Advances in Metal Forming Research at the Center for Precision Forming - Industry/University Cooperation –
T. Altan, P. Sartkulvanich, N. Kardes The Center for Precision Forming (CPF), The Ohio State University, Columbus, Ohio, USA

Abstract The demand for application for lightweight materials such as Ultra/Advanced High Strength Steels (U/AHSS), aluminum alloys, magnesium alloys and boron steels in automotive industry is increasing to reduce vehicle weight and increase crash performance. The use of these relatively new materials requires advanced and reliable techniques to a) obtain data on material properties and flow stress, b) predicting springback and fracture in bending and flanging, c) selecting lubricants and die materials/coatings for stamping and forging and d) designing tools for blanking and shearing. In addition, designing the process and tooling for a) hot stamping of boron steels, b) warm forming of Al and Mg alloys, and c) optimizing the use of servo-drive presses require advanced Finite Element based simulation methods. CPF is conducting R&D in most of these topics and also in many hot and cold forging related topics. This paper gives an overview of this research and discusses how the research results are applied in cooperation with industry. Keywords: Metal Forming, Sheet metal, Forging, FEM

1 INTRODUCTION The Center for Precision Forming (CPF) has been established with funding from the National Science Foundation (NSF) and a number of companies (www.cpforming.org). CPF is an outgrowth of the Engineering Research Center for Net Shape Manufacturing (ERC/NSM – www.ercnsm.org) and conducts research in sheet metal forming while ERC/NSM focuses on cold and hot forging related R&D projects. Both Centers work closely with industry under contract. Thus, the graduate students (MS and PhD level) as well as post doctoral researchers gain experience in working with industry and learn how best to respond to industrial needs and inquiries. This page discusses the non-proprietary aspects of the projects conducted at CPF and ERC/NSM. 2 FORGING AND CASE STUDIES

These simulations consider the variation of the die material hardness with temperature, the lubrication and cooling of the dies, the heat transfer between the forging and the die, and the die stresses that change with temperatures. Thus, using simulation results with the experimental data, obtained under production conditions it is possible to analyze the effect of forging temperatures and pressures upon die life. It is also possible to explore the use of carbide and ceramic materials for extending die life, under cost effective conditions. Finally, recommendations are developed as to the use of best die materials and coatings in both extrusion and coining dies.

2.1 Engine Valves In a 3 year study conducted for a major automotive supplier, the ERC/NSM investigating how to improve die life and the precision of hot forged valves for internal combustion engines, Figure 1. The objectives of this project include a) the prediction of abrasive wear and plastic deformation in dies used for hot/warm extrusion and forging of various valve materials, and b) the determination of “best” process parameters (die materials and coatings, billet/die temperatures, press speed, cycle time) to increase die life and precision of forged parts. To achieve the objectives, the ERC/NSM is simulating the extrusion and coining operations used in valve forging, with the commercial software DEFORM. Thus, it is possible to estimate temperatures, plastic deformation and abrasive wear in the dies, after several forging cycles.

Figure 1: Engine valve extrusion and coining tooling schematics

6th International Conference and Exhibition on Design and Production of MACHINES and DIES/MOLDS 23-26 JUNE 2011 ATILIM University, Ankara, TURKEY

2.2 Precision Forging of Gears In precision forging of bevel and spur gears, it is useful to identify the causes for premature die failure (fracture or wear). For this purpose, DEFORM simulations are used and the areas of stress concentration in the forging dies are determined. Thus, it is possible to understand the reasons for premature die fracture and to modify the die design (corner and fillet radii, as well as the shrink fit conditions and stresses of the die assembly) to improve die life. Examples of the analysis, conducted in this study, are seen in Figures 2 and 3.
Start of deformation End of deformation
Concentration of stresses and crack location on die

