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SAE TECHNICAL
PAPER SERIES
2002-01-2025
Benefits of Hydroformed Components
for BIW Applications
Francesc Perarnau
Gestamp Metalbages s.a.
Stéphane Tondo
Arcelor Auto
Reprinted From: Proceedings of the 2002 SAE International Body Engineering Conference
and Automotive & Transportation Technology Conference on CD-ROM
(IBAT2002CD)
International Body Engineering Conference & Exhibition and
Automotive & Transportation Technology Conference
Paris, France
July 9–11, 2002
400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (724) 776-4841 Fax: (724) 776-5760 Web: www.sae.org
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2002-01-2025
Benefits of Hydroformed Components for BIW Applications
Francesc Perarnau
Gestamp Metalbages s.a.
Stéphane Tondo
Arcelor Auto
Copyright © 2002 Society of Automotive Engineers, Inc.
ABSTRACT
Strategic aims of the automotive industry are the
minimisation of the costs and the optimisation of its
product’s performances. Looking for alternative
production processes to improve weight, parts reduction,
strength characteristics and rigidity, hydroforming offers
interesting
technical
and
economic
potentials.
Traditionally used in exhaust and chassis applications,
the use of hydroforming is now being rapidly extended to
Body in White application, like structural reinforcements
or closures frames.
To take benefits both from the material and the process
sides, a Spanish components supplier and major
steelmaker have developed optimised turn-key solutions
using advanced high strength steels combined with
hydroforming. Some of these solutions on modules like
the front-end module or back-doors and showing
interesting performances in terms of weight lightening
will be detailed, including description of the
manufacturing process and comparative costs
estimations.
INTRODUCTION
Currently well known by the automotive industry, but
mainly used for drive-train and chassis applications,
tubular hydroforming also offers great interest for body in
white components. The body in white (BIW), including
closures accounts for ca. 30% of the total vehicle weight,
and therefore offers one of the greatest potentials for
weight reduction and savings.
New techniques, and tubular hydroforming is a typical
example can allow designers going beyond classical
weight savings made typically by just material
replacement. The redesign of the hydroformed
components, necessary to make them feasible using this
technology, requires to redefine the perimeter concerned
by the components and often allows by gathering of
former components to reduce the number of parts. This
obviously also leads in a reduction of the amount of
“unnecessary” material like welding flanges of stamped
shells or the higher thickness that is required from
assembled hollow section to be competitive in terms of
inertia with tubes.
Based on this analysis, hydroforming could potentially
be applied to many BIW components. Most of them have
already been studied by one or another carmaker, with
more or less success, the perimeter of the function
considered and the volume of the series production
playing a major role on the profitable use of
hydroforming. However, this technique shall be carefully
considered for some applications by each new
development, and not only cars based on space frame
concepts. A non exhaustive list of applications to study
for body in white and closures could be defined as
following:
•
•
•
•
•
•
•
•
•
•
A-Pillars, single or multi-thicknesses
B-Pillars reinforcements, straight or conical
Side Roof rails, combined or not with A-pillars
Structural nodes to assemble profiles, stamped and
hydroformed components
Windshield frame upper and lower crossmembers
Instrument panel beam
Side rails
Bumper beams and shock absorbers
Door frames
Hatchback frame
However, weight saving on these components can be
achieved only using equivalent steel grades: the use of a
“softer” material would lead to an increase of the
thickness to fulfil the rigidity and energy absorption,
requirements that would compensate the benefits taken
from the close hollow section.
In the first years of the industrial use of hydroforming for
automotive components, a general trend was to use
annealed tubes. These tube offer, thanks to the heat
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treatment a high deformation potential. But they can not
reach the yield and tensile strength levels of the high
strength steels used for stamped parts (typically
between 450 and 800 MPa Yield), can not be pre-coated
(the heat treatment destroying the coating) and
furthermore are more expensive. In conclusion, they
could be used and hydroforming applied only for
components for which designers imposed hydroforming
for geometrical reasons and hydroforming was not
considered as a real weightsaving and economical
alternative.
Therefore the use of steel grades with high tensile
strength and good work hardening coefficient that lead
after the different forming steps, including tube forming,
to high mechanical properties and especially a high yield
strength appears as necessary. Such steels are typically
from the Usiphase T (Trip) and Usiphase D (DP) ranges.
