Reducing Springback using post-stretching with stake beads By Tanmay Gupta, Ali Fallahiarezoodar, and Dr. Taylan Altan #688-4 Reducing springback, especially in forming of Advanced High Strength Steels (AHSS), is a major issue in sheet metal forming. An article, published in March/April 2018 issue of Stamping Journal was devoted to this subject. In this article, the use of servo hydraulic cushion and a variable Blank Holder Force (BHF) for applying post stretching and reducing of springback was discussed. Figure 1 illustrates a simple hat bending operation. In this case, the draw-in of the sheet material is controlled by blank holder force. In this forming operation, the sheet material undergoes both bending and unbending deformation. Thus, as result of elastic recovery of the strains, the material has a natural tendency to unbend itself in the side wall as well as in the bending corners of the hat shaped part. This causes the side wall to curl that affect the dimensional accuracy and final shape of the formed part. Springback is the result of non-uniform redistribution of stresses in sheet thickness. Thus, additional stretch/tension force on the part can reduce the unbending moment and consequently the springback. Post-stretching is a well-known method for reduction of springback at parts made by drawing operation. In this method, additional blank holder force is applied toward the end of deformation which cause additional stretch on the side wall and reduce the heterogeneous stress distributions. Figure 2 illustrates how the post stretching method can drastically modify the stress distributions in the wall from tensile-compression to tensile only. As a result, appropriate post stretching can reduce the springback in the part. Post stretching can be achieved using a variable BHF, as discussed earlier. However, many North American stampers do not use servo hydraulic cushion to generate variable BHF. Another method to reduce springback by post-stretching is to use the so-called lock bead or stake bead. An example of forming a hat shaped part with stake beads is seen in Figure 3. Figure 3a shows the tool setup at the start of deformation. In Figure 3b, the upper die moved down and formed the material up to the stroke where the beads start touching the blank. After this stage the blank is formed into the beads area and stretch force is applying on the side wall during the last few millimeters of the upper die stroke. Post-stretching with stake beads is used in the industry. Usually, the die design is such that a) the stake bead would not break or wear out easily, b) the stretching of the material in the bead itself does not cause any local fracture or cracks, and c) it provides adequate stretch at the side wall to reduce springback. As the blank is formed, when the beads start touching the sheet material, higher blank holder force is required to avoid any opening between the die and the blank holder. As a result, the total load on the die and press ram increases dramatically, as seen in the example shown in Figure 4. The load versus displacement curve shown in Figure 4 is the reaction force applied on the blank holder during the deformation. The reaction force is calculated from simulation with a constant gap between the die and the blank holder. However, in reality, when nitrogen cylinders or air cushion is used the blank holder force cannot be controlled or increased toward the end stroke as it can be done with a servo hydraulic cushion. Therefore, a high blank holder force should be applied at the beginning of the deformation stroke. This is one of the limitations of using stake bead for post stretching. 1
To minimize the increase of the force in forming with stake bead, for a given part geometry and blank material/thickness, the design of a die with stake bead requires the selection, or optimization, of a) stake bead radius and height (for a round bead). This selection must eliminate the possibility of fracture of the formed part in a) the part wall and b) the bead region. It is also desired to achieve this design objective with minimum force on the die, the max force F max, seen in Figure 4, should be as small as possible. A study of forming a hat shape part from 1.2 mm Al5182-O using a servo hydraulic cushion was performed at the Center for Precision Forming (CPF) in cooperation with Hyson Metalforming Solutions. Results showed that using a variable blank holder force can significantly reduce springback and side wall curl. Figure 5 shows the profile of the experimental result compared to numerical simulations. Simulations showed with both variable blank holder force (with appropriate selection of BHF) and servo hydraulic cushion and constant blank holder force and stake bead (with appropriate selection of the bed geometry), the side wall curl can be reduced. A study for developing a method for optimization the stake bead design is being conducted at the CPF at the Ohio State University. Tanmay Gupta (gupta.854@osu.edu) and Ali Fallahiarezoodar (fallahiarezoodar.1@ osu.edu) are graduate research associates and Dr. Taylan Altan (altan.1@osu.edu) is professor emeritus and director at the Center for Precision Forming (CPF) at The Ohio State University, 1971 Neil Ave., Room 339 Baker Systems Engineering Building, Columbus, OH 43210, 614-292-5063, https://cpf.osu.edu and https://ercnsm.osu.edu. Figure 1: Schematic of hat-shape bending using a variable BHF (no stake bead) 2
Figure 2: (a) Stress distribution before post-stretching; (b) Stress distribution after post-stretching 3
Figure 3: Stake Bead for Post-Stretching in hat bending; (a) die touches the blank, (b) hat shape drawing,(c) stake bead engaging before BDC and increase stretching in the wall 4
Figure 4: Example of the load versus stroke data in forming a hat shape part with stake bead Figure 5: Experiment and simulation result of hat shape bending with and without stake bead 5