Ultrasonic Vibration Assisted Powder-mixed Electrical Discharge Surface Strengthening

Ultrasonic vibration assisted powder-mixed electrical discharge surface strengthening can be achieved by using ordinary EDM machine tools. During the strengthening procedure, the tool-electrode was assisted with ultrasonic vibration, and the powder was mixed into the dielectric, spark discharge occurred between tool-electrode and workpiece materials, in which the spark discharge pot of tool-electrode and workpiece was melted and even gasified as well as the powder in the electric discharge channel, a strengthened layer was obtained on the surface of workpiece by the effect such as ultrasonic vibration to let the melting powder mix with melting workpiece and metallurgy reaction occur. The hardness of the strengthened layer as well as the wear and corrosion resistance was improved greatly.The mechanism of powder-mixed electrical discharge surface strengthening was elaborated systematically from the stage of the apply of external voltage,the breakdown of the dielectric,the discharge channel formation and the discharge energy transformation and distribution, the migration of the electrode material to the surface of workpiece, the layer formation and the deionization in the paper. The influence of the powder in the dielectric on the breakdown of dielectric and discharge channel formation was paid more attention to study.Ultrasonic vibration assisted powder-mixed electrical discharge surface strengthening technology was proposed. The influence of ultrasonic vibration on the powder-mixed electrical discharge surface strengthening especially on the interpolar electric field and the state of discharge gap was systematically analyzed. Ultrasonic vibration can improve the discharge gap, reduce the breakdown time delay, increase the use ratio of discharge pulse. Ultrasonic vibration can accelerated the melt to reach to the workpiece surface with high speed, and improve the property of the strengthening layer eventually. The results of experiment research for studying the influence of ultrasonic vibration show that, with the increase of amplitude, the loss of tool-electrode aggravates; Ti content of the strengthening layer increases and then decreases with the increase of amplitude; After tool-electrode assisted with ultrasonic vibration, the strengthening layer have more uniform thickness and less cracks, holes. Through the analysis of five factor four level orthogonal experiment, ultrasonic vibration assisted powder-mixed electrical discharge surface strengthening can enhance the surface of workpiece under suitable condition. The thickness of strengthening layer can reach10μm, the content of the layer mainly includes the element of titanium, carbon and iron. Meanwhile, the analysis shows that the formation of strengthening layer is mainly related to the peak current and pulse duration, and eventually the chart of energy range was draw out.By analyzing the orthogonal experiment, the influence of pulse discharge energy, powder concentration and ultrasonic vibration amplitude on Ti content and performance of strengthening layer was studied, the primary and secondary order of five factors for the effect of Ti content and performance of strengthening layer was obtained. Optimal combination of parameters, which was obtained by Multiple Indicator balance method of orthogonal experiment, is peak current2.4A, pulse duration24μs, pulse interval130μs, powder concentration60g/L, ultrasonic amplitude2μm. Under the condition of optimal parameters, the performance of the sample, such as Ti content and surface roughness, is better than the samples of the orthogonal experiment, microhardness of the strengthening layer has improved markedly, even two times of the substrate. Wear resistance of the layer has also been greatly improved, the wear volume is significantly less than the matrix material, while the friction coefficient of the layer greatly reduced, as about half of the coefficient of friction of the matrix.The surface roughness of specimen obtained from Orthogonal experiment are relatively poor. For getting much better surface roughness, the four factors and three levels orthogonal test was proceeded in order to get the rules and effect of the pulse duration(A), peak current (B), the powder concentration(C), ultrasonic amplitude(D) of the surface roughness of the strengthening layer. By using the variance analysis (AVOVA), four factors and their interactions on the impact of the strengthening layer surface roughness was obtained and the optimal combination of factors was A1B1C3D1.The ultrasonic vibration assisted powder-mixed EDM surface roughness quadratic model was established by using the response surface methodology, comparative analysis of every two factors is obtained through the response surface.