Europe's best-known aluminium companies
and car manufacturers have proved that aluminium alloys are practical
alternatives to the heavier steels used in mass-production cars.
All car makers are considering using aluminium to reduce the weight
of vehicles, and thus the size of engines and the fuel consumption.
Savings would include lighter mechanical systems such as brakes, transmissions
and suspensions. Body panels and space frames could be at least 30
to 35% lighter than current steel versions, although using more aluminium
results in a much higher cost than steel.
The 14 collaborators found that aluminium alloys were suitable for
conventional production line techniques such as forming, welding and
Car makers have long been trying to develop
lighter vehicles, partly to reduce the costly fuel consumption of
their cars, but also to improve the performance under increasing
pressure to conserve fossil fuels and to minimise CO2
green house pollution. European car manufacturers also need to reduce
vehicle weight to compete with Japanese producers and to meet increasingly
stringent operating conditions, such as those set by California,
in important markets.
One option is to build cars from aluminium which is much lighter
than steel, but car manufacturers have been unwilling to switch
to aluminium, partly because of the high cost compared with steel.
Aluminium costs about 2 ECU/kg and steel about 0.5 ECU/kg. Another
concern is that they would have to change their long established
mass-production methods, that have been perfected for steel, in
favour of expensive new methods.
Some of the leading car manufacturers and aluminium companies of
Europe, led by Renault of France, embarked on this three-year project,
PABS, to determine whether aluminium alloys had properties that
made them suitable for mass-production car manufacture. Associated
Brite-Euram projects included MABS, which investigated the production
processes such as stamping and laser welding that would be needed
for aluminium, and DABS, on the design of aluminium cars and components.
SCABS, an associated non-BRITE-EURAM project by car companies and
two aluminium producers, looked at the surface treatment of aluminium
and corrosion behaviour.
The PABS participants, particularly the car manufacturers, say that
although they are all working in these areas, they would have been
unable to collaborate to such an extent without the BRITE-EURAM
structure and financial support.
All uses considered
PABS investigated the general behaviour and joining properties
of aluminium alloys already on the market, and focused on alloy
sheets for internal and external vehicle use, extruded parts for
vehicle space frames, and cast nodes. The work was split between
Alusuisse, for example, developed a database of different aluminium
companies' materials to allow alloys to be selected for sheets,
cast parts and extruded profiles. The database includes the physical
and mechanical properties of the alloys. Alcan investigated how
easily an alloy body-in-white could be recycled.
Alcan concluded that recycling was possible for all designs of bodies
but that the extent depended on how the vehicle was treated. For
example, complete shredding, without separating components and alloys,
would allow 33 to 44% of the material to be reused in its original
form, although 81 per cent of the spaceframe, free of any castings,
could be recycled. With partial separation, reuse could be as high
as 65 to 77%, while much more separation could lead to reuse of
almost all the alloys.
In the vital area of weldability, Rover subcontracted TWI of the
UK to study factors such as the effect of defects, corrosion and
surface treatment, and the suitability of various welding processes.
The results typically showed that the choice of power supply influenced
sticking and splash during resistance spot welding and surface contamination
affected the quality of arc welds.
The PABS partner Pechiney of France looked at the properties of
joints formed by arc welding, resistance spot welding, clinching
and adhesive bonding. Among its conclusions was that sheets and
profiles could be resistance spot welded and parts could be adhesive
bonded without surface preparation.
Work carried out by CRM Liège for Renault showed that the
different strength characteristics of aluminium and steel would
mean that press tools would have to be tailored specifically for
aluminium. Yet in some complex operations, such as multiple-step
stamping, aluminium could be preferable to steel because of some
residual strain hardening and ductility.
No barrier to aluminium
In general, arc welding was found possible with thin alloys up
to 1.2 mm thick using pulsed MIG equipment on as-delivered and degreased
material. Resistance spot welding was found suitable for sheet and
profiles without any pre-treatment and long life duration electrode
operating. Clinching properties depend on the alloy thickness and
properties. Adhesive bonding was found to be possible without any
surface treatment, using optimised gap for maximum strength.
The researchers also found that finite element analysis could be
used to determine fatigue strength well enough to justify the development
of the method for the future design of spot welded car bodies.
Results measure up
In the case of alloy sheet, forming needs more care than when pressing
sheet steel, with close attention paid to parameters such as surface
texture, lubrication, tooling materials and the draw bead restraining
force. The researchers were pleasantly surprised to learn that sheet
could be formed more easily than expected, although still not as
well as steel.
Aluminium also has the advantage that it is clean material and results
can be reproduced with great accuracy. In the case of cast parts,
for which most of the research was carried out by Volvo, Renault,
Alures and AMAG, particular alloys were found to have their own
advantages, depending on the specifications of the parts. Secondary
alloys are suitable for low-cost parts, primary alloy for maximum
ductility and wrought alloy for easy recycling.
A variety of casting techniques were suitable for the formation
of near net shape parts, including squeeze casting which would allow
castings to be solution treated and aged to produce strength levels
much higher than with pressure die cast parts. Squeeze cast parts
also exhibited greater ductility, would be pore-free and therefore
weldable. Additionally, squeeze casting is the most appropriate
technique to produce Aluminium Matrix Composites for high stressed
parts. One unpleasant surprise was that the fatigue limit of welded
joints is very low compared with steel. The static strength is very
much lower. Both these are important to take into account in design
because high strength would allow weight savings in joints.
Although the aluminium companies had not been asked to develop new
alloy but to select better alloy compositions, the collaborators
concluded that there is no technical barrier to the use in cars
of the aluminium alloys already on the market. However, aluminium
alloys are unlikely to replace steel rapidly because of the high
cost of the material, the higher cost of transformation processes
and the large cost of replacing or upgrading the tooling of production
lines of the car manufacturers.
However, all the manufacturers are following up on the project,
and considering substituting aluminium for steel for at least some
parts, such as car bonnets, opening parts of the car body and especially
for sport or high class cars in which mechanical lightening or down-sizing