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Change in magnesium alloy microstructure changes corrosion resistance and improves potential for transportation applications

Changing the microstructure in magnesium alloys improves their corrosion resistance, and so improves the possibilities for the transport sector to use these materials to decrease the weight of vehicles, according to work done by Mohsen Esmaily, researcher in Atmospheric Corrosion at Chalmers University in Sweden.

Magnesium is the lightest construction metal, but also the most reactive. This means that it is very sensitive to corrosion—i.e. it very easily reacts with its surroundings and rusts. This makes it difficult to use magnesium in corrosive environments, meaning that the potential to use magnesium in cars to make them lighter is limited.

For more than a hundred years, magnesium producers have worked hard to improve the corrosion characteristics by developing new, more corrosion-resistant alloys, and also by developing various coatings. Mohsen Esmaily’s research shows a completely new way to improve the corrosion resistance of the alloys by manipulating the microstructure of the material, thereby increasing possibilities to lower the weight of vehicles.

In cars where every kilo of reduced weight is important, a transition to magnesium, which is 30 percent lighter than the most common lightweight metal today, aluminium, would mean a great step forward to reduce fuel consumption.

—Mohsen Esmaily

Rheocasting at Jönköping University
Rheocasting uses a semi-solid slurry produced directly from the liquid alloy which is cooled to obtain the desired fraction of solid. The slurry is then inserted into a forming process such as High Pressure Die Casting (HPDC).
In 2004 the Engineering School at Jönköping (JTH) developed RheoMetal, a process for SSM (semi-solid metal) applications. The invention came out as a spin-off from an experiment on grain refinement of cast magnesium alloys.
By immersing a rotating steel rod, onto which a small amount of solid metal had been attached, into a liquid metal at relatively low superheat, it was shown that large amounts of high quality metal slurries, suitable for SSM casting applications, can be produced in very short time.

Studying magnesium casts produced through a casting method called rheocasting, Esmaily discovered that the corrosion resistance of magnesium alloys produced this way was up to four times better than the same material when produced by conventional high pressure die casting. This new knowledge is based on a combination of unique exposure methods and a number of advanced analytical methods.

Rheocasting of magnesium alloys was developed at Jönköping University (Sweden) in order to increase the strength of the material. Esmaily’s research shows that the technique also gives the alloys surprisingly good ability to withstand corrosion.

With his research he shows the connection between the microstructure of the alloy and its corrosion resistance. Now that the connection has been mapped, new possibilities to optimize the microstructure for even better corrosion resistance have opened up.



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