Research On Shape Precision Control Of Curved Parts In Multi-Point Forming

The forming of sheet metal is widely used in aerospace,automobile,ship and other fields.The three-dimensional curved metal parts are mainly obtained by integral die stamping in the tradition.The development of modern industry has gradually turned from traditional mass production to diversification and small batch production,while the integral die needs to spend a lot of money to to design and manufacture.Therefore,a new type of stamping process is needed to meet the requirements of diversification and small batch production,and new sheet forming process–the multi-point forming technology has emerged.The multi-point forming without blank holder is usually used to form three-dimensional plates with small curvature,while the multi-point forming without blank holder is commonly used to form three-dimensional plates with large curvature.Compared with the traditional die,the biggest advantage of multi-point die is that it can form all kinds of curved parts quickly and flexibly.However,this technique also has forming defects such as springback,and the springback seriously affects the precision of formed parts.Accurate prediction and control of springback is very important for obtaining three-dimensional curved parts with high quality.There are two methods to control the springback in multi-point forming: one is to compensate springback for the profile of the multi-point die,and the other is to use the process control method to reduce springback.In this paper,when the multi-point die without blank holder forms the cylindrical parts,its profile is compensated springback to accurately form the target part.This method can increase efficiency and save time.When the multi-point die with blank holder is usd to form the car roof,the process is optimized to control the springback.The orthogonal test method is used to optimize the process parameter for forming the car roof with high quality,which can guide the actual production.The main contents of this paper are as follows:1.Establishment of finite element model in multi-point formingBased on the principle of multi-point forming and finite element theory,the finite element models of multi-point forming for cylindrical parts and car roof were established.Establishment of geometric model,selection of material model,setting of boundary condition,selection of element type,meshing and other aspects were also discussed.Finally,the modeling method of springback analysis was studied.They laid the foundation for the numerical simulation of sheet metal forming.2.Control springback by compensating die profileThe springback compensation formula for multi-point die is given to form cylindrical parts accurately,and the determination of springback compensation coefficient was studied.The numerical simulation calculation and experimental research on the process of forming the cylindrical part before and after compensating the multi-point die profile were carried out.The node coordinates of the simulated parts and point cloud coordinates of the experimental parts were imported into Geomagic to compare with the standard parts for obtaining the error distribution nephogram and error distribution diagram.The results show that the simulated parts and experimental parts obtained by the uncompensated profile of multi-point die were greatly different from the target shape and do not meet the accuracy requirements;The simulated parts and experimental parts obtained by compensating profile of multi-point die were in good agreement with the target shape,and the overall error was small.The correctness of the springback compensation method was verified.3.Influence of curvature radius and thickness on springback amount and springback compensation coefficientSingle variable method was used to study the influence of curvature radius and thickness on springback amount and the law of springback compensation coefficient changing with curvature radius and thickness when multi-point die formed cylindrical parts.The results show that the springback amount increases with the increase of the curvature radius and decreases with the increase of the plate thickness;The springback compensation coefficient is positively and linearly related to the curvature radius;The springback compensation coefficient decreases as the plate thickness increases and the tendency of decrease gradually becomes slower.4.Improve the forming quality of car roof using process control methodIn order to form the car roof with high shape accuracy,orthogonal test was carried out by using the orthogonal test table of L16.The influence factors are elastic cushion thickness,friction coefficient,blank holder force and stamping force.Each factor takes four levels,and the test indexes are minimum thickness,maximum thickness and springback amount.Numerical simulation calculations of the forming process and springback process of the car roof were carried out.The influences of each factor on the three test indexes and the order of the influence of each factor on the springback amount of car roof were discussed.The line up grading method was used to comprehensively analyze the test results.The order of various factors affecting the forming quality and the optimal forming parameter combination were obtained by range analysis.The results show that the minimum thickness of car roof increases as the elastic cushion thickness increases,and decreases as the friction coefficient,the blank holder force,the stamping force increases;The maximum thickness increases as the elastic cushion thickness,the stamping force increases and decreases as the blank holder force increases;The variation of maximum thickness with friction coefficient is complicated;The springback amount increases as the elastic cushion thickness increases,and decreases as the friction coefficient,the blank holder force,the stamping force increases;The order of influence of various factors on the springback amount of car roof is: blank holder force > friction coefficient > stamping force > elastic cushion thickness;The order of influence on the quality of car roof is: blank holder force > elastic cushion thickness > friction coefficient > stamping force.