Magnesium alloys are utilized in engineering design mainly because of their high strength-weight ratios, excellent machinability, and relatively low cost on a piece basis. The specific gravity of magnesium is 1.74, making it the lightest structural metal. Magnesium alloys weigh about 0.064 to 0.067 pound per cubic inch (1.75 to 1.85 grams per cubic centimeter) as against approximately 0.091 to 0.108 pound (2.5 to 3.0 grams per cubic centimeter) for aluminum alloys and 0.283 pound (7.8 grams per cubic centimeter) for steel. Alloys of magnesium are found to be especially useful in transportation and portable equipment as well as for parts, which are subject to frequent and rapid changes in position.
Properties of Magnesium Alloys
Magnesium or its alloys are available in almost all the common forms in which metals are commercially used. Practically pure magnesium, 99.8 per cent, is supplied as ingot and stick for remelting, and as powder, ribbon, wire, and extruded and rolled strip. Magnesium alloys are produced in various forms including casting, sheet and plate, shapes, forgings, bar, and rod. Both standard structural shapes and sections of special design are made by the extrusion process.
Sheet is fabricated into many different articles by drawing and by spinning. Magnesium alloys are especially workable by hot forming methods. Structures in the alloys are readily assembled by welding and by riveting. The excellent machinability of magnesium alloys has been mentioned. Parts in the alloys can be machined at higher speeds and at lower costs than in most other common metals.
Cast magnesium alloys are supplied with tensile strengths up to about 40,000 pounds per square inch (≈280 MPa) and yield strengths up to about 23,000 pounds (≈160 MPa). Wrought magnesium alloys are produced with tensile strengths up to about 52,000 (≈360 MPa) pounds and yield strengths up to about 44,000 pounds (≈300 MPa).
Different combinations of mechanical properties may be obtained by suitable heat treatment of some compositions. The endurance limit of cast magnesium alloys ranges up to approximately 14,000 pounds per square inch (≈100 MPa) and of the wrought alloys up to 18,000 pounds (≈125 MPa). The modulus of elasticity of magnesium alloys may be taken as 6.5 million pounds per square inch (45 GPa) and the modulus of rigidity as 2.4 million pounds (16 GPa).
The yield strength, tensile strength, and hardness of magnesium alloys decrease with rising temperature while the elongation increases. In general, the strength, hardness, and endurance limit of magnesium alloys are substantially impaired at 300°F (150°C). However, some compositions have been developed especially for service at temperatures above 500°F (260°C). At subzero temperatures, say down to minus 100°F (minus 70°C), magnesium alloys usually gain a little in yield strength, tensile strength, and hardness but lose somewhat in elongation and impact resistance.
Apart from low density, some outstanding physical properties of magnesium are relatively high coefficient of thermal expansion, thermal conductivity, and specific heat, and relatively low electrical conductivity.
As in the case of aluminum, savings may be affected in the production of parts by specifying magnesium alloys rather than steel although the later costs less on a pound basis. The volume of metal in a part may be substantially the same irrespective of the material used, however when comparing metal costs, the price per pound of magnesium should be divided by the ratio of the specific gravities involved. For steel, this ratio is approximately 4.2 and for aluminum 1.6.
In the utilization of castings, somewhat thicker sections may be required in light alloys than in steels in order to compensate for the difference in tensile strength. At the same time, the thickness of sections is often determined by foundry limitations rather than by the mechanical requirements of design. So, the same thickness of section in a magnesium-alloy casting (as in steel) may afford enough strength for a particular purpose.
The economy arising in machining magnesium alloys may offset an unfavorable price relation with a material which costs less on a volume basis but is difficult to machine. Also, the fact that a die-casting may be made of a magnesium-base composition and used in place of another material which is not suitable for the die-casting process may lead to substantial savings.
Pure Magnesium
Magnesium materials are supplied in various compositions including the commercially pure metal (99.8 per cent minimum), alloys for casting, and alloys for the manufacture of wrought products. Ordinarily, different compositions are used for casting as contrasted with those for rolling, forging, or other working.
Substantially pure magnesium finds practically no use in engineering design or for stressed applications. The yield strength of the pure metal, as cast, is about 3,000 psi (20 MPa) with tensile strength of 12,000 psi (20 MPa), elongation of 6 per cent, and Brinell hardness of 30.
Suitably alloyed magnesium provides material with a wide range of mechanical properties. The commercially pure metal has a number of important nonstructural uses, for example, in pyrotechnics, as an alloying agent in nonferrous metallurgy, and in the cathodic protection of other metals against corrosion.
Casting Alloys
The common magnesium alloys employed for casting purposes usually contain varied percentages of aluminum, manganese, and zinc. Aluminum has been the predominant alloying element with magnesium. The maximum aluminum content does not normally exceed 10 per cent. Calcium may be added in small amount (about 0.10 per cent) to some alloys for particular purposes. New alloys developed during late years contain rare-earth metals and zirconium. These provide increased resistance to creep and stress-rupture at elevated temperatures.
Nearly all the casting alloys may be heat treated to improve the mechanical properties, a notable exception being alloy M1A.
Wrought Alloys
Several magnesium alloys have been developed especially for the manufacture of wrought products. The most important are M1A, AZ3IB, AZ6IA, AZ80A, and ZK60A. As in the case of the casting alloys, the principal added elements in the wrought compositions are aluminum, manganese, and zinc.
M1A is used for various wrought manufactures including sheet, forgings, and extrusions. In any form, the strength of M1A is relatively low. AZ3lB is employed for sheet and extrusions. AZ6lA and AZ80A find use for extrusions. Of the extrusion alloys, only AZ80A is not extruded into hollow shapes or tubing. T A54A is employed exclusively for forgings and ZK60A for extrusions.
Magnesium alloys for wrought products are classified as non-heat treatable and heat treatable. AZ80A and ZK60A are heat treated by aging the material as fabricated. The strengths are increased substantially but with loss of elongation on aging. M1A and AZ31B sheet are supplied as hot rolled, as cold rolled, and annealed.
The principal wrought manufactures which are commercially available in magnesium alloys include sheet and other flat rolled products, extrusions (bars, rods, solid shapes, hollow shapes, and tubing), forgings, screw-machine stock, and impact extrusions.
ASTM Nomenclature
The ASTM designation for substantially pure magnesium consists of the numerals showing the specified minimum purity. All digits appear, but the decimal point is omitted. A letter arbitrarily chosen is added to differentiate grades of the same purity which may have different purity requirements. The full name of the base metal precedes the designation but is omitted when the identity is obvious. Thus, the designation for 99.80 per cent minimum magnesium is 9980A.
As in the case of aluminum alloys, the ASTM design magnesium alloys consist of not more than two letters representing the alloying elements specified in greatest amounts. These ranged in order of decreasing percentages or in alphabetical order if of equal percentages, followed by the respective percentages rounded of to whole numbers.
A final letter is arbitrarily assigned in alphabetical order to differentiate alloys which otherwise result in identical designations. The full name of the base metal precedes the designation but is omitted when its identity is obvious.
The letters used to represent the alloying elements are given below:
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A - Aluminum
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M - Manganese
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B - Bismuth
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N - Nickel
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C - Copper
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P - Lead
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D - Cadmium
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Q - Silver
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E - Cerium
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R - Chromium
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F - Iron
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S - Silicon
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G - Magnesium
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T - Tin
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J - Phosphorus
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V - Arsenic
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K - Zirconium
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W - Sulfur
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L - Beryllium
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Y - Antimony
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Z - Zinc
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