Current Applications of FE Simulation for Blanking and Stamping of Sheet Materials Eren Billur, Adam Groseclose, Tingting Mao, Taylan Altan

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1 Current Applications of FE Simulation for Blanking and Stamping of Sheet Materials Eren Billur, Adam Groseclose, Tingting Mao, Taylan Altan Center for Precision Forming ( Neil Avenue, Office 339, Columbus, OH USA Abstract: Production of light weight and crash resistant vehicles require extensive use of AHSS (DP, TRIP, TWIP) and Al alloys to form complex shapes. This paper discusses the practical determination of material properties and lubricants for establishing optimum forming conditions and conducting FE simulations. Nonproprietary results of case studies from recent research at the Center for Precision Forming (CPF) are presented including: 1) determination of material properties and friction, 2) application of servo drive presses to improve formability in forming of AHSS and Al alloys, 3) FE simulation of hot stamping to obtain tailored properties, 4) predicting the pierced/blanked edge quality and its effects on fracture limits in flanging, and 5) predicting onset of necking and fracture by using FE simulations. Keywords: Finite Element Simulations, Material Properties, Advanced High Strength Steels, Aluminum. 1 INTRODUCTION The automotive industry is under considerable pressure to reduce vehicle weight and costs, and still meeting the challenging new crash standards. As a result vehicle makes and their suppliers are investing in R&D to form new materials (such as Al alloys, Advanced high strength steels (AHSS), Boron Steels) to reduce weight and improve safety performance. New materials require advances in lubrication, die materials and coatings as well as new equipment. It is a well-accepted practice to save weight and increase safety using advanced high-strength steel (AHSS), as well as hot pressed and quenched boron steels. AHSSs include dual-phase grades (DP 600 to DP 1400) with tensile strength up to 1,400 MPa (200 ksi), as well as transformation-induced plasticity (TRIP), martensitic (MS), and twinning-induced plasticity (TWIP) steels. Hot pressed boron steels may reach 1,500 to 1,600 MPa (215 to 230 ksi) tensile strength after quenching. 2 MATERIAL PROPERTIES AND FRICTION In FE simulations, the accuracy of the input data affects the results. In a typical cold forming simulation geometry and tool motions/forces have to be modeled as close as to

2 reality. Furthermore, flow stress curve and friction are very important to obtain accurate results. The die geometry can be easily modeled by importing the 3D CAD model into FE software and adding necessary boundary conditions. Flow stress and friction input, however, require experimental data. 2.1 Uniaxial Flow Stress Determination Tensile Test Typically flow stress is determined by the well-established, standard tensile test. However, because of local necking in uniaxial deformation, tensile test typically gives flow stress data until true strain, depending on the material. In most sheet metal forming operations, effective strain levels are higher than this value. Thus, the tensile data is usually extrapolated in conducting simulations. The most common method is to curve fit the raw data using Hollomon's Law (or so-called Power Law, σ = Kε n ) and extrapolate. 2.2 Biaxial Flow Stress Determination Bulge Test The biaxial hydraulic bulge test is increasingly used to determine the flow stress, Figure 1. At CPF, the bulge pressure is applied by a polymeric viscous material to avoid splashing oil when the specimen fractures at the apex. Using the measured bulge pressure and the bulge height it is possible to determine the flow stress of the sheet material by means of inverse FE analysis [Altan et al., 2006]. The bulge height fracture is an indication of material formability and can be used to evaluate the formability of various materials as well as the same material obtained from different batches and/or suppliers [Nasser et al., 2010]. Downward motion clamps the sheet Laser Sensor Test Sample Viscous Medium Pressure Transducer Continued downward motion forms the bulged sheet Stationary Punch Before Forming After Forming Figure 1; Viscous Pressure Bulge Test. The flow stress is determined by inverse FE analysis using pressure and dome height measured during the test [Nasser et al., 2010]. As seen in Figure 2, bulge test may give flow stress up to true strain. This reduces errors caused by curve fit, if not eliminates the need for curve fitting. In this particular example, flow stress obtained from the bulge test is about 10 percent lower than the extrapolated data at ~0.5 true strain. This would affect: 1) press force

