Al Biw

Aluminium in Innovative Light-Weight Car Design
821
Fig. 5 Example of chassis components made of aluminium.
Fig. 6
Weight-reduced aluminium front end of BMW 5-series GRAV (Source: BMW).
condition. For certain parts, however, that experience longterm thermal loads (as e.g. in the vicinity of the cars engine
or exhaust system) the Mg content is limited to 3 mass% to
avoid the potential danger of intergranular corrosion (IGC),
due to (slow) possible microstructural changes by Mg
precipitation along grain boundaries (known as sensitization)
at elevated temperatures (>70 C) with an exposure to
aggressive environment. Special 5000 series alloys (e.g.
alloy AA5042-AlMg3.5Mn) have been developed where
sensitization is avoided by special alloy additions and
processing steps, yielding a favourable combination of static
strength and IGC resistance, now being applied in series
production for chassis applications.
6.
Aluminium in the Vehicle Structure
In this recent 5-series cars BMW achieved a weight of

47.6 kg for their aluminium intensive front-end structure
GRAV (Fig. 6) made of extrusions, castings and sheet,
with a 30% weight saving compared to the steel. This
innovative construction fulfils all requirements regarding
strength (incl. fatigue under vibratory stresses), stiffness,
formability (crash behaviour), reliable process ability as well
as good corrosion resistance. Furthermore, the new front-end
weight balance significantly enhances driving behaviour,
furthermore crash behaviour, sufficient, could also be

fulfilled. New 5000 series sheet alloys with Mg contents
over 5% represent a direction of alloy development with
great potential for use in car body structures. They offer
substantially higher strength while, at the same time, showing
improved formability.
7.
Aluminium for Doors, Closures and Outer Panels
For these components a defect-free surface finish is

important, in addition to formability, buckling strength,
proof strength, and a suitable behaviour with regard to
thermal loads from the manufacture (e.g. age-hardening or
softening behaviour during paint baking) or during service
life of the vehicle. For car body applications (outer panels
and interior components), heat-treatable alloys of the 6000
series as well as non-heat-treatable 5000 series aluminium
wrought alloys in typical sheet thicknesses of between 0.8
and 1.25 mm are used.
Due to the strong solution hardening effect of Mg both
strength and formability are increasing with Mg content.
Thus, for applications in the structure and for painted
components where IGC has no effect high Mg 5000 series
alloys are common, as e.g. alloy AA5182 (AlMg4,5Mn0,4)
and moreover 5000 series alloys with high Mg contents
822
J. Hirsch
(a)
(b)
Fig. 7 Aluminium extrusions in bumper beams and crash boxes (a) Buckling of crushed extrusion (b).
(>5 mass%) have been developed.11) BiW exterior panels

(Fig. 1) are preferentially produced from heat treatable AlMg-Si EN-AW 6000 alloys due to their bake hardening
behaviour and optimum surface appearance.
This is also valid for hang-on-parts. Here Aluminium 6000
series alloy sheets are preferred due to their favourable
property mix of excellent surface quality after forming,
compatibility with the OEM process (i.e. strength increase in
the paint bake cycle, compatibility with joining operations
like adhesive bonding) and in-service corrosion resistance.
New 6000 series alloys with higher formability and similar
pain bake hardening give more freedom of design and offer
the potential to produce more complex parts in an efficient
and economic way.
8.
Extrusions
Another special field of Aluminium solutions and applications is the well established technology of Aluminium
extrusions. Here quite complex shapes of profiles can be
achieved allowing innovative light weight design with
integrated functions. In Europe complete new and flexible
car concepts (e.g. the aluminum space frame, Fig. 4(a)) and
complex sub-structures (e.g. in chassis parts, bumpers, crash
elements, air bags, etc.) have been developed using extrusions. Their high potential for complex design and functional
integration is most suitable for cost-effective mass production. Commonly, medium strength AA6000 and high strength
AA7000 age hardening alloys are used, since the required
quenching occurs during the extrusion process. Formability
and final strength is controlled by heating for age hardening.
Extrusions are applied for bumper beams and crash elements/boxes (Fig. 7). The main drivers in new developments
are extrudability, tolerances and strength, particularly for
strength relevant applications in the cars. New alloys are
being developed that show higher strength. Simultaneously,
it is easier to extrude and even more complex shapes can be
produced, cf. the drawing of the thinwalled shapes. Today,
extrusions are used extensively when tight tolerances can
manually be compensated. The ductility in a T7 condition
is good and the alloy should have the potential of meeting
the toughest requirements for automotive applications. Axial
crushing is a relevant test for observing the buckling

behaviour. Figure 7(b) shows a successful trial.
9.
Castings
The highest volume of Aluminium components in cars are

