| Electrochemical discharge machining(ECDM)is a non-traditional micromachining technique based on the electrochemical discharge phenomenon around the tool electrode.A conductive electrolyte solution is used as the working fluid and a non-conductive medium-gas film is generated around the tool electrode during the machining process.The presence of conductive solution and gas film allows electrochemical discharge to occur between the tool electrode and the solution.As the occurrence of electrochemical discharge is independent of the workpiece,the application of ECDM is not limited by the conductivity of the workpiece.It can be applied to machining of both nonconductive materials and conductive material,which implies that it is a micromachining technology with broad application prospects.In order to promote the development and application of this technology,it is necessary to systematically study its machining mechanism and processing technology.Firstly,the experimental observation and analysis of gas film formation and discharge in ECDM were carried out.On the experimental platform,a highspeed camera was used to observe and photograph the three processes of bubble generation,gas film formation and electrochemical discharge generation around the tool electrode,and the real-time machining voltage and current were collected by an oscilloscope.The characteristics of bubbles,gas films and electrochemical discharges were analyzed in combination with real-time voltage,current and captured images.Experimental results showed that bubble growth and coalescence occur simultaneously around the tool electrode once a voltage is applied.The evolution process of bubbles includes two stages.At the first stage,the bubbles around the electrode coalesce into a large bubble.Subsequently,the large bubble forms a thin complete gas film due to buoyancy.The electrochemical discharge in gas film can be classified into three types depending on the discharge current amplitude and the pulse duration.The geometrical properties of bubble and gas film were analyzed based on the experimental data.There is a sudden increase in bubble diameter due to bubble coalesence phenomenon.After the formation of gas film,the interface between the gas film and the electrolyte became the Taylor unstable interface.In addition,mathematical models were introduced to reveal the mechanisms of bubble growth and the variation in gas film thickness.In order to reveal the material removal mechanism in ECDM,the material removal processes were modeled and simulated for the cases of single pulse discharge on tapered tool electrode and continuous discharges on cylinder tool electrode.A tapered tool electrode was designed and fabricated by electrochemical machining(ECM)to make sure electrochemical discharges occur at the tip of the tool electrode to achieve single-point discharge.The characteristics of single pulse discharge on tapered tool electrode were analyzed,and it was found that the heat source distribution is consistent with the disk heat source.Comparison between simulation results and experimental results indicated that the disk heat source used in the single pulse discharge model is correct,and the percentage of discharge energy transfered to the workpiece in single discharge was 30.5%.In general,a cylindrical tool electrode is often used in ECDM micro-hole drilling.The discharge point at the cylindrical tool electrode starts at the edge of the electrode and gradually expands from the edge to the center of the electrode.According to the law of discharge point expansion,a material removal model was established for continuous discharges on the cylinder tool electrode.In this model,the discharge points are randomly distributed in the discharge area.A comparison was made between the simulation results of the model and the experimental results,and the number of discharges for material removal as a percentage of the total number of discharges was obtained to be about 10%.The consistency between the simulation results and the experimental results proved the validity of the model.In order to enhance the machining precision and surface quality when machining of hard and brittle materials by ECDM,a side-insulated tool electrode with diamond coating was proposed.The coating of the tool electrode meets the requirements of thin thickness,good insulation,high temperature resistance and chemical resistance.These features not only allow the side-insulated electrode to feed forward without hindrance during drilling process,but also effectively prevent the sidewall of the tool electrode from coming into contact with the electrolyte,thereby ensuring that bubble generation and discharge activity occur at the bottom of the side-insulated tool electrode.The side-insulated electrode was applied in electrochemical discharge microhole machining on quartz and comparisons were made between experimental results by side-insulated tool electrode and traditional tool electrode.It was found that the side-insulated tool electrode effectively avoided chemical etching at the entrance of the microhole and micrholes with small inlet diameter and better surface integrity were achieved.When the machining depth was increased from 50 ?m to 500 ?m,the microhole inlet diameters machined by the side-insulated tool electrode were maintained at the level of 350 ?m with fluctuation of less than 8 ?m.A comparison of machining depth of 600?m achieved by the traditional tool electrode and the side-insulated tool electrode shows that the hole taper angle decreased from 6.4° to 3.3°ECDM technology was applied to drilling of micro-holes in ceramic-coated Ni-superalloy.Different discharge patterns and material removal mechanisms were revealled when machining different layers of ceramic coated Ni-superalloy.The current waveforms are different when machining different layers of the ceramic-coated superalloy.It was found that only the electrochemical discharge occurs during the machining of the coating layer.While two kinds of discharge patterns appear when machining the superalloy substrate.One is the electrochemical discharge which takes place between the tool electrode and the electrolyte,the other is electrical discharge which occurs between the tool electrode and the superalloy substrate.Experimental results showed that both discharges affect material removal.Comparing with EDM-machined surface on Ni-supperalloy,the surface roughness of the hole sidewall machined by ECDM decreases from Ra 10.9?m to Ra 5.6?m and the mean thickness of the recast layer decreases from 38 ?m to 15 ?m. |
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