BRAZING – General Principles
The successful joining of components
by the brazing process depends on the selected brazing
filler metal,
1. Wetting the base metal without melting it,
2. Flowing into a controlled joint gap by capillary
flow,
3. Creating a adequately strong bond with the joint
faces after solidification.
The accepted definition of Brazing is met with the
above criteria being achieved with a filler metal
having a melting point above 450°C (generally
accepted demarcation between brazing and soldering).
To assist capillary flow, the joint must be designed
to ensure that the mating surfaces of the components
are
1. Clean,
2. Parallel,
3. Close enough together to assist capillary
attraction (adhesive force between solid and
liquid being greater than the cohesive forces within
the liquid).
Most brazing alloys melt over a temperature range
rather than a single temperature – the lower
temperature, which is the temperature below which
the alloy is fully solid, is called the solidus while
the upper temperature, above which the alloy is fully
molten, is called the liquidus.
Flowability is the
property of the brazing filler metal that determines
the distance it will travel away from its original
position, under the action of capillary forces. To
flow well, a filler metal must not gain appreciable
increase in its liquidus temperature even though its
composition may be altered by the addition of metal
it has dissolved.
Interactions with the base metal are minimized by
selecting the proper brazing filler metal, keeping
the brazing temperature as low as possible but high
enough to produce flow, keeping the time at brazing
temperature short and cooling the brazed joint as
quickly as possible without causing cracking or distortion.
Silver brazing alloys will not usually flow through
a capillary gap to a length of more than about 20mm
for single point application of brazing alloy, longer
overlap joint lengths can be achieved using brazing
alloy ring preforms.
A realistic rule-of-thumb for lap joints and tube
insert brazing is: "joint overlap length should
normally be between 3 and 4 times thickness of the
thinner component; it should not be less than 3 mm
nor more than 20 mm for single point application of
brazing alloy."
Brazing Temperature:
Narrow melting range filler metals do not have much
tendency to coexist as a mixture of solid and liquid
phases or to liquate. They flow readily and should
be used with small joint clearances. As the solidus
and liquidus temperatures diverge, the tendency to
liquate increases, requiring greater precautions in
brazing filler metal application. The necessity for
the brazing filler metal to melt below the solidus
of the base metal is one of the factors affecting
its selection. The brazing temperature is usually
10 to 90°C above the liquidus of the filler metal.
Fluxes
For most manual brazing in air using gas torches,
a flux is essential to prevent oxidation and continue
to maintain oxide free joint faces essential for wetting
and flowing of the filler metal. For manual brazing,
flux is applied by brushing on the flux paste liberally
on the joint faces and some distance from the joint
– it is better to use an excess of flux rather
than too little. Fluxes are intended to prevent oxidation
during brazing and not to remove dirt and contamination
already present, these should be removed prior to
brazing.
Flux compositions have been designed to be active
over a specific temperature range, and selection of
the appropriate flux for an application is based on
ensuring that the flux is active at least 50°C
below the solidus of the alloy and remains active
at least 50C higher than the alloy liquidus