Die Holder

Die

Workpiece

Detail A z Punch y

and cycle time. Coining is a closed die bulk metal forming operation, in which all surfaces of the work material is confined within the die, resulting in a well defined imprint of the die on the workpiece. Due to this inherent characteristic, coining process can be used to manufacture micro components with intricate features. Also, high production efficiency gives advantage to coining process for micro forming compared to other material removal processes, such as grinding and etching. At CPF, Finite Element Analysis (FEA) was used to help in designs of the alternate manufacturing process for surgical slit knife. The proposed manufacturing process for the knife, produced without grinding, involves a) flattening of the wire to a preform, b) coining the final geometry with flash, and c) ECMing to finish the edges and eliminate the flash. Sponsor was interested in designing the flattening and coining dies (Figure 4a). The common problem was the failure/breakage of the die due to excessive stresses. With FEA, the optimum preform that would fill the die cavity with minimal die stresses could be determined, whereas any necessary modification of coining die could be considered to reduce the die stresses. As shown in Figure 4a, metal flow and die filling at different cross section could be analyzed. In Figure 4b, amount of coining load from forming simulation was transferred to elastic-die simulation for stress analysis in the coining die to assure that the stresses are within limit.

Figure 2: An example FEA of gear forging (schematic)

Detail A

Example of effective stress distribution
1400 1200 1000 800 600 Maximum Effective Stress

Effective Stress (MPa)

(a)
40% relief of stress at critical area Current Larger Larger die Tapered Die Interference corner radius

400 200 0

Figure 3: Example of improving die design (Stresses are predicted by FEA. By modifying the size of die corner radius, it is possible to reduce stresses.) 2.3 Precision Coining of Medical Devices Conventional manufacturing process of the surgical slit knife involved coining a wire of diameter less than 1 mm to get a preform of thick flat, followed by trimming and grinding to get the sharpness of the blade. Grinding is an expensive and time-consuming process and replacement of coining would significantly reduce manufacturing cost

(b) Figure 4: (a) Metal flow and die filling at the 2D cross sections and (b) stress analysis in the coining die.

3

SHEET FORMING – MATERIAL PROPERTIES

3.1 Biaxial Bulge Test Flow stress properties are required for computer simulation and analysis of sheet forming process. Usually, these data are determined using tensile test. However, the data obtained from tensile test are only for relatively small strains (up to 0.2 for Advanced High Strength Steel (AHSS)). At CPF, Viscous Pressure Bulge (VPB) test (in Figure 5) was developed to determine the flow stress data under biaxial deformation, which the data at larger strains (up to 0.4 for AHSS) could be obtained [1-2]. In VPB test, the sheet is clamped between the lower and upper die. When the viscous medium in the lower chamber is pressurized by a punch, the sheet will be bulged into the cavity of the upper die. The sheet is only stretched, without drawing. When the deformation of the material exceeds formability, the bulged sheet will fracture at the top of the dome. VPB tests of various AHSS (e.g. DP600, TRIP 780 and DP980) and other sheet metals were conducted for several industrial sponsors. VPB tests were also used to evaluate variation of the formability of the same material but for different batches/coils.

Figure 6: Deep drawing with stretching test 4 SHEET FORMING – LUBRICAITON AND DIE MATERIALS

(a)

(b)

Figure 5: Schematic of VPB test (a) before the test and (b) after the test

3.2 Stretch Bending Research in stamping of AHSS indicates that, under certain circumstances such as stretch-bend over a small die radius, sheet metal fails at a strain level below the corresponding material forming limit. This type of failure, generally referred as shear fracture, raises manufacturing difficulty and constraints the product design. The common tests have been used to evaluate sheet bending under tension are stretch bending (SBT) and bending under tension (BUT) tests [3-5]. At CPF, preliminary bending experiments and FE simulations were conducted to establish a procedure to predict fracture (shear vs. tensile) in wedge bending and SBT. Currently, tooling of deep drawing with stretching is being developed to study the influence of punch radius and stretching level on bending failure, see Figure 6. Tooling is designed similar to BUT test tooling but allow investigation of fracture when bending over three dimensional curvature surface.

4.1 Evaluation of Lubricants New materials, such as galvanized AHSS, aluminum alloys, stainless steels, require advanced lubrication systems to enhance formability and reduce scrap rates. CPF has been evaluating lubricants as well as die materials/coatings for various automotive companies and suppliers by emulating the conditions that exist in stamping production. Two main tests were developed, i.e. cup drawing test and strip drawing test, in Figure 7. In cup drawing test [6], a round blank is drawn into a round cup. In strip drawing test, a 14”x1” strip is drawn into a hat-shaped strip. Maximum drawing load without fracture and draw-in length (or flange perimeter) are measured in both tests for evaluation of lubricated condition. The developed cup drawing and strip drawing tests were successfully used to evaluate lubricants in a laboratory setting. 4.2 Ironing to Evaluate Die Materials / Coatings With some modifications of test tooling used in evaluation of lubricants (4.1), galling of different tool materials/coatings can be evaluated through strip ironing test [7]. In this test, the blank is drawn and then ironed by reducing clearance through adjustment of the ironing die (Figure 7c). The thickness of the sheet metal is significantly reduced (~10-15%) intentionally to accelerate the tribo-test. The objectives of this project are to determine i) best lubricants, ii) best die materials and iii) coefficient of friction for use in FE simulations.

