Automakers look to lightweight metals
Oh, the problems with aluminum. It’s not as strong as steel. Its fatigue characteristics are questionable. And then there’s aluminum’s dimensional integrity.
Times do change, though. If you don’t believe it, take a look at DaimlerChrysler’s minivans. For four years, the company has used an aluminum front suspension cradle in its minivans. There, it dutifully bears some of the vehicle’s biggest dynamic loads. It also supports the ABS control unit, brake junction block assembly, leak detection pump, power steering pump oil cooler, lower control arm, sway bar, steering gear, body isolators, and a host of other items that weren’t mounted on the steel version.
Yet when the former Chrysler Corp. first announced its intention to use aluminum, cynics expressed dismay. Aluminum? In a structural application?
But aluminum has proven itself. DaimlerChrysler reports no problems with aluminum’s performance in the front cradle. And there’s good reason for that: When properly heat treated, engineers say that aluminum’s structural properties—such as yield strength and ultimate strength—nearly match those of steel (see sidebar).
What’s more, aluminum’s dimensional integrity proved not to be a concern. Last year, the company that manufactures the cradles for DaimlerChrysler, Hayes Lemmerz International Inc., reported a rejection rate of zero parts per million. In ‘98, DaimlerChrysler rejected just two out of more than 700,000 cradles—a scant 0.00028%. “I can’t remember when we last had a rejection,” notes Chad Bullock, plant manager for Hayes Lemmerz’s Bristol, IN facility.
An anomaly? Hardly. Hayes Lemmerz has had similar success with the aluminum steering knuckles that it builds for the Ford Taurus and Mercury Sable. After making more than five million steering knuckles, the firm has yet to have a rejection.
Weight the key. Clearly, the trend in the automotive industry is to use more lightweight materials. And automakers are finding that many of those lightweight materials have the strength to serve in load-bearing roles. “Aluminum may have some of its best applications in structural components,” notes David Cole, director of the Office for the Study of Automotive Transportation at the University of Michigan. “It serves very effectively there.”
That’s why more automakers are considering aluminum for structural appli- cations. At Pontiac, for example, engineers plan to use a permanent-mold aluminum casting for the rear cross member on the all-wheel-drive version of the Aztek, a vehicle that will be introduced in 2001. The vehicle will also employ an aluminum propeller shaft to deliver torque from the power take off unit in the front to the drive module in the rear.
At Saturn Corp., engineers have also made heavy use of aluminum in the company’s engines. Saturn’s Twin Cam, used on the new L-Series, employs an aluminum block. It’s not the first time Saturn has used aluminum, though. Since its inception, the company’s engineers have used lost foam casting to make aluminum blocks.
One reason for the rising interest in aluminum is its light weight. In the minivan application, engineers reduced the weight of the suspension cradle by 14 lbs. While conservative estimates held that a stamped steel cradle would have weighed 40 lbs, the aluminum cradle weighed just 26.
Aluminum also offers another advantage. Because it can be cast as a single piece, engineers say it’s far easier to add attachments to an aluminum part. To add a part, assemblers merely drill and tap a hole. Engineers can also easily incorporate accessory brackets or specialized oil passages right in the casting.
|10 challenges for the decade
Engineers in the coming decade will face an array of challenges and opportunities as they design the products that will define our way of life.
This report on harnessing new materials is the third in a series analyzing those challenges and opportunities.
That’s not to say that switching from steel to aluminum or other lightweight materials is easy. Most automakers are hesitant. If a new material doesn’t work as planned, they have a lot to lose. “With steel, the infrastructure is already in place,” explains Bernie Swanson, who served as chassis executive engineer for the minivan platform during the development of the aluminum cradle. “The stamping presses are bought and paid for, the assembly tooling is there, and you already know who the good suppliers are. There’s less uncertainty.”
Steel goes on a diet. That’s why some automakers have considered the possibility of using new, lighter weight steels.
During 1998, the American Iron and Steel Institute unveiled the UltraLight Steel Auto Body (ULSAB), which achieves weight reductions of up to 36%.
