Research On Superplastic Forming/Diffusion Bonding Of 5A70 Aluminum Alloy

Excellent mechanical properties of aluminium alloys are widely used in aerospace,rail transit,precision forming and other manufacturing industries.The research on superplastic forming and diffusion bonding of aluminum alloys is of great significance for its application in complex forming,special parts and improving the bearing capacity of equipment.The preparation of fine-grained/ultrafine-grained aluminium alloy sheets for industrial application of high strength and not heat treatable aluminium alloys,and the mechanism of grain structure,precipitated phase,cavities and submicrometer filaments in superplastic forming are important foundations for the study of Superplasticity of aluminium alloys.In this paper,the basic theory and industrial application of superplastic forming and diffusion bonding of 5A70 aluminium alloy are studied and explored.To prepare the fine-grained 5A70 aluminium alloy sheet,the requirements of controlling the dispersion distribution of precipitates has been proposed and applied during the preparation of plate.The effects of temperature and time of recrystallization treatment on the grain structure of 5A70 aluminium alloy are analyzed.The superplastic materials for large sheet metal parts in industrial application are prepared by rolling combined with heat treatment.Finally,Through continuous optimization of rolling heat treatment process,the fine-grained 5A70 aluminum alloy superplastic sheet with grain size of 8.48 μm is obtained by combining recrystallization heat treatment during 2 passes of hot rolling and 12 passes of cold rolling.To achieve the superplastic mechanism of the studied 5A70 aluminium alloy,the superplastic deformation behavior at different temperatures(400～550 ℃)and strain rates(5×10-3～5×10-4 s-1)are studied by unidirectional high temperature tensile test.The maximum superplastic fracture elongation of the alloy is 437%.Based on the analysis of strain rate sensitivity index and deformation activation energy,it is pointed out that lattice diffusion dominated the grain boundary sliding mechanism of the 5A70 alloy during superplastic deformation.Based on the relationship among dislocation,precipitated phase and grain structure,the mechanism of cavity nucleation induced by stress concentration is studied,and the formation process of cavity nucleation,growth,interlinkage and coalescence under the growth mechanism controlled by diffusion,superplastic diffusion and plastic deformation during the superplastic deformation are discussed.The composition of filaments at the superplastic fracture interface is analyzed,which is caused by the Mg-rich phase particles precipitated and Mg-rich oxides.To solve the diffusion bonding problem of 5A70 aluminum alloy,diffusion bonding systems with different temperatures(650～800K)and rough interfaces(smooth and rough)are studied by molecular dynamics simulation.The results show that the diffusion rate and the thickness of diffusion layer are strongly temperature dependent,and the behavior characteristics of interface atoms in diffusion bonding process are revealed.The temperature and pressure parameters of diffusion bonding of the alloy are investigated by thermal simulation experiments,and the superplastic forming/diffusion bonding process of a hanging product is verified by experiments.The superplastic forming properties that meet the requirements of industrialization are obtained,and the corresponding shear strength and tensile strength were 49～51 MPa and 246～255 MPa.The diffusion bonding with interfacial copper layer is analyzed and discussed,which provides an idea for industrial application of diffusion bonding of aluminium alloy.In this paper,based on superplastic unidirectional tensile test,diffusion bonding molecular dynamics simulation and thermal simulation experiments and their detection and analysis results,the main factors and deformation characteristics affecting superplastic fracture elongation in superplastic deformation of fine-grained 5A70 aluminium alloy are studied,which provided theoretical basis for industrial application.Molecular dynamics models at different temperatures and diffusion bonding interfaces are proposed.It provides a theoretical support for the pretreatment of diffusion bonding interface and has practical engineering application value.