Aluminium has riveted Jaguar’s attention and its latest models are being made from this material to save on weight, while maintaining performance

Jaguar may have been given a stay of execution by the new Ford President Alan Mulally, but a question mark still hangs over its future as part of a troubled company burdened by huge debts.

Jaguar’s spiritual home at Brown’s Lane in Coventry ceased manufacture in 2005 and now the X-Type is made at Ford’s Halewood plant near Liverpool in the UK, while the XJ and XK models are produced at Castle Bromwich in the Midlands, also in the UK.

Seventy years ago the Castle Bromwich site – Jaguar assumed control of this facility in 1980 – came to Britain’s aid in its darkest hour by manufacturing the legendary Spitfire, a fighter aircraft whose aluminium construction was a key factor in its speed and handling. Now, Jaguar in its battle for survival has also turned to aluminium for the production of its latest generation XJ and XK models at the Castle Bromwich factory whose main gates still proudly bear a propeller symbol. In keeping with the advanced technology pioneered by the Spitfire, the new XK – launched in 2006 – features what Jaguar engineers claim has become an industry-leading bonded and riveted aluminium monocoque body derived from a structure that celebrated its debut in the latest XJ saloon.

Innovative features – the shape of things to come?

The new XK’s construction features innovations such as a secondary, front bulkhead of aluminium and composite materials that, is claimed, helps reduce noise from the engine compartment, while providing a dry area under the bonnet for electrical components. In terms of longterm strength, the all-aluminium shell has a durability approaching twice that of a traditional spot-welded steel body, according to Jaguar.

Its low weight also offers a 10 per cent improvement in power-to-weight ratio, while XK is up to 90 per cent stiffer and 180kg lighter than key rivals, according to the company, with a kerb weight of 1,595kg. In Convertible form, the body-in-white weight, at 287kg, is 19 per cent lighter than the previous XK Convertible.

“The XK’s aluminium body incorporates the latest thinking in epoxy bonding and riveting techniques to produce a chassis that is safe and very light,” says Richard Else, Jaguar’s Operations Director at Castle Bromwich.

He adds that although the new XK’s aluminium chassis is significantly lighter, “It is 30 per cent stiffer than the old Coupe and 50 per cent more rigid than the Convertible it replaces.” Jaguar has opted for a unique, complete aluminium monocoque body structure, as opposed to the German marque Audi’s aluminium spaceframe with separate aluminium panels which result in a heavier body shell.

Aircraft influence

Described by Else as Jaguar’s Lightweight Vehicle Technology, the principles have been developed from aircraft industry methods, where strength is critical. “The new XK takes the concept a step further with extended use of lightweight aluminium castings and extrusions, as well as the pressed aluminium panels,” Else tells AMS. “And its strength and light weight are derived from the way the shell is constructed, using innovative jointing technologies developed by Jaguar and its suppliers.”

Another major advantage of this lightweight technology is that all the necessary stiffness is in the structure of the body shell, with large rectangular-section side sills.

“So the Convertible, even without a roof, does not need the traditional additional stiffening panels seen on many other convertibles,” says Else, “ meaning no added weight and no penalty in stiffness or refinement.”

There is only a single welded join in the new XK Coupe body, the one ‘cosmetic’ join on the roof. Apart from the obvious weight-saving benefits, a lack of welding dispenses with the need for large electric currents and in turn, no welding sparks are generated, and there is no need for water for cooling,” he adds.

Unique combination of riveting and bonding

Apart from the roof weld, all other joints in the XK shell are formed using Jaguar’s unique combination of riveting and bonding. “Most joints are effected using self-piercing rivets applied by hydraulic pressure against a fixed tool,” explains Else. Where access only to one side of the join is possible, for example in some new extruded box sections, a new riveting process has been developed.

Where particularly high stiffness is required in a join, a combination of riveting and bonding is used – with the adhesive bond in effect creating a continuous joint which is stronger than a similar, riveted-only joint.

All visible exterior panels are bonded to the underlying structure, and a new automated seam-sealing process seals all relevant areas of the shell before painting, to ensure that no gaps are missed. Else admits that most of what has been achieved, such as the vehicle’s combination of exceptionally low weight allied to substantial torsional strength, has been achieved by design, rather than the manufacturing process.

“But one of the key differences between the XK model and its competitors is that Jaguar designed it from the start as a convertible from the ground up, instead of trying to ‘add’ stiffness in a later stage. With most of our competitors, the coupe comes first and the convertible follows.”

Else concedes that using aluminium creates a ‘lot of challenges’. “To press aluminium is a more complex process than pressing steel because of the characteristics of the material and some of the effects sustained during the die process,” he tells AMS, and add that most of the panels for the XK are stamped elsewhere, by Volvo and also a Midlands supplier, Stadco.

Aluminium presents its own challenges

Painting aluminium has a different set of challenges than steel, says Else, but both go through the same paint shop. “For example, the way we put the electro coat on to steel is different to way we put it on aluminium because of the differences in the conductivity of the materials,” he tells AMS.

Another factor in using aluminium is that it requires 10 per cent more people in the body shop, while it costs some 50 per cent more than steel.

“However, a body shell accounts for only about 30 per cent of the overall cost of a vehicle,” Else points out. The decision whether to make a vehicle from aluminium depends on a number of factors, he says: the size of the vehicle, the style of the vehicle and the volumes produced.

“In terms of body styling, there are not many complex shapes on the XJ, but with XK, we have evolved the technology and so there are sharper shapes and larger panels,” Else explains.