The project, which cost $22 million and spanned four years, identified several key technologies. Among them: hydroforming; and the use of so-called “tailored blanks.”
The hydroforming process starts with a welded tube that is placed in a forming die. The tube is then filled with fluid at sufficient pressure and forced outward to conform to the shape of the die cavity. Hydroforming is ideal for use in a car’s sub-frame, engine mounts, steering column and body rails. Indeed, Saturn’s L-Series employs a front sub-frame made from a hydroformed tube. Similarly, GM’s Silverado and Sierra pickups also use a hydroformed front frame. The advantages: greater stiffness; good dimensional stability; and scrap reduction.
Tailored blanks, which involve the use of two or more material sheets, offer a different set of advantages. The material sheets, which are laser-welded together prior to forming, enable engineers to tailor a part so that each of the material’s best attributes—gage strength, yield strength, etc—are precisely located within the part.
All of the technologies used on ULSAB are already employed on production vehicles, engineers say. Porsche’s Boxster, for example, uses various grades of high strength steel, hydroforming, and tailor-welded blanks. Cadillac’s Seville employs tailor-welded body side panels and hydroformed roof rails.
More exotic metals. While automakers continue to incorporate more aluminum in their designs, researchers are advancing the state of the art on another front: magnesium manufacturing. With densities that are just 66% of aluminum’s and 22% of steel’s, magnesium alloys offer weight reductions that are matched by few other materials. Today. automakers are using magnesium wheel rims, in some cases lowering the weight of a 15-inch rim from 12 pounds to six. In the future, they also plan to use magnesium in cross-car beams, transmission cases, heater ducts, and engine control modules. Engine control modules, often made of plastic today, are actually lighter if made with magnesium.
The main problem with magnesium, however, is cost. By weight, magnesium costs about twice as much as aluminum. On a volume basis, its still costs about 30% more. And many big companies understandably shy away from such materials because of fear of the unknown.
That’s why many engineers consider a move to new materials a matter of risk management. “When you make the jump into high volume manufacturing of a new material, you have to manage it properly so you don’t get into trouble,” warns Swanson. “You have to ask yourself if you’re really willing to take the risks.”
—Charles J. Murray, Contributing Editor
Plastics may serve in hard-drive platters
Don’t look for these plastic platters at a picnic. Instead, look inside computer hard drives, where plastics have surfaced as an alternative to the glass and aluminum platters that currently hold the world’s data. As disc-drive manufacturers continue to seek ways to pack more data in less space, injection molded thermoplastics may help sate their voracious appetite for storage capacity.
In what could represent a glimpse of the future, Seagate Technology Inc. (Scotts Valley, CA) reports that it has achieved an eightfold improvement in recording densities with its Optically Assisted Winchester (OAW) drive, which primarily relies on a new opto-magnetic read- and-write head but also makes use of a patterned polycarbonate disc substrate. The OAW system recently enabled the company to squeeze 105,000 concentric tracks of data within an inch of disc space—the equivalent of 25 Gbit/square inch in a conventional hard drive.
Seagate isn’t the only manufacturer showing an interest in plastic discs. Sony Corporation revealed last year that it had developed its own plastic disc with material supplier Nippon Zeon Co. and announced plans to work with Castlewood Systems Inc. (San Francisco, CA) to commercialize the concept.
So why the interest in plastics? It boils down to a potential for reducing costs. According to Blair Souder, global marketing manager for data storage media at GE Plastics (Pittsfield, MA), a variety of engineering thermoplastics could achieve current OEM specifications for flatness and surface quality and do so for 30 to 50% less cost than aluminum discs. Souder attributes the reduction to the fact that plastic platters, unlike aluminum ones, wouldn’t need to go through an expensive polishing process prior to sputtering because they are molded to the finest surface requirements.
