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REGULAR PAPER

DOI: 10.1007/s12541-017-0107-3 ISSN 2234-7593 (Print) / 2005-4602 (Online)

Study on Hydroforming Process and Springback Control of Large Sheet with Weak Rigidity Sun Zhiying1,#

and

Lang Lihui1

1 School of Mechanical Engineering and Automation, Beihang University, Beijing, 100191, China # Corresponding Author / E-mail: [email protected], TEL: +86-010-82317062, FAX: +86-010-82316821 KEYWORDS: Aluminum alloy, Engine hood outer panel, Hydroforming, Springback compensation, Weak rigidity

The control of instability and springback about large sheet with weak rigidity is difficult in the process. The hydroforming process of aluminum alloy engine cover, springback prediction and compensation were analyzed, and the manufacturing method of parts with high precision was obtained. In this paper, the effect of drawbead, pressure versus punch stroke, and blank holder force on the thinning rate of the plate was analyzed, and the phenomenon of wrinkling and fracture was judged according to the forming limit diagram. And springback is predicted through the method of the finite element and the surface of die was compensated by the iterative method. The results show that the quality of large aluminum alloy plate with weak rigidity is improved, the cost of the mould is reduced by the compensation, and the period of mold repair and commissioning is reduced by the sheet hydroforming, which is the flexible manufacturing process. Springback of large aluminum alloy sheet with weak stiffness is within 10mm through process optimization, and the surface of die was compensated in local region. The springback not only has been effectively controlled, but the accuracy of dimensional also is more than 90% of ± 0.5 mm. Manuscript received: January 9, 2017 / Revised: January 31, 2017 / Accepted: February 24, 2017

NOMENCLATURE S0 = the desired shape of the stamp Sfi+1 = the mold shape after the I time correction Sbi = the springback size after the I time forming Si = the shape after the I time springback

1. Introduction In the field of automobile manufacturing, the use of lightweight materials to achieve lightweight has become the trend of modern industrial development. However, the formability of aluminum alloy, magnesium alloy, titanium alloy and other lightweight materials is poor, which restricts its further application. The density of aluminum is smaller than steel, the weight is light. The aluminum plate part is usually used for the automobile engine cover and the trunk lid and so on, and the aluminum is advantageous to the energy absorption, enhances the collision safety, and has certain corrosion resistance.

© KSPE and Springer 2017

However, the low elongation and elastic modulus of aluminum alloy lead to the difference of material flow, stress and strain distribution and the forming quality problems such as fracture, wrinkling and springback. The formability of aluminum sheet is poor and the springback is difficult to control. The springback of similar parts is 3 times that of steel plate. Stamping is easy to produce debris, and the structure of die is more complicated than that of steel plate. At present, the rigidity of large aluminum alloy panel is poor, and the manufacturing technology is still not perfect. The standards are lack in the aspect of mold structure. The forming technology and the research of controlling quality are studied on aluminum engine cover plate by hydroforming. The sheet hydroforming is that the fluid medium filled into the mold cavity, the sheet metal is pulled into the die filled with high pressure liquid medium by punch, the mold movement and the fluid medium pressure are controlled in a certain range. It is a processing technology in the loading path. The schematic diagram hydroforming is shown in Fig. 1. The forming properties of materials can significantly be improved by hydroforming, and the overall shape of complex parts is achieved, ensure the quality of forming, precision, and satisfy the requirements of 1

2

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Fig. 2 Engine hood outer panel

