| Titanium alloy Ti6Al4V is widely used in aircraft industry, marine and commercial applications due to its high specific strength and excellent corrosion resistance. However, Ti6Al4V is a typical difficult-to-machine material due to its poor thermal conductivity, low elastic modulus and high chemical activation. High cutting temperature during Ti6Al4V machining usually leads to fast tool wear and poor surface quality. On the other hand, most materials need to be removed from roughcast due to the design characteristics of components used in the aircraft industry. Therefore, the contradiction between increasing machining demand and poor machining performance has been one of the bottlenecks, which blocks the development of aerospace industry. In this study, the carbide tool wear mechanism during high speed milling titanium alloys Ti6Al4V was systematically researched. The cutting temperature and sawtooth chip were taken as the main influence factors of tool wear. The transition maps from sawtooth chip to continuum chip were built to guide the selections of cutting parameters. The material constitutive and failure models, friction model and heat transfer model were built to predict the maximal cutting temperature based on finite element method (FEM). The tool life in the different cutting conditions was experimentally obtained. The prediction models of tool life were built based on the exponential and quadratic equations, respectively. Based on the quadratic model of tool life, the milling parameters were optimized, taking the maximal tool life in certain material removal rate as the optimization target.The fast tool wear during Ti6Al4V cutting influences the machining efficiency and machining cost greatly. The wear mechanism of carbide tool in milling titanium alloy Ti6Al4V under the different cooling/lubrication conditions was experimentally studied by means of scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The wear mechanisms mainly include: adhesive wear, oxidation wear, abrasive wear, diffusion wear, tool fracture, tool plastic deformation,etc. The wear mechanisms interact each other. The diffusion between adhesive layer and cutting tool aggravates the adhesive wear. Ti6Al4V adhesive layer is oxidated and the oxide with high hardness supplies the abrasive particle in the abrasive wear. The thermal and mechanical cracks propagation results in the fracture of cutting tool surface. The plastic deformation of cutting tool occurs due to the oxidation of cutting tool materials. The saw-tooth chip generated in the milling process causes the cutting forces fluctuation with high frequency, which resultes in the fatigue of Ti6Al4V adhesive layer and tool materials. Cutting fluid erodes the surface of cutting tool and leaded to the thermal impact, which aggravates the fatigue of Ti6Al4V adhesive layer and tool materials. So three measurements are taken to control the tool wear: the control of sawtooth chip formation; the control of cutting temperature and the selection of reasonable cooling/lubrication methods.The formation machenisms of sawtooth chip mainly include cyclic crack theory and adiabatic shear theory. A new theory based on the dynamic cracks propagation was proposed to explain the formation of saw tooth chip. In this theory, the cracks initialize in the unmachined surface and expand towads the cutting tool point. The shearing slip aggravates and subsequently the temperature in the shearing zone increases. The failure strain increases with the increase of cutting temperature. The cracks propagation is retrained due to the increase of failure strain and finally stops. A new sawtooth chip begins to form after the formed sawtooth chip. The formation of sawtooth chip depends on the cutting speed, feed rate and cooling/lubrication methods. It found the chip was easy to be contimuun at the cutting speed 50-100m/min. The sawtooth chip was easy to form at higher or lower cutting speed. In the same cutting parameters, the sawtooth level was more obvious in the wet cutting than it in the dry cutting. The rake angle of cutting tool influences the formation of sawtooth chip greatly. Smaller the rake angle is, easilier the sawtooth chip forms. Based on the investigation of chip morphology from orthogonal turning experiments, the transition maps from sawtooth chip to continuum chip in the dry and wet cutting with rake angle 0°, 5°, 10°were built to guide the selections of cutting parameters.Cutting fluid and minimal quantity lubrication were selected to apply in Ti6Al4V machining. MQL method uses the cutting fluid of only a small amount, typically less than 50ml/h, which is typically jetted into the cutting zone with a flow of compressed cold-air. The oil amount and cold air temperature were both adjusted. The friction and heat transfer models under the different cooling /lubrication conditions were built by means of orthogonal milling experiments and heat convention experiments. The configuration of MQL nozzles and the oil amount were optimized based on the friction and heat transfer models. Split Hopkinson pressure bar (SHPB) test was employed to obtain the flow stress in the different strain, strain rate and temperature. The material constitutive model was built based on the power-law equation. The failure strains in the different stress triaxiality and temperature were measured by the torsion, tensile and compression tests. A material failure model which considered the effect of Rebinder effect, stress triaxiality and temperature on the material failure strain was built. The orthogonal turning experiments were employed to modify the built material failure model. The formation of saw tooth chip was simulated by the aid of FEM software AdvantEdge. The simulated cutting forces and temperature were compared with the experimental results and show good agreement with the experimental results.The milling processes were reasonably simplified to two dimension cutting process. The edge structure was accurately modeled in the finite element model to simulate the milling process with small uncut chip thickness. The simulated cutting forces showed good agreement with the experimental results, which validated the accuracy of finite element model. The simulated chip morphology, cutting forces, maximal cutting temperature and thermal stress under the different cooling/ lubrication conditions were extracted. It found that the sawtooth extent in wet cutting was higher than the sawtooth extent in dry and MQL cutting. The maxiamal cutting temperature in MQL cutting was almost as same as it in wet cutting. The cutting tool was cooled rapidly in the idle stroke, so the temperature gradient and thermal stress were higher in the wet cutting than in the dry and MQL cutting. The sawtooth chip leaded to the mechanical impact and temperature gradient leaded to thermal impact, which influenced the tool life greatly.The tool life tests in dry and MQL milling were performed and the tool life in the different cutting conditions was obtained based on ISO standards for too life. The exponential and quadratic equations were employed to get the predition model of tool life and analyze the effect of cutting parameters on tool life. Based on the quadratic model of tool life, the milling parameters were optimized, taking the maximal tool life in certain material removal rate as the optimization target. The effects of tool wear on the surface roughness, sawtooth chip and milling forces were analyzed. It found the tool wear had little effect on the surface roughness in the beginning of tool wear, but the surface roughness increased rapidly when tool wear reached some extent. It found the sawtooth trend increased with the increase of tool wear. The chip formation increased. The distortion even fracture of chip occurs, which increase the formation energy and cutting temperature. It found the catual rake angle of cutting tool decreased with the tool wear. So the milling forces follow the feed direction increased; the milling force direction perpendicular to the feed direction changed; the axial milling forces had little change.
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