EVALUATION OF DRAW BEADS INFLUENCE ON INTRICATE SHAPE STAMPING DRAWING PROCESS

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1 TECHNOLOGICAL ENGINEERING volume XI, number 1/2014 DOI: /teen EVALUATION OF DRAW BEADS INFLUENCE ON INTRICATE SHAPE STAMPING DRAWING PROCESS Article history: Received 10 september 2014 Accepted 4 november 2014 Available online Radek Čada, Petr Tiller Department of mechanical technology, Faculty of Mechanical Engineering VŠB-Technical University of Ostrava, Czech Republic Abstract Paper concerns evaluation of influence of shape, size and location of rectangular and semicircular draw beads on sheet-metal forming process. For analysis the simulations of forming process of selected two types of intricate shape stampings with similar ground plan and with approximately the same height from steel strip DC04 with the use of models of optimal blanks made by BSE (Blank Size Engineering) modul of simulation program Dynaform 5.7 were carried out. From simulations of forming process in simulation program Dynaform 5.7 followed that the most suitable is drawing without use of draw beads because cracks in stamping bottom corners do not arise. In the case of undesirable secondary waviness in the walls of intricate shape stamping the drawing with draw beads could be used but it would be necessary to increase the radius at the bottom of both stampings alternatively to choose another material with higher formability. Keywords forming; draw bead; simulation; finite elements method. INTRODUCTION Sheet-metal forming is one of the few technologies with minimum waste and versatile use in production. Low manufacturing costs assuming a low scrap rate in pressing shops are further important advantage of sheet-metal forming. Insufficient technological characteristic of any products, improperly designed blanks or exceeding the material formability limits are often the cause of scrap. At the production of intricate shape stampings the complicate forming conditions exist, the current presence of different stress in formed material is typical. At drawing of intricate shape stampings the unwanted phenomena can easily arise, such as loss of the plastic deformation process stability. When drawing such intricate stampings the blank area under the blankholder is often small according to total area of the formed semiproduct. For uniform shaping and stamping rigidity it is necessary to effort the local braking of sheet-metal. By various technological interventions it is possible to get a different braking intensity which allows the change of the conditions of pulling-in the formed blank sheetmetal into die space. Among the above-listed options the appropriate selection and utilization of draw beads of selected shape, number and method of placement in blankholder or die area around the stamping. It is possible globally or locally to increase the holding pressure or to increase the area of formed material under blankholder. The objective of draw bead is to prevent the secondary wrinkling of the stamping. [1, 3]. Braking by draw beads belongs to the most effective ways how to control the braking and in needed extend to increase the tensile stress. Acceptable location, geometry and of draw bead is always a question of many experiments and practical experience. An important feature of draw beads is the resistance to wear which affects the possibility of galling in sheetmetal drawing. It is necessary to choose the right material of draw bead and proper surface finishing. The usual material for production of draw beads is structural carbon steel E335. At non-rotational stampings the draw beads are produced separately and then they are fastened to the milled grooves by pressing, screwing, etc. The choice of geometry and location of the draw bead depends on experience and on the particular stamping. Support of simulation software can be found today in all steps of the manufacturing process at the draft of stamping, at the draft of stamping tools, at drawing process simulations etc. At the technical preparation of a new stamping production a various types of software specialized on sheet-metal forming simulations are used in present time. Among these programs, based on the principle of finite elements method (FEM), the simulation software Dynaform 5.7 belongs. It helps to shorten preparation time and eliminates inappropriate solutions of stamping including drawing tool, the location of the draw beads in the geometry of the drawing tool. It allows setting and