Thermodynamical Characteristics Of Tool Wear For Difficult-to-cut Materials

High speed cutting technology has been widely used in industry for its outstanding advantages. However, severe tool-work friction and plastic deformation result in huge heat generation and early tool failure. Short tool life in high speed cutting difficult-to-cut materials, such as titanium alloys and super alloys, is one of the most difficult problems for its further impliation. Frequent tool exchange decreases machining efficiency and cutting tool consumption increases cost. Most phenomena in cutting system relates with thermal or thermodynamic theory. In this research, tool wear in high speed cutting titanium alloy and super alloys have been studied based on thermodynamical theory.Thermodynamic system in high speed cutting process was developed based on non-equilibrium thermodynamic theory. Entropy generation and entropy flow were analyzed in this system. The coupling between the microscopic sub-processes of the thermodynamic system were studied. The results showed that mutual effects between different sub-progresses did exit through structure and element changes, while there is no coupling between those progresses with different tensor orders according to curie principle. The interaction between the chip tool and the workpiece were analyzed, the results showed that complicated nonlinear interaction is the remarkebale characteristic of high speed machining thermodynamic system which is different with the general thermodynamic systems whose various factors meet the Curie principle, that is different thermodynamic order tensor coupling does not exist between the factors.Orthogonal cutting simulation model was built developed with finite element analysis software DEFORM-3D. The material's flow stress behavior was described with thermodynamical constitutive equation. Separation of chips from workpiece was realized by the combination of adaptive remeshing technique and separation criterion with tool as a non-rigid solid. The material's fracture was defined by adopting Cockcroft and Coulomb friction model and shear friction model. To validate the finite element model, the orthogonal cutting tests were conducted. With this simulation model stresses, strains distribution in the chips and the workpiece can be predicted as well as cutting temperature and cutting force.Diffusion and solubility mechanisms of cemented carbide tools were studied and calculations were conducted based on thermodynamics principles. Reverse method was firstly used to calculate the excess free energy of one element dissolving into another element with binary phase diagram. For instance, with this method, the excess free energy of carbon dissolving into aluminum was calculated. Results showed that diffusion wear resistance ability of WC-based cemented carbide tool was poorer than those cemented carbide tool with more TiC,TaC and NbC when high speed cutting iron-based alloy. This analysis was proved with experiments during high speed cuting titanium alloy (Ti6A14V) and super alloy (GH907) with cemented carbide tools.The oxidation wear of cutting tool during high speed machining were studied with Gibbs free energy criterion. Oxidation products of carbide tool were calculated based on thermodynamics during high speed machining. The possible oxidation reactions for cemented carbide tool elements at different temperature were calculated with substances Gibbs free energy function method. And the orders of reaction was sorted with reaction degree as well as characteristics of every reaction were analyzed. Theoretical calculation showed that the quantities of reaction products of WO3,CO3O4 and COWO4 were large, especially,the middle reaction product Co3O4 will continue to react with WO and O2 to generate the product CoWO4. Experimental results proved the theoretical calculation. Furthermore, the oxidation products of PCBN and LT55 ceramic tools were also calculated and their wear characteristics were analyzed.The thermodynamical model of tool adhesion wear was developed. The adhesion wear was studied by turning experiments and investigated with white light interferometer, infrared imaging, and SEM-EDS and so on. Adhesion wear patterns,, wear mechanism were studied in cutting GH907 and TC4. Adhesive wear was analyzed in different velocity and cutting time. Results showed that the tool adhesion wear was more serious when cutting speed was 200m/min, while the adhesion wear was significantly reduced when cutting speed was 300m/mindue to thermal softening effect.The principle of minimum entropy production in cutting thermodynamics system was proved based on minimum energy consumption under the condition of one assumption and three constraints. The entropy generation model of shear zone and tool chip contact zone were deduced and the effects of cutting parameters on entropy generation were analyzed and proved by experiments. The results indicated that there was no minimum value of entropy generation because generalized flow and generalized force of cutting process did not satisfy the linear relationship as the complexity of cutting process and the diversity of the factors. So, the mechanisms and theoretical exploration of the machining friction thermodynamics system need to be further explored.This disseratation is supported by National Basic Research Program of China (2009CB724401),Natural Science Foundation of China (50705052),Doctorate Fund of the Ministry of Education project funding of new teachers(20070422032) and Major Science and Technology Program for High-End CNC Machine Tools and Basic Manufacturing Equipment(2009ZX04014-043,2009ZX04014-012 2009ZX04014-031).