Before Springback

After Springback

(a)

Figure 8: Prediction of springback with FEA in v-die bending (in cooperation with Cincinnati Inc.)

(b)

(c)

material: 1mm thickness: DP 780
U-flanging without stretching U-bending without stretching

Figure 7: (a) cup drawing test, (b) strip drawing test and (c) strip ironing test for evaluation of lubricants and die materials/coatings 5 SHEET FORMING – SPRINGBACK One of the major challenges in stamping AHSS is the springback that leads to dimensional inaccuracy in the formed part. Many research studies demonstrate that the elastic modulus of AHSS decreases with increasing plastic strain and it is necessary to consider the unloading modulus variation for accurate springback predictions [8-9]. At CPF, tensile tests, bending tests with V-die and S-die were conducted to determine the variation of unloading modulus as well as validate springback prediction from FE simulations. In our recent study, the effect of unloading apparent modulus (Emodulus) variation on the accuracy of springback prediction in V-die bending of DP 780 is investigated. Load-unload tensile tests were performed to obtain unloading apparent modulus variation as a function of strain. Springback in V-die bending was estimated using FEA with constant vs. variable E-modulus. Compared with experimental data, the predictions using two FEA codes (DEFORM & PAM-STAMP) gave reasonably accurate results, as shown in Figure 8. Springback in bending and flanging are also investigated using S-die, Figure 9. The purpose is to study the influence of modulus variation as well as validate springback prediction from FE simulations for three dimensional bending surfaces. Figure 9b illustrates bending and flanging experiments for various possible curvature geometries and bending modes, using S-die. 6 BLANKING AND FLANGING Blanking and shearing of AHSS presents some difficulties due to a very high shearing load that could significantly reduce tool life and yield poor quality of the sheared edge. In addition, the formability of the edges subjected to tension such as stretch flanging is highly influenced by the edge quality [10].

U-flanging with stretching U-bending with stretching

S-shape forming with – without stretching

(a)

(b)

Figure 9: Bending experiments using S-die to determine springback of various deformation modes. (a) Press and tooling, (b) various shapes formed using the S-die It is necessary to fundamentally understand shearing process of AHSS and its influence upon edge stretching. CPF has collaborated with the companies and research institutes to conduct studies on shearing, blanking and flanging of AHSS. The objectives are to understand the effect of punch/die clearance, radii, punch shape, alignment, and holder pressure upon edge quality and edge cracking due to stretching. Figure 10a shows FE simulation of blanking, applying ductile fracture criteria. Findings indicate that with proper flow stress input and fracture models, the sheared edge geometries could be determined from FE simulation. Figure 10b shows our attempt to establish a methodology to predict edge cracking in flanging by considering the sheared edge quality for various burr orientations. The study indicated that it is possible to use FEA to estimate how process variables and blanking conditions affect fracture in hole flanging. In another project supported by the sponsor, the designs of fins manufacturing and tube-fin assembly for heat exchangers were evaluated with the help of FEA. First, a pierced blank was formed into a fin using flanging and then pressed fit on a straight tube. Several process

parameters and fin geometries were varied to understand their effects on radial (clamping) force, axial force, forming stresses, etc. The optimum combination of parameters was determined and reported to the sponsor.

