One of the things acknowledged up front by Prof. Ali Fatemi and research assistant Mehrdad Zoroufi of the Dept. of Mechanical, Industrial, and Manufacturing Engineering, The University of Toledo in a study conducted on behalf of the Forging Industry Research Foundation (Cleveland; www.forgings.org) and the American Iron and Steel Institute (AISI; www.autosteel.org) is that when it comes to developing automotive components, there are not only competing manufacturing processes to be considered, but that those very processes have an effect on the mechanical properties of the component. And when the component in question is safety-critical—a suspension component, for example—then it is all the more important that those mechanical properties be thoroughly understood lest there be, well, a problem.
So the study that was conducted looked at three steering knuckles. Each was made of a different material. Each was produced by a different process. There were a 2.4-kg forged steel SAE Grade 11V37 knuckle for the rear suspension of a four-cylinder car; a 2.4-kg cast aluminum ASTM A356-T6 knuckle for the front suspension of a six-cylinder minivan; and a 4.7-kg cast iron ASTM A536 Grade 65-45-12 knuckle for the front suspension of a four-cylinder sedan. Testing was conducted using experimental, numerical and analytic tools so as to determine the effects of the production process on the components and the possible optimization.
As you might expect given the sources behind the study, the forged steel component performed well. But what you might not expect is how well it did. According to the study, the steel knuckle was stronger than the other two, with the cast aluminum being 37% of the forged steel's ultimate tensile strength and the cast iron 57%. With regard to yield strength, the cast aluminum is 42% of the forged steel and the cast iron 54%. Measuring ductility, based on elongation, the cast aluminum was 24% of the forged steel and the cast iron 48%. When the cyclic yield strength was measured, the forged steel proved to have a higher resistance to plastic deformation than the other two materials (the cast aluminum and cast iron were 46% and 55% of the forged steel, respectively).
During testing of fatigue behavior it was determined that the forged steel steering knuckle had a longer life of two orders of magnitude compared with the cast aluminum and one order of magnitude greater than the cast iron.
When they looked at the benefits that could be achieved by using processes to improve the forged steel knuckle (e.g., surface hardening; surface rolling),the researchers concluded the processes alone wouldn't provide that great of an improvement to the part as designed and that a redesign would be necessary to make cost and weight improvements.
All of which seems to show that forged steel has some considerable benefits (i.e., benefits that manufacturers need to consider).
DESIGN TO THE EXTREME. In a sponsorship of a completely different nature, the 17th annual AISI summer internship program held this past summer resulted in innovative designs by three students at the College for Creative Studies in Detroit. The designers, Chris Piscitelli, Sylvian Bryan and Sang Hoon Shin, created cars under the theme "Extreme Automotive," while keeping in mind the charter to deploy steel. Piscitelli's car employs, he says, "A dual-phased hydroformed steel space frame. The exterior panels will utilize dual-phase 490 grade, because of the steel's ability to form complex shapes while providing the performance and mass reduction needed for extreme speed." Bryan's futuristic (2050) hydrogen-powered vehicle, based on the Porsche Carrera GT platform (oddly enough, a current model that is composite-intensive), employs, in his words, "a cage of advanced high-strength steel. The steel creates an impenetrable barrier protecting the driver, passengers and hydrogen tanks in the event of a crash." And Sang Hoon Shin said of his vehicle, inspired by armor of the Middle Ages ("I decided to use the esthetics of armor, layering advanced high-strength steel blanks, to create a more aggressive styling design with an edgy appeal"): "My vehicle uses the new advance high-strength steel with hydroformed frame structure. The hydroformed frame will reduce weight and cost of manufacturing by consolidation of components and accomplishing the same functionality with less material. Advanced high-strength steel provides vehicle safety without jeopardizing agility."
Seems like they've learned their lessons well.