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Transcript  ManufactureEngineers, Part B: Journal of Engineering Proceedings of the Institution of Mechanical online version of this article can be found at: DOI: 10.1177/0954405414527954April 2014 published online 24 Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture  Sérgio F Lajarin and Paulo VP Marcondes Influence of process and tool parameters on springback of high-strength steels  Published by: On behalf of:  Institution of Mechanical Engineers  can be found at: Manufacture Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering  Additional services and information for Email Alerts: Subscriptions: Reprints: Permissions: Citations:  What is This? - Apr 24, 2014OnlineFirst Version of Record >>  by guest on April 25, 2014pib.sagepub.comDownloaded from by guest on April 25, 2014pib.sagepub.comDownloaded from   Original Article Proc IMechE Part B: J Engineering Manufacture 1–11   IMechE 2014Reprints and 10.1177/ Influence of process and toolparameters on springback of high-strength steels Se´rgio F Lajarin and Paulo VP Marcondes Abstract The advanced high-strength steels have become an interesting alternative in automotive industry to reduce vehicleweight and therefore reduce fuel consumption. However, its wide application in the automotive industry is still limiteddue to challenges in formability, tool life and springback. The springback is pointed out in literature as the main problemthat involves the mass production of structural components, and the aspects that show influence are still not fully under-stood. This work aims to statistically analyze the influence of process and tool parameters on the magnitude of thespringback on five high-strength steels. In order to do so, the U-bending test was used and two process parameters andtwo tool parameters that are mentioned in the literature as the most influential were chosen. The results of the analysisof variance pointed out the influence of the blank holder force as the parameter of greatest influence on springback, fol-lowed by the tool radius and friction condition. Keywords Springback, advanced high-strength steel, sheet forming, process parameters, tool parameters Date received: 5 April 2013; accepted: 24 February 2014 Introduction The automotive industry undergoes a constant pressurerelated to environmental requirements, which mainlyaim at reduction of emission of greenhouse gases intothe atmosphere. Thus, car manufacturers came with analternative technique to manufacture components withthinner steel sheets for mass reduction of the vehicleand consequently reduce fuel consumption and green-house gas emissions. However, to manufacture compo-nents with thinner sheets without compromising thesafety aspects, manufacturers began to replace the con-ventional steels by high-strength steels (HSSs) andadvanced high-strength steels (AHSSs).With the use of conventional steels, the main con-cern is the elimination of necking and fracture duringthe process. Nevertheless, with the use of HSS, empha-sis has been transferred to the dimensional accuracyand consistency of the products, and one of the prob-lems encountered is the springback. This phenomenoncauses a geometric distortion after forming which canbe detrimental esthetically as well as damaging theassembly of components. The AHSSs are known fortheir multi-phase microstructure which provides manyadvantages during mechanical forming, but the formingbehavior is unpredictable and is still not fully under-stood which creates challenges for efficient tool design.In recent years, various experimental techniqueshave been developed to study and characterize the phe-nomenon of springback. Ling et al. 1 carried out L-bending tests in order to study by finite element analy-sis (FEA) how die parameters like gap—between thedie and punch—die radius, step height and step dis-tance affect springback. Zhang et al. 2 proposed a V-bending test that provides a high level of springback.The advantage of this test is that it provides easy mea-surements and, also, allows the study of the sensitivityof springback relative to the tool radius and sheetthickness (r/t), mechanical properties of the sheet andcontact conditions. However, the drawback of thesetwo tests is that they cannot propitiate the realisticforming conditions