Navigant Research Blog

High-Strength Steel or Aluminum for Vehicle Body Parts: That Is the Question

David Alexander — August 10, 2015

In a presentation to the 2015 CAR Management Briefing Seminars, Eric Petersen, the vice president of research and innovation at AK Steel, outlined his plans to produce the next generation of high-strength steel. He was confident that new innovations from steel suppliers will prevent the loss of more market share to aluminum. As Petersen demonstrates, Ford’s decision to convert its F-150 to all-aluminum has prompted steel suppliers to get creative.

However, there are a lot of things to consider when choosing a material for manufacture of vehicle components. Current vehicle bodies and closures are made primarily of sheet metal, although carbon and glass fiber-reinforced plastic are also used. Original equipment manufacturer (OEM) designers and engineers have to consider both the requirements of the finished vehicle and the ability to manufacture it efficiently.

Modern production lines depend on assembly techniques that can be automated, and fixing parts together must use a process that can be done by robots both for speed and repetition. Depending on the material, this may involve spot welding, seam welding, rivets, bolts, glue, etc. If production facilities are already equipped with a certain capability, changing to a material that needs a different joining process might require a major investment, which may only be practical when the existing equipment is reaching the end of its useful life.

Part manufacture is another consideration. Many body parts have complex shapes. If they are stamped and need a deep draw, then there may be a limit on how thin the material can be. Steel that is developed to have high strength has a higher yield stress than ordinary mild steel, which is a benefit in the finished article to absorb loads, but makes it harder to form during manufacture. Other lightweight materials such as magnesium have poor formability for body panels but can be cast for uses such as the instrument panel beam in the 2015 Ford Mustang. Some materials are treated after shaping to make them harder, but that also adds cost. Different materials also require different post-manufacture treatments to prevent corrosion.

Once a part is manufactured and assembled, it has to meet various performance specifications. A vehicle body structure has to be stiff in torsion and bending, cope with fatigue loading at load points, handle rollover and side impact loads, and absorb crash energy while keeping occupants safe. In addition to meeting these structural goals, overall vehicle fuel efficiency targets mean that keeping the weight low is now critical. And, as always, there is the ever-present need to keep costs as low as possible.

No Silver Bullet

Just as with powertrains, there is no silver bullet material that is ideal for every application. Material suppliers should recognize the big-picture approach of the automotive manufacturers, which involves reducing the number of platforms to increase volume of many parts and to streamline manufacturing processes. OEMs are increasingly using multi-disciplinary optimization for part design, which considers a wide range of factors including manufacturability, assembly, structural performance, weight, functionality, and overall cost.

Future vehicles are more likely to be made from a variety of materials than to stick with one or shift entirely to another. Material suppliers should consider how to make parts with their products that not only perform better than the competition at lower cost, but also integrate easily into existing platforms.

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