An extrusion process developed at the Department of Energy's Pacific Northwest National Laboratory (PNNL), has the potential to reduce cost by eliminating the need for rare-earth elements, while simultaneously improving the material's structural properties.
Initial research found this process improves the energy absorption of magnesium by creating novel microstructures which are not possible with traditional extrusion methods. It also improves ductility –how far the metal can be stretched before it breaks. These enhancements make magnesium easier to work with and more likely to be used in structural car parts.
“Today, many vehicle manufacturers do not use magnesium in structural locations because of the two Ps; price and properties,” said principal investigator and mechanical engineer Scott Whalen. “Right now, manufacturers opt for low-cost aluminium 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 aluminium for these applications – without the added cost of rare-earth elements.”
The researchers theorised 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 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 and received a one-of-a-kind, custom built Shear Assisted Processing and Extrusion (ShAPE) machine.
With it, they have extruded thin-walled round tubing, up to two inches in diameter, from magnesium-aluminium-zinc alloys AZ91 and ZK60A, improving their mechanical properties in the process.
“In the ShAPE process, we get highly refined microstructures within the metal and, in some cases, are even able to form nanostructured features,” said 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 aluminium.”
The billets of bulk magnesium alloys flow through the die in a soft state, 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 reduction in force would enable smaller production machinery, lowering capital expenditures and operations costs for industry adopting this process.