AMERICAN JOURNAL OF SCIENTIFIC AND INDUSTRIAL RESEARCH
© 2012, Science Huβ, http://www.scihub.org/AJSIR
ISSN: 2153-649X, doi:10.5251/ajsir.2012.3.6.480.486
Effect of Sheet Thickness and Type of Alloys on the Springback
Phenomenon for Cylindrical Die
Prof. Dr. Hani Aziz Ameen
Dies and Tools Eng. Dept., Technical College- Baghdad
ABSTRACT
In this paper the effect of the springback on the bending operation of different materials are
studied. Dies were designed and constructed in cylindrical shape. Three types of alloys are used;
aluminum alloy 7020 T6, copper alloy and lead alloy. These alloys have different sheet thickness
(2 and 4) mm. Bending was done by using the press of 80 ton. The springback is calculated by
published equation. It can be concluded that, the Aluminum sheets (AA7020 T6) have the biggest
springback factor, the springback factor equal one in lead alloy of 2 mm thickness that indicated
there is no spring back in this sheet. Also concluded that the hardness of the sheet increased, the
springback back factor increased too. Also in the aluminum and copper sheets when the
thickness increased the spring back factor increased while in the lead alloy when the thickness
increased the springback factor decreased.
Keywords: Springback, forming process, aluminum alloy 7020, copper alloy, lead alloy, die
design, bending die.
INTRODUCTION
Accurate estimation of spring back in which
industries is important. Demands in bend angles can
be within a narrow range. Many studies proposed to
study spring back in bending. Johnson and Mellor
(1973), studied the springback calculated with
equation(1)
Where:
T = Thickness of the plate (mm)
E = Modulus of elasticity (N/mm²)
Ri = Internal radius curvature (before spring back)
(mm)
Rf = Real internal radius curvature (after spring back)
(mm)
σy = Yield stress(N/mm²)
This equation is derived from bending a full plasticity
and flexibility beam, and by calculating the internal
stresses near the neutral axis and in order to know to
the modulus of elasticity and the yield stress a tensile
test of a standard sample of the metal must be done.
And the amount of the value Ri/Rf depends on the
mechanical properties of the metal which will vary
depending on the value of (σyRi / ET) , if Ri/Rf=zero
so the metal is with full spring back but if Ri/Rf=1
there is no spring back in the metal.
The equation above was followed in many scientific
researches to calculate the amount of spring back.
Chan et al., (2006), studied of spring back in the Vbending metal forming process with one clamped
(edge bend) end and one free end. Spring back
occurs at the die-lip and V-region of the die model.
Different die punch parameters such as punch radius,
punch angle and die-lip radius are varied to study
their effect on spring back. Also, the effect of the
punch displacement on spring back is investigated.
The results are analyzed using Abaqus / CAE. The
analysis shows that spring back angle of the valley
region decreases with increment of punch radius and
punch angle. Thomas Schonbach (2008), presented
a new method to calculate and compensate spring
back in the early tool design phase. This method
enables to get parts inside the tolerances also for
these high strength and ultra-high strength steels. In
the first step the forming process itself is optimized
with the help of an accurate implicit solver, the spring
back after forming is calculated with the required
accuracy. Based on this simulation the robustness of
spring back is analyzed by variation in some input
parameters, e.g. material properties and lubrication. If
the spring back is not robust or it is scattering in a
wide range, the forming process must be made
robust first. This can be done by different problem
solving method, based on a robust spring back
behavior; the spring back will be compensated by an
Am. J. Sci. Ind. Res., 2012, 3(6): 480-486
integrated parametric software module. Aurelian
Albut (2008), presents the results obtained by
numerical simulation regarding spring back
phenomenon of a part manufactured from tailor
welded blanks. The final shape of the formed part is
seriously affected by spring back phenomenon. And
tries to prove the important role of the metal sheet
thickness in the spring back effect. The part has
different spring back values for each material from
the welded assembly structure. The influence of the
sheet thickness on the tailor welded stripes spring
back is examined by finite element method using
Abaqus Standard for forming process and Abaqus
Explicit for spring back of the obtained part. Hani Aziz
Ameen (2010), presented the novel derivation and
results of spring back of a sheet subjected to
combined biaxial moment and axial load. Spring back
is a common phenomenon in metal forming caused
by elastic redistribution of internal stresses during
unloading and it appears clearly in pure bending and
stretch bending of plates and sheets. Due to the
great importance of calculating accuracy, it is
necessary to assist the die designer in obtaining
spring back values easily. An exact analytical solution
for spring back due to bi-stretch bending of plate of
elastic work hardening material is presented (elasticlinear work hardening and elastic exponential work
hardening). Two cases are considered. The first is a
circular plate subjected to stretch bending and it was
found that the final curvature and the spring back
ratio (final curvature to elasto plastic curvature)
increases with increasing stretch bending. And the
second case is that of rectangular plate subjected to
stretch bending where the bending moment in the xdirection is kept constant. Jamal H. Mohammad
(2010), studied of the effect of backing pad in the Ubending process. Different backing pad values are
used to study their effect on distributions of stresses
and strains in work piece. Two examples were
analyzed one without backing pad and another with
backing pad to understand the effect of the backing
pad on the process. 3D model of U-bending was
used and analyzed by using ANSYS 11 FEM code.
