Extrusion process pushes magnesium into structural components

Magnesium is 75 per cent lighter than steel, 33 per cent lighter than aluminium and is the fourth most common element on Earth behind iron, silicon and oxygen. Despite its light weight and natural abundance, the metal requires rare elements such as dysprosium, praseodymium and ytterbium to give it the requisite strength demanded in structural components.

Initial research, described recently in Materials Science and Engineering A, and Magnesium Technology, found the Pacific Northwest National Laboratory (PNNL)-developed process greatly improves the energy absorption of magnesium by creating novel microstructures which are not possible with traditional extrusion methods. It is also claimed to improve the material’s ductility.

"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."

According to PNNL, the researchers theorised that spinning the magnesium alloy during the extrusion process would create enough heat to soften the material so it could be pressed through a die to create tubes, rods and channels. Heat generated from mechanical friction deforming the metal provides the heat required for the process, eliminating the need for resistance heaters.

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.

ShAPE facilitated the extrusion of very 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 are claimed to flow through the die in a very soft state, thanks to the simultaneous linear and rotational forces of the ShAPE machine, so 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, thereby lowering capital expenditures and operations costs for industry adopting this patent pending process. Energy is saved since the heat generated at the billet/die interface is the only process heat required to soften the magnesium.

"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."

Magna-Cosma is teaming with PNNL on this US Department of Energy's 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.