Magnesium is 75 percent lighter than steel, 33 percent 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 halted in their attempts to incorporate magnesium alloys into structural car parts. To provide the necessary strength has required the addition of costly rare elements such as dysprosium, praseodymium, and ytterbium.
A new process was developed to 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. The method is a form of extrusion in which the metal is forced through a tool to create a certain shape.
The process was found to greatly improve the energy absorption of magnesium by creating novel microstructures that 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. Currently, magnesium components account for only about 1 percent, or 33 pounds, of a typical car’s weight.
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.
A Shear Assisted Processing and Extrusion machine (ShAPE™) also was developed to successfully extrude very thin-walled round tubing, up to 2” in diameter, from magnesium-aluminum-zinc alloys AZ91 and ZK60A, improving their mechanical properties in the process. Room-temperature ductility above 25 percent has been independently measured, which is a large improvement compared to typical extrusions.
The ShAPE process creates highly refined microstructures within the metal and, in some cases, can form nanostructured features. The higher the rotations per minute, the smaller the grains become, making the tubing stronger and more ductile or pliable. The orientation of the crystalline structures in the metal can be controlled to improve the energy absorption of magnesium so that it is equal to that of aluminum.
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 process. The force is so low that the amount of electricity used to make a 1’ length of 2”-diameter tubing is about the same as it takes to run a residential kitchen oven for just 60 seconds.