INTRODUCTION
Welding
is a material joining process which can be defined as a localized coalescence
of metals or non metals produced either by heating the materials to suitable
temperature with or without the application of pressure or by the application
of pressure alone and with or without the use of filler material. The various
welding processes can be classified in many ways, such as based on energy
transfer, type of welding, or mode of welding.
1.1 CLASSIFICATION BASED ON MODE OF ENERGY TRANSFER
Energy
is converted from one form to another. Any form of energy can be converted to
heat energy, which can be used for welding.
1. Electrical Energy
converted to heat energy – SMAW, SAW, GMAW, Plasma & Resistance welding.
2.
Mechanical Energy to heat energy
Friction & Friction stir welding
3.
Chemical Energy to heat energy -
Gas, Explosive & thermit welding
4.
Beam Energy to heat energy -
LASER & Electron beam welding
5. Sound Energy to heat energy -
Ultrasonic welding
1.2 CLASSIFICATION BASED ON TYPE OF WELDING
Welding
processes shall be classified under Fusion and Solid state welding types
1.2.1 FUSION WELDING
Fusion welding involve melting of the
base metal and the filler to form a union. Fusion welds generally do not
require the application of pressure, and they may be completed with or without
the addition of filler material. All the
arc welding processes such as SMAW, SAW, GMAW, Plasma and gas welding are
covered in this category.
1.2.2 SOLID STATE WELDING
These are processes in which the two
sides of a joint are brought in to intimate atomic contact either by mechanical
deformation or by atomic diffusion or by a combination of both.
Cold
pressure welding
uses mechanical deformation at room temperature.
Hot
pressure welding uses
heat to render the metal ductile.
Friction
welding employs
rubbing at the interface to generate heat.
Pressure
butt welding used
to join bar and sections, end to end using heat generated by electric arc,
electric induction or electric resistance across the joint.
Diffusion
welding employs
modest deformation but the temperature
and its duration
are sufficient to allow atomic diffusion across the interface.
The various welding processes are
classified depending on the nature of application of these processes.
1.3
CLASSIFICATION BASED ON MODE OF WELDING:
Manual
welding: Welding
wherein the entire welding operation is performed and controlled by a welder /
operator. Here there are two movements involved. One is the feeding of the
electrode and the other is for the welding speed. If both are done by hand then
it is called manual metal arc welding.
Semi-automatic
welding: In
this case continuous wire electrode is used instead of stick electrodes, which
are not continuous. The wire is wound in the form of spool and is drawn and fed
by the motorized wire feeder into the welding torch. The welder provides only
the welding speed. Since the wire feeding is automatic and the welding speed is
manual, this type of welding process is called semi-automatic welding process.
MIG welding is an example of semi-automatic welding.
Mechanized
welding: Welding
with an equipment, which performs the welding operation under the constant
observation and control of an operator. Motors control both wire feeding and
the welding speed in mechanized manner.
The equipment may or may not perform the loading and unloading of
work. SAW is an example of mechanized
welding. In this case, the welder is called machine operator.
Automatic
welding: Welding
with an equipment, which performs the entire welding operation without constant
observation and adjustment of the controls by an operator. Welding Robots are
coming under this category. In robotics,
there are many classifications. Cartesian type, cylindrical type, polar type
and jointed arm type. Robot without vision or intelligent and robots with
vision, which can see and carry out the job by itself is another
classification.
Robots:
There are over
200,000 robots are in use in Japan alone, and equal number in USA and Europe as
on 2004. In India there are just over 500 robots are in use, mainly due to
availability of huge manpower. But it is expected that the number of robots
employed in India also will increase considerably as the demand for higher
productivity and quality is fully realized by the Indian industries nowadays.
Lights-off manufacturing is a new method where the production is done without
any manpower and it is practiced in Japan to a greater extent.
1.4
WELDING / SOLDERING / BRAZING
Depending on the temperature of
application, the welding process takes different names.