testing of various shapes, sizes and s of drawing beads and their placement in the blankholder without making expensive drawing tool prototype. [2] 1 MATERIAL FOR PRODUCTION OF INTRICATE SHAPE STAMPINGS The initial material for production of intricate shape stampings is strip from steel DC04 which is supplied in sheets of dimensions (0.9 x ) mm according to ČSN The material is killed, non ageing, with very good properties for deep drawing. During its working the anisotropy of mechanical proper-- ties must be taken into account. Measured and calculated values of the plastics strain ratio rx in directions of 0, 45 and 90 towards sheet- TECHNOLOGICAL ENGINEERING, Volume XI, number 1/2014, Unauthenticated 5

2 metal rolling direction, the values of weighted average of plastic strain ratio r, degree of planar anisotropy of plastic strain ratio Δr are listed in Tab. 1. The values of strain hardening exponent nx in directions of 0, 45 and 90 towards sheet-metal rolling direction, the strain hardening exponent mean value nm, the planar anisotropy degree of strain hardening exponent Δn are listed in Tab. 2. These values are used as input when defining the material of blank in the simulation program Dynaform 5.7 in the initial stage of the simulation process of intricate shape stamping drawing. Table 1. Values of the plastics strain ratio rx in directions of 0, 45 and 90 towards sheet-metal rolling direction, the values of weighted average of plastic strain ratio, degree of planar anisotropy of plastic strain ratio of strip from steel DC04 r0 r45 r r Δr Figure 1. Model of intricate shape stamping specimen No. 1 Table 2. Values of the strain hardening exponent in directions of 0, 45 and 90 towards sheet-metal rolling direction, the strain hardening exponent mean value, the planar anisotropy degree of strain hardening exponent of strip from steel DC04 n0 n45 n90 nm Δn DETERMINATION OF BLANK SHAPE AND SIZE FOR DRAWING OF SELECTED INTRICATE SHAPE STAMPINGS For comparison of drawing process simulations two stampings with similar shapes were chosen according to technical documentation. The stamping designated as specimen No. 1 (see Fig. 1) has an outer ground plan dimensions (330 x 350) mm and stamping height 105 mm. The stamping designated as specimen No. 2 (see Fig. 2) has an outer ground plan dimensions (206 x 224) mm and stamping height 108 mm. At preparation of models for drawing process simulation the degree of freedom in negative direction of axis (z) was chosen in order to achieve the correct establishment of drawing tools in the simulation program Dynaform 5.7. By this manner the shell model was created. In the program Dynaform 5.7 the optimal blank for both stampings was developed with the use of the BSE (Blank Size Engineering) module. This module is equiped with one-step solver M-step for rapid assessment of drawability of part in the first stage of the draft of product shape and of tool. After generating the blank contour the trimming allowance with the size 10 mm according to standard ČSN was chosen. Figure 2. Model of intricate shape stamping specimen No. 2 3 REGULATION OF MATERIAL FLOW AT DRAWING BY DRAW BEADS The draw beads are used to regulate the material flow during drawing of sheet-metal. Draw beads are used at drawing of stampings with convex bottom, larger nonrotation and intricate shape stampings. The braking by draw beads is among the most effective ways how it is possible to control the braking and in needed range to increase the radial tensile stress while reducing tangential stress. The purpose of using the draw beads is to prevent the secondary waviness of stamping walls and primary waviness of flange. The draw beads are placed in the die or blankholder of the drawing tool to restrict the blank material flow (see Fig. 3). For the most effective draw beads effect the most suitable geometric profile and dimension of the draw bead must be found. For the drawing simulation of intricate shape stamping marked as specimen No. 1 draw beads in one row in selected four planar areas (see Fig. 4) were tested. At the specimen No. 2 the draw beads were tested in two planar areas (see Fig. 4). In the technical literature [9] the draw beads distance of 20 mm to 30 mm from the die drawing edge is recommended, in case of more draw beads one after another the distance of 25 mm to 35 mm is kept. An optimal placement of the draw beads is in the flat of the drawing edge contour. Draw beads always have width 5 mm to 10 mm and 1.2 mm to 5 mm high, depending on the size and shape of stamping and on the thickness of drawn sheet-metal (see Fig. 3). Suitable 6 TECHNOLOGICAL ENGINEERING, Volume XI, number 1/2014, Unauthenticated

