Magnesium, a New Alloy for Green Cars

To improve fuel efficiency, manufacturers are trying to reduce the weight and the drag of their vehicles. To do this, they focus on structure and parts by opting for a design that contributes to reducing the drag coefficient—much like the Mercedes-Benz Bionic concept—using lighter materials.

Magnesium is 75% lighter than steel, 33% lighter than aluminum and the fourth most common element on earth after iron, silicone and oxygen. Vehicle manufacturers have failed to take advantage of its lightness and abundance in the manufacture of bodywork and parts because magnesium must be reinforced by other expensive rare earths such as dysprosium, praseodymium and ytterbium. Indeed, magnesium ductility is very low, which means that it can snap when stretched. In addition, and unlike other materials, its mechanical strength increases with heat.

What Is Ductility?

Researchers at the Department of Energy (DOE) Pacific Northwest National Laboratory (PNNL) successfully incorporated magnesium into the structural components of vehicles using a new, low-cost manufacturing process that will increase the integration of this material, which currently accounts for 1%, or about 15 kilograms of the weight of a typical car, according to one DOE report. Scott Whalen, mechanical engineer and principal investigator of the project, reports that this method has improved magnesium’s mechanical properties without having to add rare earths to such an extent that it can now replace aluminum.

Magnesium Extrusion and Spinning

The malleability is a necessary property for a material to be ductile. Consequently, researchers considered optimizing this property during the machining of parts containing magnesium. They created an extrusion-spinning method in which the metal is forced through a nozzle, or spinneret giving the material a definite shape, as usual extrusion methods are not able to adequately form magnesium.

The team designed a custom semi-automated shear processing and extrusion machine. Dubbed ShapE ™, this unique machine was tested to extrude very thin-walled cylinder tubes—up to 5 cm in diameter—from AZ91 and ZK60A aluminum-zinc magnesium alloys.

The process was efficient and improved the mechanical properties of these alloys. In fact, the researchers managed to increase ductility at room temperature by more than 25%, which is a great improvement over traditional extrusions. They also produced highly refined microstructures in some alloys: this process made it possible to create nanostructured functions.

They opted to spin the magnesium alloy during the extrusion process because it creates enough heat to make the material more malleable. With such ductility, magnesium can be pressed through nozzles to form elongated products such as tubes, rods and channels. Heat generated by mechanical friction distorts the metal and provides the required temperature for the process, thereby avoiding the use of heating elements used in traditional extrusion presses. Higher rotations produce more refined grains, making the casting more flexible and de facto more ductile. In addition, spinning allows for better control of the alloy’s crystalline structure orientation and of the optimization of magnesium’s energy absorption.

When the bulk metal passes through the nozzle using simultaneous linear and rotary forces, it only needs one tenth of the force that conventional extrusion would require. With reduced forces, the process is simpler and the machine can be smaller, thereby reducing costs, especially in energy. Consequently, the amount of electricity used to make a 30 cm long tube, 5 cm in diameter, is about the same as it takes to run a domestic oven for 60 seconds.

Automotive parts supplier Magna-Cosma is working with PNNL on this DOE-funded research project to produce large magnesium-based parts for testing at their facility near Detroit, in the United States.