Study of burr formation and phase transformation during micro-milling of NiTi alloys

Micro-milling can be defined as milling with end mills smaller than 1 mm of diameter. The top-down approach from milling to micro-milling is often used to define cutting conditions. Unfortunately geometries either for the active part or the overall shape are quite different from conventional tools, leading to inexistent problems at the macro-scale, such as a larger cutting edge radius to uncut chip thickness ratio leading to ploughing effect. Moreover, micro-milling can be used on particular material such as shape memory alloys in biomedical domain which are difficult to machine. This study focuses on burr formation during shoulder milling for two biocompatible NiTi alloys: a martensitic NiTi (shape memory effect) and an austenitic one (pseudo-elasticity effect). Design of experiment is used to highlight the influence of various parameters (cutting parameters and material phases) on the burr formation in micro-milling NiTi alloys. Burrs were observed and measured using confocal, optical and electronic microscopy and tend to be as large as shoulders dimensions. Material phase transformation was also examined. Analysis of variance emphasizes that the larger the feed per tooth and the smaller the width of cut are, the smaller the top burr is. Cutting strategy leads to different burr shape: up-milling burrs have a large curvature, whereas down-milling burrs are slightly bent. An affected layer of about 10 μm has been observed for the austenitic NiTi. The proposed experimental approach give the opportunity to study burr formation in micro-milling, the machinability of alloys or superelastic NiTi shape memory and a qualitative explanation of burr formation has been developed

Study of burr formation and phase transformation during micro-milling of NiTi alloys

Micro-milling can be defined as milling with end mills smaller than 1 mm of diameter. The top-down approach from milling to micro-milling is often used to define cutting conditions. Unfortunately geometries either for the active part or the overall shape are quite different from conventional tools, leading to inexistent problems at the macro-scale, such as a larger cutting edge radius to uncut chip thickness ratio leading to ploughing effect. Moreover, micro-milling can be used on particular material such as shape memory alloys in biomedical domain which are difficult to machine. This study focuses on burr formation during shoulder milling for two biocompatible NiTi alloys: a martensitic NiTi (shape memory effect) and an austenitic one (pseudo-elasticity effect). Design of experiment is used to highlight the influence of various parameters (cutting parameters and material phases) on the burr formation in micro-milling NiTi alloys. Burrs were observed and measured using confocal, optical and electronic microscopy and tend to be as large as shoulders dimensions. Material phase transformation was also examined. Analysis of variance emphasizes that the larger the feed per tooth and the smaller the width of cut are, the smaller the top burr is. Cutting strategy leads to different burr shape: up-milling burrs have a large curvature, whereas down-milling burrs are slightly bent. An affected layer of about 10 μm has been observed for the austenitic NiTi. The proposed experimental approach give the opportunity to study burr formation in micro-milling, the machinability of alloys or superelastic NiTi shape memory and a qualitative explanation of burr formation has been developed

Micro-end milling of NiTi biomedical alloys, burr formation and phase transformation

This paper focuses on burr formation in micro-end milling of two Nickel-Titanium shape memory alloys (SMA), an austenitic and a martensitic NiTi. Phase transformation during machining was also examined. The experimental design approach was used to study the effect of cutting parameters on burr formation. The studied parameters were cutting speed, feed per tooth, depth and width of cut, 20 machining strategy and initial material phase of the NiTi alloy. Different types of burrs were formed during micro-end milling of NiTi alloys; it was observed that top burrs are the most important. The height of top burrs can reach values close to those of the depth of cut. Burrs were observed and characterized using a Scanning Electron Microscope (SEM), confocal and optical microscopes. The affected layer under the machined surface, and phase transformation 25 were investigated by using SEM. The results of the analysis of variance showed a significant formation of burrs, deeply influenced by the feed per tooth and width of cut. An increase in the feed per tooth and a decrease of width of cut tend to decrease the height and width of the top burr. In a thin layer under the machined surface, phase transformation was observed for the martensitic NiTi

