在粘性介质压力的压边力控制 成形的金属片

Blank holder force (BHF)control in viscous pressureforming (VPF)of sheet metalLeonid B.Shulkin a ,Ronald A.Posteraro b ,Mustafa A.Ahmetoglu c ,Gary L.Kinzel d ,Taylan Altan c,*aExxon Production Research Co.,PO Box 2189,Houston,TX 77252,USAbExtrudeHone Corp.,PO Box 527,Irwin,PA 15642,USAcERC for Net Shape Manufacturing,Ohio State University,Columbus,OH 43210,USA dDepartment of Mechanical Engineering,Ohio State University,Columbus,OH 43210,USAAbstractAn eight-point BHF control system with a ¯exible blank holder is designed and built as part of an experimental viscous pressure forming (VPF)machine.This paper describes results of VPF experiments,and addresses several blank holding issues speci®c to the VPF process.FEM simulations of hydroforming with a multi-point BHF control and an elastic blank holder are conducted to ®ne-tune the control system as well as to predict the forming loads.The FEM results are compared with experimental VPF results.The viscous medium ``blow through''phenomenon between the blank holder and the sheet during VPF is described and quanti®ed.#2000Elsevier Science S.A.All rights reserved.Keywords:Viscous pressure forming;Blank holder force;Sheet metal1.IntroductionThe quality of sheet metal products is highly dependent upon the rate at which the sheet is drawn into the die.For a given blank shape and material and ®nal part geometry,it is necessary to optimize the restraining forces applied to the blank [1±4].For many parts which are non-symmetric or which have non-uniform material properties,the optimiza-tion of the BHF requires that the pressure distribution on the blank be varied spatially and as a function of time or press ram position.Precise control of the BHP makes it possible to optimize the deep drawing process,which will result in a reduction in the number of parts scrapped.The availability of this degree of ¯exible control will have important appli-cations for use with multi-gage tailor-welded blanks,dif®-cult to form materials,and small production lots.2.Viscous pressure forming (VPF)Viscous pressure forming (VPF)is a sheet metal forming process developed and patented by ExtrudeHone,Irwin,PA[5].VPF offers a potentially simple and versatile approach to ``soft''tooling forming.In its simplest version,it can be thought of as hydroforming where a highly viscous yet ¯owable semi-solid medium is used instead of water.Potential applications of VPF include prototyping and low-to-medium volume production of stretched or drawn sheet metal components,forming of hard-to-form strain sensitive materials,and scratch-free forming of painted or coated sheets [6,7].The VPF project is sponsored by the Defense Advanced Research Projects Agency (DARPA)and participating companies (Pratt &Whitney,Boeing,ALCOA).3.Design of multi-point BHF control systemA multi-point BHF control system was designed for the VPF testing machine (Fig.1(a)).A conventional,pump-driven hydraulic system (Fig.1(b))was used.This system utilizes eight short stroke hydraulic cylinders [8]with four channels of BHF control and an elastically deforming blank holder plate.Locations of the cylinders around the blank holder plate can be varied depending on the selected test partgeometry.Journal of Materials Processing Technology 98(2000)7±16*Corresponding author.0924-0136/00/$±see front matter #2000Elsevier Science S.A.All rights reserved.PII:S 0924-0136(99)00300-33.1.Test part descriptionThe geometry for the VPF test part ``sub-inner panel''was developed based on a nozzle inner panel component cur-rently produced by Pratt &Whitney (Fig.2(a)).Dimensions and shape of the test part (Fig.2(b))were selected so that the corresponding VPF tooling would ®t into ExtrudeHone's 600ton hydraulic press.The material and the corner radii were selected based on those of the Pratt &Whitney nozzle inner panel.The original blank material is Inconel 718SPF alloy,thickness t 0.76mm (0.030in.).Several other mate-rials,includingaluminum6061-O,t 0.8128mm(0.032in.),aluminum 6111-T4,t l.0l6mm (0.040in.),and AKDQ steel,t 0.8382mm (0.033in.)were used in this work.3.2.Design of blank holder plateFig.3(a)shows the shape of the blank holder (binder plate)for the sub-inner panel and the locations of the hydraulic cylinders around the blank holder.The ®nal positions of the BHF cylinders,however,were determined later based on the available discrete matrix positions for the cylinders.The outer diameter of the blank holder plate (é330.2mm (13in.))was