161976048 Advance Welding Technology

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INTRODUCTION TO WELDING

.Definition:-

The welding is a process of joining two similar or dissimilar metals by fusion or without fusion, with

or without the application of pressure and with or without filler metal. [3]

During fusion a solid union or a compact mass is formed.

If filler material is similar with the base material then this type of welding is called homogenous

welding and if filler material is different from base material then it is heterogeneous welding, where

filler material is given should have low melting temperature.

Types of welding:-

The overall welding process shown in chart below

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The welding is broadly divided into the two groups:

I. Pressure welding or diffusion welding: this welding process is done under pressure

without additional filler metals. It is classified as-

a) Hot pressure welding and b) cold pressure welding

1st state2nd state

2nd 4

II. Fusion or non-pressure welding: This process is done with additional filler metals. It is

classified as a) Gas welding, b) Thermit welding, c) Electroslage welding, d) Electron

beam welding, e) Laser beam welding and f) Arc welding.

Cold pressure welding

Cold or contact welding is a solid-state welding process in which joining takes place without

fusion/heating at the interface of the two parts to be welded. Unlike in the fusion-welding processes,

no liquid or molten phase is present in the joint.

Cold pressure welding is the establishment of an atom-to-atom bond between the two pieces to be

joined through intimate contact between oxide-free areas achieved under pressure and without the

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formation of liquid phase. In order to develop this bond, surface films have to be removed or at least

reduced in amount. Cold pressure welding is used for joining of aluminium cables, joining wires and

rods, various kitchen furniture, communication lines, and application of joining different materials

nowadays.

ADVANTAGE

Cold pressure welding of metals has the following advantages:

There is no softening of a work hardened or heat-treated metal since the process is carried out at

room temperature. This welding is suitable for electronic parts, which may be broken by heating

When dissimilar metals are welded, a brittle intercrystalline layer is not formed which is observed in

the conventional heat welding at the interface of the metals. Then, welding cannot be achieved by just

pressing two metals together since the surface of the metal is generally covered with oxide layer,

absorbed vapour layer and stained layer such as oil. Cold pressure welding is accomplished with

intimate contact of virgin metals, which appear owing to the breakdown of the surface layers, by

plastic deformation of the base metals.

PROCEDURE

Cold pressure welding can be characterized by the large number of possible metal combinations.

1. Surface Preparing

In order to reduce surface films, all the specimens were first degreased in acetone and then wire

brushed using a motor driven wire brush.

2. Deformation Amount in Lap Welding

In this welding method, deformation amount is an important parameter and named with deformation

result with the joined surface. In order to obtain bonding joint, plastic deformation of the two metals

is necessary, supposing that a basic parameter in cold pressure welding is the degree of deformation

normally expressed as the reduction R.

For lap welding is given by

where h0 is the original thickness of sheet and h1 is the instantaneous thickness at deformation

R.

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3. Surface Roughness

The fact that initially rough surfaces are required for welding suggests that bringing oxide free metals

into contact does not result in welding unless there is also some shear displacement as the two

surfaces come into contact.

The Bond Formation

Wire brushing at mechanical surface preparation forms a hard and brittle surface film

At metal surface. This layer is called as cover layer. The observations on researches show that bond

formation is realized by means of the stages given below. The stages are given in

Figure 1 shows schematically the mechanism of bonding. Deformation has not been

yet occurred in figure 1.a., and the cover layers are intact. Figure 1.b shows that a small deformation

has been resulted in fracture of the two cover layers as one layer. In figure 1.c, the surface expansion

has further increased, and extrusion of virgin material through the cracks is initiated. Real contact and

bonding have been established between the rough end surfaces of the extruded metals as shown in

figure 1.d.

Welding Dies and Welding Schematic welding dies designed for lap welding is given figure 2.Inspace

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After surface preparation, the specimens are immediately set in the welding die and then the pressure

was applied. At the beginning of the experiments, the pressure is applied at very slow rate and then at

a much higher rate. It is found that the rate of applying the pressure does not have a marked effect on

either the welding deformation or the weld strength. The welding time (time of applying and releasing

the pressure) is then set to be 1 min in all the experiments were carried out at room temperature.

Hot pressure welding

Hot-pressure-welding is a solid state process that produces joints between the faying surfaces of twobodies. It is done by application ofheat and pressure. Fusion temperature is not reached, filler metalis not needed, and substantial plastic deformation is generated. Heat is generally applied by flamesof oxyfuel torches directed on the end surfaces of solid bars or hollow sections to be joined.Alternatively, heat can be generated by eddy currents caused by electrical induction from a suitableinductor coil. As soon as the two bodies facing ends reach the correct temperature, the torches aresuddenly removed, not to stand in the way.

The bodies are brought to contact and upset together under pressure, usually by hydraulic equipment.

This variant is properly called the open joint process. If the parts are making contact under pressurebefore heat application from the outside, it is called the closed joint process. In either case flashmaterial is expelled and a bulge is formed at the joint.

Hot-pressure-welding is similar in a way to both friction welding (see Friction Welding Processes)and flash welding (see Flash Welding Process), although the source of heating is different. Forobtaining the best results the surfaces should be machined square and clean. Some beveling can beused to control the amount of upset. The process as described is performed as a manual operation.

The materials to be welded must exhibit hot ductility or forgeability. Therefore cast iron cannot beHot-pressure-welded. The materials commonly joined by Hot-pressure-welding are carbon steels,low alloy steels, and certain nonferrous metals. Certain dissimilar materials combinations are

weldable by Hot-pressure-welding. Materials that immediately form on the surface adherent oxides

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upon heating cannot be easily welded in air by this process. Typically among them aluminum alloysand stainless steels. Tests were performed in a vacuum chamber.

Advantages

Simple process

Simple joint preparation

Relatively low cost equipment

Quick weld production

High quality joints

No filler metal needed

Minimally skilled operators required

Limitations

Not all metals are weldable

Not easily automated

Length of cycle dependent on time for heating

Removal of flash and bulge required after welding.

Only simple sections readily butt weldable.

The most important parameter is the pressure sequence cycle, possibly being developed by trial anderror. Pressure in the range of 40 to 70 MPa (6 to 10 ksi) must be available.

Typical application reported, refer to butt Hot-pressure-welding of railroad rails sections and steelreinforcing bars, especially in Japan. For use in the production of weldments for the aerospaceindustry with delicate materials Hot-pressure-welding can be carried out in closed chambers withvacuum or a shielding medium. Mechanical properties tend to be near those of the base materials, butdepend upon materials composition, cooling rate and quality. Hot-pressure-welding can be aneconomic and successful process for performing butt joints of simple shapes if the materials areeasily weldable.

