5.BRAZING AND BRAZE WELDS
Mostly products used in the common manufacturing process are assembled from
more than one component. During assembly of structures joining plays an important
role. The various joining processes can be broadly directed as adhesive bonding,
soldering, brazing, pressure nonfusion welding. Each joining method has its own
practical advantages and limits. Choosing one depends upon the technical demand,
concerned economy and intended use. Among all brazing is a process which can be
applied to have a permanent joint starting from identical adherents to dissimilar
adherents. The adherents may be metals, alloys, ceramics, graphite and cemented
5.2 BRAZING AND ITS NATURE
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 filler alloy flows into the joint by capillary attraction and this necessitates that
the clearance between the base materials in the joint region be kept very small. In
brazing the parent materials are not fused, but since the temperature is high enough
as appreciable diffusion and alloying action is possible between the brazing alloy
and the parent material. The chief feature of brazing is that many dissimilar metals
can be readily joined.
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 angle. Therefore the brazing surfaces should be free from
any oxide layer or impurity. Good wetting increases the brazing efficiency.
5.4 MECHANICS OF BRAZING
Brazing involves a limited dissolution or plastic deformation of the base metal.
Brazing comprises a group of joining processes in which coalescence is produced by
heating to a suitable temperature above 450ºC and by using a ferrous or nonferrous
filler metal that must have a liquidus temperature above 450ºC and below the solids
temperature of the base metal. The filler metal is distributed between the closely
fitted surfaces of the joint.
We should take care to maintain a clearance between the base metals to allow
capillary action to work most effectively. This means a close clearance. The
following chart is based on brazing butt joints of stainless steel. It shows how the
tensile strength of the brazed joint varies with the amount of clearance between the
parts being joined.
Brazing proceeds through four distinct steps
• 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 parts.
• The molten filler metal, held in the joint by surface tension, spreads into the
joints and wets the base metal surfaces.
• 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
Recommended pickling solutions for post-braze removal of oxides
The pickling solutions recommended below may be used to remove oxides from
areas that were not protected by flux during the brazing process. In general, they
should be used after the flux residue has been removed from the brazed assembly.
Oxide removal from 10 to 25% hot sulphuric Pickling can be done at same time
copper, brass, bronze, acid with 5 to 10% flux is removed. Will work on
nickel silver and other potassium
dichromate carbon steels, but if pickle is
contaminated with copper, the
copper will plate out on the steel and
mechanically. This sulphuric pickle
will remove copper or cuprous oxide
stains from copper alloys. It is an
oxidizing pickle, and will discolor
the silver filler metal, leaving it a
Oxide removal from A 50% hydrochloric A mixture of 1 part hydrochloric
irons and steels.
acid solution, used cold acid to 2 parts water can be used for
or warm, More diluted Monel and other high nickel alloys.
acid can be used (10 to Pickling solution should be heated to
(140- finishing is necessary for bright
finishes. This HCI pickle is not like
bright dips on nonferrous metals.
removal 20% sulphuric acid, 20% This pickle is followed directly by a
stainless steels and hydrochloric acid, 60% 10% nitric dip, and then a clean
a water rinse.
temperature of 170180°F(75-80°C.)
20% hydrochloric acid, This pickle is more aggressive than
10% nitric acid, 70% the sulphuric-hydrochloric mixture
water, used at about listed above, and will etch both the
steel and the filler metal.
5.5 TYPES OF BRAZED JOINTS
A spot joint made at one point can be accomplished as easily by welding as by
brazing. But a linear joint – all other things being equal – is more easily brazed than
welded. Brazing needs no manual tracing. The filler metal is drawn through the joint
area by capillary action, which works with equal ease on any joint configuration.
There are many kinds of joints. But there are only two basic types – the butt and the
lap. The rest are essentially modifications of these two. The butt joint, both for flat
and tubular parts:
The butt joint gives the advantage of a single thickness of the joint. Preparation of
this type of joint is usually simple, and the joint will have sufficient tensile strength
for a good many applications. However, the strength of the butt joint does have
limitations. It depends, in part, on the amount of bonding surface, and in a butt joint
the bonding area can't be any larger than the cross-section of the thinner member.
If it is compared this with the lap joint, both for flat and tubular parts
For a given thickness of base metals, the bonding area of the lap joint can be larger
than that of the butt joint and usually is. With larger bonding areas, lap joints can
usually carry larger loads.
The lap joint gives a double thickness at the joint, but in many applications
(plumbing connections, for example) the double thickness is not objectionable. And
the lap joint is generally self-supporting during the brazing process. Resting one flat
member on the other is usually enough to maintain a uniform joint clearance. And,
in tubular joints, nesting one tube inside the other holds them in proper alignment
for brazing. However, suppose we want a joint that has the advantages of both types;
single thickness at the joint combined with maximum tensile strength. We can get
this combination by designing the joint as a butt-lap joint.
The butt-lap is usually a little more work to prepare than straight butt or lap, but one
can wind up with a single thickness joint of maximum strength. And the joint is
usually self-supporting when assembled for brazing.
