Chapter-1 IntroductionIntroduction:-

One of the main functions of the Engineering Works is joining of the parts. Collectively this joining is called Fastening. Fastening is generally two type are-

1. Temporary Fastening: In this process of fastening, the parts are ready to separation.

Ex-Nut and Bolts, Nut and screws, Nuts and studs are all temporary fastenings.

2. Permanent Fastening: In this process of fastening the parts not likely to require separate.

Ex-Welding.

3. Semi-permanent Fastening: In this process of fastening the parts are semi separable.

4. Ex-Soldering, Brazing and Riveting..Chapter-22.1 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 heterogenous welding, where filler material is given should have low melting temperature. 2.2. Types of welding:-The overall welding process shown in chart belowThe 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 welding1st 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.Chapter-3

Cold pressure welding3.1 Cold pressure weldingCold 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 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 vapor 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.

PRESEDURE

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.

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.

2.4. 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.

2.5. Welding Dies and Welding

Schematic welding dies designed for lap welding is given figure 2.In space

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.

3.2 Hot pressure weldingHot-pressure-welding is a solid state process that produces joints between the faying surfaces of two bodies. It is done by application of heat and pressure. Fusion temperature is not reached, filler metal is not needed, substantial plastic deformation is generated.Heat is generally applied by flames of 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 suitable inductor coil. As soon as the two bodies facing ends reach the correct temperature, the torches are suddenly 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 pressure before heat application from the outside, it is called the closed joint process. In either case flash material 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. For obtaining the best results the surfaces should be machined square and clean. Some beveling can be used 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 be Hot-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 upon heating cannot be easily welded in air by this process. Typically among them aluminum alloys and 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 and error. 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 steel reinforcing bars, especially in Japan. For use in the production of weldments for the aerospace industry with delicate materials Hot-pressure-welding can be carried out in closed chambers with vacuum or a shielding medium. Mechanical properties tend to be near those of the base materials, but depend upon materials composition, cooling rate and quality. Hot-pressure-welding can be an economic and successful process for performing butt joints of simple shapes if the materials are easily weldable. Hot pressure welding is further sub devided as-1. Gass pressure welding

2. Electric resistance welding3. Forge welding

4. Ultrasonic welding

5. Friction welding

6. Explosion welding

7. Blacksmiths forge welding3.2.1 Gass pressure weldingIt is welding method directly of raw material itself to press under the condition which is before heat melting(1,200C ~1,300C) for junction part with oxy-acetylene gas to fix on both side rebars to connect in carrying out of rebar concrete structure building. Also, it is economic splice method than folded tying (lap splice), mechanical spiral tying (mechanical splice) and also the strength of junction part is stronger than raw material.

3.2.1.1 Composition of gas pressure welding equipment

Welding units

1. Oxygen pressure controller

2. Acetylene pressure controller

3. Acetylene outlet hose

4. Oxygen outlet hose

5. Welding torch

6. Welding tip(4 edges)Compressor units

1. Pump

2. Pedal pump

3. Hydraulic pressure hose

4. RAM cylinder

5. External cylinder

6. Fixed clamp

7. Flexible clamp

8. Rebar

3.2.1.2 Working sequence to tie gas pressure welding

The method of gas compressed tying (welded splice) is classified to manual gas compression and automatic gas compression method and the principle is same but difference is by manual or automatically for splicing.

1. Grinding the section of rebar with flat by using grinder

2. Install rebar for splicing to pressure welding equipment

3. Heat and pressure

a) Heat rebar with gas mixed oxygen acetylene

b) Heat machine will be used multi-hole burner

c) Heat and pressure at the same time

d) Don't heat till the temperature of rebar melting but heating to 1,300C as rebar

e) exterior temperature (it is different from welding splice)

f) Pressure to 300~400kg/cm2 per rebar 1cm

g) Use hydraulic regularly for pressure machine

h) It shows time-passage of standard heat, pressure power as above graph.

i) If starting heat, pressure from A point, the section space will be decreased on B

j) point and finally adhere completely to D point.

k) It makes inflation by increasing pressure between F-G to keep this temperature

3.2.1.3 Required time to heat for pressure welding per diameter

3.2.2 Electric resistance welding (ERW)Electric resistance welding (ERW) refers to a group of welding processes such as spot and seam welding that produce coalescence of

surface" faying surfaces where 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).3.2.2.1 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 power sources 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 due to the current required for each application. Care is taken to eliminate contaminants between the faying surfaces. Usually, two copper electrodes are simultaneously used to clamp the metal sheets together and to pass current through the sheets. When the current is passed through the electrodes to the sheets, heat is generated due to the higher electrical resistance where the surfaces contact each other. As the electrical resistance of the material causes a heat buildup in the work pieces between the copper electrodes, the rising temperature causes a rising resistance, and results in a molten pool contained most of the time between the electrodes. As the heat dissipates throughout the workpiece in less than a second (resistance welding time is generally programmed as a quantity of AC cycles or milliseconds) the molten or plastic state grows to meet the welding tips. When the current is stopped the copper tips cool the spot weld, causing the metal to solidify under pressure. The water cooled copper electrodes remove the surface heat quickly, accelerating the solidification of the metal, since copper is an excellent

conductor" conductor. Resistance spot welding typically employs electrical power in the form 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 too low, 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 burst of molten metal is called expulsion, and when this occurs the metal will be thinner and have less strength than a weld with no expulsion. The common method of checking a weld's quality is a peel test. An alternative test is the restrained tensile test, which is much more difficult to perform, and requires calibrated equipment. Because both tests are destructive in nature (resulting in the loss of salable material), non-destructive methods such as ultrasound evaluation are in various states of early adoption by many OEMs.

