Study On Electrochemical Micromachining Technology With Assisted Electrode Using Bipolar Pulses

Cemented carbide is a composite material formed by sintering hard compound and metal binder powder through powder metallurgy process.Due to its excellent mechanical and physical properties such as high elastic modulus,high hardness,high thermal conductivity,impact resistance and wear resistance,it has been widely used in microelectronics,automobile manufacturing,biomedicine and other industries.However,these properties pose difficulties in micro machining of the extremely hard material.Electrochemical micromachining is a machining method with a wide range of machining,no tool wear and no mechanical stress,but electrochemical machining of tungsten carbide usually needs to use strong acid or alkali electrolyte,which brings serious environmental safety problems.In recent years,electrochemical machining(ECM)of tungsten carbide using bipolar pulses with a neutral electrolyte has been successfully realized,but tool wear always occurs during machining in the period of negative pulses,which loses an important advantage of ECM.To solve the problem of tool wear in electrochemical machining of tungsten carbide with bipolar pulses,this thesis proposes a method of ECM with sacrificial auxiliary electrode using bipolar pulses voltage.During the period of positive or negative pulses,the tool electrode or auxiliary electrode form an electrochemical machining circuit with the workpiece respectively,so as to realize electrochemical machining of the microstructure(micro-holes and micro-grooves,etc.)without tool wear on the surface of tungsten carbide.The main contents of this thesis include:(1)Based on the research status of cemented carbide machining at home and abroad,a method of electrochemical machining with sacrificial electrode using bipolar pulses is proposed.Bipolar pulses voltage is introduced into the electrochemical machining system composed of tool electrode,workpiece electrode,auxiliary electrode and neutral electrolyte.During the period of positive pulses,the workpiece anode and the tool cathode form a circuit for ECM,which dissolve the workpiece and form a passive film(WO₃)on the surface.During the period of negative pulses,the workpiece cathode and the auxiliary anode form a circuit for ECM,which makes the surface of the workpiece generate OH^-to dissolve a passive film(WO₃),and achieving further dissolution of the workpiece,while the tool electrode has not corrosion.The microstructure can be machined by continuously applying bipolar pulses and controlling the relative movement between the workpiece and the tool.(2)The polarization curve of YG8 cemented carbide was measured by electrochemical workstation,and the dissolution behavior of the material in neutral electrolyte was understanded.A two-dimensional axisymmetric model of machining gap composed of electric field and gas-liquid two-phase flow field was established by COMSOL Multiphysics.During the period of negative pulses,the model was used to simulate and analyze the current density,gas volume fraction and flow velocity in the ECM area.The results show that the bubbles generated on the workpiece surface and bring hydrodynamic flow,which washes the workpiece surface and help to promote the smooth removal of electrolytic products.(3)According to the principle of ECM of tungsten carbide with auxiliary electrode using bipolar pulses,based on FPGA control chip and the use of multi-channel high-speed switches composed of gallium nitride(Ga N)transistors,the method of Direct-Current(DC)voltage chopping was adopted.A bipolar pulse power supply for ECM was developed,which enables the voltage,frequency and duty cycle can be adjusted arbitrarily.(4)The ECM process of YG8 cemented carbide was studied by using the developed bipolar pulse power supply.Under the condition of neutral electrolyte,compared with experiments of ECM using unipolar pulses and ECM with assisted electrode using bipolar pulses,the experimental results showed that micro-holes can only be machined in ECM using bipolar pulses without tool wear,while there was basically no machining using unipolar pulses.It has confirmed the correctness of the methods mentioned by the thesis,which also proves the effectiveness of the development of the power supply.On this basis,the effects of negative pulse voltage,pulse frequency and pulse duty cycle on the machined micro holes were studied,the results showed that increasing the negative pulse voltage is beneficial to improve the hydrogen evolution reaction speed and obtain better micro-holes morphology;increasing the pulse frequency and decreasing the pulse duty cycle can reduce the stray corrosion on the surface of micro-holes and obtain relatively good micro-holes morphology.Finally,using tungsten wire with the diameter of 110?m,and the method of ECM with auxiliary electrode using bipolar pulses was used,micro-groove with a width and depth of 180?m and 48?m was machined on the surface of YG8 cemented carbide.