Carbon Fiber on Its Way?
Lightweight cars excite automotive engineers
With increasing concerns about fuel economy, automotive OEMs and suppliers are looking to take weight out of their cars. One approach with far-reaching consequences is using more carbon-fiber reinforced plastics (CFRP). A car made with lighter, stronger materials means better fuel economy as well as better handling and acceleration. CFRP may be the ultimate lightweight material for the job, but challenges remain.
Today, CFRP is ubiquitous in Formula 1 racers and NHRA Funny Car dragsters. It has even made some in-roads in high-end sports cars. “The Lexus LFA uses CFRP in many of its parts,” explains Ankil Shah, manager materials engineering department for Toyota (Ann Arbor, MI.) He notes that the car uses three forms of carbon-fiber parts: hand-laid pre-impregnated cloth for the main passenger cell; resin transfer molded (RTM) panels for the transmission tunnel and passenger floor module, roof and hood; and sheet molded compound (SMC) for the C-pillars and rear load floor. More parts are under development. However, the Lexus LFA is a $350,000 top-of-the-line sports car intended for limited sale. Will automakers use more CFRP in mass production cars as fuel economy regulations get tighter and the price of gas rises? There are difficulties. Material cost of carbon fiber, processing cost, and slow cycle times are current limitations, according to Shah.
Daimler AG (Stuttgart, Germany), maker of Mercedes-Benz cars, is set to make a big push into CFRP. “Vehicle weight is a critical factor for all drive types—from pure combustion to hybrids and vehicles powered by electric batteries or fuel cells,” explains Stefan Kienzle, head of materials, manufacturing, and lightweight construction in group research & advanced engineering, for Daimler. Lightening the basic structure of the car also allows engineers to add more safety or comfort features and additional drive components. Kienzle explains: “Lightweight construction is therefore an integral component of Daimler’s strategy towards sustainable mobility. We have set ourselves the development goal of reducing the body shell weight of all Mercedes-Benz vehicles by up to 10% compared to their predecessor models in order to achieve better fuel efficiency and further reduce emissions. Components made from carbon fiber, for example, weigh up to 50% less than comparable steel parts of the same strength.” CFRP parts also help create a stiffer vehicle body, increasing crash safety and making a more comfortable ride, according to Daimler.
Mercedes-Benz pioneered CFRP in automotive engineering, introducing CFRP in series production in the Mercedes-Benz SLR McLaren high-performance sports car. “The complete body and front structure were made from CFRP. We proved in 2004 that we have mastered the technology in principle. Group-wide, we now use CFRP technology in series applications in many areas, from buses to Unimogs and AMG vehicles—around 30 t are installed every year. We are working on the assumption that we will launch further CFRP parts on the market in series production from 2013 onward,” Kienzle states.
Towards that end, in January 2011 Daimler announced a joint venture (JV) with Toray Industries (Chuo-ku, Tokyo, Japan), a leading supplier of carbon fiber, for manufacturing and marketing CFRP automotive parts. There are other advantages to using CFRP instead of metal structures, according to Nobuyuki Odagiri, general manager of ACM technology department for Toray Industries. “Compared to metal structures, CFRP is not only lighter weight, but also dampens vibrations better. It does not rust or corrode, and has better fatigue properties,” remarks Odagiri. Under a joint development agreement, Toray, in addition to developing optimal intermediate materials for CFRP, has also been working on the molding processes with Daimler, which is taking the lead for designing parts and developing technologies for joining parts. Joining and fastening technologies are another critical element in the adoption of CFRP automotive parts.
A key innovation that the JV will use is a short-cycle resin transfer molding (RTM), a process developed by Toray. Toray projects using it on Daimler’s Mercedes-Benz passenger vehicles launched in 2012. Improving on traditional RTM cycle times, Toray’s target is to reduce short-cycle RTM takes to less than 10 min. Enabling breakthroughs contributing to the short-cycle RTM include a multiple gate tool design that rapidly impregnates resin throughout the mold and a quick-cure thermoset resin system.
While such a fast RTM method for molding parts will certainly contribute to future CFRP use, Odagiri points out that other manufacturing challenges remain. “[These] include automation technology for preforming, and handling of preform and parts,” he explains. “We also need to reduce manufacturing time through preforming, RTM process, assembly, and postprocesses like drilling and trimming.” He also notes that the assembly process and postproduction process will need to become more sophisticated as well.
The machine tool manufacturer MAG IAS (Erlanger, KY) is also looking at CFRP for automotive applications, using its background developing specialized machinery for aircraft parts. “[In automotive] it would make sense to use CFRP [by] leveraging its high strength-to-weight ratio,” explains Randy Kappesser, vice president and general manager for MAG. “This would seem to be in structural components, similar to where CFRP is being used in the newest commercial aircraft designs.” He points out that CFRP in aircraft is used in strength-critical applications such as wings, and not on luggage compartment doors or other parts where less expensive, less strong, lightweight materials are adequate.
There are also manufacturing advantages in using CFRP over metal. “Typically, the tooling investment for composite applications is significantly lower than for steel stampings. For instance, an inner reinforcement for a car hood will take multiple stamping tools used in sequence,” explains Kappesser. A single CFRP mold will produce the same part in a continuous fiber forming process. “[When replacing] complex multipiece steel assemblies, composites can easily justify themselves.” One example he cites is a part made using 26 separate steel components and tooling combined into a single composite-molded part.
