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汽车锁座冲压工艺及模具设计,汽车,冲压,工艺,模具设计
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Multiple-iteration springback compensation of tailor welded blanksduring stamping forming processH. Wang, J. Zhou, T.S. Zhao, L.Z. Liu, Q. LiangCollege of Material Science and Engineering, Chongqing University, Chongqing 400030, Chinaa b s t r a c ta r t i c l ei n f oArticle history:Received 24 November 2015Received in revised form 8 April 2016Accepted 10 April 2016Available online 19 April 2016The forming and springback of tailor welded blanks (TWBs) are more complicated than those of conventionalsheet metals because of the influence of weld seam. The current study investigated the microstructure and frac-tureofTWBsthrough metallographic microscopy and scanning electronmicroscopy.Tensile tests show that thinand thick materials possess similar mechanical properties. The maximum elongation and yield strength of theTWBs decrease, whereas the tensile strength of the TWBs is larger than that of the base materials. We proposedthe use of multiple-iteration springback compensation to improve the accuracy of springback compensation forthe TWBs. The simulation of the entire forming processes, which includes drawing, trimming and piercing, aswellasspringbackanalysis,wasusedtoexplorethestampingformingoftheTWBsbyautomotivefloorreinforce-ment. Forming parameters were optimised in drawing process. In addition, springback calculation was per-formed to predict the springback of the formed part. Furthermore, multiple-iteration springback compensationwas applied to drawing tools and verified by experiments. The experimental results agree with the simulationresults. 2016 Elsevier Ltd. All rights reserved.Keywords:Tailor welded blanksMicrostructureFractureSpringback compensationFinite element method1. IntroductionTailor welded blanks (TWBs)are composedof two or more pieces ofwelded sheets, which can be formed together in a single formingoperation; the welded sheets may have different thicknesses,mechanical properties or surface coatings 13. With the trend oflightweight automotive, TWBs forming has become an effectivemethod to achieve lightweight automobiles. The technique can enhancetheintegralautomotivebodystiffnessandsecuritywhilesavingmaterialsand relieving vehicle weight 4. Therefore, the application of TWBs inautomobile has been increasing annually.Numerous materials, including low-carbon steels and high-strengthsteels(HSS)and aluminum alloys, can beused as TWBs 5,6. HSS TWBsare rapidly developed in the automobile industry because of their highyield strength and good welding performance. TWBs forming hasbecome a hot issue in recent years. Abbasi et al. 7 analysed thewrinkling behavior of TWBs in deep drawing. Nguyen et al. 8improved the drawability of high-strength differential TWBs byusing the finite element method (FEM). Moreover, Bandyopadhyayet al. 9 proposed that the limiting drawing ratio and deep drawingbehavior of dual-phase steel TWBs could be improved with restrictedweld movement by controlling the initial weld line position. Mamusiet al. 10 introduced a novel approach to acquire the forming limitdiagram (FLD) for TWBs. Song 11 and Safdarian 12 investigated theeffect of thickness ratio on the formability of TWBs. The researchersemphasised that the formability and the level of FLD decrease as thedifference in the thickness ratio of TWBs increases.Previous studies on TWBs mainly focused on the microstructure,mechanical properties, wrinkling behavior, FLD, drawing ratio andformability of these materials. Few studies reported on the springbackcompensation of TWBs. As is well known, springback is a commonand ubiquitous problem in the sheet metal forming 13. Springbackappears when tools are removed from the formed part 14. An exces-sivelylargespringbackcancauseconsiderableshapedeviationbetweenthe final and designed parts, and may lead to certain difficulties in thesubsequent forming operation or assembly.Although the FEM has been successfully used to simulate thestamping forming of complex parts, the approach still does not offer asatisfactory solution regarding springback 15. To date, previousstudies on springback mainly focused on simple geometries, suchas cylindrical tooling, L-bending, U-bending and V-bending 16.However, studies on springback compensation for complex partsare limited. Furthermore, numerous tool manufacturers deal withtheproblem of springbackbasedonexperience of designers. Numeroussmall enterprises