Presentation overview
From tensile test to stamping tests…
Influent parameters on stampability and specific stainless characteristics
How to predict the feasibility of stamped parts?
Analysis of practical cases, stamping of bipolar plate
© Aperam 1
Presentation overview
From tensile test to stamping tests…
1. Mechanical characteristics of stainless steels
1.1. Tensile curve and main mechanical characteristics
1.2. Tensile based-evaluation criteria for stampability
1.3. Hardening coefficient
1.4. Anisotropy coefficients
2. Characterization of stainless steels stampability
Influent parameters on stampability and specific stainless characteristics
How to predict the feasibility of stamped parts?
Analysis of practical cases, stamping of bipolar plate
© Aperam 2
Uniaxial tensile test: principle
From tensile test to stamping tests…
1.1. Tensile curve and main mechanical characteristics
© Aperam 3
0S
F where F: the applied force
S0: initial cross-section
Necking zone
Variation of the initial cross-section Variation of the stress values
From the conventional (engineer) curve to the true curve
Str
ess, M
pa
D é formation, %
Rp0.2%
Rmax
A %
Ru
ptu
re
Ap
pa
riti
on
de
la s
tric
tio
n
D é
form
ati
on
pla
sti
qu
e
Mp
a
D é formation, %
Rp0.2%
Rmax
A %
Fra
ctu
re
A% Ag%
Conventional tensile curve
Ela
sti
c
Ho
mo
gen
eo
us P
lasti
c D
ef
Hete
rog
en
eo
us P
lasti
c D
ef E: Young modulus
Rp0.2: Yield Strength (YS)
Rm: Ultimate tensile stress (UTS)
A%: Maximal deformation
Ag%: Homogeneous deformation
r: Lankford coefficient
n: Hardening coefficient
Duplex Duplex
Austenitic
18-9ED Ferritic
K30
Martensitic
MA
UTS = 940 MPa
YS = 770 MPa
UTS = 800 MPa
YS = 650 MPa
Stress
(MPa)
Strain (%)
UTS = 500 MPa
YS = 340 MPa
YS = 300 MPa
UTS = 630 MPa
The YS/UTS ratio gives an “idea” of the grade stampability
Lower is the ratio, better is the stampability
From tensile test to stamping tests…
1.1. Tensile curve and main mechanical characteristics
© Aperam 4
4 families of stainless steels…
Martensitic Duplex Ferritic Austenitic
0,82 0,81 0,68 0,48
Conventional tensile curve
From tensile test to stamping tests…
1.1. Tensile curve and main mechanical characteristics
© Aperam 5
Curve calculation…
Engineering or conventional tensile curve: initial sample section
True or rational tensile curve: instantaneous sample section
1001lnln
1001
00
engL
Lrat
eng
engrat
L
L
l
dl
S
F
Conventional curve True Curve
εeng (%)
σeng (
MP
a)
εrat (-) σ
rat (
MP
a)
1000
0
0
L
LL
S
F
eng
eng
Rp0.2/Rm ratio stampers’ method
Hardening coefficient « n »
(for stretching solicitations)
Normal anisotropy « rN »
(for deep-drawing solicitations)
18-9ED
K30 K41
K09
Elongation (%)
Stress
(MPa)
From tensile test to stamping tests…
1.2. Tensile based evaluation criteria for stampability
© Aperam 6
AMSE AISI EN Ag (%) A (%) Rp0.2
(MPa)
Rm
(MPa) Rp0.2/Rm n rN
18-9ED 304 1.4301 48 54 310 640 0.48 0.42 0,98
K09 409 1.4512 20 34 250 420 0.59 0.26 1,49
K30 430 1.4016 17 29 330 480 0.68 0.19 0,98
K41 441 1.4509 19 31 310 470 0.66 0.21 1,36
From tensile test to stamping tests…
1.3. Hardening coefficient, n
© Aperam 7
Definition
n is defined by the Hollomon’s Law:
Represents plastic part of the true tensile curve as the Hooke’s law for the
elastic one
“physical” meaning: steel ability to homogenize strains via hardening
High n value ↔ good behavior regarding to stretching deformation mode
n
ratrat K
Hooke
εrat (-)
σrat (MPa)
Hollomon
From tensile test to stamping tests…
1.3. Hardening coefficient, n
© Aperam 8
Determination
• Between 5 and 13% for ferritic grades n ~ 0.15 - 0.25
• Between 18 and 40% for austenitic grades n ~ 0.4 -0.5
n
ratrat K )ln()ln()ln( ratrat nKln
K30
Necking
εrat (-)
σra
t (M
Pa
)
17-7C
Necking
εrat (-)
σra
t (M
Pa
)
From tensile test to stamping tests…
1.4. Anisotropy coefficients r, rN, Δr
© Aperam 9
● Isotropic material = same behavior everywhere
● Steel = rolled material preferential orientation for each grain
● Definition
• r = (width) / (thickness) r = 2/ 3 [Lankford coefficient]
● r is measured on the 3 main directions: rolling, transverse and 45°
Trend to reduce the width
Trend to reduce the thickness
From tensile test to stamping tests…
1.4. Anisotropy coefficients r, rN, Δr
© Aperam 10
Normal anisotropy rN Ability for deep-drawing deformation
• rN = (rL + 2 r45° + rT)/4
• High value of rN Good behavior for deep-drawing deformation
Planar anisotropy Δr Ability to form ears
• Δr = (rL - 2 r45° + rT)/2
• High values of Δr Formation of significant ears
r depends on the grade and the process
Cold work reduction (%)
Str
ain
ra
tio
r
AlSI 304
Presentation overview
From tensile test to stamping tests…
1. Mechanical characteristics of stainless steels
2. Characterization of stainless steels stampability
2.1. Generalities about stamping
2.2. Stretching deformation mode
2.3. Deep drawing deformation mode
2.4. To sum up…
2.5. Notion of Forming Limit Curves (FLC)
Influent parameters on stampability and specific stainless characteristics
How to predict the feasibility of stamped parts?
