CE08 - Matériaux métalliques et inorganiques et procédés associés

Microstructure design and machining of a beta metastable titanium alloy – DEMUTI

Microstructure design and machining of beta metastable titanium alloy

Contrary to the traditional approach which consists in optimizing the machining process, our approach emphasizes the role of the microstructural features of the material and of the cutting tool on the surface integrity. The data collected can then be used to determine the optimal microstructure of the material corresponding to a specification and therefore the heat treatment to be performed, or even to optimize the design of alloys.

Issues and objectif

The optimization of machining processes has been the subject, over the last decades, of technological developments aimed at improving the machinability of titanium alloys and thus controlling the cost of machining. However, it remains perfectible for improvement, especially when metastable phases appear under specific heat treatment conditions. These metastable phases, or the formation of final microstructures from a metastable phase, are most often preferred because their size is nanometric, and give the material very interesting properties to the detriment of machinability. This is particularly the case for titanium alloys. <br />The scientific objective of this project is therefore to understand and quantify the impact of microstructure elements of titanium alloys on machinability and wear of cutting tools. This objective can only be achieved by studying the chemical exchanges between the material and the tool, and the behavior at the microstructure scale of the material under extreme conditions, taking into account the chemical composition, the crystallography, and the geometry and the actual loading of each phase. The metallurgical analyses will be accompanied by instrumented machining tests and numerical and physical simulations at the scale of the polycrystalline aggregate.<br />To achieve these ambitious goals will help design ideal microstructures for titanium alloys according to a given specification, and provide essential material data (advanced rheological laws based on microstructure) to help research department in their choice of materials and to help simulate machining processes.

To highlight the role of microstructure on machinability, the originality of this work will be to design samples with predefined microstructures (called «models«) and perfectly characterized, then to be subjected to orthogonal machining (simple machining operation allowing observation and instrumentation of the location) and thermomechanical simulator to monitor the phase transformation kinetics. The microstructures selected in this program They will be in line with industrially solution and also enough diversified for a more fundamental study.. The ß-metastable titanium alloy Ti-5553 (Ti-5Al-5Mo-5V-3Cr-0.35Fe-0.15O in mass %), was selected because it offers the ability to design a wide range of microstructures. On the other hand, the crystallographic and kinetic data are known. Understanding and quantifying the role of microstructural parameters on machinability also requires taking into account the chemical exchanges between the cutting tool and the material. The objective of this approach will therefore be to evaluate the impact of the initial microstructure as well as the kinetics of phase transformation on the reactivity of the surface and to identify the factors that play a significant role in this reactivity. Finally, the role of microstructural quantities on the evolution of the stress states at the phase scale in the chip and the near-surface material will be studied. Here again, the originality of our approach, which is to design well-controlled microstructures, will allow us to identify potential phase changes and to quantify them as well as to follow the stress state by progressively complexifying the microstructure. The quantitative elements will finally be compared to a model at the crystalline scale during a machining simulation.

We have established the thermal paths for 3 model microstructures and quantitatively characterized their microstructural quantities which are the phase fractions and the characteristic sizes of the different phase morphologies. The phase dissolution kinetics have been established for heating rates up to 100°C/s. This work will be the subject of a first publication in an international journal. The design of the compression/shear device at temperature is validated by all the contributors. The one associated with the device in temperature without mechanical loading is also validated. For the task concerning the simultaneous measurement of the thermal and displacement fields, a deep learning algorithm is being finalized to determine the chip geometry more precisely. The planing tests on the model microstructures are in progress. A series of pre-tests has been performed on a Ti17 titanium alloy, to validate the kinematic acquisition procedure. The image analysis allows to analyze the chip formation, and thus to know the effect of the microstructure on the machinability. Moreover, it is then possible to observe the deformation mechanisms to be taken into account for numerical modeling. An investment obtained (outside the ANR project) will allow by the end of the year to have 2 fast cameras in order to make 3D acquisitions.

The thermal path of the other model microstructures remains to be established. The dissolution kinetics of the alpha phase from different model microstructural states is also planned for heating rates up to 100°C. The thermophysical property measurements required for the modeling in Task 4 are in progress and should be obtained by the end of 2020. Regarding chemical reactivity, the first validation tests are scheduled for mid-November 2020. The temperature compression/shear test campaign will be carried out and the microstructural and chemical characterizations of the material/tool couples will be carried out by a PFE trainee from LEM3 from February 2021 and the PhD student at CIRIMAT. The first simulations of the chemical exchanges between the titanium alloy and the cutting tool will be carried out in parallel with the first microstructural and chemical characterizations during the year 2021. The task concerning the machinability of the different model microstructures is also progressing well. A gradation of the difficulties of exploitation will be carried out for each model microstructure. It will compare the cutting effort, segmentation by deep learning and digital image correlation. The planing test campaign will be continued as well as the exploitation of the results. The thermomechanical test campaign necessary for the modeling at the crystalline scale is planned.

1 international conference and 1 national conference

Among the strategic sectors in French industry, aeronautics takes a special place both in terms of image and innovation and the number of jobs involved despite a still economic context unfavorable. To preserve this force and dynamic facing the increased competition from emerging countries, it is essential to maintain the competitiveness of our industries and the capacity for innovation. This requires the continuous development of new production processes and alloys with high added value. In the case of titanium alloys, machining is identified by industry in this sector as a critical operation despite significant technological progress over the last decade. Indeed, many finished parts are machined integrally in the mass causing a significant financial cost due to poor machinability compared to a large number of other alloys such as aluminum alloys.
To improve the machinability of titanium alloys, the traditional approach is to optimize the machining process by focusing, for example, on tool geometry or cutting forces based on turning or milling models. Our approach focuses on the role of the microstructural parameters of the material and their interaction with the cutting tool on the integrity of surfaces.
The research project is devoted to the understanding and the quantification of the role of the microstructure of titanium alloys on the machinability and wear of cutting tool. The originality of our approach is based on the design of a set "model" microstructures to understand the basic physical, chemical and mechanical mechanisms. This study will include a fine analysis of the material/tool chemical reactivity and expertize of the machining chips according to the starting microstructure as well as a crystalline scale modeling. This project is challenging because it is at the limits of the state of art, and because it is based on a multi-scale and quantitative approach taking into account fundamental physics, metallurgy, mechanical and modeling, and finally because it deals with high added value materials that are used in a competitive international business.
To achieve the goal will allow establishing the best link between the microstructure and the machinability properties and contributing through this approach to the design of "ideal" microstructures as a function of requirement specification and to provide the required material data for the simulation of the machining process. The expected progress will also help tool manufacturers to develop new coatings and even new tools to improve surface quality and reduce wear, leading to reduced cutting tool consumption.

Project coordination


The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.


LEM3 Laboratoire d'Etude des Microstructures et de Mécanique des Matériaux
ENSAM-LAMPA Ecole Nationale Supérieure d'Arts et Métiers - Laboratoire Angevin de Mécanique, Procédés et Innovation

Help of the ANR 393,725 euros
Beginning and duration of the scientific project: January 2019 - 42 Months

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