Like a third-string quarterback awaiting his moment in the spotlight, magnesium has been sitting patiently on the lightweighting sideline. But, it may finally get its turn to shine in automotive applications, thanks to several recent breakthroughs.
Magnesium is the lightest structural metal. With a density two-thirds that of aluminum and a quarter that of steel, magnesium could potentially replace heavier alloys traditionally used in the auto industry.
Currently, magnesium components account for only 1 percent, or 33 pounds, of a typical car’s weight. Most applications have been confined to expensive sports cars, such as the Porsche 911.
Magnesium is not widely used to make auto parts because it is intrinsically a brittle metal not known for malleable properties. Usually, the material is formed at elevated temperatures between 200 and 400 C.
However, engineers at Monash University in Melbourne, Australia, recently developed a way to shape pure magnesium at room temperature.
“By refining the microstructure, we have changed the deformation mechanism from intra granular (brittle) to inter granular (formable),” says Nick Birbilis, a professor of materials science engineering. “The recipe is relatively simple: Pure magnesium is pushed through a die at 80 C, then cold-rolled.
“This process changes the microstructure of the magnesium so that it is no longer brittle,” adds Birbilis. “It can be cold-rolled at room temperature to less than 1 millimeter of thickness without cracking. In fact, it simply doesn’t crack. We can even roll it to the thickness of aluminum foil and bend it 180 degrees after rolling.
“Up until now, most magnesium components in the automotive industry were made using castings,” explains Birbilis. “However, our research effort unlocks components that require formability, such as panels. With this discovery, we can expand beyond the application of cast magnesium to super-formable magnesium that can be rolled, bent or compressed into any shape, even at room temperature.”
Engineers at the U.S. Department of Energy’s Pacific Northwest National Laboratory (PNNL) have also been tackling the magnesium challenge. They recently developed a process that could make it more feasible for automotive engineers to incorporate magnesium alloys into structural components.
The method has the potential to reduce cost by eliminating the need for rare-earth elements, while simultaneously improving the material’s structural properties. The patent-pending process greatly improves the energy absorption of magnesium by creating novel microstructures that are not possible with traditional extrusion methods. It also improves ductility.
“These enhancements make magnesium easier to work with and more likely to be used in structural car parts,” claims Scott Whalen, a mechanical engineer at PNNL who served as principal investigator on the project.
“Today, many vehicle manufacturers do not use magnesium in structural locations because of price and properties,” adds Whalen. “Using our process, we have enhanced the mechanical properties of magnesium to the point where it can now be considered instead of aluminum for these applications, without the added cost of rare-earth elements.”
The PNNL engineers discovered that spinning magnesium alloy during the extrusion process creates just enough heat to soften the material so it can be easily pressed through a die to create tubes, rods and channels. Heat generated from mechanical friction deforming the metal provides all of the heat necessary for the process, eliminating the need for power-hungry resistance heaters used in traditional extrusion presses.
Whalen and his colleagues designed and commissioned an industrial version of their idea: a shear-assisted processing and extrusion (ShAPE) machine. With it, they’ve successfully extruded thin-walled round tubing up to 2 inches in diameter from magnesium-aluminum-zinc alloys AZ91 and ZK60A.
