Reduction of springback in hat-bending using variable blank holder force using servo hydraulic cushion

Transcription

1 #688-3 April 2018 Stamping Journal Reduction of springback in hat-bending using variable blank holder force using servo hydraulic cushion By Tanmay Gupta, Ali Fallahiarezoodar, Ethan McLaughlin and Taylan Altan Ever increasing needs of the automotive industry to improve fuel efficiency and crashworthiness by using lighter and higher strength materials necessitates the use of advanced high-strength steels (AHSS) and high-strength aluminum alloys. However, when using these materials, springback is one of the most difficult and important challenges, which causes geometrical and dimensional inaccuracies in the formed part. Therefore, to ensure part quality and to compensate for springback, some automotive dies are modified and re-cut many times during tryouts. Towards understanding the springback phenomenon, in this study a relatively simple and two dimensional hat-shaped bending operation is investigated by the Center for Precision Forming (CPF) at The Ohio State University, with cooperation with Hyson Metal Forming solutions. Post-stretching to reduce side wall curl Figure 1: Schematic of hat-bending operation Hat-shape bending (Figure 1) is a forming operation in which significant amount of the sheet metal used undergoes bending. Thus, the material has a natural tendency to unbend itself due to the residual bending stresses created in the side wall of the hat-shaped part. This causes the side wall to curl, causing springback. In order to remove these unwanted stresses in the final formed part, the side wall is made to undergo stretching in the final stages of the forming operation by restraining the material flow. This stretching after the bending is called as Post-stretching. This post-stretching eliminates the compressive stresses in the side wall as shown in the Figure 2, thereby reducing the springback in the final part. Many 1

2 studies conducted previously show that applying stretch force towards the end of the forming stroke leads to springback reduction. Figure 2: Stress distribution in the side wall before (left) and after stretching (right) Variable blank holder force using hydraulic cushion In this study, a 300 ton Komatsu servo press and servo hydraulic cushion, located at Hyson Metalforming solutions, were used to carry out experiments. Servo presses have the capability to increase production rate as compared to mechanical presses, through shorter cycle time (higher strokes per minute), availability of pendulum mode, and reduced stroke length. The force required to stretch the blank can be generated by draw beads and pneumatic (air or nitrogen) or hydraulic cushions. In this study, the poststretching was performed using hydraulic cushion by increasing the cushion force towards the end of the forming stroke to induce stretching (Figure 3). With servo control, the cushion force can be varied during the stroke. Thus, the BHF can be varied to obtain the optimum metal flow control, based on part geometry and sheet material thickness and properties. Servo hydraulic cushions allow the operator to change the BHF during the press stroke, within the limits of the specific inertia of the hydraulic system. This level of accuracy is made possible by the closed-loop control inherent in servo hydraulic cushion systems. The results, obtained through the finite element simulations (Figure 4), show that a variable blank holder force system can use lower binder forces at the beginning of the stroke to maintain a BHF that allows the material flow into the die cavity. The binder force is then raised at the end of the stroke to obtain the deformation required within the forming limits i.e. maximum thinning in the part should not exceed the limit value (taken as 15% for Aluminium) to eliminate springback and side-wall curl. In contrast, usage of constant BHF results in little springback reduction. FEA simulations were also performed so as to mimic the experimental conditions which also show good prediction of the springback results obtained in the experimental setup for Al5182-O (1.2 mm) (Figure 5). 2

3 Figure 3 Schematic illustrates variable and constant blank holder force (theoretical, cushion inertia effect is not considered) used in the study Figure 4: Reduction of springback in Al5182-O (1.2 mm) due to Variable Blank Holder Force using FEA simulations 3

4 Figure 5: Experimental profiles after hat-bending operation with constant BHF (400 kn) and variable BHF (100 to 700 kn, as shown in Figure 3) Factors affecting springback Studies conducted by the Center for Precision Forming (CPF) at The Ohio State University indicates clearly that in addition to part geometry (usually specified by the designer or OEM), the main parameters that affect the magnitude of springback are flow stress and E-modulus along with the friction conditions present. Estimating the overall friction coefficient plays a major role in the prediction of springback through simulations and is often difficult. Coefficient of friction (CoF) in stamping operations can be evaluated by strip drawing test, twist compression test, cup drawing test etc., however in this study a relatively simple methodology was adopted to evaluate the friction by considering drawing-in of the material between the die and blank holder. Drawing-in of the material is directly related to the CoF present in the flanges and it decreases with higher friction. Flange lengths of the experimental samples corresponding to each case blank holder force were measured and compared to the flange lengths obtained through FE simulation. The CoF which matches the experimental flange length is considered as the input in the FE simulations. In this study, effects of plastic anisotropy and the yield function (Hill 1948 and Barlat 1996 criteria) were also used in prediction of springback in FE simulation and were found to have little influence on the final results. Ethan McLaughlin (EMcLaughlin@hysonsolutions.com) works at HYSON Metal Forming Solutions, Brecksville Road, Brecksville, OH 44141, 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 4

5 The Ohio State University, 1971 Neil Ave., Room 339 Baker Systems Engineering Building, Columbus, OH 43210, , and 5