Study On Material Removal And Surface Quality In EDM Based On Thermo-hydraulic Coupling Analysis

As a non-contact material processing method,electrical discharge machining(EDM)removes the material through the extra-high temperature generated by the spark discharges between electrode and workpiece.Due to the negligible cutting force and independence on the hardness and strength of material to be machined,EDM has become an indispensable material processing technology.During the discharge process in EDM,the heated material is melted or even vaporized,thus the hydrodynamic behavior of molten or vapor material will be inevitably involved.However,the discharge in EDM generally happens in a very short time and very narrow space,and it is extremely difficult or impossible to investigate these processes in a manner of direct observation.Hitherto,there is still no unified understanding on the thermal and hydrodynamic behaviors that exist in the process of material removal.In recent years,however,the rapidly developing computer simulation technology has provided a good thought and method for the researches in this field.In this work,the material removal processes and related theories in EDM were studied based on finite element multi physical field coupling method,and the material removal mechanism and involved physical phenomena were explored from the perspective of thermo-hydraulic coupling.The simulation results were verified by single pulse and continuous pulse discharge experiments.The investigations of material removal mechanism and related physical phenomena during the discharge process can provide theoretical guidance for optimizing EDM process,and it is also of great significance to enrich the basic theory of EDM technology.Firstly,the energy conversion and transmission,as well as the jumping phenomenon of molten material surface were systematically analyzed.On the basis of synthetically considering the physical factors including heat transfer,phase transformation,fluid flow and interface movement,a novel thermo-hydraulic coupling model was established to describe the material removal during the discharge process.Level-set method was adopted to track the free interface between solid/liquid material and gas material,and the visualization of crater morphology evolution and discharge debris movement of was realized.The manner and characteristics of material removal in single pulse discharge process were studied.Results showed that the molten material is removed by vaporizing in the initial stage of discharge duration,and then mainly by splashing at the following time.Meanwhile,the material removal intensity in splashing stage is much greater than that in vaporizing stage.The temperature distribution in the heated area showed that,most of the material in molten region is remained to form the re-solidification layer on the surface of crater,and the removal ratio of molten material is only about 7.63% under the studied conditions.The size of crater including diameter and depth reached the maximum at the end of the discharge duration,and they experience a slight reduction due to the reflux of molten material after the discharge.By the good agreements between simulated crater and experimental craters,in terms of both morphology and size,the feasibility of proposed thermo-hydraulic coupling model was verified,indicating that the material removal could be explained by the thermal-hydraulic coupling mechanisms in discharge process.By analyzing the pressure distribution and velocity distribution of molten material and vaporized material,the driving forces for the molten material removal and influence factors of discharge crater morphology were revealed from the perspective of material hydrodynamics.It is found that the internal hydrostatic pressure of molten region is changed with the occurrence of thermal phase transformation.The driving force for the detachment of debris in splashing removal stage comes from the extremely large pressure difference in upper part of the molten region.Meanwhile,the profile of crater is varied due to the shearing flow of molten material,and the bulge around the discharge crater was found to be gradually smoothened during the discharge period.In addition,the material of electrode and the workpiece will migrate due to the ejection of discharge debris.The results of bipolar simulation showed that,the debris produced by electrode and the workpiece material collide in the electrode in the discharge gap.Meanwhile,the moving debris with high speed can inhibit or promote the removal of molten material.Residual stress equations and metallographic transformation equations are integrated into the thermo-hydraulic coupling model in EDM.The thermal-induced metamorphic zone generated during a single pulse discharge was studied.The thickness and metallurgical structure of thermal phase transformation layer,residual stress distribution and effects of discharge current and pulse duration on these aspects were analyzed.For a single discharge crater,the thicknesses of recast layer and heat affected zone(HAZ)are the smallest at the center of crater,and they gradually increase as approaching the periphery of the crater.Meanwhile,the recast layer undergoes a complete martensitic transformation,and this can be considered as the key reason for the enhancement of surface hardness.From the crater surface,the Von Mises stress increases first and then decreases along the depth direction,reaching its peak value near the interface of recast layer and HAZ.The maximal tensile stress has closer relationship with discharge current,and the pulse duration has greater influence on the average thickness of recast layer and depth location of maximal tensile stress.The model of random discharges,in terms of discharge location and discharge duration,was established to simulate the time-varying and dynamic material removal processes in consecutive pulse discharges.Combined with simulation and experiments,the superposition of discharge craters,as well as the characteristics and evolution of machined surface were analyzed.What is more,the temperature distribution and thickness of recast layer were studied in detail.The experimental results showed that,the calculation model of continuous pulse discharge can predict the surface roughness and recast layer thickness to some extent.