LEARN MORE

Zones



Aluminum Studies

In which we look at a material that is finding use in some of the more interesting vehicles that are beginning to emerge—as well as vehicles that we may be driving some day.

(Jaguar.) The new XJ is described by Ian Callum, Jaguar Director of Design, as “a luxury car with a true sense of gravitas.” For those of us whose Latin is a bit rusty, gravitas means “weightiness” or “substance.” Of course, Callum is describing the sense of the vehicle, because the new XJ is based on a lightweight aluminum architecture.

The body-in-white for the XJ is a study in aluminum assembly. There are stampings (bake-hardened aluminum sheet), castings (high-pressure die castings) and extrusions (high-strength alloy) employed. These pieces are assembled using a combination of self-piercing rivets with adhesives. There are 3,180 rivets and some 120 meters of adhesive applied. Eighty-eight robots are tasked to do that job. And there is no spot welding.

Yes, the XJ has a monocoque construction; the aluminum castings and extrusions are used both to minimize the overall part count while providing localized strength where required.

The body construction is performed at a new, dedicated area within Jag’s Castle Bromwich plant, where all of the aluminum stamping (with the material sourced from Alcan) is performed in a 9,600-m2 shop that’s operated and managed by what’s called a “technical partner consortium,” Polynorm Stadco (a joint venture of two companies that are stamping specialists). Final assembly is performed at the firm’s Browns Lane plant.

One of the benefits of the use of lightweight material for the vehicle is that they are able to use a 240-hp, three-liter V6 engine that provides solid performance. That is, Jaguar tested an XJ6 against the previous generation 3.2-liter XJ8. The six-cylinder version reached 0 to 60 mph in five seconds; the eight-cylinder model required 5.3 seconds. The XJ6 weighs in at 1,545 kg, which is said to be about 200 kg less than competitive vehicles. (It should be noted that there are, of course, eight-cylinder versions of the new XJ, as well: [3.5-l, 4.2-l, and supercharged 4.2-l]). 

And it is worth noting that the body of the new vehicle is bigger than its predecessor: 5,080-mm long; 1,868-mm wide; 1,448-mm high; and with a 3,034-mm wheelbase.

Which brings us back around to why the material was selected. Jag engineers had been examining aluminum-intensive body structures at the Whitley Engineering Centre (Coventry). So there was background. As XJ chief program engineer David Scholes explains, “We chose the lightweight vehicle architecture for the new XJ not because it was something new, but because it would help us deliver significant benefits for our customers. Ultimately, they may not care whether the body structure is aluminum or steel, but the Jaguar customer does care very much about performance, dynamics, fuel economy, emissions, and safety. The choice was clear.” The choice was aluminum.

Well, that’s not entirely true. You see, the cross-car beam isn’t aluminum. It’s magnesium. So are the seat frames.

The last point in Scholes’ comment: safety. While some people might imagine that a heavier material might be safer, apparently it was determined that the aluminum-intensive body offers an advantage. For one thing, because it is lighter than an equivalent steel body (e.g., it is 40% lighter than its predecessor, yet 60% stiffer), there’s less kinetic energy carried into a collision, which means less energy to absorb. (Naturally, there are front and rear crumple zones.) And the side is protected by a strong B-pillar, extruded lateral floor reinforcements, and door-beam extrusions.

The first XJ was introduced in 1968. Since then, more than 800,000 have been sold—which accounts for more than half of the Jags produced. Undoubtedly, there was due consideration given to what the seventh-generation model would be made with...

(Audi. ) Although the Audi A2 is something of a poster child for aluminum, apparently, when it goes out of production, if there is a successor, it will likely be steel, not aluminum. So, does this mean the Audi’s use of the material is soon to be behind it?

Hardly. Think only of the A8.

Since 1984, when the first A8 rolled out, Audi has invested more than $300-million in its “Aluminum Competence Center.” Much of that work has been refining the “Audi Space Frame” (ASF), which is the fundamental of the A8’s structure. With the latest generation ASF, for the new 2004 A8, the vehicle now has 60% more torsional rigidity than the previous model. What’s more (or what’s less) is that there are 17% fewer parts than in the last ASF (267 versus 334), and the weight is down by some 10%. The level of process automation is now 80%, versus 20% for the previous A8.