3 calculations, 2) springback predictions, and 3) thinning as the strain hardening is not modeled realistically [Billur and Altan, 2013]. True Stress (MPa) Uniform Strain from Tensile Test = 0.16 True Stress (ksi) 145 Tensile data with Power Law (σ=kε n ) 116 Useful strain from Bulge Test = VP Bulge Test Material, DP600, t 0 =1 mm Tensile Test True Strain Figure 2; Comparison of flow stress determined by tensile test and bulge test [Billur and Altan, 2013]. 2.3 Biaxial Flow Stress Determination Dome Test In production environment, especially at smaller companies, the conventional hydraulic bulge test may not be practical to use to evaluate the characteristics (flow stress, formability) of the incoming sheet material. Therefore, work is in progress in developing an analysis to expand the application of the well-known Limited Dome Height (LDH) test to obtain biaxial flow stress data, Figure 3. Figure 3; Schematic of the Dome Test. Optimum lubrication used between sheet and punch, allows frictionless deformation [Kardes Sever et al., 2012].

4 In this application, it is necessary to use perfect lubrication between the solid punch and the sheet sample in order to obtain the fracture at the apex of the bulge. The dome height (or punch stroke) and the deformation force, measured during the test, are used to obtain the flow stress curve, using FE based increase analysis [Kardes Sever et al., 2012] 2.4 Friction Coefficient Another very important parameter in simulation is "friction coefficient". In most cases, the friction coefficient (µ) is considered to be constant. However, in reality it depends on the surface roughness and coating of the sheet and tool material, contact pressure, sliding speed, lubrication, and temperature of the sliding surface [Roll, 2008]. Thus, to determine the friction coefficient a test that can emulate the pressure, speed and lubrication as close as to production conditions is needed. The cup draw test is used extensively to determine the friction coefficient. Several lubricants have been successfully tested with mild steels, AHSS and aluminum alloys [Ref.]. In this test, as seen in Figure 4, 305 mm (12") diameter blanks are drawn in a hydraulic press to 80 mm (3.2") depth. Positive stops are used to ensure the repeatability. Then the perimeter of the cups are measured and "average friction coefficient" is calculated by using FE simulations with several friction coefficients. The simulation that gives a flange perimeter, equal to that obtained in experiment, defines the "friction coefficient" of the lubricant used [Subramonian et al., 2011]. D die = mm Draw Die R die = 16 mm L d (draw-in length) Blankholder Punch (Stationary) R punch = 20 mm D punch = mm R i (initial radius of blank) = mm P b (Blankholder pressure) τ (Frictional shear stress) Motion of Die and Blank Holder Figure 4; Schematic of the Cup Draw Test [Subramonian et al., 2011]. 3 FORMING AHSS AND ALUMINUM IN SERVO PRESS Light weight and crash resistant vehicles require higher percentage of Advanced High Strength Steels (Dual Phase, Complex Phase, TRansformation Induced Plasticity, and TWinning Induced Plasticity) and Aluminum alloys to be formed into complex shapes.