Hot pressure welding is further sub devided as-

1. Electric resistance welding

2. Forge welding

3. Ultrasonic welding

4. Friction welding

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5. Explosion welding

Electric resistance welding (ERW)

Electric resistance welding (ERW) refers to a group of weldingprocesses such as spot and seam

welding that producecoalescence offaying surfaceswhere heat to form the weld is generated by the

electrical resistance of material vs. the time and the force used to hold the materials together during

welding. Some factors influencing heat or welding temperatures are the proportions of the

workpieces, the metal coating or the lack of coating, the electrode materials, electrode geometry,

electrode pressing force, electrical current and length of welding time. Small pools of molten metal

are formed at the point of most electrical resistance (the connecting or "faying" surfaces) as an

electrical current (100100,000 A) is passed through the metal. In general, resistance welding

methods are efficient and cause little pollution, but their applications are limited to relatively thin

materials and the equipment cost can be high (although in production situations the cost per weld may

be as low as $0.04 USD[citation needed] per weld depending on application and manufacturing rate).

Spot welding

Spot welding is a resistance welding method used to join two or more overlapping metal sheets, studs,projections, electrical wiring hangers, some heat exchanger fins, and some tubing. Usually powersources and welding equipment are sized to the specific thickness and material being welded together.The thickness is limited by the output of the welding power source and thus the equipment range dueto the current required for each application. Care is taken to eliminate contaminants between thefaying surfaces. Usually, two copper electrodes are simultaneously used to clamp the metal sheetstogether and to pass current through the sheets. When the current is passed through the electrodes tothe sheets, heat is generated due to the higher electrical resistance where the surfaces contact eachother. As the electrical resistance of the material causes a heat buildup in the work pieces between thecopper electrodes, the rising temperature causes a rising resistance, and results in a molten poolcontained most of the time between the electrodes. As the heat dissipates throughout the workpiece inless than a second (resistance welding time is generally programmed as a quantity of AC cycles ormilliseconds) the molten or plastic state grows to meet the welding tips. When the current is stoppedthe copper tips cool the spot weld, causing the metal to solidify under pressure. The water cooledcopper electrodes remove the surface heat quickly, accelerating the solidification of the metal, sincecopper is an excellent conductor. Resistance spot welding typically employs electrical power in theform of direct current, alternating current, medium frequency half-wave direct current, or high-frequency half wave direct current.

If excessive heat is applied or applied too quickly, or if the force between the base materials is toolow, or the coating is too thick or too conductive, then the molten area may extend to the exterior of

the work pieces, escaping the containment force of the electrodes (often up to 30,000 psi). This burstof molten metal is called expulsion, and when this occurs the metal will be thinner and have lessstrength than a weld with no expulsion. The common method of checking a weld's quality is a peel

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test. An alternative test is the restrained tensile test, which is much more difficult to perform, andrequires calibrated equipment. Because both tests are destructive in nature (resulting in the loss ofsalable material), non-destructive methods such as ultrasound evaluation are in various states of earlyadoption by many OEMs.

The advantages of the method include efficient energy use, limited workpiece deformation, highproduction rates, easy automation, and no required filler materials. When high strength in shear isneeded, spot welding is used in preference to more costly mechanical fastening, such as riveting.While the shear strength of each weld is high, the fact that the weld spots do not form a continuousseam means that the overall strength is often significantly lower than with other welding methods,limiting the usefulness of the process. It is used extensively in the automotive industry cars canhave several thousand spot welds. A specialized process, called shot welding, can be used to spotweld stainless steel.

There are three basic types of resistance welding bonds: solid state, fusion, and reflow braze. In asolid state bond, also called a thermo-compression bond, dissimilar materials with dissimilar grainstructure, e.g. molybdenum to tungsten, are joined using a very short heating time, high weld energy,and high force. There is little melting and minimum grain growth, but a definite bond and graininterface. Thus the materials actually bond while still in the solid state. The bonded materials typicallyexhibit excellent shear and tensile strength, but poor peel strength. In a fusion bond, either similar ordissimilar materials with similar grain structures are heated to the melting point (liquid state) of both.The subsequent cooling and combination of the materials forms a nugget alloy of the two materialswith larger grain growth. Typically, high weld energies at either short or long weld times, dependingon physical characteristics, are used to produce fusion bonds. The bonded materials usually exhibitexcellent tensile, peel and shear strengths. In a reflow braze bond, a resistance heating of a lowtemperature brazing material, such as gold or solder, is used to join either dissimilar materials orwidely varied thick/thin material combinations. The brazing material must wet to each part and

possess a lower melting point than the two work pieces. The resultant bond has definite interfaces

with minimum grain growth. Typically the process requires a longer (2 to 100 ms) heating time at lowweld energy. The resultant bond exhibits excellent tensile strength, but poor peel and shear strength.

Projection welding

Projection welding is a modification of spot welding. In this process, the weld is localized by meansof raised sections, or projections, on one or both of the workpieces to be joined. Heat is concentratedat the projections, which permits the welding of heavier sections or the closer spacing of welds. The

projections can also serve as a means of positioning the workpieces. Projection welding is often usedto weld studs, nuts, and other screw machine parts to metal plate. It is also frequently used to joincrossed wires and bars. This is another high-production process, and multiple projection welds can bearranged by suitable designing and jigging.

Seam welding

"Seam welding" redirects here. For the geometrical welding configuration, see welding joints.

Resistance seam welding is a process that produces a weld at the faying surfaces of two similarmetals. The seam may be a butt joint or an overlap joint and is usually an automated process. It differsfrombutt welding in that butt welding typically welds the entire joint at once and seam welding formsthe weld progressively, starting at one end. Like spot welding, seam welding relies on two electrodes,usually made from copper, to apply pressure and current. The electrodes are disc shaped and rotate asthe material passes between them. This allows the electrodes to stay in constant contact with thematerial to make long continuous welds. The electrodes may also move or assist the movement of the

material.

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A transformer supplies energy to the weld joint in the form of low voltage, high current AC power.The joint of the work piece has high electrical resistance relative to the rest of the circuit and is heatedto its melting point by the current. The semi-molten surfaces are pressed together by the welding

pressure that creates a fusion bond, resulting in a uniformly welded structure. Most seam welders usewater cooling through the electrode, transformer and controller assemblies due to the heat generated.

Seam welding produces an extremely durable weld because the joint is forged due to the heat andpressure applied. A properly welded joint formed by resistance welding is typically stronger than thematerial from which it is formed.

A common use of seam welding is during the manufacture of round or rectangular steel tubing. Seamwelding has been used to manufacture steel beverage cans but is no longer used for this as modern

beverage cans are seamless aluminium.

