Manufacturing of Cups by Warm Hydroforming Process

Abstract

Warm hydroforming is an innovative forming technology in which sheet metals or tube materials are formed by pressurized fluid at warm forming temperatures. Warm hydroforming ensures an additional formability increase and decreasing the necessary fluid pressure and closing force according to hydroforming. Besides the parts manufactured by warm hydroforming shows more strength then hydroforming. There are two types of warm sheet hydroforming as in sheet hydroforming. If the used tool is punch the process named as warm hydromechanical deep drawing (WHDD) and if it is die the name is warm sheet hydroforming with die (WSHF-D). If it is desired to manufacture deeper cups, warm hydromechanical deep drawing process is suitable but if the cups are shallow warm sheet hydroforming with die can be applied. WHDD process is complex than WSHF-D. Because it is necessary that the blank should have a temperature gradient in order to achieve a successful forming and the fluid pressure and blank holder force should have a path according to position of the punch in WHDD process. But in WSHF-D all areas of the sheet metal blank are heated to same temperature and the pressure is increased linearly up to maximum pressure. So implementation of WSHF-D more easy. But transition height of the cups from WSHF-D to WHDD was not reported and if a cup can be manufactured by both process, which process ensures good forming not known. Therefore in this study manufacturing of a cup having 40 mm diameter and 20 mm in depth from AA 5754 blank having 1 mm thickness were simulated by Finite Element Analysis for both of the processes. Then obtained cups were compared in terms of accuracy of geometry, maximum thinning and necessary fluid pressure and closing force. Consequently it was found that maximum thinning was lesser and thickness distribution was more uniform in WHDD process. So it was concluded that WHDD process serve better forming in terms of manufactured cups. But as the part could be manufactured with non-defect with WSHF-D and WHDD process rather complex, the choice of WSHF-D for simplicity of the process wouldn't be wrong.

Full text

Manufacturing of Cups by Warm Hydroforming Process
Mevlüt Türköz1, Ekrem Öztürk2, H. Selçuk Halkacı1, Murat Dilmeç3
1Department of Mechanical Engineering, Faculty of Engineering, Selcuk University, Konya,
Turkey.
2Department of Mechanical Engineering, S.A.C. Engineering Faculty, Necmettin Erbakan
University, Konya, Turkey
3Department of Mechanical Engineering, Faculty of Eng. and Architecture, Necmettin
Erbakan University, Konya, Turkey
(E-mail: mevlutturkoz@selcuk.edu.tr, eozturk@konya.edu.tr, shalkaci@selcuk.edu.tr,
muratdilmec@konya.edu.tr)
Corresponding Author’s e-mail: mevlutturkoz@selcuk.edu.tr
ABSTRACT
Warm hydroforming is an innovative forming technology in which sheet metals or tube
materials are formed by pressurized fluid at warm forming temperatures. Warm hydroforming
ensures an additional formability increase and decreasing the necessary fluid pressure and
closing force according to hydroforming. Besides the parts manufactured by warm
hydroforming shows more strength then hydroforming. There are two types of warm sheet
hydroforming as in sheet hydroforming. If the used tool is punch the process named as warm
hydromechanical deep drawing (WHDD) and if it is die the name is warm sheet hydroforming
with die (WSHF-D).
If it is desired to manufacture deeper cups, warm hydromechanical deep drawing process is
suitable but if the cups are shallow warm sheet hydroforming with die can be applied. WHDD
process is complex than WSHF-D. Because it is necessary that the blank should have a
temperature gradient in order to achieve a successful forming and the fluid pressure and blank
holder force should have a path according to position of the punch in WHDD process. But in
WSHF-D all areas of the sheet metal blank are heated to same temperature and the pressure is
increased linearly up to maximum pressure. So implementation of WSHF-D more easy. But
transition height of the cups from WSHF-D to WHDD was not reported and if a cup can be
manufactured by both process, which process ensures good forming not known. Therefore in
this study manufacturing of a cup having 40 mm diameter and 20 mm in depth from AA 5754
blank having 1 mm thickness were simulated by Finite Element Analysis for both of the
processes. Then obtained cups were compared in terms of accuracy of geometry, maximum
thinning and necessary fluid pressure and closing force.
Consequently it was found that maximum thinning was lesser and thickness distribution was
more uniform in WHDD process. So it was concluded that WHDD process serve better
forming in terms of manufactured cups. But as the part could be manufactured with non-
defect with WSHF-D and WHDD process rather complex, the choice of WSHF-D for
simplicity of the process wouldn't be wrong.
Keywords: Finite Element Analysis, Hydroforming, Sheet Metal Forming, Warm
Hydroforming.