5 As seen in Figure 5, only TWIP steels have formability similar to mild steels. Aluminum alloys and most AHSS typically have low formability and high springback problems. Total Elongation (%) Better Formability IF Mild BH Al CMn Al (hs) TRIP L-IP Aust. SS TWIP Lightweight Potential* Higher forces & springback* Mild Steels Conventional High Strength Steel Advanced High Strength Steel 2 nd Generation AHSS Aluminum alloys MART * Steel to steel comparison. Al has lower density (more 0 lightweight potential) and lower E-modulus (more springback) Ultimate Tensile Strength (MPa) compared to steel with similar strength. Figure 5; Banana curve, showing the advantages and disadvantages of several steel and aluminum grades [Hall, 2011]. 3.1 Servo Presses Servo drive presses offer the flexibility of hydraulic presses (control of velocity and stroke), with stroking rates that are higher than that of mechanical presses. Over the last few years many OEMs and suppliers have installed servo presses, including BMW, Audi, Honda, Toyota, Hyundai, Volkswagen and Škoda. However, at this time, most of these companies are using servo presses at forming speeds that are very similar to that of mechanical crank or link presses. However, even in this mode of operation, parts can be formed in the servo press at increased (30 to 50%) stroking rate by increasing the speed during approach and after forming [Billur and Altan, 2012; Meinhardt and Knoche, 2012]. There are experimental observations indicating that ram deceleration, cushion preacceleration and reduced forming speed in servo presses can reduce impact shock and improve formability. This can be explained by better lubrication (less splash of lubricants thanks to softer impact) and lower temperatures of the blank (slower speed in sheet/die interface). Temperature may 1) increase friction and 2) decrease flow stress thus may initiate local necking. Preliminary simulations have shown that temperatures around or above 100 C (>210 F) are common in forming of high strength steels, Figure 6. If the effect of press motion on formability can be better understood, this can help forming difficult-to-form materials such as aluminum alloys and advanced high strength steels (AHSS).

6 Contact area with die Nodal temperature - Membrane Min = 28.3 Max = 93.7 (a) (b) Figure 6; Temperatures in: (a) deep drawing of DP590 (Deform 2D) [Kim, 2009], (b) limiting dome test of HSLA 590 (PamStamp). In addition to improved formability, the free-motion of servo-press may also reduce springback by, 1) dwelling at the bottom, 2) bottoming and/or 3) re-striking motions. Experimental evidence shows that these motions reduce springback in bending AHSS [Mori et al., 2007]. Furthermore, accurate control of ram position and speed has been shown to improve the blanking conditions by reducing noise, vibrations, reverse loading and tool wear [Osakada et al., 2011]. 3.2 Forming of Al Alloys in Servo Press It is well known that aluminum alloys can save weight in areas that require: 1) dent resistance (doors, hoods, fenders, etc.); 2) energy absorption (crashboxes) and 3) high stiffness (floor panels, etc.) [Wochlecker et al., 2006]. Recently there is an increasing interest in using more aluminum in car bodies. As seen in Figure 5, aluminum alloys have limited formability and due to their low elastic modulus, they cause increased springback. CPF and its partners are currently designing a die for aluminum (5000-series) to be tested in a servo press with hydraulic cushions. The die represents forming and deep drawing challenges that could be observed in a door inner panel, Figure 7. In the design phase, several virtual tryouts are done using PamStamp simulation. Material properties are obtained from the bulge test and several lubricants have been tested with the Cup Draw Test to obtain the average friction coefficient.

7 Max Thinning, ~24% Thinning (%) Min = Max = 23.8 Figure 7; Simulation to design an experimental die to form an Al alloy in a servo press. 3.3 Warm Forming of Al Alloys Warm forming of Al alloys is also gaining attention and has some applications in the automotive industry. In the past aluminum and magnesium sheet were warm formed in servo press using a heated die and a cooled punch [Kaya and Altan 2010], Figure 8. More recently, warm hydroforming in servo-hydraulic press was investigated [Shah et al., 2011]. Recent studies by OEMs and Tier 1 suppliers focus on heating only the blank, which is more cost effective and easier to implement in mass production conditions [Harrison et al., 2013]. Experience gained in non-isothermal warm forming and hot stamping simulations can be applied into this new warm forming technology. Insulation plate Sheet Punch Die ring Heaters Blank holder Water inlet Cushion pins Figure 8; Schematic of the die-set used in warm forming Al & Mg alloys in servo-drive press [Kaya and Altan, 2010].

