A Comprehensive Study of Hole Punching for AHSS

GDIS2018

A Comprehensive Study of Hole Punching for AHSS

Chao Pu1, DJ Zhou2, Ken Schmid3, Changqing Du2, Yu-wei

Wang1

1. AK Steel Corporation

2. FCA USA LLC

3. General Motor Company

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Acknowledgements

The work discussed in this presentation was partially supported

by the A/SP Stamping Team using funds from the Auto/Steel

Partnership.

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Outline

Introduction

Experimental Procedure

• Tool Setup

• Experiment Variables and Materials

Results and Discussion

• Punching Force Studies

• Dimensional Studies

• Tool Protections

• Cutting Edge Qualities

FEA Simulations

Summary & Future works

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Introduction

Hole

Punching on

AHSS

Image source : Stamping

Journal, March, 2014

Punching Force Reduction

Dimensional Accuracy

Tool Protection

Edge Quality

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Experimental Tool Setup

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Experimental Variables

• Sample size: 254mm×254mm

• Punch rate: 10 mm/s

• Punch shapes: flat, conical, rooftop

• Punch tipping angle: 7°

(1)

(3)

(2)

Material Thickness

(mm)

Nominal

Punch Clearance

DP 1180 1.20 6.0%, 12.0%, 20.0%

DP 980 1.16 6.2%, 12.5%,20.8%

DP 590 1.31 6.4%, 12.8%,21.4%

DDS 1.38 6.1%, 12.2%,20.3%

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Material Properties

DP1180

(1.20mm)

DP980

(1.16mm)

DP590

(1.31mm)

DDS

(1.38mm)

Yield Strength (MPa) 1002.20 703.52 451.06 162.85

Tensile Strength (MPa) 1269.35 1038.99 675.05 311.23

Uniform Elongation (%) 5.40 7.16 16.45 24.71

0

200

400

600

800

1000

1200

1400

0 10 20 30 40 50

Stre

ss [

MP

a]

Strain [%]

DP1180DP980DP590DDS

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Punching Force History

• Conical shaped punch induces large deformation within the cutting area.

• The punch load is quite uniform due to gradual shearing process , similar to

scissor cutting for the rooftop punch

Punch

Blank

Rooftop Punch Shearing Process

Material deformation induced

by conical/rooftop punch

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Averaged Maximum Punch Load

0

50

100

150

200

250

Ave

rage

d M

axim

um

Pu

nch

Lo

ad (k

N) 6% 12% 20%

DP980

1.16mm

DP1180

1.20mm

DP590

1.31mm

DDS

1.38mm

~80%

~56%

• For all cases, the maximum punch load decreases as cutting clearance

increases, but the difference is trivial (about 3 to 4%).

• The rooftop punch leads to significant force reduction and it is more effective

on AHSS.

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Hole Punching Force Coefficient

• The hole punching force coefficient can be calculated as

𝐾 =𝑃

UTS ⋅ 𝜋𝐷 ⋅ 𝑡

UTS (MPa): ultimate tensile strength

P (N): hole punch force

D (mm): hole diameter

t (mm): material thickness

• This definition is similar to the shear strength index. More dependencies are

considered during the evaluation.

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Hole Punching Force Coefficient

• The hole punching force coefficient is negatively correlated to the

material strength.

• Mild steel → 1.0; AHSS: 0.7 ~ 0.8 6% Clearance

20% Clearance 12% Clearance

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Dimensional Study of Punched Hole

• Dimensional accuracy of punched holes is important in the sheet metal

forming.

• Dimensional measurements were repeated for three times for each punch

configurations (punch shape, material, and cutting clearance).

Yang, G. et al, SAE 2016

RD

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Hole Discrepancies • Conical shape leads to an uniform enlargement for diameter due to the

stress release and consequent spring back.

• The holes punched with rooftop shape exhibited oval shape with minor

axis along the ridge direction.