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1. INTRODUCTION
Warm hydroforming process has being implemented since beginning of 2000’s by combining
the advantages of warm forming and hydroforming processes [1]. Warm hydroforming
process offers part consolidation and reducing number of manufacturing steps in addition to
warm forming by increasing the formability of the materials [2]. In addition warm
hydroforming ensures an additional formability increase and decreasing the necessary fluid
pressure and closing force according to hydroforming [3].
Warm hydroforming process are performed by using male or female die. If the used tool is
punch the process named as warm hydromechanical deep drawing (WHDD) and if it is die the
name is warm sheet hydroforming with die (WSHF-D) [3]. Most of studies were conducted
on WSHF-D process and in here the sheet was formed by heated fluid after it was heated to
certain temperatures between the tools (Figure 1). In this process the pressure is increased
linearly up to maximum pressure. In WHDD process it is necessary that the blank should have
a temperature gradient in order to achieve a successful forming. So the punch is cooled and
upper and lower dies were heated (Figure 2). For a successful warm hydroforming process,
many process variables should be controlled appropriately. Besides the temperatures of the
tools the fluid pressure and blank holder force should have a path according to position of the
punch [2]. Therefore implementing of the WHDD process is rather difficult according to
WSHF-D. But if it is desired to manufacture deeper cups, WHDD process should be
performed. Because the fluid pressure between the die and blank restrains flowing of the
blank into the die and this situation make possible only manufacturing of shallow parts by
WSHF-D process.
Figure 1. Warm sheet hydroforming with die (WSHF-D) process [4]
Figure 2. Warm Hydromechanical Deep Drawing Process (WHDD) [5]
In the studies about warm hydroforming the transition height of the cups from WSHF-D to
WHDD was not reported so far. In addition if a cup can be manufactured by both process,
which process ensures good forming not known. Therefore in this study both processes were
compared with each other in terms of the quality of the manufactured cups by conducting FE
Analyses of the processes.
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2. MATERIAL AND METHODS
In the study manufacturing of a cup having 40 mm diameter and 20 mm in depth from AA
5754 blank having 1 mm thickness were simulated by Finite Element (FE) Analysis for
WSHF-D (Figure 3) and WHDD (Figure 4) processes. AA 5754 material is commonly used
by automotive manufacturers, especially for auto body interior structures. In both of the
process the blank diameter was 71 mm. The radiuses on the part were 5 mm.
Figure 3. SHF-D test setup
Figure 4. WHDD test setup and thermal conditions [6]
FE analyses were performed in commercially available FEA package Ls-Dyna solver. The FE
model of the processes were constructed in Ls-Dyna pre-post.
A quarter-model of the axisymmetric geometry of cylindrical cups, were used to take fast
solution (Figure 5a). 3D quadrilateral fully integrated shell element formulation with five
integration points through the thickness was used to model the blank. As for rigid tools,
“Belytschko-Tsay” element formulation with three integration point was assigned to shorten
the simulation time. The blank modelled with 2500 elements.
Symmetry boundary conditions was applied the edges of the blank and the fluid pressure area
was determined by mask loading condition. In addition the blank holder force was applied on
the blank from the blank holder and the encastered boundary condition was applied to the die.
The blank was modelled as elastic visco-plastic whereas punch, die and blank holder were
assumed as rigid. Elastic properties for AA 5754 blank material such as modulus of elasticity,
shear modulus, Poisson’s ratio are 70 MPa, 26.9 MPa and 0.33, respectively. Flow behaviour
of the material at elevated temperatures were taken from a previous study [7] as seen in
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Figure 5. Elastic Viscoplastic Thermal” (MAT106) material model was used by entering the
flow curves at different temperatures as tabulated.
Figure 5. a) A quarter of 3D model of the process, b) Boundary conditions of the blank [6]
Figure 5. Flow behaviour of AA 5754 at elevated temperatures [7]
While a temperature of 300 °C was assigned to the blank holder and die, the temperature of
the punch was taken as 20 °C. At the beginning of the FE analyses also the temperature of the
blank were 20 °C in WHDD process. By conducting coupled structural thermal analysis,
firstly the heat transfer between the blank and tools were calculated. So the blank had a
temperature gradient. While the centre of the blank was cold, the flange region of the blank
was heated up to the temperature of the blank holder and die. But in WSHF-D process all
region of the blank has the same temperature. Because there were not any affect that cooled
the blank. In SHF-D process 300 °C was assigned to the blank holder and die. Hence the
blank had a temperature of 300 °C.
The thermal condition of the blank was affected from the thermal coefficients used in the FE
model for contact surfaces and parts. These are heat transfer conductance (HTC), thermal
conductivity of the fluid between the two sliding surfaces (CF), critical gap distance GCRIT
under which HTC is constant. GMAX is the maximum gap above which no thermal contact is
assumed. This properties was applied as contact condition. In addition thermal conductivity
(TC) and heat capacity of the blank and tools (HC) were determined as thermal isotropic
material properties of the blank and tools. Thermal coefficients were shown in Table 1. The
effects of the thermal coefficients to the temperature condition of the blank was investigated
in a previous study [8]. The most suitable thermal coefficients given in Table 1 were
determined in this study.