The advantages of the method include efficient energy use, limited workpiece deformation, high production rates, easy automation, and no required filler materials. When high strength in shear is needed, spot welding is used in preference to more costly mechanical fastening, such as riveting. While the

strength" shear strength of each weld is high, the fact that the weld spots do not form a continuous seam 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 can have several thousand spot welds. A specialized process, called shot welding, can be used to spot weld

steel" stainless steel.

There are three basic types of resistance welding bonds: solid state, fusion, and reflow braze. In a solid state bond, also called a thermo-compression bond, dissimilar materials with dissimilar grain structure, 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 grain interface. Thus the materials actually bond while still in the solid state. The bonded materials typically exhibit excellent shear and tensile strength, but poor peel strength. In a fusion bond, either similar or dissimilar 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 materials with larger grain growth. Typically, high weld energies at either short or long weld times, depending on physical characteristics, are used to produce fusion bonds. The bonded materials usually exhibit excellent tensile, peel and shear strengths. In a reflow braze bond, a resistance heating of a low temperature brazing material, such as gold or solder, is used to join either dissimilar materials or widely varied thick/thin material combinations. The brazing material must wet to each part and possess a lower melting point than the two workpieces. The resultant bond has definite interfaces with minimum grain growth. Typically the process requires a longer (2 to 100 ms) heating time at low weld energy. The resultant bond exhibits excellent tensile strength, but poor peel and shear strength.

3.2.2.2 Projection weldingProjection welding is a modification of spot welding. In this process, the weld is localized by means of raised sections, or projections, on one or both of the workpieces to be joined. Heat is concentrated at 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 used to weld studs, nuts, and other screw machine parts to metal plate. It is also frequently used to join crossed wires and bars. This is another high-production process, and multiple projection welds can be arranged by suitable designing and jigging.

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

joints" welding joints.

Resistance seam welding is a process that produces a weld at the faying surfaces of two similar metals. The seam may be a butt joint or an overlap joint and is usually an automated process. It differs from butt welding in that butt welding typically welds the entire joint at once and seam welding forms the 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 as the material passes between them. This allows the electrodes to stay in constant contact with the material to make long continuous welds. The electrodes may also move or assist the movement of the material.

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 heated to 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 use water 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 and pressure applied. A properly welded joint formed by resistance welding is typically stronger than the material from which it is formed.

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

3.2.2.4 Percussion welding (PEW)Percussion welding (PEW) is a type of welding" resistance welding that blends dissimilar metals together. Percussion welding creates a high temperature arc that is formed from a short quick electrical discharge. Immediately following the electrical discharge, pressure is applied which forges the materials together. This type of joining brings the materials together in a percussive manner.

Percussion welding is similar to flash welding and upset welding but is generally considered to be more complex. It is considered to be more complex because it uses an electric discharge at the joint, followed by pressure being applied to join the materials together. Percussion welding is used to join dissimilar metals together, or used when flash is not required at the joint. This type of welding is limited to the materials having the same cross sectional areas and geometries. Percussion welding is used on materials that have small cross sectional areas.

Advantages of using percussion welding types include a shallow heat affected zone, and the time cycle involved is very short. Typical times can be found to be less than 16 milliseconds.

3.2.2.5 Flash welding

Flash welding is a type of

welding" resistance welding that does not use any

metal" filler metal. The pieces of metal to be welded are set apart at a predetermined distance based on material thickness, material composition, and desired

properties" properties of the finished weld.

current" Current is applied to the metal, and the gap between the two pieces creates resistance and produces the arc required to melt the metal. Once the pieces of metal reach the proper temperature, they are pressed together, effectively forging them together. ParametersSeveral parameters affect the final product. Flash time is the time that the arc is present. Upset time is the amount of time that the two pieces are pressed together. Flash time needs to be long enough to sufficiently heat the metal before it is pressed together. However if it is too long, too much of the base metal begins to melt away. The upset time is critical in creating the desired mechanical properties of the finished weld. During the upset, any impurities in the base metal are pressed out creating a perfect weld. If the upset time is too short, all of the impurities may not be burnt out of the base metal creating a defective weld. The upset time is also crucial in the strength of the finished weld because it is during the upset that coalescence occurs between the two pieces of metal. If the upset time is too short, the two pieces of metal may not completely bond. ApplicationsThe railroad industry is using flash welding to join sections of mainline rail together. This mainline rail is also known as continuously welded rail (CWR) and is much smoother than mechanically joined rail because there are no gaps between the sections of rail. This smoother rail reduces the wear on the rails themselves, effectively reducing the frequency of inspections and maintenance. [2] In other countries, continuously welded rail is used on

rail" high speed rail lines because of its smoothness. A study published in Materials Science and Design proved that flash welding is also beneficial in the railroad industry because it allows dissimilar metals and

ferrous metals" non ferrous metals to be joined. This allows crossings, which are generally composed of high manganese steel, to be effectively welded to the carbon steel rail with the use of a

steel" stainless steel insert, while keeping the desired mechanical properties of both the rail and the crossing intact. [3] The ability of this single process to weld many different metals with simple parameter adjustments makes it very versatile. Materials and Design discusses the use of flash welding in the metal building industry to increase the length of the angle iron used to fabricate joists.[1]3.2.3 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 3.2.4 Friction welding:-The heat required for welding in this process is obtained by the friction between the ends of two parts to 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).

