Steel Manufacturing Challenges and Reducing Vehicle Mass

Aug. 18, 2010
New design technologies, advanced high-strength steel, and improved manufacturing processes

By Jody Shaw and Dr. Akbar Farahani

The automobile industry is facing numerous challenges today. Clearly at the top of the list is the need to reduce vehicle weight while maintaining (or even improving) crash safety and overall performance, without impacting affordability.

The vehicle weight reduction objective is driven by a few factors, including upcoming federal corporate average fuel economy (CAFE) standards that mandate increased car fuel economy and reduced emissions. Other factors include new crash safety requirements, increasing customer demands and expectations for quality and performance, and the availability of new energy sources, such as electric/hybrid vehicles, plug-in technologies, and fuel cells.

Recent research has shown that using an innovative new design process incorporating unique optimization tools, more advanced high-strength steels (AHSS), and using advanced manufacturing technology, can reduce mass by 10 to 15 percent beyond the mass reduction that has previously been achieved with AHSS – at little or no additional total vehicle manufacturing cost.

One important segment of the research on reducing vehicle mass was led by the Auto/Steel Partnership (A/SP), as part of the Future Generation Passenger Compartment (FGPC) project. The project demonstrates that this approach is feasible now and can be used by automobile manufacturers to achieve weight reduction.

Even more dramatic mass reductions, in the 25 to 35 percent range, can be achieved with advanced manufacturing complexity. This type of development will require technical methods beyond what is utilized within the conventional mainstream of automobile makers’ design and manufacturing. Significant investments in the next wave of manufacturing innovations have to be made to meet these challenges and gain the increased mass reduction.

The FGPC project used an innovative design process called the Accelerated Concept to Product (ACP) Process, pioneered by Engineering Technology Associates Inc. Engineering Technology Associates, Inc. (ETA), the primary engineering contractor on the FGPC project. ACP uses design optimization technologies, which provides holistic design solutions resulting in the mass reduction.

The success of the FGPC work led to the FutureSteelVehicle (FSV) initiative by WorldAutoSteel, the automotive group of World Steel Association. FSV seeks to extend the positive results and develop designs that can reduce vehicle weight well beyond what was achieved in the FGPC project. Early results indicate that larger mass reductions can be achieved with a clean sheet design and by incorporating innovative manufacturing technologies. Most recently, ACP was used to identify and evaluate the most efficient manufacturing processes for achieving mass reductions in each major vehicle component. The FSV team is currently preparing a final design that incorporates optimized designs for all major components. Final results are expected in early 2011.

Three ingredients in reducing vehicle mass
Advanced steels —
Introduction of new AHSS grades is a critical ingredient in the recipe for reducing vehicle mass. Alloy additions and precise control of steel’s crystalline structure have resulted in dramatic increases in strength combined with sufficient manufacturing capability for affordable manufacturing, which improve a vehicle’s overall performance.

Whereas the average tensile-strength level of mild steel measures about 270 megapascals (MPa), high strength low alloy (HSLA) steel, introduced in the 1980s and 1990s, measures about 450 MPa. The new AHSS grades, introduced a decade ago, measure between 800 and 1500 MPa.

Advanced manufacturing technologies — Along with the improvements to steel, improved manufacturing technology is another key to producing these levels of mass reduction.

Conventional manufacturing typically begins with a steel blank, composed of one material and one thickness, which is placed in a die and stamped into a particular shape.

Today’s designs require a great deal of manufacturing flexibility, including the use of laser-welded blanks, a mix of shapes, and different passes through machinery that varies the strength level and thickness of a component. Steel is an enabler of this design flexibility. These technologies result in part consolidation thereby reducing assembly complexity and cost.

Advanced product design development process — The key ingredient in leveraging the mass reduction capability of the advanced steels and manufacturing technologies is the availability of innovative new design optimization tools, such as ETA’s ACP Process. In the conventional development process, engineers design, analyze, test and then redesign a product. Design processes that incorporate optimization tools, such as the ACP Process, can evaluate hundreds of design concepts under multiple load conditions simultaneously. This ensures that the resulting design is then optimized for mass while meeting all design and manufacturing targets.

The ACP engineering method is an innovative, holistic product development process with multi-disciplinary loading based on topology and geometry, grade, and gage (3G) optimization. Computer-aided engineering (CAE), design, and manufacturing are synchronized. Once an optimal concept is identified, the ACP process generates further design, analysis, and optimization using loading, manufacturing, material, and cost constraints. It then generates computer-aided design (CAD) data of an optimized concept design suitable for detailed design.

