Basic Research On Enhancement Of Mass Transport In The Micro Machining Gap Of Wire Electrochemical Micro Machining

As with the micro-mechanical systems fast-developing,it has been one of the topic research area that fabricating metallic micro-structures and components with high efficiency,high accuracy as well as high surface quality.Recently,electrochemical micro machining(ECMM)used for fabricating metallic materials with high quality was focused by many researchers in home and aboard,owning to its specific advantages.Wire electrochemical micro machining(WEMM),a form of ECMM,has been increasingly recognized as a flexible and effective tool for producing complex-shaped planar-contour metallic micro-structures with high aspect ratio.It inherits the basic advantages of ECMM such as able to machine almost any conductive material,good surface quality,in combination with no tool wear.etc.In addition,there is no need to fabricate a tool electrode with a complex shape for WEMM,and thus the pre-preperation time can be effectively reduced.Efficient mass transport in the micro-scale machining gap has been proved to be extremely important for improving the machining efficiency,machining stability and quality of WEMM.The aim of the thesis is to fabricating metallic micro-structures and components with high quality by enhancing the mass transport in the micro machining gap of WEMM.The major contents of the thesis are as follows:(1)A method of surface microtextured wire electrochemical micro machining has been approached.A mathematic model was established for analyzing the flow field distribution in the machining gap causing by the axial traveling wire electrode.The model indicated that velocity slippage on the traveling wire surface resulted in decreasing the flow velocity of electrolyte in the machining gap,which adversed to the mass transport in the machining gap.A model for the velocity slippage on the traveling wire surface was established.It revealed that during the electrochemical micro machining,the hydrogen bubbles attachining on the wire surface had significant influence on the velocity slippage,the value of which was greater when the size of bubbles were larger and the volume fraction of gas in the bubble layer was higher.Through force analysis for the hydrogen bubbles attachining on the axial traveling wire surface,it was found that applying surface microstructures in the wire electrode was beneficial for promoting the hydrogen bubbles detachining,resulting in the reduction of the bubble layer height and the gas volume fraction in the layer.Finally,the velocity slippage on the axial traveling wire surface decreased,and thus the mass transport rate in the micro machining gap was enhanced,effectively.(2)A process for laser scanning machining(LSM)of microstructures on the micro wire revolution surface has been designed,through analyzing the characteristics of LSM revolution surface.Using the process,ordered and uniform microstructures were fabricated on the micro wire surfaces with the diameters of 25?m and 50?m.After investigating the machining parameters of laser fluence,scaning velocity as well as scanning interval on the morphology of produced microstructures and the wettability of microstructured region,surface mictrostructured tungsten micro wire electrodes with different hydrophilicity were successfully fabricated.(3)A series of experiments of ultra-short pulses wire electrochemical micro machining have been performed.The experimental results indicated that through adopting appropriate machining parameters,the machining efficiency,the slit width homogeneity as well as the machined surface quality for wire electrochemical micro machining could be obviously improved.Applying the surface microstructured wire electrodes with greater hydrophilicity,the maximum feed rate of WEMM was higher,and the homogeneity of machined slit width was better.Using a surface microstructured wire electrode,the maximum feed rate reached to 0.5?m/s,which was 2.5 times as that of using a smooth one.(4)A method of intermittent backwards assisted axial traveling wire electrochemical micro machining has been proposed.The mechanism for the enhancement of mass transport in the machining gap by using this method was detailly analyzed by the combination of simulations and experiments.The simulated and experimental results indicated that using this method,the mass transport in the micro machining gap could be obviously enhanced,resulting in the improvement of machining efficiency and machined slit width homogeneity.The effect of the intermittent backwards parameters,namely,backwards speed,backwards distance and the each feeding length,on the width homogeneity of the machined slit and feed rate were investigated.Finally,a high quality multi-slit microstructure with the slit width of 13.8?m,standard deviation of 0.53?m and the surface roughness R_a of 0.042?m was successfully fabricated on the 304 strain steel workpiece.The width of each slit was with good consistence.(5)A method of axial high speed traveling multi-wire electrochemical micro machining has been proposed.The effect of wire traveling speed on the velocity slippage and the mass transport rate in the machining gap was analyzed.The influence of pulse frequency,duty cycle,wire electrode amount and traveling amplitude on the slit with,machining efficiency as well as stability using high frequency short pulse were investigated.Finally,WEMM using 15-wire electrode was realized,at a total machining rate of 75?m/s being achieved.Using a 15-wire electrode with the diameter of 50?m,a multiple-slit microstructure with high aspect ratio of 20 was fabricated in the substrate with the thickness of 3 mm.Using a 7-wire electrode,multiple“X”-shaped micro components with surface roughness R_a=0.128?m were produced at a time.