Research On Key Process Of Micro-hole Discharge Machining In Different Working Fluids

Various precision small holes are widely required in industrial production and precision manufacture.However,there are many problems in the processing of micro holes by the traditional machining ways,mainly because of the thermodynamic effect in processing.The use of micro-EDM(electrical discharge machining)has many excellent features not found in conventional machining.The machining force between the electrode and the workpiece in the EDM process is much smaller than that of the traditional machining.Since the electrode used for the EDM micro hole can be very small,it is relatively easy to obtain a small machining removal unit.Therefore,EDM is an irreplaceable processing method in microfabrication technology,and it is used for manufacturing various micro holes,such as fuel injection nozzles and synthetic fiber orifices.In this paper,304 stainless steel and Inconel 706 Fe-Cr alloy steels were used as research objects for micro-EDM.The preparation of electrodes in EDM is very important.The traditional preparation of micro-electrodes is prepared by WEDG(wire electricl discharge grinding).This method is timeconsuming and complicated in preparation process.Therefore,the micro electrode was fabricated by FIB-CVD(focused ion beam chemical vapor deposition)technique.The method shows that the preparation process is simple,and the time is saved compared with the WEDG method,the success rate is high,and the processing precision is very high.First,the electrode prepared by the technique is subjected to micro-hole electrical discharge machining using deionized water as a working fluid.A micropore having a pore diameter of about 25 ?m was successfully prepared by using the electrode,and the discharge gap was as small as 6.6 ?m.Secondly,the EDM oil is selected as the working fluid,and 304 stainless steel is used as the workpiece.The experimental results were optimized using Minitab software.Compared with the initial machining parameters,the MRR(materials removal rate)of the products processed by the optimized processing parameters was improved by 64%,and the main effect diagram of the signal-to-noise ratio indicated the input parameter pulse.Pulse width is the most significant factor affecting MRR.By comparing the cross-sectional topography and product surface roughness,it is found that a better surface quality can be obtained with a smaller input pulse width and a smaller peak current.Beside,304 stainless steel after micro-EDM appears degradation of the performance.Based on the application of industrial production,this work tests the fracture properties of the stainless steel workpiece after micro-EDM.The tensile test shows that the yield strength and tensile strength of micro-EDM machined 304 stainless steel are decreased,but the brittleness is increased.By comparative metallographic characterizations and microhardness,it was demonstrated that grains of the specimen machined by micro-EDM near the fracture occurred slippage and the elongation of grain was hardly observed.It is considered as the main reason that work hardening and grain refinement of the molten solidified layer formed during EDM limits the elongation of the surrounding grains.it is confirmed that the fracture form in the sample processed by micro-EDM belongs to the combination of quasicleavage fracture and ductile fracture.In order for 304 stainless steel to be better served,it is necessary to have post-treatment after micro-EDM machining to eliminate the effect of increase of brittleness which is derives from EDM processing.The EDM oil mixed with powder was used as the working fluid lastly.By comparing the surface integrity of micro holes under different concentrations of mixed working fluid,it was found that adding a certain amount of aluminum powder particles can significantly improve the surface quality of the product in micro-EDM.By performing EDS analysis on the surface of the electrode,the results show that the addition of aluminum powder in the EDM oil,less workpiece material adheres to the electrode surface,mainly because of its more stable processing.