Module 2 (Important Topics)
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Types of flames in gas welding
Types of electrodes used in arc welding
Distinguish between straight & reverse polarity
Define weldability. Mention the factors affecting
weldability
Shielded metal arc welding (SMAW)
GTAW (TIG welding)
Explain the gas metal arc welding process (GMAW)
and mention its various modes of metal transfer
across the arc
Submerged arc welding (SAW)
Electro slag welding (ESW)
• Resistance welding (spot, seam welding) or explain any
one pressure welding
• Distinguish between transferred and non-transferred
plasma arc welding
• Friction welding
• Thermit welding
• Electron beam welding (EBW)
• Laser beam welding (LBW)
• Types of solders and fluxes used in soldering
• Distinguish between soldering, brazing & welding
• Types of adhesives used in adhesive bonding
• Heat affected zone (HAZ) in welding
• Weld Defects
• Welding numerical solved problems
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CLASSIFICATION OF WELDING
• Plastic Welding or Pressure Welding :
In this type of welding the metals to be joined are to be heated
to the plastic state and then forced together by external pressure
without the addition of filler material.
Example: Resistance welding.
• Fusion Welding or Non-Pressure Welding :
In this type of welding no pressure is involved but a very high
temperature is produced in or near the joint. The metal at the joint
is heated to the molten state and allowed to solidify. The heat
may be generated by electric arc, combustion of gases or
chemical action. A filler material may be is used during the
welding process.
Example: Oxy-Acetylene Gas Welding, Arc Welding etc
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GAS WELDING
• Utilizes oxygen and a fuel gas to heat metal until it is in a
molten state and fuse multiple pieces of metal together.
• It is used for welding ferrous and non ferrous metals
particularly for thin sections up to 6 mm thick
• Flame formed by burning a mix of acetylene (C2H2) and
oxygen
• The temperature generated during the process is 33000c
• Fluxes are added to the welded metal to remove oxides
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Oxy-Acetylene Gas Welding Equipment
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Types of Flame in Oxy Acetylene Gas Welding
• Neutral flame
• Oxidizing flame
• Carburizing (Reducing) flame
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Neutral flame
Equal volume of acetylene
and oxygen
Temperature in order of about
3260oC
Consists of sharp brilliant
inner cone(short distance from
tip) and outer cone
Give a bright whitish cone
surrounded by the transparent
blue envelope
Used for welding mild steel,
stainless steel, aluminium,
copper and cast iron
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Oxidizing flame
Oxygen more in proportion
Has the highest temperature
about 34820c
Pointed inner cone, outer
envelope shorter
Used for welding brass and
copper based alloys
It is not used for welding of
steels as the excess oxygen
oxidises the metal
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Carburizing (Reducing) flame
Volume of oxygen supplied is
reduced & acetylene is more.
Temperature in order of about
3037oC
Three zones- inner cone,
intermediate of whitish colour,
bluish outer cone
Used for welding low alloy
steels and also used for surface
hardening
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Types of Flame
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Arc Welding Equipment
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Electrodes
• Consumable Electrodes :
It is consumed during welding operation. May be made
of various metals depending upon the purpose and
chemical composition of the metals to be welded.
• Non-consumable Electrodes:
May be made of carbon, tungsten or graphite which do
not consume during welding operation.
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Types of Polarity
 Straight Polarity
: In Straight Polarity, electrode is
connected to negative terminal and work-piece to positive
terminal.
• For welding thick (heavy section) work-piece
• Also known as DCSP or DCEN
 Reverse Polarity
: In Reverse Polarity, electrode is
connected to positive terminal and work-piece to negative
terminal.
- For welding thin work-piece
- Also known as DCRP or DCEP
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WELDABILITY
• Weldability is the capacity of a material to be welded under
fabrication conditions and to perform satisfactorily in the
intended service.
• Weldability depends up on Melting Point of the metal.
Thermal Conductivity
Thermal Expansion
Surface Condition.
Change in Microstructure
• A metallic material with adequate weldabilty should fulfill the
following requirements :
Have good strength after welding.
