Study On The Technology And Mechanism Of Ultrasonic Vibration Assisted Electrical Discharge Surface Strengthening

Electrical discharge surface strengthening technology can be achieved by the ordinary EDM machine tools. During the strengthening procedure, metallurgy reaction occurred between electrode and workpiece materials, in which kerosene was discomposed and a strengthened layer was obtained on the surface of workpiece materials, which was caused by the heat of pulse discharge. The hardness of the wokpiece as well as the wear and corrosion resistance was obviously enhanced. However, it has some drawbacks such as poor distribution of surface coated layers and chemical composition, the serious loss of the electrode with the type of solid, compressed powder and sintered, which severely affect the accuracy of electrical discharge surface strengthening.In the paper, ultrasonic vibration was used to assist electrical discharge surface strengthening, which could solve the problem of poor distribution of surface coated layers and chemical composition, and which has good influence on the surface quality of the strengthening layer, such as improve the surface roughness, the hardness, and the wear and corrosion resistance. In order to ensure the control accuracy of the electrical discharge surface strengthening technology, the author assisted ultrasonic vibration on the tool electrode and added specific powder in the working fluid, thus the surface strengthening layer with good performance could be formed on the workpiece surface. The technology expands the technical range of electrical discharge surface strengthening.The mechanism of electrical discharge surface strengthening technology was studied. The theory of gas impact ionization, bubble theory and impurity breakdown theory were used to analyze the surface strengthening process, which was studied from the stages of the breakdown of the dielectric, the discharge channel formation and the discharge energy distribution, the migration of the electrode material to the surface of workpiece, the layer formation and the deionization. The influence of polar effect and the electrode material on the strengthening layer formation was studied, the layer with good performance can be formed under the condition of positive strengthening and the choice of appropriate electrode material. An amorphous nano-crystalline alloy layer can be fabricated on the surface of 65Mn steel under the positive conditions and Si electrode, this is because the electron streams strike the workpiece surface with high speed, and then the temperature of the workepiece surface is dramatic raised and then cooling quickly, the condition for the amorphous formation is provided. There is no amorphous in the layer under negative conditions, this is because the heating effect of the collisions on the workpiece is weak, due to the mass and inertia of the positive ions, which cannot provide the conditions to form amorphous layer. The single pulse surface strengthening experiments were carried out, and the diameter of the discharge points increased with the increase of pulse energy, an experimental regression model of the single pulse discharge point diameter was established.Under the conditions of Si electrode and positive strengthening, the surface strengthening layer was formed on the 65Mn steel, which consisted of several pieces of overlapped round uniform plates. The layer can be formed, in which there is no cracks and cavities, and the chemical composition distribution is uniform. The strengthening layer can be divided into three zones:white layer, joining zone, and heat-affected zone (HAZ) in the vertical direction to matrix. It is an amorphous nano-crystalline structure of the white layer upside, and the columnar crystals exist between the white layer and HAZ, perpendicular to the surface of workpiece, and the columnar crystal zone changes with the pulse parameters, the microstructure of HAZ was a needle-like martensite with high carbon content.The deposition efficiency and surface roughness models of the electrical discharge surface strengthening layer were established. The law that the surface roughness, the thickness of the strengthening layers, the surface and cross section morphology varied with different processing parameters was studied, such as electrical discharge peak current, pulse duration, pulse interval and the processing time. The thickness of the layer increases with the increase of pulse duration and electrical discharge peak current, obtaining the maximum value on the condition of specific pulse interval and processing time;the surface roughness increases with the increase of pulse duration and electrical discharge peak current, micro cracks appearing on the workpiece surface at a certain parameter value. The existence of amorphous was predicted though the thermodynamic theory in the electrical discharge surface strengthening layer. The performance of the layer was studied, and the micro-hardness, wear and corrosion resistance were obviously higher than those of the matrix. Test results showed that the strengthening layer was well adhered to the matrix.Ultrasonic vibration assisted electrical discharge surface strengthening technology was proposed, the principle of which and the influence of ultrasonic vibration on the electrical discharge surface strengthening process was systematically analyzed. Ultrasonic vibration has a good influence on the surface quality of the strengthening layer, which can improve the discharge gap, cause dramatic changes in the electric field, reduce the breakdown time delay, easy to form the discharge channel. Ultrasonic vibration can increase the tool electrode material quality in alloy reaction due to the increase of the discharge explosive force and the vibration stress. The melt under ultrasonic vibration can be accelerated to high speed and transferred to the workpiece surface, and then alloy reaction occurs.The influence of ultrasonic vibration amplitude and frequency on the strengthening layer formation and performance was studied. The influence of ultrasonic vibration was weak with low frequency and amplitude;the influence of ultrasonic vibration on the acceleration of the melt, the tool electrode loss and the cavitations effect was significant with high frequency and amplitude, which formed the layer with more wrinkles and clear surface. The layers had uniform thickness, no cracks, holes and other defects with the high frequency and low amplitude. The surface roughness and Si content of the layer was lower with the medium frequency, which increased with the increase of ultrasonic amplitude. The thickness of the layer was lower with the low frequency, which decreased with the amplitude. When the tool electrode is assisted with ultrasonic vibration, the layers have uniform thickness and chemical composition, and which can decrease the cracks and holes, increase the amount of the strengthening phase and Si content, improve the micro hardness and wear resistance.The principle and process of the electrical discharge surface strengthening assisted with ultrasonic vibration and mixed powder in the dielectric was studied. The single pulse discharge process was studied from the stages of the dielectric ionizing and powder polarization, the breakdown of the dielectric and the discharge channel formation, the powder melt and participate in alloy reaction, the layer formation and the deionization. The series discharge was formed because of the powder. The powders in and around the discharge channel were melt into small particles or liquefaction, gasification under the action of the heat, which were transferred to the surface of workpiece and formed the strengthening layer under the impact of the ultrasonic vibration.The influence of the powder concentration, pulse discharge energy and ultrasonic vibration amplitude on the strengthening process was studied. The uniform thickness layer can be formed under the conditions of powder concentration 60g/l, pulse duration 24μs, pulse interval 42μs, peak current 2.4A, and ultrasonic amplitude 2μm. XRD and EDS analysis showed that there were TiC powders in the layer. The surface quality and wear resistance of the layer were studied under different conditions, and the results showed that under the condition of low pulse discharge energy, low ultrasonic amplitude and specific powder concentration, some powder can be doped into the workpiece surface and formed the layer with lower surface roughness. The wear resistance of the layer was significantly improved and the friction coefficient of the strengthened surface decreased. The powder was throw off the workpiece surface, and the surface strengthening effect was lower under the condition of the high peak current or the high ultrasonic amplitudeThe electrical discharge surface strengthening technology assisted with ultrasonic vibration and mixed powder in the dielectric was studied under low pulse discharge energy. Assisted with ultrasonic vibration can form the layer free of cracks, holes and other defects, and the surface roughness, Ti content and the wear resistance was improved. The surface quality and performance of the layer decreased with the pulse discharge energy increase when the tool electrode was assisted with ultrasonic vibration.