Experimental study of high-strength low-alloy sheet metal components with holes on the bending surfaces

Transcription

1 387 Experimental study of high-strength low-alloy sheet metal components with holes on the bending surfaces M A Farsi* and B Arezoo Department of Mechanical Engineering, Amirkabir University of Technology, Tehran, Iran The manuscript was received on 15 July 28 and was accepted after revision for publication on 8 January 29. DOI: /954454JEM1292 Abstract: Bending is one of the processes frequently used in the manufacture of sheet metal components. Springback is an important issue in bending operations since it controls the final shape of the parts. Hence, exact estimation of its value is essential for die design operations. In industry, the springback is usually obtained from empirical equations or mostly from handbook tables. These equations or tables are mainly suitable for components that have no holes on the bending surfaces, and they cannot be used for parts with any holes on the bending surfaces. In the present work, the influence of holes and their sizes on the bending surfaces of the highstrength low-alloy (HSLA) sheet metal steel using V-shape dies is studied. Different die angles, die widths, and sheet metal thicknesses are used in the experiments. The results show that all these parameters have an effect on the springback and the bending force, but their effects are not similar. An equation is derived for determining the bending forces for parts with holes on the bending surfaces. Keywords: V-bending, bending force, springback, holes on bending surfaces 1 INTRODUCTION Sheet metal bending is one of the most widely used operations in manufacturing. Although the process is simple, the bending operation presents several technical problems in production, such as prediction of springback, punch load, and fracture in the stretched surface [1]. In recent years, high-strength steel, which has higher ratios of yield strength to elastic modulus, has increasingly been used for sheet metal parts in the automotive industry [2]. When these alloys are used, the technical problems mentioned above become more important. Springback is a salient issue where controlling the shape of the component is involved. Hence, the exact estimation of its value is essential for die design operations. In industry, the springback is usually obtained from empirical equations or from handbook tables. These equations or tables are mostly suitable for *Corresponding author: Department of Mechanical Engineering, CAD/CAPP/CAM Research Centre, Amirkabir University of Technology, Hafez Street, Tehran, Iran. JEM1292 Ó IMechE 29 those components that have no holes on the bending surfaces, and they cannot be used for parts with any holes on the bending surfaces. As will be seen later, no equations or tables are available for components, of this kind, and a detailed study in this field seems relevant. 2 RELATED WORK Fedin and Kharichkin [3] determined experimentally the springback and accuracy of parts produced by bending dies. They studied the influence of material, die, and punch angle. Weinmann and Shippell [4] presented experimental results on the V-die bending of a hot-rolled, high-strength, low-alloy steel sheet. Magnusson and Tan [5] applied elementary bending theory with pure moment bending (no transverse stress) to analyse V-die bending operations. They made some assumptions and predicted that the springback angle would be as follows: u ¼ 12ð1 n2 Þ Et 3 Z W =2 M cos Proc. IMechE Vol. 223 Part B: J. Engineering Manufacture ð1þ

