1.0       INTRODUCTION

Welding is a material joining process used in making welds, and a weld is 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. They can be based on energy transfer, type of welding, or mode of welding.

1.1  CLASSIFICATION  BASED   

      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-Arc, Resistance welding.

2.                  Mechanical Energy to heat – Friction welding.

3.                  Chemical Energy to heat Gas welding, Explosive welding.

4.                  Light Energy to heat – LASER welding.

5.                  Sound Energy to heat – Ultrasonic welding.

1.2              CLASSIFICATION BASED ON

             TYPE OF WELDING

1.2.1        FUSION WELDING

These processes involve fusion of the base metal to complete the weld.  Fusion

Welds ordinarily 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 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 hand.  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: Here instead of stick electrodes, wire electrode is used.  The wire is in the form of spool and the wire is drawn and fed by the motorized wire feeder into the welding torch.  The welder gives 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.

Mechanical welding:   Welding with 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 or work.  SAW is an example of mechanized welding.  Here the welder is called machine operator.

Automatic welding:    Welding with 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 is one classification based on type of movement.  Ordinary robot or intelligent robots, which can see and carry out the job by it is another classification.

Robots:  There are over 100,000 robots are in use in Japan alone, and equal number in USA and Europe as on 2000.  In India there are just over 100 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 are a done without any manpower and it is practiced in Japan now.

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^o 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

4.      Furnace soldering 6.  Infrared 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 a liquids above 450^o C and below the solids 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 is 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, immediately a high starting current flows 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 manufacturer recommendation, which is printed on the cover of the electrode pocket.

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 1.6mm to 6.3mm and length varying from 250mm to 450mm.  The power supply required for welding may be either a transformer of 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 types 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 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^o 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 3kV 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.

Thus is an ideal process for welding of non-ferrous metals and stainless steel in limited thickness.  It is used for root pass in pressure vessels where welding from inside is not possible.

4.0              GMAW WELDING:

GMAW is abbreviation for Gas shielded Metal Arc Welding.  Here fusion is achieved by an electric arc formed between work piece and 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 as supply used is Argon or Helium, then the process is popularly known as MIG welding.  If the shielding gas is only CO_2, 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, 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, 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, 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% O_{2 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 above is enough for carbon

Steels.  CO₂ Welding is replacing shielded metal arc welding in the fabrication of structural, pipes, automobile products storage tanks and machinery 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 likeCO_{2 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 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.   It is of high capacity varying from 750 to 3000 Amps.  Since the process employs such large currents, to protect the welder from arc heat, the arc is completely submerged in the layer of flux.  Since the welder is fully comfortable, the process is successfully employed all over India.

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 2.4mm to 6.3mm.  The feed rate varies from 5 m/min to 15 m/min.  The welding wire is always connected to positive polarity of the power supply.  The wire can be solid cored or flux cored wire. The process gives very high productivity, and excellent weld quality.  This process is ideal for heavy thickness.  Generally used for welding boiler pipes, drums cross country pipelines, offshore platforms, and piping.  It is popularly used in fabrication of ships, plate girders, pressure vessels, pipes and penstocks, for welding, surfacing and strip cladding.  In strip cladding wire is in the form of strips of size say 1.6mm *75mm wide.  To achieve better deposition rates out of this process, tandem welding with 2 or more torches are used.

6.0 RESISTANCE SPOT WELDING

In this process, a spot of 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 of 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    RESISTANCE BUTT WELDING

This process is also known as upset welding or simply butts – welding.  Here, the resistance to the passage of electric current raises the temperature of the joint across the interface of the joint.  The parts to be joined (usually wires and rods) are held in clamps, one stationary and the other movable, which act as conductors for the low-voltage electric supply and also apply force.  This pressure is applied only after the abutting surfaces have reached a temperature slightly below the melting point, which results in the upsetting of the metal. Uniform and accurately mating surfaces are desirable to exclude air and give uniform heating.  The process is commonly used during rod rolling and wire drawing operations to join the ends.  Resistance wire butt welder is one of the popular variations of this process.

6.5    STITCH WELDING

This is a variation of spot welding, in which a series of overlapping spot welds are made in the same manner as stitching cloth.  Stitch welding may be performed with a normal spot welding machine or with a specially designed one, which automatically makes spot welds in a continuous series.

6.6    MULTIPLE SPOT WELDING

This is a modification of spot welding, in which two or more welds can be obtained simultaneously from each transformer secondary. For specific product welding this process is employed.

6.7    SERIES SPOT WELDING

In series welding, a portion of the secondary current bypasses any weld nugget being formed.  This shunt current passes through one of the panels being welded. Generally, two welds are made per transformer secondary.

6.8    ROLLER SPOT WELDING

In this process, a series of intermittent spot welds are made using wheels or rollers as electrodes.  The rollers are power driven and are stopped while individual welds are made.  Current is passed intermittently when the electrodes are stationary. 

6.9    FOIL BUTT – SEAM WELDING

This is a modification of seam welding, in which thin narrow strips of metal are introduced between one and both of the circular electrodes and the work piece.  The joint edges are held in the same plane instead of being overlapped.   The strips help to localize the melting and to avoid reduction of section thickness at the joint.  The process is being used successfully on the shells of rail coaches.

6.10 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 very quickly and pressure is maintained or increased until welding is complete. Friction welder helps in achieving consistently high quality of joints each within a few seconds in various similar and dissimilar metal combinations.  This 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

Friction Stir Welding (FSW) has been invented patented and developed for its industrial application by TWI, UK.  In 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 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 and without porosity or internal voids.  High integrity welds with low distortion can be achieved in many aluminum alloys even those considered difficult to weld by conventional fusion welding techniques.  Materials that have been successfully friction stir welded include a variety of aluminum alloys, copper alloys and AI – Li alloys.

