Magnesium alloys names are often given by two letters following by two numbers. Letters tells main alloying elements (A = aluminum, Z = zinc, M = manganese, S = silicon). Numbers tells nominal compositions of main alloying elements respectively. Marking AZ91 mean magnesium alloy where is roughly 9 weight percent aluminum and 1 weight percent zinc. Exact composition should be confirmed from the standards.
The designation system for magnesium alloys is not as well standardized as in the case of steels or aluminums, but most producers follow a system using one or two prefix letters, two or three numerals, and a suffix letter. The prefix letters designate the two principal alloying metals according to the following format developed in ASTM specification B275
A aluminum H thorium O silver B bismuth K zirconium R chromium C copper L beryllium S silicon D cadmium M manganese T tin E rare earth N nickel Z zinc F iron P icad
Aluminum, zinc, zirconium, and thorium promote precipitation hardening: manganese improves corrosion resistance; and tin improves castability. Aluminum is the most common alloying element. The numerals correspond to the rounded off percentage of the two main alloy elements, proceeding alphabetically as compositions become standard. Tepmer designation is much the same as in the case of aluminum. Using –F, -O, -H1, -T4, -T5, and –T6. Sand permanent-mold, and die casting are all well developed for magnesium alloys, die casting being the most popular. Athough magnesium is about twice as expensive as aluminum, its hot-chamber die-casting process is easier, more economical, and 40% to 50% faster than cold-chamber process required for aluminum. Forming behavior is poor at room temperature, but most conventional processes can be performed when the material is heated to temperatures of 450-700 F. As these temperatures are easily attained and generally do not require a protective atmosphere. Many formed and drawn magnesium products are manufactured. The machinability of magnesium alloys is the best of any commercial metal, and in many applications, the savings in machining costs more than compensate for the increased cost of the material it is necessary, however, to keep the tools sharp and to provide ample space for the chips. Magnesium alloys can be spot-welded nearly as easily as aluminum, but scratch brushing or chemical cleaning is necessary before the weld is formed fusion welding is carried out most easily by processes using an inert shielding atmosphere of argon or helium gas. Considerable misinformation exists regarding the fire hazard in processing magnesium alloys. It is true that magnesium alloys are highly combustible when in a finely divided form, such as powder or fine chips, and this hazard should never be ignored. Above 800 F, a noncombustible, oxygen-free atmosphere is required to suppress burning. Casting operations often require additional precautions because of the reactivity of magnesium with sand and water in sheet, bar, extruded, or cast form, however, magnesium alloys present no real fire hazard.
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