2 388 M A Farsi and B Arezoo where u is the springback angle, n is Poisson s ratio, E is the elastic modulus, t is the sheet metal thickness, M is the moment, f is the inclination (a function of curvature), and W is the die width. They concluded that the springback increases with the strength of the yield and decreases as the thickness increases. Garcia-Romeu et al. [6] determined the springback of sheet metals in an air bending process, based on experimental work. They presented some useful graphics to show the springback and studied the relationship of the springback with the main parameters such as material type, material thickness, die width, and radius. Other researchers have studied the influence of inelastic effects [7], the Bauschinger effect [8, 9], and sheet and material property effects [1 12] on the springback of sheet metal forming. A study of the literature shows that, although the field of sheet metal bending has been extensively researched, V-bending has received very little attention. Moreover, none of the bending components used in the literature have holes or cuttings of any kind on the bending surfaces. Many components that have some kind of cutting (circle, oblong, etc.) on the bending surfaces can be found in industry. The amount of springback and the bending force in these parts (with cutting) are different to those in parts without holes. Therefore, the values of the springback found in handbooks, etc. are not valid for parts with holes, and they should be treated separately. In the present work, the springback and bending force in parts with oblong cutting holes on bending surfaces for V-dies are studied. Figure 1 shows such a component. In this paper, the influence of die angle, die width, material thickness, and amount of cutting on the final angle and bending force of the V-die bending process of high-strength low-alloy (HSLA) steel is studied experimentally and, an equation is derived for calculating the bending forces. 3 BASIC THEORY In sheet metal forming, when sufficient load is applied to a component, it undergoes an elastic and a plastic deformation. The plastic deformation creates the form of the component. When the load is removed, some of the elastic energy is released and the component tries to go back to its original shape. The extent to which the component succeeds in returning to its original shape is called springback. In a bending process, the outer and inner surfaces of the component undergo tensile and compressive stresses respectively. Thus, it can also be said that the springback is a consequence of unbalanced stresses throughout the thickness of a sheet undergoing Fig. 2 Fig. 1 Schematic of samples Schematic illustration of springback bending-related deformation. In Fig. 2 a component is shown before and after bending. The parameters that affect the amount of springback include material properties, the geometries of the die and punch, and the process conditions. Researchers have tried for many years to determine the value of the springback in bending operations. Equation (2) [13] is one of the mathematical formulae used for determining the springback and final shape of a component R 1 ¼ 4 R 3 1Y R 1Y þ 1 ð2þ R f Et Et where Y is the yield strength, E is the elastic modulus, t is the sheet metal thickness, R f is the final radius of the sheet metal, and R 1 is the initial radius of the sheet metal. Some researchers suggest neural networks or other AI approaches to determine the value of the springback [14, 15]. However, these systems are limited and can not be used in all conditions. Another important factor that is required for die design and press selection is the bending force. The Proc. IMechE Vol. 223 Part B: J. Engineering Manufacture JEM1292 Ó IMechE 29

3 Experimental study of HSLA sheet metal components 389 bending force can be determined by equation (3) [13]. This equation shows that the force depends on the bending moment. Many parameters such as material properties and die profile affect the bending moment. An analytical approach cannot correctly describe the amount of moment in the bending process [2, 13, 16]. Thus, die designers use simple equations such as equation (4) [17] to determine the bending forces. In this equation, the die opening factor, K, is not constant and varies according to the bending conditions. Also, errors in the calculation are increased when the component has any kind of hole or cutting on the bending surfaces because the stress pattern is not normal. The stress concentration on the edges of the holes will result in more complexity and hence will make the determination of the bending force and the springback more difficult with analytical approaches. Therefore, experimental studies should be carried out for such purposes F ¼ 4M cos 2 u=2 l k 2ðR m þ R p þ tþ sin u=2 ð3þ where M is the bending moment, R p is the punch radius, R m is the die radius, l k is the die width, u is the bend angle, t is the material thickness, and Stress (MPa) Strain % Fig. 3 Result of tensile tests (ASTM E8) Table 1 Material data E (MPa) Yield stress (MPa) UTS (MPa) n Final strain F ¼ KSWt 2 =L ð4þ where K is the die opening factor ¼ (larger values for smaller R/t ratios, and vice versa; a value of 1.33 is used for a die opening of 8 times the metal thickness), W is the width of the bent-up portion, L is the die width, and S is the ultimate tensile strength (UTS). 4 MATERIAL AND EXPERIMENTAL PROCEDURE The material used for the experiments was of an HSLA type that is frequently used in the automotive industry. Figure 3 shows the result of tensile testing of this material, and Table 1 shows the material data. Tensile tests were carried out according to ASTM standards. The equipment used for the experiments was a universal test machine (Zwick machine), which is shown in Fig. 4(a) This machine can record the force and the displacement (accuracy.1 mm), making it suitable for the experiments. The sample that was used in the tests is shown in Fig. 4(b). In this investigation, V-dies with three different angles (9, 12, and 135 ) and sheet metals with two different thicknesses (.95 and 1.75 mm) were used. First, the effects of different sizes of punched holes in the bending area on springback and bending forces were studied. Second, using a 9 V-die, the effects of four different die widths on the springback Fig. 4 (a) Universal test machine and (b) test sample JEM1292 Ó IMechE 29 Proc. IMechE Vol. 223 Part B: J. Engineering Manufacture

