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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
#GDIS | #SteelMatters 3
Acknowledgements
The work discussed in this presentation was partially supported
by the A/SP Stamping Team using funds from the Auto/Steel
Partnership.
#GDIS | #SteelMatters 4
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
#GDIS | #SteelMatters 5
Introduction
Hole
Punching on
AHSS
Image source : Stamping
Journal, March, 2014
Punching Force Reduction
Dimensional Accuracy
Tool Protection
Edge Quality
#GDIS | #SteelMatters 6
Experimental Tool Setup
#GDIS | #SteelMatters 7
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%
#GDIS | #SteelMatters 8
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
#GDIS | #SteelMatters 9
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
#GDIS | #SteelMatters 10
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.
#GDIS | #SteelMatters 11
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.
#GDIS | #SteelMatters 12
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
#GDIS | #SteelMatters 13
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
#GDIS | #SteelMatters 14
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
#GDIS | #SteelMatters 16
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
#GDIS | #SteelMatters 17
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 (
%)
#GDIS | #SteelMatters 19
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
#GDIS | #SteelMatters 20
Punching Process Simulation
Flat Punch
Stress
Concentration
Crack Initiation
Spring back
Induced Gap
Conical Punch
#GDIS | #SteelMatters 21
Punching Process Simulation
0 degree
shearing angle
Material
deflection
before cutting
Maximum
Punch Load
#GDIS | #SteelMatters 22
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
#GDIS | #SteelMatters 23
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
#GDIS | #SteelMatters 24
For More Information
Yu-wei Wang
AK Steel Corporation
313-317-1301
Chao Pu
AK Steel Corporation
313-407-4784
Thank you!