However, that definition has been expanded to include any  metal joining process in which the bonding material is a  nonferrous metal or alloy with a melting point higher than  800 °F, but lower than that of the metals being joined.  Brazing requires less heat than welding and can be used to  join metals that may be damaged by high heat. However,  because the strength of a brazed joint is not as great as that  of a welded joint, brazing is not used for critical structural  repairs on aircraft. Also, any metal part that is subjected to  a sustained high temperature should not be brazed. Brazing is applicable for joining a variety of metals, including  brass, copper, bronze and nickel alloys, cast iron, malleable  iron, wrought iron, galvanized iron and steel, carbon steel,  and alloy steels. Brazing can also be used to join dissimilar  metals, such as copper to steel or steel to cast iron.  When metals are joined by brazing, the base metal parts are  not melted. The brazing metal adheres to the base metal by  molecular attraction and intergranular penetration; it does  not fuse and amalgamate with them.  In brazing, the edges of the pieces to be joined are usually  beveled as in welding steel. The surrounding surfaces must be  cleaned of dirt and rust. Parts to be brazed must be securely  fastened together to prevent any relative movement. The  strongest brazed joint is one in which the molten filler metal  is drawn in by capillary action, requiring a close fit. A brazing flux is necessary to obtain a good union between  the base metal and the filler metal. It destroys the oxides and  floats them to the surface, leaving a clean metal surface free  from oxidation. A brazing rod can be purchased with a flux  coating already applied, or any one of the numerous fluxes  available on the market for specific application may be used.  Most fluxes contain a mixture of borax and boric acid.  The base metal should be preheated slowly with a neutral soft  flame until it reaches the flowing temperature of the filler  metal. If a filler rod that is not precoated with flux is used,  heat about 2 inches of the rod end with the torch to a dark  purple color and dip it into the flux. Enough flux adheres to  the rod that it is unnecessary to spread it over the surface  of the metal. Apply the flux-coated rod to the red-hot metal  with a brushing motion, using the side of the rod; the brass  flows freely into the steel. Keep the torch heat on the base  metal to melt the filler rod. Do not melt the rod with the torch.  Continue to add the rod as the brazing progresses, with a  rhythmic dipping action so that the bead is built to a uniform  width and height. The job should be completed rapidly and  with the fewest possible passes of the rod and torch. Notice that some metals are good conductors of heat and  dissipate the heat more rapidly away from the joint. Other  metals are poor conductors that tend to retain the heat and  overheat readily. Controlling the temperature of the base  metal is extremely important. The base metal must be hot  enough for the brazing filler to flow, but never overheated  to the filler boiling point. This causes the joint to be porous  and brittle.  The key to even heating of the joint area is to watch the  appearance of the flux. The flux should change appearance  uniformly when even heat is being applied. This is especially  important when joining two metals of different mass or  conductivity.  The brazing rod melts when applied to the red-hot base  metal and runs into the joint by capillary attraction. (Note  that molten brazing filler metal tends to flow toward the  area of higher temperature.) In a torch heated assembly,  the outer metal surfaces are slightly hotter than the interior  joint surfaces. The filler metal should be deposited directly  adjacent to the joint. Where possible, the heat should be  applied to the assembly on the side opposite to where the  filler is applied because the filler metal tends to flow toward  the source of greater heat. After the brazing is complete, the assembly or component  must be cleaned. Since most brazing fluxes are water soluble,  a hot water rinse (120 °F or hotter) and a wire brush remove  the flux. If the flux was overheated during the brazing  process, it usually turns green or black. In this case, the flux  needs to be removed with a mild acid solution recommended  by the manufacturer of the flux in use.  Torch brazing of aluminum is done using similar methods  as brazing of other materials. The brazing material itself is  an aluminum/silicon alloy having a slightly lower melting  temperature than the base material. Aluminum brazing occurs  at temperatures over 875 °F, but below the melting point of  the parent metal. This is performed with a specific aluminum  brazing flux. Brazing is best suited to joint configurations that  have large surface areas in contact, such as the lap, or for fitting  fuel tank bungs and fittings. Either acetylene or hydrogen may  be used as fuel gas, both being used for production work for  many years. Using eye protection that reduces the sodium  flare, such as the TM2000 lens, is recommended.  When using acetylene, the tip size is usually the same, or one  size smaller than that used for welding of aluminum. A 1–2X  reducing flame is used to form a slightly cooler flame, and the  torch is held back at a greater distance using the outer envelope  Torch Brazing of Aluminum 5-20