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

M Chapter 2: Literature Review
to spread out over the melt surface. The gas mixtures take away the oxygen out <strong>of</strong> the
melting area <strong>and</strong> hence, provide an oxygen free atmosphere for melting.
The SF6 based shielding gas is non-toxic, odourless <strong>and</strong> corrosion free.
However, SP6 is having a heavy green house effect. Its Global Warming Potential
(GWP) is 23900 times higher than CO; <strong>and</strong> will be banned from industrial use [63].
Therefore, researchers in magnesium industries are trying to find the altematives.
Recently International Magnesium Association (IMA) found out suitable substitutes
for SF6. These are HFC-134, HFE-7100 <strong>and</strong> HFE-7200, Novecm 612 (FKS) [64].
These altematives are all fluorides <strong>and</strong> their protective effects are no less than that <strong>of</strong>
SP6 but their GWP values much less tat that <strong>of</strong> SP6 [65].
2.6 CASTING PROCESSES
Nonnally the magnesium alloys are cast by die casting method. This is to
overcome the material cost disadvantages compared to aluminium alloys [66]. All
magnesium alloys development activities for different mechanical properties are thus
also focused with die-castability. Magnesium alloy containing Al as a major alloying
element has high die castability compared to other Mg alloys. The most commercial
<strong>and</strong> widely used die cast magnesium alloys are AZ91, AM60, <strong>and</strong> AE42 etc.
The choice <strong>of</strong> a particular casting method depends upon many factors, e.g., the
number <strong>of</strong> castings required, the properties required, dimensions <strong>and</strong> shape <strong>of</strong> the part
<strong>and</strong> the castabiltiy <strong>of</strong> the alloy [67]. Although pressure die casting is predominantly
used to produce many <strong>of</strong> the magnesium alloy components, other casting processes
such as gravity <strong>and</strong> low pressure castings using s<strong>and</strong> <strong>and</strong> permanent moulds are also
familiar. Investment casting process has also presently become popular in producing
Mg alloy components. Mg-Al <strong>and</strong> Mg-Al-Zn alloys are also easy to cast in gravity s<strong>and</strong>
casting method; however they are limited in certain respects. They exhibit micro
shrinkage when they are s<strong>and</strong>-cast, <strong>and</strong> the alloy is not suitable for applications in
temperatures <strong>of</strong> above 120°C. Magnesium alloys containing RE as a major alloying
element are mainly produced in s<strong>and</strong> casting route for this purpose <strong>and</strong> they are used in
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