Increasing fuel efficiency is the goal of every major automaker these days, as the 2025 Corporate Average Fuel Economy (CAFE) standards creep ever closer. To that end, lightweighting has been the name of the game, with more than half of automotive manufacturers pursuing lightweight materials to meet CAFE standards.
It’s been a battle of steel vs. aluminum for some time, but a new method of extrusion could introduce a third player into the mix: magnesium.
The lightest of all structural metals, magnesium is 75 percent lighter than steel, 33 percent lighter than aluminum and the fourth most common element on earth behind iron, silicon and oxygen. Yet despite its light weight and natural abundance, auto makers have been stymied in their attempts to incorporate magnesium alloys into structural car parts, requiring rare-earth elements such as dysprosium, praseodymium and ytterbium to shore up its strength.
However, a new extrusion process developed at the Department of Energy's Pacific Northwest National Laboratory (PNNL), should make it more feasible for the auto industry 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.
Shear-Assisted Processing and Extrusion
The engineers and materials scientists at the DOE theorized that spinning a magnesium alloy during the extrusion process would create enough heat through mechanical friction to soften the material so that it could be pressed through a die to create tubes, rods and channels. This approach eliminates the need for the power-hungry resistance heaters used in traditional extrusion presses.
The team designed and commissioned an industrial version of their idea called the Shear-Assisted Processing and Extrusion (ShAPE) machine. They used the ShAPE machine to extrude thin-walled round tubing up to two inches in diameter made from magnesium-aluminum-zinc alloys AZ91 and ZK60A. Tests on the mechanical properties of the tubing found that it had a room temperature ductility above 25 percent, 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," said principal investigator 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 so it's equal to that of aluminum."
The billets of magnesium alloys flow through the die in a very soft state, due to the simultaneous linear and rotational forces applied by the ShAPE process. As a result, only one tenth of the force is needed to push the material through a die compared to conventional extrusion.
This significant reduction in force would enable substantially smaller production machinery, thus lowering capital expenditures and operations costs for industry adopting this patent pending process.
"We don't need giant heaters surrounding the billets of magnesium like industrial extrusion machines, said Whalen. "We are heating—with friction only—right at the place that matters."
Magnesium for Automotive Lightweighting
Currently, magnesium components account for only 1 percent of a typical car’s weight, according to a DOE report. The ShAPE machine could soon change that."Today, many vehicle manufacturers do not use magnesium in structural locations because of the two Ps; price and properties," said Whalen. "Right now, manufacturers opt for low-cost aluminum in components such as bumper beams and crush tips. 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."
Auto parts supplier Magna International, Inc. is teaming with the PNNL to advance its research on low-cost magnesium parts. The company will be testing larger tubes at a production facility near Detroit as they are developed. PNNL’s ShAPE technology is also available for licensing.
For more information, read the published research or visit the PNNL website.