Caractérisation des bavures lors du micro-fraisage d'alliages de Nickel-Titane

Le micro-fraisage est souvent défini comme une homothétie du fraisage conventionnel à des outils dont le diamètre n’excède pas 1 mm. Or, les géométries de l’outil que ce soit pour la partie active ou la forme globale sont très différentes des outils plus conventionnels, soulevant des problématiques propres à l’échelle d’étude. A ceci s’ajoute le fait que le micro-fraisage peut s’appliquer à des matériaux qualifiés d’exotiques comme les alliages à mémoire de forme ou superélastiques dans le domaine du biomédical. L’étude proposée porte sur la formation de bavures lors du fraisage d’épaulement pour deux alliages Nickel-Titane (NiTi) biocompatibles. L’approche par plan d’expériences a été utilisée pour cette étude, elle a permis de mettre en évidence l’influence de différents paramètres sur la formation de bavure lors du micro-fraisage d’alliages de NiTi. Ces paramètres concernent les conditions de coupe (la vitesse de coupe, l’avance par dent, l’engagement axial, l’engagement radial, la stratégie d’usinage) ainsi que l’état métallurgique de l’alliage NiTi (martensitique ou austénitique). Les bavures ont été observées et caractérisées géométriquement (hauteur, largeur et épaisseur) à partir d’images obtenues au MEB, aux microscopes confocal et optique. Les résultats issus de l’analyse de la variance montrent que la formation de bavures est largement influencée par l’avance à la dent et par l’engagement radial. Il est à noter que les dimensions des bavures sont de l’ordre de l’engagement de l’outil dans la matière et qu’il est souhaitable de les réduire. Une augmentation de l‘avance par dent et une diminution de l’engagement radial permettent de diminuer significativement la hauteur et la largeur des bavures. L’approche expérimentale proposée a permis d’examiner la formation de bavures en micro-fraisage et d’analyser les paramètres influents; elle a permis, de plus, d’étudier l’usinabilité des alliages de Nickel-Titane biocompatibles au comportement particulier, superélastique ou à mémoire de forme

Study of elementary micro-cutting in hardened tool steel

In order to model micro-milling cutting forces, a way is to apply a local model on discretized elements of the cutting edge and then summing on the whole edge to obtain the global cutting forces. This local model is usually obtained by numerical simulation or cutting experimentation. This paper focuses on orthogonal and oblique micro-cutting experiments of AISI 6F7 with tungsten carbide tools. Results show the influence of cutting edge sharpness on cutting forces and the existence of different mechanisms corresponding to different ranges of uncut chip thickness values. A phenomenological model has been identified to model correctly these zones. Then, by comparing experimental micro-milling forces with those deduced from these micro-cutting model and tests, a good agreement has been found. In order to complete this study, phenomenological and thermo mechanical models are being developed. The aim is to obtain an elementary cutting model that can be used for micro-milling simulation and optimization

Expérimentation de la micro-coupe élémentaire sur un acier dur et comparaison au micro-fraisage

Cet article présente des essais de micro-coupe orthogonale et oblique à partir de tournage sur un acier 40NiCrMo16. Les résultats obtenus démontrent l’influence du rayon d’acuité d’arête sur les efforts mesurés notamment aux faibles épaisseurs de copeau non déformé. Les efforts spécifiques de coupe déduits sont en cohérence avec ceux obtenus lors d’essais de micro-fraisage issus de travaux précédents. Pour compléter l’étude, cet article pose les bases de la modélisation phénoménologique et thermomécanique adaptée à la micro-coupe. Le but à terme est d’obtenir un modèle de coupe élémentaire utilisable dans le cas du micro-fraisage puis de comparer les résultats obtenus aux résultats expérimentaux

Machinability of TiNb bio-compatible alloys

The success of biomedical implantation is linked to osseointegration, depending on the mechanical loading of the bone interface. The large difference in stiffness between the host bone (30 GPa) and the usual implant material (over 100 GPa), as well as the absence of mechanical stress at the surrounding bone, induce a stress shielding effect, which leads to bone atrophy and implant loss. A recent work has shown the possibility to produce so-called second-generation titanium alloys. β-type Ti alloys have been studied for biomedical applications, due to the composition of non-cytotoxic elements. Some TiNb alloys can reach after heat treatment a Young’s modulus close to 35 GPa, which is really close to bone’s one. Unfortunately, titanium and its alloys are well known for their poor machinability due to the hardness and low thermal conductivity. Machined surfaces of titanium alloys are also easily damaged (micro cracks, build-up edge, plastic deformation, heat-affected zones, and tension residual stresses) during the process. Studying process parameters is important to avoid these phenomena. The machinability of TiNb (turning, milling) has not been studied to date. Therefore, in this study, we examined the behavior of the TiNb titanium alloy for applications as a biomaterial in micro-cutting. Orthogonal cutting tests were performed on austenite and martensite states TiNb alloys and compared with Ti40 pure titanium. The aim was to evaluate the influence of the alloys and the cutting parameters on the evolution of the cutting forces, specific cutting energy, friction coefficient which are good indicators of machinability

Vaccine breakthrough hypoxemic COVID-19 pneumonia in patients with auto-Abs neutralizing type I IFNs

