Lotus: aluminium extrusions and adhesives
Anna Kochan is European correspondent of Assembly Automation.
Spotlights the design and development of an aluminium chassis for the new Lotus Elise. Highlights the benefits of using aluminium extrusions and the reasons for choosing bonding as opposed to welding. Describes the assembly process and how the maintenance and repair drawback of a bonded design can be overcome. Notes that the largest proportion of Lotus’ sales actually come from engineering and consultancy services.
An innovative combination of adhesive and rivets is used to join the chassis of the new Lotus Elise which is made primarily from aluminum extrusions. The bonding technique was developed by the UK sports car company in partnership with Ciba Polymers of Switzerland and Hydro Aluminium, of Denmark.
When Lotus Group engineers set about developing the chassis of the new Elise sports car, the decision had already been taken that it should be made, as far as possible, from aluminum extrusions. Aluminum is much lighter than steel and, as a result, the space frame on the Elise at 68 kg is about half the weight of an equivalent one of steel (see Plate 1).
The use of extrusions as opposed to sheet aluminum also gives cost savings. “The tooling to achieve complex-shaped extrusions costs only a few thousand pounds whereas tooling to press aluminum sheet costs hundreds of thousands of pounds”, says Richard Rackham, who managed the chassis design team. Tooling cost is a significant factor considering that Lotus is a low-volume car producer. Current plans are to produce just 750 Elises a year.
Another benefit of an extrusion is that it can be made thick in some areas and thin in others to give components the strength required exactly where it is needed, he adds.
At the start of the project, Rackham’s team was uncertain of which joining technology to use and developed two chassis designs, one of which was welded, the other bonded.When tests with a bonded prototype proved successful, the welded one was dropped.
“Bonding has many advantages over welding”, says Rackham. First, it is more precise because it eliminates the distortion that comes with welding. “This is very important because in a high-performance car structure, the point where the suspension is joined to the structure has to be controlled to within 0.5 mm or there is a great variation in handling between vehicles”, he adds. Another factor in the decision was the negative effect of the heat of welding on the aluminum. Bonding enabled Lotus to take advantage of the strength-to-weight benefits offered by heat-sensitive aluminum alloys which could not be welded easily without losing properties. Bonding also spreads the loads across a greater area than welding, providing strength advantages.
Both the design of the extrusions and the design of the chassis are specially adapted to the bonding process. According to Rackham, the basic approach was to design the vehicle frame as if it were Lego. Many of the extrusions link to the neighbouring extrusion with a tongue-in-groove joint. Also, where parts have to come together, the design ensures wide flat areas for the bonded joint. The form of the extrusion itself features 5 mm-high ridges along all mating surfaces to control the gap A width. “This ensures that all the adhesive is not squeezed out from between the joint and maintains the gap width at 0.5mm. The adhesive works up to a gap width of 4 mm but, because it is expensive, we try to minimize use”, says Rackham.
Finding the right adhesive and developing the optimum bonding process for an aluminum chassis was a complex exercise. According to Ken Sears, head of vehicle engineering at Lotus, “unlike the steel industry W which shares information, the aluminum industry is very closed.” Sears found no available data on aluminum bonding technology which meant Lotus had to develop its own. Swiss company Ciba Polymers was selected to supply the adhesive and collaborated on the bonding process together with Hydro which supplied the aluminum extrusions. Hydro Aluminium is more familiar with the manufacture of aluminum extrusions for window frames and greenhouses than for sports cars. However, it was keen to move into high added-value products and has set up an automotive division specifically to handle this market.
About 35 extruded components and three sheet metal components make up the angular box-shaped aluminum chassis for the Elise. The extrusions are made of 6063 aluminum magnesium silicon alloy while panels bonded to the space frame are made from Hydro’s 3105 grade, which is derived from recycled aluminum alloys.
Hydro was able to develop extrusion tooling capable of a 2mm minimum wall thickness. However, it was not possible to reduce the wall thickness to l mm which is all that is required in some areas.The elements go through an acid etch and anodizing process prior to the adhesive being applied.
