Based on the principle of hot stamping, a simulation model of hot stamping was established. The blank contour line of U part was optimized by using a multiple iterative algorithm. By comparison with the simulation and experimental data, the multiple iterative optimization algorithm was verified. Moreover, the blank contour lines of a bumper and the B-pillar of Numisheet 2008 were optimized. The result showed that the multiple iterative method for optimizing initial blank contour line had high calculating speed and precision. The hot stamping parts’ contour lines match the ideal part contour lines quite well

Gao, P.

Jia, X.F.

Wang, Y.J.

Zhang, Z.Q.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

UDC 539.4Optimization Analysis of Initial Sheet Metal Contour Line for High-StrengthBoron Steel in Hot StampingZ. Q. Zhang,1X. F. Jia, Y. J. Wang, and P. GaoCollege of Materials Science and Engineering, Jilin University, Changchun, China1 Zhangzq@jlu.edu.cnBased on the principle of hot stamping, a simulation model of hot stamping was established. Theblank contour line of U part was optimized by using a multiple iterative algorithm. By comparisonwith the simulation and experimental data, the multiple iterative optimization algorithm was verified.Moreover, the blank contour lines of a bumper and the B-pillar of Numisheet 2008 were optimized.The result showed that the multiple iterative method for optimizing initial blank contour line had highcalculating speed and precision. The hot stamping parts’ contour lines match the ideal part contourlines quite well.Keywords: high strength boron steel, hot stamping, optimization analysis, contour line,finite element method.Introduction. Hot stamping technology is the key to the lightweight of automobileand has been increasingly used in manufacturing inside door beam, roof rail, bumper,A/B/C-pillar reinforcements, etc. [1, 2]. Accurate design of initial blank contour line canreduce the possibility of forming defects and improve the part quality [3, 4]. Scientists havegot many computing methods under different assumptions such as velocity field method[5], slip line field method [6, 7], geometric mapping [8], potential field simulation [9, 10]for cold stamping process. However, hot stamping process is a thermal-mechanicalcoupling process. One step finite element inverse algorithm is mainly used in hot stampingto get the initial blank contour line and redundant materials need to be removed by lasercutting after hot stamping. It not only wastes raw materials but also results in lowproduction efficiency. Based on multiple iterative inverse algorithms, the process of hotstamping was simulated and the optimized blank initial contour line was obtained byestablishing an optimized model, which required no further application of a cutting processafter hot stamping.1. Process of Blank Initial Contour Optimization. In the process of optimization,the border line of the part is the target object, where the border line of simulated part isregarded as comparative object, while the blank initial contour line is the optimizationobject. An optimal contour line is chosen from many calculating contour lines throughsimulation. In simulation, one-step finite element inverse algorithm is employed to getinitial blank contour line and then parameters’ editor is initialized to mesh the whole modeland set parameters, input cycle definition and related control parameters, such as number ofcycle, object choice, coefficient of thermal expansion, etc. All those procedures can help toestablish an optimized model and offer the reference for the following iterative computation.If an optimization meets the demands of presupposed forming precision, iteration will stop.If not, further optimization will go on after establishing the new mesh within the maximumiterations. The control process of the optimization model is shown in Fig. 1.2. Contour Optimization and Forming Experiment of U-Shaped Part.2.1. Contour Optimization of U-Shaped Part. The ideal U-shaped part has a length of54 mm, width of 10 mm and height of 15 mm. One-step inverse algorithm is used to get theblank initial contour line and then generate the blank with the thickness of 1 mm. Theposition of the blank and die before forming is shown in Fig. 2. The initial blank mesh was© Z. Q. ZHANG, X. F. JIA, Y. J. WANG, P. GAO, 2016ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1 91generated with 3131 nodes and 2592 elements by