- Email: [email protected]
Prototype tryout and die design for automotive parts using welded blank hydroforming
Journal of Materials Processing Technology 130±131 (2002) 121±127
Prototype tryout and die design for automotive parts using welded blank hydroforming Y.S. Shina,*, H.Y. Kima, B.H. Jeonb, S.I. Ohc a
Division of Mechanical Engineering and Mechatronics, Kangwon National University, Hyoja-dong 192-1, Chunchon, Kangwon-do 200-701, South Korea b Department of Mechanical Engineering, Induk Institute of Technology, San 76-1 Weolgae-dong, Nowon-gu, Seoul 139-749, South Korea c Division of Mechanical and Aerospace Engineering, Seoul National University, San 56-1, Shilim-dong, Kwanak-ku, Seoul 151-742, South Korea
Abstract Hydroforming is the technology that utilizes hydraulic pressure to form tube and sheet materials into desired shapes inside die cavities. It can be characterized as tube hydroforming and sheet hydroforming depending on the shape of used blank. Hydroforming offers several advantages as compared with conventional manufacturing via stamping and welding, such as part consolidation, weight reduction, improved structural strength and stiffness, lower tooling costs, fewer secondary operations, tight dimensional tolerances, low springback, and reduced scrap. In this paper, the welded blank hydroforming (WBH) technology is discussed in the aspects of formability, and the engine mount bracket and the subframe are analyzed and manufactured for the application of the technology, which have been manufactured by stamping and welding processes. Die geometry and process parameters are suggested by the computer simulation using the explicit ®nite element code, PAM-STAMPTM. Various defects, especially wrinkling in the area of big difference of die depth and large height deviation of the die ¯ange face, are investigated and analyzed. The numerical results are compared with prototype tryout results, and the wrinkling and the local thinning under different forming conditions are predicted. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Welded blank hydroforming; Wrinkling; Engine mount bracket; Subframe
1. Introduction The needs for reducing fuel consumption and emission, as well as more stringent crash requirement, lead to a stiff and lightweight design of cars. On the material side, high strength steels, aluminum alloys and tailored blanks are increasingly used. On the process side, improved deep drawing processes and new innovative processes like hydroforming have been developed, and have successfully improved in mass production. With the advent of hydroforming technology, new designs are possible where the strength to weight ratio of certain components can be greatly increased besides reducing the number of subcomponents. Hydroforming is the technology that utilizes hydraulic pressure to form tube and sheet materials into desired shapes inside die cavities. It can be classi®ed as tube and sheet hydroforming according to the used blanks [1]. Table 1 shows the classi®cations of hydroforming. Hydroforming offers several advantages as compared with conventional manufacturing via stamping and welding, such as part * Corresponding author. Tel.: 82-33-252-6317; fax: 82-33-251-6317. E-mail address: [email protected] (Y.S. Shin).
consolidation, weight reduction, improved structural strength and stiffness, lower tooling costs, fewer secondary operations, tight dimensional tolerances, low springback, and reduced scrap. Hydroforming also has some disadvantages, however, including slow cycle time, expensive equipment and lack of extensive knowledge base for process and tool design. Hydroformed parts also require new welding techniques for part assembly. Therefore, the feasibility of hydroforming versus conventional stamping and welding has to be investigated economically and mechanically for each part. The hydroforming technology was reported for the ®rst time in the 1920s, and the main technological breakthroughs were made in series production in the last 15 years [2] and recently attracted more attention of the car makers prompted mainly by the automotive industry's strong interests. Recently, a lot of studies on the hydroforming technology and the effect of process parameters have been done by many researchers analytically and/or experimentally. But most of studies on these hydroforming technologies have been mainly focused on about tube forming area rather than the sheet forming ones [3±9]. As one of few cases of sheet forming applied hydraulic pressure forming, Wang [10] hydroformed the spherical vessels by using the integrated
0924-0136/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 7 4 1 - 0
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Y.S. Shin et al. / Journal of Materials Processing Technology 130±131 (2002) 121±127
Table 1 Classification of hydroforming Tube hydroforming
Low pressure hydroforming High pressure hydroforming Sequenced pressure hydroforming
Sheet hydroforming
Hydraulic stretch forming of single blank Hydraulic stretch forming of double blanks Hydromechanical deep drawing