Life-threatening `breakthrough' cases of critical COVID-19 are attributed to poor or waning antibody response to the SARS- CoV-2 vaccine in individuals already at risk. Pre-existing autoantibodies (auto-Abs) neutralizing type I IFNs underlie at least 15% of critical COVID-19 pneumonia cases in unvaccinated individuals; however, their contribution to hypoxemic breakthrough cases in vaccinated people remains unknown. Here, we studied a cohort of 48 individuals ( age 20-86 years) who received 2 doses of an mRNA vaccine and developed a breakthrough infection with hypoxemic COVID-19 pneumonia 2 weeks to 4 months later. Antibody levels to the vaccine, neutralization of the virus, and auto- Abs to type I IFNs were measured in the plasma. Forty-two individuals had no known deficiency of B cell immunity and a normal antibody response to the vaccine. Among them, ten (24%) had auto-Abs neutralizing type I IFNs (aged 43-86 years). Eight of these ten patients had auto-Abs neutralizing both IFN-a2 and IFN-., while two neutralized IFN-omega only. No patient neutralized IFN-ss. Seven neutralized 10 ng/mL of type I IFNs, and three 100 pg/mL only. Seven patients neutralized SARS-CoV-2 D614G and the Delta variant (B.1.617.2) efficiently, while one patient neutralized Delta slightly less efficiently. Two of the three patients neutralizing only 100 pg/mL of type I IFNs neutralized both D61G and Delta less efficiently. Despite two mRNA vaccine inoculations and the presence of circulating antibodies capable of neutralizing SARS-CoV-2, auto-Abs neutralizing type I IFNs may underlie a significant proportion of hypoxemic COVID-19 pneumonia cases, highlighting the importance of this particularly vulnerable population

Experimentation and modelling of micro-cutting for micro-milling application

Les procédés de micro-fabrication connaissent actuellement une croissance importante dans les applications industrielles et pour des secteurs majeurs. Parmi les techniques d’usinage en micro-fabrication, le micro-fraisage est sans doute le plus polyvalent que ce soit en termes de matériau usiné ou de géométrie obtenue. La fabrication de micro-fraises est encore limitée par un certain nombre de paramètres (comme le rayon d’acuité d’arête) et demande alors à être optimisée. L’approche utilisée consistant à reproduire à petite échelle ce qui se fait de mieux à une échelle conventionnelle n’est alors plus forcément adaptée. Il en résulte que le micro-fraisage est un procédé encore mal maîtrisé (usure prématurée de l’outil, bris d’outil, trajectoire non maîtrisée, bavures…).L’objectif de la thèse est donc de comprendre les mécanismes mis en jeu lors de l’enlèvement de matière en micro-usinage et d’en établir un modèle permettant de prédire les efforts de coupe selon les conditions choisies et qui permettra par la suite de faciliter l’optimisation de la géométrie des outils coupantDans un premier temps, une étude expérimentale s’attache à observer la micro-coupe élémentaire d’un acier dur à l’aide de dispositifs réalisés dans le cadre de ces travaux. Un premier dispositif permet de mesurer les efforts d’usinage en micro-coupe élémentaire et un deuxième dispositif innovant permet d’étudier la formation du copeau par coupe interrompue.Par la suite, une démarche de modélisation de la micro-coupe élémentaire est proposée en complément de l’étude expérimentale. Une approche par loi de coupe basée sur les résultats des essais de micro-coupe élémentaire permet de modéliser les efforts d’usinage. En complément, des simulations numériques utilisant la méthode SPH donnent aussi des informations intéressantes sur la formation du copeau, notamment au niveau des zones de déformation.Enfin la loi de coupe associée à un modèle géométrique du micro-fraisage permet de prédire les efforts de coupe lors de l’usinage du même acier. Le modèle géométrique basé sur des travaux précédents a été complété pour prendre en compte la flexion d’outil ainsi que le faux-rond. Ce faux-rond est mesuré directement sur la machine à l’aide d’un moyen d’observation spécialement développé. Les résultats obtenus montrent une concordance entre les efforts expérimentaux et les efforts prédits.Micro-manufacturing processes are undergoing a significant growth in industrial applications and in a number of major sectors. Among the micro-machining techniques, micro-milling is probably the most versatile both in terms of machined material and in terms geometrical achievability. However, micro-end-mill manufacturing is still limited by some parameters (such as cutting edge radius) and needs improvement. The top-down approach used to reproduce what is best from conventional scale to micro-scale is not necessarily suitable. As a result, micro-milling is still a poorly controlled process (tool wear, tool breakage, path control, burrs...).The aim of the thesis is to understand the mechanisms occurring during the material removal with micro-cutting and to propose a model to predict cutting forces according to cutting conditions, which will then make the optimization of micro-end-mills geometry easier.First, an experimental study is used to observe the elementary micro-cutting operation of a hardened tool steel using specially designed devices. A first device is used to measure cutting forces in elementary micro-cutting and a second innovative device is used to study chip formation by quick-stop tests.Then, modelling approaches of elementary micro-cutting are proposed to complete the experimental study. A cutting law approach based on the results of the elementary micro-cutting tests allows the cutting forces to be modelled. In addition, numerical simulations using the SPH method investigate chip formation and particularly deformation and shear zones.Finally, the proposed cutting law combined with a micro-milling geometric model allows the prediction of cutting forces when micro-milling the same hardened tool steel. The geometric model based on previous work has been completed to consider static tool deflection and run-out. This run-out is measured directly on the machine using a specially developed device. The results obtained show a good correlation between experimental and predicted forces

