| From the September print issue
Perhaps having the atomic number 13 doesn’t help. The most abundant metal in the Earth’s crust, aluminium, has a long way to go if it is to replace steel as the default material for car body construction.
However, for some applications engineers believe that the benefits of a density one third that of steel more than outweighs the disadvantages of higher raw material cost and relatively low strength, stiffness and formability.
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Down the line:
other OEMs have not rushed to join Jaguar in all-aluminium bodies
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Jaguar has been a strong advocate of aluminium for some time, the new XK following the launch of the all-alloy XJ in 2002. Cutting the mass of the body-in-white (BIW) by 40 per cent compared to its steel predecessor provided impetus to continue with aluminium monocoque construction for its new coupé and convertible; the latter has a shell weighing only 287 kg.
The XK also makes more use of castings than the XJ in high load bearing areas such as engine and suspension mounting points. There are 20 excluding those in the doors, the majority high pressure diecast.
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| Front suspension mount is precision die cast. |
Impressive, but could the XK have been made just as light using the latest advanced high strength steels? Mark White, chief technical specialist for body-in-white, Jaguar and Land Rover, said: “You can get a weight saving with high strength steel (HSS) but it’s a small saving. Despite going from almost no HSS content in the mid 1990s to well over 50 per cent in recent offerings, most of the cars and their BIW have increased in weight.”
Trading strength against weight will always result in compromise but, White says, the benefits of using HSS are not universal: “About 15-20 per cent of the BIW are strength dominated parts; they’re the only real area where you can save weight with HSS.”
Stiffness benefits
Aluminium has low density in its favour, but its elastic modulus of 69 GPa typically is only a third that of steel. Taking a simple case, the linear stiffness of an aluminium part can be made the same by raising material thickness by half. It will still offer a weight saving of almost as much. Cost is another matter.
But nor can cost be ignored when considering HSS: they are more expensive to buy and to process than ordinary forming steels. Elevated yield strengths can reduce the suitability of the material for complex deep drawn shapes, one of the criticisms commonly levelled at aluminium. There are issues too with joining different steel grades, weld quality the most important, because of dissimilar carbon content.
When Ford outlined in July its strategies to cut emissions and boost fuel economy, a new generation of lightweight premium vehicles featured in the areas under research. No detail has been forthcoming, but White said: “We learned a vast amount from XJ that we applied to XK, and would apply to any other future products that we might do.
“Most of the learning was about production and quality maturation rather than technology: I think we knew most of the technology when we started XJ. We didn’t have to invent many things on the run; likewise the XK. We can apply that learning across the whole Ford corporation.”
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All aluminium scrap from the plant at Castle Bromwich is recyled |
Jaguar continues to review new manufacturing techniques for lightweight structures, and also new materials and production methods. Aluminium tailored blanks, first used in automotive in 2003 for the wheelarches of Lamborghini’s Gallardo, are not used by Jaguar because of concerns about the integrity and performance of the welds.
Instead, it is investigating tailor-rolled blanks to increased strength locally for regions of greater stress. The process uses profiled rollers to produce varying section thickness across a sheet of homogeneous material.
It has also looked at a multi-alloy technology from Novelis, formed last year as a spin-off from Alcan. Novelis, which supplies 150kg of 5000 and 6000 grade sheet for every XK, has developed a new method of casting ingots. Dubbed fusion technology, it allows several grades of material to be cast simultaneously to form a single ingot: a big improvement, it says, over the cladding method used for the past 60 years.
Complex process control prevents surface oxidation of the different grades, resulting in better intermolecular bonding at interfaces. The ingot is then rolled into sheet, including gauges commonly used for automotive. As-rolled thickness of the outer layers can be 5–15 per cent of gauge. The result is a material where core strength can be adjusted to offer increased formability while retaining high strength at the outer fibres together with a quality surface finish.
OEM trials
Samples have been supplied to OEMs, which are conducting trials with the new material. One application identified by Novelis is outer panels for closures, where hemming has traditionally been a problem with aluminium. It claims panel gaps can be tighter and more consistent because of better forming – especially on corners, and with improved aesthetics.