4 39 M A Farsi and B Arezoo Table 2 Die, punch, and material parameters Material Die Punch Type HSLA Width 18 mm Angle 84 t ¼.95, 1.75 mm Angle ¼ 9, 12, 135 R ¼ 1mm and bending forces were studied. The punched holes were of an oblong shape positioned in the centre of the bending area. For the sake of accuracy, the holes and the blanking of the parts were produced by a CNC punch machine. Table 2 shows more information regarding the test conditions. The bending tests comprised the following steps. 1. Aligning the die and the punch in the test machine. 2. Replacing the part on the die; bringing the punch down to a point just touching the part. 3. Moving the punch down at a constant speed and bending the component. 4. Moving the punch up. 5. Replacing the part, measuring its angle by a profile projector. Fig. 5 Deformed geometries in coining bending Fig. 6 Parts after bending In the test procedure, the bending angles were controlled by punch displacement. The angles of the parts after loading and unloading were measured by profile projector (accuracy 1 min). 5 RESULTS AND DISCUSSION Figure 5 shows the deformed geometries of the bent component on a coining 9 V-die in four stages. The sheet clearly slides smoothly and gradually bends into the die along the area R d (die radius). Figure 6 shows parts bent to Determination of the final angle All together, 28 tests were performed. Experiments showed that, when the area of the punched holes increases, the final angle of the parts and hence the values of the springback decrease. The results obtained for the two thicknesses are the same (Fig. 7, Tables 3 and 4). These results also show that the punched holes on the bending surfaces and their sizes affect the springback in a non-linear manner. This relation depends on the die angle and sheet metal thickness. For lower values of thickness, the influence of punched holes on the springback is greater. Also, as the die angle increases, this influence increases too. This behaviour shows that stress concentration in the bending zone changes the stress pattern and develops a plastic deformation zone. Thus, when the hole percentage increases, the plastic deformation also increases and the final angle decreases. Figure 8 shows the influence of the die angle on the springback where two parts, one without a hole and the other with a 6 per cent hole, are compared. It should be noted that the hole percentage here and throughout this paper refers to the ratio of width to length (W/L) in Fig. 4(b). Results of the experiments show that, under constant conditions (as the die angle is increased), the difference between the springback of the part without a hole and the part with a hole also increases. This increase for parts with a.95 mm thickness is greater than for parts with a 1.75 mm thickness. In other words, when the ratio of the value of the thickness to the radius of the bending punch increases, the region under plastic deformation becomes greater and the springback is decreased. The influence of die width on springback in 9 dies is also studied in the present work. Fifteen tests were performed using parts with and without holes (5 per cent). Figure 9 shows the results of these tests. As can be seen, the influence of the die width is very important. When the die width is increased, the final angle of the part decreases in a non-linear manner and the springback also decreases. This decrease is such that in the present tests for parts with 1.75 mm thickness, as the die width passes the value of 2 mm, the springback becomes negative. Proc. IMechE Vol. 223 Part B: J. Engineering Manufacture JEM1292 Ó IMechE 29

5 Experimental study of HSLA sheet metal components 391 Table 3 Final angle for a part with t ¼.95 mm Hole percentage Die 135, punch displacement 5.1 mm Die 12, punch displacement 4.7 mm Die 9, punch displacement 7.72 mm Table 4 Final angle for a part with t ¼ 1.75 mm Hole percentage Die 135, punch displacement 5.1 mm Die 12, punch displacement 4.6 mm Die 9, punch displacement 7.45 mm Angle (Deg) Angle (deg) Fig Die 135- punch disp.=5.1 Die 12- punch disp.=4.7 Die 9- punch disp.= Hole Percentage % (a) Die 135- punch disp.= 5.1 Die 12- punch disp.= 4.6 Die 9- punch disp.= Hole percentage % (b) Effect of cutting amount on final angle: (a) sheet metal thickness 1.75 mm; (b) sheet metal thickness.95 mm The negative springback values may be due to the bottoming effect on the parts [6]. Also, it is clear that the springback of parts with and without holes is almost equal during the bending of 1.75 mm sheet metal with a 24 mm die width. However, when the die width is greater than 24 mm or the ratio of the die width to the thickness is 14 or more, the amount of springback in parts without holes is more than in parts with holes. JEM1292 Ó IMechE 29 Different (Deg) Fig. 8 Spring back (Deg.) t = 1.75 t = Die angle Difference between springback of parts with and without holes t = 1.75 t = Hole 5% t =.95- Hole 5% t =.95 Die Width (mm) Fig. 9 Influence of die width on springback When bending a.95 mm sheet, the springback decreases with increase in die width. In all tests, the springback of parts with holes is greater than for other parts. It is clear that the relation between springback and die width for the two sheets is not the same. Proc. IMechE Vol. 223 Part B: J. Engineering Manufacture

