as the heat source rather than the inner cone. Prepare the flux  and apply in the same manner as the aluminum welding flux,  fluxing both the base metal and filler material. Heat the parts  with the outer envelope of the flame, watching for the flux to  begin to liquefy; the filler may be applied at that point. The  filler should flow easily. If the part gets overheated, the flux  turns brown or grey. If this happens, reclean and re-flux the  part before continuing. Brazing is more easily accomplished  on 1100, 3003, and 6061 aluminum alloys. 5052 alloy is more  difficult; proper cleaning and practice are vital. There are  brazing products sold that have the flux contained in hollow  spaces in the filler metal itself, which typically work only on  1100, 3003, and 6061 alloys as the flux is not strong enough  for use on 5052. Cleaning after brazing is accomplished the  same as with oxy-fuel welding of aluminum, using hot water  and a clean stainless brush. The flux is corrosive, so every  effort should be made to remove it thoroughly and quickly  after the brazing is completed. The soldering of aluminum is much like the soldering of  other metals. The use of special aluminum solders is required,  along with the necessary flux. Aluminum soldering occurs at  temperatures below 875 °F. Soldering can be accomplished  using the oxy-acetylene, oxy-hydrogen, or even an air- propane torch setup. A neutral flame is used in the case of  either oxy-acetylene or oxy-hydrogen. Depending on the  solder and flux type, most common aluminum alloys can be  soldered. Being of lower melting temperature, a tip one or  two sizes smaller than required for welding is used, along  with a soft flame setting.  Joint configurations for aluminum soldering follow the same  guidelines as any other base material. Lap joints are preferred  to tee or butt joints due to the larger surface contact area.  However parts, such as heat exchanger tubes, are a common  exception to this. Soft solder is chiefly used to join copper and brass where a  leak proof joint is desired, and sometimes for fitting joints  to promote rigidity and prevent corrosion. Soft soldering is  generally performed only in minor repair jobs. Soft solder  is also used to join electrical connections. It forms a strong  union with low electrical resistance.  Soft soldering does not require the heat of an oxy-fuel  gas torch and can be performed using a small propane or  MAPP$^{®}$ torch, an electrical soldering iron, or in some cases, a  soldering copper, that is heated by an outside source, such as  an oven or torch. The soft solders are chiefly alloys of tin and  lead. The percentages of tin and lead vary considerably in the  various solders with a corresponding change in their melting  points ranging from 293 °F to 592 °F. Half-and-half (50/50)  is the most common general-purpose solder. It contains equal  portions of tin and lead and melts at approximately 360 °F.  To get the best results for heat transfer when using an electrical  soldering iron or a soldering copper, the tip must be clean and  have a layer of solder on it. This is usually referred to as being  tinned. The hot iron or copper should be fluxed and the solder  wiped across the tip to form a bright, thin layer of solder. Normally, the parts are cleaned as for welding or brazing, and  the flux is applied according to manufacturer’s instructions.  The parts are evenly heated with the outer envelope of the  flame to avoid overheating the flux, and the solder is applied  in a fashion similar to that for other base metals. Cleaning  after soldering may not be required to prevent oxidation  because some fluxes are not corrosive. However, it is always  advisable to remove all flux residues after soldering.  Aluminum soldering is commonly used in such applications  as the repair of heat exchanger or radiator cores originally  using a soldered joint. It is not, however, to be used as a direct  replacement repair for brazing or welding. The principle use of silver solder in aircraft work is in the  fabrication of high-pressure oxygen lines and other parts that  must withstand vibration and high temperatures. Silver solder is used extensively to join copper and its alloys,  nickel and silver, as well as various combinations of these  metals and thin steel parts. Silver soldering produces joints  of higher strength than those produced by other brazing  processes. Flux is used with soft solder for the same reasons as with  brazing. It cleans the surface area to be joined and promotes  the flow by capillary action into the joint. Most fluxes should  be cleaned away after the job is completed because they cause  corrosion. Electrical connections should be soldered only  with soft solder containing rosin. Rosin does not corrode the  electrical connection.  Flux must be used in all silver soldering operations to ensure  the base metal is chemically clean. The flux removes the film  of oxide from the base metal and allows the silver solder to  adhere to it.  Silver Soldering Soldering Aluminum Soldering 5-21