1.4.1
Soldering:
This is a group of joining processes,
which produces coalescence of materials by heating them to a suitable temperature
and by using a filler metal having a melting point not exceeding 450°C and
below the melting point of the base materials.
The filler material is distributed between the closely fitted surfaces
of the joint by capillary action. Some
of the popular soldering processes:
1. Dip soldering 2.
Induction soldering, 3. Iron soldering 4. Torch soldering 5. Furnace soldering
6.Infra red soldering. 7. Resistance
soldering 8.Wave soldering
1.4.2
Brazing:
Brazing is a "group of welding
processes which produces coalescence of materials by heating them to a suitable
temperature and by using a filler metal having meting point above 450°C and
below the softening point of the base materials. The filler metal is distributed between the
closely fitted surfaces of the joint by capillary attraction". A braze is
a special form of weld, the base metal is theoretically not melted. Some of the popular brazing methods are:
1. Diffusion brazing 2. Dip
brazing 3. Furnace brazing 4. Induction brazing 5. Infrared brazing 6.
Resistance brazing 7.Torch brazing.
2.0
SHIELDED METAL ARC
WELDING
The heat required for welding is
generated by the electric arc formed between metallic electrode and base metal.
When the welder touches the plate to be welded with the electrode, a high starting
current flows instantaniuosly, which melts the tip of the electrode, thus
initiating the welding arc. If the welder maintains the arc gap, the arc
continues to remain stable and the arc heat melts the plate, electrode and the
flux coating. The flux covering of the
electrode melts in the arc heat and produces a large volume of gases and the
slag. The gases cover the arc and shield it from reacting with the atmospheric
air. The slag covers the molten metal pool until it solidifies. The slag can be
removed after welding. The flux also provides filler metal with alloying
elements to increase the strength of the weld joint. The electrode is consumed in the arc during
welding. In SMAW, the electrode polarity is chosen depending on the
manufacturers recommendation.
Shielded metal arc welding is the most
common, versatile and least expensive one and accounts for nearly 25% of total
welding in developed countries and nearly 85% of total welding in India. The electrode diameter varying from 1.6 mm to
6.3 mm and length varying from 250 mm to 450 mm. The power supply required for welding may be
either a transformer or motor-generator or transformer-rectifier supplying AC
or DC power. For achieving high quality
welding, thyristor controlled or transistor controlled or inverter based power
sources are employed. These latest solid state type of power sources is having
feedback controls to deliver the controlled amount of voltage and current to
the welding arc to achieve desired weld quality. The inverter power supplies
are the latest technology and up to date power source. The main advantage of
this power supply is the portability, lightweight and robust design. It can
give required characteristics so that we can do SMAW, GTAW, GMAW, FCAW and
Gouging with the same power supply.
3.0
GAS TUNGSTEN ARC WELDING
Gas Tungsten Arc Welding (GTAW) is
popularly known as Tungsten Inert Gas welding (TIG). In this process, an arc is
struck between a non-consumable tungsten electrode and the base metal. The
tungsten electrode is called non-consumable electrode as it melts only at
around 2800°C and in the arc heat, any other metal melts at lower temperatures.
The TIG arc is shielded by inert gas like argon, helium or a mixture of
both. A filler wire may or may not be
used. AC power supply is used for
aluminum alloys and DC for all other metals.
Normally when an arc is struck between
an anode and cathode, more heat is observed at the anode and less heat at the
cathode. Hence in TIG welding the tungsten electrode is always connected to the
negative polarity of the power source so that the electrode does not melt.
Since the tungsten has high melting
point, it is brittle and breaks easily. Hence, generally the electrode is not
touched with the plate to strike arc but a high frequency unit is used to
initiate the arc. The HF unit is actually a high voltage, high frequency unit
that generates around 3 kV voltage and 3 MHz frequency which ionizes the gap
between the electrode and the plate thus enabling the arc to jump from the electrode
to the plate. This is an ideal process for welding of non-ferrous metals and
stainless steel to a limited thickness.