limited by the design of the VPF test machine.The thickness of the blank holder was determined by iteration by using a theory-of-elasticity based computer program.Here,the blank holder is modeled as an elastic layer supported by an elastic foundation,where the founda-tion represents the combined elastic properties of the sheet metal blank,die,and press [9].The criterion for thickness calculations was formulated as follows:for the maximum distance between two neighboring hydraulic cylinders l max 130mm (5.12in.),the BHP betwcen these cylinders should not be less than 0.1p max ,where p max is the pressure on top of a cylinder.Therefore,the range of in¯uence r 0.1of one cylinder within which the BHP 10times from its maximum should be r 0.1 l max /2 65mm (2.56in.)(Fig.3(a)).Based on the above criterion,the thickness of blank holder was found to be 50.8mm (2in.).4.Simulations of hydroforming with multi-point BHF controlA series of FEM simulations of hydroforming the sub-inner panel with a rigid die and an elastically deforming blank holder with an 8-point BHF control were conducted to determine the optimal blank shape,BHF range and distribu-tion,and to predict the forming loads.The commercial 3D program PAM-STAMP [10]was used for the simulations.Hydroforming is a good approximation of the VPF process when viscous medium is applied from one side of thesheetFig.1.VPF testingmachine.Fig.2.Sub-inner panel.8L.B.Shulkin et al./Journal of Materials Processing Technology 98(2000)7±16and the deformation is slow enough to minimize strain rate effects and to result in a uniform medium pressure in the forming chamber.The values sought were the ¯uid pressure and the minimum/maximum thickness and the correspond-ing thinning/thickening of the ®nal part.Fig.3(b)shows the hydroforming simulation setup with a rigid die,a ¯at elastic blank holder,and eight BHF cylinders.The rigid punch and die,and the elastic±plastic sheet were modeled with 4-node shell elements.The blank holder and the cylinders were modeled with 8-node elastic brick elements.The working¯uid was assumed to be water with a bulk modulus E 2.18GPa (316,000psi).Fig.4shows a typical ¯uid pressure variation during hydroforming of the sub-inner panel from Inconel 718SPF (the strongest of all the materials under considera-tion)at a total BHF 8ton uniformly distributed among the eight cylinders.The maximum ¯uid pressure required to form the part reaches 67MPa (9850psi)at the end of the process (point 3in Fig.4)when forming the sharp corner (R 0.5in.,Fig.2).Therefore,the VPF test machine was designed for the maximum forming pressure of 68MPa (10,000psi).Fig.5shows how the spatial BHF control improves the part quality.When an Inconel 718SPF blank is formed with all BHF cylinders set at the same force of 1ton (Fig.5(a)),the part experiences about 60%thinning (Fig.6(a))at the upper right corner.At this point,there are still 2mm more to form in this corner (Fig.6(b)).By using the setting with ®ve BHF cylinders,0.8ton each,as shown in Fig.5(b)(this setting produced the best results),the material ¯ow in the whole part is improved.This resulted in a signi®cantly more uniform thickness distribution among the four corners with only about 22%thinning at the upper right corner (Fig.6)at the same simulation stage.No wrinkling was predicted by both simulations.However,forming the last 2mm of the corner resulted in over 50%thinning even with the BHF adjustment shown in Fig.5(b).The BHP was constant during the stroke in the above simulations.Various temporal BHF variations were tested as well.However,no improvement was detected since the failure occurred due to pure stretch-ing in the corner at the end of the process when the part is almost formed and no drawing action happens on the ¯ange.This means that the variation of BHP had no in¯uence at this stage of the process.All attempts to produce a part with less than 25%thinning were unsuccessful.The part could not be hydroformed due to an unavoidable failure at the sharp corner (Fig.6(b)).To correct this problem,two geometry changes were imple-mented in the die installed into the VPF machine:the upper right corner radius (R 0.5in.,Fig.2)was increasedfromFig.3.FEM model for VPFsimulations.Fig.4.