Good corrosion resistance after welding.
Have good weld quality.
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Shielded Metal Arc Welding (SMAW)
• Weld is produced by heating the work-piece with an arc
setup between the flux coated electrode and the workpiece.
• The coating produces a gaseous shield and slag to protect
from atmosphere
• The arc produced between these two electrodes heats the
metal to the melting temperature (about 2400-2600° C).
• Both A.C and D.C can be used.
• Also known as Flux Shielded metal arc welding or Stick
Welding
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Shielded Metal Arc Welding (SMAW)
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Inert Gas Welding
• In inert gas welding, inert gases such as argon, helium & CO2 are
used for surrounding the electric arc and keeping the atmospheric air
& other contaminations away from the molten metal
• Example: TIG & MIG Welding
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Tungsten Inert Gas Welding (TIG or GTAW)
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TIG welding uses a non-consumable tungsten electrode.
Filler metal, when required, is added by hand.
Shielding gas protects the weld and tungsten.
A constant-current welding power supply produces energy.
• TIG welding is also known as gas tungsten arc welding (GTAW)
• GTAW is most commonly used to weld thin sections of stainless
steel and non-ferrous metals such as aluminum, magnesium, and
copper alloys.
• GTAW gives stronger and higher quality welds than the welds given
by SMAW and GMAW.
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TIG (GTAW) Welding
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TIG (GTAW)
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Advantages
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Welds more metals and metal alloys than any other process
More stronger, ductile & corrosion resistant than other welds
High quality and precision
Pin point control
Aesthetic(beautiful) weld beads
No sparks or spatter
No flux or slag
No smoke or fumes
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Disadvantages
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Lower filler metal deposition rates
Good hand-eye coordination a required skill
Brighter UV rays than other processes
Slower travel speeds than other processes
Equipment costs tend to be higher than other processes
Separate filler rode required
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Applications
• Welding thin workpieces, especially nonferrous metals
• Used for welding aluminium, magnesium, stainless steel, brass,
bronze & wide range of other metals
• It is used extensively in the manufacture of space vehicles, and is
also frequently employed to weld small-diameter, thin-wall tubing
such as those used in the bicycle industry.
• Used to make root or first pass welds for piping of various sizes
• Used to repair tools and dies, especially components made of
aluminum and magnesium
• Precision welding in atomic energy, aircraft & chemical industries
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Metal Inert Gas Welding (MIG or GMAW)
• It is also known as Gas Metal Arc Welding (GMAW) or Metal Active Gas
(MAG) welding.
• A semi-automatic or automatic arc welding process in which a continuous
consumable wire electrode and a shielding gas are fed through a welding
gun.
• A constant voltage, direct current reverse polarity power source is most
commonly used
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Metal Inert Gas (MIG/GMAW)
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Modes of metal transfer in GMAW
Metal can be transferred by three methods in the GMAW process:
spray transfer
globular transfer
short circuiting
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3 Modes of metal transfer in GMAW
1) In spray transfer, small molten metal droplets from the electrode
are transferred to the weld area at a rate of several hundred
droplets per second. The transfer is spatter free and very stable.
High DC currents and voltages and large-diameter electrodes are
used with argon or an argon-rich gas mixture as the shielding gas.
2) In globular transfer, carbon-dioxide-rich gases are utilized, and
globules are propelled by the forces of the electric-arc transfer of
the metal, resulting in considerable spatter. High welding currents
are used, making it possible for greater weld penetration and higher
welding speed than are achieved in spray transfer.
3) In short circuiting, the metal is transferred in individual droplets
(more than 50 per second) as the electrode tip touches the molten
weld metal and short-circuits. Low currents and voltages are utilized
with carbon-dioxide-rich gases and electrodes made of smalldiameter wire.
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Advantages of GMAW
• The ability to join a wide range of material types and
thicknesses.
• GMAW has higher electrode efficiencies, usually between
93% and 98%, when compared to other welding processes.
