J. Cent. South Univ. Technol. (2009) 16: 0887−0891
DOI: 10.1007/s11771−009−0147−7
Deformation and defects in hydroforming of 5A06 aluminum alloy dome with
controllable radial pressure
XU Yong-chao(徐永超)1, LIU Xin(刘 欣) 1, LIU Xiao-jing(刘晓晶) 2, YUAN Shi-jian(苑世剑) 1
(1. School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China;
2. School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150001, China)
Abstract: A new process of hydroforming with controllable radial pressure was proposed to overcome difficulties in the forming of
low plastic materials and large height-to-diameter ratio workpieces. A typical 5A06 aluminum alloy dome was numerically and
experimentally investigated. The reasons for typical defects were analyzed under different radial pressures. Effects of radial pressure
on the thickness distribution were discussed and optimal radial pressure was determined. It is shown by numerical simulations and
experiment that a cup with a drawing ratio of 2.4 is formed by the new process of hydroforming with controllable radial pressure. It
is significantly effective for the forming of low plastic materials and large height-to-diameter ratio workpieces. Two typical thinning
points exit along the dome wall. With the radial pressure, thinning is decreased effectively at the two points, the dome forming is
achieved and thickness distribution is more uniform.
Key words: aluminum alloy; hydroforming; radial pressure; fracture; thickness
1 Introduction
As an advanced sheet forming process, sheet
hydroforming or hydro-mechanical deep drawing has
found a wide variety of applications in the field of
automobile and aerospace since it was invented[1−3].
Compared with conventional sheet metal deep drawing,
advantages of hydroforming include improved forming
limit, enhanced surface quality and reduced tool
cost[4−5]. Up to now, more than 300 types of workpieces
have been manufactured with hydroforming in Sweden,
Japan and Germany etc, among which the materials
mainly included low carbon steel and stainless steel, and
the thickness range of sheet metals was from 0.2 to
3.2 mm[6−8].
In China, hydroforming machines have been
developed, which has pushed the development of sheet
hydroforming greatly[9−10]. With the increasing
importance of protecting environment and saving energy,
lightweight materials such as aluminum alloy and
magnesium alloy have been applied increasingly[11−13].
5A06 aluminum alloy is widely applied in the field of
aerospace. Unlike low carbon steel, it has a poor
formability with small anisotropy parameter r and low
hardening index n[14]. 5A06 aluminum alloy dome in
aerospace is a typical complex-shaped cup with a
hemispherical bottom, and height-to-diameter ratio is
about 1.3. With conventional deep drawing, the dome
forming needs eight drawing procedures, and annealing
is necessary. Moreover, fracture is easy to occur because
of insufficient hardening and low load-carrying capacity
of material. The production efficiency is low, and the
percentage of finished product is 40%−60%, which
became a bottleneck to its application. Through
conventional hydroforming, it is still difficult to form.
Therefore, a new process of hydroforming with
controllable radial pressure was proposed for 5A06
aluminum alloy dome. Effects of radial pressure on
thickness and defects were explored, and reasonable
process parameters were obtained in this work.
2 Principle of hydroforming with controllable radial pressure
Fig.1 shows the principle of hydroforming with
controllable radial pressure. Based on conventional
hydroforming, a controllable radial pressure pr is
imposed to the rim of flange besides the chamber
pressure pc. Because sealing rings exist, the radial
pressure pr and the chamber pressure pc can be loaded
and controlled independently. Thus, the chamber
pressure pc increases the useful friction between the
punch and the blank, and the radial pressure pr aids to
push the flange to flow into the chamber when the punch
penetrates into the chamber. At the same time, the dual
fluid lubrication can be generated, which decreases the
friction between the flange and the blank holder, or
Foundation item: Projects(50525516, 50875062 ) supported by the National Natural Science Foundation of China
Received date: 2009−03−22; Accepted date: 2009−06−31
Corresponding author: XU Yong-chao, Associate professor, PhD; Tel: +86−451−86417917; E-mail: [email protected]
888
J. Cent. South Univ. Technol. (2009) 16: 0887−0891
between the flange and the die. Fluid lubrication and
radial pressure are helpful to reducing radial tensile
stress when deep drawing, therefore, serious thinning can
be eliminated and forming limit can be improved.
Fig.2 Illustration of dome hydroforming
Fig.1 Principle of hydroforming with controllable radial
pressure
3 Experimental
3.1 Dimensions and material
The dome diameter is 154 mm, and the total height
is about 190 mm. The blank diameter is about 370 mm.
