Simulation of Warm Forming Assisted Hemming to Study the Effect of Process Parameters on Product Quality

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1 07M-230 Simulation of Warm Forming Assisted Hemming to Study the Effect of Process Parameters on Product Quality Copyright 2007 SAE International Ricardo H. Espinosa Ford Motor Co. Shuvra Das, Jonathan Weaver University of Detroit Mercy ABSTRACT Current trends in the auto industry requiring tighter dimensional specifications combined with the use of lightweight materials, such as aluminum, are a challenge for the traditional manufacturing processes. The hemming process, a sheet metal bending operation used in the manufacturing of car doors and hoods, poses problems meeting tighter dimensional tolerances. Hemming is the final operation that is used to fasten the outer panel with the inner panel by folding the outer panel over the inner panel. Roll in/out is one of the main quality concerns with hemming, and keeping it under tolerance is a high priority issue for the auto manufacturers. Current hemming process technology, given the mechanical properties of current materials, has reached its saturation limit to deliver consistent dimensional quality to satisfy customers and at the same time meet government standards. Combining warm forming techniques with the traditional hemming process represents a new approach with the potential to overcome the current hemming limitation and to provide a satisfactory solution to all the requirements. The main objective of this research is to understand the effect of localized heating on the final quality in the hemming process by quantifying the influence of key geometrical and process parameters. To achieve this goal, a hemming finite element model, taking into consideration the mechanical properties as function of temperature is developed, and statistical methods to quantify the effect of key variables are employed. As an outcome to this study, the effectiveness of using warm forming techniques to improve hemming quality is assessed for A5182O aluminum, one of the most common used materials for this application. INTRODUCTION The use of materials (steel, plastic, aluminum, etc.) in the auto industry has evolved in the last century. In particular, much research has been done to understand and predict the behavior of sheet metal steel in stamping operations. Thanks to that knowledge, more aggressive exterior designs have been developed, but surface defects and dimensional variations are still a big issue [1]. With stricter passenger safety and fuel economy regulations, the auto industry faces the dilemma of reducing total vehicle weight without compromising the overall stiffness of the car. In order to fulfill both regulations, the auto industry has increased the use of aluminum as a lightweight alternative to steel in closure panels such as hoods, trunk lids, outer and inner panels, etc. [2]. Although the values of Yield Strength and Modulus of Elasticity are lower for aluminum than for steel, the specific stiffness (Modulus of Elasticity / Density) for both materials are comparable. Also, the specific strength (Yield Strength / Density) for aluminum is comparable to those of stronger steels. See Table 1 [3]. Table 1 Comparison table for different materials Material AKDQ Steel Modulus of Elasticity GPa Yield Strength MPa Density 10^3 Kg/m^3 Specific Stiffness 10^6 N-m / Kg Specific Strength 10^3 N-m / Kg HSLA AA5182O This enables replacing components made of steel with components made of aluminum with a minimum increase in thickness. Current trends in auto design to close the gaps even further between doors and the auto body [4] have in particular, complicated automation efforts for door assembly. Hemming is a secondary operation in door manufacturing that is the key contributor to the dimensional variation of doors [5]. Hemming is the final operation that is used to fasten the outer panel with the inner panel by folding the outer panel flange over the inner panel (Fig. 1). Roll in/out

2 (Fig. 2) is one of the main quality concerns with hemming, and keeping them under tolerance is a high priority issue for the auto manufacturers. Flanging Pre-Hemming Orange peeling Hemming Edge fracture Fig. 3 Aluminum superficial defects Fig.1 Flanging, pre-hemming and hemming sequences Rope Hem Roll In Roll Out Flat Hem Fig. 4 Hem types Customer perceived radius Fig. 2 Roll in/out Hemming aluminum presents two major technical challenges: minimizing orange peeling and superficial cracks (Fig. 3). The root cause for these quality defects is the lower formability of aluminum compared with low carbon steel. Flat hem is preferred by the auto industry since its craftsmanship index, a function of customer perceived hem radius (Fig. 4) and other geometrical factors, is superior to other hem types; unfortunately, due to its low formability, aluminum tends to crack if flat hemmed. Aluminum requires the use of rope hem geometry to avoid fracture; however, rope hem is difficult to control and results in pronounced aesthetical issues (roll in/out, warp / recoil, etc.) (Fig. 4) [6]. These formability problems greatly impact what has been defined as the Quality of the Looks which is an extremely relevant concept for automakers due to the importance customers pay to their perception of High Quality which is associated to what they see first, the exterior of the vehicle. In North American automotive closure panel applications, the outer panel is made of 6XXX (Al-Mg-Si) series aluminum alloys and the inner panel is made of 5XXX (Al-Mg) series aluminum alloys. This situation leads to recycling issues, since aluminum alloys need to be separated by alloy series to produce better recycled alloy. 6XXX series aluminum alloys are heat treatable and gain strength through work hardening as well as precipitation hardening and are considered medium strength alloys. The 5XXX series aluminum alloys are not heat treatable; they gain their strength through work hardening, which increases as the amount of Mg is increased.