(a)

Figure 11: Comparison of thickness distributions obtained from FE simulation and experiment (for punch stroke=1.00 in., average temperature= 150 C, and maximum pot pressure= 840 psi).
Burr Up Burr Down
Perfect Edge

(a)

(b) rpe

(c)

Die

↑

Sheet

(b)

Punch

Blankholder

Figure 10: (a) Experimental and simulated edge geometry and from blanking of DP 590, using 13.5% punch/die clearance, (b) hole flanging simulation with 3 different burr orientation using a conical punch 7 WARM FORMING – AL AND MG Aluminum and magnesium alloys are being increasingly used in automotive applications, also due to their lightweight and high strength-to-weight ratios. However, their applications are limited to shallow parts because of their low formability. Recent research demonstrated that these materials had higher formability when processed at elevated temperature. The CPF has collaborated with industrial partners to conduct research on hydroforming of these materials. Our recent attempt is an analysis of the sheet hydroforming with punch (SHF-P) of AA 5754O. FE simulations and experiments of SHF-P were conducted to determine the process parameters (blankholder force and pot pressure) that could form a part (a cylindrical cup with a reverse bulge) successfully at room and elevated temperature (about 150 C). Flow stress data of AA 5754-O used in FE simulation was obtained from [11]. This study shows that the SHF-P at elevated temperature can form a cup with larger cup height and better reverse bulge profile than forming at room temperature. Figure 11 illustrates comparison of sheet thickness distributions of a cup formed at elevated temperature, from FE simulation and experiment. 8 HOT STAMPING Hot stamping (also known as hot press forming or press hardening), developed in 1980s, has been increasingly used to manufacture crash-resistant components (e.g. side impact beam, pillar).

In this process, the boron steel blanks are first heated to austenite temperatures (900-950 C) inside a furnace and subsequently transferred to an internally cooled die set. The formed part is cooled down very rapidly inside the closed die set that is internally cooled by water circulation, completing the quenching (martensitic transformation) process. As a result, the tensile strength of hot-stamped steels can achieve up to 1,600 MPa (about 230 ksi). CPF is developing advanced process simulation techniques, using FEA, for optimizing die design and reducing cycle times in hot stamping. CPF has initiated research effort to gain knowledge of this technology by collaborating with renowned research institutes, die makers and steel suppliers worldwide. Through our collaboration, CPF was able to conduct a literature survey and establish preliminary FE simulation models to analyze the hot stamping process and to assist design of hot stamping die (with cooling channels). The input parameters (i.e. material flow stress at elevated temperature, friction, heat transfer coefficient as function of pressure), developed in various literature [12-13], were used for simulating the process. We also proposed a new methodology for modeling of hot stamping by combining the benefits of 2 FE codes, DEFORM (solid element) and PAM-STAMP (shell element). In the new methodology, forming simulation will be conducted using PAM-STAMP, while quenching simulation will be conducted using DEFORM. Conversion code are being written in order to transfer information of shell element blank (PAM-STAMP) to solid element blank (DEFORM). CPF is also currently working on proprietary projects to analyze hot stamped components for 3 different companies. Figure 12 shows one of our hot stamping simulations, conducted for the sponsor. The results shows that with sufficient blank holder force the blank can be successfully formed, with acceptable thinning and small wrinkling. However, blank temperature during forming varies from 850 to 950 C. 9 SERVO DRIVE PRESSES During the last two years, presses with 2,500 to 3,000 tons’ capacity have been developed to form large panels for automotive applications. Toyota and Honda have installed such large presses for automotive stamping in Japan. In Germany, BMW is installing a large servo press line in its Dresden plant.