Forge welding:-

This is the oldest welding process. In this process the ends of the parts to be joined are heated to a

temperature slightly below the melting point temperature and a pressure is applied so that a joint is

obtained. This is a familiar method used by the village blacksmith. The force can be applied in

repeated blows manually or by a machine or continuously by rotating roll.

Fig-4 Forge welding

Friction welding:-

The heat required for welding in this process is obtained by the friction between the ends of two partsto be joined. One of the parts to be joined is rotated at a high speed around 3000 rpm, and the other

part is axially aligned with the second one and pressed tightly against it is shown in fig---. The

friction between the two parts raises the temperature of the both ends then the rotation of the part is

stopped abruptly and the presser on the fixed part is increased so that the joining takes placed. This

process is termed as friction welding (FRW).

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Fig.-5(a) Fig.-5(b)

Fig-5 Friction welding

Ultrasonic Welding

Rub your hands together rapidly. Notice anything? They warmed up, right? If you take a hammer andpound a metal surface rapidly and repeatedly, you will find that the place where the hammer strikesthe metal warms up, too. In both these examples, the heat is due to friction. Now imagine rubbingyour hands or pounding that hammer thousands of times per second. The frictional heat generated canraise the temperature significantly in a very short time. Basically, high-frequency sound (ultrasound)causes rapid vibrations within the materials to be welded. The vibrations cause the materials to rubagainst each other and the friction raises the temperature at the surfaces in contact. This rapidfrictional heat is what sets the conditions for the materials to bind together.

Ultrasonic welding equipment has four main parts. A power supply converts low-frequency electricity(50-60 Hz) to high-frequency electricity (20 - 40 kHz; 1 kHz = 1000 Hz). Next, a transducer orconverter changes the high-frequency electricity into high-frequency sound (ultrasound). A boostermakes the ultrasound vibrations bigger. Finally, a horn orsonotrode focuses the ultrasound vibrationsand delivers them to the materials to be welded. Besides these pieces, there is an anvil upon which thewelded materials are stacked and held. There is also some method to apply force (usually air pressuresupplied by a pneumatic piston) to hold the materials together during welding.

So what materials and industries take advantage of this clever process? Ultrasonic welding ofplasticsis used widely in making electronics, medical devices and car parts. For example, ultrasonic weldingis used to make electrical connections on computer circuit boards, and assemble electronic

components such as transformers, electric motors and capacitors. Medical devices, such as catheters,valves, filters and face masks are also assembled using ultrasonic welding. The packaging industryuses this technique to make films, assemble tubes and blister packs. Even Ford Motor Company hasexplored using ultrasonic welding to make aluminum chassis in cars.

Now that you know the basics behind ultrasonic welding, let's look at the welding process itself.

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Ultrasonic Welding Step by Step

The basic process of ultrasonic welding can be described by the following steps:

1. The parts to be welded are placed in the anvil or fixture.2. The horn contacts the parts to be welded.

3. Pressure is applied to keep the horn in contact with the welded materials and to hold themtogether.

4. The horn delivers ultrasonic vibrations to heat up the materials. The vibrations move less thana millimeter either up-and-down or side-to-side.

5. The materials are welded together.

6. The horn gets retracted and the welded materials can be removed from the anvil.

The welding times, applied pressures and temperatures are controlled by a computer ormicroprocessorwithin the welding apparatus. And what actually happens during the welding processdepends on the nature of the materials. In metals, the ultrasonic vibrations are delivered parallel to the

plane of the materials. The frictional heat increases the temperature of the metal surfaces to about one

third of the melting temperature, but does not melt the metals. Instead, the heat removes metal oxidesand films from the surfaces. This allows the metal atoms to move between the two surfaces and formbonds that hold the metals together.

In the case ofplastics, the vibrations are perpendicular to the plane of the materials and the frictionalheat increases the temperature enough to melt the plastics. The plastic molecules mix together andform bonds. Upon cooling, the plastic surfaces are welded together. Welding times can vary, but thewelds can form in as little as 0.25 seconds.

The factors that vary in ultrasonic welding are the frequency of the sound waves (usually 20, 30 or 40kHz), the pressure applied to hold the materials together, and the time over which the ultrasound isapplied (fractions of a second to more than one second).

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The ultrasonic welding techniques described so far are good for materials (metals, plastics) that aresimilar. But what about materials that are not similar. Let's address this question by looking at how

New Balance has used ultrasonic welding to assemble athletic shoes.

Advantages of Ultrasonic Welding

Ultrasonic welding has many advantages over traditional methods. For one, welding occurs at lowtemperatures relative to other methods. So, the manufacturer does not need to expend vast amounts offuel or other energy to reach high temperatures. This makes the process cheaper. It's also faster andsafer.

The process occurs in fractions of a second to seconds. So, it can be done more quickly than othermethods. In fact, it can bondplasticsbetter and faster than glues. For example, the new smart keys incars have a transponder chip in them. The car can only start when it senses the chip. To make the key,one end of the metal key blank and the chip get placed into one half of the plastic top. The other halfgets placed over them and bonded to the base half. This bonding would usually be done with glue,which takes time to cure. The same task can be done with ultrasonic welding in less than a second.

Ultrasonic welding does not require flammable fuels and open flames, so compared to other weldingmethods, it's a safer process. Workers are not exposed to flammable gases or noxious solvents. Inelectronics, copper wires are usually bonded to electrical contacts on circuit boards with solder. Thesame task can be done using ultrasonic welding in a fraction of the time and without exposing workersto fumes from smoldering lead solder. Although workers' hearing may be damaged by exposure tohigh-frequency sound, this potential danger is easily reduced by enclosing the ultrasonic weldingmachine in a safety box or cage and/or using ear protection.

Finally, ultrasonic welds are as strong and durable as conventional welds of the same materials --

which is just one of the reasons the method is being used in car manufacturing. To make cars lighterand more fuel efficient, auto makers are turning to aluminum as the main metal in car bodies.Ultrasonic welding can be used to bond the metal in less time and at lower temperatures thantraditional welding.

Ultrasonic welding does have its limitations, though. First, the depths of the welds are less than amillimeter, so the process works best on thin materials like plastics, wires or thin sheets of metal.Ultrasonically welding a steel girder for a building would not be practical. Second, it does work bestwhen welding similar materials like similar plastics or similar metals. As you saw with New Balanceshoes, ultrasonically welding dissimilar materials requires an additional material -- in the case of the

New Balance shoes, it's a film that can be bonded between the synthetic suede and the mesh.

Despite these limitations, the popularity and potential of ultrasonic welding continues to grow.

3.2.6 Explosion welding

Explosive welding is a solid state welding process, which uses a controlled explosive detonation to

force two metals together at high pressure. The resultant composite system is joined with a durable,

metallurgical bond.