finite element meshing. The materialmodel should be a temperature and strain-rate dependent elastic-plastic model consideringphase transformation and thermal expand. Stress–strain curves of the material at varioustemperatures are from Numisheet 2008 [11]. Figure 3 shows the contour line position of theideal part (target), formed part (compared), and the initial blank (object to modify). Previewis the initial blank contour line obtained from multiple iterative inverse algorithms.Figure 4 shows the size deviation distribution of the formed part before and afteroptimization. Before optimization, contour line of formed part strongly deviated from thatof the ideal part. Within the tolerance 0.5 mm range, the contour line agreement of theformed part and the ideal part was 88.22% and dimensional deviation at flange boundarywas the biggest, and the maximum was 0.84 mm. After the optimization, the contour lineagreement of the formed part and the ideal part reached 100%. Dimensional accuracy of thepart was greatly improved.Z. Q. Zhang, X. F. Jia, Y. J. Wang, and P. Gao92 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1Fig. 1. Schematic diagram of the control process for optimization model.Fig. 2. Hot stamping of U-shape part.Fig. 3. Contour lines of ideal part, formed part and initial blank.2.2. Experiment on Forming U-Shaped Part. The material used in this study isB1500HS, which is manufactured by Bao steel Group. Table 1 shows the composition ofthis material.The experiment uses DNS100 electronic universal testing machine and the heatingdevice is SX2-5-12 resistance furnace. B1500HS with a thickness of 1mm was cut alongthe optimized contour line and placed into the furnace that had been heated to 930C tokeep for 3 min. The heated blank was removed quickly from the furnace to the fixed die,and then the punch descended very quickly to form the blank. The temperature of tools is20C and the stamping force is 6 kN.The dimensions of the formed part, the optimized part and the ideal part are shown inTable 2. Compared with the ideal part, the optimized part had a small contour line sizedeviation that was within 0.5 mm and it is accepted by dimensional tolerance. Small flangeand low pressure occurred in the process might cause a little spring back. Figure 5 showsthe U-shaped part.Optimization Analysis of Initial Sheet Metal Contour Line ...ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1 93aFig. 4. Contour line size deviation distribution histogram of formed part and ideal part before (a) andafter optimization (b).T a b l e 1Chemical Composition of B1500HS Steel (wt.%)C Mn Cr Si Ti B0.22 1.20 0.20 0.20 0.30 0.05T a b l e 2Comparison of the Optimized Contour and the Ideal ContourCharacteristic Tested part Simulated part Ideal part ErrorLength (mm) 53.0 53.29 53.33 0.33Height (mm0 15.4 15.00 15.00 0.40b3. Test Verification.3.1. Hot Stamping of Bumper and Optimization of the Blank Initial Contour Line.Hot stamping process of a bumper was simulated and the blank used in the study has athickness of 1.5 mm and a friction coefficient of 0.35. The blank mesh had 13,441 nodesand 35,095 elements. The blank initial contour line was obtained by finite element inversealgorithm. Formed part did not meet the demands of dimensional precision and thedeviation mainly focused on the both ends of the long axis, which is shown in Fig. 6a. Thedimensional tolerance of the bumper is 0.5 mm. At the initial step of optimization, thecontour lines of formed part had a lower agreement that was 0.85%. The dimensionaldeviation was largely from 0.5 mm to the maximum 16.31 mm, which did not meet thedemands of the dimensional precision. After four times optimized iterative computations,the contour lines of formed part had an agreement that was 95.01%. The maximumdimensional deviation was under 0.54 mm, which is shown in Fig. 6b. When thepresupposed dimensional precision was reached, iterative computation would stop, and theoptimized initial contour line of the bumper could be obtained.94 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1Fig. 5. U-shaped part after stamping.abFig. 