For further information concerning the process for welding of aluminum and aluminum alloys,
refer to Military Specification MIL-W-8604 and MIL-W-6858. Specification MIL-W-6858
established requirements for equipment, classification of welds sheer strengths, surface
preparation, testing, etc. This specification also limits the joint thickness to not more than
four times the thickness of the thinnest outer layer of material and maximum ratio between
two thickness in the pile to 3-1.
3-145. BRAZING ALUMINUM . Aluminum can be brazed although it is not commonly practiced. Utilization of brazing is
limited and confined to certain specific applications.
3-146. Aluminum is brazed by using filler metal (brazing alloy) which melts at about 100° F below the melting point of
aluminum. Thus the joining action is accomplished by the adhesion and diffusion of the brazing alloy to the base
aluminum. Brazing differs from welding in that the parent metal is not melted, however, since the brazing material is an
aluminum alloy the finished joint will have a metallurgical structure similar to that obtained by welding. The strength and
corrosion resistance is also approximately the same as that by welding.
3-147. Brazing of aluminum alloys is normally confined to the none-heat-treatable alloys 11003003-3004-5050, 5052 and
heat treatable alloys 6061 and 6062.
3-148. To properly perform the brazing operation surfaces of metal must be free of grease, oil, dirt and other foreign
matter as with welding. The oxide coating of the aluminum shall be removed to the extent that brazing flux will
effectively complete the oxide removal and clean the surface for good adhesion of the filler metal. This is especially
necessary where magnesium and silicon alloyed aluminum (5050, 5052, 6053, 6062 and 6063) are involved. The natural
oxide coating of these alloys is heavy and difficult to remove by the use of brazing flux alone.
3-149. The cleaning and oxide removal can be accomplished as previously cited in paragraphs 3-108, 3-112 and 3-115.
The requirement for flux removal is reemphasized. All traces of flux shall be removed from the finished part/product to
prevent chemical attack and corrosion. Flux removal may be aided by agitating with a non-metallic bristle brush when
washing and rinsing. The parts should then be (bright dipped) chemically cleaned by immersing in a solution of sodium
hydroxide (6 oz per gallon) for 45 to 60 seconds; followed by rinsing in hot water (150°-180° F); dipping in a 10% solution
of nitric acid (HN03); completing with another hot water rinse and drying. Final rinsing in clean water is absolutely
necessary to remove the cleaning materials. The chemical cleaning process shall be accomplished in the plating shop
area or other facility specifically designed for that purpose. Personnel utilized shall be trained to handle the chemicals
involved and the particular process. Procedure cited in paragraph 3-115 may also be used.
3-150. Flux for Aluminum Brazing. Brazing fluxes consist of fluorides and chlorides which are corrosive in nature. They
will seriously attack aluminum and must be used with care and in small quantities. Flux is normally received in powder
form in air-tight containers of glass or earthen ware. The powder shall be mixed in a glass container or earthen ware
(never steel) container by adding distilled water to form a paste (1 part water to 3 parts powder for brushing). Mix only
enough flux to last for 4-8 hrs. The unused powder should be tightly sealed after the quantity required is removed to
3-151. Aluminum Brazing Method. Aluminum may be brazed by three common methods. These methods are furnace,
dip (salt bath flux) and torch. The difference in the methods primarily is the manner in which heat is applied.
Furnace Brazing. The parts to be joined (brazed) are assembled, fluxed and then heated in a furnace with
accurately controlled temperature. This method is normally used for mass production brazing operations.
Dip Brazing. The joining of the parts is accomplished by immersing the parts into a molten flux bath. The
temperature of the bath is held slightly above the melting point of the filler metal (brazing alloy) but below the melting
point of the base metal, which permits the brazing alloy to flow into the joint. Normally unless the parts are self-
supporting they must be supported with jigs. Large parts require preheating prior to dipping to prevent reduction of flux
Torch Brazing. This method of brazing is similar to torch welding. The difference being that the special brazing
metal and flux permits the joining action to be accomplished without meeting the base metal.
3-152. The flow of the brazing alloy through closely fitted parts results from capillary attraction and gravity.
3-153. Filler metal used for aluminum brazing is normally in aluminum silicon alloy. The three common alloys contain
(1) 7½ % silicon melting point 1180° F, alloy 4043(713) (2) 10% silicon melting point 1110° F, alloy 4343 (3) 12 ½ %
silicon melting point 1060° F, alloy 4047, See Federal Specification QQ-B-655.