hydro-bulge forming technology, Zang et al. [11] formed the tapered rectangular boxes using hydromechanical deep drawing process and studied the main failure modes such as the body wrinkling, ¯ange wrinkling and corner rupture. Hein and Geiger [12] investigated the non-welded blank hydroforming (WBH) by controlling the blank holding force. Bobbert [13] dealt with the possible defects of hydroforming sheet metal pairs and focused on the failure mode of wrinkling. This paper presents an investigation of the WBH process applied to the automotive components, engine mount bracket and subframe, numerically and experimentally. An explicit FE code, PAM-STAMPTM, is used to analyze the forming process. The dies for each component in the prototype tryout are fabricated to perform the test tryout. And then, the numerical results are compared with prototype tryout ones, and the deformation of the blanks is discussed. Body wrinkling and local thinning of the blanks under different forming condition are predicted. As a result of these comparative studies, agreement between numerical and the tryout results is in good level. Finally, throughout the modi®cation of die geometry and process parameters, the sound products of two automotive components are obtained. 2. WBH Sheet hydroforming derived from tube hydroforming is an essential part of this research. In contrast to the tube hydroforming, larger variations of part intersections are permitted in the sheet hydroforming. It is particularly, applicable to any products industry-wide where the complex shapes have to be formed with a high degree of precision. It is also possible to apply to space frame, instrument panel
beam, frame and roof rails, engine cradle, and fuel tanks as some automotive parts. Sheet hydroforming can be categorized into three kinds of types according to the blank sheets, such as hydraulic stretch forming of one blank, hydraulic stretch forming of two blank, and hydromechanical deep drawing (Fig. 1). In case of the hydraulic stretch forming with a blank, a deformed shape of sheet only depend on the shaped lower die. The operation process is as follows. A blank at ®rst is placed on the lower die and, upper blank-holder clamps the blank with downward action, and then the working ¯uid in¯ow is proceeded into the die cavity as a forming process. However, in case of the second type, two blank sheets are formed into the both side (upper and lower dies) by the hydraulic pressure built up between these two blank sheets from an external pressure intensi®er. And the hydraulic stretch forming of double blank is also subdivided into non-welded blank type and welded blank one. Hydromechanical deep drawing is turn upside down the position of die con®guration against hydraulic stretch forming of one blank. Thus, the blank sheet is pressed against the punch. It does effectively cost down by the lack of female die comparing with the conventional type. On the other hand, it must be considered that a hydraulic press with high ram forces as well as counter pressure posts adapted to control of the counter pressure is required [14]. In addition to the results of the previous researchers, WBH yields more uniform strain distribution and greater draw depth than the mechanical stretch forming and/or deep drawing. 2.1. Process of WBH WBH contains the following main steps which are given in Fig. 2 that shows the WBH process sequence. The initial material for WBH is made from two ¯at or preformed sheet blanks of the same or different material and thickness with the same outer dimensions, and weld together at the edges. Preformed blank makes it possible to vary the component cross section beyond the limits of tube hydroforming [15]. 2.1.1. Pressure and force in WBH The internal pressure to hydroform a part can be calculated as follows. Assuming the plane stress condition about
Fig. 1. Hydroforming types by used blanks.
Y.S. Shin et al. / Journal of Materials Processing Technology 130±131 (2002) 121±127
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Fig. 2. Process of WBH.
the blanks for WBH, the following stress±strain relationship can be derived form the deformation theory of plasticity [16]: e sy el sl (1) 2 s e sl sy (2) ey s 2 where the equivalent stress s and the equivalent strain e are given by Eqs. (3) and (4) under plane stress condition: q s2l sl sy s2y (3) s
q 2 e p e2l el ey e2y (4) 3 where sl, el, sy and ey are the true stress and true strain components in the longitudinal and hoop direction of the welded blanks, respectively. For a thin wall tube, the hoop stress can be calculated by Eq. (5) PR (5) t where P is the internal pressure, t the tube wall thickness and R the tube corner radius. Substituting Eq. (5) into Eqs. (1) and (2), the following relationship between the internal pressure and the strain components is derived: 4ts el P ey (6) 3 R e 2 sy
Fig. 3. Schematic of closing die and welded blanks.
carried out to save the material and to improve the formability of sheet metal. Fig. 3 shows a schematic of die and welded blanks with section view. L1 is the length of blank between left and right weld line, and L2 the length between the weld line and the edge of welded blanks. The relationship between L1 and L which is the length of the blank after being formed can be expressed by the following equation: section expansion ratio
L
L1 100 L1
when being formed, if L2 is smaller than the die radius or the section expansion ratio is too small, it is impossible to place the position of weld line onto the die ¯ange face, as shown in Fig. 4. 2.2. Process limits and defects 2.2.1. Bursting and thinning The bursting and thinning is correlated. Small die radius and/or complexity of die geometry can cause a thinning of blank during process. Fig. 5 illustrates a bursting, which can occur when the formability of material exceeded or the excessive thinning raised so that it cannot be stopped due to the small die radius and even high holding force. These defects would be prevented or improved by replacing the
In WBH, the blank-holder force does not only serve for the purpose of preventing wrinkling. The blank-holder force also controls metal ¯ow from the ¯ange into the die cavity. The possibility of metal ¯ow is a key advantage in WBH [15]. The blank-holder force necessitated by the internal pressure has to be applied according to the following equation. F PA
(7)
where F is the blank-holder force, P the internal pressure, and A the projected area of the die surface. 2.1.2. Blank design A shaped blank including a pre-formed blank is used in WBH. So the optimization of welded blanks has to be
(8)
Fig. 4. Deformed shapes when L2 is smaller than the die radius.