The ACP process can incorporate the use of multiple CAE tools, including modeling tools, application-specific tools, solver technology and optimization solutions. ETA’s own advanced pre- and post- processor core finite element (FE) modeling toolset, PreSys, provides the pre/post, safety, structure, fatigue, drop test, and material handling analyses, DYNAFORM® Finite Element Analysis software, for formability analysis, die face engineering, die structure analysis, and manufacturing process simulations. The process also uses the following Finite Element Analysis software: HEEDS® from Red Cedar Technology, LS-DYNA® from Livermore Software Technology Corp., Nastran® and SFE CONCEPT® from SFE GmbH.

FGPC integrates the three ingredients
The Auto/Steel Parnership’s (A/SP) FGPC project set out in 2004 with a goal of reducing the vehicle passenger compartment structure mass by 25 percent.

In FGPC Phase 1, completed in 2007 and funded by A/SP members and the U.S. Department of Energy, the goal was to develop a lightweight passenger compartment using AHSS. The team developed a conceptual optimization methodology, which realized a mass reduction of 30 percent when compared with a typical passenger compartment of the same vehicle class.

The FGPC Phase 1 donor vehicle was the UltraLight Steel Auto Body - Advanced Vehicle Concepts (ULSAB-AVC). The vehicle packaging was adapted for both conventional diesel and hydrogen fuel cell powertrains. The 30-percent reduction was achieved while maintaining compliance with current and future (2015) crash/safety performance, stiffness and durability regulations. A series of sensitivity studies proved the robustness of the design through its ability to accommodate variations in the vehicle’s curb weight and side impact barrier height.

In addition to exceeding the original mass reduction goal by five percent while maintaining the structural parameters for stiffness and durability, the analysis predicts the optimized design improved the vehicle’s crashworthiness.

FGPC Phase 2 began in 2007 as the research group sought to prove how conceptual ideas developed in Phase 1 could be applied to a production vehicle. Using a 2008 OEM donor production luxury vehicle as the baseline product, the group set out to develop a concept design with the goal of reducing the passenger compartment’s structural mass by 20 percent.

The results showed that the mass of the passenger compartment structure could be reduced between 15 and 20 percent with minimal increased costs. This provides a cost-effective solution relative to the non-steel material solutions, which would have significantly increased the costs to achieve this type of mass reduction.

The program also proved that advanced joining technology (laser-welding or adhesive bonding) could increase the overall mass reduction to 20 percent. Sensitivity studies established the viability of the load paths and robustness of the optimized design.

Figures 1 through 3 show the results of the FGPC Phase 2 study. Figure 1 illustrates the baseline vehicle’s material content compared to that of the mass optimized concept solution. Using a preponderance of dual phase and martensitic boron steel rather than the conventional steel used in the baseline donor model is one key factor in the mass reduction.

Figure 2 shows the passenger compartment’s weight for the baseline model and the two alternative FGPC solutions. The solution that used advanced joining technology (laser-weld or adhesive bonding) was 20 percent lighter than the baseline.

Cost information shown on Figure 3 demonstrates that materials costs are lower for the FGPC solution, but forming costs are substantially higher. Total manufacturing costs for the lowest weight solution are predicted to be 5 percent higher than the baseline.

FGPC Phase 2 achieved 15 to 20 percent mass reductions, but could not reach the desired 30 percent level while adhering to the manufacturing, packaging and architectural constraints adopted by the project.

Lightweight front-end structure
Building on the positive results with the FGPC project, WorldAutoSteel, the automotive group of the World Steel Association, began a further research initiative, called the FutureSteelVehicle (FSV) Pilot Program. This program is seeking to extend mass reductions to the 30 to 40 percent range with a more aggressive use of the mass optimization technologies of advanced steel, advanced manufacturing and CAE tools.

The FSV pilot project investigated the possibility of larger mass reductions if manufacturing and packaging constraints expanded beyond today’s capability.

Using the A/SP lightweight front-end structure (LWFES) as the donor vehicle, the team took a “clean sheet” approach, meaning that the design was not based on existing geometry. The results of the fully optimized rail design achieved a 39 percent mass reduction over the original donor vehicle’s design.

The design process took advantage of ETA’s ACP Process, including the ability to generate a bandwidth of solutions that challenge those derived by engineering judgment. Additionally, the optimum load path, geometry, material, and gage are unconstrained by historical assumptions, generating non-intuitive optimized shapes and component configurations.

Figure 5 compares the baseline vehicle’s longitudinal front-end structure with several other alternatives. The alternative solutions meet the baseline performance in stiffness and crash.