Table1 Mechanical properties of 6A16 aluminum alloy

Fig. 1 Comparison of CDD and SHP

lightweight products. The side of sheet is contact with the rigid mold, the other side of sheet is contact with high-pressure flexible medium, that is rigid-flexible composite forming. Liquid are wrapped in sheet metal in the forming process, a friction-retaining effect is formed on the sheet, and ensure adequate fit between sheet and mold, and the liquid is overflowed between flange and die, which can reduce the harmful frictional resistance, it is the lubrication effect . Thereby the ultimate tensile value is greatly improved, and the springback of sheet metal is reduced. Since the 1970s, numerical simulation technology has been widely applied in sheet metal forming. After NUMISHEET’93, spring-back has been a top issue at the international conference of sheet metal forming that is held every 3 years. At present, some researches on the theory of springback have been done. The springback is analyzed from the influencing factors of the shape of the parts. The effects of fillet angle, fillet radius, fillet ratio and thickness ratio on springback were mainly studied in two-dimensional analysis of V and U shape parts, the springback is predicted from the impact of material, positive and negative springback mainly are analyzed for the tailor-welded plate between HC340 / 590DPD + Z, SPFC440 and other steel materials in the Ushaped free bending, thin plate is more prone to springback than the thin plate, and the larger gap is likely to have a negative springback. With the application of computer-aided design (CAD), the predicted springback module is developed, with the help of finite element analysis, sheet metal model and Hill'48, Yld89 and other different yield criteria, which constantly improve the springback compensation A. Ghaei et al. obtained a nonlinear unloaded and a technology. linear unloaded model to predict springback more accurately. But it cannot effectively solve the springback of large aluminum alloy cover based on the above studies. With the developing of lightweight market in automotive the application of new materials is promoted, the new manufacturing processes are putted forward. Automotive panels are important parts of the body sculpting, which are important parts of the body shape. Mainly including the top cover, hood outer panel, trunk lid outer panel, front and rear door panels, front and rear fender, side out board, which 3

4-7

8,9

10-12

13

Parameters Elastic modulus, E (MPa) Poisson's ratio, ν Yield strength, ss (MPa) Tensile strength, sb (MPa) Strain hardening, n Anisotropy coefficient, r0 Anisotropy coefficient, r45 Anisotropy coefficient, r90

Values 68.70 0.3 134 239 0.22 0.73 0.45 0.60

determine the quality of the body. Therefore, the surface quality of the outer panel is very high, but the plane dimension is large, the drawing depth is small, the surface curvature is small, the roughness is Ra0.61.3 μm, the most area of the sheet are undergone little plastic deformation in forming. However, there are few studies on the springback prediction and springback compensation of large aluminum alloy coverings. Due to its complex shape, large area and high experimental cost, there are many factors influencing the springback, which is difficult to control. In this paper, engine hood outer panel with aluminum alloy is became the research object, its shape is shown in Fig. 2, the thickness 1mm, the material is 6A16 aluminum alloy, the springback prediction and springback compensation method are studied by using hydroforming.

2. Materials Aluminum alloy is the most widely used nonferrous metal at present. It has a series of excellent characteristics, such as the low density, good mechanical properties, high resistance to corrosion, good resistance to strike and easy to recycle. It is widely used in the ships fields, the cars fields, the aerospace fields and so on. Tensile tests according to ASTM-E8 standard test were used to obtain the mechanical properties of the base material. The mechanical properties of 6016-T4 are shown in Table1. The high demand of aluminum alloy in the forming technology comes from different forming characteristics with ordinary steel. The difference mainly includes three aspects: hardening rule, failure criterion and material parameter. The hardening of the material is the main description of the relationship between the stress size in elastic plastic deformation distribution and strain distribution. The stress strain curve is used in the physical and simulation experiments. The

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Fig. 5 Flowchart of hydroforming process and springback compensation Fig. 3 Engineering stress-strain curves of the 6016-T4 program was determined, the design of punch surface was completed, and optimized the process parameters, including forming, cutting and flanging, which determine whether to meet the design requirements of qualified parts. If not qualified, according to the analysis of feedback iterations on the mold surface and process parameters were modified, while optimizing the pressure force and punch drawing speed.

4. Analysis of Hydroforming Process

Fig. 4 Numerical model for HDD process

engineering stress-strain curve of 6A16 can be got in different rolling direction from Fig. 4. Due to the characteristics of the anisotropy, the direction of 90°is used as the X direction of the parts in the view of forming difficulty.

3. Forming analysis and method of controlling springback In order to reduce the workload of commissioning, dynamic analysis of hydroforming can be observed stress distribution, wrinkling, The geometry of the sheet fracture and thickness distribution. hydroforming includes the punch, the die, the blank holder and the blank, which are shown in Fig. 4. The mold is a rigid body, and the sheet is a shell unit of Belyschko-Tsay model with the 1 mm thickness. In the experiment, the anisotropy is very important in the hydroforming process, therefore, the Baralat model with the three-parameter is used. The drawbead is integral and the position is shown in Fig. 4. Springback is a common problem in sheet forming. There are two methods to solve this problem. First, the springback are reduced by controlling and optimizing the forming process parameters (such as blank holder force, friction coefficient and fillet size). Second, springback is controlled by modifying the mold surface, which is usually springback compensation. springback still exists, but the final parts by modifying the shape forming may be consistent with the designed part. The flow chart is shown in Fig. 5. First of all, the forming process was designed according to the characteristics of parts. The first step, the hydroforming 14-16

