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Aluminium strong rival to steel

   
 
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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 paint baking.

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 the collaborators.
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 is possible.

 

Project Title:  
Low weight vehicle properties of aluminium alloys for body structures

Programmes:
Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)

Contract Reference: BE-5656

Cordis DatabaseFor more information on this project,
go to the CORDIS Database Record

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