Simulation for forming

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Simulation for forming

  1. 1. CPFSimulation and Optimization of Metal Forming Processes Taylan Altan, Professor and Director (altan.1@osu.edu) Center for Precision Forming www.cpforming.org Engineering Research Center for Net Shape Manufacturing (ERC/NSM) www.ercnsm.org The Ohio State University, Columbus, Ohio USA Prepared for Brazilian Metallurgy and Materials Association-ABM 63rd Annual Conference-July 28-31, 2008- Santos/SP-Brazil Center for Precision Forming (CPF) 1
  2. 2. Presentation Outline CPF1. Introduction2. Determination of sheet material properties  Flow stress  Bulge test as an indicator of incoming sheet quality3. Tests to evaluate lubricants for stamping  The deep drawing test  The ironing test  The modified limiting dome height (MLDH) test4. Case studies in process simulation  Multi-point Cushion Systems (MPC)  Warm forming of Al alloys, Mg alloys and High Strength Steels (HSS)5. Summary Center for Precision Forming (CPF) 2
  3. 3. CPF IntroductionStamping process as a system (e.g., the deep drawing process) 1. Workpiece material / Blank 5. Equipment 2. Tooling 6. Part 3. Interface 7. Environment 4. Deformation zone Center for Precision Forming (CPF) 3
  4. 4. CPF Introduction FE simulation is widely used in sheet metal forming as a virtual press to:  Predict material flow, stress, strain, temperature, potential failure modes  Troubleshoot a new problem  Validate tool/die designs by engineers Successful application of FE simulation depends on:  Reliable input material properties (e.g., flow stress data, anisotropy coefficients)  A good understanding of the problem (e.g., boundary conditions such as workpiece/tool temperatures, interface friction) Center for Precision Forming (CPF) 4
  5. 5. Determination of sheet material properties CPF In common practice, the uniaxial tensile test is used to determine the properties/flow stress and formability of sheet metal. Tensile test does not emulate biaxial deformation conditions observed in stamping. Due to early necking in tensile test, stress/strain data (flow stress) is available for small strains. Necking begins Engineering Stress-Strain Curve True Stress-Strain Curve = Flow stress In AHSS, the strain hardening exponent [n-value] and Young‟s modulus [E] change with deformation (strain). Center for Precision Forming (CPF) 5
  6. 6. Determination of sheet material properties CPFSchematic of viscous pressure bulge test (VPB) tooling setup at CPF Potentiometer Sheet Viscous medium Pressure transducer After forming Before forming Stationary Punch Center for Precision Forming (CPF) 6
  7. 7. Determination of sheet material properties CPF Schematic of viscous pressure bulge test (VPB) tooling setup at CPF Clamping force• Die diameter = 4 inches (~ 100 mm) Bulge/ Dome height (h)• Die corner radius = Pressurized 0.25 inch (~ 6 mm) medium Initial Stage Testing stage Pressure (P) Methodology to estimate material properties from VPB test, developed at CPF Measurement Material properties FEM based • Pressure (P) inverse technique • Flow stress • Dome height (h) • Anisotropy Center for Precision Forming (CPF) 7
  8. 8. Determination of sheet material properties CPF Bulge test (VPB) samples Before bursting After bursting 4 inches (~ 100 mm) 10 inches(~ 250 mm) Center for Precision Forming (CPF) 8
  9. 9. Determination of sheet material properties CPF Flow stress results for sample materials from the bulge testCPF has conducted a number of industrial case studies for:• Automotive - OEM,• Automotive - Tier 1 suppliers• Aerospace companies,• NASA,• Steel producers, etc., DP500 (Tensile test) DP500 (Bulge test) BH210 (Bulge test) BH 210 (Tensile test ) Center for Precision Forming (CPF) 9
  10. 10. Determination of sheet material properties CPF Bulge test as an indicator of incoming sheet qualityGraph shows dome height comparison for SS 304 sheet material from eightdifferent batches/coils [5 samples per batch]. Highest formability  G , Most consistent  F Lower formability and inconsistent  H Center for Precision Forming (CPF) 10
  11. 11. Applications of the bulge test CPF The bulge test is conducted in biaxial state of stress, thus emulating the deformation conditions in common stamping operations. True stress – true strain (flow stress) data is obtained over larger strains (nearly twice that of uniaxial tensile test). Accurate flow stress data is a necessary input to process simulation/virtual die tryouts using FEM. Dome or bulge height at bursting is a good measure of formability of the sheet material. In comparing different materials of the same sheet thickness, a larger/higher dome height at bursting, indicates better formability. Dome height at bursting can be easily used to identify variation in sheet material property which is commonly attributed to: a. different incoming coils, and b. different material suppliers. Center for Precision Forming (CPF) 11