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Y.S. Shin et al. / Journal of Materials Processing Technology 130±131 (2002) 121±127 Table 2 Mechanical properties of SAPH38P Density (kg/mm3) Young's modulus (MPa) Poisson's ratio Yield strength (MPa) K (MPa) n e0
Fig. 5. Bursting failure.
sheet material and by modifying the die geometry like as an enlargement of die radius and so on. 2.2.2. Wrinkling The major reasons for wrinkling in WBH are the different die depths between upper and lower die cavity, and the large deviation of 3D die ¯ange face (Fig. 6). The two sheets are ®xed along the weld line and undergoes a force to move simultaneously when forming is started. A wrinkling phenomenon would be caused when the curvilinear lengths for the given die section are different between upper and lower die cavity. Non-welded blanks, modifying the die parting line and equalizing the section length of die cavity can prevent wrinkling. It is possible to prevent a wrinkling due to the large deviation of die ¯ange face by means of controlling the blank-holder force and of minimizing the deviation height of die ¯ange face [13].
7.8E 06 206E03 0.3 276.91 629.6 0.184 0.002
geometry. Pressure is usually described by a pressure±time function but this may cause stability problems in some cases. However, using a ¯owing volume±time function instead of a pressure±time function eliminates the stability problem. The volumetric option is available in a few ®nite element codes such as INDEED, LS-DYNA3D, and PAM-STAMPTM (3). In this study, an engine mount bracket and subframe of automobile were taken as examples for WBH. The prototype die for each component was fabricated to perform the tryout. A used material is SAPH38P with 2.3 mm of sheet thickness and the material properties for the welded blanks are listed in Table 2. In simulation, PAM-STAMPTM is used and any effects of the weld line are neglected. Also elastic±plastic work hardening equation is used as follows: s K e e0 n
(9)
3.1. Engine mount bracket
Finite element analysis of sheet metal forming process is widely used in the automotive industry. A few software packages available in the world market are capable of simulating the hydroforming process. There are two main types of program codes, explicit and implicit. It should be carefully considered to hire a code, which can represent follower forces for numerical hydroforming analysis. A simple de®nition of a follower force is a load that must be able to follow the deformed geometry, which means that the pressure must always be perpendicular to the actual
Engine mount bracket is a part of automobile to connect engine with the subframe located at the bottom of engine. Fig. 7 shows a real product of engine mount bracket produced through WBH, while the FE model in simulation presented in Fig. 8. Through 2D section analysis of WBH process, the appropriate internal pressure, die closing velocity and the defects of die design are pre-examined, and the deformed shapes between prototype and simulation are compared in Fig. 9. Fig. 10 presents the thickness distribution of the ®nal product. The thickness variations after forming are between 1.6 and 3.23 mm. At two zones near by weld line of the part, A and B, are expected the bursting and excessive thinning, because the weld line is more stiff and strength, and less elongation than base metal. Prototype
Fig. 6. Wrinkling type for WBH.
Fig. 7. Engine mount bracket produced by WBH.
3. Simulation and prototype tryout of WBH
Y.S. Shin et al. / Journal of Materials Processing Technology 130±131 (2002) 121±127
125
Fig. 11. Section views of failure and defect.
Fig. 8. FE model for WBH.
tryout has formed under the 80 t press. Actually, to perform the prototype tryout for the engine mount, not only over 800 t press but also over 1000 bar intensi®er is required. Therefore, in order to solve this irrecoverable problem, after the laser welded blanks of the whole around edge put onto the upper and lower die, both of die were fasten with a bolt. This way was more structurally stable and economical than press holding. Fig. 11 shows the section view of failure zone and weld defects after WBH. The weld defects on the welded blanks give a bad effect to the formability of WBH. Fig. 12 illustrates the pro®les of thickness distribution along the section A±A0 , B±B0 , and C±C0 . From Figs. 11 and 12, the simulation results have a good agreement with tryout results.
Fig. 9. Comparison of deformed shapes between tryout and simulation.
Fig. 12. Comparison of thickness distribution between prototype tryout and FE simulation.
Fig. 10. Thickness distribution after FE simulation for WBH.
Fig. 13. Configuration of subframe.
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Y.S. Shin et al. / Journal of Materials Processing Technology 130±131 (2002) 121±127
Fig. 16. Deformed shapes after binder wrap and hydroforming.
Fig. 14. Wrinkling defects of WBH on subframe.
3.2. Subframe
of simulation and prototype tryout. The wrinkling by means of different die depth almost disappeared, but it remained by the deviation of die ¯ange face as before. The excessive folding pattern in the early phase during binder wrap affects to the ®nal irrecoverable wrinkling (Fig. 16). In order to eliminate the wrinkling, a simpli®ed U-channel model was adapted to modify and to determine the radius of die curvature (Fig. 17). From the results of the FE analysis using simpli®ed model, the radius, over 400 mm, of die curvature is acceptable to remove the wrinkling. Fig. 18
Fig. 13 presents con®guration of the rectangular type subframe manufactured by conventional stamping and welding process. The part, which is inside of the dash line, is the ®nal component to be produced by WBH, and the rest outside the dash line is for conventional stamping (part inside dash line in Fig. 13). Finally, two components are assembled as the bolt connection. Fig. 14 shows the results of 2D section analysis. Wrinkling occurred at section A±A0 , B±B0 , C±C0 , D±D0 , and E±E0 . The main reason of wrinkling is mentioned in Section 2.2.2. In this work, the parting line was moved to the center of die so as to equalize the section length between upper die and lower die. Fig. 15 is the results
Fig. 17. Results of FE analysis to determine die curvature.
Fig. 15. Comparison of the results between simulation and prototype tryout.
