| Micro-arc oxidation (MAO) is a novel surface modification technique to form oxide ceramic coatings in-situ on the surface of metals, such as A1, Mg, Ti and their alloys. Because the oxidation films have excellent wear and corrosion resistance, MAO has drawn extensive attention. Improving coatings performance and extending applicable scope for more types of metal are important research fields of the technique. However, with the development of MAO in industrial application, high cost resulting from high power consumption has also been a vital issue, so to reduce power consumption is another research focus. Pulse power supply is a key equipment of the MAO treatment, and its characteristics and parameters, such as form of pulse and pulse energy, have remarkable influences to the quality of the oxidation films and the power consumption. Therefore, researching the load characteristics of pulse power supply for MAO, and explicating energy efficacy of pulse, and improving the characteristics and pulse form of power supply based on aforementioned analysis, have important significance in lifting coatings performance, reducing power lossness and promoting MAO industrial application.Via analyzing the effects of current pulse amplitude, duty cycle, frequency of the square-wave power supply on MAO coatings, it is clear that the pulse power is the key factor of impacting oxidation. Based on the equivalent circuit model of porous membrane derived from electrochemical impedance spectroscopy, and adopting the method of parameter fitting for the actual waveform of load, power load equipment circuit model is set up, which can mirror load characteristics. On the basis of the aforementioned model, the energy efficacy of voltage pulse and square-wave current pulse on MAO load are analyzed, which can make clear that the consumed-energy which is useless to coatings depositing is the main reason resulting in degradation of coating quality and performance and increase of energy consumption. Above anlaysis would provide some dedign basis to improve power supply characteristics and optimize the pulse form.To improve pulse energy efficacy and control energy of a single pulse, a power supply based on flyback converter which is capable of outputting the spike current pulse with a steep front edge is proposed. The power supply is made up of several current pulse units, and it could work under different modes by assembling units in different forms to meet the technological needs. The coupled inductor is used to store and transfer energy of current pulse unit, which can realize high-frequency isolation. Peak current control and frequency auto-adjustment control of the inductor's primary winding are used to make the power supply work under Critical-DCM or DCM mode, which could ensure that the single pulse energy could be regulated. In order to lift the pulse energy and increase the output power, the coupled inductor is designed using Fe-based amorphous alloy with high saturation magnetic flux density. To ensure the controllability of output current's pulse energy, the pulse form control is analyzed. The equivalent circuit model during energy exchanging between primary winding and secondary winding of coupled inductors is established. And based on the analysis of effect of the leakage inductances of primary/secondary windings, load parameter, and its residual voltage on current pulse form, some effective methods is proposed to control pulse form.In order to meet different requirements of MAO process, some parallel combinations of current pulse units based on flyback converter, which can constitute different modes flexibly, are analyzed in detail. When the pulse units are direct parallel connected, the energy of pulses is different and pulses are asynchronous because of the difference of inductor of primary winding.The master-slave control mode is used to ensure synchronization when a large number of units are parallel connected; the amendatory peak current control is used to ensure consistency of pulse energy when a few units are in parallel. To increase output power and reach exciting arc voltage as soon as possible, the units are interleaved parallel connected during anodic oxidation phase, which also increase pulse frequency and output current and maintain zero current switch on under high pulse frequency. When a variable polarity current pulse power supply is constructed using two units in reverse parallel connection, the effect of circulating current produced in the secondary windings of two units on the output current pulse and switching condition is analyzed. And based on the analysis, a combined topology of variable polarity current pulse power supply is proposed, which utilize thyristors with self freewheeling circuit to interdict the circulating current and ouput positive and negative current pulse bidirectionally. To validate energy saving effect of power supply based on flyback converter and the energy efficacy of spike current pulse, some experiments about MAO on the surface of cast aluminum workpiece are carried out. Under the same number of workpieces and the same end processing conditions, results show that utilizing the pulse power supply based on flyback converter can decrease the unit energy comsumption remarkably. And under the same pulse amplitude and pulse width, Film thickness obtained by square-wave current pulse and spike current pulse almost is the same, but the spike current pulse supply has lower output power and better pulse efficacy.In addition, the pulse power supply based on flyback converter is also used in MAO process on surface of steel. Without necessary hot-dipped aluminizing in general methods, the micro-arc oxidation film of Q235 steel is prepared. Via technics experiment, the effects of peak current, frequency, process time on the film surface morphology, thickness, and corrosion resistance are analyzed. It is also tried to deposit films on stainless steel, and the results show that it is unable to deposit coatings because load impedance of power supply for MAO is high and it is difficult to form barrier. |
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