• All-position welding capability
• Lower heat input when compared to other welding
processes
• Suitable for ferrous & non ferrous metals
• Excellent speed of deposition
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Limitations of GMAW
• Mode of metal transfer restricts its use to thin materials.
• Difficult to weld in small corners
• Welding equipment is costly
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Applications
• MIG welding is used for welding carbon and low alloy steels, stainless
steels, copper alloys & aluminium
• It is used in aircraft & automobile industries
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Submerged Arc Welding (SAW)
• Here instead of flux covered electrode, granular flux & a
bare electrode is used
• In Submerged Arc Welding, the arc is submerged under a
layer of Flux and so the arc is invisible
• The Flux may be made of silica, metal oxides or other
compounds
• Flux is fed through a Flux Hopper
• Bare electrode (Steel stainless steel or copper etc) is fed
through the gun
• SAW is an automatic process for the production of high quality
butt welds
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Submerged Arc Welding (SAW)
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Plasma Arc Welding
• Plasma Arc Welding is an arc welding process in which
coalescence (joint) is produced by the heat obtained from a
constricted arc setup between a tungsten/tungsten alloys
electrode and water cooled nozzle or between a tungsten
electrode and the work-piece.
• Plasma Arc Welding is a shielded metal arc process.
• Plasma is a high temperature ionized gas (hydrogen or helium)
conducting electricity.
• A non-consumable tungsten electrode, water cooled copper
nozzle and gas shield (argon or argon mixtures) is employed for
the welding.
• The process employs two inert gases, one forms the arc
plasma and the second shields the arc plasma.
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Types of Plasma Arc Welding
1) Transferred Arc Process
2) Non-Transferred Arc Process
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a) Transferred
b) Non Transferred Arc
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Transferred Arc
Non-Transferred Arc
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SL.
No
Transferred Arc
Non-Transferred Arc
1
Arc is formed between work- Arc is formed between water
piece (+) and electrode (-)
cooled constricting nozzle (+)
and electrode (-)
2
Arc
is
transferred
from Plasma arc comes out of the
electrode to the workpiece
nozzle as a flame
3
Possess more energy compared Possess
to Non-Transferred Arc.
energy.
4
Make use of a current limiting Initiated by a high frequency
resistor to generate this arc.
unit in the circuit.
5
Used for cutting metals.
comparatively
less
Used for welding applications
and metal plating.
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Electro Slag Welding (ESW)
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Electro Slag Welding (ESW)
• ESW is a highly productive, single pass welding process for
thick (greater than 25mm up to about 300mm) materials in a
vertical or close to vertical position.
• An electric arc is initially struck by wire electrode that is fed
into the desired weld location (bottom of the part to be welded)
and then flux is added.
• Flux added is melted by the heat of the arc.
• When enough slag accumulates, the arc action stops and further
required heat is provided by the resistance offered by the slag to
the current flowing through it.
• The weld metal is deposited into a weld cavity between the two
pieces to be joined; the space is enclosed by two water-cooled
copper shoes to prevent the molten slag from running off.
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RESISTANCE WELDING
• Resistance Welding is a group of welding process in which joint is
produced by the heat obtained from the resistance of the work to the
flow of current in a circuit of which work is a part and by the
application of pressure.
• No filler metal is needed.
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Resistance Spot Welding
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Resistance Spot Welding
• Sheets to be welded are placed one over the other and is
placed between the electrodes. Pressure is applied to the
work-pieces by the electrodes.
• Welding current is switched on for a definite period of
time.
• As the current passes through, a small area where the
work-pieces are in contact is heated due to the resistance
offered by the materials in the contact area.
• The temperature of the weld zone is around 815 °C to 930
°C.
• Welding current is then cut off and extra electrode force is
then applied to the work-pieces. This electrode force or
pressure holds together the work-pieces.
• The electrode pressure is then released.
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Stages in Spot Welding
Spot Welding Process
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Resistance Seam Welding
• Seam Welding is a process of joining overlapping sheets by the heat
generated by resistance to the flow of electric current through the workpieces held together under force by two rotating circular electrodes.