DILIP KUMAR K. (2010), Described a finite element
for bending process of the Aluminum sheet metal
which allows for the prediction of the elastic spring
back. Material is considered as elasto-plastic after
yielding and plastic deformation is computed using
Newton Raphson method. The result obtained by
FEM is presented graphically. Experiments are
conducted for different thickness and clearance
between the die and punch and the results are
closely scrutinized with the results obtained by finite
element method.
In this paper the design and manufacturing of the
unconstrained cylindrical bending die is presented.
Also to investigate the value of spring back in
bending process, three materials alloys are used
(Aluminum, brass, and lead).
Experimental Work: The die and punches designed
and manufactured for Unconstrained cylindrical
bending die.
Specimens for spring back test: Estimating
springback value by performing a series of
experiments has been the basis of this study . in
order to perform the experiments work. The
specimens must fit the die and punch with a suitable
clearance. It should be :
1- lead sheet of 70 mm of width and 50 mm of length
and 2 mm thickness.
2- lead sheet of 70 mm of width and 50 mm of length
and 4 mm thickness.
3- aluminum sheet of 70 mm of width and 50 mm of
length and 2 mm thickness.
4- aluminum sheet of 70 mm of width and 50 mm of
length and 4 mm thickness.
5-copper sheet of 70 mm of width and 50 mm of
length and 2 mm thickness.
6- copper sheet of 70 mm of width and 50 mm of
length and 4 mm thickness.
Preparation of sheet metal: The sheets is
perpetrated by three stages , casting , rolling and
trimming as shown in Fig.(1).
481
Am. J. Sci. Ind. Res., 2012, 3(6): 480-486
A
b
c
Fig.(1 ) a. Lead sheet b. Aluminum sheet c. Copper sheet
and the chemical composition of the die and it’s
punches are shown in Table (1).
Chemical composition of the die and punches:
The die and the to punches material is (T72301)
according to the ASTM standard “A 686 – 92 R99”
Table (1) chemical composition of the die and punches
C
Si
Mn
Ni
S
P
Cr
Cu
0.95 - 1.09
0.17 - 0.33
0.17 - 0.33
max
0.25
max
0.02
max 0.03
max 0.2
max 0.25
Chemical composition of the sheets alloy: The
chemical composition of the copper sheets alloy
shown in Table(2). And the chemical composition of
aluminum sheets shown in Table(3) while for lead
alloy shown in Table(4).
Table (2) chemical composition of copper alloy
Zn%
0.01
Ag%
0.005
Pb%
0.02
Co%
0.01
Sn%
0.01
Cu%
99. 5
P%
0.007
Mn%
0.002
Fe%
0.005
Ni%
0.02
Si%
0.006
Table (3) chemical composition of aluminum 7020 alloy
Elements %
Standard value
Measured value
Si
< = 0.35
0.121
Fe
< = 0.40
0.290
Cu
< = 0.20
0.200
Mn
0.05 - 0.5
0.0764
Mg
1 -1.5
1.25
Cr
0.10 – 0.35
0.228
Zn
4–5
4.56
Ti
0.08
0.0319
Al
BALANCE
BALANCE
482
Al%
0.014
S%
0.003
Am. J. Sci. Ind. Res., 2012, 3(6): 480-486
Table (4) chemical composition of lead alloy
Sb%
As%
Bi%
Cd%
Cu%
Fe%
Mn%
0.002
0.0005
0.005
0.0005
0.003
0.001
0.0005
Ni%
Se%
Ag%
Te%
Sn%
Zn%
0.001
0.0005
0.002
0.0005
0.001
0.002
Measuring the hardness of the sheets: The
hardness of the sheets is measured by Rockwell test
using steel ball of 1/16 inch diameter for aluminum
and copper , 1/8 inch for lead, At the beginning 10 kgf
applied on the surface of the sheet to remove the
Oxides of aluminum, lead, copper and impurities and
then 100 kgf applied for Aluminum, copper and 60kgf
for lead to measure the hardness. The result of the
test shown in table(5).
