Multi-material Structures Move mpg Upward

March 1, 2016

Ryan Gehm

Multi-material vehicles are nothing new. Aluminum, plastics, and other “exotic” materials have long joined steel in the material mix of road-going cars and trucks, notably in powertrain and interior components as well as certain body applications like hoods and liftgates. But in recent years, application areas for so-called alternative lightweight materials have expanded as auto-makers and major suppliers look to gain even grams wherever they can.

Lexus offered a great example of this multi-material approach at the recent North American International Auto Show (NAIAS) in Detroit, with the reveal of its 2018 LC 500 luxury coupe. High-strength steels, aluminum, and carbon fiber converge to account for a mass reduction of 100 kg (220 lb), according to Chief Engineer Koji Sato.

“Mass management measures” include an available carbon fiber roof, aluminum door skins mounted to a carbon-fiber door inner structure, and a composite trunk floor. The LC 500 also represents the brand’s most intensive use of high-strength steel to date. (Read more about the LC 500 here.)

“Drivetrain technology alone cannot meet these [fuel economy] numbers. Lightweighting is really a dramatic lever our customers can pull with regard to meeting these standards,” said John Thomas, Global Marketing Manager for Alcoa’s Automotive Rolled Products, during an Advanced Materials/Lightweighting session at the recent SAE Government/Industry Meeting.

“Engineers are pulling on aluminum very strongly, predominantly over other solutions with regard to lightweighting, along with other multi-material solutions,” Thomas continued. “The days of a single type of material for an automotive product are gone; it’s a multi-material world.”

Alcoa claims its new “disruptive technology,” called Micromill, will allow automotive parts to be twice as formable and 30% lighter than those made from high-strength steel. The continuous-rolling process “is the fastest, most productive system right now in the world for making aluminum in sheet form,” Thomas claimed.

Several Micromill parts are featured on the Ford F-150, and its use on Ford vehicles is expected to double from 2016 to 2017. Alcoa has qualification agreements with 12 OEMs on three continents, he noted.

Parts already provided for stamping trials include hood outer and inner, tunnel pan, and a wheel well — products that customers have formed in their production dies with Micromill material.

“There’s no limit on what you can make with it with regard to automotive structures,” including Class A surfaces and high-formability areas like door inners, said Thomas. (Read more about Micromill in the April 2015 issue of Automotive Engineering.)

Of course, the steel industry is not standing idly by. “Steel can be considered its own multi-material solution because of its large range of performance capabilities,” said Roger Newport, CEO of AK Steel.

Reinforcing that notion, AK Steel’s Vice President of Research & Innovation Eric Petersen noted that in the year 2000 there were roughly 100 grades of steel, and now there are about 200. “Now [with third-generation steels] you can take something that’s over 40% elongation with the strength of 1200 MPa, and it completely opens up the design perspective for automotive engineers,” he said.

Third-generation steels combine the strength of advanced (AHSS) and ultra-high strength (UHSS) steels with enhanced formability. Gen3 grades are starting to appear in today’s cars and trucks, and Steel Market Development Institute (SMDI) estimates indicate Gen3 steels will capture roughly 10% of the automotive market by 2020.

“If you want to say anywhere is more competitive [with alternative materials] it’s the chassis and suspension area because of the very aggressive atmosphere and some of the durability requirements on a long-term basis,” said David Anderson, Senior Director, Automotive Market Long Products Program, SMDI. “But we’ve recently provided three steel solutions that equaled the weight of a benchmark aluminum forged lower control arm, for example. In fact, GM is now using a steel clamshell design — the new Malibu went from an aluminum to steel lower control arm.”

Bernhard Hoffmann, Vice President of Engineering and Product Development for Automotive Solutions at United States Steel Corp., said, “I do believe that [the industry] will be multi-material because some of the aluminum and magnesium in certain applications make more sense [due to a specific part’s] functional objectives. But the primary body structure we think will remain very much steel-dominant.”

Lightweight pickup for 2025

A recent NHTSA-funded (U.S. National Highway Traffic Safety Administration) lightweighting project set out to answer, “How much mass reduction is feasible for a 2025 model year light-duty truck? A multi-material approach was taken, with nearly every component of the baseline 2014 Chevrolet Silverado 1500 examined to determine the optimum material and design for that application. The 2014 Silverado makes extensive use of AHSS and was 150 lb (68 kg) lighter than the previous model.

