| Micro electrical discharge machining(micro-EDM)milling is a method which can be used to corrode the excess metal with the electrical corrosion phenomenon of micro-pulse discharge between micro electrode and workpiece and with the use of tool electrode for feed movement,so as to achieve the predetermined processing requirements of parts in terms of size.shape and surface quality,and it is particularly suitable for the machining of micro-groove and micro 3D structure of difficult-to-machine materials such as titanium alloy.Due to the much smaller scale of micro-EDM compared to EDM,the research on the material removal mechanism of microEDM lags behind.Meanwhile,in the micro-EDM milling of titanium alloys,the electrode face wear is particularly severe due to the small size of the electrode and the strong high-temperature corrosion resistance of titanium alloy materials.Currently,most research on micro-EDM milling can only selectively optimize either the processing efficiency or the processing accuracy,which limits the engineering application of micro-EDM milling technology.A systematic study of micro-EDM milling of titanium alloys,including the establishment of material removal theoretical models,revelation of material removal mechanisms.exploration of electrode wear compensation methods,and investigation of key process technology characteristics,is beneficial for better grasping the micro-EDM milling technology of titanium alloys,improving the predictability and controllability of the machining effect,and better serving engineering applications.In this paper,a systematic study of micro-EDM milling of titanium alloys was conducted from four aspects:theoretical models of material melting and material ejection processes,material removal mechanisms,electrode wear compensation,and ultrasonic vibration assistance.The material removal mechanism of titanium alloy micro-EDM was studied from the perspective of material melting and molten material ejection.The energy distribution of microEDM,the impact of electrons and ions,and the impact of metal vapor were mathematically analyzed.The results show that the energy released on the anode includes the impact of electrons and the impact of metal vapor,while the energy released on the cathode is mainly the impact of ions,and the impact of metal vapor is the main driving force for molten material ejection.Based on heat transfer theory,Gaussian surface heat source models and hemisphere heat source models for material melting processes were established.and a metal vapor impact velocity model based on fluid mechanics theory was developed.Single-pulse discharge experiments show that the discharge channel radius and energy distribution coefficient increase with increasing peak current and pulse width in both positive and negative polarity machining.Under the same parameters,the discharge channel radius and energy distribution coefficient based on the hemisphere heat source model are larger than those based on the Gaussian surface heat source model.Finite element numerical simulation was carried out to analyze the material removal process of titanium alloy micro-EDM.Simulation models of the material melting process were established to reveal the temperature field and molten pool evolution during the material melting process.The influence of peak current and pulse width on the molten pool morphology was analyzed.The results show that with increasing discharge time,peak current and pulse width,the radius and depth of the molten pool continuously increase,and the variation in the radius direction is faster than that in the depth direction.Simulation models of the molten material ejection process were established to reveal the evolution of the metal vapor impact velocity and the morphology of the discharge craters.The variation of the discharge crater morphology with changes in peak current and pulse width was analyzed.The results show that with increasing peak current and pulse width.the size of the discharge crater increases.and the simulation model based on the Gaussian surface heat source fits better with the material melting process of the workpiece.A study on the mechanism of electrode wear and compensation methods in titanium alloy micro-EDM milling was conducted.The analysis of electrode wear under the skin effect revealed that the skin effect indirectly affects the discharge point position,resulting in uneven distribution of discharge on the electrode end face and causing edge wear.Based on the fixedlength compensation model,the experimental results of single-factor experiments in titanium alloy micro-EDM milling demonstrated that the actual relative wear ratio of the electrode decreases with increasing pulse width.initially decreases and then increases with increasing pulse interval.decreases with increasing peak current,and initially decreases and then increases with increasing electrode rotational speed.Using orthogonal experiments,we optimized the actual relative wear ratio of the electrode and obtained the optimal combination of process parameters:pulse width of 2.4 μs,pulse interval of 13 μs,peak current of 5.6 A,and electrode rotational speed of 1600 r/mins.We investigated the application of ultrasonic vibration-assisted micro-EDM milling in titanium alloy.considering the compensation for electrode wear.The influence of additional ultrasonic vibration on the discharge channel,discharge gap,and actual relative wear ratio of the electrode was analyzed.Finite element numerical simulation was employed to study the simulation of the molten material ejection process based on ultrasonic vibration,revealing the evolution of the impact velocity and pit morphology of the molten material ejection process.The effect of ultrasonic vibration amplitude on the pit morphology was analyzed,and the results showed that the impact velocity of ultrasonic vibration varied continuously during one cycle,exerting a pumping effect on the dielectric and promoting the flow of working fluid between the electrodes,thereby facilitating the removal of eroded products and improving machining efficiency.Based on the single-factor experiments,the influence of process parameters on the effectiveness of titanium alloy ultrasonic vibration-assisted micro-EDM milling with electrode wear compensation was analyzed.We conducted a multi-objective optimization for titanium alloy ultrasonic vibrationassisted micro-EDM milling with electrode wear compensation.Regression models were established using response surface analysis for material removal rate,actual relative wear ratio of the electrode,and surface roughness.The significance of process parameters on the machining performance was explored through variance analysis,error analysis,and response surface analysis.We proposed a multi-objective optimization method combining the backpropagation(BP)neural network algorithm and normalization method.Based on electrode wear compensation,the multi-objective optimization for titanium alloy ultrasonic vibrationassisted micro-EDM milling was performed.The optimal solution was found with a pulse width of 2.4 μs,pulse interval of 10 μs,peak current of 3.2 A,and ultrasonic vibration amplitude of 6μm.Under the optimized parameter combination,the material removal rate increased by 16.48%,the actual relative wear ratio of the electrode decreased by 29.71%,and the surface roughness decreased by 25.46%.This achieved high dimensional accuracy,high surface quality,and efficient machining in titanium alloy micro-EDM milling. |
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