Thickness(mm)
2
71
Aluminum
4
68
Lead
2
36
34
Copper
2
56
Copper
4
51
The Die shape: In This work the unconstrained
cylindrical bending die and two punches is designed
and manufactured, this die designed under standard
specification, its consist of one part, normally punch
and die both are made from carbon tools steel ( A
686-92 R99) according to ASTM standard as shown
in Fig.(2).
Hardness
(HRB)
Aluminum
4
It can be indicated that the hardness according to the
table(5) would be ( 72 ,57 and 36) HRB for the
sheets of aluminum, copper and lead respectively .
Table (5) the hardness of the sheets
Material
Lead
Fig.(2) ) unconstrained cylindrical bending die
483
Am. J. Sci. Ind. Res., 2012, 3(6): 480-486
Experimental bending device: A press of 80 Ton
are used as a experimental bending device to bend
the specimens as shown in Fig.(3). Fig.(4) illustrated
the bending process.
Fig.(3) Press device
Fig. (4) Experimental work of the die
Effective Parameter on spring back in this study:
The effective
parameter which affect on the
springback phenomenon is thickness of the sheet
and type of materials alloys.
Procedure of measure the springback factor:
1. Drawing the radius of the die (25mm) on the
paper and measure the angle of the radius
as shown in Fig.(6).
st
2. Drawing the radius of 1 specimen tangent to
the die radius as shown in Fig.(6)
3. Measuring the angle of the specimen and
calculate the springback factor according to
the equation
Thickness of sheets and the type of materials are the
most effective parameter that have a big effect on the
springback .
Spring back Testing Method and Procedure: The
steps illustrated the spring back testing are as follows
1- fixing the die and punch in the compression
device. The punch is moved down.
.. (2)
2- the punch is now in contact with the sheet and the
sheet is drawn through the opening in the die.
nd
4. Repeat the same steps on the 2 to the last
specimen.
3-the punch proceeds downwards, the outer radius of
the work piece is reduced.
4- After the load is released and the tools are
removed, different radius is measured between die
radius and sheet radius. This is the value of spring
back, as shown in Fig.(5).
484
Am. J. Sci. Ind. Res., 2012, 3(6): 480-486
The die radius 25 mm
The sheet radius
Fig.(5) Different between die radius and sheet radius
Sheet radius
The die radius 25 mm
αf
αi
Fig.(6) the procedure of measure the springback factor
Table(6) springback factors
RESULTS AND DISCUSSION
No.
The springback is measured experimentally, table(6)
illustrated the springback factors for the Aluminum,
copper and lead alloy with 2 and 4mm thickness .
Fig.(7) shows the value of springback with different
thickness and materials.
485
Aluminum
sheets
thickness
Copper
sheets
thickness
Lead
sheets
thickness
Springback
factor
Ks=
1
2 mm
1.1143
2
4 mm
1.0857
3
2 mm
1.0571
4
4 mm
1.0429
5
2 mm
1
6
4 mm
1.0143
Am. J. Sci. Ind. Res., 2012, 3(6): 480-486
©Springbac
k factor (K)
Fig.(7) springback factor
REFERENCES
CONCLUSIONS
W. Johnson and Mellor ( 1973) “Engineering Plasticity” .
In the present work , it can be concluded the
following :
W.M.Chan, H.I.Chew, H.P.Lee, B.T.Cheok, "Finite element
analysis of Spring-back of V-bending sheet metal
forming processes", J. Mater. Process. Technol.
vol.172, pp 35–41, 2006.
1. The Aluminum sheets (AA7020 T6) have the
biggest springback factor .
2. The springback factor equal one in lead alloy
of 2 mm thickness that indicate there is no
spring back in this sheet.
3. When the hardness of the sheet increase the
springback back factor (K) increase
4. In the aluminum and copper sheets when
the thickness increase the spring back factor
increase while in the lead alloy when the
thickness increase the springback factor
decrease.
5. Springback could be predicted effectively by
sheet thickness.
6. Springback can be minimized by reducing
the diameter of the die.
Thomas Schonbach, “New method to calculate and
compensate spring back” , Mashhad, Iran, 2008.
Aurelian Albut “ The sheet thickness effect on springback
phenomina” , University of Galati Fascicle
Technologies in machine building , ISSN 1221-4566,
2008).
Hani Aziz Ameen “effect of the combined biaxial moment
and axial load on the springback of a sheet”, American
Journal of Scientific And Industrial Research, 2010.
Jamal H. Mohammad “study the effect of the backing pad
on the U-bending process using FEM”, 2010.
Dilip Kumar K. “thinning and spring back of aluminum sheet
metal during L-bending operation”. International
Journal of Engineering Science and Technology, Vol. 2
, No.10, P. 5120-5129, 2010.
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