Fig.-5(a) Fig.-5(b)Fig-5 Friction welding3.2.5 Ultrasonic Welding

Rub your hands together rapidly. Notice anything? They warmed up, right? If you take a hammer and pound a metal surface rapidly and repeatedly, you will find that the place where the hammer strikes the metal warms up, too. In both these examples, the heat is due to friction. Now imagine rubbing your hands or pounding that hammer thousands of times per second. The frictional heat generated can raise 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 rub against each other and the friction raises the temperature at the surfaces in contact. This rapid frictional 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 or converter changes the high-frequency electricity into high-frequency sound (ultrasound). A booster makes the ultrasound vibrations bigger. Finally, a horn or sonotrode focuses the ultrasound vibrations and delivers them to the materials to be welded. Besides these pieces, there is an anvil upon which the welded materials are stacked and held. There is also some method to apply force (usually air pressure supplied by a pneumatic piston) to hold the materials together during welding.

So what materials and industries take advantage of this clever process? Ultrasonic welding of plastics is used widely in making electronics, medical devices and car parts. For example, ultrasonic welding is 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 industry uses this technique to make films, assemble tubes and blister packs. Even Ford Motor Company has explored 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.

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 them together.

4. The horn delivers ultrasonic vibrations to heat up the materials. The vibrations move less than a 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 or microprocessor within the welding apparatus. And what actually happens during the welding process depends 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 oxides and films from the surfaces. This allows the metal atoms to move between the two surfaces and form bonds that hold the metals together.

In the case of plastics, the vibrations are perpendicular to the plane of the materials and the frictional heat increases the temperature enough to melt the plastics. The plastic molecules mix together and form bonds. Upon cooling, the plastic surfaces are welded together. Welding times can vary, but the welds 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 40 kHz), the pressure applied to hold the materials together, and the time over which the ultrasound is applied (fractions of a second to more than one second).

The ultrasonic welding techniques described so far are good for materials (metals, plastics) that are similar. 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 low temperatures relative to other methods. So, the manufacturer does not need to expend vast amounts of fuel or other energy to reach high temperatures. This makes the process cheaper. It's also faster and safer.

The process occurs in fractions of a second to seconds. So, it can be done more quickly than other methods. In fact, it can bond plastics better and faster than glues. For example, the new smart keys in cars 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 half gets 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 welding methods, it's a safer process. Workers are not exposed to flammable gases or noxious solvents. In electronics, copper wires are usually bonded to electrical contacts on circuit boards with solder. The same task can be done using ultrasonic welding in a fraction of the time and without exposing workers to fumes from smoldering lead solder. Although workers' hearing may be damaged by exposure to high-frequency sound, this potential danger is easily reduced by enclosing the ultrasonic welding machine 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 lighter and 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 than traditional welding.

Ultrasonic welding does have its limitations, though. First, the depths of the welds are less than a millimeter, 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 best when welding similar materials like similar plastics or similar metals. As you saw with New Balance shoes, 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

xplosive 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.

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

1 Explosive 2 Detonation velocity , m/s

RDX (Cyclotrimethylene trinitramine, C3H6N6O68100

PETN (Pentaerythritol tetranitrate, C5H8N12O4)8190

TNT (Trinitrotoluene, C7H5N3O6)6600

Tetryl (Trinitrophenylmethylinitramine, C7H5O8N5)7800

Lead azide (N6Pb)5010

3 Detasheet7020

Ammonium nitrate (NH4NO3)2655

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.

7) Small quantity of explosive used.

Limitations

1. 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 vibrations caused by the explosion.

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

Chapter-4 Arc welding4.1 Arc weldingArc 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 welding9. Atomic hydrogen arc welding10. Atomic arc welding4.1.1 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.

Fig The basic arc welding circuitFig.... 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 ArcTo 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.ElectrodeAn 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, etc3. Heavy coated electrode

The arc can be rendered easy to control and the absorption of atmospheric gases are reduced to minimum 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.

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 supplyWhen 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.

Fig. Straight (left) and reverse (right) polarityThe 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. TransformerA 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

FigFig. Fig

CHAPTER 5

5 Gas Welding:-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.

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.

Fig Gas flame4.1.2 Carbon arc-welding:-

Carbon arc-welding is the earliest arc-welding process. In this, the electrode is made of either carbon or graphite. In contrast to graphite electrodes, carbon electrodes are soft and therefore, cannot take up very high current densities, the arc with the carbon electrodes is more controllable. Lower currents also add to the higher electrodes life.

Though carbon or graphite electrodes are not expected to melts as the consumable electrodes, they do get heated to a red-hot temperature because of the heat from the arc which caused a slow disintegration of the electrode tip as also its oxidation. This means that this electrode is slowly consumed. For use in carbon arc welding, the electrode should be of uniform structure and, as far as possible, free from impurities. The life of a graphite electrode is higher than that of a carbon electrode.

