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