A new extrusion process could make magnesium a more viable material for the automotive industry, as Ellis Davies reports.
Magnesium alloys account for only about 1% of a typical car’s weight due to high costs and challenges with their material properties, despite being the world’s lightest structural metal. With a goal to change this, a team at the Pacific Northwest National Laboratory (PNNL), USA, has developed a new process called Shear Assisted Processing and Extrusion (ShAPE) that could make the use of magnesium more feasible for the automotive industry, as part of structural components, by eliminating the need for costly rare earth elements such as dysprosium, praseodymium and ytterbium.
The automotive industry is under pressure to produce greener vehicles. Part of this process is making cars lighter so that they can travel further using less energy, but the cost must also be comparable to current materials prices. A cost effective way to extrude magnesium could introduce new possibilities to the industry (read more about how else magnesium is used for low weight applications in the March 2017 edition of MW).
ShAPE is a thermo-mechanical process for extruding tubes, rods or other non-circular shapes from metals in the form of billets, chips, flakes or powders. Dr Scott Whalen, Senior Engineer of the Energy Processes and Materials Group at PNNL explained the process to Materials World. ‘In ShAPE, a rotating die is rammed against the metal, which results in frictional heating. The metal softens as a result and scroll features on the die face force material directly into the extrusion orifice. Material flowing through the scrolls is then mixed within the die to form a consolidated extrusion. In addition to magnesium, ShAPE is being used at PNNL to process aluminium, steel, copper, magnets and even semi-conductors,’ he said.
For magnesium, ShAPE combines linear and rotational shear forces resulting in highly refined grain size and alignment of crystallographic texture, leading to improved bulk material properties with unique microstructures not possible with conventional extrusion. The creation of smaller grains gives the tubing strength, rather than the traditional addition of rare earths.
‘The process is very effective at breaking down second phase particles within the billet, flake or powder during extrusion,' Whalen said. 'For example, as-cast Mg-2Si ingots with notoriously large second phases have been extruded into tubes and rods with evenly distributed sub-micron second phases'. This means that ShAPE enables the extrusion of as-cast materials directly into extruded products with refined and advantageous microstructures, which wasn’t possible with conventional extrusion.
Magnesium alloys processed in this way have significantly improved mechanical properties, according to Whalen. ZK60 and Mg-2Si tubes extruded using the process exhibit energy absorption properties equivalent to 6061-T6 aluminium, as well as showing room temperature ductility in excess of 20% in directions parallel, perpendicular and 45 degrees to the extrusion direction. ‘These improvements in energy absorption and ductility, without using rare earth elements, can help address the performance-cost dilemma currently facing magnesium in the automotive industry,’ explained Whalen.
As well as removing the need for rare earth elements, the process saves energy. ShAPE uses friction to provide the heat required to soften the material, which is an efficient way of locally heating only the material that is being processed. In a conventional extrusion process, a large resistance or radiant heater is required to heat entire container assemblies to bring the material to the correct temperature for extrusion. Although the team has not yet performed a production cost analysis for the ShAPE process, Whalen believes that the lower extrusion force and energy consumption will contribute to it being cost-effective. Whalen summarised, ‘The ShAPE process could be an important factor in raising the amount of magnesium used on a typical automobile. The properties achieved thus far using ShAPE could also impact other applications where strength-to-weight ratio is important.'