Phase transformation of the Ti-5553 titanium alloy subjected to rapid heating

The a -> b phase transformation upon heating in the Ti-5553 alloy with lamellar-nodular bimodal microstructure was tracked in situ with high energy X-ray diffraction. Rapid heating at 10, 50 and 100 °C s-1 from room temperature to 1050 °C was tested. Phase transformation on heating was studied by a combined analysis of the microstructural features that provides estimates of mass fractions, mean lattice parameters and full width at half maximum for the two phases. In comparison with equilibrium conditions, the experimental mass fractions reveal a shift of the transformation domain toward high temperatures when the heating rate increases. Also, the dissolution of the a phase is largely impacted by its morphology, the transformation being faster for a lamellae. The combined analysis of mean lattice parameters and full width at half maximum suggests that the a -> b phase transformation on heating is diffusion controlled. The b phase therefore inherits the solute content of the adjacent parent a phase, leading to chemical heterogeneities in the b phase regardless of the heating rate

Etude des évolutions microstructurales dans l'alliage de titane beta-métastable Ti-5553 : conditions sévères de chargements thermiques et réactivité chimique avec un outil de coupe

This PhD thesis is part of collaborative project, which aims to understand and quantify the effect of microstructural features of the Ti-5553 -metastable titanium alloy on its machinability and cutting tool wear. Thus, a set of so-called "model" microstructures were elaborated to study the effect of the initial microstructure and heating rate on the mechanisms and kinetics of phase dissolution during rapid continuous heating to simulate the local overheating at the cutting tool tip. In addition, chemical diffusion between titanium alloys and WC-Co type tools was studied to further understand mechanisms of wear assisted by chemical diffusion. Seven thermal paths were defined to obtain model microstructures with different fractions, morphologies, sizes, and spatial distributions of phase. The corresponding materials were produced, and their microstructural features were determined. Results from orthogonal cutting tests conducted by the partner laboratory LAMPA (Angers) were used to link microstructural features, microhardness, and machining forces generated during cutting. The most influencing parameters were determined. Phase transformations on rapid continuous heating up to 100 °Cs 1 were investigated primarily using in situ measurements obtained through high-energy synchrotron X-ray diffraction. As expected, the + transformation kinetics for the Ti 5553 alloy were slower compared to those reported in the literature for the Ti 64 alloy. The results also showed that the transformation kinetics depend not only on the heating rate but also on the morphology and size of the precipitates. However, the initial fraction of phase has no significant effect on the transformation kinetics but only on the starting temperature of transformation. The combined analysis of mean lattice parameters and full width at half maximum of diffraction peaks suggested that the transformation is controlled by chemical diffusion and that the low diffusivities of solute elements and the reduction in the number of / interfaces lead to chemical heterogeneities in the phase whose amplitude varies with the heating rate. Finally, numerical simulations of phase dissolution on heating were discussed in relation to experimental results. Through the study of diffusion couples under thermal loading between Ti-64 or Ti 5553 samples and WC-Co tools, the systematic formation of a TiC layer and partial dissolution of WC carbides in the tool were systematically revealed. A significantly greater diffusion length of W in the Ti-64 alloy was measured. Furthermore, for the Ti 5553 alloy, the precipitation of TiC needles up to several hundred micrometers in the matrix was observed. The diffusion mechanisms and resulting phase transformations are discussed.Cette thèse s’inscrit dans le cadre du projet ANR DEMUTI dont l’objectif a été de comprendre et de quantifier l’impact des éléments microstructuraux de l'alliage de titane beta-métastable Ti-5553 sur son usinabilité et l’usure des outils de coupe. Ainsi, différentes microstructures dites « modèles » ont été élaborée afin d’étudier l’influence de la microstructure initiale et de la vitesse de chauffage sur les mécanismes et la cinétique de dissolution de la phase lors d’un chauffage continu rapide afin de rendre compte de l’échauffement local en pointe d’outil. En complément, la diffusion chimique entre les alliages de titane et les outils de type WC-Co a été étudié pour comprendre plus avant les mécanismes d’usure assistée par la diffusion chimique. Sept chemins thermiques permettant d’obtenir des microstructures modèles avec des fractions de phase, des morphologies, des tailles et des répartitions spatiales de phase différentes ont été définis. Les matériaux correspondants ont été fabriqués et leurs caractéristiques microstructurales ont été déterminées. Des résultats d’essais de rabotage réalisés par le laboratoire partenaire du LAMPA (Angers) ont été exploités pour mettre en relation les spécificités microstructurales, la microdureté et les efforts d’usinage induits lors du rabotage. Les paramètres influents ont été déterminés. Les transformations de phases en chauffage rapide continu jusqu’à 100 °Cs-1 ont été investiguées en s’appuyant principalement sur des mesures in situ obtenues par diffraction des rayons X synchrotron haute énergie. Comme attendu, la cinétique de transformation + pour l’alliage Ti 5553 est plus lente en comparaison à celle rapportée dans la littérature pour l’alliage Ti 64. Les résultats obtenus ont également permis de montrer que la cinétique de transformation dépend non seulement de la vitesse de chauffage mais aussi de la morphologie et de la taille des précipités . En revanche, la fraction initiale de phase n’a pas d’impact sur la cinétique de transformation mais seulement sur la température de début de transformation. L’analyse combinée des paramètres de maille moyens et des largeurs à mi-hauteurs des pics de diffraction laisse supposer que la transformation est pilotée par la diffusion chimique et que les faibles diffusivités des éléments solutés et la diminution du nombre d’interfaces / conduisent à des hétérogénéités chimiques dans la phase dont l’amplitude varie avec la vitesse de chauffage. Enfin, des simulations numériques de la dissolution de la phase au chauffage sont discutées en regard des résultats expérimentaux. Par l’étude de couples de diffusion sous chargement thermique entre des pastilles de Ti-64 ou Ti 5553 et des outils WC-Co, nous avons montré la formation systématique d’une couche de TiC et la dissolution partielle des carbures WC de l’outil. Une longueur de diffusion du W significativement plus grande dans l’alliage Ti-64 a été mesurée. Par ailleurs, pour l’alliage Ti 5553, la précipitation d’aiguilles TiC jusqu’à plusieurs centaines de micromètres dans la matrice a été mise en évidence. Les mécanismes de diffusion et de transformations de phases résultantes sont discutés