8 3.4 Forming of AHSS in Servo Press CPF has been investigating the possibility to form complex AHSS parts with tight tolerances (i.e. reduced or controllable springback) using servo presses. An earlier study together with PtU (University of Darmstadt) showed that controlling the speed does affect the thinning distribution in deep drawing of DP 600 [Groseclose et al., 2013]. Recently, together with a servo press manufacturer, an OEM and a first tier part supplier, CPF is designing a new die set for forming several AHSS grades in a servo press, Figure 9. Materials of interest are 1 st generation AHSS, including DP600 and DP980 among others. Blank geometries for: 1. Straight flanging 2. Stretch flanging 3. Shrink flanging 4. U channel drawing and U bending 5. Curved U channel forming 6. Deep drawing Figure 9: Die design concept to test different springback conditions [Altan, 2007]. The design of the die set, in cooperation with a first tier supplier, requires extensive FE simulations to estimate the effects of ram speed (during the deformation stage) upon springback, temperature generation, friction and potential fracture. 4 HOT STAMPING OF BORON STEELS Direct and Indirect hot stamping of Mn-B steels are well known and practiced in industry, Figure 10. However, parts with high strength properties are difficult to weld or join and they may not provide sufficient toughness to absorb crash energy. Therefore, techniques have been developed to obtain tailored properties in a given part by using (a) Tailored Welded Blanks (TWB), (b) Tailored Rolled Blanks, or (c) by controlling the microstructure variation at various locations in the part during quenching [Rehse, 2006; Hilfrich and Seidner, 2008; Karbasian and Tekkaya 2010].

9 Total Elongation (%) >950 C (>1750 F) Austenite IF 3-5 min.s Mild in Furnace BH CMn 1 Mn-B Alloyed steel (as delivered) Ferrite & Pearlite Aust. TWIP SS Quenched in the die >27 C/s (>49 F/s) TRIP Ultimate Tensile Strength (MPa) Figure 10; Schematic illustration of hot stamping process [Billur, 2013]. L-IP MART Quenched Martensite 4.1 FE Simulation of Hot Stamping Simulation of hot stamping processes require more input parameters than conventional stamping since considerations of: 1) mechanical, 2) thermal, 3) metallurgical and 4) fluid mechanics related issues are involved in the process and are all interrelated, as shown in Figure 11 [Merklein et al., 2009]. Mechanical Field - Mechanical material properties, - Volume change due to phase transformation. Heat generation due to plastic deformation. Thermal expansion. Phase transformation depends on stress and strain. Microstructure depends on temperature. Microstructure Evolution Fluid Mechanics Thermal Field Heat transfer to the coolant medium. - Thermal material properties, - Latent heat due to phase transformation. Figure 11; Multi-physics problem of hot stamping simulation [Porzner et al., 2012]. As done by many other researchers, CPF has been developing simulation capability for hot stamping for several years. Initial studies have been done with DEFORM. Later, PAMSTAMP and LS-DYNA have been used. A typical hot stamping simulation consists of at least 4 stages: 1) gravity, 2) holding (clamping), 3) forming and 4) quenching. When a part with tailored properties is being simulated, an additional "air quenching" stage may be added to predict the final properties [Billur, 2013]. 4.2 Forming Simulations Simulation of the first 3 stages (gravity, clamping, forming) is used to predict potential failures (cracks, wrinkles) in the hot formed part. Several case studies, conducted in

