Research On Deep-type Hole Machining By Aerosol Dielectric Electrical Discharge Machining Ablation

Deep-type hole machining is one of the important processing fields of industrial manufacturing. With the development of science and technology, the renewal cycle of product is getting shorter and shorter. New deep-type hole parts of high hardness, high strength, hard machining appear constantly appeared, and higher requirements for the efficiency, machining quality and processing cost of deep-type hole machining is put forward. Electrical discharge machining(EDM) is not subject to the physical and mechanical properties of the workpiece(hardness, strength, etc.) and has no mechanical cutting force, etc. So it is suitable for the machining of difficult-to-cut materials, special and complex shaped parts. However, the corrosion energy of EDM is restricted by the output energy of pulse power supply. So the material removal efficiency is low. In addition, the difficulty of chip removal is also the key factor to restrict the application of EDM in deep-type hole machining. For the problem, the paper proposed the aerosol dielectric EDM ablation technology to machine the deep-type hole. It used aerosol formed by homogeneous mixture of "water-based-oxygen" as the dielectric. It changed the source of corrosion energy and the main energy of material removal. The technology replaced the pulse discharge energy with chemical energy released from the burning of material. With the high pressure of dielectric flow and the explosion effect, the technology promoted the chip removal and reached the purpose of machining the deep-type hole. Specific works were as follows:(1) The atomization device of "water-based-oxygen" and the deep-type hole machining system of the aerosol dielectric EDM ablation was designed and built. The pressure of oxygen and water, oxygen on-off time can adjust at any time according to processing requirements.(2) The machining mechanism of the aerosol dielectric EDM ablation was analyzed in detail, and the macro characteristics and the microscopic characteristics were studied. The breakdown model of aerosol dielectric was established, and the energy source of the aerosol dielectric EDM ablation was analyzed.(3) The comparative test of deep-type hole machining for die steel Cr12 by conventional EDM, the aerosol dielectric EDM ablation and intermittent EDM ablation were carried out under the same experimental conditions. The results show that the intermittent EDM ablation was unable to machine the hole continually because the short circuit occurred. The erosion products blocked the inner hole of electrode during machining. This phenomenon prevented oxygen from entering the machining region and induced short-circuit occurred, thereby causing the instability of deep hole machining. For the aerosol dielectric EDM ablation, the efficiency is about 5.45 times that of conventional EDM, and the relative tool wear ratio decreased by 82%. Overall, the process showed excellent surface quality and high machining accuracy.(4) The experimental research on technological rule of the aerosol dielectric EDM ablation on die steel Cr12 was carried out. The influence of electrical parameters and non-electric parameters on the characteristics of aerosol dielectric EDM ablation machining(material removal rate, relative tool wear ratio) was analyzed, and the main influence parameters were peak current, pulse-on time, the pressure of the aerosol dielectric and oxygen on-off time ratio. The results show that the material removal rate increased and the relative tool wear ratio decreased with the increasing of discharge energy. With the increasing of the pressure of the aerosol dielectric and the oxygen on-off time ratio, the oxygen concentration in the processing region increased and the intensity of combustion reaction increased, so the material removal rate increased.(5) To decrease the surface remelted layer thickness of the aerosol dielectric EDM ablation, the combined discharge machining method of aerosol dielectric EDM ablation and electrochemical machining was proposed. The homogeneous mixture aerosol formed by "electrolyte-oxygen" was used as the dielectric, and the theoretical analysis and performance tests were carried out. The typical machining process includes 5 stages namely, formation of the activated area, oxidation of the discharge point, oxidation hindered by the oxide layer, violent oxidation of the matrix metal, and micro-electrolysis of remelted layer on machining surface. The machining states include short-circuit, micro-electrolysis, spark discharge, aerosol dielectric EDM ablation- electrochemical machining and aerosol dielectric EDM ablation. The corner circular arc radius was reduced by 44.3% and surface remelted layer thickness is decreased 42.9% compared to the aerosol dielectric EDM ablation.