4.1 Influence of drawbead on the thinning rate The certain size tension along the periphery is required in the sheet forming, which comes from the binder pressure, deformation resistance and force of stamping equipment, it is one of the most effective ways by setting drawbead and binder force is controlled. So the drawbead plays an important role in the forming process of automobile panel, and its roles are as follows: (1) Increase feeding resistance. (2) Adjust feed resistance distribution. (3) It can be adjusted in a large range of the feed resistance. (4) Reduce the binder requirements. (5) Improve the wrinkle of outside sheet. Because the shape of the engine hood outer panel is irregular, and the depth is different. The deformation of sheet is complex in the hydrodynamic deep drawing process, the overall stress is not uniform, and the phenomenon of fracture and wrinkle is easy occurred. By adjusting the parameters of drawbead, the feeding resistance is adjusted at the corner of die, it promote uniform inflow, and increase the rigidity of sheet, and reduce the occurrence of instability, and improve the quality of components, while main cylinder of hydraulic press is ensured. In the hydroforming process the drawbead not only plays the role of drawing sheet, but the sealing function also is obvious, so the full drawbeads is used. The feed speed is different because of the different shapes of sheet, and the different drawing resistance is needed. So the drawbead shape was adjusted different in different locations. The distribution of drawbead is shown in Fig. 6, because the parts is symmetric, the model of 1/2 sheet was studied with the number of drawbead. In this paper, the semicircular drawbead was adopted, as shown in Fig. 7. The drawbead parameters include rounded ribs, rib height and

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Fig. 6 The distribution of equivalent drawbead

Fig. 8 The six conditions about the profile of pressure versus punch stroke

Fig. 7 The parameters of drawbead

Table 2 The drawbead height H1 Number H1 (mm)

1 5

2 4.5

3 4.8

4 0.1

5 5

groove fillet. According to the characteristics of the part, the flow resistance of sheet is influenced by the height H1 of main rib fillet. The best feed resistance was got by the adjustment of the height of drawbead H1. The influence of drawbead height on the thinning rate was optimized by the orthogonal test, the height distribution of drawbead is shown in Table 2. The maximum thinning rate is about 4-13% by the setting of drawbead parameter. The plate fully was drawed to ensure the parts without rupture and wrinkling.

4.2 The influence of the profile of pressure versus punch stroke on the thinning rate In hydroforming deep (HDD) process, adjusting liquid chamber pressure is the decisive role on forming. In hydroforming the effect of “keep friction” and “soft drawing” is caused by liquid pressure. The distribution of stress and strain is improved during the forming process, and promote the sheet is affixed to the punch. The wrinkling and fracture at vacant area effectively are suppressed by it. In the forming process, the influence of the profile of pressure versus punch stroke on the thinning rate was analyzed. The drawbead parameters were set by using the optimization results, the constant BHF is 210 tons, and the shaping stress is 14 MPa. There are 6 conditions about the profile of pressure versus punch stroke, as showed in Fig. 8. According to the Fig. 9, the pressure is used when the punch was downward, and the thinning rate is small, the phenomenon is due to the characteristics of the parts. The deep part of parts is firstly affixed to punch. The shaping stress is early added, the most deep produced pull effect, the fracture occurred. The side wall of parts is vacant, the shaping stress promote the blank to affix to the punch.

Fig. 9 The influence of the punch positions on the maximum thinning rate

Fig. 10 Different loading curves

According to effect of shaping stress on the thinning rate, the drawbead parameters were set by using optimization results. The blank holding force is 210 tons, and the forming pressure is 3 MPa. The different loading path was shown in Fig. 10. The simulation results are shown in Fig. 11, with shaping pressure

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Fig. 13 Fracture caused by excessive pressure

Fig. 11 The influence of different shaping pressure on the maximum thinning rate

Fig. 14 The profile of pressure versus punch stroke

Fig. 12 The influence of different blank holder forces on the maximum thinning rate

increased, the thinning rate is greater. The shaping pressure is less than 12 MPa, and the wrinkling is very serious at the side wall of part, when the shaping pressure is 20 MPa, the fracture occurred in vacant area. So, the optimal shaping pressure is between 12-20 MPa.