Obviously, for parts requiring only a high inertia, steels
offering a high deformation potential like the Usistamp
range (DDQ) are perfectly suited.
steel hatches in terms of weightlightening, the idea of
optimising the performance to weight ratio by using a
tubular frame has been developed for some years by the
steel industry. A first hatch concept using a tubular
frame has been presented in 2001 by the ULSAC
Consortium. This first concept was based on an
hydroformed tubular O shaped windshield frame on
which a stamped inner panel and an hydroformed outer
lower panel were assembled using laser welding (fig 1).
Compared with hatches currently under production, this
solution offers high technical performances in terms of
torsional rigidity and bending stiffness combined with a
17,6% weight reduction for an equivalent hatch surface
compared to the best solution measured. Obviously, the
use of latest production techniques like laser welding
and sheet / tubular hydroforming that allowed the
manufacturability of this hatch resulted in a significant
cost increase compared to a conventional solution.
Nevertheless, in comparison to an average market
hatch, this solution offers for 3,3 euros additional cost for
each saved kilogram a lightweight and high performance
solution.
To promote the use of its steels, the European leader for
automotive steels optimises and develops new steel
concepts for automotive modules combining the use of
hydroforming or other forming techniques with the latest
steel grades. Its Spanish partner that is currently one of
the largest hydroformed components manufacturer
validates the industrial feasibility and the economical
aspects of these concepts that can be afterwards
applied by the customers of both companies.
We will analyse hereafter two different kinds of solutions
that have been developed by Usinor Auto and
Metalbages. The first on, a hatch with tubular frame
represents a solution considered as optimum using the
current technologies and grades while the second,
hydroformed bumper beams and shock absorbers is
more a modular steel solution that shall be composed to
fit the customers requirements in an optimal way.
HATCH WITH TUBULAR HYDROFORMED
FRAME
This hatchback concept is the typical example of an
innovative steel solution that can be developed through
a steel maker and an equipment manufacturer. After a
generic concept development by the steel maker, the
parts manufacturer has validated the solution from the
industrial and economical points of view. This validation
resulted in minor changes on the geometry to guarantee
a higher productivity.
SOLUTION DESCRIPTION
Generally made out of two stamped sheets, one for the
outer panel and one as reinforcement, the classical steel
hatch meets a high competition from injection moulded
plastic part hatches. To improve the competitiveness of
Fig.1 –Hatch Concept - ULSAC
Parallel to this development, we initiated a study of a
steel hatch for which the target was to reduce both the
weight and the manufacturing costs without changing
the design or decreasing the performances requested.
The reference for this study was the hatch of a compact
that just arrived on the market. To reach this goal, two
main design constraints were defined: avoid overperformances to use only material where required and
try to avoid the use of investment consuming techniques
like sheet hydroforming or laser welding. The main
technical targets for the hatch module are a high self
rigidity and a good resistance to the strains created
during the handling of the hatch like opening and
slamming. Indeed for all of these loading condition,
plastic deformations must be avoided on closures to
ensure the perfect sealing against the car body and
harmful vibrations coming from free movement degrees.
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Obviously, a good oil-canning resistance of the lower
skin panel, a good mechanical resistance and spot
weldability of the hinges and lock areas as well as
functional surfaces for the bonding of the glass, the
support of the rear windscreen wiper and its motor were
requested.
To fulfil the mechanical requirements, the use of a rigid
frame appeared quickly as the optimum solution and a
tube offered the best stiffness to weight performance for
such a rigid frame. Nevertheless, the needs to respect
the original design and satisfy all complementary
functions by offering the requested functional surfaces
conditioned the use of the hydroforming technology to
shape the tubular frame. To also have the optimum
rigidity of the frame, especially in flexion, it has been
decided to use a U shaped integral frame, the tube
getting down up to the hatch’s bottom. The transversal
beam of the U being placed in upper position to be also
used as support for the hinges and offering a continuous
surface for the windscreen glass. Two transverse
stamped beams were added, one at the bottom of the
hatch for the lock support and one at the bottom of the
glass to support the rear windscreen wiper. The
complete principle, patented by Usinor is illustrated on
fig. 2.
