Changing consensus: Modern magnesium alloys

Everyone remembers holding a piece of magnesium foil over a Bunsen burner and watching it rapidly combust in to a brilliant white flame. It's a powerful demonstration that is etched on the mind, leaving many to automatically equate magnesium with flammability.

And this has made it difficult for opponents of the material to convince engineers to specify it use. However, magnesium is making a comeback, particular in automotive and aerospace markets, where it is being used to lightweight.

"Everyone always links magnesium with 'that' science class," says Bill Warfield, extrusion manager at Magnesium Elektron. "Our challenge representing tier 3 in the supply chain is to dispel that myth, and convince people that magnesium is not an exotic materials. There is plenty of it around, but not enough of it is used for industrial or automotive applications."

There are many examples of magnesium alloys being used by industries including aerospace and automotive, to more consumer applications. It strength, particularly in wrought alloys, are comparable to the steels used by the automotive industry, but it is the materials stiffness that offer the most potential.

Martyn Alderman ?divisional director of technology at Magnesium Elektron, says: "We always talk about specific stiffness when weight saving. A magnesium plate will be 18 times stiffer than a steel plate of the same weight."

Lightweight Design
One method of designing magnesium lightweight parts involves selectively thickening highly stressed areas. Like any material, the key to getting the greatest weight saving lies in designing for the material in question, utilising its inherent property advantages while mitigating weaknesses. Done well, magnesium has been known to offer more than 60% weight saving against steel for strength limited designs, and 50% for stiffness sensitive parts - due to its lower Young's Modulus. In the case of aluminium, it is usually 33% weight saving based on comparable strength, but 22% based on stiffness. If you can selectively stiffen the part by increasing section thickness, magnesium offers a weight saving due to the fact that aluminium is volume for volume 50% heavier than magnesium.

"In stiffness sensitive designs, as long as packaging space is not an issue, generally you'll achieve something with magnesium," says Alderman. "Bigger applications are where you tend to get the most benefit such as power trains, steering columns, steering systems and engine covers."

The Crux
Magnesium is more anodic than other structural metals and there will be a galvanic corrosion reaction unless components are kept dry. However, plain sheet corrosion rates are about the same as steel, which has been resolved by using phosphates and e-coating systems. The challenge with magnesium is to find a coating system that is as effective and as trusted.

"There are systems out there such as Anomag, Magoxid, Tagnite, Keronit," says Alderman. "But these are generally only used for aerospace. So we are working to on non-chrome conversion type treatments like a conventional powder coat.

"Lightweighting is not about one single material, it is about mixed material solutions and using the best material for the application in question. Magnesium definitely has a big part to part to play though. Today we have 4.5kg on an average vehicle, but by 2020 we want to get to over 150kg on a car."

Justin Cunningham

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