10 cooperating with hot stamping companies, illustrated that PAMSTAMP simulations were reasonably accurate in predicting fracture and/or excessive thinning. Thus, by using FE simulations, it is possible to design the dies and establish process parameters to ensure forming a defect-free part. In another application, when designing the dies for a tailored part at CPF, wrinkles were observed in the soft zone (i.e., microstructure tailored to be mostly bainite) and crack initiation in the hard zone (i.e., martensite). FE simulations showed that a two-piece blankholder could solve both problems, as seen in Figure 12. With one-piece blankholder 20 kn Crack With two-piece blankholder 15 kn 5 kn Wrinkles in the soft area Non-symmetric draw-in No wrinkles or cracks (a) (b) Figure 12; Tailored B-pillar forming simulation: (a) conventional blankholder concept, (b) two-piece blankholder concept [Billur, 2013]. 4.3 Quenching Simulations Once the part is successfully formed it is quenched (held between cooler die halves) to obtain the final microstructure, hardness and strength. In this stage, the die halves are pressed with a constant force. Several methods of simulation were developed to design quenching stage process parameters, such as: 1) duration of quenching stage, 2) number, diameter and location of cooling channels, and 3) number, power and location of heating cartridges (in case of tailored parts). In a case study, hot stamping a B-pillar with tailored properties was simulated using 1) 10 seconds of die quenching and 2) 80 seconds of air quenching. Both cycles should be as short as possible to increase productivity. However, quenching should be long enough to ensure martensite transformations are completed in the hard zone where UTS was in the range of MPa. Air quenching should be long enough to ensure all the austenite in the soft zone has been transformed at the end of the part, prepared for welding, where UTS was about MPa [Billur, 2013]. To ensure the cooling channels are designed properly for a robust production, a cyclic quenching analysis is required. In these simulations, solid dies with cooling channels are used. Figure 13 shows two different cooling systems (same cooling channels, different flow rates). In this particular example, when the flow rate was increased to 50 lt/min/cooling channel, die and punch temperatures were stabilized after 7 cycles and did not exceed 170 C. Using FE simulations, location, number and

11 diameter of cooling channels can be optimized. Furthermore, the flow rate in the channels can be determined for a robust process. Max. Temperature ( C) Die Punch Max. Temperature ( C) Die Punch Cycle Cycle (a) (b) Figure 13; Cyclic quenching simulation with: (a) low flow rate, and (b) high flow rate in cooling channels [Billur, 2013]. 5 BLANKING / PIERCING / HOLE FLANGING Edge cracking in hole flanging is mainly affected by the edge quality in the original blanked hole. The edge quality of the blanked or pierced hole is determined by: a) punch/die clearance, b) blank holder (stripper) pressure near the sheared edge, c) punch geometry, and d) punch velocity, Figure 14. Roll over zone (Z r ) Shear zone (Z r ) Fracture zone (Z f ) v p = punch velocity f b = blank holder force d p = punch diameter d d = die diameter r p = punch corner radius r d = die corner radius d b = blank holder diameter t = sheet thickness Punch-die clearance (% of sheet thickness) = (d d -d p )/2t*100 (a) Burr zone (Z b ) (b) Figure 14; (a) Schematic of blanking, (b) blanked edge (as obtained from FE simulation) [Sartkulvanich et al., 2010].

12 FE simulation of the blanking operation, although somewhat approximate, gives useful information about the process and on how best to select the process parameters. The effect of punch/die clearance in piercing round and rectangular holes has been well explained thru simulations that were compared with experiments. The preferred orientation of the blank, burrs up, reduced the possibility of edge cracking in hole flanging [Weidenmann, et al., 2009]. Modifications of the punch/die clearance at the corners, in punching a square hole, showed that stresses at the punch corner can be reduced and punch life can be increased. Investigation of high speed blanking of electronics parts also illustrated that increasing the blank holder pressure near the edge, improves the quality of the sheared surface [Subramonian et al, 2013]. This effect is also well known from the application of fine blanking. The punch shape affects (a) the stresses during blanking and (b) the resulting strain distribution at the sheared edge. FEM allows a relatively inexpensive investigation of the effects of punch geometries in blanking round holes with punches having different geometries (different punches), Figure 15. Estimation of temperatures and strains at the sheared edge, compared with available experimental data [Takahashi et al., 2013], help to illustrate how best to select the process variables to improve the quality of the blanked edge and to increase the hole expansion ratio. Flat Conical with flat tip Humped (a) (b) (c) Figure 15; Punch tip geometries investigated to determine the strains at the shear edge and their effect upon flanging (a) flat tip, (b) conical with flat tip, (c) humped tip. 6 NECKING AND FRACTURE PREDICTION In sheet metal forming simulations, the onset of fracture or localized necking is usually predicted by Forming Limit Diagrams (FLDs). This method is strain-path dependent and has limited application in: 1) Multi-stage forming operations where the strain path is always non-linear, 2) Advanced high strength steels with more than one phase (DP, TRIP, TWIP, etc). FLD s are very useful to sheet metal producers in evaluating the formability for a given sheet thickness from a well known heat or batch. However, they are not dependable in predicting the fracture in forming AHSS. Furthermore, the determination of FLD s require a number of experiments and are not very practical for use by

