Magnesium Alloys

(magnesium-based alloys), the strongest and most refractory alloys. They are prepared from a base of magnesium and a metal with limited solubility in solid magnesium. Because of the high chemical activity of magnesium, the number of metals suitable for the production of magnesium alloys is relatively small. Magnesium alloys are divided into two groups: foundry alloys, for the manufacture of shaped castings, and wrought alloys, for the production of semifinished products by pressing, rolling, forging, and stamping.

History. The first magnesium alloys appeared in the early 20th century under the trade name “Electron,” which is now obsolete. The alloys came to be regarded as valuable industrial structural materials in the late 1920’s and early 1930’s, nearly 100 years after the French chemist A. Bussy first isolated magnesium in pure form (1828). In general, until about 1950 only alloys based on Mg-AI-Zn and Mg-Mn systems were used. The discovery of the modifying and refining action of zirconium facilitated further progress in the development of magnesium alloys. Alloys based on Mg-Zn-Zr, Mg-rare earth-Zr (or Mn), and Mg-Th systems, as well as superlight alloys based on Mg-Li systems, came into use during the 1950’s. At present, the production of and demand for magnesium and magnesium alloys is steadily increasing. World magnesium production before World War II (1939-45) was about 50,000 tons; in 1969 it was about 2 million tons, 40-50 percent of which was used in the manufacture of casts and formed semifinished products.

Chemical composition. The chemical composition of the magnesium alloys most commonly used in the USSR is given in Table 1. Industrial magnesium alloys contain additives such as aluminum, zinc, manganese, zirconium, rare earths (misch metal, lanthanum, neodymium, yttrium), thorium, silver, cadmium, lithium, and beryllium. The total amount of additives in the most highly alloyed magnesium alloys reaches 10-14 per-cent. Nickel, iron, silicon, and copper are harmful impurities, which reduce the corrosion “esistance of magnesium alloys. The aluminum and silicon content is restricted in alloys containing zirconium, since in their presence zirconium does not dissolve in molten magnesium but rather forms insoluble refractory compounds with them. Impurities of iron, manganese, and hydrogen similarly reduce the solubility of zirconium in magnesium. Small quantities of beryllium (sometimes calcium) are used as additives to decrease the oxidizability of magnesium alloys in the molten state.

Physical properties. The physical properties of magnesium alloys are shown in Table 2. These alloys are the lightest metal structural materials available. Their density d is 1,360-2,000 kg/m³, depending on composition; magnesium-lithium alloys have the lowest density. The most widely used magnesium alloys have a density of 1,760-1,810 kg/m³, which is approximately one-quarter that of steel and two-thirds that of aluminum alloys. Because of their low density, articles made from magnesium have high rigidity. For example, the relative flexural rigidity for I-beams of identical weight and width is 1 for steel, 8.9 for aluminum, and 18.9 for magnesium.

Magnesium alloys have high specific heat. For the same quantity of absorbed heat, the surface temperature of components manufactured from magnesium alloys is one-half that of low-carbon steel components and 15-20 percent lower than that of aluminum-alloy components. The average coefficient of thermal expansion of magnesium alloys is 10-15 percent higher than that of aluminum alloys.

Mechanical properties. The mechanical properties of the magnesium alloys most commonly used in the USSR are given in Table 3. The optimum mechanical properties for foundry magnesium alloys are achieved in high-strength alloys based on the Mg-Zn-Ag-Zr system: yield point σ_0.2 = 260-280 meganewtons per sq m (MN/m²), or 26-28 kilograms-force per sq mm (kgf/mm²); tensile strength σ_b = 340-360 MN/m² (34-36 kgf/mm²); relative elongation δ = 5 percent. Special production processes (for example, understamping) make possible an increase in σ_b to 400-420 MN/m² (40-42 kgf/mm²). The properties of wrought magnesium alloys of highest strength are as follows: σ_0.2 = 350 MN/m² (35 kgf/mm²), σ_b = 420 MN/m² (42 kgf/mm²), σ = 5 percent. The maximum working temperature for high-strength alloys is 150°C. The most heat-resistant foundry and wrought magnesium alloys, based on Mg-rare earth and Mg-Th systems, are suitable for prolonged working at 300°-350°C and brief working at up to 400°C. High-strength cast magnesium alloys have certain advantages over aluminum alloys in terms of specific strength (σ_b/d), and the strongest wrought magnesium alloys are equal to (or somewhat inferior to) the strongest wrought aluminum alloys. Magnesium alloys have an elastic modulus of 41-45 giganewtons per sq m (GN/m²), or 4,100-4,500 kgf/mm² (60 percent of the modulus of aluminum alloys, 20 percent that of steel), and a shear modulus of 16-16.5 GN/m², or 1,600-1,650 kgf/mm². At low temperatures the elastic modulus, yield point, and tensile strength of magnesium alloys increase, and elongation and resilience are reduced; no marked decrease in plasticity (characteristic of low-alloy structural steel) is observed in magnesium alloys.

