MATETPRO - Matériaux et Procédés pour des Produits Performants

Optimization criteria of titanium alloys to improve their machinability – TITUS


Optimization criteria of titanium alloys to improve their machinability

Productivity of machining operations in titanium alloys

The large use of composites material impacted the assembly operations, including drilling, which previously consisted mainly of drilling aluminium alloys between them. Nowadays, the most common cases in last aircraft generation are the assembly of hybrid stacks, mainly composite/metal. The cost of drilling a hybrid stack is 10 times higher than the one made of metal/metal, or even more with an increase in the diameter. The optimization of composites/titanium drilling requires perfect control of titanium alloys drilling. This project is based on a global approach to control the machining cost of titanium alloys, focusing on the drilling operation with the aim to improve the machinability of titanium alloys.<br />Improved machinability is evaluated by the simultaneous combination of several criteria, such as the tool life, cutting parameters and the surface quality. By focusing on these criteria, the project aims to contribute to the optimization of cutting parameters and to the optimization of the material for three titanium alloy grades, endeavoring to develop a full analysis of physical phenomena occurring in the machining of titanium alloys. It is now established that according to their chemical composition and thermo- mechanical treatments, titanium alloys exhibit strong variations of machinability. Understanding the phenomena remains a key to further optimize machining conditions according to the material properties. The aim is to improve the basic understanding of machining phenomena, to identify materials properties playing a role in machining process (mechanical, thermal, metallurgical), and to optimize the machining conditions according to this properties.

The scientific approach adopted by the project is innovative in the field of machining. Indeed, the project is based on a multi-scale approach. The multi-scale approaches are widely used in the understanding and prediction of behaviors of materials, but less usual in the machining scientific community.
The originality of this work is also from instrumented turning tests and modeling to determine the loads on the cutting edge of the drilling tools and the material behavior in this conditions. Thus, it will be possible to transfer the results from turning to drilling operation, which is much more difficult to perform and to model.
Beyond the characterization of the material machinability to understand its behavior in response to thermo-mechanical stresses in machining, this project will highlight the effect of the material microstructure on machinability via the constitutive laws. Indeed, if we simply characterize the machinability of the material, it will not be possible to generalize the results to other titanium alloys or other microstructure of the alloy studied. The results regarding the effect of microstructure on machinability will be able to be applied for a type of microstructure or a family of material. The proposed approach should highlight the relationship between the following three properties: the microstructure, the constitutive law and machinability.

The results of the project will bring an understanding of material properties that play a role on machinability. From these elements, it is will possible to take into account this machining property in the material choice. This project will provide tools for titanium alloys optimization to predict their machinability. Heat treatments and part manufacturing can be adjusted to lead to a cost reduction in machining process. The success of these outcomes requires some preliminary results. Thus, the kinetics of alloys phases are characterized for various heating conditions, particularly during rapid heating in order to be under machining conditions. Drilling is a complex operation (variation of the angles along the cutting edge, narrow area...), preliminary results will focus on turning. This step helps to have a better understanding of the physical phenomena, particularly metallurgical transformations in the part and the chip. The drilling operations will be modelled from cutting phenomena identified during the turning test for different cutting geometries. A modeling of thermal distribution in the part for different titanium alloys will complete the study of the machinability. An important part of the modeling is devoted to the chip formation from the constitutive laws. One of the expected results of this project is to have a model to predict the machinability of the material from the bahavior law of titanium alloys.

These results will guide the development of new materials with improved machinability. Furthermore, the specific characterization of the titanium alloy may be a part of certification for the product in the future.

This project will familiarize IJL lab with the problem of microstructural evolution encountered when machining titanium under severe thermo-mechanical loads, and they can subsequently exploit the knowledge developed in TITUS for optimizing the initial microstructures of titanium alloys. These results will be published.
LAMPA lab for this project will be able to gain knowledge, firstly, on the relationship between the microstructure of titanium alloys and their behavior under extreme loads, and secondly, on the connection between the behavior of the material and its machinability. Both areas are still poorly mastered by the scientific community, which will give the LAMPA and actors of this project a significant advance. In addition, these actions will strengthen connections with IJL and LaBoMaP laboratories. During this project, the relation between LAMPA and LaBoMaP labs will be materialized by the co -supervision of a thesis.
Regarding the LaBoMaP , significant progress of this project will be a better understanding of the impact of the behavior laws of titanium alloys on the mechanisms of chip formation and on the modelling of local stresses at the mesoscale of the cutting edge . The project will also highlight the relationship between the turning and drilling processes. The promotion of these works will be carried out by scientific publications.

Further to the extension of CFRP on most of structural parts of A350 (fuselage and wing), new developments on metallic materials need to be taken into account. Till A380, Ti alloys applications were limited to structural parts on engine pylon. For compatibility reasons with CFRP, Ti alloys are now used on fuselage parts, in replacement of Al alloys. The introduction of CFRP has resulted in an increasing of Ti alloys quantity on aircraft. However, the fuselage parts of Ti alloys have a very high buy to fly ratio, producing a lot of chips. Unfortunately, Ti alloys are not so easy machinable compared to Al alloys and the production rate for Al alloys can be 30 times higher than for Ti alloys. Therefore, the plants that machine Ti alloys have to face a big issue to maintain their productivity and their cost. TITUS project is entering in this framework of improving the machining productivity of Ti alloys. The approach that is developed in TITUS is based on a fundamental understanding of the physical mechanisms occurring in machining process. The literature survey shows that there is not so much work on Ti alloys machinability: some of publications highlight different machinability between Ti6Al4V and Ti5553 or Ti54M, others try to establish relationship between microstructure and machinability but none of them explain the key material properties that master the machinability. In the TITUS project, the global objective is to establish the interaction between microstructure/cutting parameters/mechanical properties in the range of very high strain rate and temperature. One expected result of this project is a model to predict the machinability of the material from the behavior laws of Ti alloys. For addressing this ambitious target, experts in machining, in damages under extreme conditions, in Ti alloys metallurgy and in modeling are involved in this project. This project is carried out by the companies EADS IW, TIMET and AEROLIA and the laboratories IJL, LAMPA and LaBoMaP.

Project coordination

Guillaume ABRIVARD (EADS France Département Innovation Works) –

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.


Arts et Métiers ParisTech Ecole Nationale Supérieure d'Arts et Métiers
IJL Institut Jean Lamour
EADS IW EADS France Département Innovation Works

Help of the ANR 815,776 euros
Beginning and duration of the scientific project: December 2012 - 42 Months

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