| Due to the positive electrode potential of titanium alloy,the galvanic corrosion is serious when connected with the dissimilar metal.Whereas the micro-arc oxidation(MAO)ceramic coating shows high bonding strength,excellent insulation and mechanical properties.Titanium alloy treated by MAO can improve the surface contacted resistance of titanium alloy joints,and hinder the connection corrosion current circulation,which lead to a better connection corrosion performance of titanium alloy.In this paper,the effects of different negative electric parameters on the compactness of the ceramic coating of titanium alloy was studied,and the MAO process was optimized combined with the galvanic corrosion current of different titanium alloy samples and 45 steel joints and the unloading torque of titanium bolts in salt spray environment.The influence mechanism of pulse mode and cathodic pulse electrical parameters on the compactness of titanium alloy MAO ceramic coating was discussed.The results are shown as follows:Micro arc oxidation(MAO)treatment increased the surface impedance of titanium alloy,and the higher compactness of the ceramic coating,the greater the surface impedance of titanium alloy.The ceramic coating contacted resistance was effectively improved under bipolar pulse by appropriate adjustment bipolar electrical parameters.When the coating thickness is 20μm,the surface impedance of bipolar pulse MAO titanium alloy was 20.63×105 Ω,which is about 4.15 times and 19.78 times than the unipolar pulse MAO titanium alloy and original TC4 alloy,respectively.Besides,the galvanic corrosion current density was maintained on a smaller platform with the corrosion time increasing,whereas the original TC4 alloy and the unipolar MAO titanium alloy showed a gradual increase trend.The average current density under bipolar pulse are 2.55%and 0.37%for unipolar pulse micro arc titanium oxide alloy and TC4 titanium alloy respectively.Titanium Micro arc oxidation treatment can effectively improve the connection corrosion sensitivity when connect with 45 steel.And when corrosion 1000 h the corrosion sensitivity of titanium sample treatment under bipolar pulse is always blow 0 and insensitive to connection corrosion.But it is always greater than 0 of TC4 and sensitivity to connection corrosion.When the titanium bolt was connected with the 45 steel,the unloading torque of the titanium bolt was first increase and then decreased in the salt spray environment.The unloading torque of the TC4 bolt reached a maximum value of 28 N·m at 720 h,and when the titanium bolt produced under bipolar,the unloading torque is.still increasing in 1440 h:It means that increasing the surface resistance of titanium bolt can effectively increase the connection life of titanium bolt.Under bipolar pulse,the essential discharge energy and the discharge intensity is decreased,that makes the ceramic layer much more compact.When the oxidation time is 20 min,the inner coating is 87.46%of the whole coating thickness under bipolar that is 1.25 times for unipolar,and the isolation voltage is 1.24 times for unipolar when it is 421 V under bipolar.And the highest out put power under different cathodic electric parameter is 15000 W and the minimum is 9000 W.Adjusting the cathodic electrical parameters can effectively reduce the discharge energy consumption,and decrease the bigger discharge in micro arc oxidation process,that improves the compactness of the ceramic coating.Pulse mode can effect the growth character,thickness and micro-structure of MAO coating,when oxidation 20min,the coating thickness of unipolar and bipolar is 25 μm and 20μm respectively.And under bipolar the inner coating is 87.46%than overall coating thickness that is 1.24 times than unipolar. |
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