| With the increasing application of magnesium alloy, improvement of its surface properties is urgent. As a great potential surface treatment technology, the micro-arc oxidation (MAO) has been widely concerned. However, since it relates to materials science, electrochemistry, arc physics, power electronics and control systems and other disciplines and technical fields, it is a highly complex process affected by many factors. Therefore, many problems still did not give a definite conclusion. With the experimental and theoretical analysis methods, this paper aims to improve the corrosion resistance of magnesium alloy. By studying micro-arc discharge phenomenon in the MAO process, the electrical signal waveforms and the oxide coatings morphology, the relevance between process states, the membrane layer characteristics and signal characteristics will be further explored. On this basis, the separate local arc discharge mechanism and model and the requirements of power source in magnesium alloy micro-arc oxidation will be proposed. Besides, the adaptation of the load will be proposed by studying the characteristics of the load. And then a new pulse power output mode will be constructed, and the comparative study of power modes and parameters will be started. Eventually, the study of MAO power supply for industrial applications will be finished. The paper mainly includes the following:By referring to the living power supply forms, combined with the MAO process characteristics of magnesium alloy, experimental research platform with multiple output power pulse modes was designed and built. Taking MCU (microprogram control unit) as core to develop control system, the regulation of the output waveform of the power and characteristics of the voltage has been realized. Meanwhile, through the oscilloscope and computer, the load waveforms observation and data collection has been done.In experiment on all kinds of power output, the correlation between discharge phenomenon and the load voltage and current waveforms has been studied. It's considered that all magnesium MAO processes consist of anode oxidizing, micro-arc oxidizing and large-arc discharge 3 stages in turn with source voltage increasing. The former must be experienced and has been associated with the whole process; the latter in failed state should be avoided. Therefore, the definition of the arc igniting voltage U_a and large-arc discharging voltage U_L not only limit the MAO state, but also act as a quantitative stability index. With the lower arc igniting voltage and higher large-arc discharging, the process will be more stable. It is found that double-polarity pulse voltage output mode is the most stable one in the existing power modes. According to related research results, micro-arc discharge mechanism and models were put forward based on the MAO arc physics theory. The separate arc model is described as 4 courses: creating gas from electrolysis, arc discharging, metals oxidizing, cooling and shrinking of oxides. The oxygen-based gas mixtures, which coming from water electrolysis and anodic oxidation, attached to conductive channel; when the gas closed channel and stood the supply voltage and ionization, arc discharge occurs. Partially high temperature will cause the formation of intense oxidation of the metal. And the heat generated by oxidation will increasingly improve the local temperature leading to even more intense discharge. This self-reinforcing effect of the discharge and instantaneous oxidation form a so-called "avalanche effect". Local temperatures will make oxides melt and emit from channel; when gas is depleted by oxidation reactions, the discharge and oxidation will be put to an abrupt end. Large temperature difference results in sharp contraction of melt, and the metal substrate has a higher thermal conductivity, so the shrinking hole left inward. A separate local arc discharge will result in a micro-thermal cycle process, the temperature of which rises quickly and concentrated. With the arc extinguished, it rapidly transferred to the cooling process. The coating characteristics of MAO and the large-arc discharging phenomena are related with the micro-thermal cycling.Based on the inference on local micro-arc discharge and micro-thermal cycling mechanism, the arc igniting voltage will decrease with the concentration of electrolyte and temperature rising, and increase with the increase of coating thickness. The large-arc discharging tends to increase with the rising temperature of electrolyte and increasing supply voltage, and decrease with the pulse duty cycle decreasing. These were concluded by the experiments. Based on local micro-arc discharge and thermal cycling mechanism, the arcing time t_a and the cooling time t_c were defined. Large-arc discharge phenomenon is caused by a lack of cooling time. Accordingly, the basic requirement of MAO power was proposed and an ideal load voltage waveform was given.The study of effects of the double-polarity pulse of negative voltage shows that the ability to inhibit tendency of large-arc discharge results from the bypass effect of the load capacitance caused by the time sequence of the power-off chopper circuit, but has nothing to do with the negative voltage. Both theoretical analysis and experimental results show no sign for the existence of negative voltage. The only negative effect in double-polarity pulse power supply is to inhibit the tendency of