Technology. Because of magnesium’s great affinity for oxygen, the surface of the molten metal is protected with a layer of flux during the melting of magnesium alloys in air. Various mixtures containing fluorides and chlorides of alkali, as well as alkaline-earth metals, are used as flux. To prevent the metal from burning during casting, protective additives are introduced into the molding sand and the chillmolds are coated with special paints (for example, paints containing boric acid). All casting methods are used (for example, sand, shell, bar, and plaster molding, chill casting, pressure-die casting, the lost-wax method, and semiliquid stamping). For high-grade castings, the gating system is based on the principle of expanding flow.

Magnesium alloys exhibit great shrinkage (1.1-1.5 percent) upon solidification. Because of their microgranular structure, castings made from magnesium alloys containing zirconium have better and more uniform mechanical properties than castings made from alloys containing aluminum. Machining, welding, riveting, sheet-metal stamping, and closed-impression die forging are used to manufacture various components and joints from wrought magnesium alloys. These alloys have low technological plasticity at room temperature because of the hexagonal lattice structure of magnesium (slip occurs along one base plane). At high temperatures (200°-450°C) slip is observed along the supplementary planes, and the technological plasticity of most alloys increases considerably. Therefore, all pressure shaping operations on magnesium alloys are carried out in the heated state at low rates of deformation. Magnesium alloys containing 10-14 percent lithium are an exception, since they have a cubic body-centered lattice and can be worked when they are in the cold state.

Sharp notches and abrupt changes in profile are avoided in the manufacture of magnesium-alloy components. Various types of welding, riveting, and hard and soft soldering, as well as cementing, are used to join the components. Defects in cast components are corrected by welding, but alloys with a high zinc content are not welded. Most foundry and wrought semifinished products made from magnesium alloys undergo reinforcing heat treatment (hardening or aging) or annealing to relax internal stresses (casting or welding stresses). Magnesium alloys are easily cut— for example, twice as fast as aluminum alloys and ten times faster than carbon steel. Fire-safety regulations must be observed when working with magnesium alloys.

Protection from physicochemical effects. Magnesium alloys have low corrosion resistance because of their high electronegative potential and the insufficient protective properties of the natural oxide film. Such alloys are protected from corrosion by the application of artificially produced inorganic chemical or electrochemical films, in combination with paint and varnish coatings composed of a prime passivating layer and external varnish or enamel films. Proper protection ensures the reliable operation of magnesium-alloy components under atmospheric

conditions and in alkaline mediums, mineral oils, gasoline, and kerosine. Magnesium alloys of high purity (particularly in terms of iron and nickel content) are suitable for operation in marine air.

Magnesium alloys cannot be used in seawater, saline solutions, or acids, their solutions, or their vapors. The corrosion resistance of magnesium components is largely dependent upon the selection of the proper structural shape (to eliminate accumulation of moisture) and a combination of contiguous materials in the components that does not induce contact corrosion. Certain high-strength wrought magnesium alloys are prone to corrosion under stress and may be used under conditions designed to limit the magnitude of long-term tensile stresses.

Magnesium-alloy components and semifinished products are stored with the aid of chromate films, liquid neutral dehydrated oils, and special lubricants, depending on the conditions and period of storage. For prolonged storage, assembled components and spare parts manufactured from magnesium alloys paint and varnish coatings are placed in sheaths made of poly vinyl chloride or polyethylene film, with a silica-gel dessicant.

Uses. Magnesium alloys are suitable for use at cryogenic, normal, and high temperatures. Owing to their low density and high specific strength and ability to absorb impact and vibrational energy, as well as their excellent workability by cutting, they are widely used in industry, primarily to reduce the weight of components and increase their rigidity.

Magnesium alloys are used in the production of motor vehicles and tractors (crankcases, gearboxes, and wheel drums), electrical and radio engineering (instrument cases and parts of electric motors), optics (binocular and camera cases), the textile industry (bobbins, spools, and reels), printing (matrices, engraving plates, and rollers), shipbuilding (protectors), and aircraft and rocket engineering (wheel parts, control elements, parts of airplane wings, and engine block components). Cast components made from magnesium alloys are mainly used for industrial purposes. The area of use of magnesium alloys is limited only by their reduced corrosion resistance in certain mediums.