Magnesium alloy

Mixture of magnesium with other metals

Magnesium alloys are mixtures of magnesium (the lightest structural metal) with other metals (called an alloy), often aluminium, zinc, manganese, silicon, copper, rare earths and zirconium. Magnesium alloys have a hexagonal lattice structure, which affects the fundamental properties of these alloys. Plastic deformation of the hexagonal lattice is more complicated than in cubic latticed metals like aluminium, copper and steel; therefore, magnesium alloys are typically used as cast alloys, but research of wrought alloys has been more extensive since 2003. Cast magnesium alloys are used for many components of modern automobiles and have been used in some high performance vehicles; die cast magnesium is also used for camera bodies and components in lenses. Practically, all the commercial magnesium alloys manufactured in the United States contain aluminium (3 to 13 percent) and manganese (0.1 to 0.4 percent). Many also contain zinc (0.5 to 3 percent) and some are hardenable by heat treatment. All the alloys may be used for more than one product form, but alloys AZ63 and AZ92 are most used for sand castings, AZ91 for die castings, and AZ92 generally employed for permanent mold castings (while AZ63 and A10 are sometimes also used in the latter application as well). For forgings, AZ61 is most used, and here alloy M1 is employed where low strength is required and AZ80 for highest strength. For extrusions, a wide range of shapes, bars, and tubes are made from M1 alloy where low strength suffices or where welding to M1 castings is planned. Alloys AZ31, AZ61 and AZ80 are employed for extrusions in the order named, where increase in strength justifies their increased relative costs. Magnox (alloy), whose name is an abbreviation for "magnesium non oxidizing", is 99% magnesium and 1% aluminium, and is used in the cladding of fuel rods in magnox nuclear power reactors. Magnesium alloys are referred to by short codes (defined in ASTM B275) which denote approximate chemical compositions by weight. For example, AS41 has 4% aluminum and 1% silicon; AZ81 is 7.5% aluminium and 0.7% zinc. If aluminium is present, a manganese component is almost always also present at about 0.2% by weight which serves to improve grain structure; if aluminum and manganese are absent, zirconium is usually present at about 0.8% for this same purpose. Magnesium is a flammable material and must be handled carefully.

Alloy name Proportion (%) Other metals Notes Mg Al Zn Si Mn AE44 92 4 4% mischmetal Mischmetal an alloy of rare earth elements with approximately 50% cerium and 25% lanthanum AJ62A 89.8–91.8 5.6–6.6 0.2 0.08 0.26–0.5 2.1–2.8% Sr, <0.1% each of Be, Cu, Fe, Ni High temperature engine Mg alloy WE43 93.6 Y 4%, Nd 2.25%, 0.15% Zr Used in aircraft and high performance vehicles, tensile strength 250 MPa AZ8191.07 8.11 0.64 0.18 AZ31B 96 2.5–3.5 0.7–1.3 <0.05 0.2 ? Wrought alloy, good strength and ductility corrosion resistance, weldability, extrusion AMCa602 91.5 6 0.1 0.35 2% Ca Non combustible Mg alloy AM60 93.5 6 0.1 0.35 AZ91 90.8 8.25 0.63 0.035 0.22 Cu – 0.003; Fe – 0.014; Be – 0.002 Used for die castings QE22 2.5% Ag, 2% RE, 0.6% Zr Magnox (Al 80) 99.2 0.8 Non oxidizing Mg alloy

Magnesium's particular merits are similar to those of aluminium alloys: low specific gravity with satisfactory strength. Magnesium provides advantages over aluminium, in being of even lower density (≈ 1.8 g/cm3) than aluminium (≈ 2.8 g/cm3). The mechanical properties of magnesium alloys tend to be below those of the strongest of the aluminium alloys. The strength to weight ratio of the precipitation hardened magnesium alloys is comparable with that of the strong alloys of aluminium or with the alloy steels. Magnesium alloys, however, have a lower density, stand greater column loading per unit weight and have a higher specific modulus. They are also used when great strength is not necessary, but where a thick, light form is desired, or when higher stiffness is needed. Examples are complicated castings, such as housings or cases for aircraft, and parts for rapidly rotating or reciprocating machines. Such applications can induce cyclic crystal twinning and detwinning that lowers yield strength under loading direction change. The high temperature properties of magnesium alloys are relevant for automotive and aerospace applications, where slowing creep plays an important role in material lifetime. Magnesium alloys generally have poor creep properties; this shortcoming is attributed to the solute additions rather than the magnesium matrix since pure magnesium shows similar creep life as pure aluminum, but magnesium alloys show decreased creep life compared to aluminum alloys. Addition of small amounts of zinc in Mg RE alloys has been shown to increase creep life by 600% by stabilizing precipitates on both basal and prismatic planes through localized bond stiffening. or using appropriate coatings.