It is used for root pass in pressure vessels where accessability from
inner side is restricted.
4.0
GMAW WELDING
In Gas shielded Metal Arc Welding
(GMAW), fusion is achieved by an electric arc formed between work piece and a
continuous solid wire electrode, which is fed through a welding torch at
controlled speeds. Inert gas argon flows through the torch and forms a blanket
over the weld puddle to protect it from atmospheric contamination.
If the gas supply used is Argon or
Helium, then the process is popularly known as MIG welding. If the shielding
gas is only CO2, then the welding process is called CO2
welding. If the shielding gas is a mixture of argon and CO2 then the
process is called MAG welding. The wire may be solid wire or flux cored wire.
When the flux-cored wire is used for welding, the process is called FCAW.
The welding can be semi automatic or
mechanized. This is highly amenable for robot welding, or robot manipulation.
Generally a constant potential type of power source is used. This may be a
transformer-rectifier, thyristor controlled, transistor controlled or inverter
type of power source, which may be chosen depending on the quality requirement.
Generally, in both GMAW and SAW processes, welding wire is connected to
positive polarity of the power supply.
GMA welding is gradually replacing SMAW
and TIG welding. Most metals can be easily welded including aluminum, carbon
steels, stainless steels, low alloy steels, nickel, copper, magnesium and
titanium. More than 65% of the welding carried out in developed countries is
done by this process. In India as much as 10% of the welding is by MIG/MAG
welding, and nearly 85% of the welding is by SMAW only.
4.1
CO2 WELDING
CO2 Welding is a variation
of GMAW Welding in which the inert gas is replaced by gas mixtures or carbon
dioxide, which are chemically active. The carbon dioxide decomposes in the
welding arc heat and oxygen is produced. To remove this, de-oxidants are
employed in the welding wire. This produces a glassy layer of slag over the
weld metal, which can melt off in the subsequent passes, or it can be brushed
and removed.
Argon+1 to 2% O2 is used as
shielding gas for alloy steels and stainless steels. Argon+3 to 5% O2
is used as shielding gas for carbon steels and low alloy steels. CO2
alone is amenable for carbon steels. CO2 Welding is replacing SMAW
in the fabrication of structural, pipes, automobile products storage tanks and
machinery manufacture etc.
4.2
FLUX CORED ARC WELDING
This is a variation of GMAW, where
solid wire is replaced by flux cored wire. The equipments and accessories are
the same. Generally flux cored wires require additional gas shielding like CO2
and some times the large diameter flux cored wires are self shielding type and
they do not require additional gas shielding. The main advantage of flux-cored
wire is that we can easily alter the desired levels of alloying elements during
manufacture. The process will produce a
light slag, which can be easily removed.
5.0
FLUX SHILDED ARC WELDING
This process is popularly known as
Submerged Arc Welding (SAW). This is a fully mechanized welding process. The
electrode is a continuous metallic solid/flux cored wire in the form of coil.
It is fed automatically in to the arc at a constant speed. A layer of flux
covers the arc.
The power source used is generally a
constant potential power source, transformer or a transformer-rectifier with
high capacity varying from 750 to 3000 Amps. The arc is fully submerged under a
granulated flux layer and hence no radiation of heat or fumes are produced.
The welding head is mounted on a
trolley, which travels along the joint. Alternately the welding head is
stationary and the job is moved under it. The wire diameter varies from 3.15 to
6.3 mm. The feed rate varies from 2 to 15 m/min. The welding wire is always
connected to positive polarity of the power supply. The process gives very high
productivity, excellent weld quality and is ideal for heavy thickness.
Generally used for welding boiler pipes, drums, cross country pipelines,
offshore platforms, and ship building. This process is used for surfacing and cladding also. To achieve
highre deposition rates, tandem and multiple
wire welding technique are used.
6.0
RESISTANCE WELDING
Resiatnce heat energy generated shall
be used with pressure to make spot or seam welds.