13 stampers who are interested in knowing the formability of each incoming material or coil. Thus, for sheet metal forming simulations, a new and practical fracture criterion is required, which is independent from the strain history and easy to compute in simulation. The following options are being investigated: 1) Variation of thinning rate, and 2) Variation of strain rate (equivalent and/or principal) In practice, most stampers evaluate the formability of a sheet material by observing the maximum thinning before fracture in forming a given part. Often, a given material is said to be formable to a maximum thinning that is observed in die try outs or at the start of production. The objective of the ongoing study is to attempt to predict fracture in sheet forming by examining the maximum thinning variation in the part, during FE simulation. The strategy is to simulate progressively more complex geometries and determine whether the FE predictions and the experimental observations validate this method of fracture prediction. Thus, simulations and their comparisons with experiments are being conducted in: a) tensile tests, round and sheet specimens, b) bulge tests, c) dome tests, with and without lubrication (LDH tests), d) cup draw test with various lubricants, e) forming of practical automotive stampings from Al alloys and AHSS. Preliminary simulations of tensile test and limiting dome test have shown that the thinning rate could be used to estimate the onset of necking, Figure 16. [Banabic, 2010] and [Volk and Hora, 2011] have shown the characteristic point where Thickness strain increases dramatically, could be calculated to predict the onset of necking. Our preliminary simulations also show that these techniques can predict the onset of necking. Load (kn) Experiment Al 5182-O t 0 = 1.5mm Preliminary FE Tensile Elongation (mm) Thickness Strain () Tensile Elongation (mm) (a) (b) Figure 16; Preliminary tensile test simulations show that necking can be predicted: (a) by comparing load-elongation curves, and (b) by finding the characteristic point

14 This technique has also been tested using deep drawing experiments, using the tooling in Figure 8. Both DEFORM 2D and PAMSTAMP simulations have shown that the method can be used to predict the onset of necking and cracking in deep drawing of stainless steel 304 as observed in the experiments [Li et al., 2013], Figure 17. This criterion will be further tested in complex forming operations. Equivalent Strain () Safe Element Potential Necking Element Stroke (mm) Figure 17: Evaluation of strain vs. stroke in the simulation of deep drawing of SS 304 [Li et al, 2013]. 7 SUMMARY AND CONCLUSIONS The state of technology and the trends in transportation industry can be summarized as follows: 1) Requirements for lightweight and crash resistant vehicle structures demand the use of high strength aluminum alloys and AHSS. 2) Die design for blanking and stamping these alloys are routinely conducted by FE based process simulation that requires reliable input data, mainly in material properties and friction. The true stress-true strain of these alloys is best obtained by tests that emulate biaxial deformation conditions that exist in practical stamping operations. Friction conditions are best determined using tests, such as the cup draw test, that represent the metal flow conditions encountered in stamping. 3) Servo drive presses used for sheet metal forming do not only provide increase in flexibility and productivity, but have also the potential to improve formability in forming difficult to form alloys by optimizing the deformation speed and dwell at BDC. 4) Advances in simulation software, when used appropriately with the necessary input data, allow non-isothermal simulation of stamping operations such as hot stamping (press hardening) of steels and warm forming of aluminum alloys.

