Reseach On Preparation And Mechanism Of Mo_x^y- Modified Micro-arc Oxidation Coating On Aluminum Alloy Surface

In Oil industry,the high-power diesel generator's piston used in well cementing is subjected to heat-fatigue resistance,wear and corrosion.But the insufficiency of hardness,heat resistance and corrosion resistance on the surface of the aluminum alloy is restricted to the application of aluminum alloy in oil industry.The study found that the micro-arc oxidation coating formed on the surface of the aluminum alloy after the micro-arc oxidation treatment could meet the operating requirement of piston.Nevertheless,the controlling of the microstructure and the macro properties of the coating during the micro-arc oxidation preparation had not been taken into consideration which could improve the performance of micro-arc oxidation coating.Three additives,Na2WO4,Na2MoO4 and NH4VO3 containing MOxy-,are introduced into the sodium silicate-sodium hydroxide oxide liquid respectively.In this paper,ZL108 aluminum alloy piston was used as the matrix material,from the perspective of material doping modification,the influence of three kinds of modified additives on aluminum alloy micro-arc oxidation voltage,oxidation coating properties,morphology,coating structure,phase composition and the doping modified form was explored.Then the influence of current density,oxidation time and frequency on properties of modified micro-arc oxidation coating was investigated.On this basis,the mechanism of micro-arc oxidation coating on aluminum alloy surface modified by different MOxy-is depth analyzed in order to explore a new way to improve the performance of micro-arc oxidation coating on aluminum alloy piston.The research showed that with the increasing of modifier,the voltages of Na2WO4 oxidation raised;while the voltages of Na2MoO4 decreased and which of NH4VO3 increased firstly and then decreased;the thickness,hardness,surface and cross-section W,Mo,V elements content of oxidation coating increased as a whole.Meanwhile,Na2WO4 played a hole-broaden role and the corrosion resistance of the coating reduced.However,Na2MoO4 play the effect of shrinking hole.NH4VO3 can realize broadening and shrinking hole and corrosion resistance of the coating increased firstly and reduced later.W.Mo,V elements mainly existed in the form of WO3 and MoO2,V2O5 in micro-arc oxidation coating.The voltage,thickness and hardness of three kinds of modified oxidation coating increased as the current density increased.The element content of W,Mo,V,and O increased in the coating,while the content of Al element decreased.The size of pore on NaWO4 modified oxidation coating enlarged,and the micro-pore on the surface of NH4VO3,Na2MoO4 modified coating gradually disappeared.The phase composition of the micro-arc oxidation coating changed with the alteration of current density.With the increase of time and frequency,the thickness and hardness of Na2WO4 and Na2MoO4 modified oxidation coating increased,which of NH4 VO3 increased with time while decreased after the increase of frequency.In view of the establishment of MOxy-modified micro-arc oxidation model on the surface of aluminum alloy,three kinds of additives in oxidation liquid dissolved into metal acid radical anion,as WO42-.MoO42-and VO3-.They adsorbed on the surface of aluminum alloy through the effect of perturbation of high voltage direct-current electric field and bubbles on oxidation surface and,generated a thin cement layer.It generated MaOb soluble in micro-arc oxidation coating by the effect of arc remelting and oxidation reaction to alter the aluminum alloy micro-arc oxidation coating microstructure and macro properties.In this section,bubbles played a key role in the micro-arc oxidation.The water vapor bubble generated from resistance heat and oxygen bubbles from water electrolysis oxygen ions grew up rapidly under the action of heat.The internal pressure of the bubbles dropped sharply to form a low vacuum negative pressure,which generated the direct current glow discharge at the action of plus direct current field.The four stages of direct current glow discharge were corresponding to the stages of micro-arc oxidation voltage.According to the aluminum high temperature oxidation theory and ohm's law,the thickness of the aluminum alloy micro-arc oxidation coating calculation formula was derived,which involved the technological parameters related to the coating thickness in the oxidation process.The research results provided a theoretical basis for controlling to the microstructure and the macro properties of the micro-arc oxidation coating on the high-power diesel generator's piston.The purpose of further improving the performance of micro-arc oxidation coating was achieved.