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