3 location of draw beads in the blankholder is always a question of several experiments and measurements [8]. When the choice of geometry of the draw bead (radius, height etc.) and the location of the draw bead are unsuitable the stamping drawability markedly decreases. The tensile stresses may suddenly increase that cause crack near the stamping bottom or fracture of sheetmetal near the drawing edge. Draw bead may not be too high so that the metal not hardened excessively by cold deformation. High draw bead may cause at indentation to the flat blank the material waviness which consequently cannot be removed. It is therefore advantageous to use several low draw beads one after another than one high. Figure 3. Shape of rectangular and semicircular draw bead At deeper drawings the using of blankholder is often necessary for the smooth stamping process. Blankholder task is to hold the sheet-metal during the deep drawing process and to prevent its waviness. The waviness occurs in the flange (at deep drawing) or in the stamping wall (at stretching). Blankholder purpose is to prevent loss of stability in the stamping flange by the tangential tensile stress arising at drawing. Lack of pressure leads to the flange waviness while the high pressure blocks the extracting and leads to the stamping bottom breaking. For this reason the different types and designs of blankholders are used. When using the blankholder the pressing force can be calculated according to the equation (1) and it should be bigger when the ratio among initial flange width and the blank initial thickness is larger [7,10]. The size of the pressing force Fp can be determined from formula: Deformation force at the beginning of the drawing F0max which is necessary to calculate the pressing force Fp in equation (1) can be determined: F 0max 2 a h0 rmax /2/ [N], where are: a stamping finite radius [m], h0 blank initial thickness [m], σrmax maximum radial tensile stress [Pa]. 4 INTRICATE SHAPE STAMPINGS DRAWING PROCESS SIMULATION 4.1 Creation of blank and drawing tool parts models in the program Dynaform 5.7 For drawing process simulation of intricate shape stampings the models of die with a degree of freedom in negative direction of axis (z) were created. The stamping models were used for making of blanks in software Dynaform 5.7 with the use of BSE (Blank Size Engineering) Module. The model of die was used for making drawing tool parts (punch, die and blankholder). All boundary conditions, such as the shape and size of the blank, track and movement of tools, blankholder force, punch speed, material properties of sheet and friction coefficient, were set. With regard to the size of blank the size of the grid elements ( Radii ) with the parameter Radii = 4 was chosen. For the blank the material properties of strip steel DC04 with thickness of 0.9 mm were defined. When defining the drawing tool parts the separate models were assigned to already preset tool parts ( DIE, PUNCH, BLANKHOLDER ). The punch movement speed suitable for smaller presses is according to recommendation of the technical literature in range ( ) m s -1 [5, 6]. For simulations the punch movement speed of 0.6 m s -1 was chosen. Table 3. Determination of holding force for drawing of intricate shape stamping designated as specimen No. 1 Blank BSE Module of blank Si Effective area of blankholder Specific pressure pp [MPa] Holding force Fp1 [N] Table 4. Determination of holding force for drawing of intricate shape stamping designated as specimen No. 2 Blank BSE Module of blank Si Effective area of blankholder Specific pressure pp [MPa] Holding force Fp1 [N] M h 0 2 Fp 0,1 1 M F /1/ [N], According to the technical literature [9,11] (1) the specific pressure of blankholder ranges pp = ( ) MPa. 0max ( M 1) 2 b 0 For both stampings the pressure of pp = 2.14 MPa (see where are: b0 initial flange width [m], Tab. 3 and Tab. 4) was chosen. Calculation of the holding h0 blank initial thickness [m], force Fp1 was carried out like product of effective area M drawing coefficient [ ], of blankholder and specific pressure. The value of the F0 max deformation force at the beginning holding force Fp1 for intricate shape stamping designated of the drawing [N]. as specimen No. 1 is listed in Tab. 3, the value for specimen No. 2 is listed in Tab. 4. TECHNOLOGICAL ENGINEERING, Volume XI, number 1/2014, Unauthenticated 7

