(b) Alloying - A diffusion of the constituents of the filler - copper,
zinc, tin, and others - into the base metal and a corresponding diffusion of base
metal constituents into the filler metal takes place in a narrow zone at the
interface between the copper brazing alloy and the base metal. The base metal
crystallizes at the interface and, when examined under a microscope, shows a
diffusion of the copper alloy brazing material constituents into the crystal grains
and an alloying of the filler metal with the base metal.
(c) Intergranular penetration - The action of the molten copper-alloy
filler material on the base metal surface tends to open the crystal grain structure
of the base metal surface and allows the filler metal to penetrate the base metal
along the grain boundaries.
c. Brazing rod.
(1) Brazing rod is made up of a brass alloy containing approximately 60
percent copper and 40 percent zinc.
(2) About 1 percent of tin, iron, manganese, and silicon is often added to
improve the flow characteristics, decrease fumes, deoxidize, and increase the
hardness in the finished weld.
d. Fluxes. The fluxes used for braze welding should be oxidizing in
character in order to remove oxide films from the surface of the base metal.
Otherwise, the capillary flow of the filler metal will not take place without
overheating.
e. Advantages of braze welding.
(1) This process requires less preheating, permits greater welding speed,
demands a shorter cooling-off period, and is less likely to crack metals, such as
cast iron, during the braze welding operation.
(2) There is no splash or weld spatter to worry about and low temperatures
minimize distortion.
(3) The completed joint requires little or no finishing.
(4) Brazing technique does not require as much skill as the technique
required for fusion welding.
f. Disadvantages of braze welding.
(1) If the joint is to be exposed to corrosive media, the filler metal must
have the required corrosion-resistant characteristics.
(2) All brazing alloys lose strength at elevated temperatures.
(3) If the joint is to be painted, all traces of the flux must be removed.
g. Torch brazing.
(1) The oxyacetylene flame is perhaps the most common method of heating the
parts to be brazed and has the following advantages:
(a) Initial cost of the equipment is low in comparison to electric arc
welding equipment.
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