New Materials Mean Lighter, Safer and Cleaner Cars
Part 1 of Automotive IQ’s Series on Lightweight Materials
As far back as May 2009 Martin Goede, coordinator of the EU’s ‘SuperLight-Car’ project predicted that the reduction of fuel consumption and CO2 emissions would be the most important challenges facing the automotive industry in the future. "One way to reduce consumption is by reducing a car’s weight," he added.
Penalties for excess emissions have forced manufacturers to evaluate every material as a design option to reduce vehicle weight without compromising safety and performance. A 10% reduction in vehicle weight results in a 5-7% fuel saving, provided the power train is downsized (or a 3-4% fuel saving without power train modifications).
Traditionally steel has been the material of choice for car bodies which have been fabricated from stamped sheet components joined by resistance spot welding. However, facing concerted challenges from composite plastics and aluminium the steel industry is fighting back with new technology and processes such as laser beam welding, bonding and tailored blanks.
Different properties in a single sheet of steel
Commonly known as "tailored blanks", these steel blanks can combine several grades, various thicknesses and different coatings in a single sheet. These different materials are laser welded together, in order to place the best material in the best position in the right thickness to achieve a "tailor-made" pressed part.
Image credit: www.autosteel.org
This innovative technology has several advantages over monolithic pressings:
1) Mass reduction while maintaining the same technical performance
2) Improved rigidity without increasing the weight
3) Reduction in the number of pressings
4) Simplification of the vehicle assembly process
Replacing spot welds with laser welded or bonded continuous joints, further increases the rigidity of the monocoque body and structural components and subframes.
With steel being roughly three times heavier than aluminium several manufacturers have turned to aluminium for chassis and body components; particularly in higher spec vehicles, sports cars and luxury SUVs. However, the application of an all-aluminium car body is limited to particular niche applications with small to medium production runs.
For mass production, mixed material construction generally dominates. These are body designs where steel provides the strength and the stiffness whilst aluminium is used for closure parts and selected structural modules.
Aluminium-orientated design and fabrication
By way of different product forms (sheets, extrusions, castings, etc.), aluminium offers a wide variety of design options. However to realise these benefits dedicated aluminium-oriented design and unique fabrication technologies need to be followed.
Jaguar is one company that has been particularly successful in applying aluminium orientated design and light weight vehicle technology to their platforms.
As an example; Jaguar has managed to limit the weight of the current XJ (Model X351) to approximately 1800kgs through an all aluminium body construction. The body is made up of 89 % pressings, 4 % castings, 6 % extrusions and 1 % others (by part count).
For the pressed parts higher strength aluminium alloys of the EN AW-6xxx series have been used. In particular, high-strength EN AW-6111 aluminium sheet alloy is used for the outer skin including complex parts such as the complete side-body.
Of particular interest is the high strength, pre-bent and hydroformed "A" post/cantrail aluminium-extrusion assembly (made from EN AW -6082-T6 alloy).
In order to achieve optimal rigidity 2840 self-piercing rivets have been used (compared to 5000 spot welds for an equivalent steel body) with 154m of adhesive bonding which eliminates all MIG welding processes.
Interestingly the Jaguar XJ produced in the 1970’s weighed approximately 1700Kgs; albeit with less equipment than the current model. This raises the question: Are these incremental improvements in steel and aluminium design enough to meet the worlds demands for cleaner and more frugal cars?
Composites to replace steel and aluminium
Not according to BMW CEO, Norbert Reithofer: After examining trends affecting the industry he concluded that increased environmental awareness and tougher emissions regulations required a viable electric vehicle for growing cities.
However, electric cars have the reputation of being sluggish because of the heavy battery needed to achieve a range of at least 100 kilometres. In the new i3, BMW was able to offset the extra mass by using carbon fibre reinforced plastic: With five times the strength of steel and one third its weight, carbon fibre is the lightest and strongest material currently available.
Daniel Schafter, head of production of Concept BMW I, claims: "With the BMW i3, we get a reduction of 250-350 Kg using carbon fibre, which more or less compensates for the weight of the battery." This saving translates into a car weighing 20 percent less than the Nissan Leaf.
According to BMW carbon fibre expert, Dr. Dirk Lukaszewicz, carbon fibre not only saves weight but is also capable of absorbing almost four times the energy that steel or aluminium could cope with.
Image credit: www.inhabitat.com
With these impressive results, why is it that carbon fibre composites have, until now, not found a market in mainstream mass produced vehicles?
In order to find a footing as the material of choice for the construction of vehicle bodies, carbon fibre’s protagonists face several challenges:
•At around $20 per kilogram, carbon fibre is 20 times more expensive than steel (Frost & Sullivan)
•Process cycle times using conventional "Resin Transfer Moulding" methods do not allow for economical high volume mass production
•Availability: In 2007 BMW predicted a production volume of 30,000 units which, it was estimated, would account for 10% of the global production of carbon fibre filament.
BMW Chief Financial Officer Friedrich Eichiner summed up the company’s perspective on the future of carbon fibre: "The investment in carbon fibre isn't about a single vehicle, but about future-proofing our entire portfolio and therefore our business."
"There's no way around making cars lighter, and steel is reaching its limit." In BMW's Leipzig factory, steel is no longer the measure of toughness. A sign there reads "nerves of carbon fiber."
"When you take a new path, there's always risks involved," said Eichiner. "But if we succeed here, then it's a huge chance, and the competition will need time to catch up. There are not many opportunities in this industry to gain an edge like that."
Will BMW be remembered as visionaries or will steel and aluminium continue to dominate car body design and construction in the 21st century?
European Aluminium Association – Aluminium body structures
Arcelor Mittal Steel - Tailored blanks
Rocky Mountain Institute – Steel vs Carbon Fibre cost comparison
Bloomberg - BMW Makes Lone Shift to Carbon Fibre to Gain Auto Edge (Chris Reiter)
BMW AG – BMW 13 press release
Peter Els is a technical writer for Automotive IQ