Study On Preparation Processes And Properties Of Al And Al-Zn Coatings On AZ31 Magnesium Alloy

Magnesium alloys have a wide range of applications and bright prospects from aerospace and military industries to medical care due to their low density,green environmental protection,and recyclability.However,the corrosion resistance of magnesium alloys is very poor.As a structural metal,this characteristic seriously restricts its practical application.Therefore,it is of great significance and value to research on improving the corrosion resistance of magnesium alloys.Diffusing metal elements into the surface of magnesium alloys to form diffusion layers is an effective way to improve their surface corrosion resistance.In this paper,aluminizing(Al)layers and zinc-aluminizing(Al-Zn)layers were prepared on the surface of AZ31 magnesium alloy by powder embedding method to improve its surface hardness and corrosion resistance.The main works are as follows:(1)Aluminizing layers were prepared on the surface of AZ31 magnesium alloy substrate by powder embedding method.The effects of temperatures on the microstructure,composition and properties of aluminizing layers were investigated,and the formation mechanism of the aluminizing layers was clarified.(2)To study the effect of the torsion process on the aluminizing layers of AZ31magnesium alloy,aluminizing layers were prepared on the twisted AZ31 magnesium alloy samples at different temperatures,and the microstructure,composition,and properties of the aluminizing layers of the twisted(TA)and untwisted(BA)samples were compared.(3)Al-Zn layers with two different cooling methods,furnace cooling(AZ-FC)and water cooling(AZ-WC),were fabricated on the surface of AZ31 magnesium alloy substrate by powder embedding method.Through the above research,the primary conclusions are as follows:(1)Aluminizing can greatly improve magnesium alloy surfaces'corrosion resistance and hardness.For the aluminized samples,the thickening of the aluminizing layers is not obvious with the increase of the expansion temperature below the eutectic temperature.The layers are mainly composed of?-Mg₁₇Al₁₂ phase and?-Mg₂Al₃ phase compound,and layers of precipitated?-Mg₁₇Al₁₂ phase will be formed at the combination of the seepage layer and the matrix precipitation when the diffusing temperatures are close to the eutectic point.However,after reaching the eutectic temperature,the aluminizing layers thicken significantly and the organization of the layers changes to the consist of an outermost?-Mg₂Al₃ layer,a(?-Mg₁₇Al₁₂+?-Mg)eutectic layer and a(precipitated?-Mg₁₇Al₁₂ phase+?-Mg)transition layer.(2)Torsion can improve thermal diffusion efficiency,increase the thickness of the aluminizing layer,and the precipitation of the second phase.At the same time,the concentration of Al in the infiltration layer is also reduced,which affects the precipitation of the?-Mg₂Al₃phase.And the higher the diffusion temperature,the more obvious this influence is.Torsion before diffusion thickens the aluminizing layers and increases the hardness of the matrix but has little effect on that of the aluminizing layers.No?-Mg₂Al₃phase was observed in the TA-430 sample and the thickness of the aluminizing layer of this sample was significantly increased compared with others.A"chrysanthemum-like"eutectic structure was formed in the aluminizing layer of the TA-430 sample by the?-Mg and?-Mg₁₇Al₁₂ phases.(3)Different cooling methods affect the structure and composition of the Al-Zn layers.The Al-Zn layer of the AZ-FC sample is mainly composed of the?-Mg phase??-Mg₁₇(Al,Zn)₁₂phase and?-Al₅Mg₁₁Zn₄ phases.However,the content of the?phase and Mg O phase in AZ-WC sample is increased due to the excessive cooling rate and the oxidation during cooling.The?-Mg₁₇(Al,Zn)₁₂ phase and?-Mg phase precipitated during the cooling process of the furnace constitute the transition layer between the matrix and the diffusion layer.The corrosion resistance and hardness of AZ-WC sample were significantly improved.