After years of watching aluminum win smaller skirmishes with low-volume, high-end vehicles switching to the non-ferrous alloy, steel took a direct hit. At this year’s North American International Auto Show in Detroit, Ford revealed an aluminum-bodied F-150. It wasn’t a concept. It was the real deal, and it sent a very loud and clear message. From the 2015 model year, every F-150 will roll on a steel frame, but the body structure above it will be aluminum.
The announcement reverberated like thunder. This is no low-volume vehicle, but the best-selling vehicle in America. On average, Ford sells nearly 800,000 F-Series annually. (Ford doesn’t break out specific models like the F-150, but a good rule of thumb is that it accounts for about two-thirds, or 534,000 units, of Ford’s F-Series sales volume.) On the face of it, this was a big hit for steel. Ford’s announcement was followed by rumors coming out of GM weeks later that its next-generation pickups would have an alloy body architecture of its own. Only Chrysler, which continues to question the costs and benefits involved with the materials switch, has publicly stated it will stay with steel for the Ram pickup.
“We’ve had incursions like this before,” says Ron Krupitzer, vice president, Automotive Market, Steel Market Development Institute (SMDI; autosteel.org), “though in the past it was compo-sites and plastics with vehicles like the Pontiac Fiero, Saturn and GM’s APV minivans.” As Krupitzer points out, GM’s hopes for improved quality and cost didn’t pan out. “There was too much given up for the economies they thought they were going to gain.” Could the same be true of Ford’s shift to aluminum? “Certainly it has caught our attention,” Krupitzer admits, “but, to be honest, we’re a little puzzled. With the weight reduction achievable with high-strength steel—which is very, very close to aluminum in many cases—the ultimate difference in weight and fuel saved in the real world is fairly small.”
According to SMDI’s calculations, simply reducing the weight of a part on a vehicle by 10% brings a 3% reduction in fuel consumption. “That’s a tank or two of fuel over the life of most full-size vehicles,” says Krupitzer. However, starting from a clean sheet, and sizing the powertrain to give you exactly the same payload and acceleration that you had in the heavier version brings the fuel economy improvement up to 7%. That’s a pretty good return, and one Ford must have considered with the F-150, which will feature a downsized and optimized 2.7-liter EcoBoost V6 as one of its engines. Doesn’t this justify Ford’s move from metal, especially when the average weight save seen by OEMs from steel historically has been in the 3% to 4% range?
Not if the structure is optimized for steel. David Anderson, Automotive Technical Panel, Long Products Program, SMDI, cites the Cadillac ATS, which he describes as “the most optimized steel-bodied vehicle currently in production.” It has a curb weight as low as 3,319 lb., which makes it among the lightest sedans in its competitive set. The average strength of the steel in Cadillac’s smallest sedan is in the range of 410 MPa.
While steel developments continue—not only incrementally, as has been the case for years, but even fundamentally, as the steel makers begin to work hard even at the microstructure levels—Anderson observes, “Ford has made a revolutionary change with the shift to aluminum, but will others accept an equally revolutionary change with steel?” Says Krupitzer: “We’re trying to get one or more of the OEMs to make an optimized steel pickup to go against the 2015 F-150. Companies like Toyota, Nissan and Honda all see the overall cost benefit in using steel to its ultimate, while some companies seem to be oddly frustrated with the incremental change as each new steel arrives.”
What Krupitzer and Anderson suspect is that there’s insufficient appreciation for the fact that aluminum gets its strength via lower density, while steel does the same through stronger, thinner-gauge materials. This has led steel makers to invest heavily in computer-aided design and engineering. “Varying the architecture for greater efficiency,” says Krupitzer, “brought us to topology optimization; looking at the package space, subtracting that away, and optimizing the remaining structure. This results in some very interesting solutions that are quite effective. Even though the gauge is thinner, we use the geometry to restore or enhance stiffness.” This has resulted in what he describes as the front primary load path “elephant nose” structure of the Future Steel Vehicle that the WorldAutoSteel organization (worldautosteel.org) developed to show the possibilities that can be achieved with steel, and roof bows that travel diagonally instead of laterally from span-to-span. This is combined with weld bonding—welding and gluing panels at the same time—in order to get a continuous join that is many times less expensive than laser joining.
Speaking of the Future Steel Vehicle’s development, Anderson points out, “These modeling and computer tools, etc. are applicable to all OEM designs and materials, and those material providers would gain from the same clean-sheet approach and topology optimization we have used with our advanced steels.” However, whereas aluminum still uses the same 5000 and 6000 series alloys they’ve traditionally offered to both automotive and military users for years (Krupitzer likens referring to the alloys used in the 2015 F-150 as “military grade” in the Ford press release to the fishing industry’s renaming of the Patagonian toothfish as “Chilean Sea Bass”), steel potentially has more to offer on the materials front. “Our current portfolio includes 45 advanced high-strength steels, and approximately 19 manufacturing techniques like roll-forming, tailor-rolled blanks and hydroforming that other materials can’t match. And we don’t need five years to build extra capacity. We have it in place now.”