Presented work deals with springback behavior of two different aluminum alloys, one falling into 5th series (AW-5754 H22) with a thickness of 0.8 mm and other from 6th series (AW-6082 T6) with 1.0 mm thickness. These materials are used for their various applications and hardening process. The springback behavior was investigated by U-bending test. Bending tool was graduated jig with rollers and experiment was performed on R11 and R17 radii. The first series of specimens were oriented in a parallel direction and the other in a direction perpendicular to the rolling direction. Experimental results were measured with MATLAB measuring method and compared with finite element calculation carried out in PAM-STAMP. Influence of different yield functions was also examined.Рассматривается характеристика упругой отдачи двух различных алюминиевых сплавов: AW-5754 H22 толщиной 0,8 мм, принадлежащего к 5-й серии, и AW-6082 T6 толщиной 1,0 мм, принадлежащего к 6-й серии. Эти материалы используются для разнообразного применения, включая процесс упрочнения. Характеристика упругой отдачи исследовалась при испытаниях на U-образный изгиб. В качестве гибочного инструмента использовалось приспособление  с роликами, и эксперимент проводился на радиусах R11 и R17. Одна группа образцов была ориентирована в направлении, параллельном направлению прокатки, другая в перпендикулярном. Экспериментальные результаты, полученные с использованием программного комплекса MAТLAB, сравнивались с расчетом методом конечных элементов с помощью программы PAM-STAMP. Исследовалось также влияние различных функций текучести

Dvorák, M.

Slota, J.

Šiser, M.

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

UDC 539.4Experimental and Numerical Analysis of Springback Behavior of AluminumAlloysJ. Slota,a,1 M. Ðiser,a,2 and M. Dvorákb,3a Technical University of Koðice, Faculty of Mechanical Engineering, Institute of Material andTechnological Engineering, Koðice, Slovakiab Brno University of Technology, Faculty of Mechanical Engineering, Brno, Czech Republic1 jan.slota@tuke.sk2 marek.siser@tuke.sk3 dvorak.m@fme.vutbr.czPresented work deals with springback behavior of two different aluminum alloys, one falling into 5thseries (AW-5754 H22) with a thickness of 0.8 mm and other from 6th series (AW-6082 T6) with 1.0mm thickness. These materials are used for their various applications and hardening process. Thespringback behavior was investigated by U-bending test. Bending tool was graduated jig with rollersand experiment was performed on R11 and R17 radii. The first series of specimens were oriented in aparallel direction and the other in a direction perpendicular to the rolling direction. Experimentalresults were measured with MATLAB measuring method and compared with finite element calculationcarried out in PAM-STAMP. Influence of different yield functions was also examined.Keywords: finite element method (FEM), ANSYS, clinching, three sheets, steel, mechanicaljoining.Introduction. Sheet metal bending is one of the most common forming process,widely used in various production areas to create simple profiles (e.g., bending to U- orV-shape) from a flat sheet, using affordable methods. Tekiner [1] defined sheet metalbending as a homogenous material deformation around axis located in the neutral plane andorthogonal to the length direction of the specimen. Even though metal bending is basicallysimple process it is also very difficult, in particular as regards to fragile, hard to bendmaterials as titanium or aluminum alloys. These materials are often affected by springback,which is difficult to compensate. Springback is mostly affected by factors such as materialproperties (elastic modulus, Poisson’s ratio, strain-hardening exponent, yield strength ormaterial anisotropy), friction coefficient, clearance between punch and die, blankholderforce, die and punch radii, bending radii, etc. [2–6]. However, aluminum alloys oftendisplay high ratio of yield stress to elastic modulus, which also significantly affectsspringback. Springback usually leads to change in geometry of stamped part which is to becompensated, thereby leading to a rise in production costs [1]. This effect is mostcommonly compensated by over-bending of stamped part. Many currently available