PROCESS ANALYSE AND VALIDATION
A high quality 54 x 0,8 mm tube has especially been
developed, the inertia of this tube being sufficient to fulfil
the several technical requirements. The challenge was
to have a tube in these unusual dimensions with good
hydroforming ability and especially a good surface
aspect for all positions where the tube can be sawn after
assembly.
Nevertheless on the 6 bends of the tube, the bending on
the two upper hatch corners presented the main
difficulties. The choice between a multiple balls mandrel
or a spoon mandrel plays an important role, having
either influence on feasibility or on the productivity (Fig.
3). The tolerances on the perimeter and on the thickness
of the tube delivered have also a high influence on the
bending results that are critical for this kind of thin wall
tube. They should be limited to 0,3 mm for the diameter
and 0,1mm for the thickness tolerance.
Fig. 3 – Bended shape
Fig. 2 –Hatch concept – USINOR
At the contrary to the ULSAC study, our team decided to
have parallel tube ends to make its production by
hydroforming easier by allowing an easier positioning of
the sealing cylinders. It was also decided to use
conventional stamping for the outer skin panel, the
selected gauge of 0,6 mm allowing to realise the defined
shape using this technique. This typical gauge was
selected to satisfy most of carmaker’s thickness
requirements for skin panels, additional downgaging
increasing the risk of residual deformations after oil
canning.
According to the selected hatch geometry, a lateral
crushing of the sections to fit them in the hydroforming
die might be necessary. This option has been
considered for 1 of the hatches analysed (Fig. 4).
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the upper cross member is regularly spot welded on
outer skin panel; the lower cross member is crimped
against the outer skin panel; and hinges and lock are
MIG welded. Both members are filled with mastic joints
to ensure a good sealing. The dampers are fixed with
bolts that are directly riveted on the hatch (Fig 6.)
Fig. 4 – Thicknesses after preforming
The hydroforming process itself consists more of a
calibration of the section obtained during preforming and
tool closing. This is the reason why the required
pressure is limited to 1000 bar and makes no axial
feeding (unless for sealing) necessary. However, the
radii of all sections have to be optimised to answer the
design requirements without requiring a higher
calibration pressure that would occur in breaks in other
sections. The hydroforming simulation results given from
Pamstamp are quite good for this thin wall tube and the
maximal local thinning measured is about 25%, result
that can be reached in production with the selected
grade (Fig 5.).
Fig. 6 – Assembly of the bolts for damper attachment on
the hydroformed tube
All steel components are obviously galvanised and can
optionally be organic coated for higher corrosion
resistance and sealers reduction.
FUNCTIONAL VALIDATION AND TECHNICAL
ADVANTAGES
The use of a single tube that fits the complete external
shape of the hatch allows to build a very rigid frame
compared to conventional solutions: the tube benefits
from its natural continuous liaison offering an optimal
rigidity compared to discontinuous assemble sections.
Furthermore, the inertia of the tube for the same amount
of material being higher than the inertia of a stampedwelded hollow section leads in a high performance
weight-optimised solution.
The typical loads simulations for unitary loads are
described below. The real measurements have not been
realised yet.
•
Fig. 5 – Thicknesses after hydroforming
The use of low carbon high stampable steel grades for
the two cross members and for the tube makes the
welding conditions easy to define due to low charged the
chemical composition. For a higher self-stiffness of the
outer skin panel and due to the reduced gauge, a
Usiphase D 450 (DP450) has been selected.
As result, the assembly of all steel components can be
achieved with existing equipment: both cross members
are assembled on the tube by single side spot welding;
Flexion: with hinges free only in Y axis rotation,
hatch locked in translation in Z axis on the damper
attachments and unitary load centred at the lock’s
position, the maximal flexion measured is of
25N/mm (Fig. 7).
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%DVHIRUVWDPSHGVWHHOKDWFKDWGD\
•
Torsion: with hinges free only in Y axis rotation,
hatch locked in translation in Z axis at only one of
the damper attachments and a unitary load centred
at one of both lower corners of the hatch
(symmetrical), the maximal torsion measured is of
6mrad (Fig 8.).
Fig. 8 – Description of the torsion conditions
The weight of the hatch (without glass) for a typical a M1
range vehicle will be using this concept between 7,5 kg
and 8,5 kg to be compared with the average value of
11,5kg for hatches currently under production. This
represents a weight-saving potential of 25 to 35%.
