Study On The New Combined Machining Technology And Mechanism Of Ultrasonic Vibration Assisted Electrical Discharge Machining In Gas Medium

Electrical discharge machining (EDM) in gas is a new green machining technology. But its process and mechanisms need further studying. Ultrasonic vibration assisted gas medium electrical discharge machining is a new combined machining technology based on electrical discharge machining in gas. Experimental results showed that the MRR (material remove rate) was improved and the phenomena of short circuit and arc discharging were avoided effectively.The mechanism of electrical discharge machining in gas was analyzed and the theory of which was introduced. The theory of electrical discharge machining in gas is composed of three parts, which are the the conductance of gas medium, the polarity effect of electrical discharge machining in gas and the material removal process of electrical discharge machining. The discharging process during EDM in gas is composed of Townsend breakdown process and streaming breakdown process. The polarity effect is quite different from conventional EDM method. The tool electrode wear is quite small when tool electrode is cathode and workpiece is anode. And this phenomena cann't be affected by discharge pulse-on time. The process of material removal during EDM in gas is divided into three parts, which are the formation of discharge channel, the conversion of discharge energy, the materials' shoot off and deionization process.A new technology named ultrasonic vibration assisted gas medium electrical discharge machining was proposed and the principle of which was introduced. The character of the new technology is its wide application range in that the tool electrode is assisted ultrasonic vibration during machining process. The experimental equipment of ultrasonic vibration assisted gas medium electrical discharge machining was designed, which is made up of the body, the ultrasonic generation equipment, the rotary gases supply system and the pulse power generator. Both tool electrodes ultrasonic vibration assisted and workpiece ultrasonic assisted can be carried out on this experimental equipment. The design of ultrasonic rotary gas supply system is difficult in this work.The effect of discharge pulse-on time, discharge pulse-off time, discharge peak current, discharge peak voltage, gas pressure and pipe wall thickness on MRR was studied experimentally. Experimental results showed that the MRR increases with the increase of discharge pulse-on time, discharge peak current, discharge peak voltage, gas pressure; but decreases with the increase of discharge pulse-off time and pipe wall thickness. Experimental results showed that the optimal thickness of pipe wall is 0.3 mm. More over, experimental results showed that the MRR is larger when workpiece ultrasonic vibrating than that when tool electrode ultrasonic vibrating. The reason is that the small molten drops can be easily shot off on the condition of ultrasonic vibration assisted. The model of MRR was developed both taking account of effects of ultrasonic and melting on materials removal. The model showed that the MRR increases with the increase of discharge pulse-on time, discharge peak current, discharge peak voltage and amplitude of ultrasonic vibration; but decreases with the increase of discharge pulse-off time. The concept of ultrasonic vibration assisted electrical discharge milling technology was proposed and the principle of which was introduced. The compensating strategy of tool electrode wear was proposed and the track of tool electrode was optimized.The effects of discharge parameters on surface roughness were studied experimentally. The experimental results show that the larger the discharge pulse on time, the peak current and the open voltage, the larger the surface roughness. The reason is that the machined surface of EDM is composed of countless small craters, the larger the discharge energy, the larger the discharge craters. The discharge energy is defined by the discharge pulse on time, the peak current and the open voltage. The tool electrode wear rate was studied. Experimental results show that the tool electrode wear rate is very small and cann't be affected by discharge pulse-on time. The tool electrode wear rate is less than 5% when workpiece ultrasonic vibrates. The tool electrode wear rate is quite small and is hardly affected by discharge pulse on time.Ultrasonic vibration assisted gas medium electrical discharge machining in different gases medium and with different workpiece materials were carried out. Experimental results showed that the MRR of Nd-Fe-B is larger than cemented carbide. The MRR is lower when machining in N₂ than machining in pressed air. The reason is that O₂ can act with workpiece materials at the high temperature of when machining in gas, which accelerated the material removal.A single pulse experiment was designed in gas medium and ultrasonic vibration environment. The effect of discharge energy on crater shape was studied. The mechanism of ultrasonic vibration in improving machining effect was researched. The mechanism of ultrasonic vibration on discharge channels was analyzed. The effect mechanism of ultrasonic vibration on discharge gap was studied. The effect of ultrasonic vibration on MRR and discharge crater was analyzed and the effect mechanism of ultrasonic vibration in increasing MRR by stress analyzing of the small drops. The inertia force generated by ultrasonic vibration can enhance materials' removal.The microstructure and metallography structure of machined surface by ultrasonic vibration assisted electrical discharge machining in gas were studied. The component of machined surface was studied. The model of thermal stress removal was developed through observing the characteristics of micro-cracks in the workpiece's surfaces. The measures to reduce and avoid micro-cracks were proposed. The removal model of thermal stress when machining hard and crisp materials was developed. The process is composed of four phases, which are the formation of thermal stress, the formation of microstructure, the desquamating of crystal grains and the shoot off of the crystal grains.The thermal field and gas flow field during electrical discharge machining in gas were studied. The physical model and Gauss distribution model of thermal flux during single discharge pulse of electrical discharge machining in gas were developed. The thermal field and MRR were analyzed using ANSYS finite element software. The results show that the larger the discharge energy, the higher the temperature of discharging point. Also, the MRR increases with the increase of discharge energy. Moreover, the gas flow model of machining process was developed and the distribution laws of gas velocity and pressure in discharge gap were simulated by FLUENT software.