6.1
RESISTANCE SPOT WELDING
In this process, a spot weld is made
between overlapping sheets by means of two cylindrical copper alloy electrodes,
one on top and the other at the bottom, which carry a high current. The electrodes also clamp the work and apply
pressure when the metal at the joint gets sufficiently heated by electrical
resistance. A tiny button of fused metal
results at the sheet interface and it is called nugget. The electrodes are
retracted after the weld is completed.
The process is used in large scale in automobile production.
6.1
RESISTANCE SEAM WELDING
This is similar to spot welding except
that the copper alloy electrodes are in the form of circular rollers. The
overlapping sheets are held under constant pressure between the roller
electrodes, which rotate at constant speed and carry current. A series of spot welds whose nuggets are
overlapping on each other are formed which give the appearance of a continuous
seam. The process is employed in fabrication of oil / tar drums, railway
coaches, automobile industries and transformer cooling fin fabrication etc.
6.2
PROJECTION WELDING
This
is a modified method of making single or multiple spot welds. Projection welds are made by providing an
embossment or projection on one or both of the contacting base metal surfaces
to localize the pressure and current flow at a particular point. The process is
employed for fabrication of automobile components, wheels etc.
6.3
FLASH BUTT WELDING
This process is an extension of
resistance butt-welding. The parts to be
joined are gripped in the clamps and their interfaces are gradually brought
into contact to complete the secondary circuit.
When the welding voltage of up to about 10 volt is applied at the
clamps, current flows through the initial points of contact causing them to
melt. The platen on which the movable clamp is mounted is moving forward, fresh
contacts are made and then there is a continuous flashing of sparks. Flashing is allowed to continue until the
surfaces to be joined are uniformly heated or molten. At the point extra pressure is applied to the
moving platen so that the tubes are forged together and the molten metal is
expelled and weld joint is achieved. The process is used for joining of boiler
economizer tubes, rails, hanger rods etc. The process was also employed for
welding of cross-country pipelines in Russia.
6.4
INDUCTION PRESSURE WELDING
In this process, the current is induced
by high frequency induction process in the job. Due to electrical induction the
job is heated. When a suitable
temperature is reached, the weld is consolidated by a forging action of the
joint. This is generally used in
manufacture of boiler tubes. A very high level of productivity can be obtained
with this process.
7.0
FRICTION WELDING
In this process, friction is employed
to generate heat between two sliding or rotating metal surfaces. The process is usually carried out by placing
the pieces to be welded in chucks on a common horizontal axis. One part is rotated and other remains
stationary. Pressure is applied to
generate enough heat to reach a bonding temperature within a few seconds. At this point, rotation is stopped instantly
and pressure is maintained or increased until welding is complete. Friction
welding helps in achieving consistently high quality joints within a few seconds
on similar and dissimilar metal combinations. This is widely employed for
joining of tool steel to carbon steel for tooling purposes, joining of aluminum
to copper for end termination of electrical connections etc.
7.1 FRICTION STIR WELDING
In Friction Stir Welding (FSW), a cylindrical shouldered tool
with a profiled pin is rotated and slowly plunged into the joining area between
two pieces of sheet or plate material, which are butted together. The parts
have to be clamped onto a backing bar in a manner that prevents the abutting
joint faces from being forced apart. Frictional heat between the wear resistant
welding tool and the work pieces causes the latter to soften without reaching
the melting point and allows traversing of the tool along the weld line. The
plasticized material is transferred to the trailing edge of the tool pin and is
forged by the intimate contact of the tool shoulder and the pin profile. On
cooling down it leaves a solid phase bond between the two pieces. Friction stir welding can be used to join
aluminum sheets and plates without filler wire or shielding gas. Material
thickness from 1.2mm to 75mm can be welded at full penetration, without
porosity or internal voids.