Magnesium alloys harden rapidly with any type of cold work, and therefore cannot be extensively cold formed without repeated annealing. Sharp bending, spinning, or drawing must be done at about , although gentle bending around large radii can be done cold. Slow forming gives better results than rapid shaping. Press forging is preferred to hammer forging, because the press allows greater time for metal flow. The plastic forging range is Metal worked outside this range is easily broken due to lack of available deformation mechanisms.

Magnesium alloys, especially precipitation hardened alloys, are used in casting. Sand, permanent mold and die casting methods are used, but plaster of Paris casting has not yet been perfected. Sand casting in green sand molds requires a special technique, because the magnesium reacts with moisture in the sand, forming magnesium oxide and liberating hydrogen. The oxide forms blackened areas called burns on the surface of the casting, and the liberated hydrogen may cause porosity. Inhibitors such as sulfur, boric acid, ethylene glycol, or ammonium fluoride are mixed with the damp sand to prevent the reaction. All gravity fed molds require an extra high column of molten metal to make the pressure great enough to force gas bubbles out of the casting and make the metal take the detail of the mold. The thickness of the casting wall should be at least 5/32 in. under most conditions. Extra large fillets must be provided at all re entrant corners, since stress concentration in magnesium castings are particularly dangerous. Permanent mold castings are made from the same alloys and have about the same physical properties as sand castings. Since the solidification shrinkage of magnesium is about the same as that of aluminium, aluminium molds can often be adapted to make magnesium alloy castings (although it may be necessary to change the gating). Pressure cold chamber castings are used for quantity production of small parts. The rapid solidification caused by contact of the fluid metal with the cold die produces a casting of dense structure with excellent physical properties. The finish and dimensional accuracy are very good, and machining is necessary only where extreme accuracy is required. Usually these castings are not heat treated.

Many standard magnesium alloys are easily welded by gas or resistance welding equipment, but cannot be cut with an oxygen torch. Magnesium alloys are not welded to other metals, because brittle inter metallic compounds may form, or because the combination of metals may promote corrosion. Where two or more parts are welded together, their compositions must be the same. Soldering of magnesium alloys is feasible only for plugging surface defects in parts. The solders are even more corrosive than with aluminium, and the parts should never be required to withstand stress. Riveted joints in magnesium alloy structures usually employ aluminium or aluminium magnesium alloy rivets. Magnesium rivets are not often used because they must be driven when hot. The rivet holes should be drilled, especially in heavy sheet and extruded sections, since punching tends to give a rough edge to the hole and to cause stress concentrations.

A particular attraction of magnesium alloys lies in their extraordinarily good machining properties, in which respect they are superior even to screwing brass. The power required in cutting them is small, and extremely high speeds (5000 ft per min in some cases) may be used. The best cutting tools have special shapes, but the tools for machining other metals can be used, although somewhat lower efficiency results. When magnesium is cut at high speed, the tools should be sharp and should be cutting at all times. Dull, dragging tools operating at high speed may generate enough heat to ignite fine chips. Since chips and dust from grinding can therefore be a fire hazard, grinding should be done with a coolant, or with a device to concentrate the dust under water. The magnesium grinder should not be used also for ferrous metals, since a spark might ignite the accumulated dust. If a magnesium fire should start, it can be smothered with cast iron turnings or dry sand, or with other materials prepared especially for the purpose. Water or liquid extinguishers should never be used, because they tend to scatter the fire. Actually, it is much more difficult to ignite magnesium chips and dust than is usually supposed, and for that reason they do not present great machining difficulties. The special techniques that must be used in fabricating magnesium (working, casting, and joining) add considerably to the manufacturing cost. In selecting between aluminium and magnesium or a given part, the base cost of the metal may not give much advantage to either, but usually the manufacturing operations make magnesium more affordable.

Scandium and gadolinium have been tried as alloying elements; an alloy with 1% manganese, 0.3% scandium and 5% gadolinium offers almost perfect creep resistance at 350C. The physical composition of these multi component alloys is complicated, with plates of intermetallic compounds such as Mn2Sc forming. Addition of zinc to Mg RE alloys has been shown to greatly increase creep life by stabilizing RE precipitates.

Adding 10% of lithium to magnesium produces an alloy that can be used as an improved anode in batteries with a manganese dioxide cathode. Magnesium lithium alloys are generally soft and ductile, and the density of 1.4 g/cm3 is appealing for space applications. Quantum mechanical simulations have been used to predict the formation of ordered magnesium–lithium alloys. Of interest relative to manufacturing, it is predicted that addition of more than 13 at. % lithium results in ordered phases with a cubic structure.

Adding 2% of calcium by weight to magnesium alloy AM60 results in the non combustible magnesium alloy AMCa602. The higher oxidation reactivity of calcium causes a coat of calcium oxide to form before magnesium ignites. The ignition temperature of the alloy is elevated by 200–300 K. An oxygen free atmosphere is not necessary for machining operations.