(a)Fluid pressure during the stroke,(b)simulated forming stages.L.B.Shulkin et al./Journal of Materials Processing Technology 98(2000)7±169R 12.7mm (0.5in.)to 25.4mm (1.0in.)to make the ratio 2R /h less than 2(where h 38.l mm (1.5in.)is the part height)the die bottom radius was increased from R 9.525mm (0.375in.)to 12.7mm (0.5in.).5.Experimental evaluation of BHF control system 5.1.Spatial BHF control in VPFFig.7shows the ®nal locations of the BHP cylinders in the VPF machine.They are determined by the closest available discrete matrix positions and are somewhat different from the ones used for the FEM simulations (Fig.3).It should be noted that the VPF machine was designed to form parts which are of the same dimensions but mirror-symmetric with those in the FEM simulations described previously.However,this does not affect the analysis and comparisons presented in this paper.5.1.1.Pressure paper analysisPressure paper with 0.5±2.5MPa (7.4±37psi)sensitivity range was used to record the BHP distribution between asheet metal blank and the blank holder.Fig.7shows three different settings of the BHF control system and the result-ing BHP distributions.When the BHF is applied uniformly around the blank holder (Fig.7(a)and (b)),the BHP is higher at the corners due to the lower bending stiffness of the blank holder plate in these regions.The vertical marks recorded by the pressure paper are the tool marks in the top die of the VPF machine.In case (c),the setting was the same as in case (b)except the BHF cylinder on the top side of the blank holder was disconnected.The resulting BHP distribution indicates almost no pressure at the location of this cylinder and a pressure drop at the nearby corners.This shows that the range of in¯uence of one cylinder does not spread beyond the neighboring cylinders which validates the assumptions used in the design of the blank holder.5.1.2.VPF of aluminum 6061-OAluminum 6061-O blanks shaped as an octagon inscribed in a 304.8mm (12in.)diameter circle,were formed using different spatial settings of BHF cylinders.The BHF was constant during the process.Fig.8compares the fracture locations for different BHF spatial patterns.Application of a uniform BHF resulted in fracture at two corners (Fig.8(a)),Fig.5.Hydroforming simulation of Inconel 718SPF:(a)8BHF cylinders 1ton each,(b)5BHF cylinders 0.8toneach.Fig.6.Hydroforming simulation results:(a)thickness distribution comparison for two simulations with uniform and adjusted BHP (Fig.5b)maximum thinning (failure)area when there are 2mm remaining to form in the corner.10L.B.Shulkin et al./Journal of Materials Processing Technology 98(2000)7±16large at the sharp right corner and small at the left 908corner.By reducing the BHF at the corners,it was possible to improve the material ¯ow during the VPF process and to vary the fracture position and size.In case (b),where the BHF was reduced along the side and around the left corner,a large fracture occurred only at the sharp right corner (Fig.8(b)).In case (c),further reduction of the BHF around the sharp right corner resulted in a complete elimination of the fracture there;however,some wrinkles developed on the ¯ange next to this corner,and a small fracture was observed in the left 908corner (Fig.8(c)).5.2.Problems observed during preliminary VPF tests Preliminary VPF tests showed some discrepancies between the predictions of FEM simulations and the forming results and helped us to identify several design shortcomings of the VPF machine.All attempts to form parts with Inconel 718SPF were unsuccessful due to early wrinkling of the blanks which was not observed in the FEM simulations described previously.Even though the FEM simulations predicted that a total BHF 4tons would be suf®cient to form the sub-inner panel,severe wrinkling occurredduringFig.7.BHP distribution recorded by pressure paper:(a)BHF 1.25ton per cylinder,(b)BHF 0.5ton per cylinder,and (c)same as (b)with the top side cylinderdisconnected.Fig.8.VPF of aluminum 6061-O,circles indicate fracture locations for corresponding BHF spatial patterns,numbers show the force of each BHF cylinder in tons.L.B.Shulkin et al./Journal of Materials Processing Technology 98(2000)7±1611the experiments even at BHF 20tons (capacity of the system).It was concluded that the BHF control system could not produce enough BHF for Inconel 718SPF.Therefore,further tests were conducted with materials requiring lower forming pressure and BHF,such as aluminum 6061-O,aluminum 611-T4,and AKDQ steel.The blank geometry developed by the FEM (Fig.9,dashed line)was found to be too small since the ¯ange left of the blank holder in the middle of the forming process and the parts experienced severe wrinkling.In Fig.9,the