Study on the Cutting Temperature of Serrated Chip while Machining Titanium Alloy TC4 Dong LIU

Transcription

1 2016 International Conference on Applied Mechanics, Mechanical and Materials Engineering (AMMME 2016) ISBN: Study on the Cutting Temperature of Serrated Chip while Machining Titanium Alloy TC4 Dong LIU College of Mechanical and Material Engineering, North China University of Technology, Beijing, , China Keywords: Serrated chip, Cutting temperature, Titanium alloy, Cutting simulation. Abstract. The machining of titanium alloys classified as difficult machining materials. It is a major problem how to improve the machining efficiency of titanium alloys. Such problems occur during the cutting process as high cutting temperature; large specific cutting force and serious tool wear, leading to low machining efficiency. The finite element method is the new method to study the machining process. The FEM and experimental method were used to simulate the cutting temperature of serrated chip while machining titanium alloy TC4. The simulated results were compared with the experimental results. The compared results indicated that the cutting temperature of serrated chip can be simulated by the simulation model well. Introduction Titanium alloys are widely used in aerospace industry due to their excellent mechanical properties. Because of their low thermal conductivity, high chemical activity, large friction coefficient and so on, such problems occur during the cutting process as high cutting temperature, large specific cutting force and serious tool wear, leading to low machining efficiency. The high temperature is the most reason which leads to the tool failure among these problems. Therefore it has vital significance to study the variation of cutting temperature while machining TC4 for monitoring cutting process, tool wear, machine fault, chatter and then control the cutting process. Generally, two types of chip formation which were continuous chip and serrated chip were observed during machining at different cutting speed. Serrated chip is also called shear-localized chip because of intense plastic deformation within a narrow band. The chip morphology significantly influences the thermo-mechanical behavior at the workpiece-tool interface, which in turn affects the tool life. In order to increase productivity and tool life in the machining of titanium alloys, it is necessary to study the cutting temperature of serrated chip while machining titanium alloy TC4 [1,2]. The FEM models are very important in the machining process comprehension and for the reduction of experimental tests necessary for the optimization of cutting conditions, tools geometries and other parameters like the choice of the tool material and coating. None of the analytical models can predict with enough precision the adequate conditions of a machining practical situation. Numerical models are interesting candidates because they might explain the observed phenomena and help in defining the optimal cutting conditions. In order to evaluate the cutting temperature in the cutting zones, several techniques have been developed over the past 70 years such as thermal couple technique, infrared photographic technique and metallographic methods. In this paper the finite element method was used to simulate the cutting process while machining titanium alloy TC4. And the cutting temperature of serrated chips were simulated analyzed.the natural thermocouple calibration device was designed on the base of literature. The device was used to calibrate the TC4 and YS8 natural thermocouple pair and obtained the variation of electromotive force with change of temperature. The device which used to measure the cutting temperature of tool-work contact area was set up and the cutting temperature while machining TC4 was studied [3-,8].

2 Model of Simulation and Experimental Set up Material Properties The Young s modulus of titanium alloy TC4, heat conduction coefficient and heat capacity are the function of temperatures. The Poisson s ration was The Young s modulus, thermal conductivity and heat capacity varies with temperature were listed as Table 1 and Table 2. Table 1. Young S modulus, varies with temperature. Temperature ( C) Young s modulus (GMa) Table 2. Thermal conductivity and heat capacity Temperature ( C ) Heat conductivity J/m s C Heat capacity J/g C The cutting tool was defined as rigid body in the simulation and only the temperature was calculated in the simulation. Only the heat conductivity and heat capacity were need to define in the simulation model. Tool material was YS8 carbide cutting tools, its thermal conductivity coefficient: J / m s C; heat capacity value: J / g C. The carbide inserts YS8 with rake angle 20, clearance angle 7, edge roundness radius 0.05mm was used for the simulation model and orthogonal machining test. The cutting speeds were: 30m/min, 48m/min, 76m/min, 97m/min. Experimental Set up According to the interspace conductor law of the thermoelectricity, if the junction of two metals is at a uniform temperature, the emf generated is not affected by a third metal, which is at the same temperature, used to make the junction between the first two. This conclusion is tenable regardless of this conductor is inserts among a hot electrode or strings together between two hot electrodes. Therefore put this conductor between two hot electrodes and heat up it evenly, the heating temperature of the conductor can be obtained according to the output thermoelectric forces of two hot electrodes which as the Figure 1 shows. The calibration instrument schematic drawing was shown in Figure 2. The heating membrane was quickly heated by the low voltage big current transformer and the temperature of membrane increases to about one thousand degrees in several seconds. A small hole located front of the insert which diameter was 1mm was drilled using the EDM method. The standard type S thermocouple and nickel-chrome which insulated each other were embedded in the hole using the temperature-resistant glue. The junction of the frontier standard thermocouple and nickel-chrome material was formed as a heating point. The type S standard thermocouple was connected to the thermocouple amplifier 1. The type K standard thermocouple which used to compensate the cold point temperature rise was fixed at the other corner of the insert and was connected to the amplifier 2. The nickel-chrome and the wire joint from the type K thermocouple was connected to the thermocouple amplifier 2 and composed a nickel-tool natural thermocouple pair.

3 Figure 1. Principle of calibration instrument. Figure 2. Schematic illustration of calibration system. Simulation and Experimental Results In order to compare the experimental results with the simulation results, the temperature of the element in the contact area of the chip is calculated. The element node can be regarded as the power supply in parallel with a plurality of different potential due to the hot junction unit at different temperature in the calculation of the average temperature, the equivalent electromotive force according to the Thevenin's theorem for: u r u = r + u r + r (1) As the u was the parallel power equivalent electromotive force; u 1 was the equivalent electromotive force of first power supply; u 2 was the equivalent electromotive force of second power. r is the first power resistance; 1 r2 was the resistance for the power of second resistance. As shown in Figure 3. the point P1, P2, P3 and P4 as the hot junction. Suppose that the four node of the heat resistance is the same, the equivalent thermal electromotive force can be expressed as: u + u + u + u u = The equivalent temperature of the hot junction is: (2)

4 T T + T + T + T = (3) Figure 3. Average temperature of tool chip contact area. The Figure 4 was the comparison of simulation and experimental results for the average temperature of the tool chip contact area. It can be seen from the Figure 4 that both the simulation and the experimental results of average temperature of the chip was increased with the increase of the cutting speed. The error of the simulation value and the experimental value at low speed cutting error is about 10%, and the error is about 18% in high speed cutting. The reason of the error of the cutting temperature simulation and the experimental result is mainly: the System error caused by finite element simulation model; Tool workpiece natural thermocouple calibration error; Measurement error in experiment experimental results simulation results Cutting temperature Cutting speeds /m/min Figure 4. Comparison of simulation and experiment. Conclusions The FEM and experimental method were used to simulate the cutting temperature of serrated chip while machining titanium alloy TC4. The simulated results were compared with the experimental results. The compared results indicated that the cutting temperature of serrated chip can be simulated by the simulation model well. The natural thermocouple calibration device was designed and the device was used to calibrate the TC4 and YS8 natural thermocouple pair and obtained the variation of electromotive force with change of temperature. The simulation and the experimental results of average temperature of the chip was increased with the increase of the cutting speed. The error of the simulation value and the

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