Stipulations for the project were that cost could be no higher than 10% of current baseline’s MSRP (i.e., $3,805 for 2014 Silverado, equivalent to $2,625 increase in manufacturing cost), vehicle performance and functionality had to be maintained, and all recommended technologies had to be suitable for 200,000 annual production by 2022.

During the materials and manufacturing technology assessment, researchers picked technologies that they classified as mature or mid-term — currently suitable for up to 50,000 units/year — for the lightweight design, said Harry Singh, Director for Lightweighting Vehicle Projects at EDAG Inc., who provided an update on the two-year-long NHTSA project at the SAE Government/Industry Meeting.

“We split the vehicle up into 36 systems, and we looked at each system [individually],” he said. For the cab structure, for example, four options were considered: AHSS, aluminum, AHSS + aluminum, and composites (carbon fiber). The team decided on aluminum, with some steel reinforcements, because of 100-kg (220-lb) mass savings with a cost increase of $7.44/kg saved. That compares to an estimated 53-kg (117-lb) mass savings for AHSS, with a cost increase of $2.37/kg. A carbon-fiber cab structure saved 122 kg (269 lb) but at a premium of $27.19/kg.

“For the frame, the design of the mounts and the cross members is a little bit different. The best choice was to stick with steel but use some more up-to-date, higher-strength steel grades,” Singh shared. “For the doors, we thought the best option is to keep the steel on the inner panels where you get most of the strength from, but make the outer panels aluminum.”

Composites find application in the rear suspension, with two glass-fiber-reinforced plastic (GFRP) leaf springs. The engine could be downsized from 5.3-L to 5.0-L while maintaining performance as a result of the weight reduction — in turn allowing even more mass savings.

Preliminary findings (the report was still under peer review at time of presentation) show a mass savings of 17%, or 408 kg (900 lb), for the complete lightweight truck — 2024 kg (4462 lb) vs. the baseline’s 2432 kg (5362 lb). This was achieved at an incremental manufacturing cost increase of $1531, or $3.75/kg of mass saving.

Not considering the mass and cost reduction allowance for the powertrain, the mass savings for the “glider” is 20%, or 359 kg (791 lb), at a cost premium of $4.40/kg saved.

“If we apply these techniques to the fleet, we feel everything that we’re recommending is doable,” he concluded.

Multi-material decklid concept

Continental Structural Plastics (CSP) has developed a concept decklid that incorporates its TCA Ultra Lite material for the outer panel with a carbon fiber RTM (resin transfer molded) inner panel. The decklid weighs 12.11 lb (5.5 kg), a 13% weight savings compared to a similar decklid made from aluminum, according to the supplier.

The decklid concept is the result of a collaboration by the CSP R&D teams in Auburn Hills, MI, and Pouance, France. Contributing partners included Owens Corning, Compose Tooling Expert, Altair Engineering, PPE, Hexion, and Brandolph. It was developed as part of a study to compare the weight of decklids made from steel or aluminum versus a multi-material approach.

“This concept offers OEMs an affordable lightweight solution for body panels to help them achieve new CO₂ emissions regulations,” said Philippe Bonte, President of CSP’s European operations. How affordable? Bonte says the concept is “price competitive” with aluminum.

Simulation tools and advanced software allowed CSP to finalize this development in less than 16 months “from design to the first good part out of the production tool,” Bonte told Automotive Engineering.

CSP claims “a number of breakthroughs” in the use of recycled carbon fiber materials for the structural inner component. Using preformed carbon fiber mats infused with a Hexion fast-cure, epoxy-based resin enabled the companies to reduce the cycle time associated with high carbon fiber content RTM. From injection time to cure to completion, the total cycle time for this component is 2.5 to 3 minutes. Traditional cycle time using the RTM process is 20 to 30 minutes, according to Bonte.

“We have now demonstrated the possibility to mold in less than 3 minutes a large part (decklid inner) that weighs just over 2 kg,” Bonte shared. “Among many challenges [in the program], one has been to successfully impregnate a complex carbon-fiber preformed mat in less than 20 seconds. This was solved through the development of a high performance resin system, an intelligent part and tool engineering design, as well as significant process improvements using high pressure RTM technology (C-RTM).”

“The combination of our TCA Ultra Lite outer and CF RTM inner is suitable for most automotive closures applications — decklids, hoods, roofs, and doors,” Bonte said. “The technology can also apply to structural applications such as load floors.”

TCA (Tough Class A) Ultra Lite is currently used for 21 body panel assemblies on the Chevrolet C7 Corvette. It is a 1.2 specific gravity SMC formulation that can offer up to 40% weight savings compared to standard density composite materials.

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