As the lightest structural metal available, magnesium's combination of low density and good mechanical strength results in a high strength-to-weight ratio.
Because of their low modulus of elasticity, magnesium alloys can absorb energy elastically. Combined with moderate strength, this provides excellent dent resistance and high damping capacity. Magnesium has good fatigue resistance and performs particularly well in applications involving a large number of cycles at relatively low stress. The metal is sensitive to stress concentration, however, so notches, sharp corners, and abrupt section changes should be avoided.
Magnesium parts are generally used from room temperature to about 200°F or, in some cases, to 350°F. Some alloys can be used in service environments to 700°F for brief exposures.
Magnesium is widely recognized for its favorable strength-to-weight ratio and excellent castability, but deeply ingrained misconceptions often prevent designers from specifying it as a die-cast material. However, what is true of magnesium as a generic material is not true of today's die-casting alloy. The new high-purity alloy, combined with advances in fluxless, hot-chamber die-casting processing, has altered the traditional guidelines for evaluating the cost and performance of magnesium die castings.
Fabrication: Magnesium alloys are the easiest of the structural metals to machine. They can be shaped and fabricated by most metalworking processes, and they are easily welded. At room temperature, magnesium work hardens rapidly, reducing cold formability; thus, cold forming is limited to mild deformation or roll bending around large radii. Pure magnesium is usually alloyed with other elements to develop sufficient strength for structural purposes. Some alloys are heat treated to further improve properties.
Cast magnesium alloys are dimensionally stable to about 200°F. Some cast magnesium-aluminum-zinc alloys may undergo permanent growth if used above that temperature for long periods. Permanent-mold castings are as strong as sand castings, and they generally provide closer dimensional tolerances and better surface finish. Typical applications of magnesium gravity castings are aircraft engine components and wheels for race and sports cars.
Design of die-cast magnesium parts follows the same principles established for other die-casting metals. Maximum mechanical properties in a typical alloy are developed in wall thicknesses ranging from 0.078 to 0.150 in. Chain-saw and power-tool housings, archery-bow handles, and attache-case frames are typical die-cast applications.
Magnesium is easy to hot work, so fewer forging steps are usually required than for other metals. Bending, blocking, and finishing are usually the only operations needed. Typical magnesium forgings are missile fuselage connector rings.
Standard extruded shapes include round, square, rectangular, and hexagonal bars; angles, beams, and channels; and a variety of tubes. Luggage frames and support frames for military shelters are examples of magnesium extrusions.
Methods used for joining magnesium are gas tungsten-arc (TIG) and gas metal-arc (MIG) welding, spotwelding, riveting, and adhesive bonding. Mechanical fasteners can be used on magnesium, provided that stress concentrations are held to a safe minimum. Only ductile aluminum rivets should be used, preferably alloy 5056-H32, to minimize galvanic-corrosion failure at riveted joints.
Specification: Magnesium alloys are designated by a system established by the ASTM that covers both chemical compositions and tempers.
The first two letters of the designation identifies the two alloying elements specified in the greatest amount. The letters are arranged in order of decreasing percentages or alphabetically if the elements are present in equal amounts. The letters are followed by respective percentages rounded off to whole numbers, followed by a final serial letter. The serial letter indicates some variation in composition of minor alloying constituents or impurities.
The letters that designate the more common magnesium alloying elements are
A -- Aluminum
E -- Rare earths
H -- Thorium
K -- Zirconium
L -- Lithium
M -- Manganese
Q -- Silver
S -- Silicon
Z -- Zinc
For example, magnesium alloy AZ31B contains 3% aluminum (code letter A) and 1% zinc (code letter Z).
Resisting corrosion: A problem with magnesium has been its lack of sufficient corrosion resistance for many applications, particularly in the alloys used for die and sand casting. The problem has been solved by the two major supplies, Dow and AMAX; both have developed corrosion-resistant, high-purity AZ91 alloys for die casting, and both also offer a sand-casting grade.
The die-casting grade is now designated by ASTM as AZ91D and will, for all practical purposes, replace AZ91B. The sand-casting grade received the designation AZ91E from ASTM. The high-purity alloys are said to be as much as 100 times more corrosion resistant than standard magnesium alloys, and more resistant to saltwater than die-cast 380 aluminum alloy or cold-rolled steel, tested according to ASTM B117. Research in magnesium metallurgy has shown that the ability of magnesium to resist corrosion in a service environment of salt-laden air or spray depends heavily on keeping contaminants (iron, nickel, copper) below their maximum tolerance limits during all production operations.
The high-purity magnesium die-casting alloy has already replaced other metals as well as a number of plastics in a variety of U.S. passenger-car and lightweight-truck components. Examples include valve and timing-gear covers, brackets, clutch and transfer-case housings, grille panels, headlamp doors, windshield-wiper motor housings, and various interior trim parts.
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