  12. 12. Stamping lubricants in the CPF automotive industry Process with oil-based (wet) lubricant Additional Degreasing Pre-Oiling Oiling (optional)Decoiling and (optional) (optional)cutting Stacking Deep Drawing + Blanks subsequent (dry or blanking pre-oiled) operations [Courtesy: M. Pfestorf, 2005, BMW ] Center for Precision Forming (CPF) 12
  13. 13. Stamping lubricants in the CPF automotive industry Process with dry-film lubricant Deep Drawing +Decoiling / Recoiling Decoiling subsequent blankingwith Lube coating by and cutting Stacking operationsimmersion or spraying Blanks Hot bath [Courtesy: M. Pfestorf, 2005, BMW ] Center for Precision Forming (CPF) 13
  14. 14. Test to evaluate lubricants for stamping CPF The deep drawing testThe deep drawing test has been used successfully for evaluating lubricants supplied byvarious manufacturers. CPF is further developing this test for quantitative ranking oflubricants. 12 inch Initial blank 6 inch Deep drawn cup Schematic of deep drawing tooling at CPF Center for Precision Forming (CPF) 14
  15. 15. Test to evaluate lubricants for stamping CPF Schematic of the deep drawing testAs blank holder pressure (Pb) increases, frictional stress (τ) increases based onCoulomb‟s law.   Pb where  = the frictional shear stress   the coefficient of friction Coulomb’s law Pb = the blank holder pressure Center for Precision Forming (CPF) 15
  16. 16. Test to evaluate lubricants for stamping CPF The deep drawing testPerformance evaluation criteria: The maximum drawing load attained Maximum applicable Blank Holder Force (BHF) without failure of the cup Measurement of draw-in length, Ld, or perimeter of flange in a drawn cup Evaluation of lubricant build-up on the die for dry film lubricant Center for Precision Forming (CPF) 16
  17. 17. Test to evaluate lubricants for stamping CPF The deep drawing testLubricants are ranked based on the highest constant BHF that can be applied indeep drawing before the cup fails. BHF = 50 tons Test speed = 65 mm/sec Load-stroke curves of formed vs. fractured cups Center for Precision Forming (CPF) 17
  18. 18. Test to evaluate lubricants for stamping CPF The deep drawing testComparison of draw-in length for various lubricants Center for Precision Forming (CPF) 18
  19. 19. Case studies in process simulation CPF Multi-point Cushion systems (MPC) Current trends to control material flow in stamping Draw beads mainly control material flow, Blank Holder Force (BHF) avoids lift of blank holder/binder Constant BHF applied throughout press stroke, at all locations of the blank holder/binder using: • Nitrogen cylinders in the dies • Presses with hydraulic and pneumatic cushions Requirements for robust quality stamping/sheet hydroforming Variation of BHF with stroke  Springback control Variation of BHF at different locations within blank holder/binder  Enhance drawability Variation stroke to stroke, coil to coil  Allow variability in sheet material properties, thickness, lubrication and others. Center for Precision Forming (CPF) 19
  20. 20. Case studies in process simulation CPF Multi-point Cushion systems (MPC) Developments in BHF application technology• Each cushion pin is individually controlled (hydraulic/ nitrogen gas /servo control).• Offers a high degree of flexibility Die Blank holder / Binder Location of cushion pins/ Individual cylinders in the die cylinders for(Source: Müller Weingarten) each cushion pin Center for Precision Forming (CPF) 20
  21. 21. Case studies in process simulation CPF Multi-point Cushion systems (MPC) Possible variations in BHF application• Constant in location, Constant with stroke: Current practice • Each cushion pin applies same force that is kept constant in stroke • Single point cushion system, nitrogen cylinders or hydraulic cylinders• Constant in location, variable with stroke • Each cushion pin applies same force that is varied in stroke (hydraulic) • Single point hydraulic cushion system• Variable in location, constant with stroke • Each cushion pin applies different force that is kept constant in stroke • Multipoint control hydraulic cushion system, nitrogen cylinders• Variable in location, variable with stroke • Each cushion pin applies different force that is varied in stroke(hydraulic) • Multipoint control hydraulic cushion system Center for Precision Forming (CPF) 21
  22. 22. Case studies in process simulation CPF Multi-point Cushion systems (MPC) Nitrogen gas spring systems Nitrogen pressure control panelTop view of a two Top view of a three pressure-zone Individual cylinders for pressure-zone configuration each cushion pin configuration (Source: HYSON, “Nitro-dyne”) Center for Precision Forming (CPF) 22
