| Titanium alloys are the most attractive materials and widely used in different industries such as aerospace, automobile and biomedical due to its favorable properties such as high strength-to-weight ratio, low density, good heat treatment capability and exceptional resistance to corrosion. However, titanium alloys are generally considered as difficult-to-machine materials due to its poor machinability. During the machining of titanium alloy with conventional tools, tool wear progresses rapidly because of the high cutting temperature and strong adhesion between the tool and the work material, owing to their low thermal conductivity and high chemical reactivity. The objective of this study is to investigate the effect of thermal and mechanical loading on the failure progression of coated carbide tools in milling of Ti-6Al-4V alloy based on experimental and simulation analysis.A micro-scale model was developed for WC-Co cemented carbide cutting tool material based on statistical analysis of the characteristics of WC grain, such as average grain diameter, major axis, minor axis, centroid coordinate and grain orientation. Three type of distribution functions (including Normal distribution, Weibull distribution and Gamma distribution) were selected to describe the random distribution of these characteristics of WC grain, and it was found that the fitting results of Gamma function has higher accuracy by comparing the Residual Sum of Square (RSS) and correlation coefficient. Two type of simplified models were established based on the current structural theory about WC-Co carbide. Simulations were conducted to investigate the properties between stochastic model and simplified models under steady-state thermal and mechanical loads.Johnson-Cook (J-C) constitutive model was employed to predict the flow behavior of Ti-6Al-4V alloy in a wide range of temperatures and strain rates based on the experimental data derived from the Automated Ball Indentation (ABI) tests. A comparative study was made on the accuracy of different J-C constitutive fitting parameters obtained from Split Hopkinson Pressure Bar (SHPB) and uniaxial tensile or compression tests. A good agreement was observed between the prediction results by our J-C model and the experimental data. Orthogonal cutting tests were conducted, and the chip formation and cutting forces under different cutting parameters were investigated. A cutting simulation model was established based on orthogonal cutting setup, which was verified by comparing the experimental results including chip morphology and cutting forces. The mechanism of serrated chip formation was investigated during machining of Ti-6A1-4V alloy by analyzing the evolution of characteristic parameters (including Mises stress. PEEQ strain, plastic dissipated energy, temperature and SDEG, etc.) within shear zone elements. A novel method was proposed to determine the fracture energy G/by comparing the cutting forces and chip morphology.Experimental investigation was conducted to analyze the failure progression of coated carbide tools during high speed milling of Ti-6A1-4V alloy. The microstructure and compositional characterization of worn tool inserts were analyzed by using SEM with X-ray spectroscopy. It was found that coating delamination and flank wear are the main failure modes that dominate tool life at the cutting speed of 100m/min, and brittle fractures on the cutting edge and rake face caused by crack propagation are identified as the main failure modes of coated carbide tools at high cutting speeds. According to the experimental results, the failure process of coated carbide cutting tools consists of several stages:coating delamination, cobalt diffusion, attrition wear, cracking and flaking. Coating delamination firstly occurred on the cutting edge at the early stage of tool failure and then extended to the rake face and flank face as cutting process proceeds. The diffusion of binder phase (Co) was observed after coating delamination and cutting speed significantly promotes the process of cobalt diffusion. Micro cracks initiated and propagated from the voids introduced by cobalt diffusion. Finally, the WC particles were pulled out and removed from the tool matrix due to cyclic thermomechanical loadings in milling process.Based on experimental analysis and developed cutting simulation model, the transient temperature and stresses in three directions on rake face, flank face and cutting edge were investigated, and the effects of thermal and mechanical loading on the stresses of cutting tool were analyzed in cutting and idle periods. Tensile mechanical stress was found on the rake face in cutting period, and the direction of tensile stress are in an agreement with the propagation direction of mechanical cracks. In addition, it was found that the cutting heat flowing into the tool through rake face is in a stable state during cutting periods. Therefore, "Equivalent Flux Method" was proposed to simulate the evolution of cutting temperature and thermal stress in macro-scale during intermittent cutting process. And the effects of cutting speed and cutting-idle ratio on temperature and thermal stress were investigated.The plastic behavior of WC-Co carbide cutting tool material was investigated based on the developed micro-scale model by defining the properties of WC and Co binder. Intense tensile thermal stress was found on the cutting tool edge and rake face in idle periods due to the different cooling rates of tool materials. The evolution of stresses and strain for WC grain and Co binder were investigated under cyclic mechanical and thermal loadings in intermittent cutting process. It was found that tensile stresses occurred after mechanical loading and compressive stresses occurred after thermal loading within Co binder due to the plastic deformation of Co binder. In addition, stress concentration was observed within the neighboring zone between WC grain and Co binder. The rapid alteration of thermal stresses was believed to be the direct reason of micro-cracks propagation on the cutting edge. |
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