3 An observation with respect to the forming behavior of 5XXX alloys is that at ambient temperature, they suffer from dynamic strain aging effects resulting in stretcher strain marks, which limit their application to inner automotive panels [7]. However, by heating it to a temperature below the re-crystallization temperature (warm forming process), its formability is greatly improved and stretcher strains do not occur [8]. Li and Ghosh [9, 10] have reported that 5XXX series have a better response than 6XXX series alloys when formed in the temperature range of o C. A change of paradigm is needed in order to increase aluminum formability during flat hemming that will ensure a robust process from the more stringent dimensional and aesthetical point of view. Since the forming behavior of aluminum is improved at elevated temperatures, one potential answer is to use warm forming techniques in sheet metal bending processes. This paper describes a computational design of experiments (CDOE) study to assess the effectiveness of localized heating (a warm forming technique) applied to flat hemming process (a sheet metal bending operation) on A5182O aluminum to improve roll in/out (hemming quality indicator), and this aluminum type was selected due to the availability of material characterization data. LITERATURE SURVEY In the US, research work done at Ohio State University Engineering Research Center for Net Shape Manufacturing [11, 12, 13, 14, 15, 16, 17, 18] and University of Michigan [19] have focused on understanding the effect that geometrical factors have on the final hemming quality of AKDQ steel. In Europe, research work at Chalmers University of Technology [20, 21, 22], University of Twente [23] PSA Peugeot Citroen [24] has focused on developing finite element models to predict roll in/out and warp / recoil using different types of elements (shell, solid) and material models (Hill 58, Barlat & Lian). In Japan, Toyota [4, 25] has published experimental and analytical research in hemming, focusing on minimizing Warp / Recoil. Most of the published research focuses on AKDQ steel hemming; only the works of Aztema [23], Debuire [24] and Muderrisoglu [17] studied aluminum hemming and the effect of geometrical factors on the final quality. Warm forming emerged as an attractive process to make complex aluminum parts, while taking advantage of the fact that at modestly elevated temperatures ( o C) surface defects disappear and forming limits improve significantly. Most of the research in aluminum warm forming lies in three categories: experimental studies to characterize aluminum behavior at elevated temperatures [26, 27, 28, 9, 10]; experimental studies of warm forming applied to deep drawing operations [8, 29, 30, 31, 32, 33, 34]; and finite element modeling of aluminum behavior under warm forming conditions [35, 36]. Different heating mechanisms for warm forming applications have been studied: localized laser heating of the blank [36, 35], electrically heated blank [9, 10, 15, 30, 31], and electrically heated die and holder [32, 33]. Also, research efforts to improve aluminum formability for door auto panels, from the chemical composition standpoint, have taken place in parallel to warm forming studies during the last decade [2, 37, 38, 39, 40]. In the same line of action, another research effort to improve aluminum formability has been reported by General Motors [41]. They report hemming improvements for 2XXX, 6XXX and 7XXX heat-treatable aluminum alloys series by induction heating of the area to be hemmed to a temperature above 250 o C for a period of seconds and then quenching the heated area to remove the agehardening effect. In summary, the metal forming community has taken two approaches to improve the hemming quality of aluminum panels to overcome its lower formability. The first approach focuses on the optimization of the geometrical and process factors of the hemming process: punch and die radii, clearance, thickness, pre-hemming path and punch force. The second approach focuses on improving aluminum formability by modifying alloy contents. Another approach using warm forming techniques to improve aluminum formability has been successfully applied to deep drawing operations but it has not yet been extensively applied to improve aluminum hemming. METHODOLOGY A commercial software tool, Abaqus Standard [42] was used for the finite element analysis. A sequentially coupled thermal-stress analysis model using a fourmode bi-dimensional linear heat transfer solid element (DC2D4) was used to simulate the localized heating process. Heat flux was applied in the gap between punch and die to simulate localized heating along the folding line. The temperature distribution (Fig. 5) was read at the subsequent hemming simulation as a predefined field mm 12 mm 1 mm Heat flux at 10 Nodes 45 o C 55 o C 75 o C 90 o C 75 o C Fig. 5 Temperature distribution