4 4.2 Location of draw beads at drawing of intricate shape stampings For evaluation of draw bead influence on drawing process of intricate shape stamping the location on blankholder was chosen. The draw beads were placed to areas in which pulling of material take place at top speed and for this reason it is necessary to brake the material. These areas are located in the rectilinear parts of the die drawing edge contour. Fig. 4 shows for both examined specimen the triangle contours that are constructed from the die drawing edge contour reduced for the half sheet-metal thickness using the method of maximum shear stress trajectories. Thickly highlighted lines in Fig. 4 show the draw beads that can be located on blankholder model directly in simulation program Dynaform 5.7. Every this line represents the longitudinal axis of the draw bead. At the specimen No. 1 the draw beads are located in a distance of 25 mm from the die drawing edge, in the case of specimen No. 2 the draw beads are located in a distance of 20 mm from the die drawing edge (see Fig. 4). diagram FLD, True shows the boundary of the crack arising, while the allowed deformations at intricate shape stamping are located bellow the curve. For the assessment of the change of the material plasticity stock utilization due to changes in s, geometry, height, location of draw beads of rectangular and semicircular shape at both intricate shape stampings the analysis of the stamping thinning drawn from the optimal blank from steel strip DC04 (see Fig. 7 and Fig. 8) was carried out by the program ETA/Post- Processor 1.0. Figure 5 Specimen No. 1 analysis FLD, True and drawability analysis of intricate shape stamping from strip steel DC04 without use of draw beads in the drawing process Figure 6. Specimen No. 2 analysis FLD, True and drawability analysis of intricate shape stamping from strip steel DC04 without use of draw beads in the drawing process Figure 4. Triangular shape areas suitable for the location of draw beads determined with the use of the method of maximum shear stress trajectories for specimen No. 1 and specimen No. 2 5 RESULTS OF INTRICATE SHAPE STAMPINGS DRAWING PROCESS SIMULATION 5.1 Results of the intricate shape stampings drawing process simulation without draw beads Analysis of the deformations at drawing of intricate shape stampings designated as specimen No. 1 and No. 2 is shown in the forming limit diagram FLD, True, (see Fig. 5 and Fig. 6) which uses the True curve of the stress strain (true strain diagram). From analysis the tendency of wrinkling in the wall of intricate shape stamping (secondary waviness) is seen, also the primary waviness at flange surface of intricate shape stamping occurs. These trends are more pronounced at the specimen No. 2 (see Fig. 6). On Fig. 5 and Fig. 6 the yellow, lower strain limit curve in the forming limit Figure 7. Specimen No. 1 analysis of thinning of the wall of intricate shape stamping from strip steel DC04 without use of draw beads in the drawing process Figure. 8. Specimen No. 2 analysis of thinning of the wall of intricate shape stamping from strip steel DC04 without use of draw beads in the drawing process 8 TECHNOLOGICAL ENGINEERING, Volume XI, number 1/2014, Unauthenticated

5 5.2 Results of the intricate shape stampings drawing process simulation with the use of draw beads with rectangular and semicircular shape For analysis of using of the draw beads at intricate shape stamping drawing the draw beads of rectangular and semicircular shape were selected. Draw beads were placed on blankholder. Four sets of calculations were made in first set it was experimented with draw beads geometry, in second set it was experimented with a of draw beads in selected areas, in third set it was experimented with distance from the inner edge of the blankholder and in fourth set it was experimented with a draw bead height. Height of draw bead was gradually selected with the size of 2 mm, 3 mm and 4 mm. Examples of the geometry of rectangular and semicircular draw bead shape for specimen No. 1, including selected draw beads s in for selected areas, are listed in Tab. 5. Distance from the inner edge of the die drawing edge was kept after series of simulations on the optimal value 25 mm. Examples of the geometry of rectangular and semicircular draw bead shape for specimen No. 2, including selected draw beads s in selected areas according Fig. 4, are listed in Tab. 6. Distance from the inner edge of the die drawing