Analysis of practical cases, stamping of bipolar plate
© Aperam 11
blank
punch
Punch
support
Finish
product
die
blank-holder
blank-holdersupport
diesupport
blank
punch
Punch
support
Finish
product
die
blank-holder
blank-holdersupport
diesupport
The initial sheet (=blank) is introduced by force in a concave shape (= die) by
the use of a convex shape (=punch).
This operation can provoke some wrinkles on the edges of the blank. These
wrinkles can be erased thanks to a blank-holder.
All these tools are installed in a press
Characterization of stainless steels stampability
2.1. Generalities about stamping
© Aperam 12
Characterization of stainless steels stampability
2.1. Generalities about stamping
© Aperam 13
Press types
Mechanical or hydraulic
Single, transfer or with progressive tools
Single-acting, double-acting, triple acting…
ε2 = ε1
Characterization of stainless steels stampability
2.1. Generalities about stamping
© Aperam 14
Deformation modes
ε1 = ln(a/d) ; ε2 = ln(b/d)
ε1 > ε2
Plastic deformations: ε1 + ε2 + ε3 = 0
ε3 = thinning
Two adverse deformation modes
Stretching (ε1 > 0 ; ε2 > 0)
Deep drawing (ε1 > 0 ; ε2 < 0)
ε2 = 0 ε2 = -ε1/2
ε2 = -ε1
ε1 = 0
ε1
ε2
Hypothesis
r=1
In most of cases, the stamping solicitations are a mix of these two ones
Stretching
– No metal flow
– Significant thinning of the metal
Deep drawing
– Free metal flow
– Low thinning of the metal
Characterization of stainless steels stampability
2.1. Generalities about stamping
Two main deformation modes
© Aperam 15
Well adapted to austenitic grades
Stretching
Well adapted to a metal with…
High elongation A%
High hardening coefficient n
(~a low Rp0.2%/Rm ratio)
Deep drawing
Well adapted to a metal with…
A high anisotropy coefficient rN
Characterization of stainless steels stampability
2.1. Generalities about stamping
Two main deformation modes
© Aperam 16
Well adapted to ferritic grades
Erichsen Index = height of the stamped part at failure (mm)
110 mm
110 mm
© Aperam 17
Characterization of stainless steels stampability
2.2. Stretching deformation mode
Erichsen test
Erich
se
n in
de
x
(mm
)
Ferritic grades Austenitic grades
© Aperam 18
Characterization of stainless steels stampability
2.2. Stretching deformation mode
Ferritic grades Austenitic grades
Hardening coefficient Erichsen index (mm)
0
0,1
0,2
0,3
0,4
0,5
0,6
K33X K39M K41X K09X 18-9L 18-9DDQ 18-9ED
Hard
en
ing
co
eff
icie
nt
1mm-thick samples
Lubricant = Mobilux
8
8,5
9
9,5
10
10,5
11
11,5
12
K33X K39M K41X K09X 18-9L 18-9DDQ 18-9ED
Eri
ch
sen
In
dex (
mm
)
Under stretching deformation mode, thanks to higher hardening coefficients,
austenitic grades are better than ferritic ones for this deformation mode
For a given tool (d), bigger and bigger blanks are
stamped (consequently, taller and taller cups are
obtained).
We are looking for the critical blank diameter (Dmax)
for which the cup is no more stamped successfully.
