| With the rapid development of products to the direction of diversification and small batch,the shortcomings of traditional stamping technology,such as the long designing cycle,high manufacturing costs,difficult debugging and poor generality,have become increasingly prominent.On that basis the multi-point forming(MPF)is brought into being.Its main idea is to divide the curved surface of traditional stamping integral die into a series of regularly arranged and height adjustable punches.With the enveloping surface of punches,the three-dimensional shape of sheet part is realized under the controlling of computer system.Compared with the integral dies,MPF equipment has the advantages of convenient adjustment and better generality,thus saving much time,labor force and costs.After twenty-year development,MPF technology has been successfully applied in aircraft skin,high speed train head,hull plate,city sculpture,medical engineering and other fields.According to the different moving features of punches,MPF can be divided into five types,such as multi-point die forming(MPDF)and multi-point press forming(MPPF).Before loading,the punches of MPDF are adjusted through computer controlling system to form the target surface.There is no relative movement among adjacent punches in loading process,which is similar to integral die forming.For MPPF,there is no adjustment before loading.Instead of the bulk movement of upper punches in MPDF,the forming process of MPPF is accomplished through the relative movements of the all punches.Different form MPDF,in which most of the sheet metal in a suspended state during the loading process,all the punches of MPPF keep in touch with the plate in the whole forming process to develop a instantaneous forming shape,clamping and constraining the plate.Therefore MPPF can effectively inhibit the generation of dimples and wrinkles and minimize springback.The main contents and results are as follows:1.The basic principles of MPDF and MPPF were discussed.The characteristics and effects of the two forming methods were analyzed and predicted.Also the dimension selection and adjustment of punches were researched.2.Based on plasticity forming principles,finite element theories were discussed.The finite element models of MPDF and MPPF were established.And the material models,contact types,boundary conditions and mesh generation involved in finite element models were also investigated in detail.3.Models of spherical parts and saddle parts were established to compare the different effects of the two forming methods on dimples and wrinkling.Results indicate that the sheet in MPDF is in a suspended state during forming process.It's prone to generate indentations owing to the uneven pressure along thickness direction and prone to wrinkle owing to the shortage of tangential tensile.However,in MPPF,all areas of the plate were restrained by the instantaneous forming surface enveloped by punches.A state of greater tangential tension and more uniform thick stress plays an important role in suppressing wrinkling and indentations.4.Causes of springback were analyzed,and the error measurement formulas were deduced.Models of spherical parts were established to compare the different effects of the two forming methods on springback.Results show that in MPPF the springback decreases significantly,and the residual stress distribution is more uniform.Based on MPPF,iterative forming technology was proposed to investigate its influence on springback of cylindrical parts.Results show that the iterative forming technology has an obviously prohibitory effect on springback,of which the springback amount and residual stress decrease significantly.With the same forming steps,imminent forming scheme has a positive effect on the stability of the residual stress state.5.The stress state of the sheet metal in MPF was analyzed;the local bending moment formulas were deduced;the causes of local bending effect(LBF)and its influence on forming precision were investigated.Contrast analyses of correction results for LBF by increasing effective forming area(EFA)and using swinging punches were carried out.Experimental tests were employed to prove the effectiveness and realistic feasibility of the amendment by increasing EFA of MPF.Results show that the dislocation in the directions offorces from upper and lower punches leads to the generation of LBE.Method of increasing EFA can convert the edge instability into a relatively balanced state,significantly reducing the forming errors of the edge area.However it has little effect on improving the forming precision of the central zone.The application of swing units can remarkably diminish the dislocation of the force directions,resulting in a homogeneous and balanced force state.So that the forming errors can be reduced among the whole plate.Parts of high forming precision can be obtained under the combination of the two controlling methods.6.Several kinds of common configurations of punches in MPF were presented,of which three representative arrangements were employed in numerical simulations.Results of no-pair forming prove better than those of regular arrangement pair forming.The stress concentration of deformed part decreases and the strain distribution is more uniform.When lower punch matrix is larger than upper punch matrix,no-pair forming has a significant effect on improving forming precision.7.The design requirements,forming principles and characteristics of swinging punches were analyzed.The forming processes of fixed punches and swing punches were simulated respectively.Results indicate that the automatic rotation of swing punches can lead to a perfect tangency between plate and punches,so that the spherical cap radius of punch is permitted a substantial increase.Accordingly,the continuity of enveloping surface of punches increases significantly,resulting in a more stable and uniform stress state of blank.Contour lines of part's strain distribution become sparser,and the springback amount and residual stress decrease,which prove the better formability of swing punches. |
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