held in the industry. Kuwabara DEMEC, Universidade Federal do Parana´, Curitiba, Brazil Corresponding author: Se´rgio Fernando Lajarin, DEMEC, Universidade Federal do Parana´, Av.Cel. Francisco H. dos Santos, 210, Caixa Postal 19011, CEP 81531-990,Curitiba, Parana´, Brazil.Email:  by guest on April 25, 2014pib.sagepub.comDownloaded from   et al. 3 studied the springback using the stretch-bendingtest, but Raabe et al. 4 mention that this test does notallow a careful control of the stresses on the sheet dur-ing bending, and cannot exhibit bending followed byreverse bending, neither the big slide on the tool radiuscommonly observed in operations using dies. The testwhich is most widely used in the study of springback isthe U-bend test—benchmark problem from theNUMISHEET’93 conference proposed by Makinouchiet al. 5 Furthermore, it allows very realistic representa-tion of conditions of forming and the profile of thecomponent. Chen et al. 6 used the U-bending test toanalyze the influence of the tool radius in the sidewallcurling region with different AHSS. The authors con-cluded that the angular change and the sidewall curlingare more pronounced for smaller tool radii and thick-nesses. Moreover, they observed that radius larger than3 mm shows less influence on springback. Liu et al. 7 studied the blank holder force (BHF) variation in anattempt to reduce the springback phenomenon, becauseaccording to the authors, the correct BHF applicationis one of the most effective methods to solve the prob-lem. Many researchers are continuously trying to usethe Makinouchi test by means of computer simulationfor the springback evaluation. Bekar et al. 8 investigatedthe reduction of the magnitude and the variation of springback of DP600 steels in U-channel formingwithin a robust optimization framework. They per-formed a simple sensitivity analysis, and the limit of elasticity was found to be the most important randomvariable. Kang and Cheon 9 conducted an analysis of residual stresses in the coupled processes of stampingand welding by finite element methods. Xu et al. 10 stud-ied the influence of the integration points, the size of the mesh of the sheet and the punch velocity on theaccuracy and efficiency of springback simulation.Chen and Koc x 11 reported a parametric computeranalysis using FEA and design of experiments (DOE)in order to study the effects of BHF and friction onspringback variation for an open-channel-shaped partmade of dual-phase (DP) steel. The main conclusionwas that the sidewall curl is very sensitive to the contactcondition in the simulation, and in order to reducespringback variation, the standard deviations used forvariable randomization have to be decreased virtually.Lajarin and Marcondes 12 evaluated by numericalsimulation the degradation of the elastic modulus dur-ing nonlinear unloading and how the choice of numerical parameters can affect the computational pre-diction of springback for AHSSs. However, the under-standing of the influence of process and toolparameters on the occurrence of springback is stillunclear and less investigated in literature.In order to advance the subject a little further and asa contribution to the research gap still present in thestate-of-art, this experimental study aimed to statisti-cally analyze the influence of process and tool para-meters on the magnitude of the springback. In additionto BHF and friction studied by Chen and Koc x , 11 in thiswork, two additional tool parameters (tool radius andthe gap between the die and punch) that are mentionedin the literature as the most influential on the spring-back were chosen. In the present study, five HSSs— HSLA490, DP600-A and DP600-B from two differentsuppliers, DP780 and DP980—were evaluated. In orderto do so, the U-bending test was used combined withan analysis of variance (ANOVA) in order to identifywhich mean values were statistically different. All thedata are discussed separately for each one of the fivehigh-strength materials. Materials and methods Materials The materials selected for this study were steel sheetsused in the automotive industry (Table 1). The sheet of high-strength low alloy (HSLA) has been used for manyyears in the production of automotive body structures.It is a typical conventional HSS obtained primarily bymicro additions of micro-alloying