The material used is 5A06 aluminum alloy sheet metal
with 1 mm in thickness. The mechanical properties
determined by tensile test are yielding stress σs=
160 MPa, strength limit σb=340 MPa, elongation δ=18%,
anisotropy parameter r=0.85 and hardening index n=
0.23. From the above parameters, it can be seen that the
formability is poor. Usually, limiting drawing ratio is
about 2.1 (height-to-diameter ratio 1.1) for 5A06
aluminum alloy with conventional hydromechanical deep
drawing. According to the dimensions of workpiece,
drawing ratio 2.4 (height-to-diameter ratio 1.3) can be
calculated approximately. The deformation is larger, and
it is difficult to form.
3.2 Loading path
The dome is a kind of complex-shaped workpieces.
Due to the hemispherical bottom, achieving a completely
formed workpiece depends on an appropriate loading
path of chamber pressure pc for a given material and
workpiece at first. On the basis of appropriate chamber
pressure pc, effects of radial pressure pr on thickness can
be determined.
Different from a cup with a flat bottom, for the
dome with a hemispherical bottom, there is a larger
suspended region of blank between the punch and the
blank holder, and a smaller contact region between the
punch and the blank in the initial stage, as shown in
Fig.2. With the punch penetrating into the chamber,
curvature radius r0 decreases gradually in the suspended
region. The above deformation behavior demands an
appropriate chamber pressure pc with deformation. On
one side, the chamber pressure cannot be too high, in
order to avoid the serious thinning due to a large radial
stress σ1 in the suspended region. On the other side, the
chamber pressure cannot be too low, in order to establish
an effective friction (µpc) between the punch and the
blank. According to the theoretical analysis and a few
simulations, an appropriate loading path of chamber
punch stroke is about 50 mm, the hemisphere is almost
formed, and the chamber pressure maintains 20 MPa, as
shown in Fig.3(a). Effects of radial pressure on the
forming can be analyzed after the appropriate chamber
pressure is determined. The radial pressure aids to push
the rim of flange to flow into the chamber. Loading paths
of radial pressure were designed, as shown in Fig.3(b).
The radial pressures are 5, 10, 20, 30, 40, 50, 60 and 65
MPa, respectively.
3.3 FEM model
The pre/post software DYNAFORM was used to
establish model for hydroforming with radial pressure.
The finite element solver was LS-DYNA. In order to
save calculation time, only quarter of the blank and the
tool components were simulated, as shown in Fig.4.
Belytschko-Tsay shell elements were used for the blank,
and the tool was treated as rigid bodies. The fixed gap
between the blank holder and the die was 1.2 mm.
In simulation, the work hardening law of material
was σ=Kεn. Strength coefficient K=549 and work
hardening exponent n=0.23. The artificial speed of punch
was 1 m/s. Friction was modeled between the blank and
the tool interfaces using the Coulomb assumption. The
friction coefficient was assumed to be 0.05 due to the
fluid lubrication between the blank and the die, 0.12
between the blank and the punch, and 0.05 between the
J. Cent. South Univ. Technol. (2009) 16: 0887−0891
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blank and the blank holder.
The pressure loading of hydroforming with radial
pressure was dynamic. Because the chamber pressure
and the radial pressure could not be applied to the
elements automatically through the pre-processor of the
finite element software, the keyword LOAD_MASK was
used to load the chamber pressure, which was supplied
by LS-DYNA. As to the radial pressure, because shell
elements had no thickness, the radial pressure could not
be applied to the rim of elements. So, equivalent nodal
forces were imposed to the nodes of the blank rim
instead. During the forming, the equivalent node force
was reduced gradually when the diameter of flange
decreased.
4 Typical defects in hydroforming with
radial pressure
Fig.3 Loading paths for hydroforming: (a) Chamber pressure;
(b) Radial pressure
Fig.4 Finite element model for hydroforming
For the cup with a hemispherical bottom, the
deformation behavior of blank was firstly analyzed in the
chamber pressure loading path with a radial pressure of
30 MPa. Fig.5 shows the deformation process. The
chamber pressure was about 8 MPa when the punch
travel was 27 mm in the initial stage, as shown in
Fig.5(a). In this time, because of the hemispherical shape,
the small chamber pressure and contact region resulted in
an insufficient friction force between the punch and the
blank, the flange of blank did not almost flow into the
chamber although a radial pressure of 30 MPa was
applied. This illustrates that the main deformation mode
of blank is stretching in the initial stage, and thinning
cannot be avoided in this time. The chamber pressure
would be 20 MPa when the punch travel was 50 mm in
the middle stage, as shown in Fig.5(b). In this stage, the
suspended region decreased, the contact region increased,
and the chamber pressure got higher. The flange of blank
started to flow into the chamber. The deformation mode
of blank changed into deep drawing from stretching. The
chamber pressure maintained 20 MPa after the middle
stage, and the flange transferred to the straight wall of
workpiece in the final stage, as shown in Fig.5(c).
