adjoining base metals and permits the filler to penetrate
the pores of the metal.
You should carefully select the flux for each brazing
operation. Usually the manufacturers label specifies the
type of metal to be brazed with the flux. The following
factors must be considered when you are using a flux:
l Base metal or metals used
. Brazing filler metal used
l Source of heat used
Flux is available in powder, liquid, and paste form.
One method of applying the flux in powdered form is to
dip the heated end of a brazing rod into the container of
the powdered flux, allowing the flux to stick to the
brazing rod. Another method is to heat the base metal
slightly and sprinkle the powdered flux over the joint,
allowing the flux to partly melt and stick to the base
metal. Sometimes, it is desirable to mix powdered flux
with clean water (distilled water) to form a paste.
Flux in either the paste or liquid form can be applied
with a brush to the joint. Better results occur when the
filler metal is also given a coat.
The most common type of flux used is borax or a
mixture of borax with other chemicals. Some of the
commercial fluxes contain small amounts of phospho-
rus and halogen salts of either iodine, bromine, fluorine,
chlorine, or astatine. When a prepared flux is not avail-
able, a mixture of 12 parts of borax and 1 part boric acid
may be used.
Nearly all fluxes give off fumes that may
be toxic. Use them only in WELL-VENTI-
In brazing, the filler metal is distributed by capillary
action. This requires the joints to have close tolerances
and a good fit to produce a strong bond. Brazing has
three basic joint designs (fig. 6-13): lap, butt, and scarf.
These joints can be found in flat, round, tubular, or
The lap joint is one of the strongest and most fre-
quently used joint in brazing, especially in pipe work
The primary disadvantage of the lap joint is the increase
Figure 6-13.Three types of common joint designs for brazing.
in thickness of the final product. For maximum strength,
the overlap should be at least three times the thickness
of the metal. A 0.001-inch to 0.003-inch clearance be-
tween the joint members provides the greatest strength
with silver-based brazing filler metals. You should take
precautions to prevent heat expansion from closing
joints that have initial close tolerances.
Butt joints are limited in size to that of the thinnest
section so maximum joint strength is impossible. Butt
joint strength can be maximized by maintaining a joint
clearance of 0.001 to 0.003 of an inch in the finished
braze. The edges of the joint must be perfectly square to
maintain a uniform clearance between all parts of the
joint. Butt joints are usually used where the double
thickness of a lap joint is undesirable. When double-
metal thickness is objectionable and you need more
strength, the scarf joint is a good choice.
A scarf joint provides an increased area of bond
without increasing the thickness of the joint. The area of
bond depends on the scarf angle cut for the joint. Usu-
ally, an area of bond two to three times that of a butt joint
is desirable. A scarf angle of 30 degrees gives a bond
area twice that of a 90-degree butt joint, and an angle of
19 1/2 degrees increases the bond area three times.