He says that although there was no specific manufacturing breakthrough that led automatically to the use of aluminium, a number of factors were considered, such as great strides that have been made in adhesive technology in the past decade, and riveting technology. “A lot of our rivets are now self-piercing, and we don’t have to make the holes for them. Those elements are part of the equation that says this is the right business decision to make,” he adds.

“We have to keep abreast of things that are happening in the world in terms of regulation, emissions, taxation and all those sorts of things that for larger, more complex and lower volume cars mean aluminium is the right solution. The great news with XK is that we already have an all-aluminium car with XJ and we have built on what was learned in that experience to try and improve XK,” he enthuses.

This has been achieved by optimising rivet positions, rivet quantities and also the use and types of adhesives to make sure Jaguar arrived at something that’s strong, if not stronger, than a steel equivalent. Else says there are marked differences between the way in which the XK is made and, for example, Audi’s new offering. “Audi uses a type of super frame to which to which all the panels are glued, while Jaguar’s method is very similar to a steel construction in the way the body’s put together,” he tells AMS.

Starting with the supercar

Although he referred to the XJ, Jaguar’s use of aluminium can be traced to the mid-eighties with the development of the XJ220-engined ‘supercar’.

“That was all aluminium and that was when we started using the technology. We have evolved it since, as materials such as adhesives have advanced and aluminium-forming processes have moved along with casting processes,” says Else. The XJ 220 was shown in 1990 and “it provided a great learning curve.”

Although Jaguar was not the first in the market to build an aluminium body Else points out that the way the XJ was designed and the way it was built were very different: “It was based on the same principles as a steel design but it was the joining technology and the way we actually pressed the panels that was different.”

Castle Bromwich began production of XK just over a year ago, with a target of 280 units a week, but demand has now increased production to 347 a week, which has been addressed by further investment in final assembly and introducing a third shift in the bodyshop.

“The bodyshop capacity was about 140 units on each of the two shifts and with the introduction of a third shift, production starts at 6am on a Monday morning and runs until 11pm on a Friday night, but this means that maintenance can only be done at weekends,” Else tells AMS.

However, the paint shop, traditionally a potential bottleneck situation, can handle all three models –XJ, XK and S-Type – with a single shift.

Else and his colleagues are proud of the way XK, and XJ, are fixed together. “The bonding and riveting process was developed by Jaguar, though clearly we’ve used a lot of other companies as we’ve gone through this experience.

If anything, the adhesive and the riveting is very much a ‘belt-and-braces’ thing to give us the rigidity we demand.

“As we evolve the process over time, we may find the rivets give us all the rigidity we need, but what we haven’t yet got is any data to say what the adhesive performance is over time. We will look to improve our processes and in a few years’ time, we may have accumulated enough data to make changes to the adhesive, or reduce the number of rivets. We’ve done a bit of computer modelling and interestingly we’re doing some work with the old XJ220s to see how they have performed over time,” Else tells AMS, but adds that one can’t draw full conclusions from the XJ220 “because a lot of the adhesive technology has moved on since then, but we can get some general indications.”

Improving the hardness of aluminium

“Aluminium is clearly a softer metal than steel and so we’ve used an aluminium alloy that we then bake-harden. We’ve also put some processes in place in the paint shop that improve the hardness of the aluminium as it goes through the ovens, without making it more brittle. The result of that is its performance in terms of dings and dents is as good, if not better, than steel,” says Else.

“We’ve also worked with the aftermarket to see how we can optimise the repair process; if you have to change a panel that’s riveted and glued, it’s actually easier to replace than one that’s welded. So we’ve done a lot of work with the aftermarket, the manufacturing process and a lot of work in the design stages to keep the actual insurance costs of the car down and make it as good, if not better, from a repair perspective than a steel equivalent. Having said that, customers would need to use a specialist repairer, though if you're just changing a panel it can be easy because some of the stuff is actually bolted on," he reiterates.

Despite the perception that an aluminium-bodied vehicle is more vulnerable, damage to key structures such as suspension mountings that could consign the steel body to the scrap heap can be replaced through riveting and bonding in an aluminium bodyshell," Else adds.

The two Jaguar plants are the highest rated operations within the Ford worldwide production system of measurement. "In terms of our quality rating, which is the bit we really try and drive, Halewood has consistently been at two or three for the past five or six years, both in terms of Ford's own survey and also within JD Power as well, where we've been in the top five since 1989," he tells AMS.

He stresses that Ford measures efficiency not just in terms of staff-hours per car but also cost. "You can't just look at the hours-per-car figure in isolation.

Relatively modest volumes also mean that levels of automation at Castle Bromwich are low compared with Ford plants. "You can walk around the bodyshop for XK and find only 29 robots. The number of robots we have is driven by volume and we simply look at the affordable business case for them. "At this point, with our volumes, this is how much we can afford to invest."

Low volumes also meant that originally, seam-sealing processes were done manually. “But we looked at that as a standalone project to see how how it could be done more efficiently and at the same, improve the quality,” says Else. The company developed an automatic seam-sealing process in-house, in conjunction with the robot supplier and sealer supplier. We made some investments and made some savings also some quality improvements at the same time, “ he says.

Are Else and his colleagues at Jaguar pleased with Mullaly’s announcement at the Detroit Show in January that Jaguar – for the time being, anyway – is safe within Blue Oval? “As as far as we’re concerned, it’s business as usual. The team has some tight objectives they have to deliver and we’re carrying on meeting those commitments. Commitment is first rate and I still reckon I have the best job in Jaguar. It is easy to measure me because you measure my performance as to whether a car comes off the production line every two minutes.”