2.1 Introduction of magnesium (Mg) and its alloys
and magnesium alloys generally display very favourable properties, showing
their high strength to weight ratio and very low density properties. Magnesium
is one promising lightweight metal and has close-packed hexagonal structure.
With common oxidation number of +2, magnesium is representing
2.7% of the earth’s crust. It has a density of 1.74 g/cm3, which is
approximately one-fourth the density of steel and two-thirds that of aluminium
(Table 1.1). The melting point of pure Mg is 650oC and its atomic
diameter is 0.320nm, so there is favourable size factor with a various range of
the solute elements such as aluminium, cerium, yttrium and zirconium 1–4.
Table 2.1. Density of
commonly used structural materials.
The density value is
dependent on the type and amount of reinforcements.
History and production of Mg
Magnesium was discovered by Sir
Humphrey Davy over 200 years ago but officially been produced and experimented in
metallic form in 1831. However, commercial production
of electrolytic magnesium began in Germany in 1886. It has been continuously
produced by Germany until 1916. Due to high demand in military equipment’s for
flares and tracer bullets, the production of magnesium extent to US, Great
Britain, France, Canada and Russia. Unfortunately, the consumption has fall
during the war on early 1940s. The world production of metal includes Mg was
rise back in 19555–7.
Production of Mg become
possible at many locations as it is the eighth most abundant element and sixth
most abundant metal in the earth’s crust. In order to keep the Mg-end product
at lowest price, buyers are constantly looking for the lowest raw material’s
cost. Because of that, in mid the
1990’s, China has become the main Mg exporter to the world. The production of
Mg traditionally from dolomite and magnesite ore (MgO) and MgCl which contained
deposited salt. Mg was extracted by electrochemical process, then being
crushed, roasted and mixed with seawater in large tank. Once MgOH formed at the
bottom, heating, mixing and reaction with chlorine will then formed MgCl. This
will be electrolyzed to form Mg, which is a float at the surface 3.
The use of Mg is limited by
few factors, including corrosion and inconsistent mechanical properties. Therefore,
Mg was overlooked by industry for many years until the two World Wars have
started. A great interest of using Mg alloys in aviation application is because
of the low density of Mg alloys and its moderated yield strength8,9.
Unfortunately, in order to
find lower cost of raw material, manufacturer switch to aluminum metals, as it
found to be similar mechanical performance as Mg. After few decades, the
limited properties of Mg has given a huge opportunities of research idea to
develop Mg alloys and coating techniques to provide adequate properties of Mg
for specific applications particularly in automotive. With significant weight
saving, Mg has become a good material candidate for automobile. However, the application
of Mg-based components is challenging, not only because of mechanical
properties requirements, but also due to the poor corrosion resistance of the