Effect Of Homogenization Heat Treatment On Microstructure And Corrosion Resistance Of AZ31 Magnesium Composite Foams

Due to its excellent comprehensive performances,such as low density,high specific strength and good energy absorption property as well as sound absorption capacity,magnesium foam has widely been used in automobile,aerospace and other fields.Many investigations have focused on its fabrication method and mechanical properties.While,few literatures involve in its corrosion behavior,which limits its further applications.In this work,closed-cell AZ31 magnesium composite foams containing hollow ceramic microspheres（CMs）were successfully prepared by a modified melt foaming method and the foams possess homogeneous pore structures and high porosity.Geometric models of different types of pores on the surface of closed-cell metal foams were established.Based on this the superficial area formula of closed-cell metal foams was optimized,which is beneficial to calculate the foams superficial area and superficial porosity simply and accurately.The optimization is important to accurately evaluate the corrosion resistance of closed-cell metal foams.Morphology evolution of AZ31 magnesium composite foams under different homogenizing temperatures（240℃,320℃,400℃,480℃）and intervals（6 h,12 h,24 h）were examined by OM,XRD and SEM.Moreover,the effect of homogenizing heat treatment on the corrosion resistance of magnesium composite foam was studied by electrochemical experiment and hydrogen evolution experiment using 3.5wt.%NaCl solution as corrosive medium.In addition,corrosion morphologies of the foams were also observed to evaluate its corrosion behavior.The results showed that the foams with homogenizing temperature of 480℃and homogenizing intervals of 6 h exhibited optimal homogeneity and corrosion resistance,which is due to the decrease ofβ-Mg₁₇Al₁₂phase and the uniform distribution of Mg₂Si phase.Additionally,the early corrosion behavior of AZ31 magnesium composite foams with homogenizing temperature of 480℃and homogenizing intervals of 6 h clearly changed from pitting and filiform corrosion to pitting corrosion only.The major corrosion products of the foams were Mg（OH）₂,MgCl₂and Mg₂（OH）₃Cl·4H₂O.Based on all of the experiment results,the formulas of calculating the superficial area and superficial porosity of closed-cell foams were accurate and useful.