Building fat-free, high fiber autos.
Truly, it's "not your father's Oldsmobile." Ditto for Ford, Chevy...or VW, for that matter. Just check the specs - particularly the curb weight. Those bulky "landboats" of yore have given way to leaner, keener driving machines.
Eager to meet the bogeys regarding fuel economy, automakers started getting serious about fat-free engineering more than a decade ago. The payoff to date? In 1978 the typical family vehicle weighed in at 3569 lb; by 1995 it had trimmed itself to 3208 lb.
Today, engineers aim at producing even lighter machines - but machines that give up nothing in terms of structural integrity or on-the-road performance. To do so, these automotive alchemists are using such materials as aluminum, magnesium, and high strength steels in their high tech crucibles.
Take Ford Motor Co's Windstar. When the minivan debuted in 1994, it boasted three times as many high strength steel parts as its predecessor, the Aerostar. About 60% of its 165 parts for the body in white (BIW) - the unpainted, untrimmed auto body - are high strength steel stampings, reports the Auto/Steel Partnership (A/SP), Southfield, MI, an association of the Big Three automakers and major sheet steel producers. A/SP reports that Ford has saved between 5% and 10% in weight by using high strength steel throughout the Windstar's BIW. "With what's in production today, the Windstar is certainly in the forefront," says Martin A Rumel, A/SP executive director.
Ford uses electrogalvanized bake hardenable (BH) steel for the Windstar's front doors and sliding side door outer panels. It picked BH steel for these three biggest stampings because the material offers weight savings, dent resistance, and sound formability characteristics. Foregoing heavier and more costly plastic, Ford also tapped BH steel for certain unexposed structural body parts, including door reinforcements and roof headers.
A/SP also notes that Ford has been able to reduce customers' warranty claims for dent damage on the Windstar by using BH steel for exposed body panels.
Add HSLA to Windstar's list of acronyms, too. High strength low alloy steels can be found throughout the BIW. Structural underbody rails are made from HSLA steels. And ultra-high strength steels make up the side-impact door beams in the front doors and are used for front bumper reinforcement, according to A/SP.
The idea is simple, according to Mr Rumel. Today the thrust is improved fuel economy, and improved fuel economy comes about by cutting weight. "And if you can go to a higher strength steel," he says, "you can reduce the weight."
Steeled for the future
In general, the attitude steel producers display in the automotive market was best summed by Ward's Auto World in a September 1994 story headlined "Steel Hangs Tough." The magazine noted that "Steelmakers indeed have created a stunning array of engineering programs and countermeasures to answer competition from lighter-weight - and more expensive - materials." Only a year later, an international consortium of 33 steel producers boasted that its UltraLight Steel Auto Body (ULSAB) was moving farther from the drawing board and closer to the highway.
As designed in Phase I of a multi-phase project, the ULSAB cuts the BIW weight by 25% while increasing torsional rigidity by 60%. Additionally, reports A/SP, BIW cost reductions of 14% are all achievable. "The savings come from a combination of innovative design, fewer parts (169 for ULSAB vs 195 for the typical current BIW) and more efficient manufacturing," A/SP's High Strength Steel Bulletin reports.
Porsche Engineering Services Inc, Troy, MI, a North American unit of the German automaker Porsche AG, shepherded the design work of ULSAB. After benchmarking 32 current midsize four-door sedans to create an average base model, Porsche set structural performance targets and design concepts. In the end, Porsche looked to a design strategy that included the use of hydroformed tubes, high strength steels, and tailor welded blanks.
Hydroforming eliminates flanges and reduces the number of parts. It can be found in the side roof rail, the fender support rail, and the pass-through beam. Tailor welded blanks likewise help cut the parts number and reduce subassembly welding. They are seen in such parts as the front inner rail, the front shock skirt, and the quarter inner panel.
