Brazing is defined as a joining process
wherein coalescence is produced
between the adherents by heating them
to a suitable temperature above 450ºc
and by using a filler nonferrous alloy
having its liquidus temperature above
450ºC and below solidus temperature of
used base metals.
The joint is produced by diffusion of elements of filler
metal into the base metal or vice versa. Diffusion of
the elements creates bonds, which contributes to joint.
Since the filler metal is in liquid state the diffusion
rate is faster than in solids. The capillary action plays
an important role in holding the liquid filler metal
which would otherwise flow out. After soaking the
samples for a long time at brazing temperature the
samples are quenched to room temperature.
The wetting angle depends upon the free surface
energy of liquid-vapor interface, solid-vapor interface
and solid-liquid interface.
For a good wetting the wetting angle should be less
than 90º. So the free surface energy of solid-vapor
interface must be greater than solid-liquid interface.
The presence of adsorbed molecules on a metal
surface markedly decreases the surface energy of
solid-vapor interface and thus increasing the contact
Therefore the brazing surfaces should be free from
any oxide layer or impurity. Good wetting increases
the brazing efficiency.
Four Steps in Brazing
•The assembly or the region of the parts to be joined is
heated to a temperature of at least 450ºC.
•The assembled parts and brazing filler metal reach a
temperature high enough to melt the filler metal but not
•The molten filler metal, held in the joint by surface
tension, spreads into the joints and wets the base metal
•The parts are cooled or solidify, the filler metal, which
is held in the joint by capillary attraction and anchors the
parts together by metallurgical reaction and atomic
•Economical fabrication of complex and multi
•Simple method to obtain extensive joint area or joint
•Joint temperature capability approaching that of base
•Excellent stress distribution and heat transfer
•Ability to preserve protecting metal coating or
•Ability to join cast materials to wrought metals
•Ability to join nonmetals to metals
• Ability to join metal thickness that vary widely
• Ability to join dissimilar metals
• Ability to join porous metal components
• Ability to fabricate large assemblies in a stressfree condition
• Ability to preserve special metallurgical
characteristics of metals
• Ability to join fiber- and dispersionstrengthened composites
• Capability for precision production tolerance
• Reproducible and reliable quality control
>450 (less than
m.p. of base
Does not melt
Does not melt
than or equal
to m.p. of
Vacuum brazing is done by keeping the
components in an evacuated chamber with low
pressure and then applying heat.
Vacuum brazing is well suited for heat resistant
nickel- and iron based alloys that contain
aluminum or titanium, reactive metals, refractory
metals and ceramics.
The filler metal can be used as a sheet, wire or
powder paste or molten rod in the joint area.
Vacuum removes all gases and thus reduces the chance
of oxidation. The actual pressure used depends upon the
base metal, the filler metal and the degree to which
gases are expelled.
Removal of Oxides of most metals increases the brazing
efficiency. Oxides are removed by dissociation,
diffusion or chemical reaction.
The low pressure around the interface removes volatile
gases and impurities from the metals. It improves
frequently the properties of metals being brazed.