4.3 The influence of blank holder force (BHF) on the thinning rate The controlling of BHF is mainly used to prevent fracture near the punch fillet and wrinkling of flange in the forming process. The BHF has direct effects on the formability quality of sheet from Fig. 12. The blank holding force is too large, the fracture occurred or the sheet thickness was severely thinned. The blank holding force is too small, and the wrinkle occurred or drawing was not sufficient. In this paper, according to the control requirements of the automobile factory, the constant blank holder force is adopted, and the stability is good, but for the forming of the complex parts, the constant blank holder force is not conducive to forming, and the controllable area of blank holder force is small. The BHF Control is the key in the process of forming, the blank holder force is too small, then the pressure is failure. With the increase of the BHF, the thinning rate gradually wad increased, and the deformation is sufficient. When the blank holder force is more than 210

Fig. 15 The technological process

tons, the blank holder force is too large, the sheet flow was hindered, the fracture caused, as shown in Fig. 13. Therefore, it is necessary to meet the needs of the sheet material, but also make the sheet material fully deformed, and meet the technical requirements of the parts. By adjusting the blank holder force and controlling the sheet material thinning rate, the quality of the forming was improved. The profile of pressure versus punch stroke was got by the

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Fig. 16 Forming limit of hydroforming on hood Fig. 18 The wrinkle at engine outer panel

Fig. 17 The rupture at engine outer panel

optimization method of parameter, which is shown in Fig. 14. The drawing mainly occurred during the punch was down. In order to ensure the film good and get high precision parts, the forming process was divided into two stages of forming, the shaping pressure is 14MPa. The shallow drawing occurred in the forming process, a side wall with characteristics can be seen from Fig. 15. In the forming process, firstly drawing process occurred, and then the blank with the small feature is stuck to punch by hydroforming, which is done by drawing. The impact line, surface scratches and other surface quality problems are very serious by stamping molding. The state of instability was judged by the forming limit diagram. The safe area can be seen from Fig. 16. The supplementary part of the part had the tendency of wrinkling, but it does not affect to get the qualified parts. Some instability was controlled in the finite element analysis (FEA), but some assumptions were used in the simulation analysis, which cannot fully predict the debugging process. The instability is mainly divided into two kinds in the process of debugging: one, when the blank holder force is too large, the fracture occurred at the side wall when the BHF 300 tons, which was shown in Fig. 17. The other one, when the chamber pressure is too small, the wrinkle occurred at the corner parts, which was shown in Fig. 18, the shaping pressure is less than 5 MPa.

Fig. 19 The springback distribution of engine hood outer panel

increased, the springback becomes more serious. When the state of stress and strain could be accurately simulated, the elastic recovery could be accurately calculated. In addition, when the residual stress was balanced in the springback process, plastic recovery may occur, so the plastic deformation of the parts will also have an impact on the The analysis of springback was carried out after springback. unloading by hydroforming. The amount of springback in the hood outer panel was -2 to 10 mm, as shown in Fig. 19. 13,17-19

4.5 Compensator For high precision parts, the requirements of controlling springback is still not satisfied through controlling the process, so that sheet metal pieces are forming by modifying the mold surface or die structure. Therefore, the springback compensation should be adopted when solving the springback problem of high-precision large-scale curved surface. At present, the methods of compensation are mainly numerical simulation and experimental iterative method. There is a complex process of large elastic-plastic deformation in sheet forming, involving geometry, materials and nonlinearity contact, the finite element analysis (FEA) is not enough to achieve the simulation accuracy, and the FEA is Therefore, the method of experimental cumbersome and difficulties. iterative is adopted in this paper, and the quick and effective correction of the mold is accomplished by a small number of experimental iterations. The springback compensation mechanism of surface is shown in Fig. 20, Suppose S is the part shape, S is the surface after springback, then Sb is the springback size. So that, Sb = S − S . Sb is added the S in reverse, and Sf is got, so Sf = S + Sb . If Sf is as the shape of the 20-22

4.4 Springback Both elastic and plastic deformation of sheet occur in the forming process, the elastic deformation is always accompanied by plastic deformation. When the material was released from the mold, elastic recovery would cause the springback deformation. When the yield strength of the material was increased, the elastic recovery of the material

0

1

1

1

2

2

0

1

0

1

2

0

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Fig. 20 The principle of springback compensation Fig. 21 The contrast between design surface and first compensation Table 3 The springback analysis of engine hood outer panel

5. Results and Discussion The form of constraint point

There are two types of springback compensation: one is global compensation, the springback after forming was analyzed through the FEA, the amount of springback was obtained at each location, and then springback compensation was used in each node based on the principle of springback compensation. And the surface was smoothed, and the mold surface was obtained. The second is the local compensation, according to the analysis of the springback, the center of the greater spring was done the compensation treatment, and the surface between compensation area and design surface area was done smooth processing.