elements primarily tocontrol grain size. The selected AHSSs were four DPsteels with a range of ultimate tensile strengths (UTSs)of 600, 780 and 980 MPa from two different suppliers(A and B) and thicknesses of 1.5 and 2 mm.DP600 and DP780 steels showed uniform elongationsimilar to HSLA490 steel, showing excellent combina-tion of high-strength and good elongation—interestingproperties for the automotive industry. The high-strength DP980 steel (UTS = 934 MPa) still showed agood elongation of 10.4%. All the steels analyzedshowed some anisotropy.Figure 1 shows the engineering stress–strain curves.DP steel exhibits higher initial work hardening rate,higher UTS and lower yield strength/tensile strength(YS/TS) ratio than HSLA490. DP600—from suppliersA and B—showed different behavior between them. Table 1.  Mechanical properties and thicknesses of the materials.Material Supplier Thickness(mm)0.2%YS(MPa)UTS(MPa)UE (%) TE (%) E(GPa)n-value  D r r-valueHSLA490 A 1.50 415 542 14.1 20.2 208 0.116  2 0.237 0.897DP600-A A 1.57 395 620 14.9 20.0 206 0.149 0.285 0.819DP600-B B 2.06 387 605 15.8 23.0 207 0.188  2 0.377 0.909DP780 B 1.96 488 741 12.7 17.0 205 0.164  2 0.365 0.931DP980 A 1.52 828 934 7.0 10.4 208 0.078 a 2 0.134 a 0.971 a UTS: ultimate tensile strength. a Obtained for  e  = 0.06. 2  Proc IMechE Part B: J Engineering Manufacture  by guest on April 25, 2014pib.sagepub.comDownloaded from   DP600—from supplier B—showed a small yield pointelongation, while DP600—from supplier A—showed alarge initial hardening. U-bending test The simple U-bend test—benchmark problem from theNUMISHEET’93 conference—was employed to inves-tigate the influence of process and tool parameters onspringback (Figure 2(a)). The tool forms the sheet in achannel profile that is quite common in vehicle struc-tural components and is very suitable to springback.In order to study the influence of the process andtool parameters on the springback, a factorial design of 2 4 for each material was proposed. Table 2 shows thefour parameters selected with two levels each.The tool radius and gap—between the die andpunch—are found in the literature as the two tool para-meters that show influence on springback, as well asBHF and the lubrication are found as the process para-meters that also show influence. 11,13–15 For level 1, a BHF of 2.5 kN is the default value pro-posed by Makinouchi et al. 5 After preliminary tests, theBHF value of 12.5 kN was set for level 2—maximumBHF before the rupture of the material. The frictioncondition was tested with and without lubrication, andthe used lubricant was hydraulic automotive oil. Theused gap was based from the work of Sadagopan andUrban, 14 and the radius of the tool (die and punch) of 5 mm is the default value of the U-bend benchmarktool. However, the sheet thickness used for the testswith this tool is typically 0.78 mm which results in arelative radius/thickness ratio of 6.5 mm (r/t). In orderto maintain close relationship with the sheets of 1.5 and2 mm used in this work, a radius of 10 mm was alsodefined (r/t of 6.6 mm for sheets of 1.5 mm). Table 3shows the 16 arrangements of the parameters that wereused—as proposed by the 2 4 factorial design.Other parameters and test conditions were kept con-stant, such as speed of the punch (10 mm/s), displace-ment of the punch (70 mm) and test specimendimensions (35 mm  3  300 mm). The samples weretaken in the rolling direction. Measurement procedure and techniques of analysis The profile of the formed specimens was scanned with aresolution of 600 dpi (dots per inch) after the U-bendingoperation. The scanned images were analyzed in acomputer-aided design (CAD) software, and threespringback measurements were carried out following theprocedure reported in Makinouchi et al. 5 : (a) angular Figure 1.  Engineering stress–strain curves of the five testedmaterials. Table 2.  Levels for the process and tool parameters.Factor Parameter Level 1 Level 2A Blank holder force (kN) 2.5 12.5B Friction condition Lubrication WithoutC Gap 1.2t 1.5tD Die and punch radius (mm) 5 10 t: thickness. Figure 2.  Benchmark NUMISHEET’93 (Makinouchi et al., 1993): (a) U-bend tooling (mm) and (b) springback measurements. Lajarin and Marcondes  3  by guest on April 25, 2014pib.sagepub.comDownloaded from 
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