6. Contour line size deviation distribution of formed and ideal part before (a) and after (b)optimization.Z. Q. Zhang, X. F. Jia, Y. J. Wang, and P. Gao3.2. Hot Stamping of B-Pillar and Partial Optimization of the Blank Initial ContourLine. Finite element simulation model of a B-pillar was established, and the material usedin the study is 22MnB5, which has a thickness of 1.95 mm. Because the material flow ofB-pillar in the forming process is rather complicated, optimization of the whole blank initialcontour line will not get an ideal result and thus partial optimization of the blank initialcontour line can be carried out and laser edge cutting can also be reduced greatly. TheB-pillar in the study didn’t match the size of the ideal part due to deep drawing in theforming process. Three points A, B, and C were adopted along the contour line of the partand this part of contour line was ignored (red color), shown in Fig. 7. Hot stampingforming model of B-pillar was established, and the beam initial contour line was optimized.Iterations were 8 and dimension tolerance was 0.7 mm.Before the optimization, the contour line agreement of formed part and ideal part was34.68%, and their maximum deviation was 2.79 mm. After six iterations, the agreementreached 93.83%, and their maximum deviation was 1.14 mm (Fig. 8). The dimensionalaccuracy of the formed part was strongly improved.Conclusions. Based on the hot stamping principle, blank initial contour line of aU-shape part was optimized by multiple iterative inverse algorithms. The results show thatthe contour line agreement of formed part and ideal part has been greatly improved and thehot stamping experimental data of U-shape part verify the effectiveness of this method. Thetotal optimization of bumper contour line and partial optimization of B-pillar contour linealso provided good results. Partial optimization covers the shortage of whole optimizationfor complicated parts. Optimization of the blank initial contour line in hot stamping savesraw materials and cutting time and provides a vital reference for industry production.Acknowledgments. This work was supported by the National Natural ScienceFoundation of China (Nos. 51205162 and 51275203).ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1 95Fig. 7. Schematic diagram of B pillar partial optimization.Fig. 8. Contour line size deviation distribution histogram of formed part and ideal part.Optimization Analysis of Initial Sheet Metal Contour Line ...1. R. Kolleck, D. Steinhoefer, J. A. Feindt, and P. Bruneau, “Manufacturing methods forsafety and structural body parts for lightweight body design,” in: Proc. of the Conf.IDDRG (Sindelfingen, Germany, 2004), pp. 167–173.2. M. Merklein and J. Lechler, “Investigation of the thermo-mechanical properties of hotstamping steels,” J. Mater. Process. Technol., 177, No. 1, 452–455 (2006).3. Y. Q. Guo, J. L. Batoz, and H. Naceur, “Recent developments on the analysis andoptimum design of sheet metal forming parts using a simplified inverse approach,”Comput. Struct., 78, 133–148 (2000).4. Y. Q. Guo, J. L. Batoz, J. M. Detraux, and P. Duroux, “Finite element procedures forstrain estimations of sheet metal forming parts,” Int. J. Numer. Meth. Eng., 30, No. 8,1385–1401 (1990).5. K. Son and H. Shim, “Optimal blank shape design using the initial velocity ofboundary nodes,” J. Mater. Process. Technol., 134, No. 1, 92–98 (2003).6. X. Chen and R. Sowerby, “The development of ideal blank shapes by the method ofplane stress characteristics,” Int. J. Mech. Sci., 34, No. 2, 159–166 (1992).7. M. H. Parsa, P. H. Matin, and M. M. Mashhadi, “Improvement of initial blank shapefor intricate products using slip line method,” J. Mater. Process. Technol., 145, No. 1,21–26 (2004).8. R. Sowerby, J. L. Duncan, and E. Chu, “The modeling of sheet metal stamping,” Int.J. Mech. Sci., 28, No. 7, 415–430 (1986).9. X. C. Lu and B. W. Liang, “Modeling of flange deformation of irregular drawn cupsusing a fluid analogy,” Int. J. Mech. Sci., 28, No. 8, 491–497 (1986).10. Z. T. Zhang and B. W. Liang, “Determination of blank shapes for drawing irregularcups using an electrical analogue method,” Int. J. Mech. Sci., 28, No. 8, 499–503(1986).11. B. Oberpriller, L. Burkhardt, and B. Griesbach, “Benchmark 3 – Continuous PressHardening,” in: Proc. of the Conf. NUMISHEET’2008 (Interlaken, Switzerland,2008), pp. 115–124.Received 03. 08. 201596 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1Z. Q. Zhang, X. F. Jia, Y. J. Wang, and P. Gao

Optimization Analysis of Initial Sheet Metal Contour Line for High-Strength Boron Steel in Hot Stamping

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Optimization Analysis of Initial Sheet Metal Contour Line for High-Strength Boron Steel in Hot Stamping

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