solid line shows the blank geometry developed experimentally using a trial-and-error procedure.The blank is somewhat larger than the one developed by the ing this geometry,aluminum 6061-O blanks were successfully formed with the total BHF 8ton uniformly distributed among all eight cylinders.The same blank geometry was used for further experiments.5.3.VPF tests with constant uniform BHF5.3.1.VPF stages and medium ``blow through''Fig.10shows the parts formed from aluminum 6111-T4blanks with BHF 20ton.The viscous medium pressure was monitored by pressure transducers at two different points:near the die at about 1m distance from the forming chamber (dashed line in Fig.11),and at the medium delivery cylinder at about 2m from the forming chamber (solid line).The forming process was stopped at 60%,80%,and 100%of the medium pressure near the die.Pressure inside the forming chamber was not known.The medium pressure dropped by about 5MPa (735psi)between the two trans-ducers located just about 1m from each other.Therefore,the medium is expected to further drop the pressure between the die transducer and the forming chamber by 4±6MPa (600±900psi).In the future,planned tooling modi®cations will include placing a pressure transducer into the forming chamber to measure the medium forming pressure directly.No wrinkling was observed on the parts formed at 60%and 80%of the forming pressure (Fig.10(a)and (b)).A thin,transparent layer of viscous medium was present betweenthe blank holder and the sheet after both tests,covering most of the part ¯ange at the 80%test.A sudden medium pressure drop accompanied by a loud sound was heard during the 100%test when the part was formed just past 80%pressure level.Simultaneously,the ¯uid pressure in the BHF cylin-ders sharply increased and quickly reached the pressure transducer's limit of 37MPa (5500psi).At this point,the blank holder sealing was broken and the medium blew through between the blank holder and the sheet and ®lled the available space until stopped by the outer seal of the tooling where the clamping force of the press is applied (Fig.12).A sudden increase of the volume resulted in the medium pressure drop observed in Fig.11.The pressurized medium started acting against the entire blank holder area pushing it down against the BHF cylinders.Once the blank holder-sheet seal was broken and the blank holder was pushed down,wrinkling started on the ¯ange (Fig.9(c)).The medium penetrated further between the sheet and the die into the unformed corners of thedie.Fig.9.Experimentally developed blank geometry used in VPF experi-ments.Fig.10.VPF of aluminum 6111-T4,process stages at (a)60%(b)80%,and (c)100%of forming pressure (Fig.11).12L.B.Shulkin et al./Journal of Materials Processing Technology 98(2000)7±16Once the corners were ®lled,the medium pressure increased again.Just before the end of the process,the part fractured at the sharp corner (Fig.9(c)).5.3.2.Delaying medium ``blow through''by increasing BHFA tight medium sealing on the blank holder±sheet inter-face must be maintained to avoid the medium ``blow through''and subsequent wrinkling.This was achieved by increasing the BHF which offset the ``blow through''towards the end of the forming process.Fig.13shows two parts formed with different BHF levels,and Fig.14com-pares the medium forming pressure and the BHF system ¯uid pressure during the process.Increasing the BHF from 16to 20tons eliminated wrinkling by delaying the medium ``blow through''and allowing the part to be mostly formed(except for the corner regions).This way,no more ¯ange draw-in,and therefore,no more circumferential ¯ange reduction occurred,and wrinkles did not form.The part was pressed against the die walls and the viscous medium was prevented from penetrating into the die corners which,therefore,could be formed to the desired radii (Fig.13(b)).6.Estimation of medium force acting against blank holderThe medium ``blow through''happens when the pressur-ized medium penetrates between the blank holder and the sheet and pushes the blank holder down thus increasing the blank holder-die gap and allowing the sheet to wrinkle.To avoid or delay the ``blow through'',it is necessary to estimate the force acting against the blank holder plate due to the medium leakage between the blank holder and the sheet.To compensate for the force that counteracts the BHF,it is necessary to increase the BHF to provide an adequate restriction of metal ¯ow during VPF of sheet metal.6.1.Assumptions