L: 774 mm L: 549 mm L: 325 mm The springback value: The springback value: The springback value: 0-10 mm 0-8.5 mm 0-7 mm

formed part, the springback is Sb , and S is the surface after springback. If S is the desired shape, S − S should be added the surface Sf , so Sf = Sf + (S − Sb ) = S + Sb . And so on: Sb = Sf − S ; Sf = Sf + (S − S ) = S + Sb In summary: Sf = S , Sfi = S + Sbi, Sbi = Sfi − Si. When |Si − S | ≤ ε, |Sfi − Sfi | ≤ ε, the compensation calculation is end The repair principle is applied to simple curved surface. However, the method of 3D surface springback compensation is not perfect. In this paper, the trend of recovery was predicted, and the hydroforming mold is repaired. It is important to control the constrained points in the numerical simulation analysis. Three nodes are used to limit the translational freedom of the three axes. This constraint does not constrain the free deformation of the object. Based on the research result, the principle of selecting control points is summarized: 1. The distance between the nodes is small, and the stress between the nodes is neglected; 2. The control points are selected in uniform stress area; 3. The constraint point is not selected in the flange area; 4. the constraint points of the symmetry piece is in the middle position. In this paper, three schemes are simulated for the springback of hood outer shell. Because the piece is symmetrical, the area of the control point is 6 ‰ of the blank and the control point is in the center line.L (mm) is the distance between constraint point and the front edge, the analysis results was shown in Table 3. From the springback value, the maximum springback value is less than 10 mm in the three programs. When L is 549 mm, the resilience modulus is relatively small in the middle part, so the program was analyzed using the simulation analysis. 2

0

3

0

0

2

0

3

0

2

2

2

3

2

3

4

3

2

1

+1

0

2

2

0

0

5.1 Global compensation The punch surface after compensation was obtain on hydroforming through the software, as shown in Fig. 21, the red part is the compensation surface, and the yellow part is the design surface. But the punch surface was modified within the full range of part, which would take high costs of actual production, and lose the basis of design surface. There was a great risk in the next work.

0

5.2 Local compensation Combination with analysis of hydroforming and springback prediction based on design Surface, The stress-strain and springback distribution at the central position are shown in Fig. 22 in the longitudinal section. The springback process is that the stress was released. The variation of the main strain is small, but the strain of parts are within the range of 0.02 or more, which was reached the plastic deformation zone .In the hood edge, due to the more complex characteristics of the transition surface, there is a large residual stress for balancing the internal parts of the stress after its unloading, so negative springback is larger. The residual stress is very small in the middle area, the springback value is within 1 mm. The hood outer shell is shallow for forming, the middle of part is deep, and the curvature is small, so it is difficult to fully deform in the forming process. The middle position is collapse after forming, and negative springback was occurred. In the process of local compensation, firstly, the negative springback is processed. The top of the parts is lifted, which do not only solve the problem of negative springback, the springback of other locations have also been effectively controlled. The treatment of lifting surface is good at the material deformation in the forming process, and increases strain. In this paper, the surface at the largest negative rebound position was processed, and the lifted height was 1.5 mm in

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Fig. 24 The springback based on design surface and local compensation Fig. 22 The distribution of stress, strain and springback in longitudinal section

Fig. 25 The springback of design surface and compensation surface

Fig. 23 The distribution of stress, strain and springback in cross section

punch surface. The hydroforming process and springback prediction after lifting the top of punch surface were analyzed. The stress, strain and rebound distribution were shown in Fig. 23 at the cross-section of the part center. The parts are symmetrical, and the stress of central part was fully released without considering the heterosexuality, the change of main strain was small and it was more than 0.02, which was in yield zone into the plastic deformation. The residual stress is very small at the middle of the gentle region, and the scope of the springback value was within ± 1mm in the final part. The springback of hydroforming after the local compensation was shown in Fig. 24, the springback at the longitudinal section of the center part is small, and it was within the range of 2 mm or less, and the negative springback is also reduced. Through the hydroforming process, the springback were effectively controlled at longitudinal and lateral of parts. After the local compensation, the springback of the sheet metal has been effectively controlled. Through comparison with the design of the parts, the overall deviation of parts was shown in Fig. 25, and the range

