MaterialPedia

MAGNESIUM ALLOYS Although magnesium alloys are moderate in strength and rigidity, they have high specific strength and rigidity because of their low density, which, being in the range of 0.064 to 0.066 lb/in3 (1,772 to 1,827 kg/m3), is the lowest of common metals. Modulus of elasticity in tension is typically 6.5 X 106 lb/in2 (44,800 MPa), and ultimate tensile strengths range from 22,000 to 55,000 lb/in2 (152 to 379 MPa), depending on the alloy and form. Applications are due mainly to the light weight, ease of casting, and superior machinability of the alloys and include auto parts, aerospace equipment, power tools, sporting goods, fixtures, and materials-handling equipment. Both wrought and cast alloys are available, the former in sheet, plate, rod, bar, extrusions, and forgings, and the latter for sand, permanent-mold, investment, and die castings. Alloys are designated by a series of letters and numbers followed by a temper designation. The first part of the alloy designation indicates by letters the two principal alloying elements (or one if the alloy contains only one alloying element): A for aluminum, E (rare-earth elements), H (thorium), K (zirconium), M (manganese), Q (silver), S (silicon), T (tin), and Z (zinc). The two (or one) numbers that follow indicate the amounts (in percent rounded off to whole numbers) of these elements, respectively. These numbers are followed by a letter to distinguish among alloys having the same amount of these alloying elements. The temper designations that follow are similar to those for aluminum alloys: F (as fabricated); O (annealed); H10 and H11 (slightly strain-hardened); H23, H24, and H26 (strain-hardened and partially annealed); T4 (solution heat-treated); T5 (artificially aged); T6 (solution heat-treated and artificially aged); and T8 (solution heat-treated, cold-worked, and artificially aged). Thus AZ91C-T6 is the designation for an alloy containing 8.7% aluminum and 0.7 zinc as the major alloying elements. The letter C indicates that it is the third such alloy to be standardized, and, in this case, it is in the solution heat-treated and artificially aged temper. These designations, however, do not distinguish between wrought and cast alloys. And the magnesium photoengraving alloy, containing 3.3% aluminum and 0.7 zinc and made in special-quality sheet for photoengraving, goes simply by the designation PE.

Besides AZ91C, other magnesium sheet and plate alloys include AZ31B, HK31A, and HM21A, of which AZ31B is the strongest at room temperature and the most commonly used. In the H24 temper, it has an ultimate tensile strength of 42,000 lb/in2 (290 MPa), a tensile yield strength of 32,000 lb/in2 (221 MPa), 15% elongation, and can be used at service temperatures to about 200°F (93°C). The others, however, especially HM21A, are more heat-resistant and can sustain temperatures to about 600°F (315°C). For HM21A, the 100-h creep strength for 0.1% deformation is 12,500 lb/in2 (86 MPa) at 400°F (214°C) and 7,500 lb/in2 (52 MPa) at 600°F (316°C). None of the alloys are especially formable, minimum bend radii for AZ31B-O, the most formable, ranging from about 5 times thickness (5T) at room temperature to 2T at 500°F (260°C). Thus, heat is often required in forming operations, especially deep-drawing.

AZ31B is also widely used in the form of bar and other extruded shapes. The alloys for bar and extruded shapes are generally of two kinds: those alloyed principally with aluminum and zinc and those alloyed with zinc and a bit of zirconium. In the former, strength increases with increasing aluminum content, and in the latter with increasing zinc content. Some of each kind respond to artificial aging, providing in the T5 temper ultimate tensile strengths of 50,000 to 55,000 lb/in2 (345 to 379 MPa). AZ31B and many of the alloys for bar and extrusions are also suitable for forging. The hot-working range may be as low as 450 to 700°F (232 to 371°C) or 560 to 1000°F (293 to 538°C).