15 5) Simulation of blanking operations offer the potential to understand the effects of material and process variables upon the quality of the sheared edge as well as in reducing edge cracking in bending and hole flanging. 6) The prediction of the maximum thinning during forming, in function of time or punch stroke, may represent a practical and simple method for predicting fracture in AHSS. These methods may be more easily applied in practice than the commonly used fracture prediction, based on FLDs. 7) R&D in all these topics, listed above, is being conducted by many researchers, especially in forming ultra high strength steels that have limited formability and are prone to edge cracking. REFERENCES [Altan et al., 2006] Altan, T.; et al., Determination of Sheet Material Properties using Biaxial Bulge Tests, In: Proceedings of the 2nd Int. Conference on Accuracy in Forming Technology, Nov , Chemnitz, Germany. [Altan, 2007] Altan, T., Stamping Research Gains Support, In: Stamping Journal, June 2007 issue. [Banabic, 2010] Banabic, D.; Sheet Metal Forming Processes, Springer, 2010, ISBN [Billur and Altan, 2012] Billur, E.; Altan, T.; Servo-driven presses for AHSS stamping, In: Stamping Journal, Nov/Dec 2012 issue, pp [Billur and Altan, 2013] Billur, E.; Altan, T.; Determining material properties and batch-to-batch variations with bulge testing, In: Stamping Journal, Sep/Oct 2013 issue, pp [Billur, 2013] Billur, E.; Fundamentals and Applications of Hot Stamping Technology for Producing Crash-Relevant Automotive Parts, PhD Dissertation, The Ohio State University, [Engels et al., 2006] Engels, H.; Schalmin, O.; Müller-Bollenhagen, C.; Controlling and Monitoring of the Hot-Stamping Process of Boron-Alloyed Heat-Treated Steels, In: Proceedings of International Conference on New Developments in Sheet Metal Forming Technology, pp ; Stuttgart Germany [Groseclose et al., 2013] Groseclose, A.; Yoon, J.Y.; Billur, E.; Altan, T.; Optimization of Ram Velocity for Deep Drawing with Servo-Drive Presses using Gas Spring Cushion System, CPF Report 5.6/12/01, Columbus, Ohio, March [Harrison et al., 2013] Harrison, N.R.; Ilinich, A.; Friedman, P.A.; "Optimization of High-Volume Forming for Lightweight Sheet", SAE Paper , 2013, doi: / [Hilfrich and Seidner 2008] Hilfrich, E.; Seidner, D.; Crash safety with high strength steels, In: International Automotive Congress, Shanghai China, 2008.