neglect springback at the beginning of die design. Theinitial tryout parts are inspected by a special fixture, the die surface ismodified on the basis of inspection results. Complex parts usuallyrequire three or more modifications of tool shape and die tryout beforethe formed part meets the allowable tolerance requirement 17.Evidently, the traditional approaches may result in uncertainty inpredicting costs and lead times.Materials and Design 102 (2016) 247254 Corresponding author.E-mail address: jz2012 (J. Zhou)./10.1016/j.matdes.2016.04.0320264-1275/ 2016 Elsevier Ltd. All rights reserved.Contents lists available at ScienceDirectMaterials and Designjournal homepage: /locate/matdesTherefore,thepresentstudyaimstoinvestigatethesimulationoftheentire forming processes and springback compensation for TWBs. TheFEM was used to simulate the forming process and predict springback,and multiple-iteration springback compensation was introduced toimprove the accuracy of springback compensation. Finally, theproposed method was verified by experiments.2. Materials2.1. MicrostructureThe TWBs used in the present study were made by laser welding oftwo steel sheets having same chemical compositions and differentthicknesses (1.2 and 1.6 mm). The chemical composition of the basematerial (HC340/590DPD+Z) used in TWBs is presented in Table 1.The base material belongs to dual-phase HSS. Given their good perfor-mance,thesetypes of TWBsare widelyused as structuraland reinforce-ment components in the automobile industry.The base materials and the welded material significantly differ inmicrostructure and mechanical properties 18. The microstructure ofthe base materials and the weld seam zone was determined via metal-lographic microscopy 19. Microstructures of the welded joint, thebase material, the heat-affected zones (HAZ) and the weld seam of theTWBs are presented in Fig. 1. The weld joints possessed a narrowweld seam and a small HAZ, and a clear boundary exists between thebasematerials and theweld seam.The microstructure differedindiffer-entregions,becausethebasematerialswereintherollingstate,theHAZwas a gradient structure region caused by the welding thermal cycle,and the weld seam was in the casting state. The microstructure ofthe base materials were composed of ferrite and martensite, and themartensite structure was distributed in the ferrite matrix with islanddispersion (Fig. 1b). The microstructure of the HAZ was mainly com-posed of ferrite, pearlite and a small amount of martensite (Fig. 1c).The microstructure of the weld seam was composed of bainite andmartensite (Fig. 1d). The mechanical properties of the TWBs weredetermined by the microstructure of the base materials and theweld seam.2.2. Tensile testsTensile tests according to ASTM-E8 standard test 20 were used toobtain the mechanical properties of the base materials, while samplesof welded material based on Ref. 19,21 were used to get the mechanicalproperties of the TWBs material. Tensile tests were conducted at roomtemperature with a controlled displacement rate of 5 mm/min. The truestressstrain curves of the base blanks and the TWBs are presented inFig. 2. The result shows that the true stressstrain curves of the thin andthick sheets are similar. The yield strength of the base materials (thinand thick materials) are 442.09 and 432.47 MPa, respectively, whereasthat of the TWBs is 373.15 MPa. The elongation of the base materials(thin and thick materials) are 26.17% and 26.46%, respectively, whereasthatoftheTWBsis18.48%.FromFig.2,wecanconcludethatthemechan-ical properties of the thin and thick materials are similar. Moreover, theinfluence of the weld seam can be clearly observed. The welded blankdecreases the maximum elongation of the tested material. In addition,the strength of the TWBs is larger than that of the base materials. Fromthese properties, we can conclude that high strength and low elongationreduce TWB formability; increase the risk of wrinkling, cracking andspringback; and increase the need for forming forces 22.The failure conditions of the material should be identified tominimise the faults during production 23. The fracture surfacesTable 1Chemical composition of the base material used in TWBs (wt%).CSiMnPSAlFeb0.15b0.6b2.5b0.04b0.015N0.01BalanceFig. 