The baseline vehicle’s front-end structure contains 13 components, with conventional steel stampings of mild and high-strength steel with strengths of 300 to 450 MPa. The A/SP AHSS Laser Welded Blank (LWB) solution uses AHSS steel of about 800 MPa in strength. It consolidates the 13 stampings into five stamped parts, reducing flange weight and increasing overall structural integrity. The A/SP AHSS Hydroform alternative is a single tailored welded tube that consolidates all 13 components into a single unit that achieves structural integrity with no flanges. Hydroforming is a specialized type of die forming that uses a high-pressure hydraulic fluid to press room temperature working material into a die.

These two design alternatives used engineering judgment combined with design of experiment (DOE) optimization. The combination of AHSS and DOE design optimization achieved 23 percent and 32 percent mass reduction relative to the baseline, respectively, for these two designs. A cost assessment confirmed that manufacturing costs of these solutions are reduced with both the AHSS LWB (5 percent below the baseline) and AHSS Hydroform (6 percent lower than the baseline) solutions.

The DOE optimization approach is constrained by the number of parameters that can be considered as the computational requirements increase exponentially with the number of design parameters. As a result, practical computational constraints limit the number of design parameters that can be considered to 16. While the DOE approach achieved significant mass reduction, designers believed that an optimization approach allowing hundreds of design parameters could greatly improve the mass reduction.

The WorldAutoSteel FSV pilot project optimized the same component using the anticipated next generation of AHSS steels currently under development – called Gen3 AHSS – plus an optimization approach within the ACP process that enables more than 100 steel grade, thickness and geometry parameters to be considered in the optimization process. As a result, the design optimization developed a very non-intuitive design, one that would not be anticipated based on engineering judgment. This approach increased mass reduction to 39 percent compared to the production baseline.

Costs for the Gen3 version are not yet known. What we do know is that the complex geometry that contributes to such impressive weight reduction would challenge manufacturing processes in place today. Pushing the steel gages to as low as 0.5 millimeter is theoretically possible because the steel can provide more than enough strength. The challenge arises from the considerable formability and manufacturability issues that arise when such steel gages are used, but the opportunities for affordable mass reduction justifies further study into these non-intuitive solutions.

So, what do we need out of the manufacturing environment to achieve these savings in real life projects? The FGPC project has shown that significant weight reductions can be accomplished now with today’s technology. The research holds great promise of achieving even more dramatic weight reductions when we are able to address the manufacturing challenges. Only a concerted investment in new metal forming technologies will allow us to unlock the promise of weight reductions found in the new advanced steels, manufacturing technologies, and design technologies.

Future steps
The WorldAutoSteel FSV project will apply the enhanced optimization approach conducted for the pilot project to a full vehicle body structure while addressing metal forming and assembly capability. FSV will address electrified vehicle powertrains, consider a technology horizon of 2020, and set a mass reduction target 35 percent. This aggressive mass target will be achieved by leveraging the advanced steel grades, advanced steel manufacturing technologies, and advanced design optimization using ETA’s ACP process.

The latest phase of the FSV research, released in May 2010, focused on selecting the optimal manufacturing processes for each of a vehicle’s major components. For each major body structure component, multiple manufacturing processes were evaluated and ranked according to such traditional criteria as sub-system mass and cost. Beyond these criteria, FSV will consider the technologies that minimize the environmental impact of a vehicle. While today’s criteria for a vehicle’s environmental performance stresses fuel economy and tailpipe emissions, a more comprehensive approach is required to address the vehicle’s total carbon footprint. This is accomplished with a greenhouse gas (GHG) life cycle assessment (LCA) approach that measures the carbon dioxide emissions associated with all phases of a vehicle’s life. See Figure 7 for an overview of techniques evaluated and compared for the rocker. These solutions consider the full portfolio of advanced grades of steel and advanced manufacturing technologies of roll forming, hot stamping, conventional stamping, hydroforming, and extrusion processes. Each of the 14 design alternatives provide equivalent vehicle performance in crash safety and stiffness while the relative weight, manufacturing cost and impact on the vehicle’s environmental performance is calculated as shown. The approach of developing and evaluating a large bandwidth of design alternatives is repeated for all of the major body structure components.

The FSV team is selecting the technologies for each body structure component that will best meet the objectives of the FSV program for mass, cost and environmental performance. These will be further optimized within this holistic design approach. The final results of the FSV program will be released in the first half of 2011.

Stay tuned as the FSV introduces a fourth ingredient to the vehicle weight loss recipe: the ability to design geometric shapes that mimic nature’s ability to uncover the most efficient design, one that combines strength and lightness. Like the wing of a dragonfly or the dense but light structure of a bird wing, automobile designers are using powerful tools to come up with shapes that can reduce weight and improve safety and performance.

Jody Shaw is the Chairman of the FutureSteelVehicle initiative and manager of technical marketing and product development, United States Steel Corp. Dr. Akbar Farahani, vice president of Engineering and Consulting for Engineering Technology Associates Inc.

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