edge was successively selected 20 mm, 30 mm and 35 mm. Table 5. Variants of geometry of the draw beads with rectangular and semicircular shape selected for drawing process simulations of intricate shape stamping designated as specimen No. 1 Location of blankholder No. 1 No. 2 No. 3 No. 4 Type Height Draw bead parameters Angel [ ] radius 1 Groove radius 2 Bead rectangular /101 semicircular /101 rectangular /82 semicircular /82 rectangular /60 semicircular /60 rectangular /40 semicircular /40 Table 6. Variants of geometry of the draw beads with rectangular and semicircular shape selected for drawing process simulations of intricate shape stamping designated as specimen No. 2 Location of blankholder No. 1 No. 2 No. 3 No. 4 Type rectangula r Height Draw bead parameters Angel [ ] radius 1 Groove radius 2 Bead semicircular rectangular semicircular rectangular /50 semicircular /50 rectangular /30 semicircular /30 By analysis of thickness of intricate shape stamping designated as specimen No. 1 it was identified that the most suitable option of position of draw beads is area No. 3 with 60 mm and area No. 4 with 40 mm in distance 25 mm from the die drawing edge (see Fig. 9) where there is a local effect of draw beads on the corners at the bottom of the intricate shape stamping. The difference between the draw bead of semicircular and rectangular shape was not at the specimen No. 1 discovered nearly identical results at analysis of walls thickness (see Fig. 9) and at the thinning analysis were identified. Critical places of stamping wall thickness are at the bottom and the bottom corners. At selected height of draw beads of 2 mm the improvement of intricate shape stamping of the properties arises stopping the tendency of secondary waviness formation at the stamping wall and reduction of tendency of waviness arising at the flange surface. The selected heights of draw beads 3 mm and 4 mm are inappropriate owing to the separation of the bottom in drawing process. Location of draw beads in areas No. 1 and No. 2 according to Fig. 4 is not appropriate because the results of simulations a great braking of the material flow in the drawing process and the separation of the bottom of intricate shape stamping arise in both mentioned cases of draw beads shapes. Figure 9. Specimen No. 1 analysis of the wall thickness of the intricate shape stamping from strip steel DC04 with draw beads of rectangular shape in area No. 3 with of 60 mm and in area No. 4 with of 40 mm in distance 25 mm from the drawing edge At analysis of the wall thickness of intricate shape stamping designated as specimen No. 2 it was identified that the most suitable option of position of draw beads of semicircular shape with height of 2 mm (see Fig. 10) and 50 mm is area No. 1 with the 50 mm and area No. 2 with the 30 mm in the distance of 30 mm from the die drawing edge (see Fig. 4). Noticeable difference between the draw bead of semicircular and rectangular shape was not discovered. In the stamping walls thinning occur, mostly in the wall where in the area No 1 (see Fig. 4), is located draw bead with of 50 mm. Also even at this option of the location of draw beads a critical thinning of the wall of stamping occur that cause in the majority of cases in drawing process a crack in the stamping side wall (see Fig. 10). For other variants of combinations and location of the draw beads the results were not be satisfactory and during the simulations the cracks in the stamping walls and separation of bottom occur. Figure 10. Specimen No. 2 analysis of the wall thickness of intricate shape stamping from strip steel DC04 with draw beads of rectangular shape in area No. 1 TECHNOLOGICAL ENGINEERING, Volume XI, number 1/2014, Unauthenticated 9

6 with of 50 mm and in area No. 2 with of 30 mm in distance 30 mm from the drawing edge 6 CONCLUSION Evaluation of influence of shape, size and location of rectangular and semicircular draw beads on drawing process of intricate shape stampings was carried out with the use of two types of intricate shape stampings with similar ground plan and with approximately the same height (see 2) from steel strip DC04 (see 1) with the use of models of their optimal blanks made by BSE (Blank Size Engineering) modul of simulation program Dynaform 5.7 (see 4.1). At both intricate shape stampings the suitable areas of draw beads locations on blankholder were determined (see 4.2). By analysis of thickness of intricate shape stamping designated as specimen No. 1 it was identified that the