Investigation On Microstructures And Properties Of Arc-sprayed-Al/AZ91D Bimetallic Materials By Solid-liquid Compound Casting

After casting,the surfaces of magnesium alloy could be prevented from corrosion by arc-spraying.However,low bonding strength between the coating and matrix and difficulty in spraying on complex shapes limited its further application.At present,the solid-liquid compound casting process was widely used in the bonding between two dissimilar metals and has gradually become one of the research hotspots.The advantages of solid-liquid compound casting and arc-spraying were combined and developed in this research.In order to enhance the bonding between Al coating and magnesium alloy,a solid-liquid compound casting technique was adopted in this work,and the arc-sprayed-Al/AZ91D bimetals without or with Zn interlayer were fabricated by pouring the AZ91D melt into the moulds deposited with a layer of Al or Al/Zn coating,respectively.Then the microstructure and properties of the arc-sprayed-Al/AZ91D bimetals were observed and investigated by varieties of characterization methods,such as XRD,SEM,EDS,TEM,EBSD,microhardness and shear strength tests.Taken the results of casting simulation(by Any Casting)and thermodynamic simulation(by MATLAB)into account,some potential factors such as interface state,pouring temperature and Zn interlayer which might have effects on the microstructure of the arc-sprayed-Al/AZ91D bimetallic materials were analyzed,and the formation process and mechanism of interface zone were also discussed.The main conclusions are as follow:(1)The thermodynamic model results of arc-sprayed-Al/AZ91D bimetallic materials showed that with the increase of pouring temperature,the Gibbs free energy of Al-Mg binary system and the chemical potentials of Al and Mg elements all decreased continuously.With the increase of the mole fraction of Al,the chemical potentials of both Al and Mg also gradually decreased.It indicated that Al atoms tended to enrich,while Mg atoms tended to diffuse into the Al-enriched areas.Considering the thermodynamic model results of the Al-Mg-Zn ternary systems,Zn could theoretically inhibit the formation of Al-Mg intermetallic compounds because of the formation of ternary compounds.(2)In the arc-sprayed-Al/AZ91D bimetal,there was an interface zone which could be divided into three layers consisting of(Al₁₂Mg₁₇+?-Mg)eutectic structure,Al₁₂Mg₁₇ and Al₃Mg₂,respectively.The bimetallic material with untreated coating had a larger and deeper interface zone containing more Al₁₂Mg₁₇ and Al₃Mg₂ intermetallic compounds than the one with treated coating.Due to the inhomogeneous distribution of aluminum,magnesium and oxygen,the(Al₁₂Mg₁₇+?-Mg)eutectic structure presented four different morphologies(feather-like,honeycomb,radial and interdendritic structures),the hardness of which varied from 66 HV to 144 HV.The arc-sprayed-Al/AZ91D with treated coating showed higher corrosion resistance and shear strength than the one with untreated coating.The results of shear test also indicate that owing to the formation of metallurgical and chemical bonding,the adhesive strength between the as-cast AZ91D and arc-sprayed Al coating could be improved by the novel compound casting.(3)An interface zone was acquired in the arc-sprayed-Al/AZ91D bimetal by novel solid-liquid compound casting(SLCC),when the pouring temperature was not less than963 K.A mixing zone,composed of aluminum,alumina and Mg Al₂O₄,formed in the bimetal cast at 933 K.When the pouring temperature was increased from 963 to 1023 K,the acreage of the interface zone,which mainly comprised the Al-Mg intermetallic compounds,gradually aggrandized,and the?-Al₁₂Mg₁₇ intermetallic compounds in that zone tended to grow towards the magnesium alloy.The interface zones of the arc-sprayed-Al/AZ91D bimetals had greater micro-hardness than the substrate and coating metals.At a pouring temperature of 993 K,the arc-sprayed-Al/AZ91D bimetal had the thinnest interface zone,without any untransformed aluminum pieces and thread-like Mg Al₂O₄,and achieved the highest shear strength of?24.60 MPa.Furthermore,the expansion of the interface zone increased the possibility of the occurrence of brittle fracture,and thus an excessive increase in the pouring temperature did not improve the adhesive strength between the as-cast AZ91D and the Al coating.(4)Under proper arc-spraying time of Zn coating and preheat time of Al/Zn double-deck coating,the interface zone of the arc-sprayed-Al/AZ91D bimetals with Zn interlayer could be divided into two categories:one was composed of(?-Mg+Al₅Mg₁₁Zn₄)and(?-Al+Mg₃₂(Al,Zn)₄₉)structures,and the other consisted of(?-Mg+Al₅Mg₁₁Zn₄),(Mg Zn₂(main)+?-Zn)and(?-Zn(main)+Mg Zn₂)structures.If Zn interlayer was too thick,or the preheating time of Al/Zn double-deck coating was too long or too short,cracks may occur at the interface.The(?-Mg+Al₅Mg₁₁Zn₄)structure were the most abundant in the interface zone,of which the average microhardness value was 188 HV.The microhardness of Mg Zn₂ intermetallic compound almost reached 327HV.Compared with the AZ91D matrix and arc-sprayed-Al coating,the microhardness of the interface zone remained at a high level,and the increase of Zn solid solution in the region adjacent to the arc-sprayed coating reduced the microhardness of the interface zone.Most rupture of arc-sprayed-Al/AZ91D bimetals with Zn interlayer occurred at the(?-Mg+Al₅Mg₁₁Zn₄)structure and presented less features of ductility fracture.When the arc spraying time of Zn coating was 30 s and the preheat time of Al/Zn double-deck coating was 6 h,the Zn solid solution dispersed in Mg Zn₂intermetallic compound as a second phase,and the shear strength of arc-sprayed-Al/AZ91D bimetal was increased to 31.73 MPa.