Simulation for forming

CPF

Center for Precision Forming (CPF) 1

Simulation and Optimization of Metal Forming

Processes

Taylan Altan, Professor and Director ([email protected])

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

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Presentation Outline

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

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Introduction

Stamping process as a system (e.g., the deep drawing process)

1. Workpiece material / Blank2. Tooling3. Interface4. Deformation zone

5. Equipment6. Part7. Environment

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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)

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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.

Determination of sheet material properties

Necking begins

In AHSS, the strain hardening exponent [n-value] and Young‟s modulus [E] changewith deformation (strain).

Engineering Stress-Strain Curve True Stress-Strain Curve = Flow stress

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Sheet

Potentiometer

Stationary Punch

Viscous

medium

Pressure

transducerBefore forming After forming

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


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Bulge/

Dome height (h)

Initial Stage Testing stage

• Die diameter = 4

inches (~ 100 mm)

• Die corner radius =

0.25 inch (~ 6 mm)

Clamping force

Pressurized

medium

Measurement

• Pressure (P)

• Dome height (h)

FEM based

inverse technique

Material properties

• Flow stress

• Anisotropy

Methodology to estimate material properties from VPB test, developed at CPF

Pressure (P)

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


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Bulge test (VPB) samples

Before bursting After bursting


4 inches (~ 100 mm)10 inches

(~ 250 mm)

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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 (Bulge test)DP500 (Tensile test)

BH210 (Bulge test)

BH 210 (Tensile test )


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Highest formability G , Most consistent F

Lower formability and inconsistent H

Graph shows dome height comparison for SS 304 sheet material from eight

different batches/coils [5 samples per batch].

Bulge test as an indicator of incoming sheet quality


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Applications of the bulge test

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.

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Stamping lubricants in the

automotive industry

Process with oil-based (wet) lubricant

[Courtesy: M. Pfestorf, 2005, BMW ]

Stacking

Blanks

(dry or

pre-oiled)

Pre-Oiling

(optional)

Deep Drawing +

subsequent

blanking

operations

Degreasing

(optional)Additional

Oiling

(optional)Decoiling and

cutting

Stacking

Blanks

(dry or

pre-oiled)

Pre-Oiling

(optional)

Deep Drawing +

subsequent

blanking

operations

Degreasing

(optional)Additional

Oiling

(optional)Decoiling and

cutting

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[Courtesy: M. Pfestorf, 2005, BMW ]

Hot bath

Decoiling

and cutting Stacking

Blanks

Deep Drawing +

subsequent blanking

operations

Decoiling / Recoiling

with Lube coating by

immersion or spraying

Hot bath

Decoiling

and cutting Stacking

Blanks

Deep Drawing +

subsequent blanking

operations

Decoiling / Recoiling

with Lube coating by

immersion or spraying

Stamping lubricants in the

automotive industry

Process with dry-film lubricant

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Test to evaluate lubricants for stamping

Schematic of deep drawing tooling at CPF

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.

Initial

blank

Deep

drawn cup

12 inch

6 inch


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As blank holder pressure (Pb) increases, frictional stress (τ) increases based on

Coulomb‟s law.

b

where = the frictional shear stress

the coefficient of friction

P = the blank holder pressure

bP

Coulomb’s law

Schematic of the deep drawing test


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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



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Lubricants are ranked based on the highest constant BHF that can be applied in

deep drawing before the cup fails.

Load-stroke curves of formed vs. fractured cups

BHF = 50 tons

Test speed = 65 mm/sec



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Comparison of draw-in length for various lubricants



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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.

Case studies in process simulationMulti-point Cushion systems (MPC)

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Developments in BHF application technology

Die

Blank holder /

Binder

Individual

cylinders for

each cushion pin(Source: Müller Weingarten)

• Each cushion pin is individually controlled (hydraulic/ nitrogen gas /servo control).