In the carbon arc-welding practice, the required filler metal is supplied through a separate filler rod. The arc can be obtained between the carbon electrode and the work-piece or between two carbon electrodes, as shown in figure-13. In the twin carbon electrode system, there is provision for controlling the arc length by means of the welding torch whereby the electrodes can be either brought in contact with one another or taken apart.

Fig-13 Carbon welding***Atomic Hydrogen Welding

"A process in which the welding heat is generated by passing a stream of hydrogen through an electric arc between two inclined electrodes, which are usually of tungsten. The high temperature of the arc dissociates molecules of the gas into atoms, a large quantity of heat being absorbed by the hydrogen during dissociation. When the atoms leave the influence of the arc they recombine, forming molecules of hydrogen and liberating heat previously absorbed. The gas then burns in the ordinary way, taking up oxygen from the atmosphere for the purpose.The average temperature of the flame is approximately 4000 deg. C., which is higher than the maximum temperature of any other flame. The heat is concentrated chiefly at the point of recombination of the atoms, and this recombination is accelerated catalytically by contact with the surface of the metal being welded. Thus an intense flame is obtained at the point of welding. The process is, therefore, used when rapid welding is necessary, as for stainless steels and other special alloys. The hydrogen envelope prevents oxidation both of the metal and the tungsten electrodes, and it also reduces the risk of nitrogen pick-up. The non-oxidizing characteristic is perhaps the most important in practice.As a rule, the cost of welding by this process is slightly higher than with other processes, but it is sometimes the only practicable method by which a satisfactory weld can be made. An automatic atomic - hydrogen welding process has also been developed in which, instead of using hydrogen from high-pressure cylinders, the hydrogen is obtained by cracking anhydrous ammonia."

4.1.2 Electroslag Welding (ESW)

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

Prior to welding the gap between the two work pieces is filled with a welding flux. Electroslag Welding is initiated by an arc 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 electric conductivity, 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 melting the 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.

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.

4.1.3 Submerged Arc Welding (SAW)

Submerged Arc Welding is a welding process, which utilizes a bare consumable metallic electrode producing an arc between itself and the work piece within a granular shielding flux applied around the weld.

The arc heats and melts both the work pieces edges and the electrode wire. The molten electrode material 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 and forms 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.

4.1.4 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) or water 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 speed welding and cutting of Ceramics, steels, Aluminum alloys, Copper alloys, Titanium alloys, Nickel alloys.

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

Advantages of Plasma Arc Welding (PAW):

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)4.2 Thermit Welding (TW)

Thermit Welding is a welding process utilizing heat generated by exothermic chemical reaction between the components of the thermit (a mixture of a metal oxide and aluminum powder). The molten metal, produced by the reaction, acts as a filler material joining the work pieces after Solidification.

Thermit Welding is mainly used for joining steel parts, therefore common thermit is composed from iron oxide (78%) and aluminum powder (22%).The proportion 78-22 is determined by the chemical reaction of combustion of aluminum:

8Al + Fe3O4 = 9Fe + 4Al2O3

The combustion reaction products (iron and aluminum oxide) heat up to 4500F (2500C). Liquid iron fills the sand (or ceramic) mold built around the welded parts, the slag (aluminum oxide), floating up , is then removed from the weld surface.

Thermit Welding is used for repair of steel casings and forgings,for joining railroad rails, steel wires and steel pipes, for joining large cast and forged parts.

Advantages of Thermit Welding:

No external power source is required (heat of chemical reaction is utilized);

Very large heavy section parts may be joined.

Disadvantages of Resistance Welding:

Only ferrous (steel, chromium, nickel) parts may be welded;

Slow welding rate;

High temperature process may cause distortions and changes in Grain structure in the weld region.

Weld may contain gas (Hydrogen) and slag contaminations.

Try to find the picture of thermit welding4.3 Laser Welding (LW)

Laser Welding (LW) is a welding process, in which heat is generated by a high energy laser beam targeted 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), therefore diminutive 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 the atmosphere 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 spot welds 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;

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 is shielded 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, is continuously fed from a spool to the arc zone. The arc heats and melts both the work pieces edges and the electrode wire. The fused electrode material is supplied to the surfaces of the work pieces, fills the weld 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;

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 is generated 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 from atmospheric contamination.

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

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

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.

****Solid State Welding (SSW)

Solid State Welding is a welding process, in which two work pieces are joined under a pressure providing an intimate contact between them and at a temperature essentially below the melting point of the parent material. Bonding of the materials is a result of diffusion of their interface atoms.

Advantages of Solid State Welding:

Weld (bonding) is free from microstructure defects (pores, non-metallic inclusions, segregation of alloying elements)

Mechanical properties of the weld are similar to those of the parent metals

No consumable materials (filler material, fluxes, shielding gases) are required

Dissimilar metals may be joined (steel - aluminum alloy steel - copper alloy).

Disadvantages of Solid State Welding:

Thorough surface preparation is required (degreasing, oxides removal, brushing/sanding)

Expensive equipment.

The following processes are related to Solid State welding:

Forge Welding (FOW) Cold Welding (CW) Friction Welding (FRW) Explosive Welding (EXW) Diffusion Welding (DFW) Ultrasonic Welding (USW)Forge Welding (FOW)

Forge Welding is a Solid State Welding process, in which low carbon steel parts are heated to about 1800F (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 welded tubes.