16 [Karbasian and Tekkaya 2010] Karbasian, H.; Tekkaya, A.E.; "A review on hot stamping", Journal of Materials Processing Technology, Vol. 210, pp , 2010, doi: /j.jmatprotec [Kardes Sever et al., 2012] Kardes Sever, N.; Demiralp, Y.; Dykeman, J.; Altan, T.; "Determining a flow stress curve with the LDH test", In: Stamping Journal, Mar/Apr 2012 issue, pp [Kaya and Altan, 2010] Kaya, S.; Altan, T.; "Non-isothermal deep drawing of aluminum and magnesium using servo presses", In: Proceedings of International Manufacturing Science and Engineering Conference (MSEC), [Kim, 2009] Kim, H., "Prediction and Elimination of Galling in Forming Galvanized Advanced High Strength Steels (AHSS)", PhD Dissertation, The Ohio State University, [Li et al., 2013] Li, X.; Mao, T.; Rajagopal, N.; Altan, T.; "Fracture Prediction in Cup Drawing using FE Simulation", CPF Report 2.1/13/02, Columbus, Ohio, April, [Meinhardt and Knoche, 2012] Meinhardt, J.; Knoche, S.; "The New Servo Press Technology - First Experiences at BMW AG", In: Forming In Car Body Engineering, Bad Nauheim - Germany [Merklein et al., 2009] Merklein, M.; Lechler, J.; "Investigations on the Thermal Behavior of Ultra High Strength Boron Manganese Steels within Hot Stamping", In: Proceedings of ESAFORM 2009, pp [Mori et al., 2007] Mori, K.; Akita, K.; Abe, Y.; "Springback behaviour in bending ultra-high-strength steel sheets using CNC servo press", International Journal of Machine Tools & Manufacture 47, pp , [Nasser et al., 2007] Nasser, A.; Yadaj, A.; Pathak, P.; Altan, T.; "Determination of the flow stress of five AHSS sheet materials (DP 600, DP 780, DP 780-CR, DP 780- HY and TRIP 780) using the uniaxial tensile and the biaxial Viscous Pressure Bulge (VPB) tests", Journal of Materials Processing Technology 210, pp , [Osakada et al., 2011] Osakada, B.; Mori, K.; Altan, T.; Groche, P.; "Mechanical servo press technology for metal forming", CIRP Annals - Manufacturing Technology 60, pp , [Porzner et al., 2012] Porzner, H.; et al.; "Virtual Prototyping Hot Forming Engineering with Virtual Manufacturing", In: AP&T Press Hardening - The Next Step Seminar, Detroit - Michigan, [Rehse, 2006] Rehse, M.; "Flexible Rolling of Tailor Rolled Blanks", In: Great Designs in Steel, Detroit - Michigan, [Roll, 2008] Roll, K.; "Simulation of Sheet Metal Forming - Necessary Developments in the Future", In: Proceedings of the 7th International Conference and Workshop on Numerical Simulation of 3D Sheet Metal Forming Processes (NUMISHEET), pp. 3-11; Interlaken - Switzerland 2008; ISBN [Sartkulvanich et al., 2010] Sartkulvanich, P.; Kroenauer, B.; Golle, R.; Konieczny, A.; Altan, T.; "Finite element analysis of the effect of blanked edge quality upon

17 stretch flanging of AHSS", CIRP Annals - Manufacturing Technology 59, pp , [Shah et al., 2011] Shah, M.; Billur, E.; Sartkulvanich, P.; Carsley, J.; Altan, T.; "Cold and Warm Hydroforming of AA5754-O Sheet: FE Simulations and Experiments", In: Proceedings of NUMISHEET 2011, pp: [So et al., 2012] So, H.; Faβmann, D.; Hoffmann, H,; Golle, R.; Schaper, M.; "An investigation of the blanking process of the quenchable boron alloyed steel 22MnB5 before and after hot stamping process", Journal of Materials Processing Technology 212, pp , [Subramonian et al., 2011] Subramonian, S.; Kardes, N.; Demiralp, Y.; Jurich, M; Altan, T.; "Evaluation of Stamping Lubricants in Forming Galvannealed Steels for Industrial Application", Journal of Manufacturing Science and Engineering, Vol. 133 (6), pp , 2011, doi / [Subramonian et al., 2013] Subramonian, S.; Altan, T.; Campbell, C.; Ciocirlan, B.; Determination of forces in high speed blanking using FEM and experiments, Submitted to CIRP Annals Manufacturing Technology, [Takahashi et al., 2013] Takahashi Y.; Kawano O.; Horioka S.; Ushioda K.; "Improvement of stretch flangeability of high-tensile-strength steel sheets by piercing under tension using humped bottom punch", SAE Paper , 2013, doi: / [Volk and Hora, 2011] Volk, W.; Hora, P.; "New algorithm for a robust userindependent evaluation of beginning instability for the experimental FLC determination", Int J Mater Form 4, pp , [Weidenmann et al., 2009] Weidenmann, R.; Sartkulvanich, P.; Altan, T.; "Development of a Methodology to Predict Edge Cracking of Advanced High Strength Steel (AHSS) DP 590 in Hole Flanging", CPF Report 5.2/09/01, Columbus, Ohio, April [Wochlecker et al., 2006] Wohlecker, R., et al., "Mass Reduction", fka Report 56690, Aachen - Germany, November, 2006.