6 392 M A Farsi and B Arezoo deg- t = deg- t = deg -t = Displ.(mm) t =.95 1 t = Die Angle(deg) (a) Fig. 1 Relationship between bending force and the punch traverse 5.2 Determination of the bending forces In V-die coining bending processes, the bending force graph has four stages. The first stage is the elastic deformation where the force is increased suddenly. In the second stage the force is mostly constant, while in the third the force decreases because of material slip in the die. In the fourth stage the force increases very rapidly because the material is pressed between the die and the punch. Figure 1 shows the relationship between bending force and punch traverse in the experiments. Figure 11 shows the influence of the die angle and the size of the cutting area on the bending forces. An average of the bending forces in the second stage (air bending force) is used in the charts. Figure 11(a) shows that when the die angle increases, the forces are approximately constant. This may be due to the fact that the effect of the die angle is small compared with the effects of the die width and material thickness. This can be seen in equation (3). Figure 11(b) shows that, with a decrease in material in the bending area, the force also decreases. To study the influence of the die width on the bending forces, 12 tests were performed. Four different die widths with 9 angles and parts with holes of 5 per cent were used. Figure 12 shows the results of these tests. It can be seen that, when the die width is increased, the bending forces decrease. These results are also corroborated by the theoretical formula shown in equation (4). It can also be seen that the effect of die width on bending forces is greater for the thicker material than for the thinner material. 5.3 Prediction of bending forces for parts with holes Experimental observations show that, given the same conditions, those parts with holes on their bending surfaces need smaller bending forces than parts without holes. The relationship between bending forces for Fig deg- t = deg- t = deg. t = deg- t = deg- t = deg- t = Hole Percentage % parts with holes and parts without holes is of a nonlinear nature. As demonstrated in previous sections, among all the parameters that affect the bending forces, the size of the holes, the effective width, and the thickness of the parts are the most important factors. As shown in Table 5, the influence of other parameters, such as die width for parts with and without holes, is similar. Therefore, in order to determine the bending forces for parts with holes on the bending surfaces, the following equation is suggested F ¼ F perfect t t=we ð5þ R (b) Average of the bending forces in stage 2: (a) influence of the die angle (hole percentage 5 per cent); (b) influence of the hole percentage t =.95-5% hole t = % hole t = w ith out hole Die width (mm) Fig. 12 Influence of die width on bending force Proc. IMechE Vol. 223 Part B: J. Engineering Manufacture JEM1292 Ó IMechE 29

7 Experimental study of HSLA sheet metal components 393 Table 5 Influence of die width on bending forces (t ¼ 1.75 mm, angle 9 ) Die width Without hole Part with hole 5% Equation (4) (K ¼ 1.33) Suggested equation Table 6 Comparison between experiments and suggested equation (die width ¼ 18 mm) Die angle t (mm) Effective width (mm) Equation (4) (K ¼ 1.33) Part with hole (exp.) Suggested equation Total difference with experiments Mean errors Average force Error (%) experiments Eq 4 Eq Eq. 4 - t = 1.75 Exp.- t = 1.75 Eq. 5- t = 1.75 Eq. 4- t =.95 Exp.- t =.95 Eq. 5- t = Fig Effective width(mm) Bending forces: die 9, t ¼ 1.75 mm Where t is the sheet metal thickness, R is the hole radius, W e is the effective width of the part (the difference between the width of the part and the hole), and F perfect is the bending force for the part without holes (the die opening factor K is assumed to be 1.33 in equation (4)). Table 6 and Figs 13 and 14 compare the results calculated using equations (4) and (5) with experimental results. To compare the difference between calculated forces and experimental forces, the mean errors are Effective width (mm) Fig. 14 Bending forces: die 12 used. As shown in Table 6, the mean errors of the forces calculated using equation (4) and those predicted by equation (5) are about 9.5 N and 59 N respectively. When the average force is calculated using these equations and it is compared with the average experimental force, respective errors of 7.89 and 1.35 per cent can be seen. These results show a great improvement in the accuracy of equation (5) compared with equation (4). JEM1292 Ó IMechE 29 Proc. IMechE Vol. 223 Part B: J. Engineering Manufacture

