Study On The Technology And Properties Of Mg/Al Composite Plate Prepared By "Cold Spraying+Rolling" Method

Mg alloys have high specific strength and stiffness,good processability,and excellent electromagnetic interference resistance.They are essential structural materials for the production of lightweight vehicles,aerospace vehicles,and armored combat vehicles,and have been widely used in transportation,aerospace,and other industries.However,Mg alloys are very active,with a balanced electrode potential of-2.37 V,and the surface oxide film is porous,resulting in poor corrosion resistance,which greatly limits the application range of Mg alloys.Al alloys can form a dense oxide film on the surface to prevent further corrosion of the internal metal.Therefore,the preparation of Mg/Al composite plates is an effective method that can significantly improve the corrosion resistance of Mg alloys.At present,the roll bonding and explosive bonding are mainly used in industry to prepare Mg/Al composite plates.These processes have their own advantages,while they also have some limitations.For example,during the roll bonding process,the Mg/Al interface is prone to oxidation,resulting in insufficient interfacial bonding strength.Explosive bonding has problems of environmental pollution and low production efficiency.This study puts forward a process idea of "cold spraying+rolling" to prepare Mg/Al composite plate:Firstly,a 7075 Al deposition layer is sprayed on the surface of AZ31B Mg substrate by cold spraying to obtain As-Sprayed Mg/Al composite plate,achieving a tight pre bonding of the Mg/Al interface and alleviating interfacial oxidation during subsequent processing.Then,hot rolling and subsequent annealing treatment are adopted to comprehensively control the interface,microstructure and properties of the composite plate.The process is simple,reliable,environmentally friendly,and can achieve continuous production.Focusing on the development of the above process,the main conclusions of this paper are as follows:(1)After deposition,the raw powder is quasi-spherical in the XY plane,and pancake in the XZ and YZ planes.In the XY plane,the mechanical properties of the 7075 deposit in different directions are uniform.From the XY plane to the deposition direction(Z),the ultimate tensile strength of the 7075 deposit gradually decreases with the increase of the extension angle,and there is a strong anisotropy of mechanical properties.The anisotropy of mechanical properties is mainly caused by(a)the difference of crack propagation behavior in different directions caused by the anisotropy of microstructure and(b)the difference of interface bonding quality caused by the uneven distribution of deformation degree on the particle-particle interface.It is judged that 7075 deposit layer prepared by cold spraying can be used as Al layer in Mg/Al composite plate.(2)The surface of As-Sprayed and As-Rolled Mg/Al composite plates is smooth and flat,the interface is tightly bonded,and there are no defects such as interface oxidation,delamination,cracks,etc.After hot rolling,the interface of Mg/Al composite plate forms intermetallic compounds,new bonding interface and micropores.Hot rolling improves the bonding between particles in the 7075 deposit layer,and resulting in the refinement of grains in the Mg layer,thus greatly improving the ultimate tensile strength and total elongation at break of the composite plate.Due to the formation of intermetallic compounds,the shear bond strength of the composite plate after hot rolling decreases slightly.The fracture process of Mg/Al composite plate can be divided into three stages:(a)cracks always occur preferentially on the outer surface of the Al layer and extend towards the interface;(b)With the crack propagation,the stress distribution on both sides of Mg and Al in the plate is uneven,and then delamination occurs at the interface of the plate;(c)The complete fracture of the Al layer will lead to stress concentration in the Mg layer at the fracture.Subsequently,the crack propagated rapidly in the AZ31B Mg layer until the composite plate completely fractured.(3)Under lower rolling temperature and higher rolling reduction rate,cracks are easy to occur at the edge of the plate.With the increase of rolling temperature,the thickness of intermetallic compound layer increases gradually.The ultimate tensile strength of Mg/Al composite plate is positively correlated with rolling temperature and rolling reduction.The optimal rolling parameters are determined as follows:the rolling temperature is 400℃,the rolling reduction rate is 30%,at this time,the ultimate tensile strength of Mg/Al composite plate is 392.7±3.3 MPa,the total elongation at break is 4.5±0.1%,and the shear bond strength is 91.7±6.3 MPa.After heat treatment,a thick intermetallic compound layer is formed at the interface of Mg/Al composite plate.When the heat treatment system is 400℃,1h,furnace cooling,the thickness of the intermetallic compound layer reaches～65 μm.The heat treatment eliminated the dislocation density in the composite plate,promoted the mutual diffusion of Al particles,and reduced the ultimate tensile strength of the composite plate,but the fracture elongation increased significantly.The optimal heat treatment system is determined as 400℃,1h,water cooling.The Mg/Al composite plate has excellent corrosion resistance and wear resistance.Compared with the Mg/Al composite plates prepared by other processes,the properties of the Mg/Al composite plate in this study are excellent and have a good application prospect.