• Seam welding is a modification of spot welding wherein the electrodes
are replaced by rotating wheels or rollers
• Spot Welds are produced using rotating electrodes with regularly interrupted
current.
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Resistance Seam Welding
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Projection Welding
• The Projection Welding is similar to Spot Welding except that pointed
electrodes are replaced by flat and relatively large electrodes
• Small projections are raised on one side of the sheet or plate where it is to
welded to another
• The projections serve to concentrate(localize) the welding heat at these
areas and facilitate fusion
• During the welding process, the heated and softened projections collapses
under the pressure of the electrodes thereby forming the weld
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Projection Welding
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Percussion welding
• Percussion welding (PEW) utilizes the technique in which the
electrical energy for welding is stored in a capacitor
• The power is discharged within 1 to 10 milliseconds to develop
localized high heat at the joint.
• The process is useful where heating of the components adjacent to
the joint is to be avoided, as in electronic assemblies and electrical
wires.
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Percussion welding
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Stud Welding
• Stud welding (SW) is also called stud arc welding and is
similar to flash welding.
• The stud (which may be a small part or, more commonly, a
threaded rod, hanger, or handle) serves as one of the
electrodes while being joined to another component,
which is usually a flat plate.
• Polarity for aluminum is usually direct-current electrode
positive (DCEP), and for steel it is direct-current electrode
negative (DCEN)
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Stud Welding
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Friction Welding
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Friction Welding
• In friction welding (FRW), the heat required for welding is
generated through friction at the interface of the two
components being joined.
• One of the workpiece components remains stationary while
the other is placed in a chuck or collet and rotated at a high
constant speed.
• The two members to be joined are then brought into contact
under an axial force.
• After sufficient contact is established, the rotating member is
brought to a quick stop (so that the weld is not destroyed by
shearing) while the axial force is increased
• The pressure at the interface and the resulting friction produce
sufficient heat for a strong joint to form.
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Friction Welding
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Thermit Welding
• Thermit welding is a fusion process
• Thermit process is based on a chemical reaction which generates heat (Exothermic
reaction)
• Thermit is a mixture of powdered aluminum and iron oxide
• Weld is formed by pouring superheated thermit around the parts to be united
• Temperature produced by the Thermit reaction is around 3000°C
• A few Thermit reactions are
• 8Al + 3 Fe3O4 = 4Al2O3 + 9Fe (3088°C)
• 2Al + Fe3O4 = Al2O3 + 2Fe (2960°C)
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Thermit Welding
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ELECTRON BEAM WELDING (EBW)
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ELECTRON BEAM WELDING (EBW)
• In electron-beam welding, heat is generated by high velocity narrow-beam electron
• The kinetic energy of the electrons is converted into heat as they strike the workpiece
• The process requires special equipment to focus the beam on the workpiece, typically
in a vacuum
• The basic functions of any electron beam gun are to generate free electrons at the
cathode, accelerate them to a sufficiently high velocity and to focus them over a small
spot size
• Electron-beam welding equipment generates X-rays; hence, proper monitoring and
periodic maintenance are essential
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LASER BEAM WELDING (LBW)
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LASER BEAM WELDING (LBW)
• Laser-beam welding utilizes a high-power laser beam as the source of heat to produce a
fusion weld
• LASER stands for “Light Amplification by Stimulated Emission of Radiation”
• Because the beam can be focused onto a very small area, it has high energy density and
deep-penetrating capability
• The beam can be directed, shaped, and focused precisely on the workpiece
• For production of laser beam, we generally use Ruby rod in which aluminium is the main
ingredient and chromium being present as impurity in the ratio of 1 to 5000 atoms
• Flash lamps continuously bombard the chromium atoms of ruby rod and emits laser beams
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Advantages of LBW over EBW
• A vacuum is not required, and the beam can be transmitted through air
• Laser beams can be shaped, manipulated, and focused optically (by means offiber
optics), so the process can be automated easily
• The beams do not generate X-rays
• The quality of the weld is better than in EBW; the weld has less tendency toward
incomplete fusion, spatter, and porosity; and there is less distortion
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Heat Affected Zone (HAZ) in welding
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Heat Affected Zone (HAZ)
• HAZ is the region adjacent to the weld metal zone which has a microstructure
different from that of the base metal
• The heat from the welding process and subsequent re-cooling causes this change from
the weld interface
• HAZ consists of 3 regions:
 Grain growth region- region immediately adjacent to weld metal zone in which metal
is heated to above upper critical temperature
 Grain refined region (recrystallised zone) – Finest grain structure exists in which the
metal is heated to a just above the upper critical temperature
Transition region- The metal is heated to a temperature between upper and lower
critical temperature
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Weld Defects
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• Inclusions
Weld Defects
• Impurities or foreign substances which are forced into the weld puddle during the welding process. Has the
same effect as a crack. Prevented by proper technique/cleanliness.
