| Due to the limited reserves of fossil fuels and the environmental problems related to fuel emissions,the automotive industry is promoting toward the lightweight to reduce fuel consumption.The use of magnesium alloy can significantly reduce the weight of the car under the premise of meeting the structural strength,and the magnesium alloy also has great recycling potential,so it is very promising to solve the problems about environmental pollution and lack of resources.Unfortunately,Magnesium alloys is highly chemically active and is easily corroded even at room temperature,which limits their practical application.Therefore,it is urgent to find a protective coating to improve the corrosion resistance of magnesium alloys,thereby expanding the application range of magnesium alloy.Based on the Al film with good corrosion resistance and strong self-repairing ability,in this paper,the process,organization and performance of high-purity Al film preparation on the surface of AZ31 magnesium alloy are studied by using DC magnetically controlled sputtering technology.Firstly,the process parameters are optimized by orthogonal test with the grain size and thickness of Al film as evaluation indexes,and the main process parameters affecting the microstructure of Al film are obtained.Secondly,the effects of main process parameters on the microstructure and corrosion resistance of Al film are studied by single factor test.Finally,the corrosion behavior of Al film on AZ31 magnesium alloy in 3.5 wt.%Na Cl solution is revealed by immersion test.The corrosion mechanism was analyzed according to sputtering theory and electrochemical corrosion theory,and the main findings are as follows:(1)the optimal process parameters for obtaining smaller grain size and thicker Al film on magnesium alloy surface are as follows:150W sputtering power,30min deposition time,300?substrate temperature,1.5Pa sputtering pressure,1×10-3Pa vacuum degree.The aluminum film deposited under the optimal process parameters is polycrystalline and has a cube structure of the center of the face,and the optimal orientation of the crystal surface is selected in(111);The surface of the film is distributed with large and uniform triangular conical particles,the particle size is about180nm,and the film and the substrate are more closely bound,and there is a dense transition layer at the membrane-based interface,which has positive significance to improve the corrosion resistance of magnesium alloy.(2)The results of single factor experiments show that:with the increase of sputtering power,the thickness of Al film increases linearly,and the density first decreases and then increases.When the sputtering power exceeds 100W,there is no obvious effect on the orientation of Al crystal;with the increase of background vacuum degree,the density of Al film increases.When the background vacuum degree is increased to 3×10-3pa,the thickness of Al film gradually tends to be stable,and the density of Al film increases;with the increase of deposition time,the thickness of Al film increases greatly,and the grain size of the film becomes larger,and the grain stacking is not compact,and there are a lot of holes and other defects.(3)The corrosion resistance of magnetron sputtered Al film on magnesium alloy varies with sputtering process.Comparing with the low sputtering power,the Al film with high sputtering power has the best corrosion resistance.However,the excessive sputtering power will cause fine cracks in the Al film,which is unfavorable to the corrosion resistance of the Al film.Increasing the background vacuum can reduce the porosity of Al film and enhance its corrosion resistance.Although the extension of deposition time will thicken the film layer,it will increase the grain gap,resulting in an accelerated corrosion rate.Through the influence of magnetron sputtering parameters on the corrosion resistance of Al films,the optimal sputtering parameters are obtained as follows:150W sputtering power,1×10-3Pa background vacuum,60min deposition time.Comparing with AZ31 magnesium alloy substrate,the corrosion potential of Al film in3.5 wt.%Na Cl solution increased from-1.538V to-1.29V,and the corrosion current density decreased from 1.538×10-5A/cm2to 6.927×10-7A/cm2,which decreased by about two orders of magnitude,indicating that the corrosion resistance of Al coated magnesium alloy substrate was enhanced.(4)The results of immersion test show that the surface of magnesium alloy matrix has changed from smooth to uneven,and there are many obvious cracks,indicating that the matrix has been seriously corroded.In contrast,there is no obvious corrosion pit on the surface of magnesium alloy after Al plating,but with the extension of immersion test,the Al film will be corroded by alkali,resulting in the loss of its protective effect. |
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