The Research On Waveform And Negative Pulse Effect For Microarc Oxidation

Micro-arc Oxidation which is also named as micro-plasma oxidation or anodic sparkdeposition is a new method for surface treatment. The surface layer which is formed by thistechnology has excellent physical-chemical features, such as excellent bonding strength withthe substrate, high hardness, wear resistance, corrosion resistance, high thermal shockresistance, good electric insulation layer, higher breakdown voltage and so on. More and morescholars have been interested in the field of material surface treatment for the technology ofMicro-arc Oxidation.The power source is the key equipment for ceramic membrane which is produced by thetechnology of micro-arc oxidation. Most power supplies, which are used for disposing thesurface of metal in this field, are DC or unipolar pulse at present. Based on80C196KC, thedouble-pulse VVVF Micro-arc Oxidation power is designed to eliminate the deficiency ofcurrent power supply. This is a high power bipolar power supply, the parameter such asamplitude, frequency, duty ratio and number of positive and negative pulse can be adjustedcontinuously.Firstly, this paper introduces the design of the power source’s topologic structure. Themain circuit of this power source combines the SCR rectifier with IGBT chopper circuit.Through the rectifier circuit, the pulse amplitude can be adjusted; Through the chopper circuit,the output of pulse can be realized. So this structure can simplify the power structure andcontrol strategy; and also decrease the volume. As for the controller，based on combining Intel80C196KC with TC787, The trigger pulse coming fromTC787is used to control the angle oftrigger and the voltage can be adjusted consecutively. The signal coming from microcontroller80C196KC is used as drivable signal of chopper circuit to control the IGBT and thus it canattain the purpose of controlling the discharge wave-form.Secondly, this paper discusses the role of negative voltage on the micro-arc oxidationfilm. Compared with only a forward voltage, current density in process of magnesium alloymicro-arc oxidation imposed negative voltage is higher and more stable. This will bring thebeneficial effects to the magnesium alloy growth and development of the surface of theceramic coating. The negative voltage load can increase the electron current density;strengthen the role of forward voltage in the micro-arc oxidation reaction; reduce the oxidefilm surface porosity; improve its flatness and films the improvement of corrosion resistance.