Research On Diamond Cutting Of Ferrous Metals Assisted By Cold Plasma

The lack of an effective way to inhibit tool wear has been a bottleneck that restricts further progress of ultraprecision diamond cutting technology of ferrous metals.On the basis of the characteristic that active particles in cold plasma can combine with iron atoms more easily and rapidly than carbon atoms do,and its excellent cooling-lubricating effect,this thesis proposes to real-time reduce the chemical affinity on the tool-workpiece interface and improve the complex thermal action in the cutting zone,thereby restraining the diamond tool wear,stabilizing the machining process and improving the processing quality.After analyzing the mechanisms of diamond tool wear and its restraining methods,this thesis firstly develops low-temperature atmospheric flexible cold plasma jets that are easy to be transported.Then,the mechanisms of cold plasma jet assisted cutting are investigated by using theoretical and experimental methods like molecular dynamics,thermal analysis and friction and wear test.Finally,fundamental cutting experiments are conducted.The main research works of this thesis can be summarized as follows:Low-temperature atmospheric cold plasma jets that are easy to be transported to tool-workpiece interface are developed in order to restrain diamond tool wear by reducing the chemical affinity on the tool-workpiece interface and adjusting the complex thermal condition in the cutting zone.Under atmospheric pressure,nitrogen is more appropriate for generating cold plasma jet induced by bare electrode discharge.It can be found through experiments that power frequency influences volt-ampere characteristic of nitrogen discharge obviously,while gas flow has little influences on the volt-ampere characteristic.Effective transport of plasma can be realized by adopting flexible plasma jet,and length of the flexible plasma jet increases with driving voltage and gas flow.The flexible plasma jet has similar volt-ampere characteristic with the common atmospheric cold plasma jet,indicating that the flexible jet does not affect the discharge process.Pre-cooling the gas by vortex tube can lower temperature of plasma jet.The temperature of plasma jet rises with driving voltage,and increases at first then decreases subsequently along the propagation direction.When the driving voltage is 3.0 kV and the nitrogen flow rate is 50 L/min,the temperature of flexible plasma jet is about-7 °C.In order to confirm that nitrogen plasma can reduce the chemical affinity between ferrous metal and diamond,molecular dynamics simulation and thermal analysis experiments are conducted.According to the simulation results,the graphitization temperature of diamond is originally from 1200 °C to 1700 °C,but decreases to approximately 773 °C due to the catalytic effect of iron atoms.The thermal analysis experiments indicate that the graphitization temperature of NAK80/diamond interface without nitrogen cold plasma jet(NCPJ)treatment is 882℃,while the samples treated by NCPJ do not show obvious graphitization until the temperature is 1185℃.Therefore,iron atoms decrease the graphitization temperature of diamond,whereas the graphitization temperature of ferrous metals/diamond interface is increased after NCPJ treatment.The results demonstrate that a lot of active particles are absorbed on the surface and crystal boundary of NAK80,and react with ferrous metals to generate nitride compounds that have lower chemical affinity with diamond,thus restraining graphitization wear of diamond.Friction and wear tests of NAK80/diamond friction pair are separately performed under air,nitrogen and NCPJ atmospheres to ensure that cold plasma can enhance thermal conductivity of the cutting area.The results indicate that the isolation of oxygen under nitrogen and NPCJ atmospheres can alleviate oxidation wear of friction pairs,and that NPCJ atmosphere can reduce adhesive wear of friction pairs.The performance of NAK80/diamond friction pair is the best under NPCJ atmosphere.When the rotation rate is relatively lower,NPCJ cannot obviously decrease the wear of friction pairs.By contrast,when the rotation rate is relatively higher,the friction coefficient of NAK80/diamond pair under NPCJ atmosphere is lower than that under air atmosphere.The interaction between NPCJ and solid surfaces does not damage the microstructures of surfaces,and the wettability change of solid surfaces induced by NPCJ can recover after cutting.To analyze characteristics of ultrasonic elliptical vibration cutting,ultrasonic elliptical vibration device that is capable of controlling vibration characteristics is developed,and molecular dynamics simulations and cutting experiments are conducted.Cutting tool performs high-frequency vibration along the elliptical track driven by the ultrasonic vibration device while simultaneously moving along the feed direction.The combination of the two motions results in cutting tools performing periodic separating cutting.After traditional cutting,stripe-like structures form on workpiece surfaces due to ploughing effect of cutting tools.When ultrasonic elliptical vibration is added,there are "fish scale-like" vibration lines on the stripe-like structures,and the lower the cutting speed is,the denser the "fish scale-like" vibration lines are.Both cold plasma assisted cutting and ultrasonic elliptical vibration assisted cutting can obviously improve surface quality of workpieces,and surface roughness of workpieces is the lowest when the two methods are combined.Ultrasonic elliptical vibration can increase residual stress of workpiece surfaces,while cold plasma can reduce the residual stress.Fundamental cutting experiments under different atmospheres are conducted to confirm that cold plasma can alleviate tool wear when diamond tool is cutting ferrous metals.Tool wears of traditional cutting and ultrasonic elliptical vibration assisted cutting are mainly tool-edge wears.Real-time adjustment of cold plasma can significantly restrain diamond tool wear,but causes slight rake face wear.The combination of cold plasma and ultrasonic elliptical vibration can restrain diamond tool wear most obviously,but leads to slight adhesive wear of tool edge.Compared with traditional diamond tool cutting ferrous metals,cold plasma and ultrasonic elliptical vibration-assisted cutting can improve effective cutting distance by 10 times.In addition,cold plasma and ultrasonic elliptical vibration-assisted cutting can reduce diffusion wear of carbon element in diamond tool to workpieces and chips by 68.49%and 70.37%,respectively.