| Currently, as the lightest structural metallic material in practical application, magnesium alloy not only have low density, high strength to density ratio and stiffness to ratio, excellent damping properties, good electromagnetic shielding characteristic and other physical properties , but also have better merits in recovery with low energy consumption and good process ability, which make its applications have more obvious advantages than other structural metallic material in aircraft industry, automotive industry, and 3C electronic industry. However, the reason for the wide application of magnesium has been greatly limited is that standard electrode potential of magnesium is only -2.37V, the surface oxide film isn't compact, magnesium and its alloys even in the atmospheric environment in the store will soon be corrosion. In this paper, the method that Zinc-Aluminum composite coating on AZ91D alloy was obtained by depositing Zn and Al coatings successively on the alloy surface using vacuum evaporating technique and further hot pressing at certain under the constant load in the atmosphere was used to improve the corrosion resistance, and finally the metallurgical bonding between the protective coating and matrix can be obtained.The results of two types coating experiment show that: the integrated and compact coating can be formed on the surface of magnesium alloy by vacuum evaporation; the stable aluminum coating can be obtained by using hot pressing technique for 1~4h at 420℃under the constant load of 32MPa; the stable zinc-aluminum coating can be obtained without hot pressing treatment; the bonding performance between zinc-aluminum coating and matrix is better than aluminum coating after using hot pressing for 1~4h at 400℃under the constant load of 32MPa.The microstructure and morphology of the diffusion layers obtained in different processes were observed by and scanning electron microscopy(SEM),and the chemical composition and phase composition were analyzed by energy dispersive spectrometer(EDS) and X-ray diffraction(XRD). The result shows that: the depth of mutual diffusion layer between aluminum coating and matrix after hot pressing for 4h was 2~5μm, and the phase of interfacial zone among the aluminum coating and the matrix mainly includeα-Mg,α-Al, and Al3Mg2. The bonding performance between coating and matrix can be improved by using zinc as transition layer, and the depth of mutual diffusion layer between aluminum coating and matrix after hot pressing for 4h was 4~8μm; the phase of interfacial zone among the aluminum coating and zinc coating was Al2Zn3, and the phase of interfacial zone among the zinc coating and magnesium coating maybe was Mg2Zn11 or MgZn2.The microhardness of the aluminum coating sample and zinc-aluminum coating sample is 1.2 to 2.2 times relative to the matrix of AZ91D magnesium alloy, which helps to improve wear resistance of the protective coating; the result of immersion testing shows that:, the layer of the aluminum coating sample was peeled off flakily after immersing in 3.5% NaCl neutral solution for 72h, and the matrix's inside of aluminum coating have been seriously damaged; only slight black pitting occured on the surface of the zinc-aluminum coating sample after same immersion testing, and the significantly damage have not been found in the matrix's inside of zinc-aluminum coating, so only AZ91D magnesium alloy of the zinc-aluminum coating shows a significantly enhanced corrosion resistance. |
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