Fig. 18. Final sound product produced by WBH.
Y.S. Shin et al. / Journal of Materials Processing Technology 130±131 (2002) 121±127
presents the ®nal sound product after modi®cation of die face and process control. For tryout, the 2500 t hydraulic press was used, and 260 and 1200 bar were applied to initial holding pressure and maximum internal pressure of each. The holding pressure actually increases during hydroforming, because the operation of upper die was ®xed after clamping two blanks with downward action on binder wrap process. 4. Discussion and conclusion Sheet hydroforming compared with conventional stamping and tube hydroforming, is still relatively a new technology. Therefore, there is not much extensive knowledge base for tool design and process control on this technology. So the statement of WBH such as process sequence, pressure and force, friction, process limits and defects, etc. described to understand the basic knowledge for tool and process design of them. Hydroforming tryout, however, is costly very dif®cult caused by expensive equipment, even if that has large advantage and big degree of freedom in part design. FE simulation, therefore, is essential as to develop part, tool design, and process control and to reduce trial and error, part design period as well. In this work, an engine mount bracket and a subframe were taken as examples. The die design including wrinkling defects for WBH was accomplished ef®ciently by using 2D FE analysis. The wrinkling in WBH can be expected when having big difference in die depth or when there are large height deviations within the die ¯ange face. The former one can prevent the wrinkle defect by means of equalizing the curvilinear length of both side die cavity by moving the parting line. The latter case, wrinkles can be reduced by enlarging the radius of die curvature. And also the initial excessive folding in the WBH process ®nally brings an irrecoverable wrinkle. The results from simulations, especially deformed shapes, thickness distribution, wrinkling and tearing, had a good agreement with the prototype tryout results. And the WBH practically is not proper to the parts with large height deviations of die ¯ange face. Acknowledgements This research was supported by HWASHIN Co. Ltd. under project named as ``Process development for hydroforming
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technology using sheet metal'' ®nanced by Ministry of Commerce, Industry and Energy. Specially, the author is greatly indebted to B.C. Park, W.S. Lee and Y.M. Kim who helped to accomplish the prototype tryout. References [1] R. Brun, M. Lai, X. Li, A. Messina, Cutting lead-time and development costs of hydroformed parts by FEM simulation, IBEC'97 Body Assembly & Manufacturing, 1997, pp. 97±102. [2] M. Mason, Hydroform tubes for automotive body structure applications, SAE Technical Paper No. 930575, 1993. [3] M. Ahmetoglu, K. Sutter, X.J. Lee, T. Atlan, Tube HydroformingÐ Current Research, Applications and Need for Training, Engineering Research Center for Net Shape Manufacturing, The Ohio State University, Columbus, OH, 1999. [4] T.M. Srinivassan, J.R. Shaw, K. Thompson, Tubular hydroforming: correlation of experimental and simulation results, SAE Technical Paper Series No. 980448, 1998, pp. 131±137. [5] L. Wu, Y. Yu, Computer simulation of forming automotive structural parts by hydroforming process, Numisheet'96, 1996, pp. 324±329. [6] S.D. Liu, K.T. Meuleman, Analytical and experimental examination on tubular hydroforming limits, SAE Technical Paper Series No. 980445, 1998, pp. 139±150. [7] F. Dohmann, C. Hartl, Tube hydroformingÐresearch and practical application, J. Mater. Process. Technol. 71 (1997) 174±186. [8] K. Manabe, S. Nakamura, Finite element simulation of hydroforming process of pre-bent circular tubes, Numisheet'99, 1999, pp. 503± 508. [9] K. Manabe, M. Amino, S. Nakamura, FE analysis on local thinning and fracture phenomenon in hydroforming of tubular component, Advanced Technology of Plasticity, vol. II, Proceedings of the 6th ICTP, 1999, pp. 1229±1234. [10] Z.R. Wang, Numerical simulation of some new integrated hydroforming process, Invited Paper, Advanced Technology of Plasticity, vol. II, Proceedings of the 6th ICTP, 1999, pp. 1253±1260. [11] S.H. Zhang, L.H. Lang, D.C. Kang, J. Danckert, K.B. Nielsen, Z.R. Wang, Numerical simulation of the hydro-mechanical deep drawing process, Advanced Technology of Plasticity, vol. II, Proceedings of the 6th ICTP, 1999, pp. 1273±1278. [12] P. Hein, M. Geiger, Advanced process control strategies for the hydroforming of sheet metal pairs, Advanced Technology of Plasticity, vol. II, Proceedings of the 6th ICTP, 1999, pp. 1267±1272. [13] S. Bobbert, Process limits for the hydroforming of sheet metal pairs, Advanced Technology of Plasticity, vol. II, Proceedings of the 6th ICTP 1999, pp. 1261±1266. [14] S. Klaus, A. Matthias, State-of-the-art of hydroforming tubes, extrusions and sheet metals in Europe, Institute for Sheet Metal Forming Technology of the University of Stuttgrat, 2000. [15] F.J. Lenze, T. Gruszka, Application of hydroforming for Body-InWhite, Advanced Technologies & Processes IBEC'97, 1997, pp. 11±20. [16] S.D. Liu, D. Meuleman, K. Thompson, Analytical and experimental examination on tubular hydroforming limits, SAE Technical Paper No. 98044, 1998.
