Aluminium
when alloyed to small quantities of magnesium, copper or silicon,
aluminium becomes strong as well as light and is used today mostly
as a weight-saving replacement for the iron and steel used in engines,
suspensions and drivetrain components, radiators, heater cores and
air conditioning systems.
Aluminium for car bodies:
The first aluminium car structure:
One of the first designs to use a totally aluminium structure was
built by a Norwegian company called Bjerring. Only four prototypes
were ever completed after Raufoss took the company over and attempted
to transfer advanced aluminium space frame technology developed
for buses to passenger car design. The project failed simply because
it was 70 years ahead of its time.
| The
first aluminium bodied production car: |
Panhard
Dyna |
The
first production car to use aluminium as a structural material
for its body
shell was the French-made Panhard Dyna in 1954. It was an
oddball design, powered by a diminutive 850 cc two-stroke
engine, but weighed only 629 kg and could carry six people.
Truly another mileage by Panhard, which in 1894 had produced
the forerunner of the modern car (the first Panhard) with
a front mounted engine driving the rear wheels via clutch
and gearbox. |
Panhard
Dyna |
Aluminium
skin to steel frame:
Then followed the
1954 A.C.Ace and the Cobra which had full aluminium skins riveted
to tubular steel space frames, using a super-light (Superleggera)
process patented by Touring of Milan (The Aston Martin V8 models:
Vantage and Volante, use a body made from hand-formed aluminium
panels fixed to a tubular steel frame in the same kind of way).
Finally followed the Stressed aluminium bodied cars:
Honda NSX, Audi A8, Ferrari 360 Modena,BMW Z8, Audi A2...
Advantages:
1) Lighter hence fuel efficient:
Its density is only one-third that of steel. The latest Audi A2
has a body weighing only 150 kg which is 40% lighter than the body
of a similar steel car using the latest Space-frame
technology (Space-frame is also used by the Ferrari 360 Modena
and others). As car bodies contribute to approx 20 per cent of the
total weight of a car they offer a promising way to reduce the weight
of cars considerably, leading to fuel efficiency.
2) Stronger:
|
Bugatti
Royale With Caste Aluminium Wheels |
It has
a higher specific strength than steel. When the latest computer
aids and lateral thinking are brought
together into a rigid three dimensional car structure, the aluminium
body can actually be designed and built with a torsional stiffness
40 per cent better than the equivalent in steel with only 60 per
cent of the weight. In structural efficiency terms, that converts
to an efficiency improvement of 96 per cent.
After all, the costliest car built - the Bugatti Royale, had caste
aluminium wheels which could carry a truck.
3) Lesser pieces to weld:
Load bearing components are cast or forged. Less demand parts of
the frame and panels are extruded. Tailored blanks are used (ie.
pieces are developed with varying thickness) - large where strength
is required and small where strength is not required. The Audi A2
body is assembled from 240 pieces, rather than twice that number
welded into a steel body.
Drawbacks:
1) Costlier:
It costs $1500 a tonne - three times as much as steel.
2) Difficult to work with:
Its modulus of elasticity is only one-third that of steel. It cannot
be pressed into shape as easily as steel. It is also difficult to
weld pieces of aluminium.
Drawback solutions:
1) Automation & Lazors:
Four-fifths of Welding and assembly is now automated and computer
monitored resulting in extremely tight tolerances (around a 50 of
a millimetre). Also Lazor welding is used, which is not as distorting
as conventional welding.
2) Economy
of scale through space-frame technology:
Space-frame technology
should be competitive at quantities as low as
50,000 a year. Volume production
of steel cars need anything between 200,000 to 500,000 per plant
(particularly for the expensive giant presses that bash the panels
out). For low volume niche models that appeal to fashion, aluminium
just might meet the demand. Audi A2 6to be launched in June and
producing 300 cars per day in its south-west German factory is claiming
that it will achieve this economy of scale. A fact closely observed
by other players like Ford, which has several all-aluminium prototypes
priced at only 10 per cent higher than the steel cars.
A disputed advantage - Eco friendly:
Aluminium companies recon that using aluminium instead of steel
reduces carbon dioxide emissions over a vehicles lifetime by 20
per cent. However, according to MIT (Massachusetts Institute of
Technology), a program recently completed by a consortium of steelmakers
says: it would take 32-38 years of driving aluminium-intensive vehicles
to offset the amount of carbon dioxide put into the atmosphere by
the production of the aluminium needed to build those vehicles.
Space-frame technology: 
Space-frame are metal skeletons similar to those around which aircraft
are built. The body panels are hung on this skeleton about the hard
points, adding extra strength as well as making the structure aerodynamic
and keeping the wind off the driver.
Most of the space frame structure is formed from hollow section
extrusions with wall thicknesses that vary to distribute stresses
evenly. These components are extruded and bent into the required
shape, before being assembled. Their exact form depends on how they
are used, parts of the roof frame for example are shaped differently
from the door pillars or sills.
Exterior panels are pressed in special dies, taking full allowance
for the elasticity and elongation limits of aluminium compared to
steel. Special steps are also taken to compensate for the spring-back
that occurs after the panels are removed from the presses. They
are added to the inner frame by a variety of different jointing
methods and then heat treated after painting by the oven baking
system.
 Whilst
the bonding, welding and clinching processes use well-established
technology, the punch riveting adopted for fastening pressed panels
to the extrusions uses a relatively new technique that produces
a joint 30 per cent stronger than a conventional spot weld. It involves
pressing a specially coated flat-head countersunk steel rivet with
a hollow shank through the sheet aluminium into the much thicker
extruded section behind. A form tool placed behind the insertion
point spreads the hollow shank as it penetrates the outer surface
of the extrusion, generating a shape rather like the root of a human
tooth.
Positions not accessible by the punch-rivet guns, which have a relatively
short reach, are resistance spot-welded. Other joints are either
clinched, by beading over the edges of two panels and stamping a
localised geometric depression than holds them together, or arc
welded under an inert gas shield.
One of
the most influential elements affecting body stiffness is the rigidity
of the joints between the main
structural elements, which act as nodes during torsional distortion.
Extra reinforcement at these points, such as the connections between
the A-pillars and the side rails, can improve the stiffness of a
conventional steel body by as much as 20 per cent. These critical
joints are therefore replaced by die-cast aluminium parts, optimised
throughout by sectional changes, and reinforcing webs to provide
the most weight efficient structure. This results in a bare body-in-white
weight of only 247 kg on the Ferrari 360 Modena, 28 per cent less
than that of the F355 combined with a torsional stiffness increase
of 44 per cent (at 1,474 kgm/degree) and a bending stiffness increase
of 42 per cent (1,032 kg/mm). The natural frequency of first order
vibrations is also 70 per cent higher at around 60 Hz. This overall
result is a 100 kg reduction in the car's dry weight compared to
the F355, even though the body of the car is actually larger.
|
