Quantcast Brazing

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MIL-HDBK-419A
Conventional welding should be performed only by appropriately trained and qualified personnel. Consequently,
increased labor costs can be expected. In many instances, also, the welding of bonds can be much slower than
the installation of fasteners such as bolts or rivets. In such cases, the added costs of welding may force the use
of alternate bonding techniques.
An effective welding technique for many bonding applications is the exothermic process. In this process, a
mixture of aluminum, copper oxide, and other powders is held in place around the joint with a graphite mold.
The mixture is ignited and the heat generated (in excess of
reduces the copper oxide to provide a
homogeneous copper blanket around the junction. Because of the high temperatures involved, copper materials
can be bonded to steel or iron as well as to other copper materials.
Two examples of exothermic bonds are shown in Figure 7-5. The top photograph shows a 4/0 copper clad cable
bonded to a steel plate. The bottom photograph shows two 4/0 copper clad cables axially bonded together. The
tight mechanical bond established by this process is evident from these photographs.  Figure 7-6 shows
examples of the various bond configurations for which molds are readily available.
This process is advantageous for welding cables together, for welding cables to rods, or for welding cables to I-
beams and other structural members. It is particularly attractive for the bonding of interconnecting cables to
ground rods where the use of conventional welding techniques might be awkward or where experienced welders
are not available.  Because of the cost of the molds (a separate mold is necessary for each different bond
configuration), this process is most economical when there are several bonds of the same configuration to be
made.
When using this process, the manufacturer's directions should be followed closely. The mold should be dried or
baked out as specified, particularly when the mold has not been used for several hours and may have absorbed
moisture. The metals to be bonded should be cleaned of dirt and debris and should have the excess water dried
off. Water, dirt and other foreign materials cause voids in the weld which may weaken it or may prevent a low
resistance joint from being achieved. A further requirement is that the mold size must match the cable or
conductor cross sections; otherwise, the molten metal will not be confined to the bond region.
7.4.2.2 Brazing.
Brazing to include silver soldering is another metal flow process for permanent bonding. In brazing, the bond
surfaces are heated to a temperature above
but below the melting point of the bond members. A filler
metal with an appropriate flux is applied to the heated members which wets the bond surfaces to provide
intimate contact between the brazing solder and the bond surfaces. As with higher temperature welds, the
resistance of the brazed joint is essentially zero. However, since brazing frequently involves the use of metal
different from the primary bond members, additional precautions must be taken to protect the bond from
deterioration through corrosion.
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