HEAT TREATMENT OF NONFERROUS METALS (ALUMINUM ALLOYS) Aluminum is a white, lustrous metal, light in weight and corrosion resistant in its pure state. It is ductile, malleable,  and  nonmagnetic.  Aluminum  combined  with various  percentages  of  other  metals,  generally  copper, manganese,  and  magnesium,  form  the  aluminum  alloys that  are  used  in  aircraft  construction.  Aluminum  alloys are  lightweight  and  strong,  but  do  not  possess  the corrosion resistance of pure aluminum and are generally treated   to   prevent   deterioration.   “Alclad”   is  an aluminum alloy with a protective coating of aluminum to make it almost equal to the pure metal in corrosion resistance. Several of the aluminum alloys respond readily to heat  treatment.  In  general,  this  treatment  consists  of heating the alloy to a known temperature, holding this temperature for a definite time, then quenching the part to  room  temperature  or  below.  During  the  heating process, a greater number of the constituents of the metal are put into solid solution. Rapid quenching retains this condition,  which  results  in  a  considerable  improvement in the strength characteristics. The heating of aluminum alloy should be done in an electric  furnace  or  molten  salt  bath.  The  salt  bath generally used is a mixture of equal parts of potassium nitrate and sodium nitrate. Parts heated by this method must  be  thoroughly  washed  in  water  after  treatment.  The salt bath method of heating should never be used for complicated parts and assemblies that cannot be easily washed free of the salt. Heat Treating Procedures There are two types of heat treatment applicable to aluminum  alloys.  They  are  known  as  solution  and precipitation heat treatment. Certain alloys develop their full strength from the solution treatment, while others require both treatments for maximum strength. The NA 01-1A-9 lists the different temper designa- tions assigned to aluminum alloys and gives an example of  the  alloys  using  these  temper  designations. SOLUTION     HEAT     TREATMENT.—The solution treatment consists of heating the metal to the temperature required to cause the constituents to go into a solid solution. To complete the solution, often the metal is held at a high temperature for a sufficient time, and then quenched rapidly in cold water to retain this condition. It is necessary that solution heat treatment of aluminum  alloys  be  accomplished  within  close  limits  in reference  to  temperature  control  and  quenching.  The temperature  for  heat-treating  is  usually  chosen  as  high as possible without danger of exceeding the melting point of any element of the alloy. This is necessary to obtain  the  maximum  improvement  in  mechanical properties. If the maximum specified temperature is exceeded, eutectic melting will occur. The consequence will  be  inferior  physical  properties,  and  usually  a severely blistered surface. If the temperature of the heat treatment  is  low,  maximum  strength  will  not  be obtained. PRECIPITATION  (AGE)  HARDENING.—The precipitation treatment consists of “aging” material previously subjected to solution heat treatments by natural (occurs at room temperature) or artificial aging. Artificial aging consists of heating aluminum alloy to a specific temperature and holding for a specified length of  time.  During  this  hardening  and  strengthening operation, the alloying constituents in solid solution precipitate out. As precipitation progresses, the strength of the material increases until the maximum is reached. Further aging (overaging) causes the strength to decline until a stable condition is obtained. The strengthening of the material is due to the uniform alignment of the molecule  structure  of  the  aluminum  and  alloying element. Artificially  aged  alloys  are  usually  slightly “overaged” to increase their resistance to corrosion, especially the high copper content alloys. This is done to  reduce  their  susceptibility  to  intergranular  corrosion caused  by  underaging. Natural  aging  alloys  can  be  artificially  aged; however, it increases the susceptibility of the material to intergranular corrosion. If used, it should be limited to clad sheet and similar items. Quenching The basic purpose for quenching is to prevent the immediate  re-precipitation  of  the  soluble  constituents after  heating  to  solid  solution.  To  obtain  optimum physical properties of aluminum alloys, rapid quenching is  required.  The  recommended  time  interval  between removal from the heat and immersion is 10 seconds or less.  Allowing  the  metal  to  cool  before  quenching promotes intergranular corrosion and slightly affects the hardness.  There  are  three  methods  employed  for quenching.  The  one  used  depends  upon  the  item,  alloy, and properties desired. COLD  WATER  QUENCHING.—Small parts made from sheet, extrusions, tubing, and small fairings 15-44