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

Seam 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.

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