velocities encountered in jet power units. These alloys are chemically similar to the previously mentioned steels, but may also contain cobalt, copper, and columbium in varied amounts as alloying elements. NONFERROUS AIRCRAFT METALS The term nonferrous refers to all metals that have elements other than iron as their principal constituent. This group includes aluminum, titanium, copper, magnesium, and their alloys; and in addition, such alloy metals as Monel and Babbitt. Aluminum and Aluminum Alloys Commercially pure aluminum is a white, lustrous metal, light in weight and corrosion resistant. Aluminum combined with various percentages of other metals (generally copper, manganese, magnesium, and chromium) form the alloys that are used in aircraft construction. Aluminum alloys in which the principal alloying ingredients are either manganese, magnesium, or chromium, or magnesium and silicon, show little attack in corrosive environments. On the other hand, those alloys in which substantial percentages of copper are used are more susceptible to corrosive action. The total percentage of alloying elements is seldom more than 6 or 7 percent in the wrought aluminum alloys. TYPES, CHARACTERISTICS, AND USES.— Aluminum is one of the most widely used metals in modern aircraft construction. It is vital to the aviation industry because of its high strength/weight ratio, its corrosion-resisting qualities, and its comparative ease of fabrication. The outstanding characteristic of aluminum is its light weight. In color, aluminum resembles silver, although it possesses a characteristic bluish tinge of its own. Commercially pure aluminum melts at the comparatively low temperature of 1,216°F. It is nonmagnetic, and is an excellent conductor of electricity. Commercially pure aluminum has a tensile strength of about 13,000 psi, but by rolling or other cold-working processes, its strength may be approximately doubled. By alloying with other metals, together with the use of heat-treating processes, the tensile strength may be raised to as high as 96,000 psi, or to well within the strength range of structural steel. Aluminum alloy material, although strong, is easily worked, for it is very malleable and ductile. It may be rolled into sheets as thin as 0.0017 inch or drawn into wire 0.004 inch in diameter. Most aluminum alloy sheet stock used in aircraft construction ranges from 0.016 to 0.096 inch in thickness; however, some of the larger aircraft use sheet stock that may be as thick as 0.0356 inch. One disadvantage of aluminum alloy is the difficulty of making reliable soldered joints. Oxidation of the surface of the heated metal prevents soft solder from adhering to the material; therefore, to produce good joints of aluminum alloy, a riveting process is used. Some aluminum alloys are also successfully welded. The various types of aluminum maybe divided into two classes-casing alloys (those suitable for casting in sand, permanent mold, and die castings) and the wrought alloys (those that may be shaped by rolling, drawing, or forging). Of the two, the wrought alloys are the most widely used in aircraft construction, being used for stringers, bulkheads, skin, rivets, and extruded sections. Casting alloys are not extensively used in aircraft. WROUGHT ALLOYS.—Wrought alloys are divided into two classes-nonheat treatable and heat treatable. In the nonheat-treatable class, strain hardening (cold-working) is the only means of increasing the tensile strength. Heat-treatable alloys may be hardened by heat treatment, by cold-working, or by the application of both processes. Aluminum products are identified by a universally used designation system. Under this arrangement, wrought aluminum and wrought aluminum alloys are designated by a four-digit index system. The first digit of the designation indicates the major alloying element or alloy group, as shown in table 1-2. The lxxx indicates aluminum of 99.00 percent or greater; 2xxx indicates an aluminum alloy in which copper is the major alloying element; 3xxx indicates an aluminum alloy with manganese as the major alloying element; etc. Although most aluminum alloys contain several alloying elements, only one group (6xxx) designates more than one alloying element. In the 1xxx group, the second digit in the designation indicates modifications in impurity limits. If the second digit is zero, it indicates that there is no special control on individual impurities. The last two of the four digits indicate the minimum aluminum percentage. Thus, alloy 1030 indicates 99.30 percent aluminum without special control on impurities. Alloys 1130, 1230, 1330, etc., indicate the same aluminum purity with special control on one or more impurities. Likewise, 1075, 1175, 1275, etc., indicate 99.75 percent aluminum. 1-30