
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.

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.