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 Welding process, in which two work pieces are joined together at room temperature and under a pressure, causing a substantial deformation of the welded parts and providing an intimate contact between the welded surfaces.

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

Aluminum alloys, Copper alloys, low carbon steels, Nickel alloys, and other ductile metals may be welded by Cold Welding.

Cold Welding is widely used for manufacturing bi-metal steel - aluminum alloy strips, for cladding of aluminum 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 Welding process, in which two cylindrical parts are brought in contact 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.

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

Friction-stir welding (FSW):

Friction-stir welding (FSW) is a solid-state joining process (the metal is not melted) that uses a third body tool to join two faying surfaces. Heat is generated between the tool and material which leads to a very soft region near the FSW tool. It then mechanically intermixes the two pieces of metal at the place of the join, then the softened metal (due to the elevated temperature) can be joined using mechanical pressure (which is applied by the tool), much like joining clay, or dough. It is primarily used on aluminium, and most often on extruded aluminum (non-heat treatable alloys), and on structures which need superior weld strength without a post weld heat treatment.

It was invented and experimentally proven at The Welding Institute UK in December 1991. TWI holds patents on the process, the first being the most descriptive.[1]Principle of operation

Schematic diagram of the FSW process: (A) Two discrete metal workpieces butted together, along with the tool (with a probe).

(B) The progress of the tool through the joint, also showing the weld zone and the region affected by the tool shoulder.

A constantly rotated non consumable cylindrical-shouldered tool with a profiled nib is transversely fed at a constant rate into a butt joint between two clamped pieces of butted material. The nib is slightly shorter than the weld depth required, with the tool shoulder riding atop the work surface.[2]Frictional heat is generated between the wear-resistant welding components and the work pieces. This heat, along with that generated by the mechanical mixing process and the adiabatic heat within the material, cause the stirred materials to soften without melting. As the pin is moved forward, a special profile on its leading face forces plasticised material to the rear where clamping force assists in a forged consolidation of the weld.

This process of the tool traversing along the weld line in a plasticised tubular shaft of metal results in severe solid state deformation involving dynamic recrystallization of the base material.[3]

Microstructural featuresThe solid-state nature of the FSW process, combined with its unusual tool and asymmetric nature, results in a highly characteristic microstructure. The microstructure can be broken up into the following zones:

The stir zone (also nugget, dynamically recrystallised zone) is a region of heavily deformed material that roughly corresponds to the location of the pin during welding. The grains within the stir zone are roughly equiaxed and often an order of magnitude smaller than the grains in the parent material.[4] A unique feature of the stir zone is the common occurrence of several concentric rings which has been referred to as an "onion-ring" structure.[5] The precise origin of these rings has not been firmly established, although variations in particle number density, grain size and texture have all been suggested.

The flow arm zone is on the upper surface of the weld and consists of material that is dragged by the shoulder from the retreating side of the weld, around the rear of the tool, and deposited on the advancing side.[citation needed] The thermo-mechanically affected zone (TMAZ) occurs on either side of the stir zone. In this region the strain and temperature are lower and the effect of welding on the microstructure is correspondingly smaller. Unlike the stir zone the microstructure is recognizably that of the parent material, albeit significantly deformed and rotated. Although the term TMAZ technically refers to the entire deformed region it is often used to describe any region not already covered by the terms stir zone and flow arm.[citation needed] The heat-affected zone (HAZ) is common to all welding processes. As indicated by the name, this region is subjected to a thermal cycle but is not deformed during welding. The temperatures are lower than those in the TMAZ but may still have a significant effect if the microstructure is thermally unstable. In fact, in age-hardened aluminium alloys this region commonly exhibits the poorest mechanical properties.[6]Advantages and limitationsThe solid-state nature of FSW leads to several advantages over fusion welding methods as problems associated with cooling from the liquid phase are avoided. Issues such as porosity, solute redistribution, solidification cracking and liquation cracking do not arise during FSW. In general, FSW has been found to produce a low concentration of defects and is very tolerant of variations in parameters and materials.

Nevertheless, FSW is associated with a number of unique defects. Insufficient weld temperatures, due to low rotational speeds or high traverse speeds, for example, mean that the weld material is unable to accommodate the extensive deformation during welding. This may result in long, tunnel-like defects running along the weld which may occur on the surface or subsurface. Low temperatures may also limit the forging action of the tool and so reduce the continuity of the bond between the material from each side of the weld. The light contact between the material has given rise to the name "kissing-bond". This defect is particularly worrying since it is very difficult to detect using nondestructive methods such as X-ray or ultrasonic testing. If the pin is not long enough or the tool rises out of the plate then the interface at the bottom of the weld may not be disrupted and forged by the tool, resulting in a lack-of-penetration defect. This is essentially a notch in the material which can be a potential source of fatigue cracks.

A number of potential advantages of FSW over conventional fusion-welding processes have been identified:[7] Good mechanical properties in the as-welded condition

Improved safety due to the absence of toxic fumes or the spatter of molten material.

No consumables A threaded pin made of conventional tool steel, e.g., hardened H13, can weld over 1km (0.62mi) of aluminium, and no filler or gas shield is required for aluminium.

Easily automated on simple milling machines lower setup costs and less training.

Can operate in all positions (horizontal, vertical, etc.), as there is no weld pool.