Brazing has many distinct advantages:
• Economical fabrication of complex and multicomponent assemblies
• Simple method to obtain extensive joint area or joint length
• Joint temperature capability approaching that of base metal
• Excellent stress distribution and heat transfer properties
Ability to preserve protecting metal coating or cladding
Ability to join cast materials to wrought metals
Ability to join nonmetals to metals
Ability to join metal thickness that vary widely in size
Ability to join dissimilar metals
Ability to join porous metal components
Ability to fabricate large assemblies in a stress-free condition
Ability to preserve special metallurgical characteristics of metals
Ability to join fiber- and dispersion-strengthened composites
Capability for precision production tolerance
Reproducible and reliable quality control techniques
Strong, uniform, leak-proof joints can be made rapidly and inexpensively. Joints
that are inaccessible can also be joined by brazing.
• Brazed joints are high in strength. With properly designed and made brazing
joint is as strong as brazed parent material.
A brazed joint is not a homologous body but rather is heterogeneous, composed of
different phases with differing physical and chemical properties. In the simplest
case, it consists of the base metal parts to be joined and the added filler metal.
However, partial dissolution of the base metal, combined with diffusion processes,
can change the composition and therefore the physical and chemical properties of
the boundary zone formed at the interface between base metal and filler metal.
In determining the strength of such heterogeneous joints, the simplified concepts of
elasticity and plasticity theory no longer apply. In a brazed joint formed of several
materials with different characteristics of deformation resistance and deformation
speed, the stresses caused by externally applied loads are nonuniformly distributed.
Brazing gives a weaker bond than welding but brazing has its own applications. In
welding the base metal also melts and there is local casting. Welding is applicable if
both the components are of same materials. Brazing can be done for the components
of different materials and the temperature required is less than required in welding
as base metal does not melt. Brazing is widely used due to its numerous advantages.
Brazing is used in aluminum and its alloy, magnesium and its alloy, nickel alloys
and those of copper such as brasses, bronzes, copper beryllium and the copper
silicon alloys. Among the ferrous and rarer metals the list includes carbon and low
alloy steels, stainless steel, high speed steel, cast iron, cemented carbides,
zirconium, tungsten and molybdenum.
5.9 COMPARISION OF JOINING METHODS
>450 (less than
m.p. of base
Does not melt
Does not melt
than or equal
to m.p. of
5.10 BRAZING PROCESSES
Brazing processes are classified by the method used to heat the assembly. Selection
of the method for a particular job is determined by the type of equipment available,
the skill of the operator, the nature and working place of the parts to be brazed, the
relative costs of labor and materials. The processes are:
• Torch brazing
• Furnace brazing
• Vacuum brazing
• Dip brazing
• Salt-bath brazing
• Infrared brazing
• Electric blanket brazing
• Induction brazing
• Resistance brazing
• Exothermal brazing
5.10.1 Vacuum Brazing
Vacuum brazing is done by keeping the components in an evacuated chamber with
low pressure and then applying heat. It has its own benefits. 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. Two basic
types of equipment can be used, one consists of the hot wall while another cold wall
chamber. Vacuum conditions are well suited for brazing large area where solid or
liquid fluxes cannot be removed adequately from the brazing interface. The
atmospheric gases may result in embrittlement and sometimes disintegration at
brazing temperature. Many oxides disintegrate in vacuum and thus give good
Vacuum brazing has its own advantages:
1. 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.
2. Removal of Oxides of most metals increases the brazing efficiency. Oxides
are removed by dissociation, diffusion or chemical reaction.
3. The low pressure around the interface removes volatile gases and impurities
from the metals. It improves frequently the properties of metals being brazed.
One of the disadvantages of vacuum brazing is that if the filler metal is volatile then
vacuum has to be maintained at low level to prevent gasification of filler metal.
5.10.2 Role of Flux
Fluxes are applied to the brazing surface to make wetting efficient and hence
brazing. The flux used should decompose oxides without corroding the base metal
or the brazing filler metal, should be extremely active because or the short brazing
times employed, and should be easy to remove after brazing. The flux must be
capable of dissolving any oxide remaining on the base metal after it has been
cleaned and any oxide films on the liquid filler metal. Fluxes serve to suppress the
volatization of high vapor pressure constituents in brazing filler metal.
To effectively protect the surfaces to be brazed, the flux must completely cover, be
applied as an even coating and protect them until the brazing temperature is reached.
It must remain active throughout the brazing cycle.
Reaction rates of the flux with oxygen, base metals, brazing filler metals, and any
foreign materials present increase with temperature. Composition of the flux must
be carefully tailored to suit all the factors of the brazing cycle, including dwell time.
Attack of the flux on the metals must be limited, because the flux must react
promptly with metal oxides or other tarnish to enable the joint to be satisfactorily
formed. Active halides, such as chlorides and fluorides, are necessary for alloys
containing aluminum or other highly electropositive metals.
5.11 IMPORTANCE OF O.F.E. COPPER
Its common name is Oxygen free electronic copper and it is designated by C10100.
It contains 99.99 percent Cu min. OFE copper is high conductivity electrolytic
copper produced without use of metal or metalloid deoxidizers.
