Magnesium is one of the lightest of all structural metals—75% lighter than steel, 33% lighter than aluminum—and is 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. To provide the necessary strength has up to now required the addition of rare elements such as dysprosium, praseodymium and ytterbium.
Now, a new process developed at the Department of Energy’s Pacific Northwest National Laboratory should make it more feasible for the auto industry to incorporate magnesium alloys into structural components. The new extrusion process has the potential to reduce cost by eliminating the need for rare-earth elements, while simultaneously improving the material’s structural properties.
Initial research, described recently in the journals Materials Science and Engineering A and Magnesium Technology, found the PNNL-developed process greatly improves the energy absorption of magnesium by creating novel microstructures which 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. Currently, magnesium components account for only about 1%, or 33 pounds, of a typical car’s weight, according to a DOE report.
Researchers theorized that spinning the magnesium alloy during the extrusion process 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 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.
The PNNL team designed and commissioned an industrial version of their idea: the Shear Assisted Processing and Extrusion machine (ShAPE).
With it, they’ve successfully extruded very 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 large 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. 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.—Scott Whalen, principal investigator
The billets or chunks of bulk magnesium alloys flow through the die in a very soft state, due 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 would enable substantially smaller production machinery, thus lowering capital expenditures and operations 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.
Energy is saved since the heat generated at the billet/die interface is the only process heat required to soften the magnesium.
Magna-Cosma, a global auto industry parts supplier, is teaming 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.
N. Overman, S. Whalen, M. Olszta, K. Kruska, J. Darsell, V. Joshi, X. Jiang, K. Mattlin, E. Stephens, T. Clark, S. Mathaudhu (2017) "Homogenization and Texture Development in Rapidly Solidified AZ91E Consolidated by Shear Assisted Processing and Extrusion (ShAPE),” Materials Science and Engineering A, 701, 56-68 doi: 10.1016/j.msea.2017.06.062
S. Whalen, V. Joshi, N. Overman, D. Caldwell, C. Lavender, T. Skszek (2017) "Scaled-Up Fabrication of Thin-Walled Magnesium ZK60 Tubing using Shear Assisted Processing and Extrusion (ShAPE)," Magnesium Technology, 315-321 doi: 10.1007/978-3-319-52392-7_45