American Iron and Steel InstituteSouthfield, Mich.

Dec. 21, 2004
Ultralight Steel Auto Body

In 1998, the American Iron and Steel Institute, participating in a global consortium of 35 sheet steel companies from 18 countries, demonstrated that the term UltraLight Steel Auto Body has no inherent contradictions. The proof is in more than a dozen prototype car bodies that were unveiled for customers, governments, investors, and other stakeholders. Intending to show that steel's potential for lightweight auto bodies is barely touched, let alone realized, the steelmakers launched the UltraLight Steel Auto Body (ULSAB) project in 1994. Instead of optimizing individual components, they decided on a holistic approach to challenge the engineering contractor, Porsche Engineering Services Inc. (PES), Troy, Mich. For design purposes, the body structure was treated as an integrated system rather than an assembly of individual components. That perspective enabled the team to evaluate how changes in one area affect other areas and where future optimization opportunities exist. The holistic approach is better suited to the creation of a more efficient structure, emphasizes Darryl Martin, AISI's director of automotive applications. A variety of structural alternatives were considered. Early in the project, PES engineers decided against a full-frame concept because it offered no significant mass-saving opportunities. They also doubted that a full frame would meet ULSAB's structural performance criteria and were concerned about high investment costs for assembly. Although a spaceframe was considered, it was ultimately eliminated because it was not considered as mass-efficient as other approaches. The PES team ultimately narrowed its investigation to two structures -- a unibody and a hydroform-intensive body structure -- finally settling on creating a unibody or monocoque vehicle with key hydroformed parts. To save mass, PES specified high-strength and ultra-high-strength steel alloys that aren't commonly used in auto bodies. The ULSAB uses high-strength steel and ultra-high-strength steel for more than 90% of the body structure. Nearly half of the ULSAB's mass consists of parts using tailored blanks, which enable the design engineers to save weight by removing mass that does not contribute to performance. Tailored blanks promote smooth load flow, reduce structural discontinuities, and allow for the combination of thicker and higher-strength materials within the same part. The design also features hydroformed tubing for the roof rail, which saves weight and provides efficient load paths. Tubular hydroforming with its cold-working effect produces high dimensional stability and increases effective yield strength in any component, explains Martin. Use of sheet hydroforming for the roof panel also saves weight while improving dent resistance. Further weight savings were made in the spare tire tub and dash panel via a multilayered structure of thermoplastic (polypropylene) sandwiched between two thin steel skins. Martin says this material can be up to 50% lighter than a comparable sheet of homogeneous steel without compromising performance. With advanced materials and high-tech forming methods, PES was able to produce an advanced structure weighing 203 kg (447 lb), which is up to 36% less than a range of vehicles from around the world, which were benchmarked in the study. In physical tests of the structure, torsion and bending showed improvements over benchmarks of 80% and 52% respectively. The advanced materials and processes enabled the design engineers to consolidate functions in fewer parts, reducing ULSAB's part count to 96 major parts and 158 total parts, as compared with more than 200 total parts for an existing typical body structure in the same class, says AISI. Reduced part count leads to reduced tooling and assembly costs. This function consolidation also leads to mass savings and improved structural performance, AISI adds. AISI also says that computer-based analyses show the ULSAB satisfies mandated crash requirements, even at speeds exceeding U.S. government and European requirements. Best of all, ULSAB is said to cost no more to build than typical auto body structures in its class and can even yield additional cost savings. AISI points out that the benefits are possible with a material (sheet steel) that costs about 34 cents per pound. Aluminum sheet is estimated at $1.50 with a net in-place cost of about $1.05. AISI also points out that weight savings with the ULSAB requires no change in the manufacturing infrastructure. "ULSAB was manufactured and assembled using current techniques and practices including maintaining tolerances and quality standards equivalent for high volume production," says Martin. "We used no manual forming because we wanted to demonstrate clearly that you can make ULSAB right now," Martin adds. ULSAB employs about a third fewer spot welds and significantly more laser welding than a conventional body structure. Assembly includes 18,286 mm of laser welding, 2,206 spot welds, and 1,500 mm of MIG welding, the majority of which is used to attach through-pillar door hinges. An irony of the ULSAB effort is that as its concepts become prevalent in future vehicles, the market for steel will shrink, all other factors remaining constant. The major weight savings potential predicted by ULSAB would logically lead to decreased steel shipments. AISI's strategy is to raise the bar for competing materials, to improve steel's future, even in a shrinking market.

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