Prototype tryout and die design for automotive parts using welded blank hydroforming Y.S. Shina,*, H.Y. Kima, B.H. Jeonb, S.I. Ohc a
Division of Mechanical Engineering and Mechatronics, Kangwon National University, Hyoja-dong 192-1, Chunchon, Kangwon-do 200-701, South Korea b Department of Mechanical Engineering, Induk Institute of Technology, San 76-1 Weolgae-dong, Nowon-gu, Seoul 139-749, South Korea c Division of Mechanical and Aerospace Engineering, Seoul National University, San 56-1, Shilim-dong, Kwanak-ku, Seoul 151-742, South Korea
Abstract Hydroforming is the technology that utilizes hydraulic pressure to form tube and sheet materials into desired shapes inside die cavities. It can be characterized as tube hydroforming and sheet hydroforming depending on the shape of used blank. Hydroforming offers several advantages as compared with conventional manufacturing via stamping and welding, such as part consolidation, weight reduction, improved structural strength and stiffness, lower tooling costs, fewer secondary operations, tight dimensional tolerances, low springback, and reduced scrap. In this paper, the welded blank hydroforming (WBH) technology is discussed in the aspects of formability, and the engine mount bracket and the subframe are analyzed and manufactured for the application of the technology, which have been manufactured by stamping and welding processes. Die geometry and process parameters are suggested by the computer simulation using the explicit ®nite element code, PAM-STAMPTM. Various defects, especially wrinkling in the area of big difference of die depth and large height deviation of the die ¯ange face, are investigated and analyzed. The numerical results are compared with prototype tryout results, and the wrinkling and the local thinning under different forming conditions are predicted. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Welded blank hydroforming; Wrinkling; Engine mount bracket; Subframe
1. Introduction The needs for reducing fuel consumption and emission, as well as more stringent crash requirement, lead to a stiff and lightweight design of cars. On the material side, high strength steels, aluminum alloys and tailored blanks are increasingly used. On the process side, improved deep drawing processes and new innovative processes like hydroforming have been developed, and have successfully improved in mass production. With the advent of hydroforming technology, new designs are possible where the strength to weight ratio of certain components can be greatly increased besides reducing the number of subcomponents. Hydroforming is the technology that utilizes hydraulic pressure to form tube and sheet materials into desired shapes inside die cavities. It can be classi®ed as tube and sheet hydroforming according to the used blanks [1]. Table 1 shows the classi®cations of hydroforming. Hydroforming offers several advantages as compared with conventional manufacturing via stamping and welding, such as part * Corresponding author. Tel.: 82-33-252-6317; fax: 82-33-251-6317. E-mail address: [email protected] (Y.S. Shin).
consolidation, weight reduction, improved structural strength and stiffness, lower tooling costs, fewer secondary operations, tight dimensional tolerances, low springback, and reduced scrap. Hydroforming also has some disadvantages, however, including slow cycle time, expensive equipment and lack of extensive knowledge base for process and tool design. Hydroformed parts also require new welding techniques for part assembly. Therefore, the feasibility of hydroforming versus conventional stamping and welding has to be investigated economically and mechanically for each part. The hydroforming technology was reported for the ®rst time in the 1920s, and the main technological breakthroughs were made in series production in the last 15 years [2] and recently attracted more attention of the car makers prompted mainly by the automotive industry's strong interests. Recently, a lot of studies on the hydroforming technology and the effect of process parameters have been done by many researchers analytically and/or experimentally. But most of studies on these hydroforming technologies have been mainly focused on about tube forming area rather than the sheet forming ones [3±9]. As one of few cases of sheet forming applied hydraulic pressure forming, Wang [10] hydroformed the spherical vessels by using the integrated
0924-0136/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 7 4 1 - 0
122
Y.S. Shin et al. / Journal of Materials Processing Technology 130±131 (2002) 121±127
Table 1 Classification of hydroforming Tube hydroforming
Low pressure hydroforming High pressure hydroforming Sequenced pressure hydroforming
Sheet hydroforming
Hydraulic stretch forming of single blank Hydraulic stretch forming of double blanks Hydromechanical deep drawing
hydro-bulge forming technology, Zang et al. [11] formed the tapered rectangular boxes using hydromechanical deep drawing process and studied the main failure modes such as the body wrinkling, ¯ange wrinkling and corner rupture. Hein and Geiger [12] investigated the non-welded blank hydroforming (WBH) by controlling the blank holding force. Bobbert [13] dealt with the possible defects of hydroforming sheet metal pairs and focused on the failure mode of wrinkling. This paper presents an investigation of the WBH process applied to the automotive components, engine mount bracket and subframe, numerically and experimentally. An explicit FE code, PAM-STAMPTM, is used to analyze the forming process. The dies for each component in the prototype tryout are fabricated to perform the test tryout. And then, the numerical results are compared with prototype tryout ones, and the deformation of the blanks is discussed. Body wrinkling and local thinning of the blanks under different forming condition are predicted. As a result of these comparative studies, agreement between numerical and the tryout results is in good level. Finally, throughout the modi®cation of die geometry and process parameters, the sound products of two automotive components are obtained. 2. WBH Sheet hydroforming derived from tube hydroforming is an essential part of this research. In contrast to the tube hydroforming, larger variations of part intersections are permitted in the sheet hydroforming. It is particularly, applicable to any products industry-wide where the complex shapes have to be formed with a high degree of precision. It is also possible to apply to space frame, instrument panel
