Magnesium: Properties — applications — potential
Section snippets
History and properties of magnesium alloys
In the past, magnesium was used extensively in World War I and again in World War II but apart from use in niche applications in the nuclear industry, metal and military aircraft, interest subsequently waned. The most significant application was its use in the VW beetle but even this petered out when higher performance was required. The requirement to reduce the weight of car components as a result in part of the introduction of legislation limiting emission has triggered renewed interest in
Alloy development
The property profiles demanded by automobile and other large-scale potential users of magnesium have revealed the need for alloy development. A straight transfer of ‘high performance’ aircraft alloys is not possible not only on economic grounds but often the property profiles do not coincide. Fig. 3 shows the different trends in alloy development depending on the main requirement.
Specific strength
The vast majority of magnesium applications are covered by AZ91, a die-casting alloy. This alloy has insufficient creep resistance for many desirable applications at temperatures above 130°C. The aluminium system forms the basis albeit with much lower Al contents for the development of high specific strength wrought alloys. A binary alloy of 6% Al provides the optimum combination of strength and ductility. Fig. 3shows further development of Mg–Al for die-casting Mg–Al–Mn and for wrought alloys
Ductility
The previous section emphasised the need for improving ductility but concentrated on a combination of high strength with reasonable ductility. There is a need for a series of alloys with very high ductility capable of being formed by thermomechanical treatment. Ductility is determined by the number of operative slip systems. Mg being hexagonal slips at room temperature on the base plane (0001)〈〉 and secondary slip on vertical face planes () in the 〈〉 direction. This limits
Creep resistance
Magnesium melts at 650°C. Consequently, it is to be expected that there will be problems preventing creep in stressed components. Alloys containing Thorium, e.g. HZ22 show at 623K the highest service temperatures of magnesium alloys, and incidentally, compared to melting point, the highest of any material. The radioactivity of thorium has however resulted in its exclusion as an alloying element. There are various upper limits for service requirements, e.g. max. 150°C, 175°C, 200°C etc. as shown
Fibre and particle reinforced magnesium (MMCs)
Conventional alloying practice can not ensure certain properties and for these fibre and particle reinforcement must be used. The reinforcement material is usually Al2O3, SiC or carbon. The aim is to improve the elastic modulus but perhaps also wear resistance or creep-resistance. It is also possible to modify the thermal expansion. Problems arise from the reactivity of magnesium — the reinforcement can be attacked — which can impair the reinforcement. The aim of developments in this area is to
Process development
Although pressure die casting dominates the techniques currently used, magnesium can be produced by virtually all other gravity and pressure casting methods viz. sand, permanent and semi-permanent mould and steel and investment casting. The choice of a particular method depends upon many factors, e.g. the number of castings required, the properties required, dimensions and shape of the part and the castability of the alloy. Nevertheless, there is a need to develop conventional techniques
Material trends in car body manufacturing
The development of new production techniques, e.g. laser technology and the development of new materials, steels, aluminium, magnesium and plastics is influencing the ways cars are made and the materials used. Fig. 8 shows this diagrammatically. The development of constructive method is shown in Fig. 9. The diagram illustrates actual and possible developments starting from the steel unibody of the 1950's. In the drive to reduce the exhaust emissions it is necessary to reduce the weight of the
Applications of magnesium alloys
The use of magnesium alloys in the European automobile industry encompasses parts such as steering wheels, steering column parts, instrument panels, seats, gear boxes, air intake systems, stretcher, gearbox housings, tank covers etc. Some are illustrated in Fig. 15, Fig. 16.
Non-automotive applications
Magnesium based alloys have been used for numerous applications in hobby equipment e.g. bicycle frames. Interesting applications in communication engineering are shown in Fig. 17. In this case, light weight is required as well as screening against electro–magnetic radiation which plastic materials cannot offer.
Outlook
The major driving force for development is the automobile industry. Although some interesting developments have been realised for space, aircraft and other applications we have seen successful applications in automobile industry components such as steering wheels, steering column parts, instrument panels, seats, gearboxes and air intake systems. Future developments will include large body parts, cylinder blocks, door frames and petrol tank covers. Over the period 1998–2000, the increase in die
Acknowledgements
The author would like to thank members of the Institute of Materials Engineering and Technology at the Technical University Clausthal for discussions and support.
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