Generally good weld appearance and minimal thickness under/over-matching, thus reducing the need for expensive machining after welding.

Low environmental impact.

However, some disadvantages of the process have been identified:

Exit hole left when tool is withdrawn.

Large down forces required with heavy-duty clamping necessary to hold the plates together.

Less flexible than manual and arc processes (difficulties with thickness variations and non-linear welds).

Often slower traverse rate than some fusion welding techniques, although this may be offset if fewer welding passes are required.

Important welding parametersa) Tool rotation and traverse speedsThere are two tool speeds to be considered in friction-stir welding; how fast the tool rotates and how quickly it traverses the interface. These two parameters have considerable importance and must be chosen with care to ensure a successful and efficient welding cycle. The relationship between the welding speeds and the heat input during welding is complex but, in general, it can be said that increasing the rotation speed or decreasing the traverse speed will result in a hotter weld. In order to produce a successful weld it is necessary that the material surrounding the tool is hot enough to enable the extensive plastic flow required and minimize the forces acting on the tool. If the material is too cold then voids or other flaws may be present in the stir zone and in extreme cases the tool may break.

Excessively high heat input, on the other hand may be detrimental to the final properties of the weld. Theoretically, this could even result in defects due to the liquation of low-melting-point phases (similar to liquation cracking in fusion welds). These competing demands lead onto the concept of a "processing window": the range of processing parameters viz. tool rotation and traverse speed, that will produce a good quality weld.[8] Within this window the resulting weld will have a sufficiently high heat input to ensure adequate material plasticity but not so high that the weld properties are excessively deteriorated.

b) Tool tilt and plunge depth

A drawing showing the plunge depth and tilt of the tool. The tool is moving to the left.

The plunge depth is defined as the depth of the lowest point of the shoulder below the surface of the welded plate and has been found to be a critical parameter for ensuring weld quality.[9] Plunging the shoulder below the plate surface increases the pressure below the tool and helps ensure adequate forging of the material at the rear of the tool. Tilting the tool by 24 degrees, such that the rear of the tool is lower than the front, has been found to assist this forging process. The plunge depth needs to be correctly set, both to ensure the necessary downward pressure is achieved and to ensure that the tool fully penetrates the weld. Given the high loads required, the welding machine may deflect and so reduce the plunge depth compared to the nominal setting, which may result in flaws in the weld. On the other hand, an excessive plunge depth may result in the pin rubbing on the backing plate surface or a significant undermatch of the weld thickness compared to the base material. Variable load welders have been developed to automatically compensate for changes in the tool displacement while TWI have demonstrated a roller system that maintains the tool position above the weld plate.

c) Tool designThe design of the tool[10] is a critical factor as a good tool can improve both the quality of the weld and the maximum possible welding speed.

It is desirable that the tool material is sufficiently strong, tough, and hard wearing at the welding temperature. Further it should have a good oxidation resistance and a low thermal conductivity to minimise heat loss and thermal damage to the machinery further up the drive train. Hot-worked tool steel such as AISI H13 has proven perfectly acceptable for welding aluminium alloys within thickness ranges of 0.5 50mm [11] but more advanced tool materials are necessary for more demanding applications such as highly abrasive metal matrix composites[12] or higher melting point materials such as steel or titanium.

Joint designs

Improvements in tool design have been shown to cause substantial improvements in productivity and quality. TWI has developed tools specifically designed to increase the penetration depth and thus increasing the plate thicknesses that can be successfully welded. An example is the "whorl" design that uses a tapered pin with re-entrant features or a variable pitch thread to improve the downwards flow of material. Additional designs include the Triflute and Trivex series. The Triflute design has a complex system of three tapering, threaded re-entrant flutes that appear to increase material movement around the tool. The Trivex tools use a simpler, non-cylindrical, pin and have been found to reduce the forces acting on the tool during welding.

The majority of tools have a concave shoulder profile which acts as an escape volume for the material displaced by the pin, prevents material from extruding out of the sides of the shoulder and maintains downwards pressure and hence good forging of the material behind the tool. The Triflute tool uses an alternative system with a series of concentric grooves machined into the surface which are intended to produce additional movement of material in the upper layers of the weld.

FSW of two USIBOR 1500 high-strength steel sheets

Widespread commercial applications of friction stir welding process for steels and other hard alloys such as titanium alloys will require the development of cost-effective and durable tools.[13] Material selection, design and cost are important considerations in the search for commercially useful tools for the welding of hard materials. Work is continuing to better understand the effects of tool material's composition, structure, properties and geometry on their performance, durability and cost.[14]c) Welding forcesDuring welding a number of forces will act on the tool:

A downwards force is necessary to maintain the position of the tool at or below the material surface. Some friction-stir welding machines operate under load control but in many cases the vertical position of the tool is preset and so the load will vary during welding.

The traverse force acts parallel to the tool motion and is positive in the traverse direction. Since this force arises as a result of the resistance of the material to the motion of the tool it might be expected that this force will decrease as the temperature of the material around the tool is increased.

The lateral force may act perpendicular to the tool traverse direction and is defined here as positive towards the advancing side of the weld.

Torque is required to rotate the tool, the amount of which will depend on the down force and friction coefficient (sliding friction) and/or the flow strength of the material in the surrounding region (stiction).