softwareprovides the possibility to sophisticated compensation of springback [7]. Many authors[8–13] deals this by using special modules in various FEM software. In recent years, a lotof research has been done in this area, and multiple authors have analyzed the springbackproblem from many different perspectives. Thus, authors [8] considered the bending intoU-shape with a blankholder, while researchers [9] studied the impact of used material onthe prediction accuracy of numerical simulation of HSLA sheets. Authors [10, 12, 13]investigated different materials, including Ti and Al alloys, and the springback phenomenaoccurring in them. In study [11], three different materials has been used, namely TRIP RAK40/70 UHS steel, HSLA 220PD AHS steel, and DQ category DC06 alloy, which weresubjected to cyclic loading with the further Bauschinger effect investigation. This study© J. SLOTA, M. ÐISER, M. DVORÁK, 2017ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2017, ¹ 4 93deals with exploration of springback effect in two aluminum alloys, which are often usedin automotive industry. The springback effect can also depend on the specimen orientation.Specimens with loading axes oriented parallelly and perpendicularly to the rolling directioncan be distinguished by different springback angles.Experimental Procedure. The springback effect in aluminum specimens bent usingan experimental graduated bending jig with rollers was examined. Each specimen was bentto the angle of 180, and after its unloading the springback angle was measured. Themeasured results were compared with the numerical simulation data and with the results ofwork [11]. In these tests, the impact of rolling direction of the bent specimen on thespringback behavior was also investigated. Two different materials, namely AW-6082 T6and aluminum alloys different content with magnesium and silicon, which are frequentlyused in automotive industry for fabrication of car body parts, were used in this experiment.It is known that springback is strongly dependent on the type of material used [14]. TheAW-6082 T6 alloy specimens were in the state after solution heat treatment followed byartificial aging and had a thickness 1.0 mm, while those of AW-5754 H22 were work-hardened (1/4 hard) and partly annealed, and their thickness was 0.8 mm. The mechanicalproperties and rolling directions of these specimens are shown in Tables 1 and 2,respectively. Rectangular-shaped specimens with the dimensions 20 120 mm were usedfor the experiment.The experimental procedure is shown in Fig. 1. To measure the springback angle, twodifferent bend radii R11 and R17 were used. Specimens were bent to the angle of 180%around the die, then unloaded, and the springback angle was measured. The geometry ofthe experimental graduated bending jig with rollers is shown in Fig. 2.For comparison purpose, the same methodology was used as in work [11]. To measurethe springback angle, the MATLAB application was used, wherein five points per each armof specimen were applied to construct a straight line and simulated the inclination angles.The test results were compared with the numerical simulation ones obtained using anexplicit FEM code. The parameters used in the numerical simulation are listed in Table. 3.J. Slota, M. Ðiser, and M. Dvorák94 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2017, ¹ 4T a b l e 1Mechanical Properties of AW-6082 T6Rollingdirection(deg)Rp0 2. ,MPaRm,MPaA80,%n Cm,MPar0 314 342 13.7 0.087 456.12 0.52845 308 337 14.2 0.65790 313 341 12.0 0.509T a b l e 2Mechanical Properties of AW-5754 H22Rollingdirection(deg)Rp0 2. ,MPaRm,MPaA80,%n Cm,MPar0 165 250 14.2 0.196 410.69 0.54545 155 239 20.8 1.06090 160 246 19.2 0.786For aluminum alloys, the Hill90 yield function is considered to be the most appropriate[15]. It is based on the nonquadratic transition function and, in order to construct thisfunction, values from the uniaxial tensile and biaxial tests are required. This function isdefined by equation [15]:~~~~ ~~12229021 2	 	    p q cpb( )  ~~ 1 20 901, (1)where  0 is the uniaxial tensile stress in the rolling direction (MPa),  90 is the uniaxialtensile stress in the direction normal to the rolling direction (MPa),  b is stress underbalanced biaxial loading conditions (MPa), while c, p, and q are parameters defined as in[15]:Experimental and Numerical Analysis of Springback Behavior ...ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2017, ¹ 4 95T a b l e 3Parameters Defined in Preprocessing of Numerical SimulationParameter Value Parameter ValueMesh type Quadrilateral Element type ShellInitial mesh size 17 mm Element formulation E-P shellLevel of refinement 4 Yield curve Hill 90Mesh size after refinement 1.06 mm Hardening curve KrupkowskiNumber of integration points 11 Tool mesh 17 mmFig. 1. Schematic illustration of the bending process.Fig. 2. Experimental graduated bending jig with rollers.cb   0909000 902, (2)1 1 1 2120 900 900 0290    	 bb bpRRR( )( ) (1 90 9020Rc), (3)1 1 1 2120 9090 090 9020    	 bb bqRRR( )( ) ( ).1 0 0290Rc (4)Here R0 and R90 are the anisotropy values for the uniaxial tension in the rolling directionand in the in-plane direction perpendicular to the rolling direction, respectively.The above yield function was used in combination with the Krupkowski hardeninglaw expressed by the following equation [16]:   K p on( ) , (5)where  p is the plastic strain, o is the offset strain, n is the strain-hardening exponent,and K is the material constant (MPa).The applied yield functions and hardening curves for both materials are shown in Fig. 3.96 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2017, ¹ 4a bFig. 3. The Krupkowski hardening curves for AW-5754 H22 (a) and AW-6082 T6 (b), Hill90 yieldfunctions for AW-5754 H22 (c) and AW-6082 T6 (d).c dJ. Slota, M. Ðiser, and M. DvorákResults and Discussion. Tables 4 and 5 list the experimental results of springbackcompared with numerical simulation. The bending test was simulated according to theexperimental setup shown in Fig. 1. To examine the influence of rolling direction on thespringback angles, specimens were cut differently to the rolling direction. The first series ofspecimens were cut in the direction parallel to the rolling one (further RD) and the secondseries were cut in the direction perpendicular to it (further TD). Configurations of bentspecimens after springback are shown in Fig. 4.The comparison of experimental results with the numerical simulation shows that thecalculated springback values are always lower than the experimental results. This occursdue to the used material model in FEM code. If we use the experimental results from [11]instead of the original ones, the differences will be even more significant:the higherstrength of material the higher the difference. This result is affected by the applied yieldfunction. In this study, the Hill90 plasticity condition was used, which is more suitable forspecial steels and alloys [10]. The graphical representation of this yield function, incomparison with several others, is presented in Fig. 5. The discrepancy between test andISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2017, ¹ 4 97T a b l e 4Results of Springback for AW-5754 H22Die radius R11RollingdirectionExperiment SimulationRD 35.52 32.14TD 37.83 34.92Die radius R17RollingdirectionExperiment SimulationRD 39.22 36.25TD 41.87 38.72T a b l e 5Results of Springback for AW-6082 T6Die radius R11RollingdirectionExperiment SimulationRD 38.55 33.83TD 37.40 32.27Die radius R17RollingdirectionExperiment SimulationRD 52.09 47.33TD 48.15 42.71Fig. 4. Configurations of bent specimens after springback.Experimental and Numerical Analysis of Springback Behavior ...calculated results is also influenced by the used hardening curve. In the mathematicaldescription of the Krupkowski hardening law via Eq. (5), the elastic deformation componentalso appears in the formulation, and although it is much smaller than the plastic one, it isimportant in the FEM simulation of the springback [10]. Meanwhile, the Hill48 yieldfunction was used in [11] for TRIP steel in combination with the Hollomon hardeningcurve, which also contribute to large deviations between the experimental results andnumerical simulation data.The above influence of the yield function used in the numerical simulation on thecalculated results was investigated for AW-5754 H22 alloy. Two different yield functionswere used in the numerical simulation, namely, isotropic Hill48 and orthotropic Hill90yield functions. The former one considers anisotropy in all