• Segregation
• Condition where some regions of the metal are enriched with an alloy ingredient and others aren’t. Can be
prevented by proper heat treatment and cooling.
• Porosity
• The formation of tiny pinholes generated by atmospheric contamination. Prevented by keeping a protective
shield over the molten weld puddle.
• Grain Growth
• A wide T will exist between base metal and HAZ. Preheating and cooling methods will affect the
brittleness of the metal in this region
• Blowholes
• Are cavities caused by gas entrapment during the solidification of the weld puddle. Prevented by
proper weld technique (even temperature and speed)
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Soldering
• Soldering is a process in which two
or more items are joined together
by melting and putting a filler
metal (solder) into the joint, the
filler metal having a lower melting
point than the adjoining metal.
• Unlike welding, soldering does not
involve melting the work pieces.
• The filler metal flows into the gap
between
close-fitting
parts
by capillary action.
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Types of Solders
Tin lead solders- Used to join most metals. Bond
produced has good corrosion resistance.
a) 60% tin and 40% lead – used for electrical works
b) 50% tin and 50% lead – used for plumbing joints
Tin-Antimony-lead solders – Addition of antimony
increases the strength of the bond
Lead-Silver solders
Cadmium-Silver solders – Used to join aluminium to
itself or to other metals
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Fluxes used in soldering
 Inorganic acids or salts such as zinc-ammonium-chloride solutions
which clean the surfaces rapidly
Organic acid mild fluxes -Typical constituents of these fluxes are
Lactic acid, Stearic acid, Benzoic acid etc
Non corrosive resin based fluxes which are used in electrical
applications
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BRAZING
• Brazing is a metal joining process in which two or more metal items
are joined together by melting and flowing a filler metal into the
joint, the filler metal having a lower melting point than the
adjoining metal.
• Brazing differs from welding in that it does not involve melting the
work pieces
• The filler metal flows into the gap between close-fitting parts
by capillary action.
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Sl
No
Soldering
Brazing
1
Filler metal has a melting point Filler alloy has melting point
below 427 degree Celsius
above 427 degree Celsius
2
Produces weaker joints than Produces stronger joints
brazing
Soldering joints do not resist Brazed joints resist corrosion
corrosion
3
4
Suitable for thin similar or Suitable even for
dissimilar sheet metals
dissimilar metals
thicker
5
Cost is less
6
A soldering iron or small blow A furnace or heavy blow torch
torch is necessary
is necessary
Cost is more
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Adhesive Bonding
• Adhesive Bonding is the process of joining two surfaces
together, usually with the creation of a smooth bond. This
may involve the use of glue, epoxy, or one of a wide range of
plastic agents which bond either through the evaporation of
a solvent or through curing via heat, time, or pressure
• A common example of adhesive bonding is plywood, where
several layers of wood are bonded with wood glue.
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Types of Adhesives
Natural adhesives -such as starch, dextrin (a gummy substance
obtained from starch), soya flour, and animal products.
Inorganic adhesives -such as sodium silicate and magnesium
oxychloride.
Synthetic organic adhesives -which may be thermoplastics
(used for nonstructural and some structural bonding) or
thermosetting polymers (used primarily for structural bonding).
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