| In this paper, a function electrode Electrical Discharge Machining-Induced Ablation Machining(EDM-IAM) method is carried out to deal with the problem of the low removal efficiency of traditional EDM and the poor controllability and unstability of the EDM-IAM which discharges with intermittently injection of oxygen. Oxygen and working fluid pass through the independent channels inside functional electrodes simultaneously. Oxygen flow is dispersed into oxygen bubbles and distributed evenly in liquid medium through the impact of working fluid and oxygen. Then combustion atmosphere of gas-liquid mixing is formed between electrodes. This process uses the high temperature on the surface of the electrode caused by electric spark discharge as external heat source to burn the workpiece material in the oxygen bubbles, make the combustion reaction happen and remove the material of workpiece by melting and burning. EDM-IAM process including chemical energy, which is produced by metal combust is a new energy in EDM machining which can improves the ablation efficiency of it. Moreover, the function electrodes for drilling, milling, turning processing are designed and manufactured in this paper. The function Electrode EDM-IAM machining system is constructed and some analyses and experimental studies are conducted. Many control strategies and methods of the EDM-IAM are proposed. The process of function electrode EDM-IAM is explored. A good machined surface is get by trimming and in-situ synthesis of titanium alloy surface coating of TiN strengthens. The contents of specific research are:1. The machining process of the EDM-IAM which discharge intermittently through oxygen is unstable and the combustion reaction can not be carried out continuously. The function electrodes for drilling, milling, turning processing are designed and manufactured to solve this problem. Oxygen and working fluid pass through independent channels inside functional electrodes simultaneously. Combustion atmosphere of gas-liquid mixing is formed between electrodes to meet the combustion condition of metal in oxygen. By adjusting the ratio of oxygen in the reaction to control the intensity of combustion, the process of the EDM-IAM becomes efficient, controllable and stable. By using the electrode function system, experiment machine, electrode rotation drive system, working liquid cycle system and oxygen supply system, a system of the functional Electrical Discharge Machining-induced Ablation Machining is constructed with some processing methods such as drilling, mill ing, turning and so on. Some experiments of ablation are conducted, during which a highly efficient, stable and controllable continuous ablation processing is achieved.2. The machining mechanism of the functional electrodes EDM-IAM is analyzed. This paper also simulates the distribution and motion condition of medium between electrodes using simulation device. The effect of the proportion of gas and liquid pressure on the distribution and the law of motion of gas and liquid are studied. The effect of the oxygen bubbles on discharge breakdown and the electric field between electrodes is analyzed too.For studying the effect that the electrode structure and surface shape have on combustion, the structure of discharge channel, the diameter change of discharge holes and the rules of discharge channels’ migration which induced by the shape of electrode surface are studied by the single pulse experiments of electrodes with various end shape. All of these indicate that functional electrodes which are composed of thin-walled tubular electrodes are suitable for ablation machining. Through single pulse experiments of the tubular conventional EDM and ablation machining, the mechanism of combustion reaction is studied which shows that the combustion reaction is induced by the rim of discharge craters. Also, oxidation reaction which is caused by the heat from discharge can accelerate combustion. The combustion’s effect on discharge process is small. The process can run independently and play an important role in ablation. After being induced, combustion can sustain and transmits independently until the oxygen is depleted.Through the analysis of energy source and induced mechanism of EDM-IAM, we find that ablation is composed by EDM removal and combustion removal which are related and also relative independent. The precursor of combustion removal is the process of EDM which produced the external heat source of combustion. After the EDM induced combustion which relies on the release heat of itself, these two are becoming independent relatively. With lighting and heating the surrounding material continuously, combustion can be self-sustaining and transmit out independently until oxygen is depleted. The process of discharge repeats, the material is removed and the combustion reaction is induced constantly under the action of pulse power.3. Through analyzing the mechanism of EDM-IAM, in view of the features of the ablation machining composed by two relatively independent processes(EDM removal and combustion removal), and analysis of the control factors of ablation, the EDM-IAM control system which respectively control the EDM removal and the combustion removal parallelly is built. The contr ol of the EDM removal and servo feed system is realized by using the EDM servo control system. On the other hand, by controlling the average voltage between electrodes, the gas liquid ratio in the processing zone is controlled result for controlling the intensity of combustion reaction. By changing sampled servo control voltage to study the influence on EDM-IAM of it, the result can be found that the traditional EDM servo control system can meet the needs of the ablation process. Through the experiments of the pressure ratio of gas-liquid between electrodes to the conductivity, this paper studies the feasibility that the inter-electrode average voltage reflects the pressure ratio of gas and liquid, and some experiments under different press ure ratios of the gas and liquid are conducted. By studying the relations between the pressure ratios of the gas and liquid between the electrodes, inter-electrode average voltage and the material removal rate of EDM-IAM, the ratio of gas-liquid pressure can be controlled according to the average voltage between discharge gaps, and then the control of the intensity of combustion reaction is realized.4. This paper studies the influence of both the electrical parameters(pulse width, machining current, etc.) and non-electrical parameters(oxygen pressure, working fluid pressure, rotational speed of electrode.) on the function Electrode EDM-IAM. The study indicates that the processing efficiency of ablation which has significant polarity effect is tens time of that of EDM. The processing efficiency and the surface roughness value of EDM-IAM are in direct proportion to the pulse width, average current, the pressure ratio of oxygen and working fluid, while in inverse proportion to the rotational speed of the workpiece. The relative-wear of electrode is in inverse proportion to pulse width, average current, the pressure ratio of oxygen and working fluid, while in direct proportion to the rotational speed of the workpiece. This paper also analyzes the influenc e of the processing efficiency of ablation, the relative-wear of electrode and the surface roughness value. The material properties’ effect on EDM-I AM is explored by exper iments. This paper studies the proportion of the heat of combustion and melting, the effect of the properties of the oxide layer on the process ability of ablation, and the influencing factors of the relative-wear of electrode. Through the contrastive experiments of drilling, milling and turning, this paper studies the effect laws of the processing methods on the function Electrode EDM-IAM.5. To improve the surface quality of the function Electrode EDM-IAM, this paper puts forward a surface modification method with the energy gradually diminishing. According to the process redundancy and ablation depth, we select the machining parameters based on the method of the arithmetic progression which gradually reduce the ablation energy to machine the previous worse surface, and finally get truing target surface. As a result, better processed surface quality is obtained even though the processing efficiency is relatively high. To deal with the defect of oxide layer produced in ablation, we pump the Nitrogen into the processing area by using function electrode or flexible electrode which can make the Nitrogen react with the Titanium material and compound TiN oxidation protective coating on the ablation surface by using the heat generated by EDM to improve the composition and properties of the surface material. Researches indicate that the thickness of the TiN protective coating obtained on the original machining surface is more than 1 mm, and the micro-hardness is about 1859.6 HV. The TiN protective coating and the basal body are interpenetrated with each other, and the adhesive strength is good. As a result we can get a machining surface with higher hardness and better surface quality by deseaming and strengthening the ablation surface. |
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