ECONOMICAL BENEFITS OF THIS SOLUTION
The following graph describes the estimated costs for a
complete assembled hatch with hydroformed frame
(excluding glass) compared to the stamped reference
and depending on the production level. Reference for
this calculation is a stamped hatchback at a daily
production rate of 2000 pieces.
6WDPSHGVWHHOKDWFK
Fig. 7 – Description of the flexion conditions
+DWFKZLWKK\GURIRUPHGIUDPH
Fig. 9 – Cost comparison hydroformed vs. Stamped
costs savings (if value >0) or additional cost (if value <0)
for each saved kg using the solution presented instead
of a conventional stamped hatch.
The general trend cost reduction compared to a
stamped solution is mainly due to the reduction of the
material scraped during forming, even if this benefit is
partially compensated by the over-cost of the tube
compared to the blanks.
Logically, the influence of the investments required for
the production of the hydroformed component reach its
minimum at full capacity so for approximately 1700 parts
per day. And after this volume a new investment is
required. But the equilibrium compared to the stamped
solution is reached at 750 parts per day. That means
that for less of 750 parts per day, the cost of the
weightlightening varies between 1,5 and 0 Euro per
saved kg (graph 10). Over 750 parts per day, this
solution allows in any case cost savings, even taking
into account the additional investment required after
1700 units per day. In conclusion, this solution
surpasses its both targets, an average weightlightening
of ca. 30% combined with a simultaneous cost reduction
of up to 6%.
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Shock Absorber
Integrated hydroformed
6DYHG¼SHUHDFKVDYHG.J
Fig. 11 - Integrated Beam System
3DUWVSURGXFHGSDUGD\
Fig. 10 – Cost savings per each saved kg
HYDROFORMED FRONT-END SOLUTIONS
BUMPER SYSTEMS
To improve low speed damageability, special attention
has been take into account to hydroform, specially the
Bumper Systems. The Hydroform Bumper system,
which integrate the main beam with the shock
absorbers, has been designed to offer low cost and easy
repairs in the event of an accident. Insurance rates are
an important consideration when purchasing a vehicle.
The Design of Hydroforming Beam Bumpers offers an
easy reparability after low speed (15km/h.) damage.
Hydroforming designs can be used in space-frame
constructions of vehicles because they provide various
advantages. Such advantages are due to the use of
closed hollow profiles under the aspect of their loading
capacity and productivity, and the fact that special
shaped contour elements can be integrated and used as
direct joint elements to other parts. Figure 1 shows the
result of a complex simulation of a three-dimensional
hydroforming beam bumper.
They are Hydroformed beams which allows high energy
absorption over a short deformation length. The Tube
material properties used (650- 800 N/mm2) are related
to the stiffness of the structure and allow, together with
design features in the side rails and shock absorbers, a
progressive collapse of the front / rear structure. All
these features are optimised to avoid damage of
adjacent components. Bumper system has been
optimised for shape and material.
The use of high strength steel like Usiphase D600 / or
DP750 (yield strength = 600 N/mm2) enables the
system to absorb a very high kinetic energy related to its
weight and cross-section. With higher impact speed the
side rail collapses progressively. Tube lightweight
materials are used for the energy absorbing bumper
crossmembers.
To develop this integrated beam system a pre-forming
operation of tube bending is needed. It can be produced
the simulation of bending operations by using one-step
solver. Although some of the effects caused by bending
cannot be simulated using shell elements. Apart from
that, the static and dynamic properties of the component
are influenced by the geometry that is obtained during
the forming process, the resulting work hardening and
residual stresses.
The Hydroforming Beam System designed replaces five
stamping pieces (one stamping beam and two stamping
crash boxes at both ends with two welded stamping
parts each one).This design allows good impact energy
absorption (5200J) with low speed (15km/h-Crash)
damageability and reparation cost reduction.
Fig. 12 - principle of a bending machine with bend die,
mandrel and wiper die
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SHOCK ABSORBERS
To improve low speed damageability, also special
attention has been take into account to hydroform,
specially
the
shock
absorbers.
