fuel tanks, fairings, oil tanks, and for the repair of wing tips and tanks. Alloy 3003 is similar to 1100 and is generally used for the same purposes. It contains a small percentage of manganese and is stronger and harder than 1100, but retains enough work ability that it is usually preferred over 1100 in most applications. Alloy 5052 is used for fuel lines, hydraulic lines, fuel tanks, and wing tips. Substantially higher strength without too much sacrifice of workability can be obtained in 5052. It is preferred over 1100 and 3003 in many applications. Alclad is the name given to standard aluminum alloys that have been coated on both sides with a thin layer of pure aluminum. Alclad has very good corrosion-resisting qualities and is used exclusively for exterior surfaces of aircraft. Alclad sheets are available in all tempers of 2014, 2017, 7075, and 7178. CASTING ALLOYS.—Aluminum casting alloys, like wrought alloys, are divided into two groups. In one group, the physical properties of the alloys are determined by the elements added and cannot be changed after the metal is cast. In the other group, the elements added make it possible to heat-treat the casting to produce desired physical properties. The casting alloys are identified by a letter pre- ceding the alloy number. This is exactly opposite from the case of wrought alloys, in which the letters follow the number. When a letter precedes a number, it indicates a slight variation in the composition of the original alloy. This variation in composition is made simply to impart some desirable quality. In casting alloy 214, for example, the addition of zinc, to increase its pouring qualities, is designated by the letter A in front of the number, thus creating the designation A214. When castings have been treated, the heat treatment and the composition of the casting are indicated by the letter T and an alloying number. An example of this is the sand casting alloy 355, which has several different compositions and tempers and is designated by 355-T6, 355-T51, and A355-T51. Aluminum alloy castings are produced by one of three basic methods-sand mold, permanent mold, and die cast. In casting aluminum, in most cases, different types of alloys must be used for different types of castings. Sand castings and die castings require different types of alloys than those used in permanent molds. SHOP CHARACTERISTICS OF ALUMINUM ALLOYS.—Aluminum is one of the most readily workable of all the common commercial metals. It can be fabricated readily into a variety of shapes by any conventional method; however, formability varies a great deal with the alloy and temper. In general, the aircraft manufacturers form the heat-treatable alloys in the -0 or -T4 condition before they have reached their full strength. They are subsequently heat-treated or aged to the maximum strength (-T6) condition before installation in aircraft. By this combination of processes, the advantage of forming in a soft condition is obtained without sacrificing the maximum obtainable strength/weight ratio. Aluminum is one of the most readily weldable of all metals. The nonheat-treatable alloys can be welded by all methods, and the heat-treatable alloys can be successfully spot welded. The melting point for pure aluminum is 1,216°F, while various aluminum alloys melt at slightly lower temperatures. Aluminum products do not show any color changes when heated, even up to the melting point. Riveting is the most reliable method of joining stress-carrying parts of heat-treated aluminum alloy structures. Titanium and Titanium Alloys Titanium and titanium alloys are used chiefly for parts that require good corrosion resistance, moderate strength up to 600°F, and lightweight. TYPES, CHARACTERISTICS, AND USES.— Titanium alloys are being used in quantity for jet engine compressor wheels, compressor blades, spacer rings, housing compartments, and airframe parts such as engine pads, ducting, wing surfaces, fire walls, fuselage skin adjacent to the engine outlet, and armor plate. In view of titanium’s high melting temperature, approximately 3,300°F, its high-temperature properties are disappointing. The ultimate and yield strengths of titanium drop fast above 800°F. In applications where the declines might be tolerated, the absorption of oxygen and nitrogen from the air at temperatures above 1,000°F makes the metal so brittle on long exposure that it soon becomes worthless. Titanium has some merit for short-time exposure up to 2,000°F where strength is not important, as in aircraft fire walls. Sharp tools are essential in machining techniques because titanium has a tendency to resist or back away from the cutting edge of tools. It is readily welded, but the tendency of the metal to absorb oxygen, nitrogen, and hydrogen must never be ignored. Machine welding 1-32