most suitable option of position of draw beads is area No. 3 with 60 mm and area No. 4 with 40 mm in distance 25 mm from the die drawing edge (see 5.2). At the intricate shape stamping designated as specimen No. 2 it was identified that the most suitable option of position of draw beads of semicircular shape with height of 2 mm (see Fig. 10) and 50 mm is area No. 1 with the 50 mm and area No. 2 with the 30 mm in the distance of 30 mm from the die drawing edge (see Fig. 4). By simulations of drawing processes of both intricate shape stampings it was evaluated, that utilization of each variants of draw beads shapes leads to arising of considerable thinning of the stamping wall or even cracks (see 5.2). At the intricate shape stamping designated as specimen No. 1 when using separate draw beads variants (see Tab. 5) the critical thinned place in every stamping bottom corner arises where by increased radial stress the great drawing of material plasticity stock (see Fig. 9) exists. At the intricate shape stamping designated as specimen No. 2 when using separate draw beads variants (see Tab. 6) the critical thinned place in every vertical stamping wall under crossing radius between flande flange and stamping wall (see Fig. 10) arises. The positive phenomenon was that in all cases of the draw beads use by increased radial stress the partial or total elimination of waviness of side walls of intricate shape stamping arise. From drawing process simulation of explored intricate shape stampings in simulation program Dynaform 5.7 followed, that in cases, when the accurate stamping shape without existence of secondary waviness of walls must be kept and less depend on stamping walls thinning, using of draw beads at drawing is useful, but their location and geometry must be chosen so that in any place at the stamping the depletion of material plasticity stock and crack arising must not occur. In cases, when minimal stamping walls thinning must be kept and less depend on the accurate stamping shape without existence of secondary waviness of walls the drawing without draw beads (see 5.1) can be used, because smaller risk of arising of cracks in stamping exists (see Fig. 5 and Fig. 6). At drawing of intricate shape stampings with draw beads the risk of excessive thinning of their walls or depletion of material plasticity stock and arising of cracks ban be decreased by increasing of radius at the stamping bottom, alternatively by selection of different material with higher formability (e.g. steel DX54D, or DX56D). Acknowledgements Results in the contribution were achieved at solving of specific research project No. SP2014/193 with the name Research and Optimization of Technologies for Higher Useful Properties of New Materials and Engineering Products solved in year 2014 at Faculty of Mechanical Engineering of VŠB Technical University of Ostrava. Literature [1] Cada, R. Formability of steel sheets: technical book monograph (in Czech). Reviewers: L. Pollak and P. Rumisek. 1 st ed. Ostrava: REPRONIS, s. [2] Cada, R. Flat forming of metal materials (in Czech). 1 st ed. Ostrava: VSB TU Ostrava, p. ISBN [3] Cada, R. Formability of materials and nonconventional forming methods: Flat formability: instructions for exercises: script (in Czech). 1 st ed. Ostrava: VSB TU Ostrava, s. [4] Evin, E., et al. CAE support in designing manufacturing moldings. Transfer of innovation [online] [cit ], s On line: < archiv/transfer/9-2006/pdf/73-76.pdf>. [5] Evin, E., Hrivnak, A., Kmec, J. Gaining of material data for numerical simulation (in Slovak). In: Proceedings of 7th International Conference TECH- NOLOGY 2001: Part I. Bratislava: Slovak Technical University in Bratislava, 2001, pp [6] Evin, E., Tomas, M., Vyboch, J. Prediction of local limit deformations of steel sheets depending on deformation scheme. Chemical journal, 2012, Vol. 106, No. S3, pp [7] Kotouc, J. Tools for cold forming (in Czech). 3 rd ed. Prague: Czech Technical University, p. [8] Mielnik, E. M. Metalworking Science and Engineering. 1 st title, 2 nd series, United States of America: McGraw-Hill, Inc. ISBN [9] Primus, F. Forming theory of sheet-metal and pipes: script (in Czech). 2 nd ed. Prague: Czech Technical University in Prague, p. [10] Tisnovsky, M. and MÁDLE, L. Deep drawing of sheetmetal at presses (in Czech). 1 stt ed. Prague: SNTL, p. [11] Zebala, W. Matras, A.: Aspects of fibre composite machining. In Technological engineering 2/2012, ISSN TECHNOLOGICAL ENGINEERING, Volume XI, number 1/2014, Unauthenticated