So what’s the difference? For one thing, there are larger castings: long continuous profiles and straight extruded sections. The ASF has a front and rear section. The forward structure is a large, single casting (previously eight pieces) that supports the air conditioning system, pedal mount, and crossbeam (connecting the A-pillars). The A-pillars are also cast shell halves. The rear structure has two central castings. One of them supports the rear subframe and connects the sills at the rear. The other connects the C- and D-pillars, serves as a suspension mount, and forms the outside edge of the roof frame. Two straight extruded sections are used to transversely attach the rear castings with the rear shelf. There are two other straight sections at the rear, these providing vertical support for the upper and lower planes while also serving as the portal for the air suspension’s strut mount. (The rear section of the ASF had to be redesigned in order to accommodate the new adaptive air suspension system for the A8.)

The ASF has front and rear longitudinal members that are joined by a casting. The forward and rear sections of the ASF are joined with the roof frame rails, the sills, the seat cross members, the B-pillar, and the floor panels. The side of the roof frame is hydroformed and has varying cross sections to accommodate different load requirements along its length. The B-pillar is a single casting; it had previously been eight different pieces. There are single-section side panels and a single-section roof.

The various pieces of the A8 are put together in a variety of ways. For example, laser welding is used to attach the roof and side panels to the support structure; in all, there are 20 meters of laser-welded seams. They’re also using a combination of laser/MIG welding. The front longitudinal members are bolted in place to facilitate repair in case of a front-end crash. Riveting is also used for joining.

The weight of the A8 body is estimated to be approximately 50% lighter than an equivalent steel body. The weight of the A8 3.7 quattro is 3,894 lb.

(Land Rover.) Maybe it’s just marketing hype. But the word on the Range Rover is that it is “the most capable vehicle in the world.” It is presently in its third generation. That’s three generations, each of which last about 10 years. The current model (i.e., the ’03) has a new steel monocoque body with integrated chassis, as well as three steel subframes, all of which are said to contribute to high levels of torsional stiffness. That said, there are aluminum closure panels: the front quarter panels, hood, and doors (as well as an all-aluminum V8 under that hood). So, with the steel structure, why the aluminum panels?

According to a Land Rover spokesman, “Land Rovers have used aluminum since 1948 in their bodies for its low weight and anti-corrosion benefits.” When you get out in the bush, apparently, some of the slogging can be messy, and aluminum is a means by which it can be handled.

(SUV'S. ) The sounds on Capitol Hill are raucous. Listen, for example, to Joan Claybrook, president, Public Citizen, addressing the Senate committee on Commerce, Science and Transportation (February 26, 2003): “SUVs are basically gussied-up pickup trucks, and most have never been comprehensively re-designed to be safely used as passenger vehicles. In a crash, the high bumper, stiff frame and steel-panel construction of SUVs override crash protections of other vehicles. Due to their cut-rate safety design, SUVs often fail to adequately absorb crash energy or to crumple as they should, so they ram into other motorists and shock their own occupants’ bodies. Endangering their occupants, SUVs may also slide over roadside guardrails, which were designed for cars. And their high profile and narrow track width create a tippy vehicle, which, when combined with their weak roofs and poor crash protection, places SUV drivers at risk of death or paralysis in a devastating rollover crash.”

And that’s in just the third paragraph of her statement, which goes on for pages.

Jeffrey W. Runge, administrator of the National Highway Traffic Safety Administration (NHTSA), was somewhat more measured in his comments to the same committee on the same day. Yet Dr. Runge’s observations are, perhaps, more disconcerting. As in: “A more complex fleet, including vehicles such as minivans and SUVs that scarcely existed before, has replaced the fleet that was once dominated by passenger cars. There are now over 79 million light trucks on the road—including pickups, minivans, and SUVs—representing about 36% of registered passenger vehicles in the United States. With light trucks now accounting for nearly 50% of new vehicle sales, their share of the total fleet is growing steadily.

“While the overall fleet is safer, the new fleet composition presents new safety issues. Two issues stand out. Rollover is one issue. Pickups and SUVs are involved in a higher percentage of rollovers than passenger cars—the rate of fatal rollovers for pickups is twice that for passenger cars and the rate for SUVs is almost three times the passenger car rate.” (The other issue, incidentally, is compatibility).

It should go without saying that the Alliance of Automobile Manufacturers—which consists of BMW, DCX, Ford, GM, Mazda, Mitsubishi, Nissan, Porsche, Toyota, and VW—roundly rejects such criticisms, rolling out its own interpretation, which indicates that SUVs are not only “as safe as cars,” but actually “have a safety record that surpasses that of cars in the most common crashes.”

