| Magnesium(Mg)has the characteristics of low density,high specific strength,shock absorption and noise reduction,and is mainly used in aerospace,rail transit and electronic products industries.However,because the crystal structure of Mg is hexagonal and the electrode potential is low,there are few movable slip systems,and its strength is low,plasticity is poor and corrosion resistance is not good.Aluminum(Al)is abundant,and Al alloy has good plasticity and corrosion resistance,so it is widely used in engineering.Mg alloy and Al alloy can be made into Mg/Al clad plates by rolling,which can effectively give play to the advantages of the two metals and improve their comprehensive properties,and has great application prospects.However,the unclear rolling bonding mechanism and poor mechanical properties of Mg/Al are still common problems faced by scholars at home and abroad.In this paper,the rolling bonding mechanism of Mg/Al dissimilar metals at small reduction ratio is studied by work function,and the rolling composite of Mg/Al laminates at large reduction ratio is studied,also the evolution law and strengthening mechanism of its microstructure and properties are analyzed.The specific research contents are as follows:(1)ABAQUS software was used to simulate the rolling process of different metals of Mg and Al at small reduction ratios(10%,14.5%,19%,23.5% and 28%),and the variation laws of temperature,normal stress,frictional shear stress and equivalent plastic strain in rolling deformation zone were studied,and the rolling composite state and plastic deformation behavior of the two metals were analyzed,and the relationship between composite state and stress field at small reduction rate was explored.And the different composite states of Mg/Al clad plates and their corresponding stress-strain conditions in the small reduction ratio range are clarified.(2)The work function of different crystal faces of Mg/Al dissimilar materials was used to analyze the bonding mechanism.Based on the software platform of Materials Studio,the unit cell modeling of magnesium and aluminum with different crystal planes is carried out by using the CASTEP(Cambridge sequential total energy package)module based on density functional theory.VASP(The Vienna ab initio simulation package)software is used to calculate the electron exchange correlation energy of each supercell,and the geometric structure is optimized based on the generalized gradient approximation(GGA)theory.Finally,the above crystal structure is fully relaxed,and the vacuum energy level and Fermi energy level of each crystal plane are obtained by self-consistent and non-self-consistent calculations,and then the work functions of each crystal plane of Mg and Al are calculated.(3)A single pass hot rolling with small reduction(10%-30%)was used to carry out the rolling composite experiment of Mg/Al clad plates.The results show that there are three bonding states and two critical bonding points for Mg and Al dissimilar materials,that is,when the rolling reduction is less than 14.5%(critical bonding point I),the Mg/Al dissimilar metals are not bonded at all;When the reduction ratio is bstween14.5% and 21.75%,Mg/Al dissimilar metals are partially bonded;When the reduction ratio is more than 21.75%(critical bonding point II),the Mg/Al dissimilar metals are completely bonded.The microstructure characteristics of the interface at the critical composite point were analyzed by electron backscattering diffraction technology.It was found that the critical bonding point I was mainly composed of substructure,and the critical bonding point II was mainly composed of deformed structure.In addition,with the increase of reduction,the substructure of metastable state decreases,and shear band appears at critical bonding point II.Because of the small plastic deformation,the main deformation mechanism of the two critical bonding points is dislocation slip.(4)In order to further study the bonding mechanism of small reduction ratio,based on two critical bonding points obtained from rolling experiments,the ratio of crystal planes at the Mg/Al bonding interface was counted and the electronic force at the critical bonding point was obtained,and the variation law of work function of Mg and Al was calculated,and the response characteristics between macro normal stress and micro electronic force under different reduction conditions were analyzed.Based on the principle of minimum energy and Kubo formula,the linear relationship between electronic force and external disturbance(normal stress)is constructed,and partial recombination is achieved at the critical bonding point I to overcome the Mg barrier,and recombination is achieved at the critical bonding point II to overcome the Al barrier,which is consistent with the experimental critical point of small reduction ratio.(5)The rolling deformation behavior and microstructure and properties of hot rolled Mg/Al clad plates with 40%,50%,58% and 66% large reduction ratios were studied.The results of microstructure analysis show that with the increase of reduction ratio,the microstructure evolves from tensile twin and compressive twin to recrystallization.The test results of mechanical properties show that the tensile strength and plasticity of the clad plates gradually increase with the increasing reduction ratio.At 66% reduction ratio,the yield strength of the Mg/Al clad plates reaches 328 MPa,and the elongation reaches 14.65%,which is 60% and 7.6%higher than that of the original Mg plate respectively,also 80% higher than and 4.7% lower that of the original Al plate respctively.The mechanism that strength and plasticity of Mg/Al clad plates increase synchronously with the increase of reduction ratio is studied and analyzed.It is found that the appearance of bimodal structure of magnesium layer leads to long-range back stress near the interface,and then the high density of interlayer interface enhances back stress hardening and Mg/Al heterogeneous deformation-induced hardening leads to strength increase.The increase of ductility with the increase of reduction ratio is mainly due to the enhancement of local necking inhibition and delay of local necking,while the uniform deformation is enhanced due to the increase of mutual constraints. |
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