Titanium alloys support a wide range of practical applications due to their excellent mechanical properties. These include high strength-to-weight ratio, high mechanical strength at elevated temperatures and remarkable oxidation resistance. Machinability investigations so far have been intentionally focused on Ti-6Al-4V, which is commonly used in the aerospace research and development. However, a new classes of titanium alloys are also being developed for these applications. Ti-5Al-5Mo-5V-3C, also known as Ti5553, is included in this new category of titanium grade alloys. It corresponds to a near beta titanium alloy and generally it is employed on the production of high strength parts. Its high tensile strength combined with low weight (compared to Ti64) makes Ti5553 a suitable choice for landing gear parts and advanced structural components. However, due to the previously mentioned mechanical properties of Ti5553, machining processes can be difficult. During the cutting tests, the cutting zone experiences high cutting temperatures, and combined with a low rate of heat transfer, it generates stress and premature tool failure. By using several distinct experimental approaches, this work presents a comparison between different machining conditions (combinations of tools and coolants) to diagnose wear processes and identify better cutting parameters. The main objective of this research is to establish an understanding of how these parameters affect tribological aspects when machining Ti5553. The results of machining studies demonstrate different wear behaviour for CBN and PCD tools under various cutting environments (different coolant modes). These operating conditions can considerably affect the cutting forces leading to an increased tool life and improved surface integrity by decreasing, the residual stress and roughness, as well as work of hardening the workpiece during machining operations. / Thesis / Master of Applied Science (MASc)

Les travaux présentés dans cette thèse s'inscrivent dans le contexte de la modélisation et la compréhension des phénomènes liés à l'usinage du Ti5553. L'objectif des travaux est de lever certains verrous scientifiques et d'améliorer les connaissances sur l'usinage de ce matériau afin de proposer des conditions de coupe et d'engagement optimales à l'entreprise Messier-Dowty. Nous avons identifié plusieurs modèles nous permettant de décrire au mieux le processus de coupe en tournage et en fraisage. Au niveau de l'usure des outils et des possibilités d'amélioration de l'usinabilité de l'usinage du Ti5553, l'ensemble des travaux a montré un manque d'informations dû à la rareté du matériau. Pour définir les constantes de la loi de Johnson-Cook et de fait caractériser le matériau, notre démarche s'est focalisée vers le développement d'une méthode basée sur des essais d'usinage et des essais de traction permettant ainsi d'obtenir une loi ajustée à l'usinage du Ti5553. La loi de comportement a ensuite été utilisée dans les modélisations d'efforts de coupe en tournage et en fraisage avec pour objectif de modéliser l'ensemble de la gamme des outils employés par l'entreprise Messier-Dowty (formes, géométries de coupe. . .). Ces modèles nous ont ensuite permis de mettre en avant les difficultés liées à l'usinage du Ti5553 et de proposer des conditions de coupe et d'engagement adaptées. La troisième partie est consacrée à l'analyse des phénomènes d'usure des outils coupants où les différents modes de dégradation en tournage et en fraisage ont été identifiés. Leurs relations avec le processus de coupe ont été définies afin de proposer un critère d'évaluation de l'usure des outils. Une partie de cette étude est également consacrée à l'identification de l'influence des conditions d'engagement, des géométries de coupe, des conditions de coupe sur l'usure. Le dernier chapitre concerne la valorisation industrielle où nous présentons les applications et les extrapolations de nos travaux dans le contexte industriel. Nous avons également testé l'augmentation de la température qui constitue une des pistes d'amélioration de l'usinabilité mais sans résultat probant. Enfin, nous avons abordé la problématique de la surveillance d'usinage où nous avons mis en avant les difficultés liées aux particularités du processus de coupe. / The work presented in this thesis fits the context of modeling and understanding of phenomena related of Ti5553 machining. The goal is to improve knowledge about material to provide optimal cutting conditions, optimal engagement and cutting geometry. We have identified several models allowing to describe the cutting process in tunining and milling. In terms of tool wear and usinability improvement in Ti5553 machining all studies showed a lack of information. Our approach focused towards in developing an alternative method to define the Johnson-Cook law constants based on machining tests and tensile tests to obtain the constitutive law adjusted to Ti5553 machining. The constitutive law was used cutting forces modeling for turning and milling involving the full range of tools used by the company (shapes, cutting geometries...). These models were allowed us the highlights the machining difficulties and propose suitable conditions. The third part focused on wear cutting tools mechanisms analysis where the differents degradation stage in turning and milling have been defined. Their relationships with the cutting process has been identified in order to propose a criterion of wear evolution. Part of this study is to identify the effect of engagement, cutting geometries and cutting conditions on tool wear. The last part concerns the industrial development where we present our work applications and extrapolations in our industrial context. We also tested one of the prospects of machinability improving (increase temperature) but without result. Finally, we addressed the problem of process monitoring on Ti5553 machining where we highlighted the difficulties related to the specifics on cutting process.

