Study On Corrosion Resistance Of Vacuum Die-Cast AZ91 Magnesium Alloy

Magnesium alloys are the lightest metallic structural materials. They are widely used in the fields of automobile, aerospace, and electronic information owing to high specific strength and high specific stiffness, good damping capacity, castability and mechanical properties. However, their relatively lower corrosion resistance properties limits their further popularization and application. In this paper, effect of vacuum die-casting, Ca,Y alloying and phosphate pretreatment on the microstructure and corrosion resistance property of AZ91 magnesium alloy was investigated.The vacuum die casting process can decrease the gas content and porosity size, and make alloy microstructure more uniform distribution and densification. The effect of vacuum die-casting on the corrosion resistance property of AZ91 magnesium is study by corrosion weightlessness test, electrochemical test and salt spray test. Results show that vacuum die-casting process can improve the corrosion resistance of AZ91 magnesium alloy, corrosion weightlessness test show that the corrosion rate of vacuum die casting alloy drop to 33.75% of conventional die casting alloy immersed in 3.5% NaCl aqueous solution 18 hours. Electrochemical test results show that vacuum die casting can rise the corrosion potential by 2.67%, reduce the corrosion density by 15.92% and decrease the corrosion rate by 15.92%.The microstructures of the alloys are refined by adding Ca alone or adding Ca combined with Y element, the grain sizes of AZ91-1.5Ca-1.0Y are much finer. Forming some stringer Al2Ca phases and block Al2Y phases, which connect with reticular Mg17Al12 phases, and distribute around the grain boundaries. The experimental results show that adding Ca combined with Y element can increase the corrosion electric potential and decrease the corrosion current density of AZ91 magnesium alloys, and the corrosion current density of AZ91-1.5Ca-1.0Y magnesium alloy drops one order of magnitude.Phosphate pretreatment on the surface of AZ91 magnesium alloy was study in this dissertation, the optimal pretreatment was determined after large number of experiments. Corrosion weightlessness test results show that the corrosion rate of alloy after phosphate pretreatment was decreased 46.28%. Electrochemical test results show that vacuum die casting can increase the corrosion potential by 19.23%, reduce the corrosion density by 62.86% and decrease the corrosion rate by 59.78%. Salt spray test results display that the corrosion rate can reach to 0.3703mg·cm-2·d-1, compared with unphosphating alloy, which decreased by 35.49%.