castings, such as engine blocks, cylinder heads, wheels and
special chassis parts. The substitution of cast iron engine
blocks continues, even for diesel engines, which in Europe
have gained a substantial increase in market share in Europe.
However, due to the high requirements on strength and
durability, cast iron is still often being used. Significant
progress in Aluminium alloy development (Al-Si-Cu-MgFe-type) and better process control and casting methods
improved material properties and functional integration that
enables Aluminium to meet the specific high requirements.
Aluminium castings are also gaining acceptance in the
construction of space frames, axle parts and structural
components. Complex parts are produced by high integrity
casting methods that ensure optimal mechanical properties
and allow enhanced functional integration.5) For high
pressure die cast HPDC new AlSiMgMn alloys have been
developed with enhanced strength and ductility combination.
A typical BIW casting (A-pillar) is shown in Fig. 8(a),
integrating functions connecting the B-pillar instrument
panel and the front-end structure with ribs for stiffness
purposes.
Crash-and strength relevant alloys (e.g. AlSi9MgMn)
tailored for chassis- and BIW applications have been
developed. Considering the highly integrated geometries
of high-pressure die-castings (HPDC), it is a prerequisite
that material models are developed in parallel to the alloy
development to capture the mechanical behaviour of the
castings. Figure 8(b) shows a diagram of force as a function
of displacement during Arcan testing of AlSiMgMn HPDC
material characterizing its fracture and crack propagation
behaviour.
10.
Advanced Multi-Material MM Design Concepts
Advanced design concepts have been developed by

the main European car manufacturers (OEMs) evaluating
Aluminium in Innovative Light-Weight Car Design
(a)
823
(b)
Fig. 8 Complex Aluminium automotive parts produced by high pressure die casting. (a) AlSiMgMn high integrity A-node casting
(b) Fracture force evolution (simulations and experimentally measured crack propagation)
Fig. 9
Final BiW and selected materials for the innovative multi material SLC concept.35)
Aluminium solutions in competition with (new) steels,

magnesium and plastics or composites. The multi-material
MM design approach should yield the best material for
the appropriate function, considering all technical, economical and ecological aspects, i.e. material and manufacturing
costs (including suitable joining technologies), performance
and safety, life-cycle-analysis LCA (incl. CO2 emissions
and recycling).2) The SLC (full title: Sustainable Production Technologies of Emission Reduced Light weight Car
concepts) produced a Golf V BiW as reliable benchmark for a
mass produced car. The result was a 34% weight reduction
within a cost increment of 7,8 /kg saved, using assembling
technologies feasible for a high volume production, (Fig. 9)
and providing similar crash performance in a 5-star rating
regime.3)
The SLC Project included a broad pre-competitive
material and technology screening and testing. It generated
newly developed special Aluminium alloys and reference
studies for BIW structural application (in 5000, 6000 and
7000 alloys) with high strength, formability, energy absorption and excellent crash characteristics.11,12) Suitable technologies were evaluated and developed by up-scaling of most
promising and improving of existing Aluminium sheet and
warm forming technologies,8) HPDC for complex structural

parts, forming technologies, Aluminium-steel TWBs for
crash-beams, joining and assembling for multi-material high
production.13)
The full body-in-white prototype was constructed by the
consortium (Fig. 10) and is being displayed on numerous
congresses and exhibitions (e.g. Ref. 5, 9)). It receives
highest attention by experts and even the prestigious
InnoMateria award.14)
11.
Summary
Todays European cars contain an average of 132 kg of

Aluminium components and industry is working on new
improved Aluminium alloys and solutions for automotive
applications: In the body structure and for chassis and
suspension parts, presently Aluminium is mostly used in
sport and luxury cars, but they can also find their place
in smaller mass produced cars. Especially designed and
processed Al-Mg-Mn 5000 alloys and newly developed high
Mg contents improve mechanical and corrosion properties,
and roll clad materials provide innovative alternatives. For
hang-on parts like hoods, doors, and fenders, precipitation

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Aluminium in Innovative Light-Weight Car Design

Fig. 5 Example of chassis components made of aluminium.

Weight-reduced aluminium front end of BMW 5-series GRAV (Source: BMW).

Aluminium in the Vehicle Structure

In this recent 5-series cars BMW achieved a weight of

furthermore crash behaviour, sufficient, could also be

Aluminium for Doors, Closures and Outer Panels

For these components a defect-free surface finish is

(>5 mass%) have been developed.11) BiW exterior panels

crushing is a relevant test for observing the buckling

The highest volume of Aluminium components in cars are

Advanced Multi-Material MM Design Concepts

Advanced design concepts have been developed by

Aluminium in Innovative Light-Weight Car Design

Aluminium solutions in competition with (new) steels,

warm forming technologies,8) HPDC for complex structural

Todays European cars contain an average of 132 kg of

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