The adhesive is a single-part, heat-cured epoxy paste (XB 5315) which is more often used tar bonding oily steel. It has a tensile strength of 35 MPa and an E-modulus of 2,700 MPa. Curing takes about 40 minutes at 200°C. Until cured, it has a paste-like consistency and is very stable.
Because adhesive-bonded joints are strong in shear but weaker in peel, each joint is reinforced by thread-forming rivets to prevent the onset of peel during a crash. The ejot rivets selected for the task are self-swaging and selftapping drive screws. They are made from mild steel coated with a high-performance corrosion-resistant finish called Dacromet. A zinc aluminum coating, Dacromet gives a significant 480 hours of salt-spray resistance
Where a rivet is to be inserted, a hole of 8 mm diameter is drilled in the top element, and one of 4 mm diameter directly underneath. The 6 mm diameter rivet is then rotated at high speed by the special insertion tool and introduced into the larger hole. As it is driven down into the smaller hole, it melts the aluminum around the sides, and the displaced material is drawn up into the larger hole. As a result, thread engagement along the length of the rivet is ensured.
A major exercise in corrosion prevention has led Lotus to adopt Xylan and Delta finishes on components where an aluminum element comes into contact with a steel element. In some places, a coated 0.5 mm thick shim is inserted between the aluminum component and the steel component so that it protrudes from the joint by 5 mm. This effectively provides a l O mm-long path between the two metals which is sufficient to prevent corrosion, says Rackham. A finish, however, cannot be applied in every case, he adds. For example, there is a circlip to hold in bearings, which cannot be coated. Here, the engineers have employed an aerospace-grade grease.
The assembly of the complete Elise chassis is performed by Hydro in Denmark in a specially-constructed controlled-environment building. The clean atmosphere ensures that really nasty contaminants such as silicon cannot get anywhere near the bonding process, says Sears. The adhesive is manually applied to the extrusions, and the more than 130 rivets are inserted before the chassis is loaded to an oven for curing. The rivets also hold the chassis together so that it can be transferred to the oven without falling apart.
The completed aluminum chassis is delivered to the Elise assembly line which has been set up parallel to the Elan line at Lotus’ Hethel factory. During the final assembly process, other elements to be bonded to the chassis are joined to it using a cold-cure adhesive. The Elise assembly line is gradually building up to an output of four cars a month, the first car having come off the line at the end of May.
In developing the Elise, Lotus has aimed to provide a technology shop window for its engineering division which offers a consultancy service to the automotive industry in general. In fact, about two-thirds of the company’s sales currently come from its engineering and consultancy activities, and only one-third from selling cars. The technologies developed for the Elise have been selected on the basis that volume car manufacturers will be interested in adopting them, says Tony Shute, Elise program manager. “If we thought that Ford or Opel might not be interested because it was too exotic, we didn’t use it”, he adds.
However, if the Elise chassis structure with bonded aluminum extrusions is to be adopted by volume car manufacturers, a number of developments will have to take place, says Rackham. “The curing oven is an expensive nuisance. Adhesives technology is evolving and within a few years cold-cure adhesives will become available for this type of application”, he believes. Also, the Elise chassis structure is very simple. No complex joints are involved and only straightforward surfaces have to be bonded. “In most cars, complex joints would have to be tackled, and that would require some development”, he adds.
One disadvantage of the bonded design of an aluminum chassis still remains, however. Maintenance and repair is not quite as easy as with steel. Should such a chassis be damaged in an accident, it cannot be repaired by any back-street garage. The bonding conditions are too exacting for that. However, the Elise chassis has been so designed that extra bolt holes have been formed in those areas susceptible to damage. These holes are there so that a plate can be bolted on to the existing structure to compensate for the damage, should it occur. In the event of a serious collision, the body and the chassis can be separated and whichever is beyond repair can be replaced.
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