“Influence of Si, Sb and Sr Additions on the Microstructure ...

_ g _ Chapter 2:Literature Review
Volkswagen attempted the use <strong>of</strong> Mg-Al-Ca alloys in the l970’s <strong>and</strong> claimed an
improvement in creep resistance with the addition <strong>of</strong> about l% Ca to magnesium
alloy AZ8l. However, the application <strong>of</strong> this alloy to die casting was not possible
owing to die sticking <strong>and</strong> hot cracking. Later, a through investigation on the optimum
amount <strong>of</strong> Ca addition to Mg~Al alloy to avoid these problems was carried out by
Institute <strong>of</strong> Magnesium Technology (ITM) in Canada <strong>and</strong> General Motor [87, 88]. It
is found that Mg-A1-Ca based alloys, which contain Ca more than 1%, are susceptible
to the hot cracking problem.
Mg-Al-<strong>Sr</strong> (AJ) alloys are new addition to the creep-resistant Mg-Al based
magnesium alloys [89]. Various alloy compositions such as AJ5l (Mg-5Al-1<strong>Sr</strong>),
AJ52 (Mg-5Al-2<strong>Sr</strong>) <strong>and</strong> AJ62 (Mg-6Al-2<strong>Sr</strong>) have been developed. The creep
resistance <strong>of</strong> theses alloys is found to be better than many Mg-Al alloys [90, 91].
However still, the microstructure <strong>and</strong> its creep mechanism are not fully understood.
2.7.2.1 AZ91 (Mg-9% Al-1% Zn-0.2% Mn) alloy
AZ91 alloy is one <strong>of</strong> the old but most commercially used Mg-Al based alloys.
Among 90% <strong>of</strong> Mg alloy components used in transport industries are made from
AZ91 alloy [14]. Currently, this alloy is being used in manufacturing instrumental
panels, steering column, steering wheel, seat frame etc in automotive vehicles. This
alloy provides wide range <strong>of</strong> properties <strong>and</strong> eastability. Of course, AZ9l is the bench
mark alloy for castability compare to all Mg alloys.
AZ91 alloy contain 8.1-9.3% Al, 0.4—l% Zn, 0.3 % max Mn, other impurities
like <strong>Si</strong>, Fe, Cu <strong>and</strong> Ni. Aluminum in this alloy <strong>of</strong>fers mechanical, corrosion <strong>and</strong>
foundry properties like castability. Zinc <strong>of</strong>fers little solid solution strengthening <strong>and</strong>
corrosion resistance. Manganese neutralizes the ill effect <strong>of</strong> Fe <strong>and</strong> hence provides
corrosion resistance. However, AZ91 alloy does not exhibit good creep resistance,
which is a major setback [15].
20