(too many force required failure)
Cup diameter d = 33mm
58.5 62.5 66 70 74 77 80 Blank diameter D (mm) 55
Drawing Ratio 1.66 1.77 1.89 2 2.12 2.24 2.33 2.42
LDR = Dmax / d
© Aperam 19
Characterization of stainless steels stampability
2.3. Deep drawing deformation mode
© Aperam 20
Characterization of stainless steels stampability
2.3. Deep drawing deformation mode
LDR graphical determination
F12T - 0,98mm - Clée 040185
3000
3500
4000
4500
5000
5500
6000
6500
7000
1.80 1.90 2.00 2.10 2.20 2.30 2.40 2.50 2.60
Drawing ratio
Sta
mp
ing
fo
rce
(d
aN
)
Successful cupsBroken cupsExtrapolated line (successful cups)Extrapolated line (broken cup) LDR
LDR=2.28
Drawing ratio
Ferritic grades Austenitic grades
© Aperam 21
Characterization of stainless steels stampability
2.3. Deep drawing deformation mode
Under deep-drawing deformation mode,
ferritic grades are better than the austenitic ones
Limit Drawing Ratio
1mm-thick samples
Lubricant = Mobilux
1,75
1,8
1,85
1,9
1,95
2
2,05
2,1
2,15
2,2
K09X K33X K39M K41X 18-9ED 18-9DDQ 18-9L
LD
R
Ferritic grades
Austenitic grades
© Aperam 22
Characterization of stainless steels stampability
2.3. Deep drawing deformation mode
L.D.R. vs. normal anisotropy
Thanks to their high anisotropy, ferritic grades are better than austenitic ones
under deep-drawing deformation mode
11 mm 316L
Deep drawing deformation mode Stretching deformation mode
1.05
1.8
1.0
Anisotropy Coefficient
rN
0.50
0.20
0.18
Hardening
Coefficient n
1.95 – 2.00 11.5 mm 304
8.6 mm 444
2.15 – 2.20 9.6 mm 430Ti
2.05 –2.10 8.7 mm 430
LDR
Dmax/d
Erichsen
(t= 0.8mm)
Grade
Well adapted to
austenitic grades Well adapted to
ferritic grades © Aperam 23
Characterization of stainless steels stampability
2.4. To sum up…
Rétreint Expansion
Mixte
Stretching
1 = 2
Plane
tension
3 = - 1
Uniaxial
tension
3 < 0
Shear
2 = - 1
3 = 0
Plane strain
compression
3 = - 2 2
1
0
FLC
Reduction of thickness
Increasing of
thickness
Rétreint Expansion
Mixte
Stretching
1 = 2
Plane
tension
3 = - 1
Uniaxial
tension
3 < 0
Shear
2 = - 1
3 = 0
Plane strain
compression
3 = - 2 2
1
0
FLC
Reduction of thickness
Increasing of
thickness
Rétreint Expansion
Mixte
Stretching
1 = 2
Plane
tension
3 = - 1
Uniaxial
tension
3 < 0
Shear
2 = - 1
3 = 0
Plane strain
compression
3 = - 2 2
1
0
FLC
Reduction of thickness
Increasing of
thickness
Stretching
1 = 2
Plane
tension
3 = - 1
Uniaxial
tension
3 < 0
Shear
2 = - 1
3 = 0
Plane strain
compression
3 = - 2 2
1
0
FLC
Reduction of thickness
Increasing of
thickness
Stretching
1 = 2
Plane
tension
3 = - 1
Uniaxial
tension
3 < 0
Shear
2 = - 1
3 = 0
Plane strain
compression
3 = - 2 2
1
0
FLC
Reduction of thickness
Increasing of
thickness
© Aperam 24
Characterization of stainless steels stampability
2.5. Notion of Forming Limit Curve
Whatever the kind of solicitations, there is a domain not to go further
if necking wants to be avoided
The FLC is the border of this domain in the deformation plane e1, e2
© Aperam 25
Characterization of stainless steels stampability
2.5. Notion of Forming Limit Curve
Experimental determination: Principle
Use of samples enabling to
warranty constant 1/ 2
The failure of the sample enables to
determine 1 critical value
Forming Limit
Curve
The FLC goes through all critical
points determined and constitute the
boarder of the “safe” domain
Uniaxial tensile state
ε2 - Minor strain (%)
ε1 - Major strain (%)
© Aperam 26
Characterization of stainless steels stampability
2.5. Notion of Forming Limit Curve
Experimental determination: Nakazima method
Hemispherical punch (100mm), clamped blank (drawbead + blank-holder force=400kN)
Blanks with the same length (200mm) but different widths different ε2/ε1 ratios
Samples with small width : close to uniaxial tension
Symmetrical blank : equibiaxial tension (=stretching strain state)
ε2 - Minor strain (%)
ε1 - Major strain (%)
Characterization of stainless steels stampability
2.5. Notion of Forming Limit Curve
ARAMIS System composed of:
2 lenses
2 LED lights
Left lens Right lens
- The use of 3D Digital Image Correlation (DIC)...
