Research On Magneto-Vibration Springback Control And Electromagnetic Incremental Forming Technology Of Aluminum Alloy Skin

Aluminum alloy skin is a thin-walled part of an aircraft and a cladded section of internal structural frame.The quality of material forming directly affects the flight performance and service life of aircraft.Moreover,there are problems associated with conventional stamping such as high springback and easy wrinkling.Electromagnetic forming is a forming method that uses transient strong Lorentz force to deform the workpiece at a high rate.It has the advantages of improving the forming limit of rigid materials and reducing the springback action of formed parts.However,the existing electromagnetic forming technology is mainly used in small-size tubular and plate parts.How to achieve the precise manufacturing of large-size aluminum alloy thin-wall skin is a key scientific and technical challenge,which requires immediate resolution.Considering 3003 aluminum alloy sheet as the research object,a new process involving the combination of large-sized,skin pulse magnetic field vibration forming and dynamic incremental forming was proposed in this paper.Theoretical analysis,finite element simulation and experimental methods were used to reveal the influence mechanism of magnetic vibration and magnetic field incremental loading on material dynamic deformation behavior,springback control and wrinkle suppression.The work provides theoretical and technical support for the precise manufacture of large-size curved and deep drawing skin parts.The main research contents and conclusions of this thesis are as follows:(1)Focusing on the challenge that the traditional electromagnetic forming needs to set discharge coil at the bending part to reduce the springback,which leads to the complicated mold structure,a new process of loading the pulse magnetic field end of L-shaped bending part was proposed.The influence law of blank holder gap,discharge voltage and the length of the sheet press zone on the springback of the formed part was analyzed by finite element simulation and experiment.It was found that the blank holder gap was supposed to be wider than the thickness of the sheet,which would be conducive to the high frequency vibration effect of the sheet in the process of coil discharge,causing repeated tensile and compression deformations at the bending angle,and finally leading to the tangential stress at the bending angle,which initially increased then decreased subsequently.The larger the length of the sheet press zone is more conducive to the reduction of the springback angle of the sheet,but it is easy to appear the phenomenon of distortion and bending.With the increase in discharge voltage,the vibration amplitude of sheet increased under the action of pulsed magnetic field,and both the tangential stress and the overall elastic strain energy at the bending angle decreased.When the thickness of the sheet was 1mm and the blank holder gap was 2 mm,the springback capacity of the L-shaped part after conventional stamping was recorded as 13°,and the springback capacity of the sheet after 10,000 V discharge voltage was 2°.The simulation and experimental errors were less than 7%,which proved the validity of the numerical simulation model and the effectiveness of the magnetic vibration reduction of springback.(2)Based on the mechanism that magneto-vibration can effectively reduce the springback of parts,a new electromagnetic vibration incremental forming process with elastic pad for large-size curved skin parts is proposed,which can enable the parts experience weak plastic deformation and reduce springback action under high-frequency vibration.The springback control model of bending sheet under the action of electromagnetic force and inertia force and the coupling simulation model of electromagnetic field and structure field suitable for coil moving incremental discharge were established.It was observed that the inertia force caused by high speed deformation was beneficial to reduce the tangential stress and springback at the bending of sheet.The plate vibrates at a high frequency under the action of electromagnetic force and elastic pad,resulting in a minute increase in plastic strain,but significant decrease in tangential stress and elastic strain energy.When the planar racetrack coil is used,the corresponding plates on both sides of the coil move towards each other under the action of electromagnetic force and squeeze the central part to produce local bulge.Incorporation of a 304 stainless steel shielded coil side magnetic field effectively mitigated the bulge defect and improved the surface quality of the formed parts.The sheet deflected and deformed under conventional stamping,and the springback height was 496 mm,which was almost horizontal.When the elastic pad thickness,H = 10 mm and discharge voltage,U = 7000 V were used,the shielding discharge coil was discharged at 6 different positions along the sheet surface,and the springback height of the sheet was 156 mm.(3)In order to further reduce the springback of large-size curved skin parts,a forming process combining pre-deformation and electromagnetic incremental vibration was proposed,and the effects of elastic pad thickness H and discharge voltage U on the forming quality of the parts were discussed via simulation.It was found that with the increase in elastic pad thickness H or discharge voltage U,the vibration amplitude on the plate increases after coil discharge,and the springback of the forming part decreased.However,when the thickness of the elastic pad H or the discharge voltage U exceeded the critical value,the height of the bulge on the sheet increased significantly.In the pre-deformation stage,the sheet exhibited plastic deformation,and the springback height of the sheet was 87 mm.On the basis of pre-deformation,elastic pad thickness,H = 10 mm and discharge voltage,U = 7000 V were adopted.After the coil was discharged at six positions,elastic strain energy of sheet was reduced by 89.4%,elastic strain was transformed into plastic strain,and the springback height of sheet was 15.8 mm.(4)With respect to the challenge of large gap between the die and the part after springback in the electromagnetic incremental vibration forming process,a new process of large plastic deformation of the part driven by electromagnetic force and multi-pass stamping was proposed.Each die stamping height of 20 mm and the sheet were stretched and bent,then the coil in the set discharge position continuous discharge twice,so that the sheet after stretching and bending in each layer is closely fitted with the die.The simulation results showed that: the local area of the sheet under the action of electromagnetic force collided with the die at a high speed and adhered to the die,increasing the plastic strain significantly;the elastic deformation of the material transformed into plastic deformation;the stress oscillated and attenuated,and the springback of the parts was obviously inhibited.After 5 stamping actions and 10 discharge sessions,the maximum gap between the part and the die after springback was 1.5 mm.The forming accuracy of the part was improved significantly.The deformation profile of the sheet obtained by numerical simulation was in agreement with the experimental results.(5)Considering the setback that the deep drawing of large-size thin-wall ellipsoid parts was susceptible to wrinkling,a composite process combining electromagnetic incremental forming,multi-pass deep drawing and variable blank structure was proposed.Under conventional deep drawing conditions,the sheet area near the flange was subjected to circumferential compressive stress.When the depth of drawing exceeded120 mm,the sheet wrinkled apparently.A waist coil was used to discharge in turn along the 3D path.Furthermore,the sheet material corresponding to the coil has a large plastic deformation and is thinned by the thick compressive strain.After 10 series of pull and 86 discharge,the forming height of the part was 150 mm,and there was no fold in the plate area in contact with the mold.With respect to conventional stamping,electromagnetic composite forming can reduce the corrugating of sheet and increase the deformation height of smooth zone.In summary,the new composite technology combining conventional stamping and electromagnetic incremental forming proposed in this thesis exhibits the benefits of reducing the springback of large-sized thin-walled skin parts and suppressing wrinkling,which provides a new methodology for the realization of high-efficiency and high-precision manufacturing of large-sized skin parts.