| Micro-arc oxidation (MAO) is a new surface treatment technique applied to valve metalsand their alloys to form the ceramic membrane on the surface with a high-quality performance.It has a wide application in the aviation, aerospace, mechanical, electronic and biologicalmaterials etc. as the resistance to abrasion, corrosion resistance and insulation properties ofthe alloys forming the protective film are greatly improved. However, since it relates tomaterials science, electrochemistry, arc physics, power electronics and control systems andother disciplines and technical fields, it is a highly complex process affected by many factors.Therefore, many problems still did not give a definite conclusion.This paper aims to manufacture the power supply which apply to MAO process. With theexperimental and theoretical analysis methods, studying the relevance between process states,the membrane layer characteristics and signal characteristics. On this basis, the separate localarc discharge mechanism and model and the requirements of power supply in MAO will beproposed. Besides, the adaptation of the load of power supply will be proposed by studyingthe equivalent load model. Accordingly, designing a micro-arc oxidation pulse power supplywith multiple output modes. The main research work is as follows:It’s considered that the MAO process consists of three stages, such as anode oxidizing,micro-arc oxidizing and macro arc discharge in turn with source voltage increasing, whichcan be distinguished by the micro-arc igniting voltage and the large-arc discharging voltage.The model is described a local arc discharge as four courses: water electrolyzing, arcdischarging, metals oxidizing, and cooling and shrinking of oxides, from which result a localthermal cycle make the tendency of large-arc discharge all along in MAO. The importantpurpose of pulse supply is to break the large-arc discharge by intervening proper cooling time.The load characteristics of MAO presents highly capacitive, it made load voltage noteasy to meet the basic requirements of MAO, meanwhile, the power supply system will becaused the current impact, so adaptive problems to the power source must be solved in thesetwo aspects. The former can be achieved by increasing mandatory discharge circuit to releaseload voltage quickly; the latter should consider the absorbing or protective measures in designof the power supply.A new micro-arc oxidation pulse power supply with multiple output modes was designed,which includes main circuit and control system, based on the requirements of the load voltagewaveform and its load characteristics of capacity. The main circuit includes rectifier circuit,filter circuit, chopper circuit, the discharge circuit and protection circuit. So that, it can outputmultiple modes. The control system consists of two parts: power master control system andhuman-machine interface (HMI) systems. The80C196KB MCU was serving as the core ofcontrol system, which not only can collect the voltage and current signals and the parametersset in manual mode, but also can generates the trigger pulse of SCR and drive pulse of IGBTand monitor the working status to protect them. The AT89C52MCU was serving as the core of the HMI system, which can realize the preset of power parameters, power output mode andthe working methods. It also can display the power supply parameters, time and work statusin real-time during the process of the MAO. The data and command can be exchanged usingserial interface communication between the two control systems.Two ways can be selected to load: constant voltage increment and constant current. Thepulse power supply for MAO also can provide five kinds of output modes: the DC mode, thesingle-polarity pulse mode, the double-polarity pulse mode, the pulse power supply withdischarge loop mode and the AC square-wave mode. Besides, the process control of micro-arcoxidation was achieved, and it can provide technical support for the further study of theprocess and mechanism of MAO. |
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