Figure 14: Compared here are the slide motions of a 1,100-ton mechanical and servo-driven press for identical slide velocity during forming [15] Figure 12: Preliminary hot stamping simulation using PAMSTAMP™ 10 SUMMARY / CONCLUSION The developments in metal forming industry are responding to the requirements of the changing environment and markets. Continuous pressures for lowering costs, increasing safety, reducing vehicle weight require that the industry develops new light weight and high strength materials as well as modern production methods and machines. Thus, continuous R&D is a must. Research Centers, located at universities, can best contribute to the industry needs and activities by close cooperation with the practicing engineers and by considering “real world” conditions. Such cooperation also assists in the education and training of engineering students who are better prepared for the real world by becoming familiar with the challenges faced by the industry. 11 REFERENCES [1] Gutscher, G., Palaniswamy, H. and Altan, T., 2004, Flow Stress Determination Using the Viscous Pressure Bulge (VPB) Test, J. of Materials Processing Technology, Vol. 146, pp.1-7. [2] Al-Nasser, A., Pathak P., Yadav, A. and Altan, T., 2009, Determination of Flow Stress of Five AHSS Sheet Materials (DP600, DP780, DP780CR, DP780HY and TRIP 780) Using the Uniaxial Tensile Test and Biaxial Viscous Pressure Bulge Test, CPF Report No CPF-2.1/09/01, CPF, The Ohio State University, Columbus, Ohio [3] Sriram, S., Wong, C., Huang, M., and Yan, B., 2003, Stretch Bendability of Advanced High Strength Steels, SAE Paper No. 2003-01-1151, pp.107 - 115, 2003. [4] Hudgins, A., Matlock, D., Speer, J., Fekete, J., and Walp, M., 2007, The Susceptibility to Shear Fracture in Bending of Advanced High Strength Steels”, Materials Science and Technology (MS&T), Sep. 16-20, 2007, Detroit, MI. [5] Walp, M.S., Wurm, A., Siekirk, J.F., Desai, A.K., 2006, Shear Fracture in Advanced High Strength Steels, SAE Paper No. 2006-01-1433, 2006. [6] Kim, H., Yan, Q. and Altan, T., 2008, Investigation of Galling in Forming Galvanized Advanced High Strength Steels (AHSS), Sixth Progress Report: Evaluation of Stamping Lubricants in Forming of AHSS Using Deep Drawing and Ironing Test”, CPF Report No CPF-2.3/07/03, CPF, The Ohio State University, Columbus, Ohio [7] Kim, H., Han, S., Kim, K. and Altan, T., 2008, Investigation of Galling in Forming Galvanized Advanced High Strength Steels (AHSS), Sixth Progress Report: Evaluation of Die Coatings and

Figure 13: Servo-driven presses off the flexibility of slide motion [14] Figure 13 shows the difference between the motion of mechanical and servo presses [14]. In addition to the flexibility of controlling the slide motion, servo-driven presses also offer considerable energy savings, especially in large-capacity presses. In these machines, the installed motor power is larger than in comparablecapacity mechanical presses. However, during a stamping operation (deep drawing, blanking, coining), the servomotor power is used only while the press is moving, since there is no continuously rotating flywheel and clutch/brake mechanism as there is in a conventional mechanical press. In addition, the braking energy is transferred back into the power system during the dynamic braking operation of the servomotors. It also is possible to install an external energy storage capability to compensate for energy peaks and reduce the nominal power drawn from the local power supply system, if it is economically justified. The best way to illustrate the cost effective application of modern servo-driven presses is to make actual production comparisons. In one case, a 1,100-ton conventional crank press was compared with a 1,100 ton servo-driven press. [15] The reduction of cycle time and the increase of productivity, while maintaining the same slide velocity during the deformation process, are shown in Figure 14.

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

Stamping Lubricants Using the Strip Drawing Test, CPF Report No CPF-2.3/08/02, CPF, The Ohio State University, Columbus, Ohio Fei, D. and Hodgson, P., 2006, Experimental and Numerical Studies of Springback in Air V-bending Process for Cold Rolled TRIP Steels, Nuclear Engineering and Design, 236:1847-1851. Cobo, R., Pla, M., Hernandez, R. and Benito, J. A., 2009, Analysis of the Decrease of The Apparent Young’s Modulus of Advanced High Strength Steels and Its Effect in Bending Simulations, IDDRG, 1-3 June 2009, Golden, CO, USA, 109-117. Konieczny, A. and Henderson, T., 2007, On Formability Limitations in Stamping Involving Sheared Edge Stretching, SAE Technical Paper No. 2007-01-0340 Abedrabbo, N., Pourboghrat, F. and Carsley, J., 2007, Forming of AA5182-O and AA5754-O at elevated temperatures using coupled thermomechanical finite element models, International Journal of Plasticity 23, 841-875 Eriksson, M. et al., 2002,“Testing and evaluation of material data for analysis of forming and hardening of boron steel components, Modeling and simulation in material science and engineering, pp. 277-294 Akerstrom,P. et al., 2005, Material parameter estimation for boron steel from simultaneous cooling and compression experiments, Modelling and simulation in material science and engineering, Vol. 13, pp. 1291-1308 Miyoshi, K., 2004, Current Trends in Free Motion Presses, Proceedings from the 3rd Japan Society for Technology of Plasticity (JSTP), Int. Seminar on Precision Forming, Nagoya, Japan, March 2004, (PAGE NUMBER Unknown). Bloom, T., Servo Drive Presses for the Next Generation Press Shop, presentation prepared by Schuler-Weingarten A.G., p. 13