There are several magnesium die-casting alloys but more than a dozen magnesium sand-casting alloys and magnesium permanent-mold casting alloys. One composition, AZ91, is available in four grades: AZ91A, B, C, and D. AZ91C, for sand- and permanent-mold castings, contains 8.7% aluminum, as opposed to 9 in the die-casting alloys (A and B). Each also contains 0.7% zinc and 0.13 manganese. As die-cast, AZ91A and AZ91B provide an ultimate tensile strength of 33,000 lb/in2 (228 MPa), a tensile yield strength of 22,000 lb/in2 (152 MPa), and 24% elongation. For AZ91C-T6, these values are 40,000 lb/in2 (276 MPa), 21,000 lb/in2 (145 MPa), and 6%, respectively. AZ91D is a high-purity version, containing extremely low residual contents of iron, copper, and nickel, which markedly improves corrosion resistance, precluding the need for protective treatments in certain applications, such as auto underbody parts. The benefits of high purity are apparently applicable to other alloys as well, such as AS41, a more heat-resistant die-casting alloy. AM50A and AM60B, with 5 and 6% aluminum, respectively, are ductile (6 to 10% elongation) die-casting alloys. Their tensile strengths are 30,000 lb/in2 (207 MPa) ultimate and 17,000 lb/in2 (117 MPa) yield.
The sand- and permanent-mold casting alloys generally use either aluminum, zinc, and manganese or zinc, thorium, zirconium, and, in some cases, silver and rare-earth elements, in the way of alloying elements. Almost all these alloys respond to artificial aging or solution heat-treating and artificial aging. The strongest, ZE63A, in the T6 temper, and ZK61A, in the T5 or T6 temper, have an ultimate tensile strength of about 45,000 lb/in2 (310 MPa), a tensile yield strength of about 28,000 lb/in2 (193 MPa), and about 10% elongation. The alloys containing zirconium and rare-earth elements— ZE33A, ZE41A, and ZE63A—are more creep-resistant at higher temperatures than the aluminum-, zinc-, and manganese-bearing alloys, but are more difficult to cast. The magnesium-silver alloys QE22A and QH21A, also are superior in elevated-temperature performance and have good castability and weldability, but are quite costly.

Like their base metal, magnesium alloys have outstanding machinability, providing faster cutting speeds and greater depths of cut at less power than all commonly machined metals. However, dust, chips, and turnings can pose a fire hazard, necessitating special precautions.

In general, media that are basic, neutral, or contain fluorine cause little or no corrosion, but those which are acidic attack magnesium. Thus, most acids, including fruit juices, attack the metal, although pure chromic, oleic, and dry stearic acids at room temperature are exceptions, as is 5 to 60% hydrofluoric acid. The metal also resists distilled water and acid-free rain but is attacked by boiling water, carbonated water, steam, and seawater. Pure compounds to which it is also generally resistant at room temperature include butyl, ethyl, isopropyl, and propyl alcohols; ethyl acetate, ethyl benzene, ethyl cellulose, ethyl chloride, and ethyl salicylate; methyl cellulose, methyl chloride, and methyl salicylate; and, in any concentration, sodium carbonate, sodium cyanide, sodium dichromate, sodium fluorine, sodium hydroxide, sodium phosphate (tribasic), and sodium silicate. Pure media at room temperature which attack the metal include methyl alcohol, ethyl bromide, most ammonium salts, milk, and nitrous gases. Pure chlorine and most chlorides in any concentration are corrosive, as are most heavy-metal salts, hydrogen peroxide, iodides, all nitrates, nitroglycerin, most sulfates, and tanning solutions. Methylene chloride and vinylidene chloride, however, are not generally corrosive. Because magnesium is at the anodic end of the galvanic series, it will be corroded when coupled with many other metals,necessitating protective measures. Over the years, many protective coatings and treatments have been developed to improve the metal’s suitability in many environments.

Although lithium is no longer used in magnesium alloys, a series of magnesium-lithium alloys, developed for aerospace applications and ammunition containers, were noted for their extremely light weight and moderate ultimate tensile strengths—14,500 to 36,000 lb/in2 (100 to 248 MPa). One such alloy, LA141A, has a density of only 0.045 lb/in3 (1245 kg/m3), or three-quarters that of magnesium. Melram 072, from Magnesium Elektron in England, is a silicon carbide–reinforced zinc-copper-manganese magnesium alloy for extruded tubing. Its density is 0.04 lb/in3 (1107 kg/m3) and the tensile modulus is 9.1 X 106 lb/in2 (62,745 MPa).

Magnesium-nickel is a master alloy of magnesium and nickel used for adding nickel to magnesium alloys and for deoxidizing nickel and nickel alloys. One such alloy contains about 50% of each metal, is silvery white in color, and is furnished in round bar form. Magnesium-Monel contains 50% magnesium and 50 Monel nickel-copper alloy. Alloys of magnesium with nickel, Monel, zinc, copper, or aluminum, used for deoxidizing nonferrous metals, are called stabilizer alloys.