Investigation On Micro-arc Oxidation Ceramic Film Preparation And Growth Process Mechanism On The Surface Of Aluminum Alloy By AC

Micro-arc oxidation is also called the micro plasma oxidation process or anode spark deposition process.Through the micro plasma discharge on valve metal surface,involving electrochemical,chemical and thermal plasma chemical processes,which are occurred on the surface of metal to produce ceramic film.Because of oxide film layer structure is growth in situ,it has good thermal shock,high wear resistance and corrosion resistance.So micro-arc oxidation process is especially suitable for the high speed movement parts and components.Micro-arc oxidation process has following advantages:equipment is simple,low requirements for parts pretreatment,using alkaline electrolyte which the environmental pollution is small,processing parts ability as well.As the new material surface treatment technologies,Russia,Ukraine,the United States,Britain,Japan and other countries have given close attention to the research of this technology.China has developed this technique in recent years.Micro-arc oxidation process test was studied in the surface of 2A12 aluminum alloy using AC power.According to the experimental data of process parameters,it is concluded that micro-arc oxidation process parameters change rule:in ceramic oxide film growth process,the change rule of parameters can be divided into three periods;in the first period,initial oxide film is broken-down,the value of current slowly declining and the value of voltage between initial oxide film slowly rising;in the second period,The second time to oxide film breakdown is happened,value of current rapidly rising and voltage between initial oxide film rapidly falling;the third period for stable oxide film growth period,the current and the voltage between the film are stability.Initial oxidation film breakdown belongs to the solid dielectric breakdown,tests to estimate its breakdown field strength order of magnitude about 2000 kv/cm,which associated with the dielectric properties of oxide film,electrolyte composition and concentration have no effect on it.Base on the rule of parameters change by micro-arc oxidation process test,and the surface morphology of the ceramic oxide film that generated in different period,the growth process of ceramic oxide film is divided into three stages:The first stage of the initial oxide film breakdown discharge,initial oxidation film breakdown is solid dielectric breakdown;It lead to the aluminum oxide,and generate 1-5μm in diameter size ceramic oxide granules.The second stage for the formation of porous ceramic oxide film and breakdown discharge,with the increase of the number of ceramic oxide granules,granules are full of sample surface,and gradually form ceramic oxide film;Because the ceramic oxide film is formed by ceramic oxide granules,the structure of film is porous film structure;In the process of micro-arc oxidation,oxygen and hydrogen molecules are adsorbed in the pore of ceramic film,and formed gas film.Gas film is broken-down and occurrence micro-arc discharge,when the gas film is broken-down,a large current through the ionized gas pass to the initial oxidation film.Joule heating effect produce high temperature that make alumina oxidize and generate aluminum oxide,Aluminum oxide melt at high temperature.At the end of the micro-arc discharge,fused alumina are rapidly cooled by electrolyte.In the process of cooling,crystallization change of alumina occurs and generatesα-Al₂O₃ andγ-Al₂O₃.Third stage for ceramic oxide film growth stage,micro-arc discharge continues in the discharge hole burning-out zone,which make burning-out area expands and thicken unceasingly;At the same time new burning-out area also appear constantly;Burning-out area connect and fusion each other,finally form the whole ceramic oxide film on the sample surface.With ceramic oxide film thickening,the field intensity between the film gradually weakened,finally is lower than gas film breakdown field intensity;Micro-arc discharge gradually stopped;At this time,ceramic oxide film stop growing,the film thickness is the maximum thickness.According to the mechanism of ceramic oxide film growth process,in this paper,micro-arc oxidation process parameters are designed and validation tests are curried on.Current density of aluminum alloy electrode surface is the important factor that affect the growth of ceramic oxide film:when the current density of aluminum alloy electrode surface is less than 30 A/dm²,it is difficult to form large-scale gas film breakdown discharge phenomenon on the surface of aluminum alloy electrode,ceramic oxide film can’t continue to grow;When current density of aluminum alloy electrode surface between 30-70A/dm²,there are mass gas film breakdown phenomenon on the surface of aluminum alloy electrode,ceramic oxide film can be sustained growth;When current density of aluminum alloy electrode surface is larger than 70A/dm²,the large discharge energy will destroy the ceramic oxide film.Through the validation tests,ceramic oxide film can be obtained,film thickness is more than 100μm,and the micro-hardness is more than 1500 HV.The quality of ceramic oxide film is excellent.So the micro-arc oxidation by AC power process has engineering application value,it may widely use in the aluminum alloy surface treatment field which is not high demand on the film thickness.The method that can improve the ceramic oxide film thickness is increasing current density,but due to the 50HZ frequency of the AC power,the single cycle discharge time is too long,if use higher current density,easy to produce larger quantity of heat,which can destroy the ceramic oxide film.In the later study,AC inverter will adopted,study on the influence of ceramic oxide film growth by different frequency,which can expand the application scope of alternating current on the micro-arc oxidation technology.