The Mechanism Of Micro-discharge Of Micro-arc Oxidation On Magnesium Alloy

Micro-Arc Oxidation is a new surface treatment technique,developed from anodic oxidation,it uses micro-arc discharge in the metal oxide ceramic membrane surface can increase the corrosion resistance of magnesium alloy.In this paper,the Micro-Arc Oxidation processing on magnesium alloy AZ91D,for the purpose of the mechanism of micro-arc discharge and requirements of its impact on the power and the impact of electrical parameters on micro-arc oxidation.Analysis of the different forms of power and electrical parameters of magnesium alloy micro-arc oxidation process and characteristics of the phenomenon,as well as the film growth rate,morphology, structure etc,on the test platform which build with self-developed high-power, multi-output of the pulse.Made of mechanism of micro-discharge and model of micro-arc oxidation on magnesium alloy,obtain requirements of its power,based on this supply a new form of power-with the pulse discharge power circuit,and focus on the impact of the pulse width and over-voltage of micro-arc oxidation on magnesium alloy.Study shows that:Micro-arc oxidation process can be divided into three stages, namely:anodic oxidation stage,the micro-arc oxidation stage and continuous arc stage. Ceramic layer components by Mg,MgO,MgAl₂O₄,Mg₂SiO₄ phase,the key elements is the O,Mg,Al,Si,F.The process of micro-arc oxidation on magnesium alloy essentially is statistical results for a single micro-discharge.The surface of magnesium alloy grow the layer of insulating film thin and transparent immediately after electricity,when the a weak bubble department has been in strong breakdown voltage, this part of gas was dissociation use energy of electron impact when breakdown,form of discharge channels,generated plasma,there is the phenomenon of gas discharge, and then a series of high-temperature reaction,generate to ceramic film.Power form plays a crucial role in of micro-arc oxidation process on magnesium alloy,the form of DC pulse will cause continual arc and loss by burning test specimen,AC pulse can inhibit the producing of continual arc because of the existence of negative pulse,however,the existence of negative voltage does not benefit on the micro-arc oxidation process.The design principle of power form with discharge circuit is adding a discharge circuit at both ends of load after positive pulse, took experiments for micro-arc oxidation of this power can inhibit the producing of continual arc,and the rate of film growth is higher than in AC pulse power form. Control pulse width can regulator the way of a single arc extinction,avoid to mutual interference between arc,study shows that bubbles will be fully ionized at the same time there will be no extra energy loss,and generate a smooth surface layer of high corrosion resistance at pulse duty cycle for 200～300μs range.Over-voltage regulation is to control the parameters of energy,film thickness increased with the over-voltage increase,however,the film surface roughness and the corrosion resistance will reduce at excessive over-voltage,study shows that the coating layer is relatively thick and have a high degree of corrosion resistance of at over-voltage is 100～150V range.