8.0
EXPLOSIVE
WELDING/ CLADDING
In this process, two pieces of metal
are impacted together at an extremely high velocity of impact achieved by the
detonation of an explosive charge. The
result is a solid-state weld completed in microseconds without any noticeable
deformation. The process has been used
to prepare clad plates involving dissimilar metals, and in welding tubes to
tube sheets in heat exchangers.
9.0
ELECTRON BEAM WELDING
In this process fusion is achieved by
focusing a high power density beam of electrons on the area to be joined. Up on
striking the metal, the kinetic energy of the high velocity electrons changes
to thermal energy causing the metal to melt and the beam passes through the
thickness of the plate thus making a keyhole. The electrons are emitted from a
tungsten filament heated to approximately 3000°C. The electron gun the job and
the fixtures are kept in a vacuum chamber. Very high welding speeds, high
purity of welds and a very good control of weld parameters are possible to
obtain by using this process. EB welding is highly suitable for welding of
aluminium, copper, refractory metals such as tungsten, molybdenum, columbium,
tantalum and metals, which oxidize readily such as titanium, beryllium, and
zirconium.
10.0
LASER WELDING
In this process, fusion is achieved by
directing a highly concentrated beam to a narrow spot. The word LASER is an abbreviation of Light
Amplification by Stimulated Emission of Radiation. High-energy output is
available from CO2 Laser and Nd-YAG Lasers. A high voltage of the
order of 20 to 30 KV is applied and the lasing medium is excited to emit photons with same direction, amplitude
and frequency. The emitted photons traveling in the same direction, hits the
mirror in one direction, reflected back hits the mirror in the opposite
direction. Within a short time a narrow, coherent beam of laser light is formed,
a part of which comes out through the partially transparent output mirror. This
beam is further focused to achieve the welding or cutting etc.
Depending on the lasing medium, we have
solid, liquid, gas and semiconductor lasers. CO2 Laser is the most
popular gas laser and Nd-YAG is the most popular Solid laser used for
industrial applications. Laser beam can be used to weld difficult to weld
materials such as nickel, tungsten, steel, titanium and columbium. This can also be used for cutting non metals,
rubber, plastics, paper, textile etc.
Lasers are used in medical, electronic, aerospace, defence,
communication and fabrication industries.
11.0
ULTRASONIC WELDING
This is a solid state welding process
for joining similar or dissimilar metals by application of high frequency
vibratory energy to work pieces held together under moderate static pressure.
The equipment consists of a frequency converter, which changes 50Hz power to
the high frequency 15-60 KHz required by ultrasonic head. The head has a piezo-electric transducer,
which converts the high frequency electrical output into vibratory mechanical
motion and wave-guides, which transmits the vibratory waves to the tools and
into the weldment. The job is clamped
while welding. The process is used to
weld plastics, thin foils and dissimilar metals. The process is employed for
manufacturing of toys for children
12.0
ELECTROGAS WELDING
This process is an extension of MIG /
MAG welding, designed for single pass vertical welding of steel plates in the
thickness range of 10 to 50mm. The two plates are held vertically with a gap of
12mm. regardless of plate thickness. The
wire electrode is introduced downwards into the cavity formed by two plates to
be joined and two movable water-cooled copper chill blocks. The cavity is
shielded with a mixture of argon and CO2. The welding head is
suspended from elevator mechanism, which provides automatic control of vertical
travel speed during welding. This technique is used in shipyards, and in
fabrication of storage tanks and large diameter pipes. Two such machines has
been developed in WRI and installed at BHEL units.
13.0
ELECTROSLAG WELDING
This process is similar to the electro
gas welding process designed for making butt welding in the vertical position
in one single pass and the plate thickness can vary from 12.5 mm to unlimited
thickness. The welding heat is provided by a small quantity of flux which is
converted in to a conductive molten slag by its resistance to the electric
current passing between the continuously fed wire or wires (up to three wires
for 200 mm plates) and the parent plates. The hot molten slag melts the filler
wire and the joint surfaces and also shields the weld pool which mover up along
full cross section of the joint as welding progresses. There is no arc and welding is quiet and
spatter free. A pair of water-cooled copper shoes fitted on each side of the
joint retaining the molten metal and slag pool.
