| Magnesium alloy is one kind of structural materials which has high specific strength and ratio of strength, and has been widely applied in many industries including the aerospace, automotive, military, electronic instead of steel, aluminum alloy and other materials. However, due to the structure of its surface is soft and corrosion easy in the presence of chloride or humid environment. These greatly limit the popularization and application of magnesium alloy. In this paper, the elector-spark deposition is used to prepare alloy strengthening coatings on surface of AZ91 magnesium alloy. The influence of the electro-spark deposition voltage, frequency and duty ratio process parameters on cross- section morphology, thickness, surface roughness of deposited coating is analyzed by the means of scanning electron microscopy and energy spectrum analysis(EDS). In addition, immersion corrosion test of different time and electrochemical-polarization curve test of AZ81-La alloy coating, aluminum alloy coating and tungsten-remelting modified layer are studied. Also, corrosion resistance, corrosion mechanism, elements distribution and phase structure of AZ81-La alloy coating and tungsten-remelting modified layer were discussed. The results showed that:①With the deposition voltage increased from 25 V to 45 V, the amount of melting of electrode wire and base metal increased with pulse energy increases. The coating thickness increases, internal defects, cracks and holes are obviously,uniformity improved then gradually deterioration trend. At the same time, the coating roughness value is the lowest when the deposition voltage is 30 V.②Electro spark deposition frequency increases from 150 Hz to 350 Hz,the thickness of the coating is composed of 190 um reduces to 120um; The decrease of melting rate and deposition rate resulted in outside air involved in the internal coating. It was got a increase in hole, defect and roughness, and got a loose deposit coating. the droplet fusion and roughness of the coatings exhibits fine.③ Electro spark deposition duty accounted for 40%, The coating filled with many holes, point defect, rough surface and a thinner deposit coating, when the duty cycle is 50%, the electrode-material and substrate,molded appropriate, it was got a decrease in hole, defect and roughness, and a smoother deposit coating, when the duty ratio reached 55%, 60%, the fluidity of liquid metal speed, direction, Coating is roughness with grey orange peel.④Aluminum is lower among α-Mg than β-Mg17Al12 phase and more lively in AZ91 D, the corrosion occurred in α-Mg, then gradually expand to the surrounding, and ending at the bent of high aluminum content of β-Mg17Al12 phase. At immersion experiment of aluminum coating, chloride ion adsorption into crack aperture, and due to the activation state- tiny corrosion cell passivation state of electrode potential difference in corrosion crack inside- outside, which formed a huge corrosion morphology in the end.⑤Magnesium alloy tungsten electrode remelting modified layer through immersion test of 12, 24, 36 hours, the coating surface appeared corrosion pits and holes that keep original coating morphology then developed become a tiny porous corrosion morphology; AZ81- La coating although different times of corrosion, its surface morphology change is not big, the corrosion surface is still smooth, corrosion area is smaller than other.⑥Electrochemical polarization curve test results show that the free corrosion potential of the tungsten electrode remelting modification and AZ81-La coatings is positively shifted to- 1.49 V,-1.33 V, which is positively moved about 120 mv, 280 mv compared with the matrix, respectively, the corrosion tendency is reduced about two and one order of magnitude; Show that both have a certain degree of increase of alloy corrosion resistance.⑦Electro-spark deposition AZ81-La coating, La element were mainly concentrated in the second phase. The Al-La-Mn(Al8La Mn4) and Al11La3 appearance improved the corrosion potential in Na Cl solution of substrate material from; La element appearance increased β phase, they are small and thin shape. In addition, the plate structure of Al-La-Mn(Al8La Mn4) and Al11La3 phase not easy to fall off, can effectively reduce the possibility of corrosion.⑧ The microhardness test results show that hardness value of electrospark deposition coating was obviously higher than substrate, AZ81-La coating hardness value reaches 181.2 Hv, tungsten electrode remelting of modified layer and the aluminum coating hardness value were 104.8 104.8 Hv and Hv. |
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