beam, frame and roof rails, engine cradle, and fuel tanks as some automotive parts. Sheet hydroforming can be categorized into three kinds of types according to the blank sheets, such as hydraulic stretch forming of one blank, hydraulic stretch forming of two blank, and hydromechanical deep drawing (Fig. 1). In case of the hydraulic stretch forming with a blank, a deformed shape of sheet only depend on the shaped lower die. The operation process is as follows. A blank at ®rst is placed on the lower die and, upper blank-holder clamps the blank with downward action, and then the working ¯uid in¯ow is proceeded into the die cavity as a forming process. However, in case of the second type, two blank sheets are formed into the both side (upper and lower dies) by the hydraulic pressure built up between these two blank sheets from an external pressure intensi®er. And the hydraulic stretch forming of double blank is also subdivided into non-welded blank type and welded blank one. Hydromechanical deep drawing is turn upside down the position of die con®guration against hydraulic stretch forming of one blank. Thus, the blank sheet is pressed against the punch. It does effectively cost down by the lack of female die comparing with the conventional type. On the other hand, it must be considered that a hydraulic press with high ram forces as well as counter pressure posts adapted to control of the counter pressure is required [14]. In addition to the results of the previous researchers, WBH yields more uniform strain distribution and greater draw depth than the mechanical stretch forming and/or deep drawing. 2.1. Process of WBH WBH contains the following main steps which are given in Fig. 2 that shows the WBH process sequence. The initial material for WBH is made from two ¯at or preformed sheet blanks of the same or different material and thickness with the same outer dimensions, and weld together at the edges. Preformed blank makes it possible to vary the component cross section beyond the limits of tube hydroforming [15]. 2.1.1. Pressure and force in WBH The internal pressure to hydroform a part can be calculated as follows. Assuming the plane stress condition about
Fig. 1. Hydroforming types by used blanks.
Y.S. Shin et al. / Journal of Materials Processing Technology 130±131 (2002) 121±127
123
Fig. 2. Process of WBH.
the blanks for WBH, the following stress±strain relationship can be derived form the deformation theory of plasticity [16]: e sy el sl (1) 2 s e sl sy (2) ey s 2 where the equivalent stress s and the equivalent strain e are given by Eqs. (3) and (4) under plane stress condition: q s2l sl sy s2y (3) s
q 2 e p e2l el ey e2y (4) 3 where sl, el, sy and ey are the true stress and true strain components in the longitudinal and hoop direction of the welded blanks, respectively. For a thin wall tube, the hoop stress can be calculated by Eq. (5) PR (5) t where P is the internal pressure, t the tube wall thickness and R the tube corner radius. Substituting Eq. (5) into Eqs. (1) and (2), the following relationship between the internal pressure and the strain components is derived: 4ts el P ey (6) 3 R e 2 sy
Fig. 3. Schematic of closing die and welded blanks.
carried out to save the material and to improve the formability of sheet metal. Fig. 3 shows a schematic of die and welded blanks with section view. L1 is the length of blank between left and right weld line, and L2 the length between the weld line and the edge of welded blanks. The relationship between L1 and L which is the length of the blank after being formed can be expressed by the following equation: section expansion ratio
L
L1 100 L1
when being formed, if L2 is smaller than the die radius or the section expansion ratio is too small, it is impossible to place the position of weld line onto the die ¯ange face, as shown in Fig. 4. 2.2. Process limits and defects 2.2.1. Bursting and thinning The bursting and thinning is correlated. Small die radius and/or complexity of die geometry can cause a thinning of blank during process. Fig. 5 illustrates a bursting, which can occur when the formability of material exceeded or the excessive thinning raised so that it cannot be stopped due to the small die radius and even high holding force. These defects would be prevented or improved by replacing the
In WBH, the blank-holder force does not only serve for the purpose of preventing wrinkling. The blank-holder force also controls metal ¯ow from the ¯ange into the die cavity. The possibility of metal ¯ow is a key advantage in WBH [15]. The blank-holder force necessitated by the internal pressure has to be applied according to the following equation. F PA
(7)
where F is the blank-holder force, P the internal pressure, and A the projected area of the die surface. 2.1.2. Blank design A shaped blank including a pre-formed blank is used in WBH. So the optimization of welded blanks has to be
(8)
Fig. 4. Deformed shapes when L2 is smaller than the die radius.
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Y.S. Shin et al. / Journal of Materials Processing Technology 130±131 (2002) 121±127 Table 2 Mechanical properties of SAPH38P Density (kg/mm3) Young's modulus (MPa) Poisson's ratio Yield strength (MPa) K (MPa) n e0
Fig. 5. Bursting failure.
sheet material and by modifying the die geometry like as an enlargement of die radius and so on. 2.2.2. Wrinkling The major reasons for wrinkling in WBH are the different die depths between upper and lower die cavity, and the large deviation of 3D die ¯ange face (Fig. 6). The two sheets are ®xed along the weld line and undergoes a force to move simultaneously when forming is started. A wrinkling phenomenon would be caused when the curvilinear lengths for the given die section are different between upper and lower die cavity. Non-welded blanks, modifying the die parting line and equalizing the section length of die cavity can prevent wrinkling. It is possible to prevent a wrinkling due to the large deviation of die ¯ange face by means of controlling the blank-holder force and of minimizing the deviation height of die ¯ange face [13].
