Magnesium And Its Alloys
Magnesium is a light metal along with density of 1.74 g/cm3. Magnesium is more costly than aluminium as density 2.74 g/cm3 along with which it compares for lightness. Magnesium in its molten state burns readily, thus it is complicated to cast the alloys of magnesium. Magnesium alloys contain low corrosion resistance and presents poor creep and fatigue behaviour. Their h.c. p. structure does not allow deforming readily at room temperature as only three slip systems exist in h. c. p. at room temperature. The best benefit that magnesium alloys offer is such of low density and a lot of aircraft parts are made in these alloys.
Al while added to Mg in the range of 3 to 10% along with minute amounts of Zink and Mn raises strength, castability and hardness. Addition of Mn as 1.2% along with small amount of C does not raises strength but enhances corrosion resistance. Mg-Al-Zink alloys contain good corrosion resistance and mechanical strength. Such alloys are good casting material and usually used at high temperature like 250oC. Extrusions and forgings for normal purpose are made in such alloys are utilized in aircraft, radio, instrument and automotive industries.
Several magnesium alloys are illustrated in Table no.21 with their applications.
Table no.21: Magnesium Alloys
|
Composition (% )
|
Condition
|
UTS N/mm2
|
YS N/mm2
|
Elongation
%
|
Application
|
|
Wrought alloys
|
Mg, 3 Al, 1 Zn, 0.2 Mn
|
Annealed
|
228
|
-
|
11
|
Air borne cargo equipment
|
|
Mg, 2 Th, 0.8 Mn
|
T8
|
228
|
198
|
6
|
Missile and aircraft sheets upto 427oC
|
|
Mg, 6 Zn, 0.5 Zr
|
T5
|
310
|
235
|
5
|
Highly stressed aerospace uses, extrusions, forgings
|
|
Cast alloys
|
MG, 6 Zn, 3 Al, 0.15
Mn
|
As-cast
|
179
|
76
|
4
|
Sand casting requiring good room temperature strength
|
|
Mg, 3 Re, 3 Zn, 0.7 Zr
|
T6
T5
|
235
138
|
110
97
|
3
2
|
Pressure tight and permanent mould castings used at
150-260oC
|