Other processes were also deemed critical to the design. They include hotforming, rollforming, and traditional stamping. As the A/SP notes, "Hotforming offers the advantages of very good formability of ultra high strength steels (over 160 KSI yield strength past forming), improved tolerance control with no springback, and good potential in working with tailor welded blanks."
The results of this 451 lb design "overjoyed" the engineers involved in the project, reports Peter T Peterson, director of product applications for US Steel Corp's Flat Rolled Products and a member of the Automotive Applications Committee of the American Iron and Steel Institute. "The long and the short of it," he says, "is that we took 140 lb out of the vehicle, which is something we couldn't have done with alternative materials in the hood, fenders, and deck lids." Structurally as well, the design is impressive: torsional rigidity, 19,056 Nm/deg; bending rigidity, 12,529 Nm/mm; first BIW mode, 51 Kz. In short, it's a vehicle "stiffer than a pump handle," Mr Peterson tells T&P.
A HEAVYWEIGHT SUMMARY ON THE ULTRALIGHT STEEL AUTO BODY
Reference ULSAB Diff.
Mass (Kg) 271 205 -66 Torsional Rigidity (Nm/deg) 11531 19056 +7,525 Bending Rigidity (Nm/mm) 11902 12529 +627 First BIW Mode (Hz) 38 51 +13 Cost (US)(*) 1116 962 -154
Source: IBIS ASSOCIATES, WELLESLEY, MA.
The design shaved dollars as well as pounds. This structure would cost $154 less to manufacture than a current conventional design.
ULSAB now moves into Phase II, the building of demonstration hardware. After crash modeling is completed, the development of tooling will begin, says Mr Peterson. "We want to validate that every part in the vehicle is manufacturable using current technologies," he explains. "That means dies and presses running at current production speeds. When we're finished, we will have what we call demonstration hardware to validate the manufacturing processes and assembly sequences." This phase is scheduled to end in 1997.
The final phase, scheduled for early 1998, will take ULSAB to regional automakers. According to Mr Peterson, the consortium will encourage automakers to build in and build on the knowledge gained by this project.
CAFE au aluminum
And while steel may be hanging tough, it's not hanging out alone. If the federal Corporate Average Fuel Economy (CAFE) standards increase to 35 mpg in the next decade, expect to see increases in the use of aluminum along with plastics/composites. Some industry observers see the increase in the use of aluminum going as high as 20%. Again, it's a matter of weight-watching.
"The use of aluminum in cylinder heads and blocks in passenger car engines is expected to increase dramatically to 90% and 33% respectively by the year 2005," reports the Delphi VIII analysis produced by the Office for the Study of Automotive Transportation at the University of Michigan Transportation Research Institute, Ann Arbor, MI. "A smaller, but growing, use of aluminum for cylinder heads and blocks in light trucks is also envisioned in the next decade. The substitution of aluminum for cast iron in engine blocks and cylinder heads is a significant source of weight reduction, apparently with an acceptable value/price trade-off."
Probably nowhere is aluminum being trumpeted more today than in GM's new electric car, the EV1. Weighing in at 2970 lb, it's the first production vehicle in North America boasting an all-aluminum structure.
The EV1 uses the Aluminum Vehicle Technology (AVT) system of Alcan Rolled Products Co, Farmington Hills, MI. "It's a proprietary system that's not only material technology in terms of the alloy, but a proprietary coating and lubricant on the material. When combined with an adhesive we developed with Ciba-Geigy Corp, you have the ability to bond and weld a structure together," explains David Rinehart, Alcan's program manager-automotive.
The EV1 uses a sheet-based body frame that integrates the functions of the car body and chassis like the unibody or a body-frame integral. AVT is used to create the weld-bonded aluminum structure. All the mechanical components of the EV1 and the composite material panels are attached to it. Extrusions, castings, and other sheet fabrications are integrated into this structure.