and derivationThe medium penetrates between the blank holder and the sheet and stops at a distance L from the edge of the blank holder opening (Fig.15).Assume,that there is no energy loss within the medium,therefore the medium pressure drop is due to friction only.Hence,to stop the medium at a distance L,the friction force (per unitwidth)Fig.11.Viscous medium forming pressure in VPF of aluminum 6111-T4parts shown in Fig.10.Fig.12.Illustration of the viscous medium ``blow through''between the blank holder and thesheet.Fig.13.VPF of AKDQd steel with different BHF:(a)16tons,and (b)20tons.Fig.14.Viscous medium forming pressure during VPF of the parts shown in Fig.13.Fig.15.Medium leakage between the blank holder and the sheet.L.B.Shulkin et al./Journal of Materials Processing Technology 98(2000)7±1613on the medium±sheet and medium±die interfaces should be equal to the force (per unit width)due to the medium pressure,i.e.F fric F press X(1)The friction force (per unit width)at the two interfaces can be expressed asF fric 2"L 0p d x Y (2)where "is the Coulomb friction coefficient and P 0is themedium pressure in the forming chamber.The force (per unit width)due to the medium pressure is F press P 0 Y(3)where is the gap between the blank holder and the sheet.Equating the two forces (Eqs.(2)and (3))givesL 0p d xP 02"X (4)The total force exerted upon the blank holder by the medium within the distance L can be estimated asF BH CL 0p d x Y (5)where C is the blank holder cavity bining(Eqs.(4)and (5))gives F BH 12P 0 "C X(6)6.2.Sample calculation and comparison with experiments To measure the counter force exerted by the viscous medium upon the blank holder,the BHF control system was modi®ed as shown in Fig.16.The pressure control andsolenoid valves of the original system (Fig.1)were by-passed and the BHF cylinders were connected directly to the pump which could produce up to 34MPa (5000psi)of hydraulic pressure.The pressure was set by using the internal pressure relief valve inside the pump unit.This way,each BHF cylinder could be loaded up to 2.5tons thus providing a total BHF 20tons.The pressure,monitored by the pressure transducer,was maintained by the check valve which created a closed,pressurized volume consisting of the eight cylinders and the corresponding hoses and ®ttings.After each forming cycle,the pressure in the system was relieved through manual draining.Fig.17shows the ¯uid pressure and the forming medium pressure near the die during VPF of aluminum 6111-T4,t l.0l6mm (0.040in.)and AKDQ steel,t 0.899mm (0.035in.).A substantial ¯uid pressure increase can be seen after the ``blow through''.Since the cylinders were con-nected to a closed volume,the viscous medium counter-pressure led to further compression of the hydraulic ¯uid which resulted in the pressure increase observed in Fig.17.In both cases,the system was set at BHF 20ton,which corresponds to 34MPa (5000psi)of ¯uid pressure.In the case of aluminum 611l-T4,the pressure increased by 22MPa (3200psi),which corresponds to a 12.8ton increase of the BHF (eight cylinders,each with 6.5cm 2(1in.2)of piston area).In the case of AKDQ steel,the pressure increased by 54MPa (8000psi),which corresponds to a 32ton increase of the BHF before the process was stopped by the operator when the BHF system pressure reached 89MPa (13,000psi),thus exceeding the 68MPa (10,000psi)design limit of the system.FEM simulations of hydroforming showed that the sheet thickened as much as 25%at the ¯ange.Therefore,for aluminum 611l-T4,t 1.016mm (0.040in.),the gap between the blank holder and the sheet can be as much as 0.254mm (0.01in.).Assuming the gap 0.254mm (0.01in.)and friction " 0.03[11]the blank holder cavity perimeter C 700mm (27.5in.),and the medium forming pressure P 0 30MPa (4400psi)(Fig.17),the total force acting against the blank holder is F BH 8.8tons (Eq.(6)).Considering all assumptions and simpli®cations used in deriving Eq.(6),the calculations can be considered toagreeFig.16.Simplified BHF control system for VPFmachine.Fig.17.Increase of BHF system fluid pressure after ``blow through''.14L.B.Shulkin et al./Journal of Materials Processing Technology 98(2000)7±16reasonably well with the experimental results(12.8ton)and, therefore,are applicable as a part of the blank holder system design methodology.It should be noted that the blank holder counterforce will be higher than predicted by Eq.