| With a rapid development of global economy as well as science and technology,some issues such as environmental pollution,ecological imbalance,resources depletion and dramatic increase in energy consumption have become increasingly prominent.How to achieve a sustainable development has become a common problem of human.The use of lightweight components is one of the important ways to reduce energy consumption and environmental pollution to achieve sustainable development.As typical lightweight components,hollow aluminum alloy profiles have been widely used in the fields of rail transportation,vehicles and ships,building structures and aerospace.The porthole die extrusion process,which involves thermoplastic deformation and solid-state welding,is the key technology to manufacture hollow aluminum alloy profiles.In recent years,owing to the rapid development of high-speed trains in China,the demand for high-performance hollow aluminum profiles with large scale and complex cross-sections for high-speed train bodies has been increasing.The special requirements of high-speed trains,such as high-speed,light-weight,comfort and safety,make the aluminum alloy profiles for high-speed train bodies significantly different in structure,function and performance from those for conventional train bodies.Microstructure and performance of aluminum alloy profiles,especially welds quality,become very important technical requirements.Therefore,the study on porthole extrusion welding behavior,interfacial structures evolution and welds quality control method of aluminum alloy profiles has become one of the key scientific issues for the manufacturing of the high-performance hollow aluminum alloy profile with large scale and complex cross-section for high-speed trains.At present,in terms of porthole dies extrusion process of aluminum alloy profile,researchers have carried out some studies on the material flow behavior,the effect of extrusion process parameter and die structure on the porthole die extrusion welding process,extrusion welding mechanism,microstructure and mechanical properties of extrusion welds as well as welds quality control and prediction of extrusion profiles.However,there are still several critical issues that need to be studied and solved:(1)Physical simulation methods of solid-state welding process,quantitative characterization methods of interfacial bonding degree and performance,and quantitative relationship between solid-state welding conditions and interfacial bonding degree.(2)Formation process,microstructure and corrosion resistance of transverse welds,longitudinal welds and back-end defects in porthole die extrusion processes,and their influences on properties of extruded profiles.(3)Intrinsic relationship between porthole die structure and process parameters,extrusion welding behavior,microstructure of welds and mechanical properties of profiles,and control methods of welds quality.(4)Solid-state welding mechanisms,dynamic evolution of microstructure and interface structure of welds,and their influences on the welding quality of profiles.(5)Universal quantitative assessment methods of welds quality of the profiles extruded by porthole dies.Focusing on the above-mentioned key issues that need to be studied and solved,this thesis carried out the following research work and obtained the following research results:(1)A new idea of comparative study between hot isothermal compression bonding and conventional hot isothermal compression was proposed,and the hot deformation and solid-state welding behavior of 6063 aluminum alloy in the deformation temperature range of 673 K-803 K and the strain rate of 0.001 s-1-0 s-1 were studied.The flow stress behavior under two experimental conditions was compared.The constitutive equation of 6063 aluminum alloy during solid-state welding was established.The power dissipation and plastic deformation energy during hot deformation and solid-state welding were compared.The grain structure,grain boundary characteristics,texture,and the evolution of bonding interface structure during hot deformation and solid state welding under different deformation temperatures and strain rates were studied.A method to characterize interface bonding degree was proposed,and the quantitative relationship between interface bonding degree,deformation temperature and strain rate was established.The influence of deformation temperature and strain rate on the micro-hardness and shear strength of the material was studied,and an interfacial performance characteristic parameter that can reflect the bonding degree of bonding interfaces was proposed.(2)Modular porthole dies that can be flexibly adjusted were designed and manufactured,and plate-shaped profiles were obtained through extrusion experiments.Microstructure,corrosion resistance and fracture behavior of longitudinal welds,transverse welds and back-end defects in the profiles extruded by the porthole dies were studied by microstructural characterization,chemical etching,and in-situ observations of tensile processes based on digital image correlation techniques.Strain concentration phenomenon was found at the locations of longitudinal welds,transverse welds and back-end defects.Two fracture modes were found during tensile tests:"Uniform plastic deformation ? Strain concentration ? Fracture" and"Uniform plastic deformation ? Strain concentration ? Debonding ? Fracture".Finite element numerical simulation models of porthole die extrusion processes were established to reveal the formation process and mechanism of back-end defects and transverse welds.(3)Extrusion experiments with different extrusion die structure parameters were carried out and microstructure observation,tensile tests and fracture surface morphology analysis were carried out.The influence of the shape of bridge and the depth of welding chamber on the evolution and length of transverse welds and on the formation process,microstructure and mechanical properties of longitudinal welds were studied.The distribution of the ratio between welding pressure and material flow stress as well as the material flow behavior were studied,and the concept "effective welding surface" was proposed.(4)Effects of extrusion speeds and temperatures on the grain structure,interface structure,aging precipitated phase and mechanical properties of longitudinal welds were studied,and interfacial micro-voids and the phenomenon that grains grow across bonding interfaces were found.(5)Evolution of grain structure,micro-texture,and mechanical properties of the aluminum alloy material along a welding path during porthole die extrusion were studied.It was found that during the porthole die extrusion,the evolution mode of grains is "Equiaxed fine grains? Strip-shaped coarse grains ? Equiaxed fine grains",while the evolution mode of micro-textures is "Shearing texture ? Goss and copper textures? Cube texture.It was also found that the coarse grains and special grain orientation appearing in the welding zone will make the decrease of micro-hardness and tensile properties of longitudinal welds.(6)Fine structures of the bonding interfaces formed in the presence/absence of gas-pockets in extrusion dies were studied,and interfacial amorphous layer and interface migration phenomenon were found.Solid-state welding mechanisms corresponding to the presence/absence of gas-pockets were respectively revealed from the micro-nano scale.(7)Material flow behavior was studied to reveal the formation process of longitudinal welds and macroscopic welding defects in extruded profiles.Tracking methods of material particles during porthole die extrusion were studied,and welding paths were determined.A new solid-state welding creation considering stress triaxiality,equivalent strain rate,temperature and contact time was proposed,and welds quality was successfully predicted based on the creation and welding paths.(8)Extrusion die design and manufacture as well as extrusion experiments of an incompletely symmetrical hollow aluminum alloy profile were carried out.Grain structure,interfacial structure and mechanical properties of longitudinal welds of this profile were studied,and a general method for the evaluation of welds quality was proposed. |
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