"What we really have is a hybrid approach to how we tool up the body structure," explains Tom Lobkovich, body structure systems manager for the EV1. Tooling processes, he says, are fashioned for the most advantageous manufacturing strategy. "In fact, that has been one of the drivers on the design strategy relative to the various components," adds Bill Szkodzinski, manager of manufacturing engineering. "Each one is looked at on its own merit, to determine whether it makes sense to do a stamping, a folding, an extrusion, or a casting."
For the assembly techniques themselves, GM uses both the rivet-and the resistance spot weld, says Mr Szkodzinski. Spot welding creates a whole set of challenges on aluminum, he adds. "When you spot weld aluminum vs steel, it requires three times the current and somewhere between one and two times the pressure, depending on the metal combination," he points out. "And there is a lot closer alignment requirement between the electrodes. That equates to a weld gun that is typically larger than the equivalent steel weld gun." Alcan points to such successes as EV1 as proof that aluminum is (pardon the expression) making more of a dent in the automotive market. Aluminum currently has a 5.8% content share of the North American market. An annual growth rate of 4% is anticipated.
"While the EV1 is a limited-production vehicle," says Mr Rinehart, "the AVT system also lends itself to scaling up for volume production and for utilizing the current high volume production infrastructure."
From Mg onward
The Delphi VIII analysis predicts that "aggressive" CAFE requirements will increase usage not only of aluminum, but magnesium as well. In fact, if one looks at the recent evolution of the typical family vehicle, magnesium usage jumped fivefold from 1978 to 1995. According to Diemakers Inc, Monroe City, MO, magnesium diecastings in particular have higher strength-to-weight ratio than aluminum diecastings or car body steels.
Initially, automakers took a somewhat cautious approach toward magnesium applications. In the early '80s, Ford used magnesium for a brake and clutch pedal support bracket on its Bronco and Ranger pickups. The weight difference compared with aluminum was telling: less than half. By the mid'-80s the automaker was looking to magnesium for its design of a new steering column in such 1990 models as the Taurus and Crown Victoria, says Diecasters, which worked with Ford's engineers on this application.
While weight reduction is certainly an advantage in using magnesium, the material's low heat content and low reactivity to steel give it a longer die life compared with aluminum diecastings, according to Peter Caton, technical director of Diemakers Ltd, Slough, England. At a presentation to materials engineers in England late last year, Mr Caton noted that the "good fluidity of molten magnesium allows complex and fine detailed components to be cast and the low heat content also results in lower thermal distortion, enabling close tolerances to be held [and] often eliminating secondary machining operations."
While the relative merits of aluminum and magnesium parts get debated, engineers certainly haven't forgotten that their strategic concoctions also include iron, copper, plastics, powdered metals, and so on. One recent case in point: Amodel polyphthalamide (PPA), produced by Amoco Polymers, Alpharetta, GA, as a replacement for a diecast aluminum part used as the base for an ABS pump and motor assembly. The part can be found in GM's C/K light trucks. According to Amoco, the changeover from diecast aluminum to PPA eliminates five secondary machining operations.
How significant any particular material will be to future production runs is impossible to predict. But it's fair to say that the issue of curb weight will greatly influence the automotive menu, with the recycling issue coming in a close second. (Currently, 75% of a car's material content is recycled, reports the American Automobile Manufacturers Association.)
Of course, the most important thing to remember is that engineering turns these materials into road-worthy, marketable vehicles. "The whole key to this is holistic design," Mr Peterson insists. "Basically, light-weighting a vehicle is a systems engineering problem. You can't light-weight components and get a light-weight vehicle, because some of the components don't weigh enough already and don't do their jobs in the systems context as well as they should. Soon you have to start putting patches on the parts that are failing. But holistic design may indicate to you that if you provide some stiffening in one area of the structure, you may take the stress off a component that's failing, and you may be able even to lighten it up.
"But you've got to look at the entire structure," he stresses.
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|Title Annotation:||materials for lightweight automobiles|
|Author:||McKenna, Joseph F.|
|Publication:||Tooling & Production|
|Date:||Apr 1, 1996|
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