The document summarizes a presentation given at the Numisheet 2008 conference about a new method for reducing springback in sheet metal forming. The method involves locally heating the formed part above room temperature before removing the forming tools. Experimental tests on aluminum and magnesium alloys showed the method can effectively reduce springback. Finite element modeling was able to simulate the thermo-mechanical phenomena involved. Further studies are needed to better understand the underlying mechanisms and apply the method to more complex geometries. The research was funded by the Italian Ministry of Research.

1. September 1-5
Numisheet 2008
Interlaken - Switzerland
Localized warming of sheet metal parts
for the reduction of springback
Matteo Strano Antonio Del Prete
m.strano@unicas.it antonio.delprete@unile.it
Alessia Mentella Teresa Primo
alessia.mentella@unicas.it teresa.primo@unile.it
UNIVERSITY of CASSINO UNIVERSITY of SALENTO
Department of Department of
INDUSTRIAL ENGINEERING INNOVATION ENGINEERING
Cassino (FR), ITALY Lecce (LE), ITALY

2. September 1-5
Numisheet 2008
Interlaken - Switzerland
TABLE OF CONTENTS
PURPOSE SIMPLE STRETCH
THE BENDING CONCLUSIONS
SPRINGBACK OF THE BENDING
WORK TEST TEST
What is the Presentation Experimental Experimental Further
springback of the new equipment equipment developments
method
Springback
compensation Experimental Experimental
and reduction model model
methods
State of the art of some Some Test Some Test
compensation methods Results Results
for Al and Mg
speciments FEM Model FEM Model
and Results and Results
2
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3. September 1-5
Numisheet 2008
Interlaken - Switzerland
THE SPRINGBACK 1. Flat sheet
Empirical handbook rules
Predictive Numerical simulation 2. Sheet bending with nominal
techniques dimension punch
Analytical algorithms Xitool
Die
3. Elastic recovery of the sheet
SPRINGBACK after the load release
Target
COMPENSATION Δyi
Xi+1tool = Xitool + Δyi
4. Compensation of the
springback with an
Process adjustment method over-dimensioned punch
Corrective
methods Warm forming (tool heating) Die Δyi
Warm forming (workpiece heating) Xi+1tool
Xitool
3
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4. September 1-5
Numisheet 2008
Interlaken - Switzerland
THE SPRINGBACK
Warm forming: diffused heating
AZ31
AZ31
Source: F.K. Chen, T.B. Huang - Formability of stamping magnesium-alloy AZ31 sheets, Journal of Materials
Processing Technology 142 (2003) 643–647
4
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5. September 1-5
Numisheet 2008
Interlaken - Switzerland
THE SPRINGBACK
Warm forming: diffused heating
Al 1050
Source: Keum Y.T., Han B.Y. [3]
5
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6. September 1-5
Numisheet 2008
Interlaken - Switzerland
THE SPRINGBACK
Warm forming equipment
6
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7. September 1-5
Numisheet 2008
Interlaken - Switzerland
PURPOSE OF THE WORK
Springback
reduction
Locally increasing the
temperature of the cold
formed part, before
removing the forming
tools
7
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8. September 1-5
Numisheet 2008
Interlaken - Switzerland
PURPOSE OF THE WORK
• Cold forming a sheet metal part to a specified shape
1
• Locally heating the part up to a warm temperature
2
• Waiting until the part cools down
3
• Removing the loads and tools and let the part
4 springback
8
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9. September 1-5
Numisheet 2008
Interlaken - Switzerland
THE BENDING TESTS
SIMPLE BENDING TEST STRETCH BENDING TEST
9
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10. September 1-5
Numisheet 2008
Interlaken - Switzerland
EXPERIMENTAL SETUP
10
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11. September 1-5
Numisheet 2008
Interlaken - Switzerland
SIMPLE BENDING TEST
Experimental model
Size Material Initial Thickness
110×10 mm Aluminium alloy AA 1050-0 t0 = 0.6 mm; Thermocouple supports
t0 = 1mm
Magnesium alloy AZ31 t0 = 1.6 mm C W
Low carbon steel (zinc coated) t0 = 1.5 mm
Fictitious Active thermocouple
thermocouple
SANDWICH
heating
K-type
thermocouple
11
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12. September 1-5
Numisheet 2008
Interlaken - Switzerland
SIMPLE BENDING TEST
Test procedure
• Rectangular strips are cut and clamped on one short side with
1 maximum BHF.
