Research On The Method And Mechanism Of Pulsed Magnetic Treatment Of Micro-tools

Micro cutting tool is the main component of micro cutting technology. In micro cutting, the tool deformation, friction and wear, the stress field, temperature field, had occurred in a small scale. It puts forward higher requirements for cutting edge strength. So, the micro tools must be strengthened. Based on the problem and difficulty in strengthening treatment of micro tool, propose using pulse magnetic field to strengthen the micro tool. In this study, based on scientific theories and methods including knowledge of material, electromagnetism, metallography, dislocation theory and theory of metal cutting, the pulsed magnetic treatment of micro cutting tool and its mechanism are investigated in many aspects. The obtained outcomes lay the foundation on the application of pulsed magnetic treatment technology.The main research contents are listed as follows:1. Influences of pulsed magnetic treatment on the material microstructure and mechanical properties are clarified. The changes of carbide, martensite and residual austenite structure of high speed steel material before and after pulsed magnetic treatment are analysed using metallographic in situ observation method. Results show that after pulsed magnetic treatment the carbide increase significantly, the residual austenite is lower and the martensite structure and crystalline grain are refined. The carbide of high speed steel presents precipitation, fragmentation and deformation three types of change. Based on the martensitic phase transformation kinetics, the magnetic stress may induce residual austenite transform to martensite in the form of fcc-hcp(γâ†'ε). Magnetic field increases the free energy difference between austenite phase and martensite phase, increases the phase transformation driving force. The changes of microstructure led to changes of mechanical properties, of which the maximum increase of Rockwell hardness of high speed steel is 2.9 HRC, the maximum increase of microhardness of carbide is 66 HV. The red hardness of high speed steel can be increased at low temperature.2. The micro tool material strengthening mechanism of pulse magnetic treatment is investigated. The changes of micro tool material dislocation density and dislocation configuration before and after pulsed magnetic treatment are analysed by the way of XRD and TEM. Results reveal that the dislocation density of high speed steel material increases by 24% after pulsed magnetic treatment. The dislocation multiplication and tangles and form the dislocation cell structure. The dislocation motion driving force and resistance under pulsed magnetic treatment are analyzed, and the material flow stress model is established. Results reveal that the micro tool material strengthening mechanism of pulsed magnetic treatment are the results of joint action of dislocation strengthening, dispersion strengthening and fine-grain strengthening.3. Effect of pulsed magnetic treatment on the cutting performance of micro milling cutter is studied. A special micro milling cutter for cutting and friction is designed and made by CNS7 d micro cutting tool CNC grinding machine. The changes of tool wear, milling force and surface quality are analysed. Three different magnetic properties materials of ferromagnetic, paramagnetic and diamagnetic are selected as the workpiece material. Results reveal that there are two reasons for micro tool strengthening in the process of pulsed magnetic treatment. The first one is the change of mechanical properties of tool materials, the second one is the effect of cutting tool magnetic on the contact status of tool, chip and workpiece. Micro cutting tool and workpiece can form a closed magnetic circuit when milling the ferromagnetic material. Three body friction state is formed among tool, chip and workpiece under the effect of magnetic adsorption. The tool wear is decreased and the surface quality is improved. The reinforcement is both work together. When milling the paramagnetic material, reinforcement is mainly the change of the cutting tool material mechanical properties. But when milling the diamagnetic material, disturbances are generated in the process of cutting as the result of tool magnetic circuit and workpiece magnetic circuit in the opposite direction. So the tool wear is increaseed and the surface quality is decreased.4. The optimized strengthening process of micro cutting tool pulsed magnetic treatment is investigated. The effect factors of micro tool pulsed magnetic strengthening treatment are analyzed, and the process parameters selection principle of micro tool pulsed magnetic strengthening treatment are determined. The effects of cutting speed, cutting depth and feed rate on the micro tool pulsed magnetic strengthening treatment are studied using orthogonal experiments. Results show that feed rate is the most significant factor of surface roughness in micro tool pulsed magnetic strengthening treatment technology, and cutting depth is the most significant factor of machining surface burr. When using the optimized process parameters, the micro tool wear can decreases by 30% and the surface roughness increases by 8% after pulsed magnetic strengthening treatment.