Research On Material Deformation Mechanism In Aluminum Alloy Stretch Bend Forming With Low Frequency Vibration Assisted

Aluminum alloy stretch bend forming structural parts are widely used in the manufacture of high-speed rail vehicle body structural parts due to their light weight,high strength,good corrosion resistance and long service life.With the continuous improvement of the operating speed of high-speed trains,the aerodynamic shape of the car body requires the aluminum alloy bending structure to have more complex curves,higher forming accuracy and lightweight structure with complex sections.In the traditional stretch bend forming process,there will be serious quality defects such as cross-section distortion,uneven deformation,and large springback,which will seriously affect the manufacturing accuracy,operation safety and comfort of the car body.The low frequency vibration assisted stretch bend forming process can improve the plastic flow performance of the material and the friction state of the contact interface during the stretch bend forming process of aluminum alloy,and accelerate the release of residual stress,thereby improving the forming accuracy and surface quality of the stretch bend forming structural parts.Therefore,it is of great significance to study the materials deformation behavior and mechanism of low frequency vibration on aluminum alloy stretch bend forming,which is of great significance to improve the quality of stretch bend forming and enrich the processing technology of high-strength aluminum alloy.In this paper,by analyzing the causes of quality defects of aluminum alloy with high strength and complex cross-section,the high precision requirements of aluminum alloy for high-speed trains were analyzed.The low frequency vibration assisted stretch bend forming process method and the variable incremental clamp trajectory design method were proposed respectively,and the materials deformation behavior and mechanism of low-frequency vibration on aluminum alloy stretch bendi forming were studied.The main research contents and results are as follows:(1)Aiming at the problems of large deformation resistance and uneven deformation of high strength and complex section aluminum alloy,a low frequency vibration assisted stretch bending process was proposed.The mechanical models of the loading process were established for the three stages of pre-stretching,bending,and re-stretching of low frequency vibration assisted stretch bend forming,respectively.The mechanism of low-frequency vibration on the stress superposition of the stretch-bend forming deformation process and the stress-strain distribution of the workpiece section were revealed.The unloading springback process was equivalent to the reverse elastic bending process.The analytical model of springback and springback radius of low frequency vibration assisted stretch bend forming was established,and the prediction of springback and springback radius of low frequency vibration assisted stretch bend forming was realized.(2)Based on the viscoelastic-plastic theory,the volume effect of low-frequency vibration of aluminum alloy was studied.The viscoelastic-plastic deformation of materials was equivalent by rheological theory,and a series-parallel hybrid viscoelastic-plastic material constitutive model composed of elastic elements,plastic elements and viscous elements was established.The stress-strain characterization method of viscoelastic-plastic material constitutive was obtained.Taking the strain history of low frequency vibration as the strain boundary condition,the constitutive model of viscoelastic-plastic material under low frequency vibration load is established,and the influence of low frequency vibration volume effect on material softening and stress relaxation was analyzed.The influence of low-frequency vibration on the elasticity,plasticity and strain rate sensitivity of materials was analyzed by low-frequency vibration uniaxial tensile test system.It is proved that low-frequency vibration had significant volume effect and could improve the plastic deformation performance of materials.(3)Based on Hertz contact theory and elastic-plastic deformation theory,the plastic sliding friction mechanism of contact interface in low frequency vibration assisted stretch bend forming was studied,and the plastic sliding model of double rough contact surface under low frequency vibration was established.The analytical expression of plastic sliding friction coefficient of stretch bend forming about low frequency vibration frequency and forming contact pressure was obtained,and the mechanism of low frequency vibration improving plastic sliding friction of stretch bend forming is revealed.Through the simulation analysis of low frequency vibration sliding friction of double rough contact surface,the influence of low frequency vibration frequency and forming contact pressure on the contact stress,friction force and friction coefficient of asperity on the contact surface was obtained.(4)Aiming at the influence of the friction resistance of the contact interface on the tensile plastic deformation,an analytical model of the friction shear stress and tensile stress distribution at the contact interface of the stretch bending die was established.The influence of friction shear stress on the plastic deformation of the die-attached section of the bending specimen was analyzed by the equivalent bending test.The proportional relationship between the deformation of the die-attached section and the free section of the bending specimen under different friction coefficients and cladding angles was obtained.Taking the uniform plastic deformation of the die segment as the optimization goal,a tensile compensation method based on friction resistance was proposed,and an analytical calculation model of the variable increment stretch bending clamp trajectory was established to realize the accurate compensation of the tensile increment during the stretch bend forming process.(5)Based on the loading characteristics of discrete cladding in stretch bend forming,the finite element simulation model and physical experiment of low frequency vibration assisted equivalent stretch bend forming of aluminum alloy are established by using the discrete equivalent principle.The finite element simulation analysis method which can accurately simulate the low frequency vibration assisted stretch bend forming and predict the springback process is obtained.Taking the door column of L-section aluminum alloy profile with variable curvature as the research object,the finite element model of low frequency vibration assisted variable incremental stretch bend forming and springback process of L-section aluminum alloy profile was established.The influence of vibration parameters and variable incremental stretch bend trajectory on stress-strain,residual stress,springback and section distortion in the forming process was obtained.