Formability in Non-Symmetric Aluminium Panel Drawing Using Active Drawbeads

Formability in Non-Symmetric Aluminium Panel Drawing Using Active Drawbeads Rui Li, Klaus J. Weinmann (1) Michigan Technological University, Houghton, USA Received on December 21,1998

Abstract Aluminum is expected to gain popularity as material for the bodies of the next generation of lighter and more fuel-efficient vehicles. However, its lower formability compared with that of steel tends to create considerable problems. A controllable restraining force caused by adjusting the penetration of drawbeads can improve the formability of aluminum. The paper describes a spatial and temporal FE model for the analysis of this technique, especially in non-symmetric panel forming. Comparison of the results of numerical simulations with the corresponding experimental results shows that the predictions of strain distribution on the panel are in excellent agreement. Furthermore, FLD analysis indicates that the active drawbead concept is beneficial to the formability of Al 61 1 1-T4. Keywords: Aluminum Sheet, Drawbeads, Formability

1 INTRODUCTION

Driven by environmental and economic concerns, a global effort exists to develop a new vehicle which will be able to travel three times farther than today's comparable automobiles on the same amount of fuel. This challenge has resulted in increased attention directed to reducing the weight of vehicles. Aluminum, especially, Al 611 1-T4, is expected to be the next generation car panel material. However, aluminum suffers from considerably lower formability than steel. In sheet metal stamping, the quality of the final product depends mainly on being able to efficiently control metal flow. Failure by either tearing or wrinkling occurs due to excessive or insufficient restraint, respectively, of the blank. The drawbead has been widely utilized in sheet metal stamping operations for many years as a mechanism for providing proper restraining force to a sheet. However, today's industry usually employs fixed drawbeads in the tooling. It is known that the stamping process involves many parameters, such as lubrication properties, sheet geometry, blankholding conditions, etc. Many of them may keep varying during the process. Therefore, the drawbead penetration does not have to be constant. Accordingly, the required drawbead restraining force (DBRF) could be allowed to adjust frequently with the varying of local conditions. Results of strip drawing tests used to simulate a crosssection of deep drawing operations demonstrated the effectiveness of the active drawbead for controlling the DBRF [I], [2]. In 1996.Xu and Weinmann [3]extended the research to a rectangular box by using 3 0 finite element modeling. The work predicted that the drawability of the sheet could be very sensitive to the choice of drawbead control schemes and early

Annals of the ClRP Vol. 48/7/7999

penetration of the drawbead led to improved draw depth. This numerical conclusion was later experimentally verified by Bohn [4]. However, these studies were limited to symmetric part forming. In this paper, a finite element investigation of active drawbead technology in non-symmetric panel forming is described, which simulates an automotive front fender panel. A typical panel formed is shown in Fig. 3. The four corners of the panel have different combinations of punch and lower die radii and the four unsupportedwalls have different slopes with respect to each other. Geometric details about a similar tooling system were given by Hishida and Wagoner [5]. 2 Test Configuration.

Since early drawbead penetration, commencing when punch/blank contact is established, results in improved draw depth, a series of trajectories with different maximum drawbead penetrations was chosen as shown in Fig. 1. C1 through C4 refer to Cases 1

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through 4, respectively. Based on initial tryouts with Al 61 11-T4, a common blankholder force (BHF) of 36.65 kN was selected for all tests in order to produce successful parts without wrinkles and tears in all cases. The mainly mineral oil based lubricant was only applied to the flange regions of the blank allowing dry contact with the punch. The application density of lubricant was 1.55 g/m2. The average draw velocity was set at 12 mm/sec. Blanks, which are initially in rectangular shape with a size of 406.4 x 304.8 x 1 mm, were covered with circular grids with 2.54 mm diameter and 3.8 mm center spacing on both sides. After forming, the grids were measured to analyze strains in the plane of the sheet. The major strain EI occurs in the direction of sheet flow into the die, while the minor strain ~2 is normal to it. Note that the rolling direction of the sheet was oriented parallel to the length of the blank. Each test was intentionally stopped at a draw depth of 30 mm, which represents the maximum safe depth for the most sensitive of the trajectories studied (61.

anisotropy parameters ro and rs0 are 0.65 and 0.80 respectively. Isotropic elasticity is assumed with a Young's Modulus of 70,000 N/mm2 and a Poisson's Ratio of 0.31. The coefficient of friction between the sheet and the punch was taken as 0.25 due to the dry friction, and those between the sheet and the lower tooling, drawbeads, and blankholder were 0.1, It was assumed that the friction coefficients would remain constant during the operation.

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3 FINITE ELEMENT MODELING

Due to the non-symmetric shape of the part, the die tooling and the sheet had to be simulated totally without any simplification of the geometry. A corresponding finite element model of the tooling is illustrated in Fig. 2.

Table 1: FEA Process Parameters The simulation was ended at a draw depth of 30 mm for each case so that the FEA results could be directly compared with the experimental results. The simulation parameters are summarized in Table 1. Note that in order to reduce the simulation time, the punch speed was set as 120 mm/sec, which is 10 times the real draw velocity. Earlier studies about explicit finite element techniques for analysis of sheet metal forming have shown that the results after increasing the forming velocity by a factor of 10 are virtually indistinguishablefrom quasi-static results 171. 4 RESULTS AND ANALYSIS

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To examine the agreement of the finite element analysis with experimental data, strains are measured in five zones, A through E, as shown in Fig. 3.

