Magnesium is 36 per cent
lighter per unit volume
than aluminium and 78
per cent lighter than iron, which
indicates that significant
automobile weight reduction can
be achieved through replacing
aluminium and some steel
components by those made using
magnesium alloys. Reducing the
automobile weights by certain
amount will result in similar
percentage of improvement in fuel
economy. For a typical vehicle, this
represents a fuel saving of about
0.5 liters per 100 kilometers for
every 100 kilograms of weight
reduction. Reduction in weight is
best accomplished through a
combination of innovative
structural design and increased use
of lightweight materials.
Magnesium alloys are very
attractive materials for weight
reduction in automobile
applications. Currently, the average
vehicle in North America uses only
0.25 per cent magnesium (3.8 kg)
compared to eight per cent
aluminium (120 kg).Magnesium
and its alloys are important for
structural application and demand
for them will increase considerably
in the coming years. However,
significant research is still needed
on magnesium processing, alloy
development, joining, surface
treatment, corrosion resistance, as
well as,mechanical properties
improvement. Since high cost is a
major obstacle to greatly increased
magnesium use in autos,
developing new alloys, which have
better formability could enable
major cost reduction. New
magnesium alloys are also needed
to meet the automobile and
aerospace requirements for
elevated-temperature strength and
creep resistance. This research area
is presently one of the most active
areas in light metals technology.
Industrial demand for highly
qualified personnel in magnesium
alloys area is a fact for years
to come.
Magnesium and its current use
Magnesium is the eight most
abundant metal on the earth
surface at approximately 2.5 per
cent of its composition. It is an
alkaline earth element that
crystallises in a hexagonal
structure.Magnesium is the
lightest metallic material used for
structural applications with a
density of 1.738 g/cm3 in
comparison with the densities of Al
(2.70 g/cm3) and Fe (7.86 g/cm3).
Magnesium also has a very good
strength to weight ratio of
common structural metals and has
the exceptional die-casting
characteristics [Gradinger and
Stolfig, 2003; Pekguleryuz et al.,
2003]. In addition, on a per-pound
basis,magnesium costs more than
aluminium, whereas on a volume
basis the price of both becomes
approximately the same. This
makes magnesium alloys one of
the most promising lightweight
materials for automotive
applications. Emerging goals for
reduced emission and fuel
economy in passenger vehicles is
exerting a driving force for
expanding the use of magnesium
[Mordike, 2002]. The current use of
magnesium in automotive
application includes cross-car
instrument panel beams, steering
wheel armatures, cam covers and
valve covers. The market for
automotive magnesium parts has
grown rapidly, nearly 15 per cent
per year during the 1990s. The
average magnesium content in the
2002 model cars was 4 kg. Figure 1
shows the North American
automotive magnesium usage for
different car manufacturers [Das,
2003]. Die-casting is one of the
most effective fabrication methods
to produce magnesium
components in automotive
industry. However, the number of
available Mg-based alloys for
die-casting is very limited. AZ91
(Mg-9 wt. per cent Al-1wt. per cent
Zn) is the principal alloy, which
represents 80 per cent of the
magnesium casting components
[Zhang and Couture, 1998].
These alloys are not suitable for
applications over 95°C such as
powertrain components in
automobile applications because
of their restricted creep properties
which limited the current
application of magnesium to
non-critical parts such as valve
covers and instrument panels. In
contrast to steels and many Alalloys,
the conventional Mg-alloys
have a relatively low resistance to
creep [Blum et al. 2001].
High temperature behaviour of
magnesium and its alloys
Grain boundary sliding has been
observed to be the main creep
mechanism in magnesium alloys in
the stress-temperature ranges of
interest for automotive application.
Magnesium seems to creep even at
low temperature by a
stress-recovery mechanism.
The creep mechanisms at low
temperature are basal slip within
the grains and sub-grains
formation while at the higher
temperatures diffusion-dependent
mechanisms become predominant
[Blum et al. 2001].
Mg-Al alloys are one major
group among the magnesiumbased
alloys. The strength of these
alloys is improved by forming a
solid solution where 11.5 atomic
per cent Al are soluble in the Mgmatrix
at 437°C. The microstructure
of these alloys is characterised by
the Mg-g (Mg17Al12) eutectic at
the grain boundaries.
The non-stoichiometric phase g
is incoherent with the a-Mg matrix.
In addition to this poor coherency,
if Mg-Al alloys are exposed to
elevated temperatures (>150°C) for
longer time, the supersaturated Mg
solid solution transforms to
Mg-matrix with coarsely dispersed
g precipitates and contributes to
grain boundary migration and
creep deformation. g is also prone
to aging and has poor
metallurgical stability, which
limited the application at higher
temperature [Aghion et al. 2003;
Pekguleryuz and Renaud, 2000].
Development of creep resistant
magnesium alloys
Early developments in improving
the creep properties of magnesium
were made in the 1960’s by
Volkswagen. It was based on the
Mg-Al-Si system.These alloys exhibit
marginally improved creep
resistance but are quite difficult to
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