7.8E 06 206E03 0.3 276.91 629.6 0.184 0.002
geometry. Pressure is usually described by a pressure±time function but this may cause stability problems in some cases. However, using a ¯owing volume±time function instead of a pressure±time function eliminates the stability problem. The volumetric option is available in a few ®nite element codes such as INDEED, LS-DYNA3D, and PAM-STAMPTM (3). In this study, an engine mount bracket and subframe of automobile were taken as examples for WBH. The prototype die for each component was fabricated to perform the tryout. A used material is SAPH38P with 2.3 mm of sheet thickness and the material properties for the welded blanks are listed in Table 2. In simulation, PAM-STAMPTM is used and any effects of the weld line are neglected. Also elastic±plastic work hardening equation is used as follows: s K e e0 n
(9)
3.1. Engine mount bracket
Finite element analysis of sheet metal forming process is widely used in the automotive industry. A few software packages available in the world market are capable of simulating the hydroforming process. There are two main types of program codes, explicit and implicit. It should be carefully considered to hire a code, which can represent follower forces for numerical hydroforming analysis. A simple de®nition of a follower force is a load that must be able to follow the deformed geometry, which means that the pressure must always be perpendicular to the actual
Engine mount bracket is a part of automobile to connect engine with the subframe located at the bottom of engine. Fig. 7 shows a real product of engine mount bracket produced through WBH, while the FE model in simulation presented in Fig. 8. Through 2D section analysis of WBH process, the appropriate internal pressure, die closing velocity and the defects of die design are pre-examined, and the deformed shapes between prototype and simulation are compared in Fig. 9. Fig. 10 presents the thickness distribution of the ®nal product. The thickness variations after forming are between 1.6 and 3.23 mm. At two zones near by weld line of the part, A and B, are expected the bursting and excessive thinning, because the weld line is more stiff and strength, and less elongation than base metal. Prototype
Fig. 6. Wrinkling type for WBH.
Fig. 7. Engine mount bracket produced by WBH.
3. Simulation and prototype tryout of WBH
Y.S. Shin et al. / Journal of Materials Processing Technology 130±131 (2002) 121±127
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Fig. 11. Section views of failure and defect.
Fig. 8. FE model for WBH.
tryout has formed under the 80 t press. Actually, to perform the prototype tryout for the engine mount, not only over 800 t press but also over 1000 bar intensi®er is required. Therefore, in order to solve this irrecoverable problem, after the laser welded blanks of the whole around edge put onto the upper and lower die, both of die were fasten with a bolt. This way was more structurally stable and economical than press holding. Fig. 11 shows the section view of failure zone and weld defects after WBH. The weld defects on the welded blanks give a bad effect to the formability of WBH. Fig. 12 illustrates the pro®les of thickness distribution along the section A±A0 , B±B0 , and C±C0 . From Figs. 11 and 12, the simulation results have a good agreement with tryout results.
Fig. 9. Comparison of deformed shapes between tryout and simulation.
Fig. 12. Comparison of thickness distribution between prototype tryout and FE simulation.
Fig. 10. Thickness distribution after FE simulation for WBH.
Fig. 13. Configuration of subframe.
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Y.S. Shin et al. / Journal of Materials Processing Technology 130±131 (2002) 121±127
Fig. 16. Deformed shapes after binder wrap and hydroforming.
Fig. 14. Wrinkling defects of WBH on subframe.
3.2. Subframe
of simulation and prototype tryout. The wrinkling by means of different die depth almost disappeared, but it remained by the deviation of die ¯ange face as before. The excessive folding pattern in the early phase during binder wrap affects to the ®nal irrecoverable wrinkling (Fig. 16). In order to eliminate the wrinkling, a simpli®ed U-channel model was adapted to modify and to determine the radius of die curvature (Fig. 17). From the results of the FE analysis using simpli®ed model, the radius, over 400 mm, of die curvature is acceptable to remove the wrinkling. Fig. 18
Fig. 13 presents con®guration of the rectangular type subframe manufactured by conventional stamping and welding process. The part, which is inside of the dash line, is the ®nal component to be produced by WBH, and the rest outside the dash line is for conventional stamping (part inside dash line in Fig. 13). Finally, two components are assembled as the bolt connection. Fig. 14 shows the results of 2D section analysis. Wrinkling occurred at section A±A0 , B±B0 , C±C0 , D±D0 , and E±E0 . The main reason of wrinkling is mentioned in Section 2.2.2. In this work, the parting line was moved to the center of die so as to equalize the section length between upper die and lower die. Fig. 15 is the results
Fig. 17. Results of FE analysis to determine die curvature.
Fig. 15. Comparison of the results between simulation and prototype tryout.
Fig. 18. Final sound product produced by WBH.
