Another lightweight material that is slowly being adopted by automakers is magnesium. It is attractive to engineers because it’s 36 percent lighter than aluminum and 78 percent lighter than steel, yet it has a better strength-to-weight ratio than either metal. Magnesium is also highly moldable, and parts can be die-cast with walls as thin as 0.8 millimeter.
“Magnesium has the potential to be a real enabler,” says Dr. Mark Verbrugge, director of GM’s Materials and Processes Laboratory. “It has the ability to enhance design freedom and performance, in addition to reducing mass.”
Magnesium is of particular interest for automotive applications, because it can be joined by a wide variety of assembly processes, including adhesives, fasteners and welding. Components made from the lightweight material include engine cradles, radiator supports, instrument panel cross-bar beams, transmission housings, seat structures, support brackets, brake and clutch pedals, bumper support beams and airbag housings.
However, the typical American-made vehicle contains less than 12 pounds of magnesium. Once the correct market infrastructure is in place, Verbrugge believes magnesium content could increase to as much as 350 pounds per vehicle by 2020.
“The anticipated use of magnesium in engines, transfer units and transmissions where aluminum is currently being used will achieve a further 33 percent increase in weight savings,” adds Gomes. “Several chassis applications in the future could be manufactured from magnesium.”
According to Gomes, using threaded fasteners in magnesium presents several challenges to manufacturing engineers. “The lack of ductility in magnesium limits the ability of thread-forming screws to function,” he points out. “The cold flow and thermal expansion rate of magnesium creates challenges in maintaining joint clamp load in applications subject to extreme heat cycles. Magnesium applications using steel screws have to be designed to prevent accelerated stress corrosion.”
“In the early stages of studies that are being conducted, the issues and advantages of magnesium appear to be similar to aluminum,” says Henkel’s Liddiard. “With both aluminum and magnesium, the oxide layers will still play a role in determining the efficacy of a durable bond. Corrosion of magnesium in an automotive application comes primarily from galvanic action in connection with steel.
“Adhesion to magnesium depends on the alloy composition,” notes Liddiard. “Therefore, application-specific testing is recommended. The advantage of adhesives is that they will not only bond components but will also interrupt the corrosive action.”
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