Due to a large deformation, the workpiece was
difficult to be formed without radial pressure. However,
Fig.5 Deformation process of dome hydroforming: (a) Initial stage; (b) Middle stage; (c) Final stage
J. Cent. South Univ. Technol. (2009) 16: 0887−0891
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when a small radial pressure of 5 MPa, or smaller than
10 MPa was applied, fracture occurred at the bottom of
the hemisphere. The main reason was that, for the
enlarged blank, due to the small radial pressure, although
the useful friction force existed in the contact region
between the punch and the blank, the larger tensile stress
could not still be eliminated partly at the bottom. The
deformation mode was completely stretching, and
serious thinning caused the occurrence of fracture. Fig.6
shows the results of both simulation and experiment
under a radial pressure of 5 MPa.
When a large radial pressure 65 MPa was applied,
the tensile stress could be eliminated partly, and
load-carrying ability of blank was improved at the
bottom of hemisphere. So the hemisphere could be
formed in the middle stage without fracture. However,
due to a radial pressure 65 MPa, the average pressure
was larger between the blank holder and the blank, and a
caused unhelpful friction force was increased between
the die and the blank. It was difficult for the flange to
flow into the chamber. So, the fracture occurred near the
die profile when the tensile stress reached the strength
limit. Fig.7 shows the results of both simulation and
experiment under a radial pressure of 65 MPa.
5 Thickness distribution
Fig.8 shows the photo of a formed workpiece and
the thickness distribution obtained by numerical
simulation and experiment under a radial pressure of 30
MPa. In Fig.8(b), horizontal coordinate describes the
linear distance to the centerline, and vertical coordinate
describes the thickness at different positions. From Fig.8,
there are two lower thinning points in the thickness
distribution curve. The first lower thinning point is at the
bottom of the hemisphere, and the thickness is about 0.84
mm. The second lower thinning point is near the straight
wall, and the thickness is about 0.89 mm. The results
also show that it is easy to fracture at the two lower
thinning points, as analyzed in the above defects. It can
be seen from the experimental results that, the average
thickness is 0.86 mm at the two points, and the
experiment is in a good agreement with the simulation.
Fig.9 shows the effect of radial pressure on the
average thickness of two lower thinning points. Judging
from the average thickness, thinning ratio decreases
gradually when the radial pressure increases in the range
from 10 to 40 MPa, but thinning ratio increases when the
radial pressure increases in the range from 40 to 60 MPa.
The optimal radial pressure is 40 MPa, and the average
thickness is 0.88 mm at two typical thinning points. The
results demonstrate that radial pressure has positive and
negative effects on the thinning, which depends upon the
useful pushing force on the rim of flange and the friction
force between the blank and the die. When the positive
effect is more than the negative one, the flowing resistant
Fig.6 Fracture type under radial pressure of 5 MPa: (a) Simulation result; (b) Experimental result
Fig.7 Fracture type under radial pressure of 65 MPa: (a) Simulation result; (b) Experimental result
J. Cent. South Univ. Technol. (2009) 16: 0887−0891
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Fig.8 Photo of hydroformed workpiece (a) and thickness distribution (b)
radial pressure is 40 MPa, and the average thickness of
0.88 mm can be achieved.
References
[1]
[2]
[3]
[4]
Fig.9 Effect of radial pressure on average thickness
is decreased in the flange, the radial tensile stress is
reduced and serious thinning is suppressed. On the
contrary, the radial tensile stress is increased, and serious
thinning or fracture occurs.
[5]
[6]
[7]
6 Conclusions
(1) A cup with a drawing ratio of 2.4 is formed by
the new process of hydroforming with controllable radial
pressure. It is significantly effective for the forming of
low plastic materials and large height-to-diameter ratio
workpiece.
(2) For the dome hydroforming with radial pressure,
the main deformation is stretching in the initial stage,
and changes into deep drawing from stretching in the
middle stage. Thinning occurs, and two lower points are
respectively located at the bottom of hemisphere and the
straight wall.
(3) Fracture occurs at the bottom of hemisphere
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straight wall when the radial pressure is above 60 MPa.
(4) Thinning ratio decreases gradually when the
radial pressure increases in the range from 10 to 40 MPa,
but thinning ratio increases when the radial pressure
increases in the range from 40 to 60 MPa. The optimal
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