• Punch is moved up to 40-60 mm with nearly constant speed
2 (trapezoidal profile) and sheets are bent at room temperature.
• Punch is stopped and samples are heated up to a known maximum
3 temperature Tmax, for a know heating time, with local heating.
• Heater is removed and punch holds until the specimen is cooled to
4 about 30°C.
• Punch moves down, the strip is unclamped and the final bending
5 angle aw is measured.
12
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13. September 1-5
Numisheet 2008
Interlaken - Switzerland
SIMPLE BENDING TEST
Some test results
Al 1050-O W Al 1050-O W
wall thickness 0.6mm Wall thickness 1mm
W – 230°C W – 160°C 42°21’
45°76’
C – 29°C C – 29°C
36°8’
35°49’
C C
29° C 230° C 230° C
29° C
13
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14. September 1-5
Numisheet 2008
Interlaken - Switzerland
SIMPLE BENDING TEST
Some test results
30%
0.6 mm
25%
α w − αc
1 mm
20%
15% ∆α % = ⋅100
αc
∆α%
10%
5% Al 1050-0
0% 25%
50 100 150 200 250 t0=1.55 mm
20%
Temperature (°C)
15%
∆α% 10%
AZ31
5% Carbon Steel
0%
100 150 200 250 300 350
Temperature (°C)
14
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15. PLAIN STRAIN
2D MODEL
(4 NODE BRICK ELEMENTS)
Alessia Mentella
2° PIOLA-
KIRCHHOFF
STRESS TENSOR
September 1-5
THERMO-
MECHANICAL
ANALYSIS
GREEN-
LAGRANGE
STRAIN TENSOR
Finite Element Model
Numisheet 2008
SIMPLE BENDING TEST
Interlaken - Switzerland
ELASTIC-PLASTIC
MATERIAL
(ISOTROPIC
JOHNSON-COOK
HARDENING)
15

16. September 1-5
Numisheet 2008
Interlaken - Switzerland
SIMPLE BENDING TEST
Finite Element Model
Material data used for AZ31
m
e  T − 18 
A B C
170 412.16 0.0223
n
0.223
m Tmelting
0.06943 618 (
s = A + Ben )  &
1 + C ln 0.01  1 − T
 


− 18 
 melting
T [C°] E [MPa] poisson thermal exp [mm/mm*C°]
0 49500 0.30 0.00002400
20 49000 0.31 0.00002415 THERMAL BC:
100 46000 0.32 0.00002505 • NO HEAT SOURCE
200 43000 0.34 0.00002580 • NO HEAT EXCHANGE
300 40000 0.36 0.00002655 • CONDUCTION THROUGH
400 37000 0.38 0.00002715 THE WORKPIECE
500 34000 0.4 0.00002760
16
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17. September 1-5
Numisheet 2008
Interlaken - Switzerland
SIMPLE BENDING TEST
FEM Results
20%
AZ31 – wall thickness 1.6mm
53.55°
19% Tmax=320°C 46.45°
αW αC
%
Δα%
18%
17% ∆α%= 15% AZ31
16%
100 200 300 400
Temperature (°C)
17
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18. September 1-5
Numisheet 2008
Interlaken - Switzerland
SIMPLE BENDING TEST
FEM Results
heating 120
100
Stress [Mpa]
80
Stress [Mpa]
60
40
20
20
0
0
0
0 0,02
0,02 0,04
0,04 0,06
0,06 0,08
0,08
Cold forming Heating
Cold forming
Cold forming Equivalent total strain
Cooling Springback total strain total strain
Equivalent Equivalent
Heating
18
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19. September 1-5
Numisheet 2008
Interlaken - Switzerland
STRETCH BENDING TEST
Experimental model
Size Material Initial Thickness
220×10 mm Aluminium alloy AA 1050-0 t0 = 1mm
Magnesium alloy AZ31 t0 = 1.6 mm
Aluminium alloy AA 2024-T3 t0 = 0.3 mm
19
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20. September 1-5
Numisheet 2008
Interlaken - Switzerland
STRETCH BENDING TEST
Test procedure
• Rectangular strips are cut, painted and clamped on both sides with
1 maximum BHF.