4 Maximum temperatures at the center of the folding line varied from 25oC (ambient temperature) to 285oC. The elastic and plastic mechanical properties of A5182O aluminum were characterized as a function of temperature (Fig. 6). The material model used in the finite element simulations was a linearly elastic linearly strain hardening plastic. Maximum temperature for this simulation was kept below 300oC, since heating A5182O aluminum to temperatures above 300oC may cause metallurgical problems due to structural changes within the aluminum blank [36]. The hemming model was validated for AKDQ steel at ambient temperature with published experimental results for vertical flanging force (Fig. 10), horizontal flanging force (Fig. 11), vertical hemming force and roll in/out. A5182-O Flow Stress vs. Temperature 4.5E E E+08 True Stress (Pascals) E E E E E E E Fig.7 Flanging model 0.20 True Strain Pre-Hemming Fig. 6 Material characterization The three main phases of the hemming process were simulated using a single punch tool with three different profiles. For the flanging simulation, the punch profile was described by its radius (Rp). Punch radius was kept at 10 mm for all flanging simulations. Abaqus Standard was also used to perform the stress analysis of the hemming process using a four-node-bidimensional incompatible mode generalized plain strain element (CPEG4I) to model the blank. This element is enhanced with incompatible modes to improve bending simulation. For the flanging simulation (Fig. 7), holder and punch were simulated using analytical surfaces, and the die was modeled using rigid elements (R2D2) with contact pairs defined between punch and blank, die and blank, holder and blank. For the pre-hemming simulation (Fig. 8), a 45 degree pre-hem tool following a vertical path was used. Contact pairs were defined for the prehemming tool and blank, inner panel and blank, holder and blank. Pre-hem displacement was defined as the vertical distance traveled by the pre-hem tool until it was tangent to the flange. The hemming stage (Fig. 9) was simulated using a flat hemming tool following a vertical path. Contact pairs were defined for the hemming tool and blank, inner panel and blank, holder and blank. The hemming stroke was defined as the distance traveled by the hemming tool that positioned it at three times the blank thickness above the reference plane (holder upper surface). Pre-Hemming Tool Face Angle = 45 Pre-Hemming Stroke Initial contact position Pre-Hem Displacement Inner Panel Final contact position Holder Fig. 8 Pre-hemming geometry Hemming Tool Initial contact ii Hemming stroke Final contact position 3 * blank thickness Inner Panel Holder

5 Fig. 9 Hemming geometry The discrepancy between the analytical and experimental results for horizontal flanging force (Fig. 11) is attributed to the fact that Abaqus simulations could not incorporate a clearance self-adjustment performed by the punch while it was increasing its contact with the blank, as reported by the authors of the published experimental data that observed the same condition on their finite element simulations [14, 15, 16]. In summary, the shapes of the simulations for vertical flanging force, horizontal flanging force and vertical hemming force closely resembled the shape of the experimental recorded data and the hemming model results for roll in/out were in good agreement with experimental results. The main factors affecting the hemming quality can be grouped into three main categories: process variables, geometry and material properties. Hemming literature [11 to 17] reports that for a given material and a prehemming path, die radius and clearance between punch and die are the two most critical factors affecting final hemming quality. These two factors plus maximum blank temperature along the folding line were the input variables under study for the hemming process with localized heating simulation. To isolate