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Explosive welding under high velocity impact was probably first recognized by Garl in 1944. It has

been found to be possible to weld together combinations of metals, which are impossible, by other

means.

The Process

This is a solid state joining process. When an explosive is detonated on the surface of a metal, a high

pressure pulse is generated. This pulse propels the metal at a very high rate of speed. If this piece of

metal collides at an angle with another piece of metal, welding may occur. For welding to occur, a

jetting action is required at the collision interface. This jet is the product of the surfaces of the two

pieces of metals colliding. This cleans the metals and allows to pure metallic surfaces to join under

extremely high pressure. The metals do not commingle, they are atomically bonded. Due to this fact,

any metal may be welded to any metal (i.e.- copper to steel; titanium to stainless). Typical impact

pressures are millions of psi. Fig. 1 shows the explosive welding process.

Explosives

The commonly used high explosives are

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Applications

1) Joining of pipes and tubes.

2) Major areas of the use of this method are heat exchanger tube sheets and pressure vessels.

3) Tube Plugging.

4) Remote joining in hazardous environments.

5) Joining of dissimilar metals - Aluminium to steel, Titanium alloys to Cr Ni steel, Cu to

stainless steel, Tungsten to Steel, etc.

6) Attaching cooling fins.

7) Other applications are in chemical process vessels, ship building industry, cryogenic industry, etc.

Advantages

1) Can bond many dissimilar, normally unweldable metals.

2) Minimum fixturing/jigs.

3) Simplicity of the process.

4) Extremely large surfaces can be bonded.

5) Wide range of thicknesses can be explosively clad together.

6) No effect on parent properties.

14

1 Explosive 2 Detonation velocity ,

m/s

RDX (Cyclotrimethylene trinitramine, C3H6N6O6 8100

PETN (Pentaerythritol tetranitrate, C5H8N12O4) 8190

TNT (Trinitrotoluene, C7H5N3O6) 6600

Tetryl (Trinitrophenylmethylinitramine, C7H5O8N5) 7800

Lead azide (N6Pb) 5010

3 Detasheet

7020

Ammonium nitrate (NH4NO3) 2655


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7) Small quantity of explosive used.

Limitations1. The metals must have high enough impact resistance, and ductility.2. Noise and blast can require operator protection, vacuum chambers, buried in sand/water.

3. The use of explosives in industrial areas will be restricted by the noise and ground vibrationscaused by the explosion.

4. The geometries welded must be simple flat, cylindrical, conical.

4.1 Arc welding

Arc welding is a process of joining of metal where heat is produced by generating an electric arc

without the application of pressure and with or without the use of filler metal.

The filler metal is used or not depends on base plate thickness.

The various arc welding are-

1. Shielded metal arc welding (SMAW) or Flux shielded metal arc welding or Manual metal arc

welding (MMAW).

2. Gas shielded arc welding.(GSAW)

3. Submerged arc welding.

4. Electroslag welding.

5. Plasma arc welding

6. Arc spot welding.

7. Stud (Arc) welding.

8. Carbon Arc welding

9. Atomic hydrogen arc welding

10. Atomic arc welding

1.1.1 Manual metal arc welding (MMAW)

This process uses consumable flux-coated electrodes to produce by arcing, as well as to supply filler

material to the weld zone.

This welding is very suitable for mild steel but it is also applicable in cast iron, wrought iron etc.

Principle of MMA Welding:

An electric arc is generated between a piece of wire called electrode and the work pieces which is to

be welded. The heat required for the welding is generated from this arc and fuses the electrode

gradually thus molten metal is formed falls in the gap between work pieces.

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Fig The basic arc welding circuit

Fig.... shows the electrode and work pieces. The electrode is connected to the ()ve pole and work

pieces is connected to the (+) ve pole of a arc generating machine. The electric energy is change into

arc which generate heat and light (spark). The electrode and work spices melt into molten state and

solidify to make join by forming metallurgical bond or union.

Electric Arc

To generate the arc the electrode is touched to the work pieces as well as electric current is flow then

the electrode is withdrawn from work pieces and maintained 2-3 mm gap between work pieces and

electrode to maintain an air resistance between them but the electron do not stop to flow, it ionized the

air and a cannel of electron is produced. This cannel of electron is called arc which generate heat for

welding. The heat of the arc produces the temperature approximately 3000C to 3500C

The welding current may vary from 20 to 600 Amp in MMA Welding. When AC current is used the

heat is developed equally at work pieces and electrode as the electrode and work pieces are changing

polarity continuously.

Fig.2 illustrates the shielding of the welding arc and molten pool with a Stick electrode. The extruded

covering on the filler metal rod, provides a shielding gas at the point of contact while the slag protects

the fresh weld from the air.

Electrode

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An electrode is a metal core wire with flux-coated (insulating covering). The MMA welding

consumable flux-coated electrode is used to supply filler material to weld zone.

MMA welding is used for steel, alloy-steel, structural steel, heat resistant steel, cast iron , mild steel

and other metal alloys.

Metal electrodes are may be three types as-

1. Bar electrode

Bar electrode is a carbon steel filler rod without coating which have limited used for welding of

wrought iron and mild steel. When the globules of metal flow from the electrode to the work pieces,

they are exposed to the oxygen and nitrogen in the surrounding air and thus decreases the strength and

ductility of the metal. If a bar wire is used as the electrode it is found that the arc is difficult to control

and the weld tends to be porous and brittle. With bar wire electrodes, much metals is lost by

volatilisation which turning into vapour

2. Coated electrode

A coated electrode is a carbon steel filler rod that has been covered by same form of fluxing material.

Coated (covered) electrodes reduce the loss of metal by volatilisation. Materials of coating for arc

welding are mainly Borax, Ammonia, Sulphur, Cellulose, Calcium carbide, Dolomite, Rutile, mica,

clay, Slica, Manganese dioxide, Iron powder, Fero-silicon, Sodium silicate, Potassium silicate, etc

3. Heavy coated electrode

The arc can be rendered easy to control and the absorption of atmospheric gases are reduced tominimum by heavy coated electrode. Under the heat the coating react to form a slag which is liquid

and lighter then the molten metal. It rises to the surface, cools and solidifies, forming a protective

covering over the hot metal while cooling and protect it from the atmospheric effect of the weld

metal.

The coating of the welding electrodes serve several purposes are

1. Establish and maintain the arc.

2. Protect the molten metal from oxygen and nitrogen.

3. Increases the rate of cooling.

4. Provide alloying element to the join.

5. Influence the shape of the bead.

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Types of coated electrode: The electrode are classified as per the core material as follows

a. Mild steel electrodeb. Cast iron electrode

c. Inconel electrode

Welding power source

The power sources of welding supply can be A.C. Transformer or D.C. Generator. In D.C set, an

electrode connects to the (+) ve pole, which will burn away 50% faster than if connected to the (-)ve

pole. As a result the bar electrodes or medium coated electrodes are connected to the (- ve) pole as

heat requires to burn this electrode is less where as heavily coated electrodes are connected to the (+)

ve pole, due to extra heat required to melt the heavy coating of electrode.

