Brazing is a group of welding processes which produce coalescence of
materials by heating them to a suitable temperature and by using a
filler metal having a liquidus above 450oC and below the solidus of
the base metals. The solidus is the highest temperature at which the
metal is completely solid, that is, the temperature at which melting
starts. The liquidus is the lowest temperature at which the metal is
completely liquid, the temperature at which freezing starts. The solidus
and liquidus for a particular alloy are defined values.
The filler metal is distributed between closely fitted surfaces of the
joint by capillary attraction. To avoid confusion it is necessary to
explain braze welding which is different since the filler metal is not
distributed by capillary attraction.
Filler materials for brazing are covered by an AWS specification. They are
classified according to analysis: aluminum-silicon, copper, copper-zinc,
copper-phosphorus, copper-gold, heat-resisting materials, magnesium, and
silver are the basic groupings. Filler metal selection is based on the
metal being brazed.
Certain brazing filler metals contain cadmium in significant amounts.
When these are used adequate ventilation is required.
Filler metals are available in many forms, the most common is the wire
or rod. Filler metal is also available as thin sheet, powder, paste,
or as a clad surface of the part to be brazed.
Brazing of aluminum alloys was made possible by the development of fluxes
that disrupt the oxide film on aluminum without harming the underlying
metal and filler metals (aluminum alloys) that have suitable melting
ranges and other desirable properties, such as corrosion and mechanical
The aluminum-base filler metals used for brazing aluminum alloys have
liquidus temperatures much closer to the solidus temperature of the base
metal than those for brazing most other metals. The non-heat treatable
wrought aluminum alloys, such as the 1xxx, 3xxx, and 5xxx (low-magnesium)
series, have been brazed successfully. Alloys that contain higher magnesium
contents are more difficult to braze by the usual flux methods because of
poor wetting and excessive penetration by the filler metal. Filler metals
that melt below the solidus temperatures of most commercial, non-heat
treatable wrought alloys are available.
The most commonly brazed heat-treatable wrought alloys are those of 6xxx
series. The 2xxx and 7xxx series of aluminum alloys have low melting points
and therefore are not normally brazeable. Alloys that have solidus
temperatures above 595oC are easily brazed with commercially binary
aluminum-silicon filler metals. Higher-strength, lower-melting-points
alloys can be brazed with proper attention to filler metal selection
and temperature control, but the brazing cycle must be short to minimize
penetration by the molten filler metal.
Sand and permanent mold casting alloys with high solidus temperatures are
brazeable. Commercial filler metals for brazing aluminum are
aluminum-silicon alloys containing 7 to 12 wt% Si. Brazing fillers
with lower melting points are attained, with some sacrifice in resistance
to corrosion, by adding copper and zinc. Filler metals for vacuum brazing
of aluminum usually contain magnesium.
Most filler metals are used for any of the common brazing processes and
methods. Two alloys, 4004 and 4104, have been developed exclusively for
use in fluxless vacuum brazing.
Brazing of aluminum to copper is difficult, because of the low melting
temperature, 548oC, of the aluminum-copper eutectic and its extreme
brittleness. The eutectic is formed due to dissolution of aluminum
during brazing. By heating and cooling rapidly, however, it is possible
to make reasonably ductile joints for applications such as copper inserts
in aluminum castings for electrical conductors.
Copper and Copper Alloys
Most copper and copper alloys can be brazed satisfactorily using one or
more of the conventional brazing processes: furnace, torch, induction,
resistance, and dip brazing. Their brazeabilities are rated from good to
Brazing of Tough Pitch Coppers. Tough pitch coppers are subject
to embritllement when heated at temperatures above 480oC in reducing
atmospheres containing hydrogen. Phosphorus-deoxidized and oxygen-free
coppers can be brazed without flux in hydrogen-containing atmospheres
without risk of embritllement, provided self-fluxing filler metals
Brazing of Red and Yellow Brasses. Red and yellow brasses are readily
brazed with a variety of filler metals. Flux is normally required for best
results, especially when the zinc content is above 15 wt%. Low-melting
filler metals should be used to avoid dezinfication of the yellow brasses.
If added to red brass or yellow brass, lead forms a dross on heating that
can seriously impede wetting and the flow of filler metal. Consequently,
in brazing leaded brasses, the use of a flux is mandatory to prevent dross
formation in the joint area.
Brazing of Phosphor Bronzes. Phosphor bronzes contain small amounts
of phosphorus, up to approximately 0,25 wt%, added as deoxidizer. Although
susceptible to hot cracking in the coldworked condition, alloys in this
group have good brazeability and are adaptable to brazing with any of
the common filler metals that have melting temperatures lower than that
of the base metal.
In the selection of a brazing process for a nickel-base alloy, the
characteristics of the alloy must be carefully considered. The nickel-base
alloy family includes alloys that differ significantly in physical
metallurgy (such as precipitation-strengthened versus solid-solution
strengthened) and in process history (such as cast versus wrought).
These characteristics can have a profound effect on their brazeability.
Precipitation-hardenable alloys present several difficulties not normally
encountered with solid-solution alloys. Precipitation-hardenable alloys
often contain appreciable (greater than 1 wt%) quantities of aluminum and
titanium. The oxides of these elements are almost impossible to reduce in
a controlled atmosphere (vacuum, hydrogen). Therefore, nickel plating or
the use of a flux is necessary to obtain a surface that allows wetting by
the filler metal.
Titanium and Titanium Alloys
Titanium is one of the chemical elements that reacts readily with oxygen
to form an adherent and stable oxide. This oxide gives titanium and
titanium alloys excellent corrosion resistance. Properties such as
corrosion resistance, light weight, and high strength make titanium
especially attractive in aerospace and chemical applications.
Since titanium and titanium alloys are very resistant to corrosion,
the filler metal should be selected carefully to avoid galvanic corrosion.
Titanium and its alloys are usually brazed by induction or furnace
processes with a protective atmosphere. The brazing atmosphere is usually
a vacuum less than 13 mPa or an inert atmosphere with dew point
lower than -55oC. Vacuum brazing with filler metals that contain
silver or gallium must be performed with an argon back pressure to avoid
the vaporization loss of these elements.
In induction brazing, the chemical elements of the filler metal must
alloy readily with titanium, due to the fast heating cycle. However,
furnace brazing requires a filler metal with a chemical composition
that does not alloy excessively with titanium, due to longer period
at elevated temperatures. Torch brazing is not usually done because
special fluxes and a skilled operator are required.
b and a+b titanium
alloys can be strengthened by heat treatment. The basic
heat treatment consists of heating in single-phase field (b alloys) or in
a two-phase field (a+b alloys), followed by a quenching. The aging is
performed between 480 and 650oC. During the aging, a or other compounds
will precipitate in a b matrix, increasing the strength and toughness of
the alloy. Thus, the thermal cycle during brazing may affect the mechanical
properties of the base metal. The b alloys should be brazed at a temperature
close to the solubility temperature. Brazing at higher temperatures will
reduce the ductility of these alloys.