• Punch is moved up to a maximum dome height h1 (10-30 mm) with
2 nearly constant speed and sheets are bent at room temperature.
• Punch is stopped and samples are heated up to a known maximum
3 temperature Tmax, for a know heating time, with local heating.
• IR heater is switched off and punch holds until the specimen is
4 cooled to about 30°C.
• Punch moves down, and the residual dome height h2 is measured;
5 strips are unclamped.
20
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21. September 1-5
Numisheet 2008
Interlaken - Switzerland
STRETCH BENDING TEST
Some test results
AZ31 ~160 °C
23 °C
10%
( h1 − h2 ) − ( h1 − h2 )
cold warm
∆h % = 8%
( h1 − h2 )cold 6%
heating ∆h% 4%
s= 12 mm
2% s= 14 mm
s= 15 mm
0%
50 70 90 110 130 150
Tmax [°C]
21
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22. September 1-5
Numisheet 2008
Interlaken - Switzerland
STRETCH BENDING TEST
FEM Model
EXPLICIT APPROACH FORMING PHASE
• Quadrangular shell element (2mm side lenght) +
• 7 integration point through the thickness POST-DEFORMATION HEATING
IMPLICIT APPROACH
• Temperature assigned to the nodes SPRINGBACK SIMULATION
• No heat transfer with the environment
. T . T (°C) 20 500
ε ij = α T δ E [MPa]
ij Poisson
69000
0.33
60000
0.36
Thermal exp. [mm/mm*C°] 240x10-7 274x10-7
Yield Stress (MPa) 110 48
Plastic hardening moduli 0.20 0.20
22
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23. September 1-5
Numisheet 2008
Interlaken - Switzerland
STRETCH BENDING TEST
FEM Results
IR heater
Without post-deformation heating
With post-deformation heating
23
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24. September 1-5
Numisheet 2008
Interlaken - Switzerland
CONCLUSIONS
A new method for the reduction of springback has been
presented, based on warm heating of the sheet metal blanks after
forming and before springback.
1. The experimental evidence has shown the method is effective for simple
bending, stretch bending and draw bending;
2. Numerical simulation run with a fully implicit integration; a small strain
formulation and solid (brick) elements is able to provide a thermo
mechanical description of the phenomena involved in the process;
3. Mixed explicit/implicit simulation with shell elements did not provide
satisfactory results.
24
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25. September 1-5
Numisheet 2008
Interlaken - Switzerland
FURTHER DEVELOPMENTS
1. Deeper investigate the reasons and the main mechanisms of
the phenomenon;
2. Test the procedure on a more complex geometry workpiece;
3. Deeper understand the location of the heating points.
Acnowledgement
The Italian Ministry of research (MUR) founded the present research under the
“PRIN ‘05” program titled SMART.
25
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26. September 1-5
Numisheet 2008
Interlaken - Switzerland
Localized warming of sheet metal parts
for the reduction of springback
Matteo Strano Antonio Del Prete
m.strano@unicas.it antonio.delprete@unile.it
Alessia Mentella Teresa Primo
alessia.mentella@unicas.it teresa.primo@unile.it
THANK YOU FOR YOUR ATTENTION!
UNIVERSITY of CASSINO UNIVERSITY of SALENTO
Department of Department of
INDUSTRIAL ENGINEERING INNOVATION ENGINEERING
Cassino (FR), ITALY Lecce (LE), ITALY