Research On Wrinkle-free Forming Process For Sheet Parts Based On Solid Medium

Sheet parts are widely used in the automotive,aerospace,instrumentation and other industrial fields.Wrinkle defects are prone to occur during the forming process for the thin-plate parts.In this case,blank-holder device is usually used to supress the forming wrinkle defects,and the device is only suitable for the specified part,which is not versatile.Furthermore,the blanking areas are reserved for avoiding the wrinkle formation of the sheet material greatly to reduce the material utilization rate.Regarding the issue above,a solid medium plate wrinkle-free forming method is proposed.The forming process using medium materials for forming sheets is developed rapidly in the field of sheet forming,and the forming medium usually uses gas,liquid,viscous medium or urethane rubber as the load transmitting medium.Applying pressure to the sheet during the forming process to shapeit into the desired shape,which is advantage for the formation of complex shaped parts.At the same time,the forming medium can change the forming stress state in the sheet and provide a beneficial tangential force for the sheet.In this paper,a based solid medium forming method to suppress wrinkles for sheet metal is studied,and low-melting alloy and paraffin wax is employed as force medium.The main contents and conclusions are shown as follows:1.Analyzed the stress of sheet metal in forming process,researched the wrinkle-fress forming mechanism of solid viscoelastic material for sheet,and compared the forming process of two different solid medium materials with low melting point alloy and paraffin;design experimental device according to sheet forming principle;The stress-strain curve of the low melting point alloy was obtained by the tensile test.The tensile test results indicated that the mechanical strength of the low melting point alloy decreased with the increase of the tensile test temperature,and the elongation increased.The mechanical properties of the low-melting alloy were the highest at 65 °C,which is suitable for anti-wrinkle forming for sheets.2.Analyzed the characteristics of solid viscoelastic materials and sheet metal forming.Based on the theoretical knowledge of FEM,Lagrangian-Eulerian(CEL)coupling analysis method is used to solve severe distortion when the grid of Lagrangian mesh was used in the simulation.After comparing expremetal and numerical simulation the results,it showed that the CEL coupling method can accurately simulate the deformation dynamics of the sheet,and the test results are consistent with the numerical simulation results.The expremental test and simulation results show that the anti-wrinkle forming method using the solid viscoelastic material sheet is feasible.The reverse scanning of the test piece was carried out to analyze the forming error,and the result of analysis showd the maximum forming error of the formed part was-0.581~0.581 mm.3.Designed and manufacture a plate anti-wrinkle forming experimental device,which consists of a punch,a die,a punch ring and a guide sleeve.The stress and strain of the sheet were analyzed with the punch stroke to study the forming process of the low-melting alloy for sheet,and the influence of the low-melting alloy temperature,the thickness of the viscoelastic material and the mold structure on the stress and strain of the formed sheet was analyzed.The results showed that the increase of forming force causes the plastic deformation of the sheet to increase,which increases the average stress value of the sheet.The increase of the thickness of the low-melting alloy’s layer can effectively reduce the average stress of the sheet and make the stress and strain distribution of the sheet more uniform.Increasing the required forming force can increase the average stress of the forming sheet;the increase of the die overflow hole’s diameter can reduce the forming force required for sheet metal forming,and the sheet stress should be more evenly distributed as the diameter of the overflow hole increases.4.The wrinkle-free forming mechanism of paraffin particle media sheet was studied.The stress and strain distribution of sheet using paraffin particle media was analyzed.The stress and strain changes of the formed part along the warp direction were further analyzed.The stress and strain distribution of the part at different punch strokes were discussed.In the case,08 AL and Al 1060 were selected as the research objects.The influence of the thickness of paraffin wax’s layer and the average size of paraffin particles on the stress and strain distribution of the plate was discussed.The results show that the distribution of latitudinal compressive stress is uniform and the average value is small during the forming process.The paraffin particle medium provides the normal pressure on the sheet to reduce the compressive stress in the zonal distribution of the sheet,which can effectively inhibit the occurrence of wrinkling.Increasing the thickness of the paraffin particle’s layer and reducing the particle size of the paraffin particles can reduce the latitudinal stress of the sheet and make the stress distribution more uniform.5.Investigated the effects of solid viscoelastic materials on sheet metal forming process from three important aspects: sheet wrinkling,thinning rate and rebound.The relationship between the latitudinal stress of the sheet and the deformation of the sheet was analyzed.The relationship between the wrinkling and the latitudinal stress of the sheet was studied.The distribution law of the wall thickness of the formed sheet and the forming factors affecting the wall thickness of the formed part were investigated.The influence of process parameters on the springback and shape error of the formed parts was studied.The results show that the latitudinal stress distributed along the warp direction of the workpiece gradually changes from tensile stress to compressive stress from the center to the edge of the workpiece,and the formed part is subjected to latitudinal compressive stress at the edge of the workpiece.The increase of stress,wrinkle defects were occurred in the formed sheet,increase the thickness of solid medium’s layer,reduce the size of paraffin particles and increase the temperature of low melting point alloy can reduce the latitudinal stress during sheet forming,due to the smaller the latitudinal stress,the number and height of wrinkles is reduced;the wall thickness distribution of the formed part is different using low-melting alloy and paraffin particle medium.When the low-melting alloy medium is used,the maximum thinning position of the sheet is located at the waist of the formed part.The central part of the formed part is thinned and the wall thickness of the edge is increased.When the sheet is formed by paraffin particle medium,the maximum thinning position is located in the center of the workpiece,and the wall thickness of the sheet gradually increases from the center to the edge of the sheet;The increase of thickness of the solid medium’s layer and the forming pressure can reduce the amount of rebound of the sheet and forming error.Increasing the size of the paraffin particles and the temperature of the low-melting alloy will increase the amount of rebound of the formed part and reduce the forming accuracy of the workpiece.The mold overflow hole has a certain influence on the shape accuracy of the plate.As the diameter of the overflow hole increases,a stepped crease appears at the waist of the plate.