• Offers a high degree of flexibility

Location of cushion pins/

cylinders in the die


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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


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Individual cylinders for

each cushion pin

(Source: HYSON, “Nitro-dyne”)

Nitrogen pressure

control panel

Top view of a two

pressure-zone

configuration

Top view of a three

pressure-zone

configuration

Nitrogen gas spring systems


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(Source: IFU, Stuttgart)

IFU flexible Blank holder / Binder

hydraulic control unit

Erie binder unit (hydraulic system)

with liftgate tooling inside press

Hydraulic systems

(Source: USCAR)


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MPC is routinely used in deep drawing of stainless steel sinks

(Source: Dieffenbacher, Germany)

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

double sink.

Application of MPC die cushion technology in stamping


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Previous work at CPF in

Blank Holder/Binder Force (BHF) determination

Inputs required

FEA Software

(PAM-STAMP, LS-DYNA)

Software developed at

CPF for BHF

determination

• Tool geometry (CAD)

• Material properties

• Process conditions

• Quality control parameters (wrinkling, thinning)

• No. of cushion cylinders (n)

BHF at each

cushion pin as

function of punch

stroke

• 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)


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Estimation of Blank Holder Force (BHF)

varying in each cushion pin & constant

in stroke, using FE simulation coupled

with numerical optimization, developed

at CPF.

Die

Beads

Sheet

Inner

Binder

Punch

Outer

Binder

Cushion Pin

Geometry : Lift gate inner

Material : Aluminum alloy, AA6111-T4

Initial sheet thickness : 1 mm

Segmented blank holder

[Source: USCAR / CPF - OSU]

FE model


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0

20

40

60

80

100

120

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15Pin numbers

Bla

nk

ho

lde

r fo

rce

(k

N)

Pin 1 2 34

6

5

7

891011

14 12

13

15

BHF predicted by FE simulation in individual

cushion pins for forming Aluminum alloy

(A6111-T4, sheet thickness = 1 mm)

Pin locations and

numbering


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Experimental validation of BHF prediction by FE simulation

Aluminum alloy

(A6111 – T4, t = 1 mm)

Minor wrinkles, no tears

Bake Hardened steel

(BH210, t = 0.8 mm)

No wrinkles, no tears

Dual Phase steel

(DP600, t = 0.8 mm)

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.


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In cooperation with IUL, Dortmund

Segmented

elastic blank

holder with

multipoint

cushion

system

Die

Sheet Hydroforming with Die

(SHF-D) processStamping

In cooperation with

IWU Fraunhofer Institute, Chemnitz

Die

Blank

Blank

holder

Punch

Cushion

pins

Ongoing work


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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.


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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.

Potential future work in BHF estimation for MPC systems


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Case studies in process simulationWarm forming of Al alloys, Mg alloys

and High Strength Steels (HSS)

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.

Challenges in process simulation

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Case studies in process simulation

Elevated temperature formability study:

Schematic of warm forming tooling at AIDA America, Dayton

Stage 1 Stage 2 Stage 3

Heated toolCooled

punch

Die Holder

Upper Tool

Die Ring

Lower Tool

Cartridge Heaters

Blank Holder

Punch

Cartridge Heaters

Warm forming of Al alloys, Mg alloys and

stainless steels

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Elevated temperature formability study:

Servo Press at AIDA America, Dayton

Power Source Balancer tank Main gear

Servomotor

Capacitor

Drive Shaft

Warm forming of Al alloys, Mg alloys

and stainless steels

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[In cooperation with AIDA America, Dayton]

• Material Al5754-O,

t = 1.3 mm

• Forming velocity = 5mm/sec

• Influence of temperature on the

deep drawability of round cups

(Ø 40 mm) was investigated.

2.4

2.5

2.6

2.7

2.8

2.9

3

250 275 300

Die and Blank holder temperature (deg C)

Lim

itin

g D

raw

ing

Ra

tio

(L

DR

)

• Similar studies were conducted

for higher forming velocities of

15 mm/sec and 50 mm/sec.

Results of elevated temperature formability study

Warm forming of Al alloys, Mg alloys

and stainless steels

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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).

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Center for Precision Forming (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

37

Initial stage Final stage

Tool Inner race

Spindle

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-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.

(Blank thickness = 0.1 mm; Final blade thickness = 0.01 mm)

38

Formed part Initial blank

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-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.

39

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-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.

40

Original Finisher Forging Final Forging with Reduced Flash

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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.

Summary

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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: [email protected], Ph: + 1-614-292-5063

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