| Electrochemical machining(ECM)is currently one of the more widely used special machining methods for forming cooling holes.Material removal is the continuous anodic dissolution of the workpiece in the electrolyte in the form of ions through the electrochemical reaction.During the ECM process,the Joule heat generated by the electrification and the gas and heat accompanying the electrochemical reaction will affect the current density on the surface of the workpiece,thereby changing the size and morphology of the cooling hole.Therefore,this thesis uses multi-physics coupling simulation and experimental observation to study the forming mechanism of ECM of cooling holes.According to the mathematical model of convection heat transfer and the theory of gas-liquid two-phase flow,the distribution of gas volume,temperature and current density in the ECM area is analyzed,and further study the influence of different process parameters on the dynamic forming law of the cooling hole size and morphology under the multi-physics coupling.Through the study of ECM mechanism,a simulation model of through-mask ECM is established,the influence of through-mask structure on the current density distribution on the surface of the workpiece is analyzed,and the electric field distribution and size characteristics of cooling holes under different through-mask conditions are studied.The results show that the through-mask structure can greatly improve the electric field distribution in the entrance area of the cooling hole and reduce the current stray corrosion.The entrance radius and taper of the cooling hole will decrease with the reduction of the diameter of the through-mask,and the change of the height of the through-mask has little effect on the forming accuracy of the cooling hole.Based on the mathematical model of convection heat transfer and the theory of gas-liquid two-phase flow,a multi-physics coupling simulation model is constructed to study the distribution of gas-liquid two-phase flow field,temperature field and electric field under the balanced state of ECM of cooling holes,and to analyze the temperature,gas volume fraction and current density distribution in the machining gap under different process parameters.The simulation results show that as the machining voltage and electrolyte concentration increase,the electrolyte temperature and gas volume inside the machining gap will increase,and accumulation will form at the "corner" of the machining area,resulting in poor current density distribution uniformity.The increased flow rate of electrolyte in turbulent condition is beneficial to the renewal of heat and gas,but too fast flow rate will cause violent "vortex effect" to affect the renewal rate of electrolyte.According to the multi-physics coupling simulation model,the current density distribution on the surface of the workpiece is obtained,and the mathematical model of the anode boundary motion on the current density is established to analyze the dynamic forming process of size and morphology of cooling hole electrolytic processing under different process parameter conditions.The results of simulation and experiment show that the size of the inlet of the cooling hole is larger,the size of the outlet is smaller,and the middle part is relatively stable.The size and morphology of the cooling hole is greatly affected by the processing voltage,electrolyte concentration and electrode feed speed.The change of the electrolyte flow rate in the turbulent state has little effect on the dynamic formation of the cooling hole.Finally,a central composite experimental study was carried out with the help of the response surface module in Design Expert to establish the second-order response models of input variables such as machining voltage,electrolyte concentration,electrode feed rate and performance indexes such as mean radius and taper.Error analysis is carried out through the ECM experiment of cooling holes and the simulation regression equation,which verifies the validity of the multi-physics coupling simulation model. |
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