On the other hand, when alternative current (A.C.) is used, the heat generates equally at work pieces

and electrode as the electrode and work pieces are changing their polarity at the frequency of the

supply

When the job is connected to the (+)ve pole and electrode is connected to the (-)ve pole, the

arrangement is said to be of straight polarity. On the other hand, when the job is connected to the

(-)ve pole and the electrode is connected to the (+)ve pole, the arrangement is said to be of Reverse

polarity.

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Fig. Straight (left) and reverse (right) polarity

The D.C. output units are used for both steel and non-ferrous metals and welding can be obtained in

current ranges of 300-400 amps, as required. Thev A.C. output unit (A.C. transformer) supply current

is usually from 80 to 100A and voltage 80-100V from mains supply.

A.C. Transformer

A transformer consists essentially of high magnetisable silicon iron core and two windings wound

upon the core with insulated wire. One of the winding is connected to the supply line which is called

primary winding and the other winding delivers the desired voltage or current, which is called the

secondary winding. The voltage supplied to the transformer is termed the input voltage, while that

supplied by the transformer is termed the output voltage. If the output voltage is greater than the input

voltage, it is a set-up transformer, while if the output voltage is less than the input; it is a set-down

transformer. Transformers for welding purpose are always set-down. The transformer may be of the

dry type(air cooled)or it may be immersed in oil(oil cooled) contained in the outer container. Oil-

cooled transformers have a lower permissible temperature rise than the dry type and therefore, their

overloaded capacity is much smaller. Welding transformers are available up to 450-500 amps. A

circuit diagram of a transformer is shown here

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Fig

Fig.

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Fig

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CHAPTER 5

5 Gas Welding:-

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It is the types of fusion welding, in this welding the heat is obtained by the combustion of fuel gas.

The most widely used gas combination for producing a hot flame for welding metals is oxygen and

acetylene. The approximate flame temperature produced by oxy-acetylene flame is 3200oC. [3]

Fig Gas welding process

Gas welding equipment:-

The basic equipment required to carry out oxy-acetylene gas welding is as follows-

i. Welding torch:-

It is also known as blow pipe. It is a tool for mixing the oxygen and acetylene in the desired

volumes and burning the mixture at the end of tip, which produces a high temperature flame.

The welding torches are commercially available in the following two types:

a. Injector or low pressure type; and

b. Positive or equal pressure (also known as high pressure) type.

ii. Welding torch tip:-

The tips are made of high thermal conductivity material such as copper or copper alloy. The

interchangeable tips for the various thicknesses are usually provided with each welding

torch.

iii. Pressure regulators:-

There are two gauges on the body of the regulator, one showing the pressure in the cylinder

while the other shows pressure being supplied to the torch. The desired pressure at the

welding torch for oxygen is between 70kN/m and 280kN/m and for acetylene it is between

7kN/m and 103kN/m.

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iv. Hose and hose fittings:-

The standard colour for oxygen cylinder is black and for acetylene cylinder it is red.

v. Gas cylinder:-

The standard colour for oxygen cylinder is black and for acetylene cylinder it ismaroon.

Fig Gas welding equipment

Gas flame:-

The following three type of flame are use for gas welding.

i. Neutral flame:-

The neutral flame, as shown in fig.-20 is obtained by supplying of oxygen and acetylene. It

has the following two sharply defined zones.

a) An inner luminous cone (3200oC), and

b) An outer cone or envelope of bluish colour (1250oC).

The most oxy-acetylene welding (e.g. welding of steel, cast iron, copper, aluminium etc.) is

done with the neutral flame.

ii. Oxidising flame:-

The oxidising flame, as shown in fig.-20, is obtained when thereis an excess of oxygen. It is

used for welding brass and bronze.

iii. Reducing or carburising flame:- The reducing flame, as shown in fig.-20, is obtained when

there is an excess of acetylene. It is used for welding of molten metal, a certain alloy steel,many of non-ferrous, hard surfacing material is such as satellite.

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Fig Gas flame

Electroslag Welding (ESW)

Electroslag Welding is awelding process, in which the heat is generated by an electric current passingbetween the consumable electrode (filler metal) and the work piece through a molten slag coveringthe weld surface.

Prior to welding the gap between the two work pieces is filled with a welding flux. ElectroslagWelding is initiated by anarc between the electrode and the work piece (or starting plate). Heat,generated by the arc, melts the fluxing powder and forms molten slag. The slag, having low electricconductivity, is maintained in liquid state due to heat produced by the electric current.

The slag reaches a temperature of about 3500F (1930C). This temperature is sufficient for meltingthe consumable electrode and work piece edges. Metal droplets fall to the weld pool and join the work

pieces.

Electroslag Welding is used mainly for steels.

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Advantages of Electroslag Welding:

High deposition rate - up to 45 lbs/h (20 kg/h);

Low slag consumption (about 5% of the deposited metal weight);

Low distortion;

Unlimited thickness of work piece.

Disadvantages of Electroslag welding:

Coarse grain structure of the weld;

Low toughness of the weld;

Only vertical position is possible.

1.1.2 Submerged Arc Welding (SAW)

Submerged Arc Welding is a welding process, which utilizes a bare consumable metallic electrode

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producing an arcbetween itself and the work piece within a granularshielding flux applied around theweld.

The arc heats and melts both the work pieces edges and the electrode wire. The molten electrodematerial is supplied to the surfaces of the welded pieces, fills the weld pool and joins the work pieces.

Since the electrode is submerged into the flux, the arc is invisible. The flux is partially melts andforms a slag protecting the weld pool from oxidation and other atmospheric contaminations.

Advantages of Submerged Arc Welding (SAW):

Very high welding rate;

The process is suitable for automation;

High quality welds structure.

Disadvantages of Submerged Arc Welding (SAW):

Weld may contain slag inclusions;

Limited applications of the process - mostly for welding horizontally located plates.

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Plasma Arc Welding (PAW)

Plasma Arc Welding is the welding process utilizing heat generated by a constricted arc struck

between a tungsten non-consumable electrode and either the work piece (transferred arc process) orwater cooled constricting nozzle (non-transferred arc process).

Plasma is a gaseous mixture of positive ions, electrons and neutral gas molecules.

Transferred arc process produces plasma jet of high energy density and may be used for high speedwelding and cutting ofCeramics, steels, Aluminum alloys, Copper alloys, Titanium alloys,Nickelalloys.