the effect of temperature on final hemming quality indicators, an adiabatic hemming process was assumed. The temperature distribution obtained during the heating stage was kept for the whole hemming process: flanging, pre-hemming and flattening. During the last step of the simulation, the material was cooled down to ambient temperature and measurements for roll in/out were recorded (Fig. 12). AKDQ Steel Comparison of Vertical Flanging Force for Clearance = 1.09 mm Abaqus Standard (friction =0.0, 0.1), Abaqus Explicit (friction=0.1) and Experimental Data Abaqus Explicit µ = 0.1 Abaqus Standard µ = 0.1 Vertical Flanging Force (Newtons / mt) Abaqus Standard µ = 0.0 Experimental a) Blank at ambient temperature Abaqus Standard friction=0.0 Abaqus Standard friction=0.1 Abaqus Explicit friction=0.1 Experimental 273 o C 285 o C 210 o C 190 o C Fig. 10 Vertical flanging force model comparison for AKDQ steel AKDQ Steel Comparison of Vertical Flanging Force for Clearance = 1.09 mm Abaqus Standard (friction =0.0, 0.1), Abaqus Explicit (friction=0.1) and Experimental Data b) Temperature distribution from the localized heating simulation Abaqus Standard µ = o C Horizontal Flanging Force (Newtons / mt) Abaqus Standard µ = 0.1 Experimental 285 o C 210 o C 190 o C Abaqus Explicit µ = 0.1 c) Geometry after flanging phase o C Abaqus Standard friction=0.0 Abaqus Standard friction=0.1 Abaqus Explicit friction=0.1 Experimental Fig. 11 Horizontal flanging force model comparison for AKDQ steel 285 o C 210 o C 190 o C

6 d) Geometry after pre-hemming phase 273 o C 285 o C 210 o C 190 o C e) Geometry after hemming tool is at its final position Flange Length L = {12} mm Clearance C = {1.00, 1.09, 1.34, 1.50} mm Die Radius Rd = {0.5, 3.0} mm Die 25 o C 25 o C 25 o C 25 o C f) Hemmed part is cooled down to ambient temperature Punch Punch Radius Rp = {10} mm Holder Blank Thickness t = {0.838} mm RESULTS Fig. 12 Hemming sequence simulation An analytical design of experiments (ADOE) using a 4X2X5 General Linear Model was developed to assess the effect of clearance, die radius and temperature on hemming roll in/out for A5182O aluminum. A total of 40 finite element analysis simulations were run. Table 2 shows the combination of parameters simulated and Table 3 shows the codes for the plotted curves. Geometrical parameters used in this study are shown in Fig. 13. Table 2 Parameter values for 4X2X5 General Linear Model Material A5182O Clearance (mm) 1.00, , 1.50 Die Radius (mm) 0.5, 3.0 Temperature ( o C) 25, 89, 154,, 285 Table 3 Coding for 4X2X5 General Linear Model Clearance Die Radius Code Clearance Die Radius Code rhc RHc rhc RHC rhc RHC rhc RHC150 Fig. 13 Hemming parameters Roll in/out is defined as the distance that the bent knee moves in or out from the flanging plane. This is a critical parameter that affects the quality of the hem. One of the main hypotheses to be proven with this study is to find out if there is a beneficial effect on roll in/out by using localized heating on the hemming process. Fig. 14 shows that at ambient temperature baseline geometry, rhc100, roll in/out is at the lowest level ( mm). Main effects plot for roll in/out after hemming (Fig. 15) shows that clearance, die radius and temperature are statistically significant factors. The interactions plot (Fig. 16) shows that there is a strong interaction between die radius and temperature for large die radius. On the other hand, the other two factor interactions, clearance and die radius and clearance and temperature have no effect on roll in/out since their lines are approximately parallel. Roll In / Out (mm) RHC150 RHC109 rhc RHC100 rhc rhc109 rhc A5182O Roll In / Out (mm) rhc150 RHC150 rhc134 RHC134 rhc109 rhc100 RHC109 RHC Temperature (C) rhc100 rhc109 rhc134 RHC100 RHC109 RHC134 rhc150 RHC150

