Study On Processing And Microstructure Of Aluminum Alloy Coating On The Surface Of Magnesium Alloy By Hot Dipping

The magnesium alloys have became the most potential lightweight materials forcommercial use in the21st century because of the advantages of small density, specificstrength, high specific stiffness, good shock absorption, high thermal conductivity, easyto recycle ect.But, as a structural material, magnesium alloy corrosion resistance is poor,severely restricting its large-scale application.At present, magnesium alloy plating onthe surface of metal mainly focus on metal powder penetration, hot (cold) spraying,electroplating, electroless plating.This experimen adopts the method of hot dip plating,plating aluminum and its alloy layer on the surface of AZ31magnesium alloy.Theorganization, morphology and phase composition was analyzed and tested by usingscanning electron microscopy (SEM),X-ray energy spectrum analyzer (EDS) andoptical microscope (OM).The polarization curve was determined,and immersioncorrosion test with NaCl solution was carried out.This paper studies the influence ofProcess parameters such as the immersion plating pretreatment solution,dip coatingtemperature, immersion plating solution composition, ect on the coating thickness andcoating phase composition and microhardness of coating,of coating microstructuremorphology, etc,and analyzed the principle of the coating forming.Research results show that in hot dip plating,appropriate immersion platingpretreatment is necessary in order to increase the wettability of the magnesium alloysubstrate and solution.Through Comparison Testing,this paper adopted the method ofpre plating a layer of low melting point metal-tin on the surface of magnesiumalloy,whcih remove the oxide film on the surface of the magnesium alloy so as toimprove the wettability.Compared with hot dipping of steel,the formation of magnesiumalloy plating has its particularity.Through SEM and metallographic microscopeobservation,we find that the formation of the coating consists of three stages-diffusionreaction,solidification and final formation.Among them, the diffusion of Mg elementdetermines the formation of coating.By EDS, the microstructure of the coating isobtained.The coating is mainly composed of transition layer, rich tin layer andaluminum alloy outer layer.The phase composition of the transition layer includesmagnesium aluminium intermetallic compounds and a few Mg, Si, Mg, Sn and Mg, zincintermetallic compounds which are distribute on transition layer.Immersion plating timeand hot-dip temperature field have a significant impact on the thickness of transition layer.The longer,the greater the thickness of transition layer,but increasing rate ofthickness of coating slows down with the extension of immersion plating time.Theinfluence of temperature field on element diffusion mainly focused on the initial stagesof coating formation.The thickness of aluminum outer layer is not affected by theimmersion plating time,and basicly was related to the extraction speed and solutiontemperature.In hot dipping experiment, this paper designs5metal composition of coating thatincludes Al-10Si，Al-10Si-10Bi，Al-10Si-10Zn，Al-10Si-20Zn，Al-10Si-10Zn-10Bi.Studies have found that Si can reduce solution melting point,increase liquidity inthe plating solution, make the coating appearance is smooth.Zinc can control thetransition layer growth speed, making the columnar crystal of transition layer becomessmall,and further reduce melting point,but the size is limited.Bi together with zincsignificantly reduces the coating metal melting point,which can significantly speed upthe transition layer growth, and outer layer become thinner.