A non-linear finite element code (ABAQUWExplicit) was used to simulate the forming process. All contact surfaces of the tooling were simulated using 4-node three-dimensional rigid surface elements (type R3D4), including the punch, the blankholder, the lower die and the drawbeads. All dimensions were modeled exactly as they exist in reality. The sheet was modeled with 4node bilinear finite-strain shell elements (type S4R), and five Gauss points were chosen for shell section integration. The von Mises yield criterion was used to model the plastic deformation characteristics of Al 6111-T4. It is assumed tha! the Al 6111-T4 blank satisfies the constitutive equation in the form of the power law:

where .the reference stress value K is 540 MPa and the work-hardening exponent n is set as 0.245. The

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Figure 3: FEA panel showing selected zones with element locations for strain analysis

Zone A is located in the corner most prone to fracture. Fourteen elements were selected from the middle of the flange, extending over the die shoulder into the side wall and down to the punch face. Zone B wraps around the die shoulder, along the centerline of the long parallel drawbead side of the panel, starting in the middle downstream from the drawbead location, running over the die shoulder and into the middle wall. Similarly. Zone C extends along the centerfine of the shortest straight edge of the cavity, starting from the edge of the flange, following the material into the die cavity and ending in the side wall as well. Zones D and E are located along the punch nose radii at those corners, where defects usually are first observed.

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Figure 7: Comparison of both FEA and experimental data with the FLD of At 6111-T4 in (a) Zone D and (b) Zone E, (m-measured, p- predicted) The strains in Zones A through C are illustrated in Figures 4 to 6 in accordance with the traverses shown in Fig. 3. In Zone A, the finite element predictions agree very well with the experimental data. Element A1 to A3 locate on the up-stream side of the die shoulder. The material experiences compression in this area since the absolute values of minor strain are higher than the major strains for all cases. Continuing with element A4, the major strains become higher than the absolute values of minor strain, meaning that near uniaxial tension dominates from the die shoulder into

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the side wall. Case 3 (5 mm trajectory) indicates more material flowing into the die cavity because of a relatively high strain level corresponding to this trajectory, which means that the 5 mm trajectory has the potential to produce a part with the deepest draw depth of the four cases. Element A12 is located right on the punch nose. A considerable jump of the strain value for this element and its near plane strain mode imply severe thinning deformation. The small strain values of elements A13 and A14 show that the punch face imparts only slight deformation under near biaxial tension conditions. The strains in Zone B are very small. The numerically higher major strains indicate only thinning deformation. Since both major and minor strains remain almost constant from 81 to 610, the thinning effect should distribute uniformly in this region. Excellent agreement is also obtained for the deformations in Zone C. Thickening defonnation is obvious on the outer flange area far from the die shoulder. However, right after the material passes over the die shoulder (Element 6), thinning dominates the mode of deformation. Since D and E are the most critical zones in which fracture can occur, the Forming Limit Diagram (FLD) generated by Xu and Weinmann [8] is employed here to study the strains in these zones in order to shed light on the formability of A1 6111-T4 for different drawbead trajectories. The results are plotted in Fig. 7. Only the two points most susceptible to tearing in each case were shown for clarification. Good agreement belween FEA results and experimental data can again be seen in both figures, although the FEA values usually tend to be smaller than experimental data due to !he limitation of FEA software on predicting necking and tearing. Note that both experiments and simulation were stopped at a safe depth of 30 mm. Therefore, no data were recorded above the forming limit curves. Case 3 generated the lowest strain levels for both FEA prediction and experimental measurement in both zones, while Case 1 shows the highest. Strain values of Cases 2 and 4 are lying between Cases 1 and 3. This phenomenon clearly indicates first that active drawbeads are indeed able to improve the formability of Al 6111-T4, and second that a 5 mm active drawbead trajectory (Case 3) results in the deepest draw for the cases considered in this study.

5 CONCLUSIONS A FE model was used successfully to study the effect of active drawbeads on formability of Al 6111-T4 in non-symmetric panel forming. The excellent agreement between the predicted and experimental data validates this model. Comparing both experimental and predicted strains to the FLD is further evidence that active drawbeads can be effective in improving sheet metal formability.

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6 ACKNOWLEDGMENT

This work was supported by the U.S. Department of Energy and ALCOA. 7 REFERENCES Weinmann, K.J., Michler, J.R., Rao, V.D. and Kashani, A.R., 1994, Development of a ComputerControlled Drawbead Simulator for Sheet Metal Forming, Annals of the CIRP, 43/1: 257-261 Michler, J.R., Kashani, A.R., Bohn, M.L. and Weinmann, K.J., 1995, Feedback Control of the Sheet Metal Forming Process Using Drawbead Penetration As the Control Variable, Trans. NAMRIISME, 23: 71-76 Xu, S.G. and Weinmann, K.J., 1996, An Investigation of Drawbead Control in Rectangular Box Forming by Finite Element Modeling, Trans. NAMRVSME, 24: 137-142 Xu, S.G, Bohn, M.L., and Weinmann, K.J, 1998, Drawbeads and their Potential as Active Elements in the Control of Stamping Operations, "Neuere Entwicklungen in der Blechumformung", University of Stuttgart, MAT-INFO WerkstoffInformationsgesellschaftmbH, 269-303 and Wagoner, R.H., 1993, Hishida, Y.. Experimental Analysis of Blankholder force Control in Sheet Metal Forming, SAE Technical Paper No. 930285 Li, R., 1998, Improving the Formability of A1 67 1 7 T4 by means of Drawbeads in Non-Symmetric Panel Forming, Master's Thesis, Michigan Technological University, Houghton Hibbitt, Karlsson and Sorensen, Inc. 1998 ABAQUS/Explicit User's Manual, version 5.6, HKS Xu, S.G.and Weinmann K.J., 1998, On Predicting the Forming Limit Diagram for Automotive Aluminum Sheet, Annals of the CIRP, 47/1: 177180