Constitutive Model Of 2A12 Aluminum Alloy Sheet And Deformation Behavior Of Tailor Welded Blank In Hot Gas Forming

With the development of aerospace and aircraft industries,the new generation transportation are facing requirements of large-scale,lightweight,long service life and high reliability,which results in a series of large-scale,complex-shaped and high performance complex integrated thin-wall components.Due to the dimension limitation of current thin sheet blank,tailor welded blanks are required to manufacture such large-scale integrated thin-wall components.In addition,in terms of lightweight materials,such as aluminium alloys,with poor ductility at room temperature,forming at elevated temperatures is needed to increase the formability.Nevertheless,tailor welded aluminium alloy blanks exist the severe problems,mainly incompatible deformation of weld beam and easy to crack during hot forming process.The thesis investigates the deformation characteristics and constitutive model of aluminium alloy blanks under hot conditions firstly,then the hot deformation of tailor welded blanks of this alloy were also investigated for both uniaxial and biaxial stress states.Aiding by the obtained fundamentals,a new hot gas pressure forming process of thin-walled tailor welded tubes with irregular cross-section geometries was proposed and investigated.In order to precisely analyze the deformation characteristics of aluminum alloy at high temperatures,a device utilizing Digital Image Correlation(DIC)was established enabling the instant strain field during hot uniaxial tensile to be measured and recorded.Using such a device,the deformation behaviors at a temperature range between room temperature and 475 °C and strain rate range between 0.001-0.1 s-1 were thoroughly investigated.Subsequently,the ultimate tensile strength,strain to failure and anisotropic properties were obtained.The whole strain filed during the high temperature uniaxial tensile process was investigated to analyze the variations of strain distributions at different temperature and strain rates.Different gauges and local points were selected to analyze the strain rate and flow stress evolutions of different deformed zones.In order to precisely describe the flow behaviors of aluminium alloys at high temperatures,a new methodology of constructing constitutive model using deformation history of local points was proposed,which enables the characteristics of instant strain rate variation to be accurately captured.Based on this method,the traditional double power function model was used for 2A12 aluminium alloys to describe the stress-strain curves at high temperatures.The results have shown that,using the data of local points with constant strain rate,the Fields-Backofen model exhibits higher prediction accuracy compared to the Johnson-Cook and Arrhenius models.In addition,data of different local points with various strain rates can be used to establish a simplified double-power function constitutive model using the same specimen,which can be used to reflect the flow stress variation due to instant strain rate change within gauge in traditional uniaxial tensile.Using the above high temperature uniaxial tensile method,the deformation behavior under uniaxial stress state of both friction stir welded and fusion welded 2A12 aluminium alloy sheets were investigated.The results have shown that,under different welding parameters of hot deformed friction stir welded blank,mechanical properties of thermal-mechanical,thermal affected zones are similar to those of base material,in comparison,the Nugget Zone(NZ)is the weakest resulting in locally concentrated deformation.While for the fusion welded sheets,the strength of NZ is higher than the base material,and the base material zone becomes weakest zone occurred localized deformation.In order to the high temperature deformation behavior of tailor welded aluminium alloy blanks under plane stress states,a hot gas bulging device based on DIC system was established,which contributes to measuring the complete straining history during hot gas bugling.The results have shown that,unlike the uniaxial stress state,the weld beam was always the weak zone during bulging and occurred localized deformation.Fracture was experienced within the zone of welding beam,and the ultimate bulging height was half of the base material approximately.While for the fusion welded sheet,the zone of base material is the weak zone experiencing localized deformation.However,due to the poor ductility of welding beam,fracture still locates at the welding beam during bulging.In order to address the problems of incompatible deformation and local fracture in hot gas forming friction stir tailor welded blanks,according to the deformation behaviors of tailor welded blanks under complex stress states in hot gas forming,a new hot gas forming process was proposed dedicated for large-scale thin-walled aluminium alloy tubular components with irregular cross-sections.Through the finite element simulation,the preformed tube blanks with optimized shape and dimensions were obtained enabling the wrinkling and cutting defects successfully avoided.Through the experimental investigation,the deformation mechanism of such irregular cross-section tailor welded tube under hot gas bulging condition was proposed,which has been used to guide the manufacture of particular irregular cross-section tubular prototype component.