1. Microstructures of (a) cross-section of the welded joint, (b) base material, (c) HAZ, and (d) weld seam.Fig. 2. True stressstrain curves of the base materials and the TWBs.248H. Wang et al. / Materials and Design 102 (2016) 247254were characterised by TESCAN VEGA3 LMH scanning electron micros-copy (SEM) equipped with an energy dispersive X-ray spectrometerand magnification of up to 1,000,000.SEM micrographs of the fracture surfaces after the tensile test arepresented in Fig. 3. The images show numerous isometric dimples onthe fracture surface of the base material. This result indicates that thefracture type of the base materials is a typical ductile fracture (Fig. 3aand b). As illustrated in Fig. 3c, the fracture surfaces of the weld seamof TWBs show the river pattern and numerous dimples surroundingthe cleavage facets, evidently indicating the presence of ductile andcleavage fractures.3. MethodsThe forming process simulation and multiple-iteration springbackcompensation are illustrated as a flowchart in Fig. 4. Firstly, analysisand designoftheprocesswerecarried out on thebasisof the character-istics of the part. In the first stage, the forming processes of the partshould be planned. Once the process design is finalised, the addendumand binder designs, which are called die surface design, can be carriedout. The simulation of the entire forming processes, which includesdrawing, trimming, piercing and flanging, is performed to verify theformability of the designed processes and determine whether or notthe designed surface can be formed without any defects. Tooling geom-etry and process parameters may need to be modified or redesignedthrough several iterations based on feedback from the simulation. Simu-lation results are fed back to designers, and the designs of the processanddiesurfaceareadjustedandmodified inaccordancewiththesimula-tionfeedback.Meanwhile,processparameters,suchasblankholderforceanddrawingvelocity, areoptimisedinthefinite element(FE)simulation.Springbackbecomesamajorproblemwhenformingdefects,suchaswrinkles and cracks, were solved. Springback calculation was carriedout to predict the springback after the tools are removed from theformed part. The deviation distance between the springback part andthe design model can be obtained and used to estimate whether ornot the springback part can meet the allowable tolerance. If the tolerancerequirement cannot be satisfied, the die surface should be compensated.Fig. 5 shows the schematic of the displacement adjustment method inmultiple-iteration springback compensation.Displacement adjustment method is an effective approach towardspringback compensation 24. The principle of this method includescalculating the distance between the springback and reference shapes.The compensation shape is obtained with the same distance in theopposite direction of the springback. As shown in schematic of thedisplacement adjustment method, the geometry of the part isdefined as a FE mesh with n nodes in 3, the coefficient i is a node inFig. 3. SEM micrographs of the fracture surfaces after tensile test: (a) thin material, (b) thick material and (c) weld seam of TWBs.Fig. 4. Flowchart of forming process simulation and multiple-iteration springback compensation.249H. Wang et al. / Materials and Design 102 (2016) 247254theFEmesh. R!isthereferenceshape(ordesignedshape)ofthepart, S!isthespringbackshape,and C!isthecompensationshape.Thereferenceand springback shapes can be defined as follows:R!ri!r!?3;1 i nno1S!si!s!?3;1 i nno:2Theshapedeviation !oftheformedpartcanbecalculatedasfollows:! S!R!:3Ifthemaximumshapedeviation !exceedsthedesignrequirementsof the component tolerance , the tools should be compensated. Thecompensated shape C!is expressed asC! R! aS!R!?4ci! ri! a si!ri!?;1 i n:5The factor a is called the compensation factor, which is generallynegative and varies between 2.5 and 1. The first compensatedshape C!is defined as C!1. With this compensated geometry, a newforming process simulation is carried out. Ct!is the compensationshape in the t-th times. S!tis the springback shape in t-th times ofspringback analysis in the FE simulation.C!1 R! aS!1R!?6C!2 C!1 aS!2R!?7C!t C!t1 aS!tR!?;1bt btmax:8As shown in Fig. 4, the process needs to be repeated for two or moretimes to meet the design requirements of component tolerance.Multiple-iteration springback compensation is completed when themaximum shape deviation meets the design requirements of thecomponent tolerance . The discriminant function can be expressed asfollowsS!tR!?maxb:94. Simulation4.1. Finite element modelAsshowninFig.6a,theautomotivefloorreinforcementcomposedofHSSTWBswasstudiedintheexperiment.Theinitialblankoutlineofthepart can be obtained by unfolding the part, adding a certain