• Velay, Vincent Lionel Sébastien
• Vidal, Vanessa
• Julien, R.
• Dahan, Y.
• Salem, Mehdi
• Rezai-Aria, Farhad
• Forestier, R.

Abstract

Ti-6Al-4V is a titanium alloy widely used for aircraft components (Banerjee & Williams 2013). Its manufacturing processes comprise thermo- mechanical treatments including cooling steps during which residual stresses can be generated and can thus make more difficult the machining steps (Gunnberg et al. 2006). This contribution deals with a global approach undertaken to get better insight into the thermomechanical behaviour and the microstructural evolution during heat treatments, and particularly during the cooling operations from the (α + β) phase field, of the high temperature forged Ti-6Al-4V titanium alloys. A new experimental facility using conventional hydraulic testing machine and induction heating was developped to investigate differents temperature-time histories representatives of the conditions undergone in parts during quenching. Specific thermo-mechanical tests of the Ti-6Al-4V titanium alloy were then carried out as a function of the temperature, the strain rate and the cooling rate. Tensile mechanical tests performed on Ti-6Al-4V alloy exibits a predominant viscosity at high temperature whereas a significant hardening mechanism occurs at low temperature. Tensile tests with strain dwell time allow to determine the effect of both viscous and non viscous stress levels whatever the temperature and the cooling rate considered. Moreover, as the mechanical properties, at a specific temperature, can be strongly related to the microstructure (phase transformation) (Roy & Suwas 2013), post-mortem optical and scanning electron microscope observations was carried out to study the metallurgical evolution (fraction, size and morphologies of the α and β phases). Hence, an image analysis protocol was developed to study the surface fraction of phases and grain sizes. It shows that they mainly evolve in a particular domain of temperature (950 to 700 °C) and for a specific range of cooling rate (5 to 60 °C/min). The grains belonging to the high temperature β phase transform progressively into lamellar grains (β + αII) where the surface fraction of β evolves linearly with the cooling rate. Furthermore, a high cooling rate may favors the nucleation of αII lamellae which can lead to a large population of thin lamellae. It is shown that cooling rate effect induce a material hardening due to a wide number of interfaces. References Banerjee, D. & Williams, J.C. (2013): Perspectives on Titanium Science and Technology. - In: Acta Materialia, 61: 844–879. Gunnberg, F., Escursell, M. & Jacobson, M. (2006): The influence of cutting parameters on residual stresses and surface topography during hard turning of 18MnCr5 case carburised steel. - In: Journal of Materials Processing Technology, 174: 82–90. Roy, S. & Suwas, S. (2013): The influence of temperature and strain rate on the deformation response and microstructural evolution during hot compression of a titanium alloy Ti–6Al–4V–0.1B. - In: Journal of Alloys and Compounds, 548: 110–125.

Topics

• impedance spectroscopy
• surface
• compound
• grain
• grain size
• phase
• steel
• viscosity
• titanium
• titanium alloy
• quenching
• lamellae