Non-transferred arc process produces plasma of relatively low energy density. It is used for weldingof various metals and forplasma spraying (coating). Since the work piece in non-transferred plasmaarc welding is not a part of electric circuit, the plasma arc torch may move from one work piece toother without extinguishing the arc.

Advantages of Plasma Arc Welding (PAW):

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Requires less operator skill due to good tolerance of arc to misalignments;

High welding rate;

High penetrating capability (keyhole effect);

Disadvantages of Plasma Arc Welding (PAW):

Expensive equipment;

High distortions and wide welds as a result of high heat input (in transferred arc process).

End of arc welding (may be)

Laser Welding (LW)

Laser Welding (LW) is a weldingprocess, in which heat is generated by a high energy laser beamtargeted on the work piece. The laser beam heats and melts the work pieces edges, forming a joint.

Energy of narrow laser beam is highly concentrated: 108-1011 W/in2 (108-1010 W/cm2), thereforediminutive weld pool forms very fast (for about 10-6 sec.). Solidification of the weld pool surrounded

by the cold metal is as fast as melting. Since the time when the molten metal is in contact with theatmosphere is short, no contamination occurs and therefore no shields (neutral gas, flux) are required.

The joint in Laser Welding (Laser Beam Welding) is formed either as a sequence of overlapped spotwelds or as a continuous weld.

Laser Welding is used in electronics, communication and aerospace industry, for manufacture of

medical and scientific instruments, for joining miniature components.

Advantages of Laser Welding:

Easily automated process;

Controllable process parameters;

Very narrow weld may be obtained;

High quality of the weld structure;

Very small heat affected zone;

Dissimilar materials may be welded;

Very small delicate work pieces may be welded;

Vacuum is not required;

Low distortion of work piece.

Disadvantages of Carbon Arc Welding:

Low welding speed; High cost equipment;

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Weld depth is limited.

***Metal Inert Gas Welding (MIG, GMAW)

Metal Inert Gas Welding (Gas Metal Arc Welding) is a arc welding process, in which the weld isshielded by an external gas (Argon, helium, CO2, argon + Oxygen or other gas mixtures).

Consumable electrode wire, having chemical composition similar to that of the parent material, iscontinuously fed from a spool to the arc zone. The arc heats and melts both the work pieces edges andthe electrode wire. The fused electrode material is supplied to the surfaces of the work pieces, fills theweld pool and forms joint.

Due to automatic feeding of the filling wire (electrode) the process is referred to as a semi-automatic.The operator controls only the torch positioning and speed.

Advantages of Metal Inert Gas Welding (MIG, GMAW):

Continuous weld may be produced (no interruptions); High level of operators skill is not required;

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Slag removal is not required (no slag);

Disadvantages of Metal Inert Gas Welding (MIG, GMAW):

Expensive and non-portable equipment is required; Outdoor application are limited because of effect of wind, dispersing the shielding gas.

***Tungsten Inert Gas Arc Welding (TIG, GTAW)

Tungsten Inert Gas Arc Welding (Gas Tungsten Arc Welding) is a welding process, in which heat isgenerated by an electric arc struck between a tungsten non-consumable electrode and the work piece.

The weld pool is shielded by an inert gas (Argon, helium, Nitrogen) protecting the molten metal fromatmospheric contamination.

The heat produced by the arc melts the work pieces edges and joins them. Filler rod may be used, ifrequired.

Tungsten Inert Gas Arc Welding produces a high quality weld of most of metals. Flux is not used inthe process.

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Advantages of Tungsten Inert Gas Arc Welding (TIG, GTAW):

Weld composition is close to that of the parent metal;

High quality weld structure

Slag removal is not required (no slag);

Thermal distortions of work pieces are minimal due to concentration of heat in small zone.

Disadvantages of Tungsten Inert Gas Arc Welding (TIG, GTAW):

Low welding rate;

Relatively expensive;

Requres high level of operators skill.

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Forge Welding (FOW)

Forge Welding is a Solid State Weldingprocess, in which low carbon steel parts are heated to about1800F (1000C) and then forged (hammered).Prior to Forge Welding, the parts are scarfed in order to prevent entrapment of oxides in the joint.

Forge Welding is used in general blacksmith shops and for manufacturing metal art pieces and weldedtubes.

Advantages of Forge Welding:

Good quality weld may be obtained;

Parts of intricate shape may be welded;

No filler material is required.

Disadvantages of Forge Welding:

Only low carbon steel may be welded;

High level of the operators skill is required;

Slow welding process;

Weld may be contaminated by the coke used in heating furnace.

Cold Welding (CW)

Cold Welding is a Solid State Weldingprocess, in which two work pieces are joined together at roomtemperature and under a pressure, causing a substantial deformation of the welded parts and providingan intimate contact between the welded surfaces.

As a result of the deformation, the oxide film covering the welded parts breaks up, and clean metalsurfaces reveal. Intimate contact between these pure surfaces provides a strong and defect less

bonding.

Aluminum alloys, Copper alloys, low carbon steels,Nickel alloys, and otherductile metals may bewelded by Cold Welding.

Cold Welding is widely used for manufacturing bi-metal steel - aluminum alloy strips, for cladding ofaluminum alloy strips by other aluminum alloys or pure aluminum (Corrosion protection coatings).Bi-metal strips are produced by Rolling technology. Presses are also used for Cold Welding.

Cold Welding may be easily automated.

Friction Welding (FRW)

Friction Welding is a Solid State Weldingprocess, in which two cylindrical parts are brought incontact by a friction pressure when one of them rotates. Friction between the parts results in heating

their ends. Forge pressure is then applied to the pieces providing formation of the joint.

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Carbon steels, Alloy steels,Tool and die steels, Stainless steels,Aluminum alloys, Copper alloys,Magnesium alloys,Nickel alloys,Titanium alloys may be joined by Friction Welding.

An another special type of friction welding is friction-stir welding which is explained below

Explosive Welding (EXW)

Explosive Welding is a Solid State Welding process, in which welded parts (plates) aremetallurgically bonded as a result of oblique impact pressure exerted on them by a controlleddetonation of an explosive charge.

One of the welded parts (base plate) is rested on an anvil, the second part (flyer plate) is located abovethe base plate with an angled or constant interface clearance.Explosive charge is placed on the flyer plate. Detonation starts at an edge of the plate and propagates

at high velocity along the plate.The maximum detonation velocity is about 120% of the material sonic velocity.The slags (oxides, nitrides and other contaminants) are expelled by the jet created just ahead of the

bonding front.

Most of the commercial metals and alloys may be bonded (welded) by Explosive Welding.