trimmingallowanceand rounding the outline(Fig. 6b). The leftsideisa rectangu-lar blank with a thickness of 1.6 mm, and the right side is an isoscelestrapezoidal blank with a thickness of 1.2 mm.On the basis of the structural characteristics and forming processanalysis of the automotive floor reinforcement, the forming process ofthis part includes drawing, trimming and piercing. The addendum andbinder surfaces of the part were designed using a 3D software. Toanalyze and optimize the forming process, the FE model of the entireforming processes were built in Fig. 7 using the FE software Autoform.The initial blank was set as a dual-phase HSS (HC340/590DPD+Z)withthicknessesof1.2and1.6mm.TheMaterialpropertiesandprocessparameters used in the FE simulation are shown in Table 2 25. Theflow curve which describes the hardening behavior of the materialwas obtained in the tensile tests. Triangular element was used in theFE simulation, and the nodes and elements number of the initial blankwere 285 and 487, respectively. To achieve the accuracy and saveFig. 5. Schematic of the displacement adjustment method: (a) springback compensationin the first time, (b) springback compensation in the t-th time.Fig. 6. (a) Automotive floor reinforcement, and (b) blank outline.250H. Wang et al. / Materials and Design 102 (2016) 247254calculating time, the adaptive mesh was carried out to refine theelements of part during the forming process. The FE model of theTWBs is more complicated than the conventional sheet metal formingmodel because the mechanical properties of the TWBs differ fromthose of the base materials 26. The weld seam was considered asrigidlinks(spotwelds)intheFEmodelbecausethismodelisparticularlyefficient for dealing with laser-welded blanks, especially the highstrength of the weld metal in HSS TWBs 27.The simulation of the entire forming processes, which includesdrawing, trimming and piercing as well as springback analysis, wasused to explore stamping forming. The drawing process was dividedinto three stages: gravity loading stage, blank-holder closing stage anddrawing stage. The gravity loading stage was adopted to elastic shellelements. In addition, the elastic-plastic shell elements were appliedto the remaining two stages. The die, punch and binder were set asrigid bodies in the drawing process. A constant coefficient of friction, = 0.125, was used for the contact interface between blank and rigidstamping tools; moreover, the velocities of closing and drawing wereboth 1 mm/s during the simulation. Fig. 7b demonstrated the trim andpiercing lines in the trimming and piercing processes. As shown inFig. 7c, three points (F1, F2and F3) were used to constrain the formedpart by compulsory restraint condition during springback analysis.4.2. Multiple-iteration springback compensationSpringback is a common problem in the stamping forming process,and TWBs forming is no exception. To solve this problem, two kinds ofmethods are developed in practice. The first method is controlling andoptimizing the forming parameter (i.e., forming temperature, blankholder force, contact friction, tool radius) 28. The second method iscalled springback compensation, which is conducted by modifying theshape of stamping tools 29. Springback compensation is commonlyused to develop these tools. The springback may remain after forming,but the final shape of the part after springback would closely conformto the designed shape.Springback compensation can be usually divided into three types,namely, compensating all stamping tools, compensating a portion ofstamping tools or only compensating drawing tools. In general, thestrategy of compensating drawing tool is more economical than theother strategies. Hence, we select this approach in this study. Asshown in Fig. 8, the drawing tool surface is defined by three regions:the direct compensation region, the fixed compensation region andthe transition region. The final shape of the part is defined as the directcompensation region, and this region is compensated with the inversevector of the springback simulation result. The binder surface is definedasthefixedcompensationregion,whichisstationaryandunchangeableinspringbackcompensation.Theareabetweenthedirectcompensationregion and the fixed compensation region is defined as the transitionregion, which is used to guarantee the continuity of the compensationsurface without a negative draft angle.Fig. 7. FE model of the entire forming processes: (a) drawing, (b) trimming and piercingand (c) springback