Research On Technology And Properties Of Nano-Al₂O₃ Modified VW75 Mg Alloy Micro-arc Oxidation Coatings

As the third metal tructural material after steel and aluminum alloys,magnesium has been applied in automobile,electronic equipment,aerospace and other fields.Magnesium and its alloys can meet the lightweight structure requirements of the high-speed development of manufacturing industry at the present stage.However,magnesium alloys have poor corrosion resistance and corrosion can easily occur without surface protection,making structural parts and components failure and damage and resulting in economic losses.The characteristics,application,corrosion condition of magnesium alloy were summarized.Combined with the development trend of surface protection technology of magnesium alloy,we selected VW75 Mg alloy as base material to make an environment friendly ceramic coatings on the surface by micro-arc oxidation.The micro-arc oxidation process was exploited by means of scanning electron microscope,optical microscope,coating thickness tester and electrochemical test.We studied the action mechanism of nano-Al2O3 additive in electrolyte and the failure behaviors of magnesium alloy in thermal shock experiment.By testing the electrochemical and thermal shock resistance of the alloy after micro-arc oxidation treatment,the modification mechanism of micro-arc oxidation ceramic coatings by nano-alumina was revealed and the research on the protective performance of micro-arc oxidation ceramic coatings on the VW75 Mg alloy was improved.The influences of voltage and oxidation time on the thickness,porosity and corrosion resistance of ceramic coatings were revealed:at the same oxidation time,with the increase of voltage,the thickness,porosity and the average pore diameter increased gradually of VW75 Mg alloy surface micro-arc oxidation coatings,and the microcracks and ablation appeared at 400 V.At the same power supply voltage,with the increase of oxidation time,the thickness of the coatings increased,but the porosity decreased under low pressure,there was the phenomenon of"self-sealing hole".The thickness and porosity of the coatings play a key role on corrosion resistance,and the corrosion resistance of the alloy increased by 4?5 orders of magnitude after micro-arc oxidation treatment.After that,nano-Al2O3 was added to modify the coatings.The order of influence factors obtained by orthogonal test analysis.Nano-Al2O3 can enter the membrane by discharge channel and participate in film formation.After modification,the thickness of the coatings increased and the number of microcrack reduced.Nano-Al2O3 promoted the"self-sealing hole" and corrosion resistance of the coatings was improved.It was found that the bonding force between the ceramic coatings and substrate had a certain relation with the compactness of the coatings:the greater the compactness,the stronger the bonding force.The density of the ceramic coatings after modification was improved,and the adhesion between the ceramic coatings and substrate was correspondingly increased while the thickness of the coatings increasing.In the process of thermal shock cycle,the coatings generateed cracks and spalling due to the stress generated by the thermal stress and the internal reaction product accumulation.In the initial stage of thermal shock cycle(<60 times),the oxide inside the unmodified coating reacted with air and water,and the products accumulated in the costings.However,the ceramic coatings modified by nano-Al2O3 had fewer reaction products in the initial stage,and the weight gain of the test piece was not obvious.Thermal shock resistance was superior that of the unmodified coatings.