Simulation for forming

CPF

Simulation 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

Presentation Outline CPF

1. Introduction
2. Determination of sheet material properties
 Flow stress
 Bulge test as an indicator of incoming sheet quality

3. Tests to evaluate lubricants for stamping
 The deep drawing test
 The ironing test
 The modified limiting dome height (MLDH) test

4. Case studies in process simulation
 Multi-point Cushion Systems (MPC)
 Warm forming of Al alloys, Mg alloys and High Strength Steels (HSS)

5. Summary


CPF
Introduction
Stamping 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


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)


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).

Schematic of viscous pressure bulge test (VPB) tooling setup at CPF

Potentiometer

Sheet

Viscous
medium

Pressure
transducer After forming
Before forming
Stationary Punch

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


Bulge test (VPB) samples

Before bursting After bursting

4 inches (~ 100 mm)
10 inches
(~ 250 mm)


Flow stress results for sample materials from the bulge test
CPF 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 )


Bulge test as an indicator of incoming sheet quality
Graph shows dome height comparison for SS 304 sheet material from eight
different batches/coils [5 samples per batch].

Highest formability  G , Most consistent  F
Lower formability and inconsistent  H


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.


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 ]


Stamping lubricants in the CPF
automotive industry
Process with dry-film lubricant

Deep Drawing +
Decoiling / Recoiling Decoiling subsequent blanking
with Lube coating by and cutting Stacking operations
immersion or spraying Blanks

Hot bath

[Courtesy: M. Pfestorf, 2005, BMW ]


Test to evaluate lubricants for stamping CPF
The deep drawing test
The deep drawing test has been used successfully for evaluating lubricants supplied by
various manufacturers. CPF is further developing this test for quantitative ranking of
lubricants.

12 inch Initial
blank

6 inch

Deep
drawn cup

Schematic of deep drawing tooling at CPF

Schematic of the deep drawing test
As blank holder pressure (Pb) increases, frictional stress (τ) increases based on
Coulomb‟s law.

  Pb

where  = the frictional shear stress
  the coefficient of friction
Coulomb’s law Pb = the blank holder pressure


Performance 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


Lubricants are ranked based on the highest constant BHF that can be applied in
deep drawing before the cup fails.

BHF = 50 tons
Test speed = 65 mm/sec

Load-stroke curves of formed vs. fractured cups


Comparison of draw-in length for various lubricants


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.


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


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

Nitrogen gas spring systems

Nitrogen pressure
control panel

Top 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”)


Hydraulic systems

IFU flexible Blank holder / Binder
hydraulic control unit
(Source: IFU, Stuttgart) Erie binder unit (hydraulic system)
with liftgate tooling inside press
(Source: USCAR)

Application of MPC die cushion technology in stamping

Sample cushion pin configuration (hydraulic MPC unit) for drawing stainless steel
double sink.

(Source: Dieffenbacher, Germany)

MPC is routinely used in deep drawing of stainless steel sinks


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)


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
Binder
Punch


BHF predicted by FE simulation in individual
cushion pins for forming Aluminum alloy
(A6111-T4, sheet thickness = 1 mm)
120 11
10 9 8
Blank 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


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 was
formed successfully - with three different materials/sheet thicknesses in the same die -
by only modifying BHF in individual cushion pins.

Ongoing work

Sheet Hydroforming with Die Stamping
(SHF-D) process
In cooperation with
In cooperation with IUL, Dortmund IWU Fraunhofer Institute, Chemnitz
Punch

Die

Segmented
elastic blank
holder with
multipoint Blank
cushion
system Cushion
Blank pins
Die
holder


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.


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.

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.


Warm forming of Al alloys, Mg alloys and
stainless steels
Elevated temperature formability study:
Schematic of warm forming tooling at AIDA America, Dayton
Punch
Die Ring
Die Holder Blank Holder

Cartridge Heaters Cartridge Heaters

Upper Tool
Lower Tool

Cooled
Heated tool
punch
Stage 1 Stage 2 Stage 3

and stainless steels
Elevated temperature formability study:
Servo Press at AIDA America, Dayton
Power Source Balancer tank Main gear

Capacitor

Servomotor
Drive Shaft


and stainless steels

Results of elevated temperature formability study
3
Limiting 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]


CPF
Process Modeling Applications
-Progressive Die Design-

A process sequence was designed for the part shown. The existing design
was improved through FE simulation to reduce the potential for failure in the
formed part (excessive thinning and wrinkling).


CPF
-Incremental Forming-

Orbital Forming of Wheel Bearing Assembly: .
Determine the influence of various process parameters such as axial feed, tool
axis 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 can
withstand

Tool
Inner race

Spindle

Initial stage Final stage


CPF
-Microforming of Medical Devices-

Microforming of a Surgical Blade:
•Using FEA with die stress analysis, the flash thickness was reduced such that
grinding of flash was replaced by electro-chemical machining (ECM).
•The designed tool geometry was successfully used in production to coin this
part.

Initial blank Formed part
(Blank thickness = 0.1 mm; Final blade thickness = 0.01 mm)


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.


CPF
-Material Yield Improvement in Hot Forging-

Material yield was increased by ≈15% through preform optimization, with
an additional 3-4 % improvement through
blocker die design.

Original Finisher Forging Final Forging with Reduced Flash


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.


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


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