Magnesium has long been known as the lightest of our engineering metals. This metal, silvery white in color, has a specific gravity of only 1.74. Aluminum, the next lightest structural metal, is l.5 times heavier; zinc is 4 times heavier; iron and steel are 4.5 times heavier; and copper and nickel are 5 times heavier. Magnesium does not occur in the free state but is very abundant in nature, constituting 2.5 pct of the earth's crust in the form of various ores. It is the third most abundant structural metal, being exceeded only by iron and aluminum.

Magnesium is unique, however, in that in the form of magnesium chloride it also exists in the oceans. Sea water is the source most widely used for production in the United States but magnesium is also commercially produced from magnesite, dolomite, and other ores as well as from certain inland brines.

Not only is magnesium potentially very abundant but it is in addition a very versatile metal and can be shaped and worked by practically all methods known to the art of metal working. It can be cast by sand, die, and the various permanent-mold methods; extruded into an endless variety of shapes and rolled into sheet, plate, and strip. Magnesium is readily forged and can be formed into useful shapes by drawing, bending, spinning, impact extrusion, and other standard methods. The joining of magnesium is accomplished by gas, arc and electric-resistance welding methods and by brazing, bolting, and riveting. The machinability of magnesium is unsurpassed by that of any other structural metal and magnesium is often selected for an application because of this characteristic. The chemical and electrochemical properties of mag­nesium also provide the bases for important commercial uses that will be discussed later.

Because of the two outstanding characteristics, availability and workability, magnesium is now considered to be destined to become one of the world's common structural metals. This is in contrast to conditions existing only about a decade ago, before sea-water plants had been developed and before common metal-working techniques had been sufficiently adapted to magnesium fabrication.

Production Processes. Two types of production processes are in use in the United States today for the extraction of, metallic magnesium. One is based on the electrolytic decomposition of fused magnesium chloride and the other is based on the thermal reduction of magnesium oxide by an agent such as ferrosilicon. Sea water containing about 0.13 pct magnesium is pumped into large settling tanks, where it is mixed with lime produced by roasting oyster shells that are dredged from the bottom of the ocean. Reaction of the lime with magnesium chloride in sea water yields magnesium hydroxide. The magnesium hydroxide cake is. converted into magnesium chloride by reaction with hydrochloric acid. The magnesium chloride is evaporated to dryness and is fed to electrolytic cells where it is decomposed by electricity into magnesium metal and chlorine. In the ferrosilicon process, dolomite ore is calcined and mixed with pulverized ferrosilicon and then briquetted. The briquetted pellets are heated under vacuum in large cylindrical retorts, which attain a temperature of 2,200°F at the hot end. The silicon reacts with the magnesium oxide to yield silicon dioxide and metallic magnesium vapor. This vapor condenses into magnesium crystals at the cooled end of the retort. These crystals in turn must be remelted and cast into solid ingots.

Workability. The workability of magnesium is an important factor in its practical utilization in many fields of use. The term "workability" as used here refers to the facility with which the metal can be converted into useful forms such as castings, extrusions and sheet, and the ease of additional fabrication into final shape by machining, drawing, forging, bending and other processes.

Casting. Most metals can be fabricated by casting processes, of which casting in sand molds is probably the oldest and most common. Magnesium, however, has been cast by practically all known methods.

These include sand, permanent mold, die, plaster mold, investment, and centrifugal casting. The sand casting process has been used for years but it has been difficult for magnesium sand castings to be price competitive with other cast metals. Low metal efficiency and the need for special fluxes and sand addition agents account for this extra cost. Die casting of magnesium, on the other hand, is competitive with the die casting of other metals such as aluminum and zinc. Molten magnesium can be handled in iron equipment without reaction with the iron and as a consequence there is no corrosion of the equipment, nor is there any iron pickup in the alloy. This advantage permits pumping and metering arrangements so that production die-casting operations can be mechanized. The fact that molten magnesium does not attack the die allows rapid operation of the die-casting machine because a minimum of die surface treatment is required. Magnesium does not solder to the die and it is therefore possible to cast small cored holes and slots without draft. Dimensional accuracy is good and tolerances of 0.0015 in. per inch are readily maintained on production runs. Magnesium die castings have good dimensional stability and are not subject to growth.

Extruding. Magnesium is readily extruded on conventional hydraulic extrusion presses into an almost endless variety of shapes. These include round rods, square bars, angles, channels, I-beams, zees, tees and other standard structural shapes. By means of the extrusion process, shapes can be produced that are designed to replace assemblies of smaller parts. A substantial saving in cost can thus be accomplished.

The most commonly used method of extrusion begins with a solid cast ingot, which is placed into the heated container of the extrusion press. Pressures up to 100,000 psi are applied to force the metal through a steel die. Shapes emerge from the die at speeds ranging from 5 ft. to as much as 100 ft. per minute, depending upon the shape and size of the section. The extrusion of powdered magnesium is also possible. In this process, atomized magnesium particles are charged into the container of the extrusion press and are compressed and extruded from the die as solid metal possessing unique properties. The process makes it possible to produce alloys that cannot be made by melting and casting in the ordinary manner. Magnesium-aluminum alloys containing zirconium are an example, since zirconium will not alloy with molten magnesium when aluminum is present. However, powdered alloys of magnesium-aluminum and magnesium-zirconium can be co-extruded. A heat treatment then serves to produce an "interference" precipitation with a resultant increase in strength and hardness. Similarly, it is possible by the co-extrusion of metal powders to produce a dispersion of anodic and cathodic constituents which serves to give the alloy corrosion protection by electrochemical action.

Rolling. For many years, magnesium sheet and plate were produced by rather primitive methods, consisting of initial breakdown on two-high hot breakdown mills with frequent retreating between passes, until a thickness of 10 to 20 pct of the final gage was reached. At this point, a short-time anneal was given followed by cold-rolling on a two-high or preferably four-high finishing mill. This method of rolling was practiced by all mills rolling magnesium in the United States through 1951. However, shortly after World War II, pilot-plant tests on continuous rolling were carried out and the tests were successful.

Machining. The unsurpassed machinability of magnesium permits machining operations at extremely high speeds — usually at the maximum obtainable on modern machine equipment. Heavier depths of cut and higher rates of feed than are used on other metals are possible and the life of high-speed-steel cutting tools on magnesium equals the life of carbide-tipped tools when machining other metals. There is no tendency for magnesium to tear or drag during the machining operation and as a result an excellent surface finish can be obtained. Close dimensional tolerances can be held and the free-cutting characteristics of magne­sium produce well-broken chips which do not obstruct the cutting tool or the machine. Indicative of these good machining characteristics of magnesium is the fact that the power required to remove a given amount of metal is lower for magnesium than for any other structural metal.

Joining. Most of the common joining methods used with other metals are used on magnesium. Bolting, riveting, brazing, and the various types of welding are applicable. The welding methods used include arc, gas, and electric resistance; spot, seam, and flash welding. Arc welding probably is the most widely used method of joining magnesium by fusion methods and. with this process welds are made in plate as thick as 0.5 in. in just one pass. Another method of joining, which first found its most extensive use in the aircraft industry, is adhesive bonding. This method of joining is particularly well adapted to magnesium. Brazing is a relatively new development in joining methods for magnesium and, although limited as to alloys that can be joined, the process has been successfully used in commercial practice. Soldering has had only limited use as a method of joining magnesium, but this process is used as a means of filling dents and other surface imperfections.