“In the grand scheme of things, the manufacturing challenges are not as significant as other challenges faced by CFRP [in automotive],” says Daniel Allman, director of automotive composites for MAG IAS, adding that vehicle system and part designers do not have the knowledge base for predicting performance of CFRP parts during the life of the part. They have limited knowledge of how a CFRP vehicle will handle a crash. “The OEM’s are risk adverse relative to replacing steel in a vehicle skeleton that surrounds the occupant environment,” Allman states. In response, Automotive OEM’s are seeking predictive simulation models of the forming process they can trust. “Accurately predicting what happens to fiber orientation during the molding process is a key requirement,” he states. He goes on to note that engineers creating complex shapes will need to predict fiber deformation during forming and curing.
“The largest barrier to seeing advanced composites utilized in automotive architectures is the legacy metals-based design culture and legacy assets in place to support metals-based technology. These are proven systems and the metals industry has done an excellent job of developing the next generation of high-strength metals that the OEM’s have utilized in today’s vehicle platforms. However the OEM’s also state they feel they are hitting the wall relative to the level of lightweighting that can be achieved with metals technology,” says Kappesser.
The automotive industry is hypersensitive to cost, a fact not lost on the carbon fiber supplier community. The good news is that many automotive applications do not require carbon fibers with as high a strength or modulus of elasticity (stiffness) as that used in aerospace, according to research conducted by the Oak Ridge National Laboratory (ORNL; Oak Ridge, TN.) “Typical aerospace grade fibers are considered to be about 700 KSI tensile strength and above with 33 MSI or more of tensile modulus,” explains Cliff Eberle composite materials technology development manager with ORNL. “Our [automotive] strength requirement for many applications is 250 KSI and our stiffness requirement is 25 MSI.” He notes that this strength requirement is not only much less than aerospace-grade carbon fiber, but is less than half of most industrial-grade fibers. He envisions parts using this carbon fiber in a chopped form, much like the current sheet molding compound (SMC) that uses fiber glass for nonstructural automotive parts. He goes on to stress that strength and stiffness are dependent on the part and its use. A critical chassis component may require higher strength and need a higher grade of fiber or continuous fiber with precision orientations.
Can they predict the ultimate cost that might make CFRP attractive to automotive? “We believe a price range of $5–$7 per pound is needed,” states Eberle. Perhaps as important as absolute price is price volatility, which Eberle characterizes as common in the carbon-fiber market. “The current global market demand is more or less a 100 million lb [45,359 t] of carbon fiber shipped annually,” he explains. With aerospace taking up much of that current supply, events that affect that industry as well as the price of oil can affect the supply and demand driven price. “I have seen the same carbon fiber fluctuate between $7 and $14 a pound in three years,” he relates. “You just cannot handle that in large volumes such as you see in automotive.” Piggybacking off a ‘precursor’ industry may be one path for CFRP in automotive. Eberle speculates that the wind energy market, where CFRP for longer wind turbine blades would have advantages, might spur more capacity. ”It will help to grow the carbon-fiber composites industry so that you have a larger base and less volatility.” Clearly, more capacity is needed. ORNL estimates using as little as 6 lb (2.7 kg) of carbon fiber per new automobile in 2011 would max-out the current supply capability. ORNL is currently funded to research a number of ways of producing less expensive carbon fibers from a number of precursors, including lignin from wood products.
In the final analysis, cost savings in manufacturing rather than reducing weight may be the primary driver for using composites, according to David Hwang, research analyst for Lux Research (Boston, MA). “Automakers will adopt composites ostensibly for reducing weight,” explains Hwang, “but in reality, composites will find the broadest adoption in applications where they help cut manufacturing costs—specifically in complex nonload bearing parts for low-volume production and electric vehicles.” Why? He points out that despite lowering overall vehicle weight, composites are not as effective in driving down fuel consumption compared to powertrain-related technologies, at least in the near term. Engine downsizing (paired with turbochargers) or electric hybridization improves fuel efficiency up to 50% with relatively low cost. Composites, which are commonly cited to improve fuel economy 6% to 7% for every 10% in reduced weight, require automakers to pursue a fairly dramatic overhaul of their car and production lines to hit those marks. Additionally, aluminum and high-and ultra-high-strength steel, which do not impose changes to existing production, supply, and recycling infrastructure, also lower weight and will steal market share from composites.
“All this low-hanging fruit means that composites will need to leverage benefits apart from fuel-efficiency to find adoption,” says Lux. He stresses the advantages composites have in fixed cost through tooling equipment cost reduction and part consolidation. “Composites are actually cheaper than steel at low production volumes.” However, because composites are a more expensive material, they start to lose their edge as production volume increases. “Right now, we see composites making a lot of sense for volumes less than about 2000/year. Although composites will see some success in high-volume applications with SMC body panels, most growth will be in low-volume production where part consolidation grants composites a cost advantage over steel, or in electric vehicles where even small reductions in weight translate into large reductions in battery usage and cost.”
Daimler’s Kienzle concurs with the idea that CFRP has its place among other solutions. “Our motto is: Use the right material in the right place,” he remarks. “To give an example [on] our new Mercedes-Benz SLS AMG, the aluminum spaceframe weighs just 241 kg and is a major component in the overall concept. For the first time not only the outer skin, but also the complete bodyshell structure is entirely of aluminum, leading to a low DIN kerb [curb] weight of 1620 kilograms.” ME
This article was first published in the September 2011 edition of Manufacturing Engineering magazine. Click here for PDF.