Dissimilar metals may be joined by Explosive Welding:

Coppertosteel;

Nickel to steel;

Aluminum to steel;

Tungsten to steel;

Titanium to steel;

Copper to aluminum.

Advantages of Explosive Welding

Large surfaces may be welded;

High quality bonding: high strength, no distortions, no porosity, no change of the metalmicrostructure;

Low cost and simple process;

Surface preparation is not required.

Disadvantages of Explosive Welding:

Brittle materials (low ductility and low impact toughness) cannot be processed;

Only simple shape parts may be bonded: plates, cylinders;

Thickness of flyer plate is limited - less than 2.5 (63 mm);

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Safety and security aspects of storage and using explosives.

Explosive Welding is used for manufacturing clad tubes and pipes, pressure vessels, aerospacestructures, heat exchangers, bi-metal sliding bearings, ship structures, weld transitions, corrosion

resistant chemical process tanks.

to top

Diffusion Welding (DFW)

Diffusion Welding is aSolid State Welding process, in which pressure applied to two work pieceswith carefully cleaned surfaces and at an elevated temperature below the melting point of the metals.Bonding of the materials is a result of mutual diffusion of their interface atoms.

In order to keep the bonded surfaces clean from oxides and other air contaminations, the process is

often conducted in vacuum.No appreciable deformation of the work pieces occurs in Diffusion Welding.

Diffusion Welding is often referred more commonly as Solid State Welding (SSW).

Diffusion Welding is able to bond dissimilar metals, which are difficult to weld by other weldingprocesses:

Steel to tungsten;

Steel to niobium;

Stainless steel to titanium;

Gold to copper alloys.

Diffusion Welding is used in aerospace and rocketry industries, electronics, nuclear applications,manufacturing composite materials.

Advantages of Diffusion Welding:

Dissimilar materials may be welded (Metals, Ceramics, Graphite, glass);

Welds of high quality are obtained (nopores, inclusions, chemicalsegregation, distortions).

No limitation in the work pieces thickness.

Disadvantages of Diffusion Welding:

Time consuming process with low productivity;

Very thorough surface preparation is required prior to welding process;

The mating surfaces must be precisely fitted to each other;

Relatively high initial investments in equipment.

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Ultrasonic Welding (USW)

Ultrasonic Welding is a Solid State Weldingprocess, in which two work pieces are bonded as aresult of a pressure exerted to the welded parts combined with application of high frequency acousticvibration (ultrasonic).

Ultrasonic vibration causes friction between the parts, which results in a closer contact between thetwo surfaces with simultaneous local heating of the contact area. Interatomic bonds, formed underthese conditions, provide strong joint.

Ultrasonic cycle takes about 1 sec. The frequency of acoustic vibrations is in the range 20 to 70 KHz.Thickness of the welded parts is limited by the power of the ultrasonic generator.

Ultrasonic Welding is used mainly for bonding small work pieces in electronics, for manufacturingcommunication devices, medical tools, watches, in automotive industry.

Advantages of Ultrasonic Welding:

Dissimilar metals may be joined;

Very low deformation of the work pieces surfaces;

High quality weld is obtained;

The process may be integrated into automated production lines;

Moderate operator skill level is enough.

Disadvantages of Ultrasonic Welding:

Only small and thin parts may be welded;

Work pieces and equipment components may fatigue at the reciprocating loads provided by

ultrasonic vibration;

Work pieces may bond to the anvil.

Make Cut chat of solid state welding (above)

4 Welding stainless steels

Dr. Dmitri Kopeliovich

Welding austenitic stainless steels

Welding ferritic stainless steels

Welding martensitic stainless steels

Welding austenitic-ferritic (Duplex) stainless steels

Welding precipitation hardening stainless steels

According to the AISI classification Stainless steels are divided onto groups: austenitic,ferritic,

martensitic,austenitic-ferritic (Duplex) andprecipitation hardening steels.

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Most stainless steels may be welded by different welding processes:

Shielded Metal Arc Welding (SMAW)

Submerged Arc Welding (SAW)

Metal Inert Gas Welding (MIG, GMAW)

Tungsten Inert Gas Arc Welding (TIG, GTAW)

Resistance Welding (RW)

Gas Welding (GW)

Friction Welding (FRW)

Electron Beam Welding (EBW)

Laser Welding (LW)

Brazing

Electric resistance welding:-

It is a type of pressure welding. It is used for joining pieces of sheet metal or wire. The welding heat is

obtained at the location of the desired weld by the electric resistance through the metal pieces to a

relatively short duration, low voltage (from6 to 9 volts only) high amperage (varying from 60 to 4000

amperes) electric current. The amount of current can be regulated by changing the primary turns of

the transformer. When the area to be welted in sufficiently heated, the pressure varying from 25 to 55MPa is applied to the joining area by suitable electrodes until the weld is solid.

Fig-6 Electric resistance welding[9]

Principle Of Electric resistance welding:-

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In resistance welding, a low voltage (typically 1v) and very high current (typically 15000A) is passed

through the joint for a very short time (typically 0.25s). this high amperage heats the joint, due to the

contact resistance at the joint and melts it. The pressure on the joint is continuously maintained and

the metal fuses together under this pressure. The heat generated in resistance welding can be

expressed as---[5]

[5]

H = the total heat generated in the work, J

I = electric current, A

t = time for which the electric current is passing through the joint, s

R = the resistance of the joint, ohms

and k = a constant to account for the heat losses from the welded joint.

The resistance of the joint, R, is a complex factor to know because it is composed of the

a. Resistance of the electrodes,

b. Contact resistance between the electrode and the work-piece,

c. Contact resistance between the two work-piece plates, and

d. Resistance of the work-piece plate.

The amount of heat released is directly proportional to the resistance. It is likely to be released at all

of the above-mentioned points, but the only place where a large amount of heat is to be generated to

have an effective fusion is at the inter face between the two work-piece plate. Therefore, the rest of

the component resistances should be made as small as possible, since the heat released at those places

would not aid in the welding.[5]

Types of resistance welding:-

The various types of electric resistance welding are as follows:

i. Spot welding :-

It is used for welding lap joint, joining component made from plate material having

0.025mm to 1.25mm is thickness. The plates to be joined together are placed between the

two electrode tips of copper or copper alloy. It may be noted that[3]

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Fig-7 Spot welding

The electrode tip diameter (d) should be equal to where t is the thickness of plate

to be weld.

The distance between the nearest edge of plate and centre of weld should be at least

1.5d.

