Magnesium Alloys in Lightweight Construction
Magnesium is the lightest metallic construction material. However, its alloys have a small market share. Current research should change that.
Magnesium alloys, as the lightest metallic construction material, with a density that is about 30 % lower than that of aluminium alloys, still have a low market share compared to aluminium alloys. This is due to many different reasons: On the one hand, primary production is currently concentrated in China, which is not considered as a safe business location by all OEMs. This is remarkable because magnesium, as the seventh most abundant element on earth, could be exploited in sufficient quantities and can even be extracted from seawater.
Corrosion is also mentioned as a potential disadvantage. In fact, magnesium has the lowest electrochemical potential compared to all other construction metals and therefore galvanic corrosion occurs in contact with other metals and when covered with an electrolyte. However, this is comparable with other metal pairs and can be controlled by coating or appropriate design.
Furthermore, there is an old and vague fear of the fire hazard that could arise from the use of magnesium alloys. This aspect too has long since been refuted. The FAA, the American Aviation Authority, has conducted comparative studies with a magnesium alloy from MEL and a 2024 aluminum alloy, which concluded that modern magnesium alloys are safe and may be used in seat structures.
Casting Alloys Available for Any Application Temperature
More than 90 % of the magnesium alloys used today are processed by means of die casting. For the use of the parts at room temperature, good and affordable alloys are available, especially AZ91 and AM50 or AM60 — the former with a slightly better strength and AM alloys with higher ductility.
All types of die-casting alloys contain > 4 % aluminium, which improves castability. Additional die-castable alloys are available for use at elevated temperatures, such as the AS21/AS41, AE42/AE44 or AJ62, which are used to manufacture a large number of gear cases or crankcases. With a further increase in operating temperatures, only aluminium-free magnesium alloys can be used, which can then only be processed in sand casting or gravity die casting. Typical examples of these alloys are WE43, WE54, QE22, Elektron21 or SC1, which are used to cast helicopter gearboxes and engine blocks.
Today, magnesium alloys also offer potential benefits in their development and use as wrought alloys, semi-finished products and their derivatives. However, they are currently only used as a material of niche products. The main reason for this is the limited formability of magnesium and many of its alloys, mainly due to the underlying crystal structure. Although semi-finished products can be presented as sheets, profiles or forgings, the production of the desired products is cost-intensive.
Research on Wrought Alloys with Improved Molding Characteristics
Current research work, which deals with the development of alloys with improved molding abilities as well as the optimization of the processes used, offers new solutions for wrought alloys.
A typical application is a metallic pencil sharpener made of extruded profiles. For this application, the material's easy precision machinability is utilized. Conventional wrought alloys use aluminium as an alloying element to improve formability and workability, but also to control stability. Examples include the alloys AZ31, AZ61 or AZ80. AZ31 is the typical magnesium sheet alloy. Alternatively, alloys with manganese as an additional element are being discussed, e. g. AM50 or AM60. More recent developments exploit microstructural effects, which are achieved by adding rare earth elements to the alloy, significantly increasing the formability of sheet metal. This type of alloys use zinc as the main alloy element, for example in ZE10, ZW41, ZE41 or grain-refined variants ZEK100, ZWK100. In some cases, the above-mentioned alloys of the WE series are also used or combinations with manganese, e. g. ME20 or ME21. Examples of alloying elements that also reduce the flammability of magnesium are yttrium and calcium.
An improved molding ability of the alloys ensures an optimized production process of wrought alloys. Using the example of casting-rolling processes as an example, it is possible to increase the efficiency of the process control more and reduce production costs.
Casting-Rolling Process Optimizes the Production of Wrought Alloys
A special kind of process optimization that is currently being discussed internationally is the combination of casting and rolling, the so-called casting-rolling process. In this process, a melt is poured over a specially designed die into a rolling gap where solidification and molding take place in direct combination with each other. In this way, a thin strip is formed as the starting material, which can be rolled to the desired thickness in just a few rolling steps.
On the one hand, this results in a significant reduction of rolling effort. On the other hand, this process also makes it possible to use other alloys which, if necessary, cannot be processed into sheet metal in a complex rolling process.
The ASTM designation B275 is the globally accepted designation of magnesium alloys today. According to this definition, magnesium alloys are named with two letters and two digits. Each letter represents an alloying element in the order of its addition in mass:
- A Aluminium H Thorium S Silicon
- B Bismuth J Strontium T Tin
- C Copper K Zirconium V Gadolinium
- D Cadmium L Lithium W Yttrium
- E Rare Earths M Manganese X Calcium
- F Iron Q Silver Z Zinc
These two letters are followed by two numbers describing the concentration of the alloying elements. These numbers can be followed by letters indicating the state of development of the alloy and separated by a hyphen, the heat treatment state is added.
- Dr. Hajo Dieringa, Deputy Chairman of the Supervisory Board Head of Magnesium Process Technology Department;
- Dr. Jan Bohlen, Deputy Chairman of the Supervisory Board Head of the magnesium wrought alloys department, both Magnesium Innovation Centre, Helmholtz-Zentrum Geesthacht.
This article was first published by konstruktions praxis.
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