Sheet made this way will cost more, but Novelis believes that the automotive industry will be a big market for fusion. Michael Bull, director of technology, auto division, said: “Price is obviously an issue but the way we look at it is that there will be great pressure to lightweight vehicles. Fusion adds value throughout the supply chain: downgauging, robustness at high volume and ease of forming are all key attributes. Taking an outer panel from 1.1 to 0.8mm changes the business case considerably.”
But the business case at present is very much of a targeted use of aluminium. White cites the BMW 5 Series as an example where aluminium used in the front end of an otherwise steel monocoque has reduced weight of the BIW. Similarly, the new Audi TT couples a development of the aluminium space frame, first seen in the A8, with steel. The resulting bodyshell weighs 206kg, aluminium accounting for 140kg.
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Joining remains an issue in aluminium. This is BMW's 5-Series bulkhead in a sticky situation |
This view on the future for aluminium is echoed at Novelis. John Gardner, European vice president, communications, said: “Ten years ago aluminium intensive vehicles were the Holy Grail; we were looking at a number of them, and that was it. Our view today is that there will be niche all-alloy cars like the Audi A8, Jaguar XJ and others, and that aluminium for these applications will work very well. But for mass volume cars I’m not sure that the market is willing to accept it. But what we do want, and what we are seeing the growth in, are multi-metal cars.”
He adds: “Clearly cost is a very big driver. We’re working very hard with our customers, and I think the success we’ve had with them is where they haven’t just looked at the initial material cost, but at the cost of the whole system. There is now a much better understanding of the total value that you can add by lightweighting.”
Total value includes environmental gains. The need to reduce CO2 emissions and fuel consumption will put increasing pressure on engineers to make cars lighter. In so doing, the trend towards downsizing can accelerate.
Power savings
Jaguar’s reintroduction of six cylinder engines into the XJ is attributed to the aluminium body. Light weight also reaps benefits in dynamics too, as White explains: “If you look at the XK performance relative to its competitors, we get away with a smaller engine and less power. But the customer benefits when they’re not using all of that power in everyday use, in terms of fuel economy and emissions. And agility: never underestimate the effect of light weight on agility.”
But aluminium also has competition. White says: “Aluminium is the material of choice to get us the significant weight saving we need on products like the XJ and XK – the very large cars. The guys who worked on these programmes are very good body engineers, but I want to foster that ability to look not just at aluminium but at magnesium, plastics and HSS – and to do it because it’s the right reason.”
BMW has been active in looking at a variety of materials too – its 6 Series coupé mirrors the aluminium front/steel BIW of the 5 Series but adds aluminium bonnet and doors, plastic front wings and a composite boot lid.
Of course, what all of these cars have in common is that they are premium vehicles. This makes recovery of the additional cost of these materials easier than with a B or C-segment vehicle. Also bigger cars permit the use of larger sections, an effective way of stiffening the structure instead of using thicker sheet. So BMW’s 3 Series, a more recent design than the 5 or 6 Series, does not use the mixed metal approach for BIW. It relies instead on HSS and still offers a mass reduction and increased torsional stiffness over its predecessor.
Downmarket gains
Elsewhere, aluminium is making more of an impact in the lower market segments. Low intensity use at high volumes is increasing sheet aluminium consumption, through gradual substitution for steel in discrete parts such as closures. The Renault Clio III has an aluminium bonnet and uses aluminium in the doors for side impact protection.
A report commissioned by the auto and light truck group of the Aluminium Association found that nearly 50 vehicles, accounting for two million units this year, will contain more than 225kg of aluminium each. Of those almost 100,000 will have all-aluminium body structures. The association also says that two thirds of aluminium used for BIW and closures during 2006 will be in Europe and that European use of sheet aluminium for closures – the highest in the world – has increased by 400 per cent since 2000.
So there is growth. But maybe just not the kind that was at one stage anticipated.
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