So what does this have to do with aluminum? Well, Dr. Richard L. Klimisch, vice president, The Aluminum Association, who has been monitoring this situation rather carefully, believes that aluminum can help solve some of the problems related to SUVs, such as the aforementioned “tippy” condition described by Claybrook. “The issue with SUVs is how to make them safer and to get better fuel economy at the same time,” observes Klimisch, who adds, “The conventional wisdom is that you have to compromise one in order to get the other. You can get both with aluminum.”

What Klimisch suggests is that if aluminum was used to produce SUVs, then the vehicles could still be comparatively sizable, include energy-absorbing crush zones, yet be lighter than steel vehicles, thereby providing both safety and fuel economy. Or, consider the rollover issue as it relates to the problem of achieving greater roof strength. Klimisch says that by using aluminum, increased strength can be achieved without putting more weight above the beltline, thereby not contributing to a high center of gravity. He suggests that thin-wall casting, a process that is used by Audi for the A8, would be a good way to create components like stronger B-pillars, which could address this issue.

Aluminum and SUVs (more than the amount used for, say, the Range Rover) have real technical potential. Two papers were presented at the 2003 SAE World Congress by engineers from Ford about aluminum bodies and frames for SUVs.

In “A Design Concept for an Aluminum Sport Utility Vehicle Frame” by Michael W. Danyo, Christopher S. Young, Henry J. Cornille, and Joseph Porcari (SAE paper 2003-01-0572), the authors describe a study that was conducted under the Partnership for a New Generation Vehicle (PNGV) program along with Alcan Aluminum and The Budd Company: “The specific objective of the study was to assess the capability of an aluminum frame to achieve equivalent performance to the 2002 Ford Explorer frame, but at a 40% weight reduction.” It wasn’t just a matter of determining whether they could devise a lighter frame, but a frame that was actually compatible with other ’02 Explorer elements, like the body mounts, powertrain, closures, and so on.

The research concluded that it is possible to make a suitable frame, although there would be both product and process changes required vis-à-vis switching from steel to aluminum. For example, there would be an increase in the size of the components, be it in thickness, section height or section width. Even with the proposed size increases, the authors note, “These dimensions will yield a proposed design that yields a 44% weight savings for the frame relative to the production steel frame while maintaining equivalent global torsion and bending stiffness equal to or greater than the steel frame.”

As for the process, there is a big change. They looked at various ways to create the frame. MIG welding components was considered, but it was determined that the amount of MIG welding needed to be minimized because of the distortion that can occur due to the thermal conductivity of aluminum. That is, aluminum’s thermal conductivity is about three times that of high-strength steel, which means that it quickly dissipates heat, which requires that more energy be applied, which can result in distortion. And that’s not desirable.

They looked at hydroforming side rails, but there were problems associated with that approach related to such things as the required variety of section sizes falling outside the limits of hydroformed aluminum tubes, the necessity of MIG welding of the cross members and other components, and the inflexibility of hydroforming tools as related to possible model changes.

They came up with another approach, which involves stamping components, then using self-piercing rivets and adhesives to attach most of the frame parts, with just a minimal amount of MIG welding being required. The construction approach is described as “inside-out,” with assembly of the inner components first, then attaching the outers.

The frame is not the issue with the SUV that is described in “The P2000S Unitized Sport Utility Vehicle Body Structure” by Henry J. Cornille, Jr., Michael W. Danyo, and Christopher S. Young (SAE paper 2003-01-0573). The P2000S is a unibody sport utility vehicle. The exterior resembles a 1998 Ford Explorer; the interior includes a foldaway third row like the one in the 2002 Explorer. The Ford engineers had experience to draw on for this program, as the corporation had built 40 “Aluminum Intensive Vehicles”, replicas of the 1992 Mercury Sable, as well as developing the P2000 Sedan, a Taurus-sized vehicle that resembles the ’95 Contour, as part of a PNGV project. With that basis, they moved forward on the P2000S, which resulted in the production of 10 bodies-in-white, eight of which were transformed into running vehicles. The vehicles achieved a 59% body structure weight reduction as compared with a body-on-frame steel vehicle. While the preponderance of the components is aluminum (86.9% are 5000 series alloy; 5.4% are 6000 series), there is some steel, 7.7% of the body-in-white components, which mainly take the form of fasteners and seat-belt anchorages.

Clearly, if weight reduction is the goal, then aluminum is an answer. But even Klimisch acknowledges that nowadays, the whole focus is on cost. And in that context, steel remains more competitive.