60mm

59.9

60

60.1

60.2

60.3

60.4

60.5

60.6

DP1180 DP980 DP590 DDS

Pu

nch

ed H

ole

Dia

met

er (

mm

)

6% Cutting Clearance

12% Cutting Clearance

20% Cutting Clearance

6% Cutting Clearance

12% Cutting Clearance

20% Cutting Clearance

Ridge Direction Transverse Direction

60mm

59.9

60

60.1

60.2

60.3

60.4

60.5

DP1180 DP980 DP590 DDS

Pu

nch

ed H

ole

Dia

met

er (

mm

)

6% Cutting Clearance

12% Cutting Clearance

20% Cutting Clearance

#GDIS | #SteelMatters 15

Tool Protections: Snap-through Load

• Snap-through load, i.e. reverse tonnage, leads to severe press machine

damage.

• Rooftop punch can provide an effective solution for press machine protection

and noise reduction

Load History

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Tool Protection From Enlarged Hole

Abrasive

Friction

Abrasive

Friction Abrasive

Friction Flat Punch

Piercing Punch Pushing Pulling Back

Conical Punch Abrasive

Friction

Enlarged Hole

Abrasive

Friction

Abrasive

Friction

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Cutting Edge Quality

Rooftop Punch

Rolling

Flat Punch Conical Punch

Transverse

Double

Burnish Zone

Fracture

Zone

• The cutting surface was examined using optical microscope with 200X magnification.

#GDIS | #SteelMatters 18

In-plane Hole Expansion Test

• In-plane hole expansion tests were conducted to evaluate the edge damage

due to the punch geometry during the punching stage.

• The conical shaped tool can produce a punched hole with higher edge

stretchability, while rooftop punch results in the most severe edge damage.

RD

Flat Conical Rooftop

6% Clearance 12.7 12.9 10.1

20% Clearance 13.1 14.6 6.93

0

2

4

6

8

10

12

14

16

In-P

lane

HE

R (

%)

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FEA Model

Flat & Conical Punch Model

(Axisymmetric)

Binder

Button

Rooftop Punch Model

(3D Quarter)

Blank Mesh

• Mixed-Voce-Swift model

600

800

1000

1200

1400

1600

0 0.1 0.2 0.3 0.4

Tru

e S

tre

ss (

MP

a)

Plastic True Strain

EXP DP1180 Simulation DP1180

+

* Test data is provided by WSU through ASP Fracture Project

• All-strain Based Modified Mohr-

Coulumb (eMMC) Fracture Model

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Punching Process Simulation

Flat Punch

Stress

Concentration

Crack Initiation

Spring back

Induced Gap

Conical Punch

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Punching Process Simulation

0 degree

shearing angle

Material

deflection

before cutting

Maximum

Punch Load

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Summary

Punching Force Reduction

Dimensional Accuracy

Tool Protection

Edge Quality

Flat Punch Conical Punch Rooftop Punch

No Effect;

Force Coefficient:0.7~1

No Effect;

Force Coefficient:0.7~1

Significant reduction

(56%~80%);

Force Coefficient:0.15~0.4

Accurate

Uniformly enlarged

diameter; could be

compensated

Oval shape with minor axis

along the rooftop ridge

Large snap-through load;

Multiple abrasive wearing;

Large snap-through load;

Reduced abrasive

wearing;

Significantly reduced snap-

through load;

Inconsistent edge surface

condition

Smooth and Consistent

Edge Surface

Localized material

deformation; Inconsistent

edge surface at small

clearance

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Future Studies

• In-plane hole expansion tests will be continued to study the sheared edge

damage mechanism.

• A numerical damage model will be developed to simulate the edge cracking.

• The punch shape and geometry will be optimized to achieve the goals of load

reduction and dimensional accuracy.

Roundness Measurement

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For More Information

Yu-wei Wang

AK Steel Corporation

313-317-1301

Yu-wei.Wang@aksteel.com

Chao Pu

AK Steel Corporation

313-407-4784

Chao.pu@aksteel.com

Thank you!