7 Fig. 14 Roll In/Out versus temperature for all simulations For small die radius geometries, temperature has little effect on roll in/out values; as temperature increases, roll in/out values remain almost constant. But for large die radius, temperature has a significant effect on reducing roll in/out as temperature increases. On average, a 0.6 mm reduction in roll in/out is achieved for large die radius geometry at any clearance. Roll In / Out Roll (mm) Fig. 15 Roll in/out main effects plot Clearance (mm) Main Effects Plot for A5182 Roll In / Out Clearance (mm) Die Radius (mm) Temperature ( o C) Clearance Die Radius Temperature Interactions Plot for A5182O Roll In / Out In this study, the effect of temperature on roll in/out for flat hemming on A5182O aluminum was investigated. For that purpose, a sequential heat transfer stress analysis finite element model was developed to simulate localized heating applied to the flat hemming process. The results from this model were validated at ambient temperature using experimental data for AKDQ steel. The input variables for this analysis were Temperature, Die Radius and Clearance. A 4X2X5 General Linear Model was used to statistically analyze a total of 40 simulations for all the different combinations of input variables. Roll in/out results from the statistical analysis indicate that as temperature increases geometrical parameters continue being relevant. Specifically, for small die radius geometries, A5182O aluminum material is insensitive to temperature changes; in this case, geometrical factors are more relevant. On the other hand, for large die radius, A5182O aluminum material is very sensitive to temperature changes. Roll in/out is reduced 62% for a 1.0 mm clearance and 38% for a 1.5 mm clearance. In order to fully assess the impact of localized heating on roll in/out, follow up work includes: studying the effect of temperature in warp / recoil behavior, another critical hemming quality indicator; and the effect of localized heating in reducing the stresses along the folding line, a critical area where cracks appear while folding A5182O at ambient temperature. Although no experimental data was available to validate the hemming model with localized heating for AKDQ steel and A5182O aluminum, the results from the simulations are directional and show that there is a potential to improve the quality indicators by using warm forming techniques applied to the hemming process with A5182O aluminum. Future work also includes validating the results from the finite element simulations by collecting data from an experimental setting for localized heating applied to the hemming process Die Radius (mm) 25 o C Temperature ( o C) 285 o C Fig. 16 Roll in/out interactions plot CONCLUSION Roll In / Out (mm) REFERENCES 1. Dutton, T, Pask, E (1998), Simulation of surface defects in sheet metal panels, Simulation of Materials Processing: Theory, Methods and Applications, Huetink & Baaijens (eds), Balkema, Rotterdam, pages Miller, W, et al (2000), Recent development in aluminum alloys for the automotive industry, Material science and technology, A280 pages Mangonon, P (1999), The principles of materials selection for engineering design, Prentice Hall. 4. Umehara, Y (1990), Technologies for the more precise press-forming of automobile parts, Journal of Materials Processing Technology 22 pages Eagle, B (1994), An evaluation of the relationships of automobile door distortions to variations in material properties and production operating parameters, MS

8 Thesis MIT Sloan School of Management, Cambridge, MA. 6. Friedman, P (2001), Craftmanship in hemming of aluminum closure panels, Presented at 2001 Windsor workshop, Windsor, Ontario, June 5, Katgerman, L, Eskin, D (2003), Hardening, annealing and aging, Handbook of aluminum, CRC. 8. Bolt, P, et al (2001), Feasibility of warm forming of aluminum products, Journal of Materials Processing Technology 115 pages Li, D, Ghosh, A (2003), Tensile deformation behavior of aluminum alloys at warm forming temperatures, Materials Science and Engineering, A352 pages Li, D, Ghosh, A (2004), Biaxial warm forming behavior of aluminum sheet alloys, Journal of Materials Processing Technology, 145 pages Livatyali, H, Larris, S (2004), Experimental investigation on forming defects in flat surfaceconvex edge hemming: roll, recoil and warp, Journal of Materials Processing Technology, Vol , pages Livatyali, H et al (2004), Experimental investigation of forming defects in flat surface-convex edge hemming, Journal of Materials Processing Technology, Vol. 146, no. 1, pp Feb Livatyali, H et al (2003), Computer aided die design of straight flanging using approximate numerical analysis, Journal of Materials Processing Technology, 142 pages Livatyali, H et al (2002), Prediction and elimination of springback in straight flanging using computer-aided design methods. Part 2: FEM predictions and tool design, Journal of Materials Processing Technology, 120, pages Livatyali, H et al (2001), Prediction and elimination of springback in straight