Machinability of titanium metal matrix composites (Ti-MMCs)

RESUME Les composites a matrices metalliques de titane (Ti-MMCs), en tant que nouvelle generation de materiaux, ont plusieurs applications potentielles dans les domaines industriels de l’aerospatiale, et l’automobile. La presence de particules de ceramique ameliore les proprietes mecaniques et physiques de la matrice. Cependant, ces particules sont dures et abrasives par leur nature, ce qui cause de multiples difficultes pour leur usinabilite. L’usure severe des outils ainsi que la vie utile tres breve de ceux-ci representent les inconvenients les plus importants de leur usinabilite. La litterature indique tres peu d’etudes concernant leur usinabilite, en particulier pour l’estimation de la duree de vie utile et de l’usure des outils de coupe. Les outils de diamant polycristallin (PCD) semblent etre le meilleur choix pour usiner les MMCs du point de vue des chercheurs. Malgre ceci, en raison de leur cout eleve, des alternatives abordables sont fort souhaitables. Les plaquettes de nitrure de bore cubique (CBN) demeurent les outils les plus durs apres celles de PCD, demontrent d’excellentes qualites dont une grande resistance a l’usure, une bonne durete a temperature elevee, un bas coefficient de friction et une temperature de fusion elevee. Toutefois, les outils de CBN n’ont pas encore ete etudies dans le contexte de l’usinage des Ti-MMCs. La presente etude elabore en profondeur les mecanismes d’usure des plaquettes de CBN durant le tournage des Ti-MMCs. La morphologie unique des faces usees des outils a ete etudiee pour la premiere fois, menant a une nouvelle comprehension dans l’identification des mecanismes d’usure chimiques durant l’usinage des Ti-MMCs. De plus, la pleine exploitation de la vie utile des outils de coupe est critique en raison des depenses importantes liees au remplacement non-optimal des outils de coupe. Ceci encourage fortement le developpement d’une modele fiable pour l’estimation de la vie utile pour toutes conditions de coupe. Cette etude elabore une nouvelle methode decoulant de la methodologie d’analyse de survie (survival analysis) pour estimer les etats d’usures successives des outils pour toutes conditions d’usinage des Ti-MMCs. Ce modele statistique prend en compte la duree de l’usinage en plus des effets des parametres de coupe. De cette maniere, les resultats obtenus ont demontre un excellent accord avec les resultats experimentaux. De surcroit, un modele plus avance a ete developpe en rajoutant l’usure des outils comme variable au modele precedent. Il en sort un nouveau modele propose pour estimer la duree de vie utile restante des plaquettes usees pour des conditions d’usinage variables, en incluant l’usure actuelle des outils dans les donnees d’entree. Les resultats de ce modele ont ete valides par experimentation et le modele concorde tres bien avec les essais experimentaux. ----------ABSTRACT Titanium metal matrix composites (Ti-MMCs), as a new generation of materials, have various potential applications in aerospace and automotive industries. The presence of ceramic particles enhances the physical and mechanical properties of the alloy matrix. However, the hard and abrasive nature of these particles causes various issues in the field of their machinability. Severe tool wear and short tool life are the most important drawbacks of machining this class of materials. There is very limited work in the literature regarding the machinability of this class of materials especially in the area of tool life estimation and tool wear. By far, polycrystalline diamond (PCD) tools appear to be the best choice for machining MMCs from researchers’ point of view. However, due to their high cost, economical alternatives are sought. Cubic boron nitride (CBN) inserts, as the second hardest available tools, show superior characteristics such as great wear resistance, high hardness at elevated temperatures, a low coefficient of friction and a high melting point. Yet, so far CBN tools have not been studied during machining of Ti-MMCs. In this study, a comprehensive study has been performed to explore the tool wear mechanisms of CBN inserts during turning of Ti-MMCs. The unique morphology of the worn faces of the tools was investigated for the first time, which led to new insights in the identification of chemical wear mechanisms during machining of Ti-MMCs. Utilizing the full tool life capacity of cutting tools is also very crucial, due to the considerable costs associated with suboptimal replacement of tools. This strongly motivates development of a reliable model for tool life estimation under any cutting conditions. In this study, a novel model based on the survival analysis methodology is developed to estimate the progressive states of tool wear under any cutting conditions during machining of Ti-MMCs. This statistical model takes into account the machining time in addition to the effect of cutting parameters. Thus, promising results were obtained which showed a very good agreement with the experimental results. Moreover, a more advanced model was constructed, by adding the tool wear as another variable to the previous model. Therefore, a new model was proposed for estimating the remaining life of worn inserts under different cutting conditions, using the current tool wear data as an input. The results of this model were validated with the experimental results. The estimated results were well consistent with the results obtained from the experiments.