Fig. 26 Test part

of the deviation is 94% of + 0.5 mm in engine hood outer panel. Aluminum alloy 6A16 automobile hood outer plate was verified by the experimental, and the experimental result was shown in Fig. 26, the deviation of springback between the actual parts and simulation analysis is less than ± 0.5 mm by reverse fitting analysis, which met more than 90% of the part.

6. Conclusions In order to get the mold surface with high precision, springback

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prediction is very important after forming. At the same time it provided an effective basis for springback compensation. Based on the analysis of sheet hydroforming, springback prediction and repair the local punch, the part was got combined with the finite element analysis and experiment, the results are shown that: 1. In the hydroforming of aluminum alloy automobile engine hood panel, the drawbead shape, the profile of pressure versus punch stroke and punch position are important parameters. The qualified specimens were obtained through the optimization of process parameters. The hydroforming is used to reduce the forming pass, reduce the mold cost, make the sheet fully attaching die, and improve the quality of the engine hood plate surface. 2. The analysis and the optimization of hydroforming process are studied by the FEA, and the forming process was predicted. The test results verify the rationality of the process parameters, which was shown that the numerical simulation technology of process parameters has an important guiding role in test. In the optimization of process parameters, the forming pressure is later, then the forming is better. The shaping pressure is best between 12-20 MPa, and the blank holding force is between 180 tons and 210 tons. 3. The surface quality of engine cover plate with aluminum alloy effectively wad controlled by hydroforming, while the springback was reduced after unloading. Considering springback compensation in the mold design process, the mold repair and manufacture cost were reduced, and the mold repair time was reduced, the cost of debugging was reduced. 4. Through the local compensation to reduce the risk of mold repair, and the springback was effectively controlled. The deviation range between the design parts and the part after springback is within the allowable range ± 0.5 mm. The springback was controlled through the process of numerical simulation experiments, prediction and springback compensation. The simulation analysis is consistent with the experiment. In short, the quality of the large aluminum alloy hood outer panel with weak rigidity was improved by hydroforming, which solved the major problems of lightweight materials, and reduced the production cost, save debugging time, improve the size precision of aluminum alloy engine hood plate.

ACKNOWLEDGEMENT The authors gratefully acknowledge the financial support from National Science and Technology Major Project with Grant No. 2014ZX04002041.

REFERENCES 1. Park, D.-H. and Kwon, H.-H., “Development of Warm Forming Parts for Automotive Body Dash Panel using AZ31B Magnesium Alloy Sheets,” Int. J. Precis. Eng. Manuf., Vol. 16, No. 10, pp. 21592165, 2015. 2. Lang, L.-H., Wang, Y.-M., Xie, Y.-S., Yang, X.-Y., and Xu, Y.-Q., “Pre-Bulging Effect during Sheet Hydroforming Process of Aluminum

