Investigation On Microstructures And Properties Of Galvanized-Q235/AZ91D Bimetallic Materials By Solid-liquid Compound Casting

Nowadays,magnesium alloys were employed in the automotive and aerospace fields for economic savings and ecological protection due to the lightest materials than mostof construction material and many advantages such as good machinability and cast ability as well as high damping capacity.However,their corrosion and wear resistance were poor which limited its further application.By contrast,galvanized steel had the better corrosion and wear resistance,relieving the drawbacks of magnesium alloy.Therefore,if realizing the combination of them effectively,it will enlarge the application further.In this paper,the galvanized-Q235/AZ91D bimetallic material was fabricated via the combination of hot-dip galvanizing and solid-liquid compound casting technology which is via casting the molten AZ91D into the mold where the galvanized-Q235 was inserted.After that,the heat treatment technology was applied to change the microstructure and mechanical properties in the interface zone between galvanized-Q235 and AZ91D.At last,the formation mechanism of the galvanized-Q235/AZ91D interface zone with and without heat treatment were illustrate via the thermodynamic model..Based on the analysis of microstructure,chemical composition,microhardness,shear strength and fracture characteristics at the interface of composite castings,the effect of the heat treatment and galvanizing coating on the microstructure and mechanical properties of the Q235/AZ91D bimetal material produced by solid-liquid compound casting,and The main conclusions are as follows:(1)As for the pure-Q235/AZ91D bimetallic material,there are white oxide products and intermediate gaps at the interface of AZ91D and Q235,and no intermetallic compounds are formed.The oxide product is partially bonded to the concave and convex part of the rough surface of Q235 steel,resulting in mechanical bonding at the interface rather than metallurgical bonding.(2)The interface zone between galvanized Q235 and AZ91D could be divided into three different layers.The layer adjacent to the AZ91D(layer I)was mainly composed of(?-Mg+Mg Zn)eutectic structure and a black block phase(?-Mg).The layer close to the Q235(layer III)was mainly comprised of small white block Fe Al₃ and black block?-Mg,and the intermediate layer(layer II)consisted of a white uniform lamellae phase(?-Mg+Mg Zn)eutectic structure.(3)After heat treatment,the(?-Mg+Mg Zn)eutectic structure was transformed into the Al₅Mg₁₁Zn₄which may promote the microhardness from 139.2 HV to reach298.8 HV.Besides,the?-Mg and(?-Mg+Mg ₁₂Al ₁₇)in the AZ91D substrate were transformed into(?-Mg+Al₅Mg₁₁Zn₄)and Al 5 Mg 11 Zn 4 respectively,resulting in the increasement of the thickness of the interface zone and the increasement of microhardness from 59.5 and to 173.4 HV and 294.2 HV,respectively.(4)The microhardness of interface zone between galvanized-Q235 and AZ91D was higher than the AZ91D substrate,and the that of the interface close to Q235displayed more than the Q235 substrate.From Q235 to AZ91D,the microhardness was first decreased from 294.2 HV to 174.3HV due to the vanishment of Fe Al₃ as well as formation of eutectic,and then decreased to 139.2 owing to the formation of?-Mg.(5)After heat treatment,the shear strength of the galvanized-Q235/AZ91D bimetallic was increased from 11.23 MPa to 24.63 MPa because of the vanishment of Mg Zn and the formation of Al₅Mg₁₁Zn₄,and fracture morphology was translated from to the complete brittle fracture to brittle fracture with some ductile fracture.(6)In the process of the pure-Q235/AZ91D solid-liquid compound casting,the oxidation product that was reaction formation of the magnesium alloy and air,on the one hand formed a metallurgical bond with AZ91D,on the other hand,formed a mechanical bond with the concave and convex part of Q235 steel.As for the hot dip galvanized-Q235/AZ91D solid-liquid compound casting,the liquid magnesium alloy melted the galvanized coating fully,and then rapidly solidified.At this time,reaction layer II,III were formed.After that,under the heat of the liquid magnesium alloy,the Mg,Al and other elements in the AZ91D was diffused into the reaction II,resulting in partly translation from the reaction II into I.During the heat treatment,the Mg,Al and other elements in the AZ91D was diffused into the interface zone,resulting in partly translation from the reaction II and I into the layer close to AZ91D.At the same time,the Zn,Al elements in the interface zone were diffused into the AZ91D substrate,resulting in partly formation of the layer adjacent to AZ91D.