8 394 M A Farsi and B Arezoo 6 CONCLUSION According to the results obtained from the experiment, the following conclusions can be drawn. 1. The springback is a function of material thickness, die angle, and the size of the punched hole. 2. The size of the hole on the bending surface has an effect on the bending angle. When the size of the hole increases, the final angle of the part decreases. 3. The influence of the size of the hole on the springback in the 135 die is greater than that in the 12 die, and the influence of the 12 die is greater than that in the 9 die. 4. Under the same conditions, the springback is greater for the thinner part than for the thicker one. 5. When 1.75 mm sheet is used and the ratio of die width to thickness is 14 or more, the amount of springback in parts without holes is greater than in parts with holes. However, when this ratio is less than 14, the amounts of springback are almost equal. 6. When the material in the bending area decreases, the bending force also decreases. This means that, under the same conditions, parts with holes always need less bending forces than parts without holes. 7. When the die width is increased, the bending forces decrease. Also, the effect of die width on bending forces is greater for the thicker material than for the thinner material. As the die width increases, the bending force decreases in a nonlinear manner. This behaviour is the same for parts with and without holes. 8. When the die width is increased, the final angle of the part and the springback decrease. The results of this study show that components that have any kind of hole on the bending surfaces behave differently to components without holes. Among all other factors, the springback is the most important, since the final shape of the component depends on it. This factor becomes even more important when the component has closer tolerances. Also, the bending forces in parts with and without holes are dissimilar. Using the experimental results of the present work, an empirical equation has been derived. This equation predicts the bending forces with a 1.35 per cent average error compared with experimental results. The results obtained in the present work show that parameters such as thickness t, effective width W e, and hole radius R of the part play a most important role in addition to the parameters used in equation (4). Furthermore, the equation derived in this work (equation (5)) can be used to calculate the bending forces for other materials, as long as the bending force for the same material without a hole, (F perfect ) is available. REFERENCES 1 Huang, Y. M. and Chen, T. C. Influence of blank profile on the V-die bending camber process of sheet metal. Int. J. Adv. Mfg Technol., 25, 25, Carden, W. D., Geng, L. M., Matlock, D. K., and Wagoner, R. H. Measurement of springback. Int. J. Mech. Sci., 22, 44(1), Fedin, Y. A. and Kharichkin, E. S. Determining the spring-back angle and accuracy of parts made in bending dies. J. Chem. and Petrol. Engng, 1974, 1, Weinmann, K. J. and Shippell, R. J. Effect of tool and work piece geometries upon bending forces and springback in 9 degree V-die bending of HSLA steel plate. In Proceedings of 6th North American Metal Working Research Conference, USA, May 1978, pp Magnusson, C. and Tan, Z. Mathematical modeling of V-die bending process. In Proceedings of 16th Biannual IDDRG Congress, Stockholm, Sweden, May 199, pp Garcia-Romeu, M. L., Ciurana, J., and Ferrer, I. Springback determination of sheet metals in an air bending process based on an experimental work. J. Mater. Process. Technol., 27, 191, Cleveland, R. M. and Ghosh, A. K. Inelastic effects on springback in metals. Int J. Plasticity, 22, 18, Chun, B. K., Jinn, J. T., and Lee, J. K. Modeling the Bauschinger effect for sheet metals, part I: theory. Int. J. Plasticity, 22, 18, Chun, B. K., Jinn, J. T., and Lee, J. K. Modeling the Bauschinger effect for sheet metals, part II: application. Int. J. Plasticity, 22, 18, D Urso, G., Pellegrini, G., and Maccarini, G. The effect of sheet and material properties on springback in air bending. Key Engng Mater., 27, 344, Fei, D. and Hodgson, P. Experimental and numerical studies of springback in air V-bending process for cold rolled TRIP steels. Nucl. Engng. and Des., 26, 236, Özgu, R. T., Ulvi, S. E., and Özdemir, A. Determining springback amount of steel sheet metal of.5 mm thickness in bending dies. Mater. and Des., 26, 27, Vukota, B. Sheet metal forming processes and die design, 24 (Industrial Press, New York). 14 Inamdar, M. P., Date, P., Narasimhan, K. S., Maitil, K., and Singh, U. P. Development of an artificial neural network to predict springback in air V-bending. Int. J. Adv. Mfg Technol., 2, 16, Roy, R. Assessment of sheet-metal bending requirements using neural networks. Neural Comput. and Applic., 1996, 4, Marciniak, Z., Duncan, J. L., and Hu, S. J. Mechanics of sheet metal forming, 22 (Butterworth Heinemann, GB). 17 Suchy, I. Handbook of die design, 2nd edition, 26, pp (McGraw-Hill, USA). Proc. IMechE Vol. 223 Part B: J. Engineering Manufacture JEM1292 Ó IMechE 29