Research On The Method And Mechanism Of Cathodic Plasma-assisted Electrochemical Micromachining

Electrochemical micromachining(micro-ECM)has essential applications in fabricating high-surface quality microstructures in difficult-to-process materials.Its unique advantages include no tool wear,no surface damage,and no contact,etc.Meanwhile,further development of micro-ECM technology meets challenges in avoiding lateral stray corrosion,improving material removal rate,enhancing mass transfer in the small inter-electrode gap,and achieving processing of chemically inert materials.To solve the problems presented in micro-ECM,a cathodic plasma-assisted micro-ECM method was proposed in this research,which introduced cathodic plasma into the conventional micro-ECM to realize the coupling of plasma and electrochemical reaction.Investigations were conducted on the plasma generation mechanism,bubble and plasma behavior,and the interaction between electrical parameters and plasma physicochemical properties.Further,the optimal coupling method and machining characteristics under the application of cathodic plasma are studied in-depth to reveal the principle and mechanisms of cathodic plasma-assisted micro-ECM.Direct observation of the machining area by a high-speed camera and infrared camera shows that the gas film,discrete air bubbles,and electrolytes are arranged in sequence around the electrode.The gas film is broken down under high electric field strength,forming a discharge plasma.The light emitted by the discharge irradiates the bubbles around the tool electrode,creating a plasma-illuminated area.The thickness of the discharge plasma layer is about 0.1 mm,but the plasma-illuminated area reaches > 1 mm.The power pulse signal regulates the discharge plasma behavior,and the plasma cannot be ignited when the pulse width is < 3 μs.The pulse amplitude affects the discharge plasma energy.The larger the amplitude,the higher the discharge energy.However,when the applied voltage amplitude is too large,the discharge mechanism transforms into spark discharge by directly breaking the gap between electrodes.The pulse width affects the plasma energy and stability.As the pulse width increases,the plasma intensity and diameter increase.The cooling effect of cathodic discharge can be adjusted by pulse duty ratio,and a high pulse duty ratio leads to sufficient ionization.Meanwhile,increasing the pulse frequency can interrupt the discharge process and obtain a stable and mild discharge plasma,which improves the machining accuracy and stability.The gas/plasma film induced on the tool electrode surface has the characteristics of a sidewall insulation effect,which can ensure the machining accuracy of microECM at high machining voltages.Moreover,the induced dynamic bubble flow and plasma expansion can promote the removal of electrolytic products in the small machining gap,achieving high machined surface quality even without flushing.In addition,the energy generated by the discharge plasma can increase the electrolyte temperature,thus increasing the conductivity of the electrolyte and activating the workpiece surface to realize in-situ thermal promoted ECM.The cathodic plasma can be controlled by applied pulse waveform to optimize the synergistic coupling effect of plasma-electrolysis.In this research,a bipolar pulse waveform is applied to cathodic plasma-assisted ECM,which generates plasma in negative pulse to promote the anodic dissolution in the subsequent positive pulse.Based on this method,a micro-rod with a diameter of 18 μm and an aspect ratio of55:1 was successfully fabricated within 5 s from the initial diameter of 200 μm,achieving a maximum machining efficiency of 36.4 μm/s.Further,a high-and lowvoltage hybrid waveform is designed to realize cathodic plasma-assisted micro-ECM.Plasma and bubbles induced by the high voltage pulse form a self-excited flow field,efficiently removing the electrolytic products and ensuring the smoothness of the machined surface.In addition,the gas/plasma film of high resistance forms a sidewall insulation effect on the tool electrode surface,shielding stray currents.Finally,the structured micro-rod and the microstructure can be fabricated.On the other hand,it is verified that the cathodic electrolytic plasma can achieve the machining of Si C single crystal in an alkaline solution,implying the plasma’s strong oxidation capacity due to its high temperature and many free radical ions.The generated oxides are etched in the high-temperature alkaline solution caused by plasma.A micro-hole with a diameter of 220 μm and depth of 350 μm is fabricated on the Si C surface.Further,the cathodic plasma assisted-etching method effectively fabricates nanoporous structured Si C by scanning the cathode tool.Adjustment of nanoporous structure is realized by controlling the plasma intensity and the tool scanning speed.The achievable maximum etching rate and etching depth are 540nm/min and 10 μm,respectively.The cathodic gas and plasma formation can occupy the entire machining gap,causing a severe lack of electrolytes for ECM.To solve this problem,a tube electrode enabling internal supplying,i.e.,flushing,of electrolytes to the machining area is applied.The plasma ignition principle and control method under tube electrode conditions is analyzed.The violently generated plasma gas on the end surface of the electrode can isolate the electrode from contacting the electrolyte,confining the current distribution inside the electrolyte jet and realizing a process environment similar to electrochemical jet machining in air.Based on this method,microstructures with high surface quality can be fabricated.With tool electrode feeding,a deep microhole can be machined with an inlet diameter of 980 μm,outlet diameter of 750 μm,and a depth-to-diameter ratio of 5.1:1.Meanwhile,no recast layer presents on the hole sidewall.