14.0
PLASMA WELDING
This is an extension of TIG welding. In
plasma welding torch, plasma energy is concentrated and ensures its most
efficient utilization for welding cutting and spraying. The tip of the tungsten
electrode is located within the torch nozzle while the nozzle has a small
opening which constricts the arc. As the gas (argon) is fed through the arc, it
becomes heated to the plasma temperature range (30000° F to 60000° F). The
plasma tail flame from the torch nozzle acts as a jet of tremendous velocity.
The plasma arc is of two types. Transferred arc and non-transferred arc. In the
latter the arc is formed between the electrode and the orifice inside the
torch. The process is used for welding of steels stainless steels, copper,
aluminum, and titanium. Plasma process is used for stainless steel cutting very
efficiently
14.1
MICRO-PLASMA WELDING
This is a modified plasma welding
process using DC current range of 0.1 -10 amps. It is capable of welding
extremely thin sheets and foils in the thickness range of 0.05-1.0mm.
15.0 DIFFUSION BONDING
In
this process, union between specially prepared mating surfaces takes place as a
result of diffusion, which occurs due to high temperature and pressure, exerted
for sufficiently longer durations. The pressure is low enough to ensure that
there is no plastic flow or deformation. The extended time of several minutes
at elevated temperatures (which will cause oxidation) requires that the joint
be made in protective atmosphere or vacuum. Thin inserts are placed between the
mating surfaces to speed up diffusion and ensure strong welds.
16.0
OTHER WELDING PROCESSES
16.1
CAPACITOR DISCHARGE WELDING
This is used for welding of studs,
thermocouples etc on a substrate. A low volt and high current electrical
discharge from a capacitor, supplies the arc energy. The current creates an
arc, which melts the entire face of the stud, and a similar area of the work.
The stud is then driven at a high velocity in to the molten pool.
16.2
STUD WELDING
This is an arc welding process in which
the arc is struck between a metal stud or similar part and the base metal. The arc heats the mating ends to a required
temperature after which they are brought together under pressure. Operator
positions the stud, held in a portable pistol shaped tool called stud gun. Once initiated, welding time and final
driving home of stud to complete the weld are controlled automatically by a
timing device. A ceramic ferrule is used with each stud. The ferrule
concentrates the heat, prevents oxidation of molten metal and confines the
molten metal to the weld zone. The stud welding can also be carried out by
resistance welding or friction welding.
16.3
GAS WELDING
In this age-old process, the melting of
the base metal is achieved by means of a gas flame, which derives its intense
heat from the combustion of fuel with oxygen. The most common gases used are
acetylene and hydrogen. Filler metal may or may not be used. Equipment for gas
welding consists of oxygen and acetylene cylinders, pressure regulators that
reduce the high cylinder pressure to the required working pressure, a torch
where two gases are mixed and hoses, which connect the regulators to the torch.
This is used for welding metals of low melting points and operations such as
soldering, brazing and thermal spraying. The main advantage of this process is that
it does not need power supply.
16.4
THERMIT WELDING
This process utilizes the exothermic
heat developed during the reaction between iron oxide and aluminum. When a
mixture of three parts of iron oxide and one part of aluminum by weight is
ignited with Mg tape, a chemical reaction takes place which proceeds rapidly
through the mass, resulting in the formation of aluminum oxide and iron and
produces considerable amount of heat. The heat is sufficient to melt iron and
oxide slag. For welding, Thermit mixture is placed in a refractory crucible
above the pieces to be welded. The molten metal is guided to the joint to be
welded by a sand mould, which is fastened around the work. By virtue of its
superheat the Thermit metal melts a portion of base metal with which it comes
in contact. This process is ideal for welding rails and tor steel rods.