Physical properties of copper are significantly affected by small amounts of
impurity elements. The oxygen is relatively insoluble in copper and it is present in
the form of an oxide. Large amount of oxygen ( more than 1%) can make copper
brittle. Even trace amount of oxygen can cause embrittlement over about 370ºC.
Oxygen free coppers are not subjected to embrittlement at elevated temperatures and
can be readily welded. Pure non-phosphorus oxygen free electrolytic copper has
high degree of electron mobility. In comparison of welding, brazing of copper is a
widely used process. The brazing of copper to copper is applicable in manufacturing
heat exchanger equipment refrigeration and air conditioning industry, and in
OFE copper is used in busbars, waveguides, lead-in wire, anodes, vacuum seals,
transistor components, glass-to-metal seals, coaxial cables, klystrones, microwave
tubes. Heating in oxidizing atmospheres should be avoided while using.
Melting point of OFE copper is 1083ºC and its coefficient of linear thermal
expansion is 17μm/m.k at 20 to 100ºC. Its specific heat is 385J/kg.k at 20ºC and
thermal conductivity is 391 W/m.k at 20ºC. OFE copper can be readily soldered,
brazed, gas tungsten arc welded, gas melt arc welded. Its capacity for being oxyfuel
gas welded is fair. Shielded metal arc welding methods are not recommended.
5.12 BRAZING ALLOYS
Brazing filler alloys are largely classified on basis of their chemical composition
rather than mechanical properties requirement. The major categories have various
classification in each category and these are:
• Aluminum silicon
• Copper and copper-zinc-tin
• Copper-phosphorus with or without silver
• Gold-copper and gold-nickel-palladium
• Silver-copper with or without zinc
Silver based brazing alloys play a major role in the field of metal joining. This is
because of their low melting point. They also have wide metallurgical compatibility.
Circumstantially these can be used for most of the engineering materials with the
exception of aluminum and its alloys, magnesium and its alloys with all methods of
5.13 DESCRIPTION OF ANALYTICAL EQUIPMENTS
5.13.1 Furnace :
The furnace used is high vacuum high temperature furnace. It has a digitally
controlled programmer. One can fix the heating rate and soaking temperature. The
furnace is double walled and can sustain a maximum of 1600 ºC and can produce a
vacuum of 10-6 torr. The furnace also has a provision of quenching the samples in
gas. It has a heat exchanger, which cools the gas. The furnace is water-cooled with
three sets of heater. Three sets are provided for homogeneous heating area inside the
furnace. The heater and the shields are made of molybdenum, as molybdenum is
very good reflector of heat and can bear high cyclic temperature in inert or vacuum
environment. Three vacuum pumps are attached to the furnace to give a good
vacuum. The pumps are namely: Roughening pump, Root pump, diffusion pump.
The valves attached to the furnace work with pneumatic principle. The temperatures
are reached with the help of corresponding appropriate thermocouple.
5.13.2 Tensile testing machine :
The universal tensile testing machine that we used to test the tensile samples of two
brazed and one unbrazed (reference) samples of copper. It is an automatic tensile
testing machine. The software it works on directly measures the engineering stressstrain of the test-samples. The machine is capable of performing compression as
well as the bend tests.
Experimental details :
The samples for vacuum brazing were made from the cold drawn OFE copper rod
having diameter 15 mm. The brazing samples had length of 40 mm. One sample of
length 80 mm was also cut for reference. The samples for brazing were turned and
made planar in C.N.C lathe. Then the samples were polished to give a perfect planar
surface. The filler metal chosen was silver copper eutectic alloy. Eutectic alloy was
used as it melts congruently and has lower melting point than copper. The samples
to be brazed were fixed in the jig made from stainless steel. Stainless steel was
chosen as it has higher melting point than both the filler and base metal and has
coefficient of linear expansion less than copper so the samples would not slip from
its position during brazing. After fixing the samples in the jig the whole
combination was put in the furnace for brazing at a high vacuum. The brazing
temperature was 800 ºC and the soaking time was 25 min, after the soaking time the
samples were quenched with argon gas. The brazed samples and the reference were
turned in C.N.C lathe to get A.S.T.M standard tensile specimen. The tensile testing
was carried out and the different mechanical properties were obtained.
The filler metal and the base metal were also seen under microscope.
Joint efficiency = strength of brazed joint / strength of parent metal
= 202.23 / 359
With this tensile joint design it is found that the ultimate tensile strength of
cylindrical oxygen free electrolytic copper, brazed at 800°C for 25 min, is 202 Mpa
where as that of parent metal is 359Mpa. The joint efficiency of the brazed sample
at 800°C is 56.33%. Investigation shows a number of plastic instability on the
brazed tensile sample compare to parent sample. Failure occurred at the brazed joint
indicating a weak region. As ductility concerns, the brazed joint exhibited 21.41%
elongation compare to that of pure copper(17.6 %).Tensile testing of brazed sample
showed a abrupt decrease in strength. This indicates there is a possibility of
improving the designed joint strength with further process optimisation.