Principle of the 3D DIC
Sample Lubricant
system
Lens
LED lights
Characterization of stainless steels stampability
2.5. Notion of Forming Limit Curve
Determination of the displacement fields according to the 3 main directions (X, Y & Z)
Left lens Right lens
- The use of 3D Digital Image Correlation (DIC)...
Displacement field according to Z direction
ARAMIS System composed of:
2 lenses
2 LED lights
ROI
Characterization of stainless steels stampability
2.5. Notion of Forming Limit Curve - The use of 3D Digital Image Correlation (DIC)...
From the determination of the displacement fields Calculation of the equivalent strain fields
ε1 – Major strain field visualization
ε2 – Minor strain field
visualization
Section 0
Section 1
Section 2
Length (mm)
Ma
jor
str
ain
(%
)
Determination of
the major and minor strains
before necking appeared...
Necking phenomenon ??
ARAMIS software was developed to respect the ISO 12004-2 standard
for the determination of the FLC
Characterization of stainless steels stampability
2.5. Notion of Forming Limit Curve - The use of 3D Digital Image Correlation (DIC)...
This approach is used for each sample and each deformation path...
ε1 Major strain field
ε2 Minor strain field
Other example: Sample corresponding to a stretching strain state (200 x 200 mm2)
ε2 - Minor strain (%) ε
1 -
Ma
jor
str
ain
(%
)
0
45%
0
45%
Linearity of the deformation path
Accurate determination of the strain levels
Possibility to follow each step of the
deformation path
Characterization of stainless steels stampability
2.5. Notion of Forming Limit Curve - The use of 3D Digital Image Correlation (DIC)...
ε2 - Minor strain (%)
ε1 - Major strain (%) Uniaxial tensile state
From minimum 7 different samples, it possible to obtain an accurate FLC...
Safe area
Risks of splits
Global FLC curve
© Aperam 32
Characterization of stainless steels stampability
2.5. Notion of Forming Limit Curve
Influent material characteristics
• Hardening coefficient (n)
o The most significant coefficient
o An increase of n leads to an increase of the FLC-level
• Strain rate sensitivity coefficient (m)
o Similar effect as n, but less significant
o In the necking area, strain rate increases if m-value is high, which slows
down the localization
o m is inversely dependent with Rp0.2 (for ferritics) and Rm (for austenitics).
• Normal anisotropy coefficient (rN)
o Second order effect
o When r increases, stretching abilities are reduced
• Thickness (t)
o Thicker materials lead to an increase of the FLC-level
Blanking Gridding Stamping Strain evaluation
Rectangular blanks
L=210mm,
l = 50 210mm
Electrolytic gridding
Squares of 2mm
Nakazima stamping
Hemispherical punch
ASAME analysis
3D reconstitution of the sample
© Aperam 33
Characterization of stainless steels stampability
2.5. Notion of Forming Limit Curve
Older experimental method:
This technique is still used on complex geometry to appreciate strain level
induced by a forming process
Presentation overview
From tensile test to stamping tests…
Influent parameters on stampability and specific stainless characteristics
1. Influent parameters
How to predict the feasibility of stamped parts?
Analysis of practical cases, stamping of bipolar plate
© Aperam 34
Sheet
Blank
Final part
Blanking
Forming : bending, stamping
(stretching, drawing), ...
Surface finish (roughness, ...)
Orientation of the blank under the press
Quality and nature of tools (hardness, roughness) Lubrication conditions
Mechanical characteristics Rheology Metallurgy
Cutting burrs and orientations of burrs
Blank holder pressure Stamping rate, ...
© Aperam 35
Influent parameters on stampability and specific
stainless characteristics
1. Influent parameters on stampability
A good lubricant decreases the
friction between the sheet and the
blank-holder
Less punch force required
Higher LDR
AutoForm = software for stamping simulations
Aims of the software :
Evaluate the feasibility of a part
Localize the risky areas
Input data:
Geometry of the part to stamp (CAD-file type IGS),
True stress/strain curve extrapolated to high deformations
Anisotropy coefficients in the 3 directions
Forming Limit Curve
Blank-holder pressure
Stamping rate
Lubrication coefficient
Output (for all points on the part):
Strain paths, thinning, plastic strain, wrinkles sensitivity, …
MATERIAL data
PROCESS data
DESIGN data
© Aperam 36
Study of Complex designs: Stamping Simulations with AutoForm
How to predict the feasibility of stamped parts ?