In order to prevent tool fracture and to minimize excessive wear and tear on the tool and associated machinery, the welding cycle is modified so that the forces acting on the tool are as low as possible, and abrupt changes are avoided. In order to find the best combination of welding parameters, it is likely that a compromise must be reached, since the conditions that favour low forces (e.g. high heat input, low travel speeds) may be undesirable from the point of view of productivity and weld properties.

d) Flow of materialEarly work on the mode of material flow around the tool used inserts of a different alloy, which had a different contrast to the normal material when viewed through a microscope, in an effort to determine where material was moved as the tool passed.[15] [16] The data was interpreted as representing a form of in-situ extrusion where the tool, backing plate and cold base material form the "extrusion chamber" through which the hot, plasticised material is forced. In this model the rotation of the tool draws little or no material around the front of the pin instead the material parts in front of the pin and passes down either side. After the material has passed the pin the side pressure exerted by the "die" forces the material back together and consolidation of the join occurs as the rear of the tool shoulder passes overhead and the large down force forges the material.

More recently, an alternative theory has been advanced that advocates considerable material movement in certain locations.[17] This theory holds that some material does rotate around the pin, for at least one rotation, and it is this material movement that produces the "onion-ring" structure in the stir zone. The researchers used a combination of thin copper strip inserts and a "frozen pin" technique, where the tool is rapidly stopped in place. They suggested that material motion occurs by two processes:

1. Material on the advancing front side of a weld enters into a zone that rotates and advances with the pin. This material was very highly deformed and sloughs off behind the pin to form arc-shaped features when viewed from above (i.e. down the tool axis). It was noted that the copper entered the rotational zone around the pin, where it was broken up into fragments. These fragments were only found in the arc shaped features of material behind the tool.

2. The lighter material came from the retreating front side of the pin and was dragged around to the rear of the tool and filled in the gaps between the arcs of advancing side material. This material did not rotate around the pin and the lower level of deformation resulted in a larger grain size.

The primary advantage of this explanation is that it provides a plausible explanation for the production of the onion-ring structure.

The marker technique for friction stir welding provides data on the initial and final positions of the marker in the welded material. The flow of material is then reconstructed from these positions. Detailed material flow field during friction stir welding can also be calculated from theoretical considerations based on fundamental scientific principles. Material flow calculations are routinely used in numerous engineering applications. Calculation of material flow fields in friction stir welding can be undertaken both using comprehensive numerical simulations[18]

HYPERLINK "http://en.wikipedia.org/wiki/Friction_stir_welding" \l "cite_note-19" [19]

HYPERLINK "http://en.wikipedia.org/wiki/Friction_stir_welding" \l "cite_note-seidle_stwj-20" [20] or simple but insightful analytical equations.[21] The comprehensive models for the calculation of material flow fields also provide important information such as geometry of the stir zone and the torque on the tool.[22]

HYPERLINK "http://en.wikipedia.org/wiki/Friction_stir_welding" \l "cite_note-mehta_torque-23" [23] The numerical simulations have shown the ability to correctly predict the results from marker experiments[20] and the stir zone geometry observed in friction stir welding experiments.[22]

HYPERLINK "http://en.wikipedia.org/wiki/Friction_stir_welding" \l "cite_note-24" [24]Generation and flow of heatFor any welding process it is, in general, desirable to increase the travel speed and minimise the heat input as this will increase productivity and possibly reduce the impact of welding on the mechanical properties of the weld. At the same time it is necessary to ensure that the temperature around the tool is sufficiently high to permit adequate material flow and prevent flaws or tool damage.

When the traverse speed is increased, for a given heat input, there is less time for heat to conduct ahead of the tool and the thermal gradients are larger. At some point the speed will be so high that the material ahead of the tool will be too cold, and the flow stress too high, to permit adequate material movement, resulting in flaws or tool fracture. If the "hot zone" is too large then there is scope to increase the traverse speed and hence productivity.

The welding cycle can be split into several stages during which the heat flow and thermal profile will be different:[25] Dwell. The material is preheated by a stationary, rotating tool to achieve a sufficient temperature ahead of the tool to allow the traverse. This period may also include the plunge of the tool into the workpiece.

Transient heating. When the tool begins to move there will be a transient period where the heat production and temperature around the tool will alter in a complex manner until an essentially steady-state is reached.

Pseudo steady-state. Although fluctuations in heat generation will occur the thermal field around the tool remains effectively constant, at least on the macroscopic scale.

Post steady-state. Near the end of the weld heat may "reflect" from the end of the plate leading to additional heating around the tool.

Heat generation during friction-stir welding arises from two main sources: friction at the surface of the tool and the deformation of the material around the tool.[26] The heat generation is often assumed to occur predominantly under the shoulder, due to its greater surface area, and to be equal to the power required to overcome the contact forces between the tool and the workpiece. The contact condition under the shoulder can be described by sliding friction, using a friction coefficient and interfacial pressure P, or sticking friction, based on the interfacial shear strength at an appropriate temperature and strain rate. Mathematical approximations for the total heat generated by the tool shoulder Qtotal have been developed using both sliding and sticking friction models:[25](Sliding)

(Sticking)

where is the angular velocity of the tool, Rshoulder is the radius of the tool shoulder and Rpin that of the pin. Several other equations have been proposed to account for factors such as the pin but the general approach remains the same.