To meet the challenges posed by hard drives, GE Plastics is pursuing a wide range of material development strategies, including modification of existing materials with new additives and also new combinations of materials—whether as copolymers or through multi-component injection molding. It might come as a surprise that the company is not focusing its platter development efforts on polycarbonate, which completely dominates as the disc material for seemingly related optical media applications. But as Souder explains, disc-drive platters represent a “first-surface media” application in that the data-carrying metal layer resides on top of the disc rather than between layers of plastic. “These substrates don’t have an optical function,” he says. “They have only an information carrying function.” With optical properties out of the picture, GE has been free to experiment with other disc materials that have better stiffness and dimensional stability. “We’ve been sampling materials from our entire resin portfolio,” Souder says.
Early on in the development process, GE successfully molded plastic discs in its media lab from highly filled grades of Ultem PEI, Souder reports. But in recent months, the company has been evaluating even more promising materials that take advantage of GE’s new vibration-damping technology. First developed for Noryl PPO and later applied to Valox PBT and Xenoy PBT/PC, this additive-based technology is now in the process of being extended to Ultem PEI. Souder won’t give specifics about which of these vibration-resistant materials—or related copolymers—will most likely be company’s offering for hard drives.
Even if plastics could qualify as a drop-in replacement for aluminum discs given today’s OEM requirements, their real promise relates to their use in tomorrow’s drives. Souder points to an emerging strategy for increasing the density of data on a disc: Advances in head-positioning technology allow data to be arranged in tighter tracks on pre-patterned discs. Consider, for example, Seagate’s OAW drive, whose polycarbonate disc is pre-patterned with the tightest of track densities. “Plastics are an enabling technology,” Souder says, because they allow patterns to be molded into the disc surface.
Producing injection molded discs for the next generation of hard drives won’t be easy. “The requirements are increasingly severe,” says Tom Feist, GE’s technical manager for global media. He cites smoothness and flatness specifications measured in angstroms over surfaces as large as 130 mm across and sputtering process temperatures now at 300C and rising.
Making matters worse, the recording heads fly closer and closer to the platter surface as disc technology evolves, and surface-quality requirements continually tighten as a result. Feist notes that in some new designs the heads fly as close as 6 microinches away from the surface of the disc. “At that distance, even hundred angstrom differences over the surface of a disc look like the Rocky Mountains,” he says. And in what could be the biggest challenge for plastics, the increasing proximity of heads and discs also intensifies the need for disc materials that resist nearly all axial displacement—even while the disc spins at 4,500 rpm or faster.
Whatever material does win out as the best candidate for disc drives, it will likely be “vibration modified.” According to Feist, inherent vibration damping properties will help plastics defy axial displacement in spite of flexural modulus disadvantage compared with aluminum. For example, he points to one vibration-damped material that produced discs with less displacement than aluminum, even though the metal is much stiffer. “We’re working right now to understand the effect of inherent damping properties on displacement,” Feist says.
—Joseph Ogando, Materials Editor
Ceramic bearings roll on
The hybrid bearing’s foothold gets stronger as machinery speeds climb higher, loads increase, tolerances tighten, and operational environments become more harsh and demanding. During the 1990’s, the hybrid bearing with ceramic balls and steel races began replacing its all-steel counterpart in many high-speed machine-tool applications. To a lesser extent, they appeared in aircraft wing-flap actuators and precision instruments.
But today, thanks to advances in materials and manufacturing, higher quality, less-expensive ceramic balls make hybrid bearings suitable for even more applications. Some examples include recirculating ball screws for aerospace and industrial applications, and turbine-driven air tools.
In general, recirculating ball screws (RBSs) convert rotary motion into linear motion while transmitting power. Characteristics include high precision, near-zero play in pre-loaded systems, and conversion efficiency between 92 and 94%. Balls rolling in suitably shaped races make contact between the screw and nut threads more efficient, then at the end of the race path recirculate back to the beginning. In aviation-related applications, RBSs impart motion to the movable control surfaces on aircraft. In industrial systems, RBSs reside in a variety of electromechanical actuators to displace numerically controlled machine-tool slides.