analysis.Table 2Material properties and process parameters used in the simulations.Material propertiesYielding stress (MPa)432Work-hardening exponent0.15Strength coefficient957Youngs modulus (MPa)2.1 105Poissons ratio0.33Density (N/mm2)7.9 105Process parametersElement typeTria.Initial element number of blank487Drawing velocity (mm/s)1Closing velocity (mm/s)1Friction coefficient0.125Fig. 8. Different regions of the part in springback compensation.Fig. 9. Forming limit diagram of the part after drawing.251H. Wang et al. / Materials and Design 102 (2016) 247254Multiple-iteration springback compensation was employed asdepicted in the flowchart shown in Fig. 4. When the first compensa-tion is finished, the compensated die surface should be validated inthe simulation of entire forming processes. Whether or not theshape deviation of the second time of the springback part satisfiedthe allowable tolerance should be checked. If it cannot satisfy theallowable tolerance = 0.7 mm (as the requirement of assemblingby the automaker for this part), the next compensation must be carriedout until the shape tolerance of the final part meets the allowabletolerance.5. Results and discussion5.1. Formability and thickness distributionForming parameters were optimised using the single-factor experi-ment in the FE software. For example, the blank holder force is simulatedevery5tons(T)from60Tto100Tinthedrawingprocess.OnthebasisoftheviewingresultsofFLDandthicknessdistributionofthesimulationsinthe postprocessor, the optimal range of the blank holder force wasdefined as 7585 T. The forming limit diagram at the end of the drawingprocess (the blank holder force equals to 80 T) is showninFig. 9. The FLDclearly shows that the drawing part does not possess any defects, such aswrinkles and cracks. Fig. 10 shows the thickness distribution diagramafter trimming and piercing. As the initial blank is a TWBs, the thicknesson the left side is larger than that on the right side.5.2. Springback predictionSpringbackbecomesamajorproblemwhenformingdefects,suchascracks and wrinkles, are solved. Therefore, springback analysis wascarriedouttopredictspringback.Resultsofspringbackpredictioncalcu-latedbyFEsimulationarepresentedinFig.11.Thefigureshowsthatthemaximum value of springback is 3.04 mm. In terms of the HSS TWBswith different thicknesses, blank thickness is an important influencingfactor. In general, the thinner the blank is, the larger the springbackvalue and shape error will be.5.3. Springback compensationFor automotive floor reinforcement, thespringback part satisfies theallowable tolerance when the iteration springback compensation andthe simulation of entire forming processes are implemented for thethird time. Fig. 12 shows the compensation results of the drawing toolby using multiple-iteration springback compensation. The originalshape and shapes after springback compensation were shown in thesections. From the compensation result, we can concluded that thedirectcompensation region and thetransition region exhibit an evidentchange and the side wall of the compensated part exhibit an inwardmovement.When multiple-iteration springback compensation is finished, thefinal compensation surface can be exported to a 3D software (e.g. UGand CATIA). In addition, the compensation surface is used to manufac-ture tools. As the upper layer of the part, the compensation surfacecan be used to directly manufacture the female die, and the materialthickness should be offset when manufacturing the male die. Fig. 13shows the tryout part in the experiments.Fig. 10. Thickness distribution diagram after trimming and piercing.Fig. 11. Results of springback prediction calculated by FE simulation.Fig. 12. Result of the drawing tool by using multiple-iteration springback compensation.252H. Wang et al. / Materials and Design 102 (2016) 247254To inspect shape deviation, the tryout part was placed on a specialfixture, and 32 measuring points of the part were inspected. The specialfixturewasdesignedandfabricatedtomeasuretheholes,shapedeviationand trim deviation of the formed part. The work surface of the specialfixture was offset surface from the reference surface of the design part,sothegapbetweenthespecialfixtureandformedpartcanbeeasilymea-sured using a gap gauge. The shape deviation can be calculated from themeasuring results. Fig. 14 shows the positions of the 32 measuring pointsand the inspection result of the tryout part on the special fixture. Asshown in Fig. 14, the area between the red lines indicates