  23. 23. Case studies in process simulation CPF Multi-point Cushion systems (MPC) Hydraulic systemsIFU flexible Blank holder / Binder hydraulic control unit (Source: IFU, Stuttgart) Erie binder unit (hydraulic system) with liftgate tooling inside press (Source: USCAR) Center for Precision Forming (CPF) 23
  24. 24. Case studies in process simulation CPF Multi-point Cushion systems (MPC) Application of MPC die cushion technology in stampingSample cushion pin configuration (hydraulic MPC unit) for drawing stainless steeldouble sink. (Source: Dieffenbacher, Germany) MPC is routinely used in deep drawing of stainless steel sinks Center for Precision Forming (CPF) 24
  25. 25. Case studies in process simulation CPF Multi-point Cushion systems (MPC) Previous work at CPF in Blank Holder/Binder Force (BHF) determination• CPF in cooperation with USCAR consortium developed software to program MPC die cushion system in stamping. Methodology for BHF determination (Numerical optimization techniques coupled with FEA) Inputs required BHF at each• Quality control parameters Software developed at cushion pin as (wrinkling, thinning) CPF for BHF function of punch• No. of cushion cylinders (n) determination stroke • Tool geometry (CAD) FEA Software • Material properties • Process conditions (PAM-STAMP, LS-DYNA) Center for Precision Forming (CPF) 25
  26. 26. Case studies in process simulation CPF Multi-point Cushion systems (MPC) FE model Die Estimation of Blank Holder Force (BHF) varying in each cushion pin & constant in stroke, using FE simulation coupled with numerical optimization, developed at CPF. Sheet Geometry : Lift gate inner Material : Aluminum alloy, AA6111-T4 Beads Initial sheet thickness : 1 mm Inner Segmented blank holder Binder [Source: USCAR / CPF - OSU]Cushion Pin Outer BinderPunch Center for Precision Forming (CPF) 26
  27. 27. Case studies in process simulation CPF Multi-point Cushion systems (MPC) BHF predicted by FE simulation in individual cushion pins for forming Aluminum alloy (A6111-T4, sheet thickness = 1 mm) 120 11 10 9 8Blank holder force (kN) 100 13 80 15 7 60 6 40 14 12 5 2 4 20 Pin 1 3 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Pin numbers Pin locations and numbering Center for Precision Forming (CPF) 27
  28. 28. Case studies in process simulation CPF Multi-point Cushion systems (MPC) Experimental validation of BHF prediction by FE simulation Bake Hardened steel (BH210, t = 0.8 mm) No wrinkles, no tears Aluminum alloy Dual Phase steel (A6111 – T4, t = 1 mm) (DP600, t = 0.8 mm) Minor wrinkles, no tears No wrinkles, no tears Using a hydraulic MPC system installed in mechanical press, the auto-panel wasformed successfully - with three different materials/sheet thicknesses in the same die - by only modifying BHF in individual cushion pins. Center for Precision Forming (CPF) 28
  29. 29. Case studies in process simulation CPF Multi-point Cushion systems (MPC) Ongoing work Sheet Hydroforming with Die Stamping (SHF-D) process In cooperation withIn cooperation with IUL, Dortmund IWU Fraunhofer Institute, Chemnitz Punch Die Segmented elastic blank holder with multipoint Blank cushion system Cushion Blank pins Die holder Center for Precision Forming (CPF) 29
  30. 30. Case studies in process simulation CPF Multi-point Cushion systems (MPC) Potential future work in BHF estimation for MPC systems Even with predicted optimum BHF, there can be inconsistency in metal flow in production. This inconsistency can be attributed to the variations in:  sheet material property (variations in incoming coil/different supplier) &  process conditions such as lubricant behavior (smearing), tool temperatures, etc. A methodology is needed to modify/adjust the BHF (by modifying nitrogen gas/hydraulic pressure) in individual cushion pins during production, such that the obtained draw-in (flange outline) matches the draw-in (flange outline) for a good part. Center for Precision Forming (CPF) 30
  31. 31. Case studies in process simulation CPF Multi-point Cushion systems (MPC) Potential future work in BHF estimation for MPC systems Schematic shows mismatched draw-in (flange outlines) seen in top view for a sample part. An „imaging system‟ could be used as feedback to obtain and compare flange outlines. Center for Precision Forming (CPF) 31
  32. 32. Case studies in process simulation CPF Warm forming of Al alloys, Mg alloys and High Strength Steels (HSS) Challenges in process simulation Lack of reliable input data for FE simulation • Flow stress of sheet material at relevant strain, strain rate and temperature • Thermal properties of sheet material at different temperature • Interface friction coefficient at higher temperature between dissimilar metals in contact • Interface heat transfer coefficient between dissimilar metals in contact Lack of knowledge on the yield surface to describe yielding behavior of metals at elevated temperature in FE codes. Lack of knowledge on the strain softening behavior exhibited by metals at elevated temperature to consider in FE simulation. Center for Precision Forming (CPF) 32
