Research On Material Removal Mechanism And Formation Of Surface Denatured Layer In Electrical Discharge Machining

Although the technology of electrical discharge machining(EDM)has developed for more than 70 years,the micro-physical process,how the molten materials are removed in the discharge process,has not been clarified so far.As a result,two big bottleneck problems: low machining speed and poor surface quality,have not been solved fundamentally so far.To solve the above problems,in this thesis,the material removal process and mechanism were investigated by using the advanced simulation technology and experimental equipment.The elucidation of the above micro-physical process can contribute to understand the basic principles of EDM,thereby improving the machining speed and surface quality of EDM by designing the pulse power,servo control system and machine tool,etc.more reasonably.The single discharge process was simulated by molecular dynamics simulation(MD simulation).From the simulation results,it was found that a p art of materials inside the molten pool were vaporized,resulting in the increase of pressure inside the molten pool.As a result,the molten materials expanded outward and a swell was formed.When the difference of pressure inside and outside the molten p ool exceeded the surface tension of the molten pool,the swell exploded and the molten materials were removed.The above process is the material removal mechanism caused by pressure inside the molten pool.Furthermore,it was found that the interaction of jets in the gap caused the generation of radial shear force acting on the surface of the molten pool.This radial shear force helped remove the molten materials.This is the material removal mechanism of radial shear force caused by metal vapor jets in the gap.From the simulation results,it was also found that there were three kinds of mechanisms in the forming process of the ringed bulge.The first mechanism was the pressure gradient inside and outside the molten pool,causing the flow of the molten materials along the edge of the discharge crater;the second mechanism was the shear force caused by the metal vapor jets,causing the shear flow of the molten materials along the radial direction of the discharge crater;the third mechanism was the debris falling back to the electrode surface and the spattering materials from the opposite electrode,forming a part of the ringed bulge.The evolution process of microstructures of the materials inside the denatured layer was investigated by building MD simulation model of the monocrystalline and polycrystalline coppers.It was found that as for the discharges on the(100)and(110)planes,the stacking fault existed mainly in the form of the triangular pyramid.But,on the(111)plane,most of the stacking faults were bedded.After the discharge,there generated lots of bedded stacking faults concentrated under the discharge crater and bulge of the polycrystalline copper.Moreover,discharge on the polycrystalline copper caused much more stacking faults and dislocations than the discharge on the monocrystalline copper,which indicates that much better surface quality can be obtained with discharge on the monocrystalline copper.A two-dimensional and large-scale simulation model was built based on the periodic boundary conditions of MD simulation,then,the free electron heat conduction was achieved in the MD simulation by combining with the two-temperature model.At last,a submicron discharge crater was obtained.The simulation results of two-dimensional and large-scale electrode show that the surface denatured layer generated in the monocrystalline copper was the least while the surface denatured layer generated in the polycrystalline copper increased with decreasing the grain size.The measurement experiment of wire vibration was implemented to prove the existence of discharge reaction force caused by the interaction of jets in the gap,which can verify the validity of the theory of interaction of jets in the gap.The theory of interaction of jets can be described as follows.In the discharge process,both the tool electrode and workpiece emit the jets into the gap,then,the interaction of jets in the gap causes the generation of discharge reaction force.Moreover,electrode materials which show the ease of boiling results in stronger interaction of jets in the gap;thereby,the discharge reaction force will be larger.The discharge reaction force acts on the surface of the molten pool directly and applies a pressure on the molten pool,which has great influence on the topography of the discharge crater.Furthermore,based on the experimental results of wire vibration,the discharge reaction force was quantitatively calculated by solving the inverse problem of the wire vibration equation,and the result shows that compared with the discharge reaction force caused by the bubble,the discharge reaction force caused by the interaction of jets was significantly smaller.The material removal mechanism of the pressure inside the molten pool and material removal mechanism of radial shear force caused by metal vapor jets in the gap,which were proposed by the simulation results,were verified through the single pulse discharge experiments.Furthermore,the movement of the molten pool and the removal of the molten material were directly observed by high-speed cameras.The observation results revealed the influence of flow distribution caused by the interaction of jets in the gap on the material removal.That is,molten materials were removed along the tangential direction of the electrode surface in the discharge process.Moreover,the movement law and reason of the molten pool in the discharge process were found.Namely,the movement of the molten pool was closely related with the movement of the plasma.The forming mechanism of the discharge craters were also revealed.The shape of the discharge crater was influenced together by the pressure inside and outside the molten pool.Last,the influencing mechanism of the discharge gap on the material removal process was elucidated.The discharge gap can influence the removal of the molten material by affecting the shear force acting on the molten pool.Therefore,on the basis of the former research results,this paper proposed to use the materials which were easy to boil as the tool electrode to improve the material removal amount of single discharge crater on the workpiece.The experimental results show that this method can not only improve the machining speed of EDM significantly,but also decrease the wear of tool electrode.The discharge process of CFRPs was observed by a high-speed camera.It was found that in the discharge spot of CFRPs,a large amount of gaseous jets were emitted into the gap under the extremely high temperature of the arc plasma,generating the jetting force.When the discharge occurred on the parallel surface of CFRPs,the carbon fibers around the discharge spot were crashed continuously by the high-speed jets flying into the gap,then,broken and threw into the gap by the jetting force to form the carbon fiber debris;however,when the discharge happened on the vertical surface of CFRPs,the high-speed jets had little influence on the removal of carbon fiber.