Expérimentation et modélisation de la micro-coupe pour une application au micro-fraisage

Micro-manufacturing processes are undergoing a significant growth in industrial applications and in a number of major sectors. Among the micro-machining techniques, micro-milling is probably the most versatile both in terms of machined material and in terms geometrical achievability. However, micro-end-mill manufacturing is still limited by some parameters (such as cutting edge radius) and needs improvement. The top-down approach used to reproduce what is best from conventional scale to micro-scale is not necessarily suitable. As a result, micro-milling is still a poorly controlled process (tool wear, tool breakage, path control, burrs...).The aim of the thesis is to understand the mechanisms occurring during the material removal with micro-cutting and to propose a model to predict cutting forces according to cutting conditions, which will then make the optimization of micro-end-mills geometry easier.First, an experimental study is used to observe the elementary micro-cutting operation of a hardened tool steel using specially designed devices. A first device is used to measure cutting forces in elementary micro-cutting and a second innovative device is used to study chip formation by quick-stop tests.Then, modelling approaches of elementary micro-cutting are proposed to complete the experimental study. A cutting law approach based on the results of the elementary micro-cutting tests allows the cutting forces to be modelled. In addition, numerical simulations using the SPH method investigate chip formation and particularly deformation and shear zones.Finally, the proposed cutting law combined with a micro-milling geometric model allows the prediction of cutting forces when micro-milling the same hardened tool steel. The geometric model based on previous work has been completed to consider static tool deflection and run-out. This run-out is measured directly on the machine using a specially developed device. The results obtained show a good correlation between experimental and predicted forces.Les procédés de micro-fabrication connaissent actuellement une croissance importante dans les applications industrielles et pour des secteurs majeurs. Parmi les techniques d’usinage en micro-fabrication, le micro-fraisage est sans doute le plus polyvalent que ce soit en termes de matériau usiné ou de géométrie obtenue. La fabrication de micro-fraises est encore limitée par un certain nombre de paramètres (comme le rayon d’acuité d’arête) et demande alors à être optimisée. L’approche utilisée consistant à reproduire à petite échelle ce qui se fait de mieux à une échelle conventionnelle n’est alors plus forcément adaptée. Il en résulte que le micro-fraisage est un procédé encore mal maîtrisé (usure prématurée de l’outil, bris d’outil, trajectoire non maîtrisée, bavures…).L’objectif de la thèse est donc de comprendre les mécanismes mis en jeu lors de l’enlèvement de matière en micro-usinage et d’en établir un modèle permettant de prédire les efforts de coupe selon les conditions choisies et qui permettra par la suite de faciliter l’optimisation de la géométrie des outils coupantDans un premier temps, une étude expérimentale s’attache à observer la micro-coupe élémentaire d’un acier dur à l’aide de dispositifs réalisés dans le cadre de ces travaux. Un premier dispositif permet de mesurer les efforts d’usinage en micro-coupe élémentaire et un deuxième dispositif innovant permet d’étudier la formation du copeau par coupe interrompue.Par la suite, une démarche de modélisation de la micro-coupe élémentaire est proposée en complément de l’étude expérimentale. Une approche par loi de coupe basée sur les résultats des essais de micro-coupe élémentaire permet de modéliser les efforts d’usinage. En complément, des simulations numériques utilisant la méthode SPH donnent aussi des informations intéressantes sur la formation du copeau, notamment au niveau des zones de déformation.Enfin la loi de coupe associée à un modèle géométrique du micro-fraisage permet de prédire les efforts de coupe lors de l’usinage du même acier. Le modèle géométrique basé sur des travaux précédents a été complété pour prendre en compte la flexion d’outil ainsi que le faux-rond. Ce faux-rond est mesuré directement sur la machine à l’aide d’un moyen d’observation spécialement développé. Les résultats obtenus montrent une concordance entre les efforts expérimentaux et les efforts prédits