A major difficulty in applying these equations is determining suitable values for the friction coefficient or the interfacial shear stress. The conditions under the tool are both extreme and very difficult to measure. To date, these parameters have been used as "fitting parameters" where the model works back from measured thermal data to obtain a reasonable simulated thermal field. While this approach is useful for creating process models to predict, for example, residual stresses it is less useful for providing insights into the process itself.

ApplicationsThe FSW process is currently patented by TWI in most industrialised countries and licensed for over 183 users. Friction stir welding and its variants friction stir spot welding and friction stir processing are used for the following industrial applications:[27]

Friction stir welding was used to prefabricate the aluminium panels of the Super Liner Ogasawara at Mitsui Engineering and Shipbuilding

Shipbuilding and OffshoreTwo Scandinavian aluminium extrusion companies were the first to apply FSW commercially to the manufacture of fish freezer panels at Sapa in 1996, as well as deck panels and helicopter landing platforms at Marine Aluminium Aanensen. Marine Aluminium Aanensen subsequently merged with Hydro Aluminium Maritime to become Hydro Marine Aluminium. Some of these freezer panels are now produced by Riftec and Bayards. In 1997 two-dimensional friction stir welds in the hydrodynamically flared bow section of the hull of the ocean viewer vessel The Boss were produced at Research Foundation Institute with the first portable FSW machine. The Super Liner Ogasawara at Mitsui Engineering and Shipbuilding is the largest friction stir welded ship so far. The Sea Fighter of Nichols Bros and the Freedom class Littoral Combat Ships contain prefabricated panels by the FSW fabricators Advanced Technology and Friction Stir Link, Inc. respectively.[29] The Houbei class missile boat has friction stir welded rocket launch containers of China Friction Stir Centre. The HMNZS Rotoiti in New Zealand has FSW panels made by Donovans in a converted milling machine.[30]

HYPERLINK "http://en.wikipedia.org/wiki/Friction_stir_welding" \l "cite_note-NZ-31" [31] Various companies apply FSW to armor plating for amphibious assault ships [32]

HYPERLINK "http://en.wikipedia.org/wiki/Friction_stir_welding" \l "cite_note-Stotler-33" [33]

Longitudinal and circumferential friction stir welds are used for the Falcon 9 rocket booster tank at the SpaceX factory

AerospaceBoeing applies FSW to the Delta II and Delta IV expendable launch vehicles, and the first of these with a friction stir welded Interstage module was launched in 1999. The process is also used for the Space Shuttle external tank, for Ares I and for the Orion Crew Vehicle test article at NASA[dated info] as well as Falcon 1 and Falcon 9 rockets at SpaceX. The toe nails for ramp of Boeing C-17 Globemaster III cargo aircraft by Advanced Joining Technologies[36] and the cargo barrier beams for the Boeing 747 Large Cargo Freighter[36] were the first commercially produced aircraft parts. FAA approved wings and fuselage panels of the Eclipse 500 aircraft were made at Eclipse Aviation, and this company delivered 259 friction stir welded business jets, before they were forced into Chapter 7 liquidation. Floor panels for Airbus A400M military aircraft are now made by Pfalz Flugzeugwerke and Embraer used FSW for the Legacy 450 and 500 Jets [37]

The centre tunnel of the Ford GT is made from two aluminium extrusions friction stir welded to a bent aluminium sheet and houses the fuel tank

AutomotiveAluminium engine cradles and suspension struts for stretched Lincoln Town Car were the first automotive parts that were friction stir at Tower Automotive, who use the process also for the engine tunnel of the Ford GT. A spin-off of this company is called Friction Stir Link, Inc. and successfully exploits the FSW process, e.g. for the flatbed trailer "Revolution" of Fontaine Trailers.[37] In Japan FSW is applied to suspension struts at Showa Denko and for joining of aluminium sheets to galvanized steel brackets for the boot (trunk) lid of the Mazda MX-5. Friction stir spot welding is successfully used for the bonnet (hood) and rear doors of the Mazda RX-8 and the boot lid of the Toyota Prius. Wheels are friction stir welded at Simmons Wheels, UT Alloy Works and Fundo[39] Rear seats for the Volvo V70 are friction stir welded at Sapa, HVAC pistons at Halla Climate Control and exhaust gas recirculation coolers at Pierburg. Tailor welded blanks[40] are friction stir welded for the Audi R8 at Riftec.[41] The B-column of the Audi R8 Spider is friction stir welded from two extrusions at Hammerer Aluminium Industries in Austria.

The high-strength low-distortion body of Hitachi's A-train British Rail Class 395 is friction stir welded from longitudinal aluminium extrusions

4.3.1.1 Explosive Welding (EXW)

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

One of the welded parts (base plate) is rested on an anvil, the second part (flyer plate) is located above the 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:

Copper to steel;

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 metal microstructure;

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);

Safety and security aspects of storage and using explosives.

Explosive Welding is used for manufacturing clad tubes and pipes, pressure vessels, aerospace structures, heat exchangers, bi-metal sliding bearings, ship structures, weld transitions, corrosion resistant chemical process tanks.

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4.3.1.2 Diffusion Welding (DFW)

Diffusion Welding is a Solid State Welding process, in which pressure applied to two work pieces with 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 welding processes:

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 (no pores, inclusions, chemical segregation, 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 hi