Magnesium is the lightest of the structural metals with a density of only 1.74 g.cm-3. However, magnesium is used as a structural metal in an alloyed form and most magnesium alloys have a density slightly higher than this. Magnesium is a reactive metal and is usually found in nature in the form of and oxide, carbonate or silicate, often in combination with calcium. This reactivity is one of the reasons why the production of magnesium metal requires large amounts of energy.

Magnesium alloy developments have traditionally been driven by aerospace industry requirements for lightweight materials to operate under increasingly demanding conditions. Magnesium alloys have always been attractive to designers due to their low density, only two thirds that of aluminum. This has been a major factor in the widespread use of magnesium alloy castings and wrought products.

A further requirement in recent years has been for superior corrosion performance and dramatic improvements have been demonstrated for new magnesium alloys. Improvements in mechanical properties and corrosion resistance have led to greater interest in magnesium alloys for aerospace and specialty applications, and alloys are now being specified on programmes such as the McDonnell Douglas MD 500 helicopter.

Magnesium alloy sand casting are used in aerospace applications because they offer a clear weight advantage over aluminum and other materials. A considerable amount of research and development on these alloys has resulted in some spectacular improvements in general properties compared with the earlier AZ types. Although there has been, and still is, a large volume of castings for aerospace applications being produced in the older, conventional AZ-type alloys, the trend is toward the production of a greater proportion of aerospace castings in the newer zirconium types.

Although the magnesium-aluminum and magnesium-aluminum-zinc alloys are generally easy to cast, they are limited in certain respects. They exhibit microshrinkage when sand-cast, and they are not suitable for applications in which temperatures of over 95°C are experienced. The magnesium rare earth-zirconium alloys were developed to overcome these limitations. Sand castings in the EZ33A alloy do in fact show excellent pressure tightness. The greater tendency of the zirconium-containing alloys to oxidize is overcome by the use of specially developed melting processes.

For normal, fairly moderate temperature applications (up to 160°C), the two alloys ZE41A and EZ33A are finding the greatest use. They are very castable and can be used to make very satisfactory castings of considerable complexity. In addition, they have the advantage of requiring only a T5 heat treatment.

When a demand arose in some aerospace engine applications for the retention of high mechanical properties at higher elevated temperatures (up to 205°C), thorium was substituted for the rare earth metal content in alloys of the ZE and EZ type, giving rise to the alloys of the type ZH62A and HZ32. Not only were there substantial improvements in mechanical properties at elevated temperatures in these alloys, but good castability and welding characteristics also were retained. The thorium-containing alloys, however, exhibited a greater tendency for oxidation, requiring greater care in meltdown and pouring.

A further development aimed at improving both room-temperature and elevated-temperature mechanical properties produced an alloy designated QE22A. In this alloy, silver replaced some of the zinc, and the high mechanical properties were obtained by grain-refinement with zirconium and by a heat treatment to the full T6 condition (that is, solution heat treated, water quenched, and precipitation aged). However, problems were experienced with both of these alloys. The use of thorium has become increasingly unpopular environmentally, and the price of silver has become very unstable in recent years. Hence, there has been a considerable amount of research and development work on alternative alloy types.

The most recent alloy emerging from this research was an alloy containing about 5.0% Y in combination with other rare earth metals (that is, WE54A), replacing both thorium and silver. This alloy has better elevated-temperature properties and a corrosion resistance almost as good as the high-purity magnesium-aluminum-zinc types (AZ91C). The alloys used for investment casting are very similar to those used for the sand casting process.

In motor racing, RZ5 is generally used for gearbox casings although MSR/EQ21 alloys are also being used increasingly due to their superior ambient temperature properties or because of increased operating temperatures. RZ5 wheels have been shown to have significantly better performance than Mg-Al-Zn alloy wheels under arduous racing conditions. Due to the high operating temperature of racing engines, WE54 castings have been used for a variety of Formula 1 engine parts and are used for engine components for a limited edition road car. Forged WE54 pistons offer great future potential for motor racing and other applications will exist for other wrought products.

Magnesium alloys are also used in many other engineering applications where having light weight is a significant advantage. Magnesium-zirconium alloys tend to be used in relatively low volume applications where they are processed by sand or investment casting, or wrought products by extrusion or forging. Zirconium-free alloys, principally AZ91 but also other alloys, are used in automotive and various other high volume applications.