Audi TT (2007 year). Manual — part 257

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permitted unless authorised by AUDI AG. AUDI AG does not guarantee or accept any liability
with respect to the correctness of information in this document. Copyright by AUDI AG.

5

Technological concept

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Aluminium castings

Sheet-aluminium parts

Aluminium extruded sections

Sheet-steel parts

Sheet-steel parts are used for the first time in the
ASF of the new Audi TT in addition to aluminium
castings, aluminium extruded sections and alumin-
ium sheet-metal parts. Collectively, they make up
the body structure. Vehicle weight distribution has
been optimised by using sheet-steel parts in the
rear body section. This has a direct bearing on
sporting characteristics, such as driving dynamics
and acceleration as well as safety characteristics,
such as stopping distance and driving stability
In spite of the partial use of sheet-steel parts, the
total body weight of 277 kg, including attachments
such as doors and lids, is considerably less than
that of a comparable all-steel body.

Although the new TT has grown in size, the gross
weight of the vehicle has been reduced through the
use of the aluminium-steel bodyshell.

The body structure of the new Audi TT has higher
strength and 50 % higher torsional rigidity than its
predecessor.

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permitted unless authorised by AUDI AG. AUDI AG does not guarantee or accept any liability
with respect to the correctness of information in this document. Copyright by AUDI AG.

6

Audi Space Frame of the Audi TT

383_004

Contact corrosion

Aluminium has a passivating oxide surface layer
which protects the material underneath from corro-
sion. This is why an unpainted aluminium part nor-
mally does not corrode.

However, if there is contact between aluminium and
a metal higher up in the electrochemical series than
aluminium and if an electrolyte, such as salt water,
is present in this region, contact corrosion will
occur.

The greater the difference in potential, the greater
the contact corrosion. As aluminium is normally the
lesser noble metal, it degrades.

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Contact corrosion can only be prevented by taking
measures to stop the flow of electrical current
induced by the potential difference between
the two metals. This is best achieved by painting the
surfaces. However, the risk of corrosion is high even
if a tiny amount of paint damage occurs.

In the case of the Audi TT, the following anti-corro-
sion measures have been taken:

– coating of all steel screws and fasteners such as

self-piercing rivets

– Galvanising of all sheet-metal parts (zinc and alu-

minium have a smaller potential difference than
steel and aluminium)

– Insulation by means of adhesive
– Sealing of aluminium-steel joints

Reference

For detailed information on contact corrosion,
please refer to Self-Study Programme 239 "Audi
A2 - Body" .

Schematic of contact corrosion with example

Contact corrosion

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permitted unless authorised by AUDI AG. AUDI AG does not guarantee or accept any liability
with respect to the correctness of information in this document. Copyright by AUDI AG.

7

Joining steel and aluminium

Thermal joining processes such as MIG welding can
be ruled out because it is not possible by these
means to make a joint which has the requisite struc-
tural and dynamic strength and will not result in
contact corrosion.

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Steel

Aluminium

One of the challenges for the development of the
Audi TT bodyshell was the attachment of the sheet-
steel rear-end parts to the aluminium body assem-
blies.

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Aluminium-steel connections

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permitted unless authorised by AUDI AG. AUDI AG does not guarantee or accept any liability
with respect to the correctness of information in this document. Copyright by AUDI AG.

8

Audi Space Frame of the Audi TT

This is ensured by non-thermal joining of parts
using coated self-piercing rivets and special screws
in combination with bonding.

Special requirements are made with regard to the
strength and corrosion protection of the joints
between aluminium body parts and steel parts.

Adhesive bonding is the basis for the corrosion pro-
tection of corrosion-susceptible aluminium/galva-
nised steel joints in the bodyshell of the Audi TT.In
this way, the mating materials are superficially insu-
lated, thus suppressing corrosion processes at the
point of contact. As a further measure, all compos-
ite joints either sealed with PVC or coated with wax
preservative after the cataphoretic dip coating (KTL)
process.

Aluminium and steel joints with faulty corrosion pro-
tection can exhibit much higher corrosion rates than
all-aluminium joints or all-steel joints.When making
aluminium-steel joints, therefore, highest standards
of quality must be maintained at all times during the
production process and in the service workshop.

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Adhesive

Adhesive