BRAZING

BRAZING

Brazing is a procedure traditionally used for metals (but also ceramics), where molten filler
metal (brazing alloy) reaches the joint. A wide range of heat sources are also used to categorise
brassing according to the heating system. Some methods heat up locally (only in the joint area)
to achieve brazing temperature, others heat the whole setup (diffuse heating).
The workpiece material is not melted as in welding, but metal parts are joined using filler metals
that have a melting point above 450°C, but below the melting point of the materials being joined.
The molten filler metal is drawn into the weld gap by capillary attraction across the joint,
eventually solidifying to form the bond. There are various ways to apply the heat.
Brazing collar onto tube

Methods of pre-placing the brazing alloy for furnace or induction brazing

1. Surfaces to be joined are degreased, abrasive grit blasted and pickled.
2. Components are assembled or jigged, very often with a ring of braze metal charge

placed in position.
3. Components are heated by torch, in a conveyor furnace or by induction, to

required temperature. Joint is filled by capillary action.

The following method is the most commonly used approaches.
Localised Heating Techniques

 Torch Brazing
In this method, The fuel gas flame supplies the required heat for melting and flow filler
metal. The gas may be acetylene, hydrogen, and propane and is used to generate a flame
in combination with oxygen or air. This process is automated easily and requires low
investment in resources. The use of a flux is necessary if torch brazing is to be cleansed
after brazing.

 Induction Brazing

High frequency induction heating for brazing is clean and rapid, giving close control of
temperature and location of heat. Heat is created by a rapidly alternating current which is
induced into the workpiece by an adjacent coil.

 Resistance Brazing
This is a process in which heat is generated from resistance to an electrical current (as for
induction brazing) flowing in a circuit which includes the workpieces. The process is
most applicable to relatively simple joints in metals which have high electrical
conductivity.

Diffuse Heating Techniques
 Furnace Brazing
Furnace brazing offers two key advantages: safety atmosphere brazing and a capacity to
reliably check all the heating and refrigeration cycles stages (where high purity gases or
vacuum negate a flux requirement). Heating is by elements or the fire of gas.

 Dip Brazing

This involves immersion of the entire assembly into bath of molten braze alloy or molten
flux. In both cases the bath temperature is below the solidification point of the parent
metal, but above the melting point of the filler metal

Manufacture:
1. Surface preparation as soldered joints (see L02).
2. Assembly/jigging:
 Jigs restrain thermal expansion, absorb heat, deteriorate and are expensive. If
possible joints should be self-locating. Jigs should be easy to load, have
minimum contact with parts, have a low thermal mass, keep alignment, and be
heat resistant. During assembly/jigging flux is positioned as a paste, and
brazing material as a “shaped preform” of wire, or foil, or as a paste.
3. Heating:
 torch – most common hand-held or fixed heating method
 induction – mainly for ferrous parts
 furnace – batch or continuous. Can be done under Ar, N2, H2, cracked
NH3, or vacuum with no flux

 resistance – heating is local, and electrodes can act as jigs
 dip (salt bath) – assembly is immersed in molten flux.
4. Finishing – flux or excessive braze material is removed before inspection.

The real skill lies in the design and engineering of the joint, but if you do not follow the correct
brazing procedures even a properly built joint will turn out to be poor. These procedures have
been reduced to six fundamental steps. Although it's normally easy to do, no one should be
forgotten. Steps to successful brazing:

1) Ensure good fit and proper clearances.
In order to spread molten metal filler between basal metals surfaces, Brazing uses
capillary action. Therefore, retain a distance between the base metals when brazing, so
that capillary action can work most effectively. This means a close clearance in almost
every situation.
Clearances must not be unnecessarily accurate to achieve an adequately solid joint.
There are a number of clearances under which capillary action is applied, so you have
some leeway. Bear in mind that joint strength decreases typically with increasing

clearance.

Note that brazed joints are rendered at brazing temperature rather than at room
temperature. Consider the thermal expansion coefficient of metals, particularly tubular
assemblies.

2) Clean the metals.
Only with clean metal surfaces can the capillary action work properly. You should
eliminate these contaminants or shape a barrier between the base metal surfaces and the
brazing materials, if covered with grease, grate, roost, scale or soil.

An acid pickle solution for removing rust and scale should be removed first, for the
grated surface will not work. The pollutants can be extracted chemically or mechanically
if the metal surfaces have an oxide or scale coating.

You can speed up the cleaning process by using a molten wheel, file or grit, followed
by a rinsing operation particularly when repairing brazing where components are highly
dirty or heavily rusted. Until the pieces have been washed thoroughly, they flow and
braze so that they can be removed by manufactured dust or body oils from the surfaces.

3) Flux the parts
Flux is a material that is added before brazing to the joint surfaces. Its use is critical for
the atmospheric brazing process, with only a few exceptions. The heating of a metal
surface is the result of a chemical reaction of hot metal with oxygen in the air, since oxide
formation has been speed up. The surface is protected by flux-coating on the joint area,
avoiding the formation of oxides. It dissolves and absorbs oxides that have not
completely been removed during heating or washing.
Fluxes on the joint may be applied in every way as long as the joints are protected
entirely. Usually, if necessary, you add flux before brazing so that the pieces are drying
and flattening or are knocked off. Select a flux formulated for your brazing application
particular metals, temperatures and conditions.

4) Assemble for brazing.
Keep them in brazing position until the pieces are washed and fluxed. The easiest
method of holding them together is gravity, if the shape and weight of the pieces allow.
Also, by adding additional weight, it can support gravity by not adding too much. You
can not keep clearance when adding too much weight and forcefully pull the brazing
filler metal out of the joint area. Recall, adding weight to the pieces raises the mass and
increases the time needed to lift the pieces to the brazing temperature.

5) Braze the assembly.
The brazing itself consists of heating the assembly at the brazing temperature and flowing
through the joint of the filler metal. Make sure that you don't heat the container to the
base of materials at a brazing temperature. First, the heating process:
 In brazing, apply heat broadly to the base metals. If you're brazing a small
assembly, you may heat the entire assembly to the flow point of the brazing filler
metal. If you're brazing a large assembly, heat a broad area around the joint. A
variety of fuels—natural gas, acetylene, propane, propylene—can be combusted

with either oxygen or air. Be mindful that all metals should be heated to the
maximum possible amount in the assembly so that the brazing temperature can
be reached at the same time. Keep the torch moving and the rod or wire against
the joint area carefully in manual brazing. To boost flow, add some flux at the
end of the filling rod.

Be careful: The external base metal surfaces in the heated assembly can be slightly hotter
than the internal joints. Be careful to place the filler metal in front of the joint
immediately.

6) Clean the brazed joint.
Clean the assembly after you have brazed. Due to the Corrosive nature of most brazing
fluxes, cleaning is necessary. Usually, cleaning is a two-step procedure:

 Remove the flux residues.
 Remove any oxide scale formed during the brazing process by pickling.

Immerse the assembly while it's still hot, but ensure that the filler metal has solidified
until it is quenched. The traces of the glass stream normally break up and fade away.
Clean them gently with a wire brush when the body stays in the hot water.

If you haven't used enough to start with or have overheated the parts during brassing, it
might have trouble removing stream. Then the stream is fully oxidised and turns normally
green or black. In this case, a mild acid solution must eliminate the flux.