(6)once the``blow through''occurs and the sheet wrinkles.In this case,the medium pressure acts upon a larger area of the blank holder and,in the most critical case,the force may be as high as the full forming pressure over the entire blank holder area.An approximate analysis shows that the force exerted by the forming medium upon the blank holder is proportional to the forming pressure.Therefore,it is necessary to compen-sate for this force by increasing the BHF proportional to the forming pressure during the VPF process.This can be done by including a feedback loop into the BHF control system.A more practical approach would be to simply include a temporal BHF pro®le in the control program.VPF experi-ments show that the medium``blow through''between the sheet and the blank holder happens at an intermediate stage of the process once the medium pressure reaches a certain critical level.In addition,simulations and experiments demonstrated that the blank holder has no in¯uence at the later process stages when corners are formed and,hence, there is no advantage in increasing the BHF after two level of forming pressure.Therefore,it is reasonable to start the process at a low constant BHF to promote the material draw-in,then linearly increase it to a higher level,and ®nish the process at this second constant level.A similar concept is being tested in VPF experiments conducted at the ERC/NSM.7.ConclusionsFEM simulations of hydroforming and VPF experiments demonstrated that spatial BHP control is instrumental in in¯uencing the material¯ow in critical forming areas and improving formability of sheet metal parts,especially at the early and intermediate stages of the forming process.By optimizing the three design parameters,namely the blank holder thickness,the number,and the relative settings of BHF cylinders,a desired BHP distribution can be obtained for a speci®c part geometry.The application of an elastic, instead of a rigid model,of the blank holder adds a new dimension to the accuracy of FEM simulations of deep drawing and hydroforming.The next logical step will be to model the elastic properties of not only the blank holder but also the die and various press components whose elastic de¯ections affect the part quality.A sample calculation shows that the force exerted by the medium against the blank holder can be as high as several tons for a relatively small part such as the sub-inner panel used in this work.This may partially explain discrepancies between FEM simulations(where leakage is not modeled)and experiments(where leakage is always present).In addition,the medium layer signi®-cantly reduces friction between the sheet and the blank holder.Therefore,the metal¯ow restriction in the VPF experiments is less and,consequently,the required blank size is larger than predicted by the FEM simulations of hydroforming.Thus far,for the part geometries considered,the VPF process showed no clear advantage in comparison with deep drawing and hydroforming processes in terms of sheet metal formability,forming loads,and speed.In fact,the VPF results in more material thinning in sharp female corners,requires a large clamping force to prevent die separation under the forming pressure,has the medium``blow through''problem which adds another variable to consider when designing a BHF control system, and is much slower than conventional deep drawing with a rigid punch.However,the VPF process may have a cost advantage over deep drawing and hydroforming for prototyping and niche product shops due to lower cost of the tooling,easier and safer handling of viscous medium,and quieter operation.AcknowledgementsThe VPF project is partially sponsored by the Defense Advanced Research Projects Agency(DARPA).Authors also wish to thank the project leader,Ralph L.Resnick, ExtrudeHone Corp.,and participating companies,ALCOA, Boeing,Concurrent Technologies,Erie Press,and Pratt& Whitney,for their valuable support.References[1]M.A.Ahmetoglu,G.L.Kinzel,T.Altan,Improvement of part qualityin stamping by controlling blank-holder force and pressure,J.Mater.Process.Technol.33(1992)195±214.[2]E.Doege,G.Stock,1995.Lock toolÐdeep-drawing process andexamples of operations with elastic blank-holders,SAE Paper no.950917.[3]K.Siegert,E.Dannenmann,S.Wagner,A.Galaiko,Closed-loopcontrol system for blank holder forces in deep drawing,Ann.CIRP 44(1995)251±254.[4]K.J.Pahl,New developments in multi-point die-cushion technology,Proceedings of Second International Conference on Sheet Metal Forming 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