the allowabletolerance range. A total of 24 measuring points were found in the rangeof allowable tolerance, indicating that the qualified rate of this part is75% in the first die tryout.In practice, the die surface of unqualified points should be manuallymodified by the technician in accordance with the inspection result ofthe tryout part to improve the qualified rate of the parts. The remainingunqualifed measuing points can be subjected to a simple manual diesurface modification because the unqualified area and the maximumdeviationofthepartaresmall.Thedifferencebetweentheexperimentaland simulation results is attributed to numerous factors, such aselement type, material model, constitutive equation, friction modeland solution algorithm.Inconsiderationofthelowaccuracyofthespringbackcompensationfor a complicated automotive panel by the traditional method, three ormore repetitions are usually required to compensate for the die surface,remanufacture tooling and debug tooling until the formed part meetsthe allowable tolerance requirement. Although the proposed methodcannot supply a highly accurate compensation result, it can effectivelyreduce the number of die tryouts and tryout time in comparison withthe traditional methods.The proposed springback compensation fortheTWBswiththesamematerial anddifferent thickness was simulated in theFE software in thecurrent study.Inaddition, theTWBswithdifferent mechanicalpropertiesand thickness canalso be defined andimplemented inthis FEsoftware tocompensate springback 30,31. Dissimilar base materials in TWBs willbe assigned the corresponding material parameters, respectively, inthe FE software. In addition, the following springback compensationmethod can be performed the same as TWBs with same mechanicalproperties.6. Conclusions1. Tensile tests show that the mechanical properties of the thin andthick materials are highly similar. The weld seam decreases the maxi-mum elongation and yield strength of the TWBs. The tensile strengthof the TWBs is larger than those of base materials.2. The fracture of the base materials involves a typical ductile fracture,whereas that of the welded blank includes ductile and cleavagefractures.3. The simulation of the entire forming processes is used to analyze theforming process of the TWBs. Experimental results indicate that thesimulation results agree with the experimental results. Thus, thesimulation can provide guidance to actual production.4. The maximum value of springback is 3.04 mm without springbackcompensation. When the multiple-iteration springback compensationand the simulation of the entire forming processes are implementedfor the third time, the formed part after springback satisfies theallowable tolerance.5. Inspectionresultsshowthat24ofthe32measuringpointscansatisfygeometrical tolerances. Thus, the qualified rate of production canreach 75% by using multiple-iteration springback compensation atthe first time of die tryout. Therefore, the number of compensationand tryout time can be decreased by using the proposed method.AcknowledgmentsThe authors are pleased to acknowledge the support of this work bythe Fundamental Research Funds for the Central Universities of China(No. 2012ZX04010-081) and the Chongqing University PostgraduatesInnovation Project (No. CYB15012).References1 F.I. Saunders, R.H. Wagoner, Metall. Mater. Trans. A 27 (1996) 26052616, http:/dx./10.1007/BF02652354.2 M. Merklein, M. Johannes, M. Lechner, A. Kuppert, J. Mater. Process. Technol. 214(2014) 151164, /10.1016/j.jmatprotec.2013.08.015.3 N.B.K. Babu, M.J. Davidson, A.N. Rao, K. Balasubramanian, M. Govindaraju, Mater.Des. 55 (2014) 3542, /10.1016/j.matdes.2013.10.004.4 B.L. Kinsey, 7 - Tailor Welded Blanks for the Automotive Industry, in: B.L. Kinsey, X.Wu (Eds.), Tailor Welded Blanks for Advanced Manufacturing, Woodhead Publishing2011, pp. 164180.5 F. Xu, G. Sun, G. Li, Q. Li, J. Mater. Process. Technol. 214 (2014) 925935, http:/dx./10.1016/j.jmatprotec.2013.11.018.6 R. Padmanabhan,M.C. Oliveira, L.F. Menezes, Mater. Des. 29 (2008) 154160, http:/10.1016/j.matdes.2006.11.007.7 M. Abbasi, M. Ketabchi, T. Labudde, U. Prahl, W. Bleck, Mater. Des. 40 (2012)407414, /10.1016/j.matdes.2012.04.015.8 N.T. Nguyen, K. Hariharan, F. Barlat, M.G. Lee, Int. J. Precis. Eng. Manuf. 15 (2014)22732283, /10.1007/s12541-014-0591-7.9 K. Bandyopadhyay, S.K. Panda, P. Saha, G. Padmanabham, J. Mater. Process. Technol.217 (2015) 4864, /10.1016/j.jmatprotec.2014.10.022.
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