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13/10/2010
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Tom Shelley reports on latest developments in magnesium alloys
 New magnesium alloys and processing methods are offering the capability to make stronger, higher integrity magnesium parts for aerospace that save weight at acceptable production costs.
Shearline is working towards transferring the established technology of Thixomoulding magnesium alloys to processing new compositions. Engineering quality magnesium alloys have been used since before World War II. The Volkswagen Beetle, for example, launched in 1936, used nearly 25kg of magnesium castings in its transmission housing and air cooled engine. During the war, forged magnesium alloys were used to make engine bearers for most German aircraft as well as other parts.
Thixomoulding - semi solid metal casting - is today used to produce millions of thin walled magnesium alloy laptop computer and mobile phone enclosures, because the strength per unit weight and rigidity of magnesium alloys is greater than that of plastics. Charles Maltby, technical and commercial director of Shearline explained that the basis of the process is to load the metal alloy as a solid powder, and put it through a machine that is , "Very similar to a plastic injection moulding machine, except that it has a clever modification in the form of a screw system that is much stronger." The granules are put into the hopper of the machine at room temperature, after which it is subjected heat, with more energy being put in by the shearing action of the screw, raising its temperature to one that at which the alloy starts to melt, but below the melting point of magnesium, which is 650°C.
The resulting semi-solid slurry is forced into a mould, where it solidifies to form a structure made up of equiaxed grains, with very little porosity, unlike a conventional die cast structure which is likely to have greater porosity and columnar grains growing inwards from the cooled, mould face, resulting in potential lines of weakness. Parts made by thixomoulding should always be of superior quality "provided the mould tool has been well designed and taken advantage of the known benefits of injection moulding to improve net shape adherence and reduce mating part count wherever possible", according to Maltby.
While magnesium alloys were extensively used in military aircraft during WWII, there is traditional concern about using them today because of a possible fire risk, although to get it to burn easily, magnesium has to be in the form of either powder or thin ribbon. Despite the fact that there are no reports of mobile phone or laptop computer enclosures catching fire, there have been one or two accounts of magnesium alloy sports car wheels catching fire in competition when tyres are punctured and rims were dragged along the track. However, it is possible to come up with alloy compositions that resist ignition even more than normally, but these alloys, some of which can additionally offer enhanced fatigue and/or corrosion resistance, have hitherto not been thixomoulded. Machines for thixomoulding tend to be somewhat expensive, so Shearline has been working with the University of Sheffield, and the Advanced Manufacturing Research Centre (AMRC), and has secured a Knowledge Transfer Partnership and a KTP Associate – Rachel Peachey – in order to develop the process and ensure that the machines will not suffer damage or degradation working with the different alloys.
Shearline already has experience in handling thixomoulded AZ91D magnesium alloy parts for an electric folding bike. AZ91D is 9% aluminium, at least 0.15% manganese and 1% zinc. The new alloys are variants of the basic magnesium 9% aluminium composition, but with other additions and reduced aluminium.
The AMRC is closely linked to Boeing, as well as having partnerships with a number of other leading edge companies, including Rolls Royce and BAE Systems. Maltby points out that thixomoulding is inherently a more energy efficient and eco-friendly process than die casting as well as producing better quality parts.
Pointers
* Thixomoulded magnesium alloy parts are already widely used for making mobile phone and laptop computer enclosures, where they show superior properties relative to die cast parts
* The process is in the process of being extended to slightly different alloys suitable for use in aerospace, with superior fire, fatigue and corrosion resistance
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Author
Tom Shelley
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