Magnesium, the lightest of all structural metals, has a lot going for it in the quest to make ever lighter cars and trucks that go farther on a tank of fuel or battery charge. It is 75% lighter than steel, 33% lighter than aluminum, and the fourth most common element on Earth behind iron, silicon, and oxygen.
But despite its light weight and natural abundance, automakers have been stymied in their attempts to incorporate magnesium alloys into structural car parts. Providing the necessary strength has required the addition of costly rare elements such as dysprosium, praseodymium, and ytterbium.
However, a new process developed at the Department of Energy’s Pacific Northwest National Laboratory could make it more feasible for auto companies to incorporate magnesium alloys into structural components. The method could reduce cost by eliminating the need for rare-earth elements, while simultaneously improving the material’s structural properties. It’s a new twist on extrusion, in which the metal is forced through a tool to create a certain shape.
The trademarked process, Shear Assisted Processing and Extrusion Magnesium (ShAPE), came about when researchers theorized that spinning a magnesium alloy during extrusion would create just enough heat to soften the material so it could be easily pressed through a die to create tubes, rods, and channels. Heat from the mechanical friction deforming the metal also provides all of the heat necessary for the process, eliminating the need for power-hungry resistance heaters common in traditional extrusion presses.
The billets or chunks of bulk magnesium alloys that flow through the die are soft, thanks to the simultaneous linear and rotational forces of the ShAPE machine. This means only one tenth of the force is needed to push the material through a die compared to conventional extrusion.
This significant reduction in force should enable substantially smaller production machinery, thus lowering capital expenditures and operational costs for industry adopting this patent-pending process. The force is so low that the amount of electricity used to make a one-foot length of two-inch diameter tubing is about the same as it takes to run a residential kitchen oven for just 60 seconds.
The PNNL team designed and commissioned an industrial version of their idea, a custom-built Shear Assisted Processing and Extrusion machine (while coining the ShAPE acronym). With it, they’ve successfully extruded thin-walled round tubing, up to two inches in diameter, from magnesium-aluminum-zinc alloys AZ91 and ZK60A, improving their mechanical properties in the process. For example, room temperature ductility above 25% has been independently measured, which is a significant improvement compared to typical extrusions.
“In the ShAPE process, we get highly refined microstructures within the metal and, in some cases, are even able to form nanostructured features,” says team member and mechanical engineer Scott Whalen. “The higher the rotations per minute, the smaller the grains become, which makes the tubing stronger and more ductile or pliable. Additionally, we can control the orientation of the crystalline structures in the metal to improve the energy absorption of magnesium to be equal to that of aluminum.”
All of these enhancements make magnesium easier to work with and more likely to be used in structural car parts. Currently, magnesium components account for only about 1% (or 33 lb.) of a typical car’s weight, according to a DOE report.
“Today, many vehicle manufacturers do not use magnesium in structural locations because of price and properties,” says Whalen. “Right now, manufacturers opt for low-cost aluminum in components such as bumper beams and crush tips. Using our process, we have improved magnesium’s mechanical properties to the point it can be considered instead of aluminum for these applications, and without the added cost of rare-earth elements.”
Magna-Cosma, a global auto industry parts supplier, is working with PNNL on this DOE-funded research project to advance low cost magnesium parts and, as larger tubes are developed, will be testing them at one of their production facilities near Detroit.
PNNL’s ShAPE technology is available for licensing and could help to make a dent in the auto industry’s magnesium target, all the while slimming down cars which currently weigh an average of 3,360 lb.