The spacing between two spot-weld should not be less than 3d.

ii. Roll spot and seam welding :-

When the spot welds on two overlapping pieces of metal are spaced, the process is known

as roll spot welding. If the spot welds are sufficiently made close, then the process is

called seam welding. This process is based adopted for metal thickness ranging from

0.025mm to 3mm.[6]

Fig.-8(a) Roll welding Fig.-8(b) Seam welding

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Fig-8 Roll & seam welding[8]

iii. Projection welding :-

It is similar to spot welding except that one of metal pieces to be welded has projection on

its surface at the points where the welds are to be done. In other words, it is a multi-spot

welding process. It may be noted that [1]

When two pieces of different metals are to be welded by projection welding, then the

projection should be made on the metal pieces with the higher conductivity.

When the two metal pieces are of different thickness, the projection should be made

on other metal pieces.

Fig-9 Projection welding [7]

Arc welding:-

In welding, generation of heat by an electric arc is one of the efficient methods. Approximately, 50%

of the energy is liberated in the form of heat. The electric-arc welding process makes use of heatproduced by the electric arc to fusion-weld metallic pieces. This is one of the most widely used

welding process, mainly because of the ease of use and high production rates that can be achieved

economically.[2]

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Fig-11 Arc welding[8]

Principle of Arc:-

As arc is generated between two conductor of electricity, cathode and anode, when they are touched

to establish the flow of current and then separated by a small distance. An arc is a sustained electric

discharge through the ionized gas column, called plasma, between the two electrodes.[2]

It is generally believed that electrons liberate from the cathode move towards the anode and are

accelerated in their movement. When they strike the anode at high velocity, a large amount of heat is

generated. Also when the electrons are moving through the air gap between the electrodes, also called

the arc column, they collide with the ions in the ionized gas column between the electrodes.The

positively charged ions, move from the anode and impinge on the cathode, thus liberating heat. About65 to 75% of total heat is liberated at the anode by the striking electrons. A temperature of the order of

6000cis generated at the anode.[2]

In order to produce the arc, the potential difference between the two electrodes (voltage) should be

sufficient to allow them to move across the air gap. The larger air gap requires higher potential

differences. If the air gap becomes too large for the voltage, the arc may be extinguished. Here, we

may make use of an analogy of a person walking on the road. Suddenly, when a deep trench comes in

his way, the person would try to jump across it, if it is a short one. But if it is a broader one, then he

may move back a little; come running towards the trench and try to jump over it. If it is too broad, he

may abandon the idea of jumping across it. The energy spent in jumping is much more than what is

spent while normal walking. In the case of an arc, the extra energy spent crossing the air gap isliberated as heat.[5]

For convenience of explanation, we have chosen a direct current arc for the above description. But

even with an arc of the alternating current (AC), it would be similar, with the main difference that the

cathode and anode would change continuously and as a result, the temperature across the arc would be

more uniform compared to a DC arc.

Arc-welding Equipment:-

The main requirement in an arc-welding set-up is the source of electric power. They are essentially of

two types:

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(a) Alternating current (AC) machines

(i) transformer

(ii) motor or engine-driven alternator

(b) Direct current (DC) machines

(i) Transformer with DC rectifier

(ii) Motor or engine-driven generator

Fig-12 Arc welding equipment[8]

The transformer does not have any moving part and as a result operates with less maintenance cost

and also has higher efficiency. The power used is also less expensive and practically there is no noise

in operation of the welding transformer. In AC welding, normally only transformer are used.

In DC arc-welding, a rectifier or a generator can be used to supply the required DC power. In the

rectifier type, the power supply is first stepped down by means of a transformer to the required

voltage and then silicon controlled rectifiers (SCR) are used to convert AC to DC. These rectifiers are

very compact, highly reliable and have high efficiency. The other type is a DC generator, which is

driven by either an induction motor running on AC or an oil engine. This combination is less efficient,

more expensive and noisy in operation.[5] [3]

4.1.3. Arc-welding processes:-

The following are the various arc-welding processes commonly used in Engineering practice:

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5 Metal arc welding :-

Metal arc is initiated by touching coated electrode with work piece. Then a gap is maintained between

electrode and work-piece with in the gap, between electrode and work-piece, free electrons and holes

will be formed in gas.

In metal arc welding, the arc is produced between the metal electrode (also called filler rod) and the

work-piece. During the welding process, the metal electrode is melted by the heat of the arc and fused

with the work piece. The temperature produced by the heat is about 2400oc to 2700oc.[5]

Fig-14 Metal arc welding machine[8]

6 Tungsten inert gas welding (TIG) :-

In this technique non consumable tungsten electrode will be used to generate the arc to increase the

thermal resistance of tungsten alloying elements like Thorium and Beryllium are added.

This will be used for less thickness material without using filler material.

Molten weld pool is protected by inert gas atmosphere (Helium, Ne, Ar, N2,& CO2etc).

N2is preferred for welding copper.

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The power supply will be AC or DC depending upon work piece material for all over

material except Al, Mg alloys DCSP will be used so that heat concentration will be more

on work-piece side and depth of penetration is more.

For Al and Mg alloys if we use straight polarity due to high temperature oxide formation is

severe and this will not allow electrons from the electrode. To overcome this DCRP is used

but the melting rate of the electrode is more when compared to melting of the work-piece

due to this depth of penetration is reduced. To overcome this AC welding will be used for

welding of Al and Mg alloys.[3]

Fig-15 TIG[7]

TIG welding of Al and Mgalloys in Aerospace industries and automobile industries. Since weld bead

thickness is less it can be used for welding in any position.

7 Metal Inert Gas welding (MIG) :-

In this technique consumable electrode in the form of spool wire will be supplied.

DCRP will used for welding of all materials increase the weld deposition rate.

By increasing the current supply (100amps to 300amps) we can increases the depth of

penetration and we can weld high thickness material also.

Constant voltage type of power source is used since the control of movement of electrodeand feeding of the electrode is done automatically.

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It is used for welding stainless steels, Aluminium, Mg, Cu and Ni alloys in aircraft and automobile

industries.[6]

Fig.-16(a)[8] Fig.-16(b)[7]

Fig-16 MIG[8]

8 Submerged Arc Welding :-

Submerged arc welding is semiautomatic version of shielded metal arc welding. It is used for long

weld run.

In this technique arc will be submerged inside the molten weld pool any length of welding

with high thickness of materials can be done by using this technique in a single run. High

welding current will be used to increase the melting rate of electrode and work-piece (200-

2000 amps).

Best suited for high deposition rate, high welding speed and high depth of penetration.

The flux material contains CaO and CaF2.

The arc is produced between a bare metal electrode and the work-piece. It is used for non-ferrousmetal. It is mostly done on low carbon and steel alloy.

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Fig-17 Block diagram of saw

Types of welded joints:-

The types of welded joints are

I. Lap joint: The lap joints are employed on plate having thickness less than 3mm, shown in

fig-1(a).

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Documents

161976048 Advance Welding Technology