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Table 1. Thermal coefficients used in FE simulations
HTC
(W/m
2
.K)
CF
(W/m.K)
HC
(J/kg.K)
TC
(W/m.K)
GCRIT
(m)
Thermal coeff.
1400
10
Tools=4200
Blank=900
Tools=50
Blank=220
0.005
“Forming_One_Way_Surface_To_Surface” contact algorithm was used for determining
contact behaviour between the blank and tools. Coulomb friction model with a friction
coefficient of 0.05 was determined for the interacting interfaces between the blank-blank
holder and blank-die.
The fluid pressure and blank holder force (loading profiles) should have a path according to
position of the punch in WHDD process but the fluid pressure increases linearly and blank
holder force can be fixed in WSHF-D process. The loading profiles seen in Figure 6 were
used in the FE analysis of WHDD process. In order to obtain a successfully formed part, it is
necessary to use convenient loading profiles. So the convenient loading profiles could be
determined traditionally by many trial and error experiments. But this would need long time,
manpower, and high cost. Alternatively, computer simulations coupled with certain methods,
such as design of experiments, can be conducted to determine proper loading profiles and
minimize experimental trials [9]. The loading profiles given in this study were determined by
adaptive FE analyses coupled with fuzzy control algorithm which was taken from study of
Türköz [10].
Figure 6. Loading profiles used in WHDD process FE analysis
The loading profiles used in WSHF-D FE analysis were given in Figure 7. In here the
pressure of 20 MPa was sufficient to obtain target part. The same blank holder force profile
with in WHDD process was used to compare the parts.
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Figure 7. Loading profiles used in WSHF-D process FE analysis
3. RESULTS AND DISCUSSION
The parts obtained by WHDD and WSHF-D processes were given in Figure 8. All dimensions
and materials of the part is the same with each other. The difference was the blank took the
shape of the punch in WHDD process and it took the shape of the die in WSHF-D process.
Although the part had taken the same geometry quality of the parts was different from each
other. The quality of the parts were compared by change of the thickness of the blank as seen
in Figure 9. The thickness of the parts reduced on the base of the cups and it increased on the
flange region. While the maximum thickness reduction was 5% on the base radius of the cup
which was formed by WHDD, it was 26% on the centre of the cup which was formed by
WSHF-D process. The failure criteria was assumed as thickness reduction of the blank 30%
as in the study of Choi et al. [2]. So the part formed by WSHF-D did not fractured. While the
difference of the thickness change on the part formed by WSHF-D was 42%, it was 29% on
the part formed by WHDD. Hence the thickness distribution was more uniform on the part
formed by WHDD. So it was concluded that the quality of the part formed by WHDD was
better than the part formed by other method.
a) b)
Figure 8. The part formed by a) WHDD and b) WSHF-D processes
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Figure 9. Thickness distributions of the parts formed by WHDD and WSHF-D processes
While the part took the shape of the tool with a pressure of 17 MPa in WSHF-D process, the
necessary fluid pressure was 32 MPa in WHDD process. In addition implementation of the
WHDD process was more complex because of the difficulty of determining the convenient
fluid pressure curve. In the study it was concluded that as the part could be manufactured with
non-defect with WSHF-D and WHDD process rather complex, the choice of WSHF-D for
simplicity of the process wouldn't be wrong. But if it is desired to form deeper parts, a unique
choice is WHDD process. Because the part with 20 mm height could be manufactured hardly
in it’s forming limit.
4. CONCLUSION
In the study a part was formed by warm sheet hydroforming which is an innovative forming
technology. Warm sheet hydroforming has two types according to used tool is male or female.
When the male tool was used the process named as Warm Hydromechanical Deep Drawing
(WHDD). If the used tool is die it is named as Warm Sheet Hydroforming with Die (WSHF-
D). Forming of a cup having 40 mm diameter and 20 mm in depth with 5 mm base and flange
radiuses were analysed by FE method for both of the processes. And obtained parts were
compared by fracture situation, thickness distribution and necessary fluid pressure. It was
concluded that the part formed by WHDD process serve better forming in terms of
manufactured cups because of thickness reduction was considerably less. But as the part could
be manufactured with non-defect with WSHF-D, necessary fluid pressure was less in WSHF-
D and WHDD process rather complex, the choice of WSHF-D for simplicity of the process
wouldn't be wrong.
ACKNOWLEDGMENT
This work is supported by The Scientific and Technological Research Council of Turkey
(TÜBİTAK). Project number: 112M913. Project Title: “Investigations on the Integral Effects
of Temperature, Pressure and Blank Holder Force Variation on the Warm Hydromechanical
Deep Drawing Process and Parts Produced”. TÜBİTAK support are profoundly cknowledged.
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