The properties of titanium alloys are based on alloy compositions and microstructures that consist of mixtures of the two allotropic modifications of titanium, the low temperature α (hcp) and the high temperature β (bcc) phases. This thesis deals with the hot working behaviour of three commercial titanium alloy compositions designated IMI834, Ti17 and Ti5553 with a focus and detailed analysis of the Ti5553 alloy. These alloys represent the differing uses of titanium alloys in the aerospace industry. IMI834 is a near α alloy used in high temperature creep resistant applications as compressor discs and blades in aeroengines. Ti17 is a high strength alloy α+β used at intermediate temperatures in fan and compressor discs of aeroengines, while Ti5553 is a high strength-high toughness metastable β alloy used in the undercarriages of aircraft. The three alloys have widely differing β transus temperatures (related to α phase stability) and compositions. Titanium alloys are vacuum arc melted and thermomechanically processed. This process involves ingot breakdown in β (bcc) phase, and subsequent thermomechanical processing in two-phase α+β (hcp+bcc) region at temperatures that typically involve volume fractions of α in lath or plate form ranging from 15% to about 30%. The thermomechanical processing breaks down lath α to spheroidal particles, a process known as globularisation. Chapter I of this thesis reviews the current understanding of the hot working of titanium alloys and microstructure evolution during the hot working process. Chapter II summarises the main experimental techniques used: the hot compression test, and subsequent microstructure and microtexture analysis by scanning electron microscopy and related electron back scattered diffraction techniques (EBSD), transmission electron microscopy and related precession electron diffraction techniques (PED) for orientation imaging. The starting structure in the α+β domain of hot work is generally not a random distribution of the 12 variant Burgers Orientation Relationship (BOR) between the α and β phases, (11̅0)β || (0001)α and <111>β || <112̅0>α . A variety of morphologies and distributions ranging from the typical colony structures of near α and α+β alloys to the fine distributions of variants arranged in a triangular fashion are observed with specific growth directions and habit planes. Chapter III describes a quantitative evaluation of α distribution that are typical of some of the starting structures for the hot working conditions used in this thesis, specifically in the Ti5553 alloy. For this purpose, a Matlab based script has been developed to measure the spatially correlated misorientation distribution. It was found that experimental spatially correlated misorientation distribution varies significantly from a random frequency for both pair and triplet wise distribution of α laths. The analysis of these structures by established techniques of analysis of self-accommodated structures based on strain energy minimisation shows that the observed variant distribution arise from the residual strain energy accommodation of the semi-coherent α plates. The hot working process has been examined through hot compression tests of the 3 alloys at strain rates ranging from 10-3 s-1 to 10 s-1 over a temperature range designed to maintain constant volume fractions of the α and β phases during deformation ranging from about 30% α to a fully β structure. Since extensive prior work has been carried on the processing of titanium alloys, Chapter IV focuses on a comparative study of hot deformation behaviour of the three alloys with an emphasis on isolating microstructural and other effects. The macroscopic flow behaviour has been analysed in terms of conventional rate equations relating stress, strain, strain rate and temperature. The three alloys show very similar features in their stress-strain behaviour. β phase deformation exhibits yield points whose magnitude varies with strain rate and temperature. The flow stress curves are typical of materials undergoing dynamic recovery and recrystallization processes. The stress-strain behaviour in the α+β domain of hot work exhibits significant flow softening in the early stages of deformation with a subsequent approach to steady-state behaviour at true strain of about 0.5. Activation energy analysis of the steady state condition suggests that the rate controlling mechanism is related to recovery in the β phase in both α+β and β processing. Zener-Hollomon plots of the flow stress in the three alloys indicate that their flow stress can be normalized to a temperature-compensated strain rate and they differ only in the slopes of the plots that are related to the stress exponent. Empirical constitutive models were developed for a predictive understanding of the flow stress as a function of strain, strain rate and temperature using conventional rate equations for the flow stress Chapter V and VI examine the evolution of microstructure and microtexture in detail during hot deformation and subsequent heat treatment in Ti5553. A combination of EBSD (micron and submicron scale) and PED (nano meter scale) is used in orientation imaging to examine the globularisation process of the α phase and the recovery and recrystallization in the β phase in both supertransus and subtransus hot compression. The understanding of these processes is enhanced by tracking the same starting β grain through the deformation process. The effect of strain, strain rate and temperature on the evolution of subgrains in α and its fragmentation into spheroidal α is quantified. In the absence of shear bands, the globularisation process is seen to evolve from a strain driven Raleigh instability of the plate α, by subgrain formation in α and β phases. The related microtexture evolution is analysed. The analysis of recovery and microtexture evolution in the β phase described here has not been attempted earlier in the literature. The overall evolution of structure and texture is seen to result from the complex interplay between recovery and recrystallisation in the α and β phases in substranus deformation. While the Burgers orientation relationship between α and β is lost in the early stages of deformation, it appears to be restored at large strains as a consequence of ‘epitaxial’ recrystallisation processes that seem to result from the discontinuous nucleation of recrystallization of either phase at interphase interfaces in the Burgers orientation. The effect of substranus deformation on β texture following supertransus post deformation heat treatment is also examined and compared with β textures resulting from alternative strain paths such as friction stir processing. Finally Chapter VII summarises these results and the new insights into the evolution of structure and microtexture during hot deformation of titanium alloys and suggests directions for future work.

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