directions ( r 1), while thelatter (Hill90) function is described by the Lankford coefficients, which are different in alldirections. Therefore, differences in the equivalent stress are minimal in the RD direction,but quite considerable in the TD direction. The respective results shown in Fig. 6 indicatethat the orthotropic material model Hill90 provides a better correlation with theexperimental springback results than the isotropic Hill48 one. In case of AW-5754 H22alloy, which is age-hardened, one can observe that springback efffect for specimens withthe orientation parallel to the rolling direction (RD) is less pronounced than for specimensoriented perpendicular to the rolling direction (TD). The respective difference in theequivalent stresses was 7.5% for RD specimen with R11 radii, 39% for TD specimen withR11 radii, 10.6% for RD specimens with R17 radii, and 36% for TD specimen with R17radii. On the other hand, for AW-6082 T6 alloy, higher springback angles are observed inRD specimens. The difference in the equivalent stress for R11 radii was 2.2% in RD and2.5% in TD cases. For R17 radii, the respective differences were 3.7% in RD and 1–3% inTD cases. Besides the fact that AW-6082 T6 alloy specimens had higher thickness andstrength than AW-5754 H22 ones, this material was heat-treated, and its higher strengthinvoked a stronger springback effect (Fig.7). In addition, during its heat treatment process,the so-called Guinier–Preston zones (GPZ) were created by the segregation. These areas98 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2017, ¹ 4Fig. 5. Comparison of different yield functions for material AW-5754 H22.J. Slota, M. Ðiser, and M. DvorákISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2017, ¹ 4 99Fig. 6. Influence of yield function on the equivalent stresses after springback.Fig. 7. Results of numerical simulation for two materials, radii, and directions.Experimental and Numerical Analysis of Springback Behavior ...100 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2017, ¹ 4Fig. 8. Comparison of equivalent stresses before and after springback.J. Slota, M. Ðiser, and M. Dvorákhave increased concentration of intermediate phase in solid form. Since GPZs are coherentwith the solid solution, they create grid stresses that increase the hardness and strength ofthe material in the direction parallel to the rolling one.Figure 8 shows the equivalent stresses in specimens before and after springback. Thespecimens were cut in the center of their width (in the middle layer). It is easy to see theresults of the stress drop after springback, which invoked the stress relaxation in thespecimen under study. For AW-6082 T6, the patterns of stresses in RD and TD are verysimilar. For AW-5754 H22, there is a visible stress drop in the TD case, in contrast to RDone. This is due to the material structure shown in Fig. 9. AW-57584 H22 is Al+Mg alloy,while a higher content of Mg cause the creation of Mg5Al8 intermetalical phase in the grainboundaries, and the generation of strains in the material microstructure. The precipitationprocess has a negative effect on the mechanical properties, especially strength. On the otherhand, the presence of Mg and Si in the alloy AW-6082 T6 alloy leads to the formation ofMg2Si precipitates, which improves the material strength.Conclusions. In this study, the springback behavior of aluminum sheets wasinvestigated using the FEM simulation and static bending tests. The most importantfindings can be concluded as follows:1. The application of isotropic yield function yields similar calculation results as thatof orthotropic yield function for specimens with cutting direction parallel to rolling one(RD), but different resultys for the perpendicular direction (TD). In the latter case, theequivalent stresses estimated via the isotropic model are higher than those obtained via theorthotropic one, which results in a larger deviation of simulated and experimental data. Toachieve a better calculation accuracy, the application of more advanced yield functions isexpedient.2. A more pronounced springback effect was observed in the AW-6082 T6 alloy, incontrast to AW-5754 H22, which had a lower strength. For both materials, the increase inthe die radius caused a stronger sprigback