large-arc discharge, so that it meets part of the requirements of the MAO.The film of MAO is the accumulation of metal oxides and caused by a series of discrete separate local arc discharge. The thermal and light radiation created by instantaneous arc discharge within the small area, combined with heat released by severe oxidation of the metal, resulted in the instantaneous formation of local high temperature in discharge area, making oxide molten. Experiencing sharply cold and hot, accompanied by melting and recrystallizing, the metal oxide coating with non-equilibrium structure is obtained. With the increase of voltage and time, the growth of MAO film is can be divided into early, middle and late period. Because of the early existence of a large number of shallow conductive channels, the arc spots are small and numerous, the surface of film is smooth and dense and the holes are uniform and fine. But the deposition rate is lower. In the middle period, shallow channel gradually closed, and the arc spots expended and went thinning. The uniformity and density of film declined and the roughness increased. Meanwhile, the size of pores became larger and the number decreased. So the rate of deposition is highest at this time. The late period focus on deep conductive channel discharge. Arc spots became larger and less and the surface of film became uneven, rough and loose. Probably accompanied by the melt of ridges and cracks caused by stress concentration, the deposition rate will decline.Thermodynamic analysis shows that, MAO makes the free energy of magnesium alloy reduce and the overall reaction is exothermic. Power consumption of electricity and heat produced by the reactions will eventually cause the temperature rise in the electrolyte transformation. Kinetic analysis shows that the voltage is the working conditions of MAO, and current is the reflection of its status and extent. The growth rate of film in a certain range increased monotonously with voltage and current, but simple kinetic model cannot accurately describe it.The load characteristics of MAO presents highly capacitive, and three resistors and two capacitors are available to be the equivalent circuit model, which can be further simplified to two resistors and one capacitor forming a first-order RC system. This conclusion has been tested by the simulation data analysis on the load voltage and current waveform. Strong capacitive made load voltage not easy to meet the basic requirements of MAO, meanwhile, the power supply system will be caused the current impact, so adaptive problems to the power source must be solved in these two aspects. The former can be achieved by increasing mandatory discharge circuit to release load voltage quickly; the latter should consider the absorbing or protective measures in design of the power source.Based on the requirements of the load voltage waveform and its load characteristics of capacity, a new MAO power supply mode was put forward. That is, the pulse power supply with a discharge loop, and has been realized on the experimental platform. By constructing the discharge circuit, the new type power supply supports the natural discharge, not full discharge and full discharge modes. The arcing time and the cooling time of the latter can be precisely setup by pulse parameters to solve the problem of large-arc discharge effectively.Comparative study shows that the pulse power supply with discharge loop is more suitable for magnesium alloy MAO process than the double-polarity one. Mainly because of good stability in the process, excellent controllability parameters, uniform arc spots; its efficiency of filming and the quality of coating surface and corrosion resistance in most cases are better than the latter.The influence of power loading and the parameters in MAO process was significant. Two ways can be selected to load: voltage increment and constant current. When the voltage increment is in 5V/min ~ 15V/min, or constant current in 4A/dm2 ~ 12A/dm2, the process is the best overall performance. The film quality will be deteriorated when the value is too large; the efficiency will be low when the value is too small. The reasonable range of pulse frequency and duty cycle of is 500Hz ~ 800Hz, respectively, and 20% to 30%.For industrial production, the main circuit design of the new type high-power pulse power supply of MAO, with IGBT parallel and protection measures, solves the problems of the impact load current and leads to reliable operation of power electronic devices. Taking the 16-bit high performance microcontroller 80C196KB as the core, the hardware of power supply control system has been developed. The development of system control software has been finished with the output pulse shape and voltage characteristics adjusting algorithms as the key. Taking AT89C52 microcontroller as the core to develop the process control system, human interface has been designed, guarantying the ease use of device, flexibility and maneuverability. With the designed communication protocol, the two MCU systems exchange data via serial asynchronous communication, and connect with the host computer through the RS-232C communication interface to provide support for MAO processing centers in the future. The equipment has been tested to achieve the expected technological goal, and has passed the preliminary assessment of practical application.
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