- Published in Q&A.
The aluminium XJ is lighter than the smaller, steel-bodied XF: what can you do to narrow the gap?
“ We had the conversation at the last EuroCarBody conference on how much more we think we can do with steel. The consensus was that reducing a steel body’s weight by 10% is possible. At a stretch, applying the latest materials and joining technology, we might get to 20% – but that’s optimistic.
Steels have improved dramatically in the past decade. Do you see the rate of improvement continuing?
“ There are still a few advances to come but the past five years have been more a consolidation – ensuring that we’ve got consistent mechanical properties, and solving some of the problems that really high-strength steels have brought with them. They make forming aluminium look quite easy. We’re at the end of the strength development with boron steels up at 1,400MPa. Cold-formed steels might reach 1,000MPa. They’re not going to be able to compete with boron steels; instead, they’ll improve in formability.
This XJ uses better aluminium alloys than its predecessor – what further advances would you like to see?
“ We’re trying to focus all of our development work with partners such as Novelis on making the grades we have more formable, and increasing the strength of the 6000-series alloys. I think we can push the 6000 series up to 300MPa and maybe beyond. I’d like to push formability too, but design isn’t a limitation – we’re now achieving quite tight radii. I don’t want to go to 7000-series alloys in the short to medium term because that brings issues with corrosion, joining and recycling.
Jaguar favours structural adhesives and self-piercing rivets for aluminium: do other joining methods have potential?
“ Cold-joining technologies are better from an environmental point of view than traditional, fusion-joining technologies. MIG and laser welding are quite energy-intensive, and require cooling water and air extraction. A rivet gun uses a tenth of the electricity that a spot-weld gun uses, so as well as getting a lower carbon footprint for the car by reducing weight we can reduce the factory’s carbon footprint too. That doesn’t mean we’ll never use anything other than rivets and glue – there are emerging technologies, such as ultrasonic welding, that I’m very interested in.
How would you design a mixed-metal body?
“ I’d want to minimise the interface joints between steel and aluminium. And I’d use steel for the box sections and aluminium where I’d get the biggest weight saving for the minimum on-cost: large sheet parts such as floorpans, dash panels, door inners and outers and roof panels.
Some OEMs are investing in carbon-fibre composites for their electric city cars – what are the pros and cons?
“ The big issue is clearly cost. A number of city cars are going towards lightweight carbon and aluminium structures because they have to offset the weight of the battery. With a steel body and a very large battery, range and performance would be very limited. Another disadvantage is repairability – we may start to see vehicles with sacrificial aluminium crash structures and the tubs in carbon.
Crash tests are becoming more stringent – can you keep producing safer cars that are also lighter?
“ You can do that using alternative materials and architectures. The XJ has one of the slimmest A-pillars, and we’ve got a full panoramic roof, but we more than pass the new US roof-crush test at three times the gross vehicle weight. We do that by integrating a high-strength hydroformed aluminium beam made from 6082-T6 into the pillar itself.
How can you achieve further weight reductions?
“ Jaguar Land Rover has put all of the interior trim and door components with the body-in-white door group so, effectively, they own the whole door, maximising system integration and subsequent weight and cost reductions. There’s also better understanding of lightweight materials. As we grow in confidence with them, we may be able to use fewer rivets or less adhesive. We can look at further downgauging. On the latest XJ we’ve taken the front fender from 0.93mm, which is very competitive for aluminium, down to 0.86mm. That’s industry-leading in terms of gauge and equivalent to some steel fenders in mass-produced cars.