flanging using computer-aided design methods. Part 1: Experimental investigations, Journal of Materials Processing Technology, 117, pages Livatyali, H et al (2000), Improvement of hem quality by optimizing flanging and pre-hemming operations using computer-aided die design, Journal of Materials Processing Technology, 98, pages Muderrisoglu, A, et al (1996), Bending, flanging, and hemming of aluminum sheet-an experimental study, Journal of Materials Processing Technology, 59, pages Wang, Ch, et al (1993), Mathematical modeling of plane-strain bending of sheet and plate, Journal of Materials Processing Technology, 39, pages Zhang, G, Wu, X, Hu, S (2001), A study on fundamental mechanisms of warp and recoil in hemming, ASME Journal of Engineering Materials and Technology, October 2001, Vol Svensson, M, Mattiasson, K (2002), The influence on the roll-in hemming. A comparison between FEsimulations and practical tests.numisheet Svensson, M, Mattiasson, K (2002), Threedimensional simulation of hemming with the explicit FE-method, Journal of Materials Processing Technology, 128, pages Svensson, M (2002), Hemming simulation, Simulation of materials processing: theory, methods and applications, Huetink & Baaijens (eds), Balkema, Rotterdam. 23. Atzema, E, Baartman, R, Klomp, A (1998), Finite element simulations of the hemming process, Simulation of materials processing: theory, methods and applications, Huetink & Baaijens (eds), Balkema, Rotterdam. 24. Debuire, F, Zwilling, V (2002), Experimental and numerical approaches of hemming: application on steel and aluminum 6016, SAE paper Iwata, N, et al (1995), Improvements in finiteelement simulation for stamping and applications to the forming of laser-welded blanks, Journal of Materials Processing Technology, 50, pages Altan, T, et al (2002), Warm forming of aluminum alloys, thefabricator.com, article published on January 31, Krajewski, P (2005), The warm ductility of commercial aluminum sheet alloys, SAE paper Morris, L, George, R (1977), Warm forming highstrength aluminum automotive parts, SAE paper Cavaliere, P (2002), Hot and warm forming of 2618 aluminum alloy, Journal of light metals, 2 pages Naka, T, et al (2001), The effects of temperature and forming speed on the forming limit diagram for type 5083 aluminum-magnesium alloy sheet, Journal of Materials Processing Technology, 113 pages Naka, T, et al (2003), Effects of temperature on yield locus for 5083 aluminum alloy sheet, Journal of Materials Processing Technology, 140 pages Moon, Y, et al (2003), Effect of tool temperature on the reduction of the springback of aluminum sheets, Journal of Materials Processing Technology, 132, pages Moon, Y, et al (2003), Tool temperature control to increase the deep drawability of aluminum 1050 sheet, International Journal of Machine Tools and Manufacture, 41, pages Schuocker, D (2001), Mathematical modeling of laser-assisted deep drawing, Journal of Materials Processing Technology 115, pages Van den boogaard, A, Bolt, P, Werkhoven, R (2003), Modelling of AlMg sheet forming at elevated temperatures, Material forming processes, Anne Marie Habraken (eds), ISTE Publishing Company.

9 36. Takuda, H, et al (2002), Finite element simulation of warm deep drawing of aluminum alloy sheet when accounting for heat conduction, Journal of Materials Processing Technology, 120 pages Lahaye, Ch, et al (1999), Improved AA5182 aluminum alloy as preferred choice for critical forming operations, SAE paper Lahaye, Ch, et al (2000), Aluminum alloy selection for closures with respect to functional demands, SAE paper Daniel, D, et al (1999), Development of 6XXX alloy aluminum sheet for autobody outer panels: bake hardening, formability and trimming performance, SAE paper Chen, Z, et al (2004), Numerical and experimental investigation of 5XXX aluminum alloy stretch flange forming, SAE paper Krajewski, P, et al (1999), Method for improving the hemmability of age-hardenable aluminum sheet, U.S. Patent 5,948, Abaqus User s Manual Version 6.3. CONTACT Ricardo H. Espinosa, Ford Motor Co, ATNPC MD 214, Plymouth Road, Livonia, MI 48150, , respino4@ford.com Shuvra Das, Professor, Mechanical Engineering Department, University of Detroit Mercy, 4001 W. McNichols Road, Detroit, Mi , , dass@udmercy.edu Jonathan Weaver, Associate Professor, Mechanical Engineering Department, University of Detroit Mercy, 4001 W. McNichols Road, Detroit, Mi , , weaverjm@udmercy.edu