For the displacement of movable wing surfaces, where control systems are particularly exposed to atmospheric agents, the only way to ensure that the RBS last for the aircraft’s entire life cycle is to use a stainless steel. But due to brittleness and poor corrosion resistance, 440C stainless is unusable. In Germany, FAG, VSG, Bochum University, Boeing, and Umbra Cuscinetti developed Cronidur 30® for RBS applications. Cronidur 30 is a rolling bearing steel from the family of high-nitrogen steels. Hardness is 58 HRC, and the steel exhibits very good high-temperature strength and corrosion resistance, and holds up better if there is a problem with lubrication, mixed-friction conditions, and pollution particles in the lubricant. Test results qualify Cronidur 30 for the likes of Boeing flap programs, and fuel pumps in the Space Shuttle.
The king of stainless steels. Because of its performance in these applications, Cronidur 30 is hailed as the king of stainless steels for recirculating ball screws. Chemically speaking, explains Umbra Cuscinetti’s Luciano Pizzoni, the presence of a significant percentage of nitrogen and a reduced carbon content distinguishes this tempering and induction material from other stainless steels such as 440C. Since nitrogen has an effect similar to that of carbon during heat treatment, the concept of equivalent carbon is commonly used to compare material composition. Cronidur 30 has a lower equivalent carbon content than 440C. The presence of nitrogen, instead of carbon, reduces chromium carbide precipitation during heat treatment, making the material more corrosion-resistant.
While the percentage of chromium in Cronidur 30 is practically identical to that of 440C, the nitrogen maintains chromium content after heat treatment to ensure corrosion resistance. Introducing high percentages of nitrogen depends on addition of silicon nitride (Si3N4) in the form of ceramic pellets in a pressurized remelting process.
Steel vs. Aluminum
How does aluminum stack up against steel? The A-356-T6 aluminum used in many automotive applications, such as suspension cradles, actually offers some mechanical properties that nearly match those of steel.
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While Cronidur 30 arouses great interest in aviation, its implications for the industrial world are nothing to scoff at. One interesting application on an electromechanical press uses a Cronidur 30 ballscrew with an idler-ball configuration. Idler-ball designs used in aviation to eliminate sliding friction at the ball contact points, separate the loaded balls with a smaller-diameter one that doesn’t roll on the race. Instead, the spacer ball remains suspended like an idler wheel between the two loaded balls. To this effect, the spacer ball rolls on the loaded balls, preventing sliding contact.
The RBS replaced a planetary roller screw in the press application with no change in outside dimensions or peak load (50,000 N). The material’s ability to operate at contact pressures of 3,570 MPa without damage, even after millions of revolutions under load, makes it possible to state that RBSs made of Cronidur 30 can replace planetary-roller screws while retaining essentially the same overall geometry within the assembly. The enormous advantage is the increase in efficiency, from an average of 0.7 for the planetary-roller screw to 0.9 for ground RBSs, making it possible to use motors with power and torque levels reduced by a factor of 1.286. The bottom line: potential to reduce installed power by approximately 30%.
Ceramics take on turbine drives. Just as the turbine has revolutionized the defining industries of our time, Air Turbine Tools (Boca Raton, FL) brings the cutting edge of technology to modern manufacturing. “Traditional vane tools are heavy, vibrate, and don’t last as long as turbine-powered tools,” says Air Turbine Tools VP Bill Mitchell. Vane tools have more moving parts and vanes can wear out if not properly oiled. In contrast, turbine tools have only two moving parts: hybrid ceramic bearings supplied by Peterborough, NH-based New Hampshire Ball Bearings Inc. that use Cerbec ceramic balls from Norton Advanced Ceramics (East Granby, CT).
ATT built its success on the back of hybrid bearings that handle speeds up to 65,000 rpm, and its proprietary turbine technology. The company’s sales have grown by 20% every year for the last five years. Turbines convert a moving stream’s energy into mechanical energy. ATT’s design uses an “O” ring to control airflow between two chambers working as a governor to maintain constant speed at rated power. Air coming into the center of the turbine disk blows out through the nozzle openings causing the tool to spin on two bearings that are pre-packed with grease and sealed, so the spindle or tool requires no lubrication or maintenance.
—John Lewis, Northeast Technical Editor