Finite Element Analysis And Optimization Of High Flexible Stretch Forming Process

Stretch forming is the main forming method of big size and small curvature skin parts.Compared with the traditional die stamping forming,it has the advantages of small springback and high forming precision.However,it needs high manufacturing cost and generally only used in aviation and aerospace fields.With the rapid development of aviation and aerospace industry,demands for skin parts develop towards individuality,complicate and rapidness,which makes it a new challenge for traditional stretch forming.The traditional stretch forming technology has the following two aspects of disadvantages.On the one hand,it involves large quantity of whole die,long production cycle,high cost of storage and transportation,and low repeating utilization rate.On the other hand,it is featured by low flexibility of stretch forming machine,die fitness difficulty of sheet metal,low material utilization rate,uneven deformation,and great tendency of cracking and wrinkling.It is visible that the traditional stretch forming technology has been unable to meet the production requirements under the new situation.In addition,the domestic stretch forming equipment is mainly imported from foreign countries.The core manufacturing technology of stretch forming equipment is controlled by Europe and the United States and other developed countries.Besides,the high equipment selling price makes it unavailable for small and medium-sized enterprises.Therefore,it is urgent to produce flexibility stretch forming equipment with independent intellectual property rights for the development of China industry.High flexible stretch forming equipment is a new type stretch forming equipment independently developed.It includes high flexible die and high flexible stretch forming machine.By replacing the traditional whole die with digital multi-point die,the high flexible die has the capability of forming many different parts with only one set of die.Besides,it can reduce the die manufacturing cost and production cycle.High flexible stretch forming machine realizes the sheet metal attaching the die through the self-coordination motion of discrete clamps,and can improve the forming quality,material utilization and production efficiency.In addition,the simple control systemand relatively low manufacturing cost of the high flexible stretch forming equipment make it more acceptable for small and medium-sized enterprises.Furthermore,it is helpful for the application and promotion of the technology in other areas.In the paper,taking the high flexible multi-point die and the high flexible stretch forming machine as the main line,the sheet metal stretch forming process was analyzed through the method combining the finite element numerical simulation and experiment so as to optimize the processing technology,improve the forming quality of parts,and provide scientific guidance for industrial production and the development of the new stretch forming equipment.The main contents and conclusions are as follows:1.Principle and mechanics analysis of high flexible stretch forming processThe principle,technical characteristics and forming process of high flexible stretch forming were discussed.Besides,the variation of stress and strain in sheet metal was analyzed.The internal pull force and springback of sheet metal forming process were calculated.Selection basis of sheet metal size and quality evaluation criterion of sheet metal were given.2.The establishment of high flexible stretch forming process finite element modelThe solution algorithm of dynamic explicit formulation and finite static implicit formulation of finite element numerical simulation as well as the elastoplastic constitutive relation of material were discussed.In addition,the finite element model of high flexible stretch forming and springback model of sheet metal forming were established.3.Structure analysis and numerical simulation of high flexible stretch forming machineStructural analysis was conducted on the main parts of high flexible stretch forming machine,including clamping mechanism,stretch mechanism and universal structure.The effect of clamp discretization,clamp gap,and loading amplitude on the forming results was analyzed through the finite element numerical simulation method.The following analysis results were obtained: with the increase of clamp number,the sheet metal better attached to the die;and the deformation became more homogeneous.With the increase of clamp gap,uniformity degree of stress and strain distribution of formed part increased first and then decreased;too small clamp gap caused attaching die difficulty and too large gap made forming area appear wrinkles.Under the conditions of the sheet metal with the width of 1200 mm and ten clamps,when the gap between clamps equal to 1/9 of the width of the clamp,the forming effect is the best.When forming force was loaded with reasonable amplitude,the forming quality could be improved.4.Research on the arrangement form of basic unitsThe indentation forms and its forming reasons of the high flexible stretch forming parts were analyzed.The multi-point dies with conventional arrangement,declining staggered arrangement and parallel staggered arrangement were constructed.Taking spherical and saddle parts as the research object,the effect of basic units arrangement modes on the forming results was studied through the finite element numerical simulation and experiment.The analysis results were as follows: the punch top dimple and stretching groove were the main defects of the forming part surface;the staggered arrangement multi-point die can suppress the indentation defect,improve the surface quality and forming precision.Compared to conventional arrangement,the staggered arrangement multi-point die had better forming effect.5.Research on the effect of ball head formThe critical geometric condition of the tangency between ball head and sheet metal was deduced.The finite element model of stretch forming with swing ball head was established,and then the effect of swing ball head on the forming results was analyzed.Analysis results were as follows: when the basic element ball head radius was less than the tangent critical radius of sheet metal and ball head,the forming quality improved with the increase of ball head radius;when the ball head radius was larger than the critical radius,the surface quality degraded with the increase of ball head radius.When the swing ball head die was used to replace the fixed ball head die,it could effectively inhibit the indentation on the surface of the formed part,and reduce the thickness of elastic cushion and springback amount.The combination of swing ball head and staggered multi-point die made the forming effect better.6.Research on the effect of friction conditionsThe friction coefficients of the different friction contact pairs in dry friction and oil lubrication were determined through friction test.The effect of friction conditions between sheet metal and die on the forming results was studied through finite element numerical simulation method.Besides,the influence of multi-point die and elastic cushion,as well as the friction conditions between elastic cushions on the forming results was studied through the finite element numerical simulation and experiment.The research results show that: the effective forming zone of formed parts had better forming effect when sheet metal and die had dry friction,but the stress and strain of transition area were larger,which could be effectively reduced through oil lubrication;When multiple friction contact pairs exist simultaneously,dry friction condition was maintained between the friction contact pairs,the effective forming zone had better forming effect;when oil lubrication was used for elastic cushions,forming effect was poorer.7.Research on the effect of elastic cushion on high flexible stretch formingTaking spherical part as the research object,the effect of thickness,hardness and layer number of elastic cushion on the formed part surface quality and forming error was studied through the finite element numerical simulation.Moreover,the determination method of sheet metal attaching die and the realization method of the error compensation were discussed.The study results were as follows: with the increase of elastic cushion thickness,the formed part surface quality improved,but the forming error increased;the greater the elastic cushion hardness,the higher surface quality and forming precision the formed part had.Comparing elastic cushions with the same thickness,the formed part surface quality and forming precision reduced with the increase of layers,but the springback decreased at the same time.Profiling method was simple to determine sheet metal attaching to the die.Multi-point die could quickly realize error compensation through the method of die profile correction.