Mechanism Study Of Blasting Erosion Arc Machining Based On Hydrodynamic Arc Breaking Mechanism

Blasting Erosion Arc Machining(BEAM)characterized by powerful multi-hole inner flushing is a new type electrical erosion process,which removes material by utilizing the heat effect of the electrical arcing instead of sparks as in Electrical Discharge Machining(EDM).The practice shows that BEAM not only inherits the advantages from conventional EDM,such as without megascopic cutting force,regardless of material hardness etc.,but also greatly improves the material removal rate(MRR).Therefore,BEAM is a promising processing in the rough machining field of tough metal material.However,as a new machining technology,the processing mechanism of BEAM is still not fully understood.In order to promote the applications and popularize of BEAM,it is necessary and essential to start a comprehensive study of the processing mechanism.In this study,an experimental setup,which can produce the single arc discharge inside a powerful flush and enables the occurred phenomena to be observable,was specially designed.Based on this setup,the processing mechanism of BEAM was deeply explored and researched from phenomenon to essence.The arcing phenomena in the presence of high-velocity flush were observed and recorded by a high-speed video camera,and the arc temperature was calculated based on the spectrums obtained by a spectrograph.The calculated results show that the arc temperature is much higher than that of the EDM generated spark,which indicates that arc is a more efficient heat source.On the other hand,the obtained videos and images indicate that the hydrodynamic arc breaking mechanism(HABM)can effectively prevent the formation of a steady arc which is apt to burn the workpiece.The high-speed photography also recorded a significant phenomenon that a debris cloud formed on the downstream side of the electrode during arcing.This phenomenon demonstrates that debris expelling in BEAM process occurs in the discharge duration rather than at the end of discharge as the conventional EDM does.Furthermore,this paper compared the craters obtained in negative polarity BEAM with that in positive polarity BEAM,thereby explains why the former gains a bigger MRR and the latter results in a better machined surface.In this research work,the discharge channel expanding rule after the breakdown of the dielectric was studied theoretically and mathematically.Based on the view that the discharge channel is a variable mass,cylindrical plasma,under proper assumptions,a theoretical discharge column expansion formula has been derived using the infinitesimal method according to the conservation of energy.The single pulse discharge experimental results prove that the derived expansion formula can accurately predict the crater radius.Moreover,comparing with the previous empirical formulas,the derived expansion formula can not only point out the influences of maintaining voltage,peak current and pulse-on time on the discharge channel expansion,but also reveal the influences of breakdown radius and dielectric properties on the discharge channel expansion.As a result,the new derived expansion formula has larger scope of application.The derived expansion formula points out that,as the spark column expands with time,the total heat transfer time decreases radially from the center to the periphery at the spark/anode interface rather than equals to the pulse on time everywhere.This phenomenon can be considered as the time integration effect(TIE)of the expanding spark.Based on the expansion formula,this paper derived a quantization expression of the TIE.According to the quantization expression,this paper developed a transient thermal model considering TIE of the expanding discharge channel to simulate the material erosion process of EDM in water.Comparing with the previous thermal model using Gaussian heat source instead of considering TIE,The new model is able to predict the crater sizes more precisely.Moreover,the practice indicates that the new model is more competent at simulating the material removal under larger pulse duration.The modeling and simulation of flow field in the flushing tube were conducted using computational fluid dynamics software so as to ulteriorly study the mechanisms behind the captured phenomena.The fluid analysis results show that a negative pressure zone formed by flushing on the downstream side of electrode,resulting in negative-pressure suction effect.With the help of negative-pressure suction effect,the high-velocity flushing can continuously take the molten metal out of the molten pool during the discharge,leading to the reduction of the overheating of the molten metal,and improve the efficiency of the discharging energy,thereby promotes the MRR.The observing results demonstrate that,due to the HABM,the discharging process in BEAM can be divided into two stages according to the morphologic change of discharge channel,namely,channel expanding stage and channel distorting stage.Based on this discovery,this paper summed up the research results of the previous chapters and developed a transient thermal model at last to analyze the material removal process of BEAM.In this model,the channel expanding stage is modeled with the aforementioned thermal model considering TIE,and the channel distorting stage is simulated with a travelling heat source with a gradually decreased radius.The verified experiment indicates that this model is capable of simulating the trailing craters obtained in BEAM,and therefore can be used to explain the mechanism of BEAM.