effect. This is attributed to the equivalentstresses, which are lower for smaller die radii. The results obtained also depend on theorientation of specimens to the rolling direction. For AW-5754 H22 alloy, greater springbackangle was measured in TD case, than in RD one, while AW-6082 T6 alloy exhibited anopposite trend.3. For AW-6082 T6, the patterns of stress distribution in RD and TD specimens arevery similar. For material AW-5754 H22, there was a visible stress drop in the TDspecimens, in contrast to RD case. The material structure and type of hardening are shownto control this effect.4. Although measured and simulated data exhibited a good fit, the applied materialmodel has a vital effect on the springback angle computation results. To improve theISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2017, ¹ 4 101a bFig. 9. Microstructure of AW-6082 T6 (a) and AW-5754 H22 (b).Experimental and Numerical Analysis of Springback Behavior ...calculation accuracy, modern material models with comprehensive hardening laws arerequired.Acknowledgments. This contribution is the result of project APVV-14-0834implementation.1. Z. Tekiner, “An experimental study on the examination of springback of sheet metalswith several thicknesses and properties in bending dies,” J. Mater. Process. Tech.,145, 109–117 (2004).2. Y. E. Ling, H. P. Lee, and B. T. Cheok, “Finite element analysis of springback inL-bending of sheet metal,” J. Mater. Process. Tech., 168, No. 2, 296–302 (2005).3. N. Nanu and G. Brabie, “Analytical model for prediction of springback parameters inthe case of U stretch–bending process as a function of stresses distribution in the sheetthickness,” Int. J. Mech. Sci., 64, 11–21 (2012).4. R. H. Wagoner, H. Lim, M. G. Lee, et al., “Advanced issues in springback,” Int. J.Plasticity, 45, 3–20 (2013).5. C. J. Su, G. H. Zhang, S. M. Guo, et al., “Influence of friction on springback ofquadrangle parts bending,” Adv. Mater. Res., 217–218, 619–624 (2011).6. Y. Hu, Simulating the Die Gap Effect on Springback Behavior in Stamping Processes,SAE Technical Paper 2000-01-1111 (2000), doi: 10.4271/2000-01-1111.7. Springback Compensation with AutoForm Version 4.1, EuroBlech (2006).8. W. Phanitwonga and S. Thipprakmasa, “FE-analysis of channel width effects onspring-back characteristics in the U-bending process,” Procedia Engineer., 183,17–22 (2017).9. H. L. Dai, H. J. Jiang, T. Dai, et al., “Investigation on the influence of damage tospringback of U-shape HSLA steel plates,” J. Alloy. Compd., 708, 575–586 (2017).10. A. Gisario, M. Barletta, and S. Venettacci, “Improvements in springback control byexternal force laser-assisted sheet bending of titanium and aluminum alloys,” Opt.Laser Technol., 86, 46–53 (2016).11. J. Slota, M. Jurèiðin, and M. Dvorák, “Experimental and numerical analysis ofspringback prediction in U-bendings of anisotropic sheet metals,” Zeszyty NaukowePolitechniki Rzeszowskiej, 288, 525–533 (2013).12. J. Zhou, B. Wang, J. Lin, and L. Fu, “Optimization of an aluminum alloy anti-collision side beam hot stamping process using a multi-objective genetic algorithm,”Arch. Civ. Mech. Eng., 13, No. 3, 401–411 (2013).13. A. Zajkani and H. Hajbarati, “Investigation of the variable elastic unloading moduluscoupled with nonlinear kinematic hardening in springback measuring of advancedhigh-strength steel in U-shaped process,” J. Manuf. Process., 25, 391–401 (2017).14. J. Slota, M. Jurèiðin, and E. Spiðák, “Numerical and experimental springbackdetermination of sheet metals in an air bending process,” Acta Metall. Slovaca, 18,No. 4, 200–209 (2012).15. S. Bruschi, T. Altan, D. Banabic, et al., “Testing and modelling of material behaviourand formability in sheet metal forming,” CIRP Ann. - Manuf. Techn., 63, 727–749(2014).16. C. Arwidson, Numerical Simulation of Sheet Metal Forming for High Strenght Steels,Luleå University of Technology, Luleå (2005).Received 01. 09. 2017102 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2017, ¹ 4J. Slota, M. Ðiser, and M. Dvorák

Experimental and Numerical Analysis of Springback Behavior of Aluminum Alloys

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Experimental and Numerical Analysis of Springback Behavior of Aluminum Alloys

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