Phase transformation of the Ti-5553 titanium alloy subjected to rapid heating

The $\alpha \to \beta$ phase transformation upon heating in the Ti-5553 alloy with lamellar-nodular bimodal microstructure was tracked in situ with high energy X-raydiffraction. Rapid heating at 10, 50 and 100 C s$^{-1}$ from room temperature to1050 C was tested. Phase transformation on heating was studied by a combinedanalysis of the microstructural features that provides estimates of mass fractions,mean lattice parameters and full width at half maximum for the twophases. In comparison with equilibrium conditions, the experimental massfractions reveal a shift of the transformation domain toward high temperatureswhen the heating rate increases. Also, the dissolution of the $\alpha$ phase is largelyimpacted by its morphology, the transformation being faster for $\alpha$ lamellae. Thecombined analysis of mean lattice parameters and full width at half maximumsuggests that the $\alpha \to \beta$ phase transformation on heating is diffusion controlled.The $\beta$ phase therefore inherits the solute content of the adjacent parent a phase,leading to chemical heterogeneities in the $\beta$ phase regardless of the heating rate

Influence of the microstructure of a Ti5553 titanium alloy on chip morphology and cutting forces during orthogonal cutting

Titanium alloys, largely used for aeronautical applications, are difficult to machine. High cutting forces, chip serration and important tool wear reflect this poor machinability, limiting productivity. One way of improving the machinability of titanium alloys consists of controlling their microstructure. In the present work, the impact of the microstructure of the Ti5553 alloy on chip formation and cutting forces is investigated. For this purpose, a novel experimental approach is proposed. Orthogonal cutting tests are performed on eight different microstructures, which allows studying the impact of the α-phase fraction as well as the size and shape of α particles. Also, an original post processing method based on machine learning provides chip morphological information from images recorded with two high speed cameras. Such information is completed with the cutting forces measured with a dynamometer. In contrast with commonly used approaches, the proposed method is not limited to the formation of a few segments, but uses the full dataset acquired during a test. The results obtained for the different microstructures indicate that no direct link can be established between the cutting forces and their hardness as minimal cutting forces are obtained for microstructures with an intermediate hardness. For microstructures providing low hardness, high cutting forces result from a significantly thick chip. In opposition, for the microstructures leading to high hardness, an important flow stress generates high cutting forces. This study also suggests that chip morphology is primarily affected by the α-phase fraction while the size and morphology of α-phase particles have little influence