only affects the magnitude of the threshold stress and the athermal part of the flow stress but not the functional dependence of the thermally activated part of the flow stress on temperature and strain rate. Dealing with forging, Hu and Dean  investigated the hot-forging of Ti–6Al–4V for the production of blade forms near to net-shape. More recently Majorell et al. [8,9] have given a new impulse to the study of the mechanical behaviour of Ti– 6Al–4V under plastic deformation extending the temperature range for...
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