Lightweight materials compete for automakers' attention

  • 24-Jun-2013 03:25 EDT
EDAG_Leichtbau_1.jpg

Automakers looking to lightweight their new car designs are studying advanced steels, aluminum alloys, and plastic composites. (EDAG)

From the automakers’ viewpoint, the federal government’s toughest CAFE standards ever—fleet-wide fuel-economy averages of 54.5 mpg—are only a few new-car cycles away. As the 2025 start-date approaches, it is with rising trepidation that industry planners face the fact that meeting the mandated mileage mark will require cars made of materials other than the traditional, lowest-cost technology: formed mild-steel sheet.

The hard engineering reality is that other fuel-saving measures, including advanced internal-combustion engines, hybrid and electric powertrains, slicker aerodynamics, efficient HVAC units, as well as stop-start, regenerative braking, and energy-recovery systems, won’t be enough to carry the corporate fuel-efficiency numbers beyond the threshold.

Introduction of high-strength-to-weight structural materials such as high-strength steels (HSS), aluminum, and carbon-fiber-reinforced polymer (CFRP) composites elicits deep concerns among risk-averse and cost-wary company planners, who worry over uncertain manufacturing costs. The need to shave substantial weight while ensuring full crashworthiness and durability must be balanced with other requirements such as maximizing the use of existing production infrastructure, dealing with questions regarding unfamiliar materials, processing and joining methods, and establishing a strong supply base.

Time to diet

To meet stringent standards, planners know that their “next-next-generation” vehicle designs must shed as much as 30% of their current mass without adding major costs, since the car-buying public will brook no big price rises, said Carla Bailo, Senior Vice President for R&D at Nissan Americas, speaking recently on the car industry talk show Autoline. That’s up to half a ton depending on the vehicle size and the efficacy of other efficiency measures.

Today’s average passenger car is about three-quarters metal, the great majority of which is mild-steel sheet. But the ferrous fraction is falling with each model year as more aluminum and nonferrous metals (approximately 8%) and plastics (about 11%) are incorporated into new vehicle designs.

Cutting a car’s mass by 10% can raise fuel-efficiency numbers by somewhere around 6 to 8%, says the U.S. Department of Energy, so it’s no surprise that new lightweight nonstructural materials and mass-saving interior designs are already well on the way from subsystem suppliers. But shaving pounds from the car structure itself—the most fundamental of automotive subsystems and one of the most finely optimized in terms of delivering function at the lowest production cost—is never an easy call.

“We put the right material in the right place at the right price,” a top Volkswagen engineering manager told AEI a couple of months ago in Mexico, but what’s the “right” choice in every case remains a remarkably fluid concept.

Material moves

Nissan recently announced that it intends expanded use of a high-formability, advanced high-strength steel (AHSS) alloy with a tensile strength of 1200 MPa in up to one-quarter of all body parts. Meanwhile, VW said in a press release that it is “giving up aluminum for the high tensile steel, which is up to six times stronger than conventional steel. The new material not only made the new Golf about 100 kg lighter but also helped the company reduce costs.” VW added that this approach “confound[s] forecasts that aluminum would be the metal of choice for reducing weight. Aluminum is about a third of the weight of conventional steel but costs three times as much.”

Whatever structural solutions are available, be they new steels, aluminum, or some combination including polymer composites, the important thing to remember is that “the U.S. auto industry has developed around using metals,” stressed Matthew Zaluzec, Global Materials and Manufacturing Researcher for Ford Motor Co. and a member of the U.S. Automotive Materials Partnership, in testimony to the National Research Council's Committee on Fuel Economy of Light-Duty Vehicles in Ann Arbor, MI, last February.

“Automakers buy steel, stamp it, shape it into frames and body parts, weld it, paint it, and send out finished cars,” he emphasized. “To disrupt that flow is a real leap of faith. For 2016, you're going to be driving cars that are made mostly of steel and aluminum. It's all about the infrastructure. I can buy it now, and I know I'll be able to buy it five years from now.”

Dueling feasibility studies

Such a basic change in the practice of automotive design and manufacturing, of course, offers a potential bonanza for suppliers of mass-efficient engineering materials. As automakers sift through their lightweighting options, the perennial grudge match between the steel and aluminum industries to promote their products has only escalated, with each releasing its latest reports to show their metal’s capabilities and make their opposing sales points.

The mass-shedding efforts of both WorldAutoSteel, the automotive group of the World Steel Association (see study at http://www.worldautosteel.org/projects/future-steel-vehicle/), and the Aluminum in Transportation Group of The Aluminum Association (see study at http://www.drivealuminum.org/news/new-study-aluminum-superior-to-steel-to-drop-weight-boost-mpg) cut from 35 to 40% of the weight out of (different) base vehicles.

(Go to http://www.sae.org/mags/aei/12263 to read about a recent study conducted by ArcelorMittal evaluating the various lightweighting materials’ abilities to help OEM fleets meet the 2025 target.)

A year or two before those studies, a European Commission-funded consortium of seven European automakers (led by VW) as well as suppliers and engineering centers set out to study and refine a multimaterial design—SuperLightCar—which achieved a 35% weight savings (http://www.superlightcar.com/public/index.php?option=com_frontpage&Itemid=1).

Although the three studies have different constraints and goals, and their baselines do not entirely jive, the separately sponsored (but related) studies were conducted by one company, EDAG Engineering + Design AG of Fulda, Germany, which has wide experience in new product development, production facility development, and tool development for the automotive industry. This commonality of approach will hopefully enhance the validity of the comparison.

Cost estimates

A related study was conducted by Auburn Hills, MI-based EDAG for NHTSA (National Highway Traffic Safety Administration) using a 2011 Honda Accord sedan as its baseline vehicle and focusing on the potential weight savings and costs deriving from alternative structural materials. EDAG targeted changes that would add no more than 10% to the overall production costs, which were limited to $1500.

Making a car with extensive use of aluminum, it concluded, would cut weight by 35% and cost $927 more. In contrast, a car using a combination of advanced steel, aluminum, and polymers in targeted locations would cut weight by 24.5% but only add $319 to production costs. A CFRP-intensive vehicle would cut weight by half, the EDAG study said, but it would add $2700 in cost.

Some idea of the complexities of this issue emerged recently when Honda released some “observations” of the lightweight design used in the NHTSA-sponsored study. Honda engineers challenged EDAG’s conclusion that it had achieved performance parity with the 2011 Accord in the areas of crashworthiness, performance, and drivability (ground clearance) or took into account certain business conditions (platform commonality) and the need to add mass to correct performance and platform issues. (See a Honda presentation regarding the NHTSA study at http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CC0QFjAA&url=http%3A%2F%2Fwww.nhtsa.gov%2Fstaticfiles%2Frulemaking%2Fpdf%2FMSS%2F4-Thomas-Honda_Report.pdf&ei=5MbBUZ_mCuXZ0QH0moHwCw&usg=AFQjCNH6nF604Pl1_Wxlq30W6bDPT4pfWQ&sig2=8w9l1AyxsxFb_nTdbIL3dw&bvm=bv.47883778,d.dmQ&cad=rja

The difficult discussions continue as the clock ticks away.

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