Homogeneous
wallthickness is needed around the hydroformed piece
and low material deformation to guarantee the Allianz
Test (Crash impact at 15km/h).
New hydroforming design concepts have been made in
order to integrate in one hydroforming piece the function
of two spotwelded stamping pieces. It can be obtain a
single piece HSS (high strenght steel) crash box with
low cost and weight reduction.
Fig. 13 - one-step simulation results (thinning) with a
thickness reduction of 14% at both edges. Initial tube
material needed is 80mm diameter and thickness 1,8.
Another front-end concept consists of a bumper beam
which is also manufactured using hydroforming, but
associated with separated shock absorbers. The shock
absorbers can be either hydroformed, stamped or
profiled and some examples will be detailed in the next
chapter. The beam is realised out of a high strength
steel tube, for example Usiphase D 600 (DP600) which
is expanded during hydroforming to realise the optimum
profile and cross sections and benefit simultaneously
from the high work hardening from this kind of
metallurgy. The result is a very rigid beam compared to
its weight and that offers a very cost competitive solution
for small to medium range vehicles in middle to high
volume production. One example of this beam is shown
on fig 14.
Fig. 15 - Two spotwelded stamping pieces integrated in
one hydroforming piece
This concept design allows several advantages : Good
energy absorption and cost ratio ( No assembly, low cost
due to multi-parts production and easy process, regular
energy absorption and avoid initial peak, obtain 2
hydroforming pieces from 1 initial tube); suitable for high
easy adjustable to energy
volume productions;
absorption required (thickness, grade, length); various
shapes allowed (round-round, round-square, squaresquare).
The use of high strength steel DP600 / DP750 (yield
stress = 600 N/mm2) enables the system to absorb a
very high kinetic energy related to its weight and crosssection. HSS DP600 is suitable to be used for
hydroforming crash boxes.
Fig. 14 - Hydroformed beam for front-end module with
separated shock absorbers (non figured)
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the shock absorber. Work force is about 70KN, from this
moment the deformation appears on the rail side. On the
first 40ms the energy absorption of the shock absorber
is 6000J (50%). At the end the energy absorption due to
the impact is 9250J (77%) by the crash box and 2250J
(19%) by the vehicle.
CONCLUSION
Fig. 16 - Wallthickness reduction results by calculation
with PAM-STAMP
Hydroforming shall not be considered only for low
volume productions where it can obviously offer a higher
weight saving than stamped solutions, but often requires
an extra charge for each saved kilogram. This technique
reaches its bottom costs for volumes of about 1500 to
1800 parts per days in case of single components
productions. We saw that for volumes over 750 parts per
day, hydroforming offered in ant case a cheaper solution
that the original stamped one.
This hydroforming design concept is suitable to produce
the shape with only hydroforming process directly. No
pre-forming operation is needed. To get this final shape
is needed a close force of Fz=15.153 (KN) and internal
pressure Pi=2.536 bar.
Associated with the use of dedicated high strength
steels, hydroforming also offers very high technical
advantages and especially in terms of weightlightening
in comparison to hollow components made from
discontinuously assembled stamped components.
Tube diameter of 72mm, wallthickness of 3.0mm, and
material specified as St34-2 mod.NBK (Re=280 N/mm2,
Rm=380 N/mm2, A=50%, Ag=36%) made feasible its
hydroforming process and it can be guaranteed its
behaviour in front of Crash impact to assume the energy
absorption.
Both advantages combined allow us to develop new
solutions offering a high price/performances/weight
competitiveness for BIW and closure applications on
parts that would have traditionally been manufactured
either as stamped steel solutions or using different
materials.
This concept design allows good impact energy
absorption (9500J) with low speed impact (15km/hCrash) as required the automotive industry.
CONTACTS
•
Francesc Perarnau – R&D Manager
Gestamp metalbages
Tel: (+34) 93 827 3940
•
Stéphane Tondo – Product Manager Hydroforming
Usinor Auto
Tel: (+33) 3 44 55 72 66
DEFINITIONS, ACRONYMS, ABBREVIATIONS
HSS: high strength steel. Steel having generally
speaking a yield strength over 300 Mpa
BIW: Body in white
Fig. 17 - Hydroforming Crash Box and crash simulation
This concept design allows good energy absorption
advantages: untill 40ms the deformation is controlled by