Micro-arc Oxidation Process And Mechanism Of 7E04 Aluminum Alloy For Petroleum Drill Pipe

Aluminum alloy drill pipe is widely applied in the field of oil drilling currently.Considering the poor surface properties of the aluminum alloy drill during practical application,this project puts forward to solve these problems through the surface treatment of micro arc oxidation technology.In this paper,7E04 aluminum alloy which was developed for drill pipe use was applied as matrix material.Under the silicate electrolyte system,we investigated the effects of four main process parameters:sodium silicate addition,oxidation current density,duty cycle and oxidation time on the basic properties of micro-arc oxidation ceramic coatings by single factor method,then we found out the best basic parameters.With the optimization process,the influence of Na2W04 concentration on the performance of the micro-arc oxidation ceramic coatings studied by these analysis methods:SEM,EDS and XRD.The modification mechanism of Na2W04 during micro-arc oxidation process was discussed ultimately.The result of the basic process of micro arc oxidation showed that,at a temperature of 20 ? and a frequency of 100Hz,the optimal parameters of micro arc oxidation for 7E04 aluminum alloy are these,the concentration of Na2SiO3 is 15.Og/L,the best current density is from 4A/dm2 to 8A/dm2,the best duty ratio is form 20%to 60%and the best oxidation time is 30min.In this condition,the thickness,micro-hardness and corrosion resistance of micro arc oxidation ceramic coatings can be improved a lot.During the Na₂WO₄ modified process,we adopted the oxidation current density of 6A/dm2 and duty cycle of 40%respectively.The addition of Na2W04 was 2.0g/L,4.0g/L,6.0g/L,8.0g/L,10.Og/L respectively and then we prepared five groups of micro-arc oxidation modified ceramic coatings.The investigation exhibited that the color of the ceramic coatings gradually deepened with the increase of Na₂WO₄ concentration,which varied from grey to black gradually.During the micro arc oxidation process,oxidation voltage decreased slightly and the thickness of the coatings was gradually decreased;however the content of W showed an upward trend in the coatings and its main forms in the coatings are WO3 and W,which have high melting points and hardness,at the same time,the surface morphology of the coatings gradually smooth and dense,all these makes the micro-hardness and corrosion resistance of a substantial increase.During the micro arc oxidation process with Na2W04,the plasma discharge caused the high temperature and high pressure,so the WO42-can be gradually transformed iinto WO3,which mixed with the substrate oxide and formed in the coatings.Due to the high melting points of WO3,the weak areas of the coatings decreased,and then the intensity of the discharge decreased.The spark discharge is finer and more homogeneous,which restrained the generation of surface micro-pore size during the discharge.