Owing to excellent service performance (high specific strength and good corrosion resistance) 1, titanium alloys are attractive materials for developing lightweight structures for aeronautics applications. Ti-6Al-4V (TA6V, grade 5) is the most common alloy used in aeronautics, and is usually formed by high temperature processing. Equipment manufacturers, for both semi-finished products as billets and forging parts, request fully globularized microstructures of TA6V. For some products, specifications are more stringent. For instance if some macrozones with a common local orientation (texture) are detected during ultrasonic testing, the product has to be rejected.

The industrial forging processes involve a long series of plastic deformations followed by rotations to decrease the section of bars down to obtain long product. Ingot metallurgy processing of conventional alpha-beta titanium alloys often comprises a series of steps each of which has specific microstructural goal 2. This project aims at investigating the last step of the process: cross forging in alpha + beta range to obtain a globularized structure.

In a first-hand, we will introduce the results of uniaxial and plane strain hot isothermal compression tests. These tests are realized with a servo hydraulic compression system Schenck™ 3. We focus on the effect of the following parameters on globularization kinetics: compression temperature, strain, strain rate, initial orientation of the lamellae. This new set of data is completed with the available data from the literature. We aim first at extracting the constitutive law of the flow stress at high temperature of this alloy. These results will be use to simulate, with a great accuracy, thermomechanical conditions of the test (strain rate and flow stress). Secondly, we will discuss about the texture evolution and alpha lamellae globularization mechanism.

In a second-hand, we will show the first results of a new experimental procedure aiming at simulating experimentally the complex deformation paths of industrial processes, involving temperature and strain gradients. For instance, during open-die forging, several rotations of 90 degrees can occur with holding time between each step, and their effect on globularization is not well documented. It is possible to recreate the condition of this industrial process using the multi-axial deformation module MaxStrain, which is part of the physical simulation system “Gleeble 3500” 4. MaxStrain module opens new prospects in the understanding of the physical phenomena taking place during complex thermo-mechanical treatments representative of the industrial ones.

Thanks to this machine, the effect of changing compression direction on the material flow and on the microstructural transformations will be investigated. Moreover, we first aim at characterizing the range of the deformation and temperature gradients created by the complex thermomechanical paths into the sample. As an outcome, we will discuss on the possibilities to take the benefit of these gradients to study the hot working of metals. Heterogeneous deformation tests provide information for a large panel of deformation conditions in a minimum of specimen, based on a full-field examination of the stress and microstructure gradients.

References:

1. Fan, X. G. et al. Acceleration of globularization during interrupted compression of a two-phase titanium alloy. Mater. Sci. Eng. A 720, 214–224 (2018).

2. Semiatin, S. ., Seetharaman, V. & Weiss, I. Flow behavior and globularization kinetics during hot working of Ti–6Al–4V with a colony alpha microstructure. Mater. Sci. Eng. A 263, 257–271 (1999).

3. Montheillet, F. & Desrayaud, C. Essais rhéologiques à chaud. 20 (2009).

4. Khmelevskaya, I. Y., Kawalla, R., Prokoshkin, S. D. & Komarov, V. S. Effect of multiaxial deformation Max-strain on the structure and properties of Ti-Ni alloy. IOP Conf. Ser. Mater. Sci. Eng. 63, 012108 (2014).