  33. 33. Case studies in process simulation CPF Warm forming of Al alloys, Mg alloys and stainless steels Elevated temperature formability study: Schematic of warm forming tooling at AIDA America, Dayton PunchDie Ring Die Holder Blank HolderCartridge Heaters Cartridge Heaters Upper Tool Lower Tool Cooled Heated tool punch Stage 1 Stage 2 Stage 3 Center for Precision Forming (CPF) 33
  34. 34. Case studies in process simulation CPF Warm forming of Al alloys, Mg alloys and stainless steels Elevated temperature formability study: Servo Press at AIDA America, Dayton Power Source Balancer tank Main gearCapacitorServomotor Drive Shaft Center for Precision Forming (CPF) 34
  35. 35. Case studies in process simulation CPF Warm forming of Al alloys, Mg alloys and stainless steels Results of elevated temperature formability study 3Limiting Drawing Ratio (LDR) • Material Al5754-O, 2.9 t = 1.3 mm 2.8 • Forming velocity = 5mm/sec 2.7 • Influence of temperature on the deep drawability of round cups 2.6 (Ø 40 mm) was investigated. 2.5 • Similar studies were conducted 2.4 for higher forming velocities of 250 275 300 15 mm/sec and 50 mm/sec. Die and Blank holder temperature (deg C) [In cooperation with AIDA America, Dayton] Center for Precision Forming (CPF) 35
  36. 36. CPF Process Modeling Applications -Progressive Die Design- A process sequence was designed for the part shown. The existing designwas improved through FE simulation to reduce the potential for failure in the formed part (excessive thinning and wrinkling). Center for Precision Forming (CPF) 36
  37. 37. CPF Process Modeling Applications -Incremental Forming-Orbital Forming of Wheel Bearing Assembly: .Determine the influence of various process parameters such as axial feed, toolaxis angle, etc., on the residual stress in the bearing inner race of the assembly,deformed geometry of the spindle, and the axial load that the assembly canwithstand Tool Inner race Spindle Initial stage Final stage Center for Precision Forming (CPF) 37
  38. 38. CPF Process Modeling Applications -Microforming of Medical Devices-Microforming of a Surgical Blade:•Using FEA with die stress analysis, the flash thickness was reduced such thatgrinding of flash was replaced by electro-chemical machining (ECM).•The designed tool geometry was successfully used in production to coin thispart. Initial blank Formed part (Blank thickness = 0.1 mm; Final blade thickness = 0.01 mm) Center for Precision Forming (CPF) 38
  39. 39. Process Modeling Applications CPF -Material Yield Improvement in Hot Forging-Hot Forging of Suspension Components:• A study was conducted for a tier one aluminum forging supplier to optimize the preform and die (blocker and finisher) designs, forging temperatures as well as flash dimensions. Center for Precision Forming (CPF) 39
  40. 40. CPF Process Modeling Applications -Material Yield Improvement in Hot Forging-Material yield was increased by ≈15% through preform optimization, withan additional 3-4 % improvement throughblocker die design. Original Finisher Forging Final Forging with Reduced Flash Center for Precision Forming (CPF) 40
  41. 41. CPF Summary Process simulation using FEA is state of the art for die/process design. Determination of reliable input parameters [material properties /interface friction conditions] is a key element in successful application of process simulation. For practical application, stamping lubricants should be evaluated in the laboratory under near-production conditions (speed, temperature, interface pressure). Reliable friction coefficient values needed for process simulation can be obtained from these laboratory tests. Multi-point control (MPC) die-cushion systems offer high flexibility in process control, resulting in considerable improvement in formability. MPC systems demonstrate good potential in forming light weight/high strength materials. Reliable flow stress data at elevated temperature is required as an input for accurate FE simulation of the warm forming process. Considerable research on warm forming process and its application to production is in progress. Intelligent use of process modeling saves time & costs and increases precision of formed parts. Center for Precision Forming (CPF) 41
  42. 42. CPF Questions / Comments Contact information: Taylan Altan, Professor and Director Center for Precision Forming - CPF(formerly, Engineering Research Center for Net Shape Manufacturing – ERC/NSM) www.cpforming.org / www.ercnsm.org The Ohio State University, Columbus, Ohio USA Email: altan.1@osu.edu, Ph: + 1-614-292-5063 Center for Precision Forming (CPF) 42

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