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Alloy Box with Unequal Height and Flat Bottom,” Transactions of Nonferrous Metals Society of China, Vol. 22, Suppl. 2, pp. s302s308, 2012. 3. Xu, Y., Chen, Y., and Yuan, S., “Influences of Loading Paths on Thickness of Aluminium Alloy Cup with Hydro-Mechanical Deep Drawing,” International Journal of Materials and Product Technology, Vol. 38, No. 2-3, pp. 173-183, 2010. 4. Phanitwong, W. and Thipprakmas, S., “Development of Anew Spring-Back Factor for a Wiping Die Bending Process,” Materials & Design, Vol. 89, pp. 749-758, 2016. 5. Choi, S.-W. Lee, M.-S., and Kang, C.-G., “Effect of Process Parameters and Laminating Methods on Spring-Back in V-Bending of CFRP/CR340 Hybrid Composites,” Int. J. Precis. Eng. Manuf., Vol. 17, No. 3, pp. 395-400, 2016. 6. Thipprakmas, S., and Boochakul, U., “Comparison of Spring-Back Characteristics in Symmetrical and Asymmetrical U-Bending Processes,” Int. J. Precis. Eng. Manuf., Vol. 16, No. 7, pp. 14411446, 2015. 7. Zang, S.-L., Lee, M.-G., Sun, L., and Kim, J. H., “Measurement of the Bauschinger Behavior of Sheet Metals by Three-Point Bending Springback Test with Pre-Strained Strips,” International Journal of Plasticity, Vol. 59, pp. 84-107, 2014. 8. Wang, H., Zhou, J., Zhao, T. S., Liu, L. Z., and Liang, Q., “MultipleIteration Springback Compensation of Tailor Welded Blanks during Stamping Forming Process,” Materials & Design, Vol. 102, pp. 247254, 2016. 9. Leu, D.-K., “A Simplified Approach for Distinguishing between Spring-Back and Spring-Go in Free U-Die Bending Process of SPFC 440 Sheets,” Materials & Design, Vol. 94, pp. 314-321, 2016. 10. Peng, X., Shi, S., and Hu, K., “Comparison of Material Models for Spring Back Prediction in an Automotive Panel Using Finite Element Method,” Journal of Materials Engineering and Performance, Vol. 22, No. 10, pp. 2990-2996, 2013. 11. Eggertsen, P.-A. and Mattiasson, K., “On Constitutive Modeling for Springback Analysis,” International Journal of Mechanical Sciences, Vol. 52, No. 6, pp. 804-818, 2010. 12. Park, D.-H. and Kwon, H.-H., “Development of Warm Forming Parts for Automotive Body Dash Panel Using AZ31B Magnesium Alloy Sheets,” Int. J. Precis. Eng. Manuf., Vol. 16, No. 10, pp. 21592165, 2015. 13. Ghaei, A., Green, D. E., and Aryanpour, A., “Springback Simulation of Advanced High Strength Steels Considering Nonlinear Elastic Unloading-Reloading Behavior,” Materials & Design, Vol. 88, pp. 461-470, 2015. 14. Palumbo, G., “Hydroforming a Small Scale Aluminum Automotive Component Using a Layered Die,” Materials & Design, Vol. 44, pp. 365-373, 2013. 15. Meng, B., Wan, M., Wu, X., Yuan, S., Xu, X., and Liu, J., “Inner

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Wrinkling Control in Hydrodynamic Deep Drawing of an Irregular Surface Part Using Drawbeads,” Chinese Journal of Aeronautics, Vol. 27, No. 3, pp. 697-707, 2014. 16. Wang, J., Fu, M., Yu, J., Wang, X., and Yang, W., “Study of the Flow Behavior and Defect Formation in Forming of Axisymmetrically Flanged and Multi-Scaled Parts,” Int. J. Precis. Eng. Manuf., Vol. 17, No. 10, pp. 1341-1349, 2016. 17. Luo, L. and Ghosh, A. K., “Elastic and Inelastic Recovery After Plastic Deformation of DQSK Steel Sheet,” Transactions-American Society of Mechanical Engineers Journal of Engineering Materials and Technology, Vol. 125, No. 3, pp. 237-246, 2003. 18. Han, K., Van Tyne, C., and Levy, B., “Effect of Strain and Strain Rate on the Bauschinger Effect Response of Three Different Steels,” Metallurgical and Materials Transactions A, Vol. 36, No. 9, pp. 23792384, 2005. 19. Groche, P. and Bäcker, F., “Springback in Stringer Sheet Stretch Forming,” CIRP Annals-Manufacturing Technology, Vol. 62, No. 1, pp. 275-278, 2013. 20. Zhang, Z., Wu, J., Zhang, S., Wang, M., Guo, R., and Guo, S., “A New Iterative Method for Springback Control Based on Theory Analysis and Displacement Adjustment,” International Journal of Mechanical Sciences, Vol. 105, pp. 330-339, 2016. 21. Bałon, P. and Świątoniowski, A., “Springback Compensation in Cold Forming Process for High Strength Steel / Kompensacja Sprężynowania W Procesie Formowania Stali Na Zimno,” Archives of Metallurgy and Materials, Vol. 60, No. 4, pp. 2471-2478, 2015. 22. Mole, N., Cafuta, G., and Štok, B., “A 3D Forming Tool Optimisation Method Considering Springback and Thinning Compensation,” Journal of Materials Processing Technology, Vol. 214, No. 8, pp. 1673-1685, 2014.