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. As the beam is moved the  keyhole also is moved leaving behind  the molten metal to solidify. 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     refractory metals such as tungsten, molybdenum, columbium, tantalum and metals, which oxidize readily such as titanium, beryllium, and zirconium.  It can also be used to join aluminum steel and ceramics.  EB machine is highly expensive but its use is justified for  critical applications such as nuclear and aerospace  components fabrication.

10.0  LASER WELDING

In this process, fusion is achieved  by  directing a highly concentrated beam to a fine 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.  In CO 2 lasers, CO2 is the lasing medium.  A high voltage of the order of 20 to 30 KV is applied through a pair of electrodes.   This high voltage excites the gas particles.  When a fundamental particle of light such as photon hits the excited gas CO2 gas atom, the excited CO2 gas atom returns to normalcy.  But in the process it emits another photon with same direction,  amplitude and frequency.  This achieves the light amplification by stimulated emission of radiation. 

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 other difficult to weld metals such as nickel, tungsten, steel,  titanium and columbium.  This can also be used for cutting hula, rubber, plastic, paper, cloth 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 50 Hz power to the high frequency 15-60 KHz required by ultrasonic head.  The head hasa 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 38mm.  The two plates are held vertically with a gap of 12mm. regardless of  plate thickness.  The wire electrode is introduced downwards in to the cavity formed   by two plates to be joined and two movable water-cooled copper chill blocks.  The cavity is kept free of air by the shielding gas usually a mixture of  argon and CO2.  Wire diameter may be1.6mm to 2.4mm. The welding head is suspended from elevator mechanism, which provides automatically control of vertical travel speed during welding. The welding head is automatically  raised as the molten  is building up.  The welding is completely automatic.  This technique is used in shipyards, and in fabrication of storage tanks and large  diameter pipes.  One such machine has  been developed in WRI and they are used in BHEL, Bhopal and Haridwar  for fabrication of structural by welding in the vertical direction.

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 12.5mm to 500mm.  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 500mm 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 and acts as a mould to cool and shape the weld surfaces.  The copper  shoes move automatically along upward as welding progresses.  Many years ago the process was  employed for welding  of boiler drums and thick plates in India and Russia.  Now it has become obsolete.

14. SPECIAL WELDING PROCESSES

14.1    CAPACITOR DISCHARGE

WELDING:

This is used for welding  of studs, welding of thermocouples etc on base metal.  A low voltage, high amperage 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 pool.

14.2    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 proper  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 the 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 in flux of air to the 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.

14.3    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 commonly gas is acetylene  and sometimes  hydrogen is used.  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 as soldering, brazing and thermal spraying.  The main advantage of this process is it does  not need power supply.  In India, even  today, the bicycles are produced with gas welding process only.

14.4ATOMIC HYDROGEN WELDING

In this process an arc is struck  between  two tungsten electrodes using AC power supply. Streams of hydrogen gas are passed from orifices around the electrodes in to the arc. Here  the  molecules of hydrogen dissociate in to molecules at a point a few millimeters away from the arc and liberate intense heat, which melts the base metal.  The hydrogen gas acts as a carrier of heat.   Filler metal may or may  not be used.  As on date  this process is obsolete.

14.5  THERMIT WELDING

This process utilizes the intense heat developed during the reaction between iron oxide and aluminum.  When a mixture of three parts of iron oxide and one part  of a aluminum by weight is  locally with a special ignition powder a  vigorous reaction takes place which  proceeds rapidly through the mass, resulting in the formation of aluminum oxide and  iron  and a considerable amount of heat.  The heat is sufficient to melt iron and oxide slag.  In carrying out Thermit welding the Thermit mixture is placed in a refractory crucible above the pieces to be welded.  The molten metal from reaction 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 of railway tracks.

14.6  ARC SPOT WELDING

In this  process, coalescence at the overlapping surfaces is produced  in one spot by heating with an electric arc between an electrode and the work.  The weld is made without preparing a  hole in either member.  Filler metal or a  shielding gas or flux may or may not be used.  Using either of the manual metal arc, TIG and GMAW  processes produces the arc. Arc spot welding by the CO2 process is widely used today, for which the equipment is provided with the necessary controls to achieve  consistent spot welds. While electric resistance spot welding requires access from both sides of the overlapping plates, arc spot welding can be  made from one side  only.

14.7   FIRECRACKER WELDING

This is a semi-automatic version of MMA welding.  A specially designed heavy – coated electrode, which can be of any length  up to 2 meter, is laid on the seam of a grooved butt joint or along the root  of a tee joint.  It is then clamped down with a water-cooled  copper bar or heavy square section, which is grooved to accommodate the electrode.  The copper bar is nearly   as  long as the electrode.  The bare end is clamped in a holder.   The arc is struck at the striking end, and the electrode gradually consumes by itself, making the butt or fillet weld of the  same length as the electrode.   The copper block helps to prevent overheating of the electrode  and to maintain intimate contact between the electrode and the joint.

15.0  PLASMA WELDING:

    

Add caption

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  gas (argon) is fed through the arc it  becomes heated to the plasma temperature range (30000 °F to 60000 °F).  The plasma tail flame issues 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, titanium, model and income.  Plasma Mig welding is a development of plasma  welding process.  Plasma process is used for cutting applications very  effciently.

15.1          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 – 1.6mm.

16.0          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 a sufficiently long time.  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.  Sometimes thin inserts are  placed between the mating surfaces to  speed up diffusion and ensure strong welds.