Y.S. Shin et al. / Journal of Materials Processing Technology 130±131 (2002) 121±127
presents the ®nal sound product after modi®cation of die face and process control. For tryout, the 2500 t hydraulic press was used, and 260 and 1200 bar were applied to initial holding pressure and maximum internal pressure of each. The holding pressure actually increases during hydroforming, because the operation of upper die was ®xed after clamping two blanks with downward action on binder wrap process. 4. Discussion and conclusion Sheet hydroforming compared with conventional stamping and tube hydroforming, is still relatively a new technology. Therefore, there is not much extensive knowledge base for tool design and process control on this technology. So the statement of WBH such as process sequence, pressure and force, friction, process limits and defects, etc. described to understand the basic knowledge for tool and process design of them. Hydroforming tryout, however, is costly very dif®cult caused by expensive equipment, even if that has large advantage and big degree of freedom in part design. FE simulation, therefore, is essential as to develop part, tool design, and process control and to reduce trial and error, part design period as well. In this work, an engine mount bracket and a subframe were taken as examples. The die design including wrinkling defects for WBH was accomplished ef®ciently by using 2D FE analysis. The wrinkling in WBH can be expected when having big difference in die depth or when there are large height deviations within the die ¯ange face. The former one can prevent the wrinkle defect by means of equalizing the curvilinear length of both side die cavity by moving the parting line. The latter case, wrinkles can be reduced by enlarging the radius of die curvature. And also the initial excessive folding in the WBH process ®nally brings an irrecoverable wrinkle. The results from simulations, especially deformed shapes, thickness distribution, wrinkling and tearing, had a good agreement with the prototype tryout results. And the WBH practically is not proper to the parts with large height deviations of die ¯ange face. Acknowledgements This research was supported by HWASHIN Co. Ltd. under project named as ``Process development for hydroforming
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technology using sheet metal'' ®nanced by Ministry of Commerce, Industry and Energy. Specially, the author is greatly indebted to B.C. Park, W.S. Lee and Y.M. Kim who helped to accomplish the prototype tryout. References [1] R. Brun, M. Lai, X. Li, A. Messina, Cutting lead-time and development costs of hydroformed parts by FEM simulation, IBEC'97 Body Assembly & Manufacturing, 1997, pp. 97±102. [2] M. Mason, Hydroform tubes for automotive body structure applications, SAE Technical Paper No. 930575, 1993. [3] M. Ahmetoglu, K. Sutter, X.J. Lee, T. Atlan, Tube HydroformingÐ Current Research, Applications and Need for Training, Engineering Research Center for Net Shape Manufacturing, The Ohio State University, Columbus, OH, 1999. [4] T.M. Srinivassan, J.R. Shaw, K. Thompson, Tubular hydroforming: correlation of experimental and simulation results, SAE Technical Paper Series No. 980448, 1998, pp. 131±137. [5] L. Wu, Y. Yu, Computer simulation of forming automotive structural parts by hydroforming process, Numisheet'96, 1996, pp. 324±329. [6] S.D. Liu, K.T. Meuleman, Analytical and experimental examination on tubular hydroforming limits, SAE Technical Paper Series No. 980445, 1998, pp. 139±150. [7] F. Dohmann, C. Hartl, Tube hydroformingÐresearch and practical application, J. Mater. Process. Technol. 71 (1997) 174±186. [8] K. Manabe, S. Nakamura, Finite element simulation of hydroforming process of pre-bent circular tubes, Numisheet'99, 1999, pp. 503± 508. [9] K. Manabe, M. Amino, S. Nakamura, FE analysis on local thinning and fracture phenomenon in hydroforming of tubular component, Advanced Technology of Plasticity, vol. II, Proceedings of the 6th ICTP, 1999, pp. 1229±1234. [10] Z.R. Wang, Numerical simulation of some new integrated hydroforming process, Invited Paper, Advanced Technology of Plasticity, vol. II, Proceedings of the 6th ICTP, 1999, pp. 1253±1260. [11] S.H. Zhang, L.H. Lang, D.C. Kang, J. Danckert, K.B. Nielsen, Z.R. Wang, Numerical simulation of the hydro-mechanical deep drawing process, Advanced Technology of Plasticity, vol. II, Proceedings of the 6th ICTP, 1999, pp. 1273±1278. [12] P. Hein, M. Geiger, Advanced process control strategies for the hydroforming of sheet metal pairs, Advanced Technology of Plasticity, vol. II, Proceedings of the 6th ICTP, 1999, pp. 1267±1272. [13] S. Bobbert, Process limits for the hydroforming of sheet metal pairs, Advanced Technology of Plasticity, vol. II, Proceedings of the 6th ICTP 1999, pp. 1261±1266. [14] S. Klaus, A. Matthias, State-of-the-art of hydroforming tubes, extrusions and sheet metals in Europe, Institute for Sheet Metal Forming Technology of the University of Stuttgrat, 2000. [15] F.J. Lenze, T. Gruszka, Application of hydroforming for Body-InWhite, Advanced Technologies & Processes IBEC'97, 1997, pp. 11±20. [16] S.D. Liu, D. Meuleman, K. Thompson, Analytical and experimental examination on tubular hydroforming limits, SAE Technical Paper No. 98044, 1998.