production technology notes

PRODUCTION TECHNOLOGY NOTES

Electron Beam Welding (EBW)

In EBW process, the heat is generated when the electron beam impinges on work piece. As

the high velocity electron beam strikes the surfaces to be welded, their kinetic energy

changes to thermal energy and hence causes the workpiece metal to melt and fuse. A

schematic setup of the electron beam welding is shown in Fig. This process employs an

electron gun in which the cathode in form of hot filament of tungsten or tantalum is the

source of a stream of electrons. The electrons emitted from filament by thermionic

emission are accelerated to a high velocity to the anode because of the large potential

difference that exists between them. The potential differences that are used are of the

order of 30 kV to 175 kV. The higher the potential difference, higher would be the

acceleration. The current levels are low ranging between 50 mA to 1000 mA. The electron

beam is focused by a magnetic lens system on the workpieces to be welded. The depth of

penetration of the weld depends on the electron speed which in turn is dependent upon the

accelerating voltage.

When the high velocity electron beam strikes the work-piece all the kinetic energy is

converted to heat. As these electrons penetrate the metal, the material that is directly in the

path is melted which when solidifies form the joint. Electron beam welding has several

advantages which may not be found in other welding processes. The penetration of the

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beam is high. The depth to width ratios lies between 10:1 to 30:1 can be easily realized

with electron beam welding. It is also possible to closely control this penetration by

controlling the accelerating voltage, beam current, and beam focus. The process can be

used at higher welding speeds typically between 125 and 200 mm/sec. No filler metal or

flux needs to be used in this process.

The heat liberated is low and also is in a narrow zone, thus the heat affected zone is

minimal as well as weld distortions are virtually eliminated. It is possible to carry out the

electron beam welding in open atmosphere. For welding in vacuum, the work-piece is

enclosed in a box in which the vacuum is created. When electron beam moves in the normal

atmosphere, the electrons would be impinging with the gas molecules in the atmosphere

and would thus be scattered. This scattering increases the spot size of the electron beam

and consequently there is lower penetration. As the vacuum increases, the scattering effect

of the electron beam decreases and hence, penetration increases. The other advantage of

using vacuum is that the weld metal is not contaminated.

The EBW process is mainly used for welding of reactive metals (nuclear reactor

components), titanium, zirconium, stainless steel, etc. for aero-space and automotive

industries.

SOLID STATE WELDING PROCESSES

In these processes, the base materials to be joined are heated to a temperature below or

just upto the solidus temperature and then continuous pressure is applied to form the

welded joint. No filler metal is used in solid-state welding processes. The various solid-

state welding processes are-

(1) Forge Welding

(2) Cold Pressure Welding

(3) Friction Welding

(4) Explosive Welding

(5) Diffusion Welding

(6) Thermo-compression Welding

Some of the above important welding processes are discussed as under,

Forge Welding



In this welding process, the work-pieces to be welded are heated to the plastic condition

(above 1000°C), and then placed together and forged while hot by applying force. Force

may be applied by hammering, rolling, drawing or squeezing to achieve the forging action.

Forge welding was originally the first process of welding. In this process the two metal

pieces to be joined are heated in a forge or furnace to a plait condition and then they are

united by pressure. The ends to be joined are heated in a furnace to plastic condition and

formed to the required shape by upsetting. Then they are brought together and hammered,

so as to get the finished joint similarly, a butt joint can be prepared by forge welding as

shown in Fig. Before joining the two pieces, their ends are formed to the required shape

according to the type of joint.

The forge welding is a manual process and is limited to light work because all forming and

welding are done with a hand sledge. It is a slow process and there is considerable danger

of an oxide scale forming on tile surfaces. The tendency to oxidize can be counteracted

somewhat by using a thick fuel bed and by covering the surfaces with a fluxing material,

which dissolves the oxides. Borax in combination with salt ammoniac is commonly used as

flux. The forge welding is recommended to such metals, which have a large welding

temperature range like low carbon steel and wrought iron. By the increase of carbon

content, this range decreases rapidly. High carbon steels alloy steels require considerably

more care in controlling temperature and producing the welds. Large work may be welded

in hammer forges driven by steam. Welded steel pipe is made mechanically by running the

preheated strips through rolls, which form the pipe to size and apply the necessary

pressure for the weld.

Friction Welding

In this process, the heat for welding is obtained from mechanically induced sliding motion

between rubbing surfaces of work-pieces as shown in Fig. In friction welding, one part is

firmly held while the other (usually cylindrical) is rotated under simultaneous application

of axial pressure. As these parts are brought to rub against each other under pressure, they

get heated due to friction. When the desired forging temperature is attained, the rotation is

stopped and the axial pressure is increased to obtain forging action and hence welded joint.



Most of the metals and their dissimilar combinations such as aluminium and titanium,

copper and steel, aluminium and steel etc. can be welded using friction welding.

Explosive Welding

In explosive welding, strong metallurgical bonds can be produced between metal

combinations which cannot be welded by other methods or processes. For example,

tantalum can be explosively welded to steel although the welding point of tantalum is

higher than the vaporization temperature of steel. Explosive welding process is shown in

Fig. It is carried out by bringing together properly paired metal surfaces with high relative

velocity at a high pressure and a proper orientation to each other so that a large amount of

plastic interaction occurs between the surfaces. The work piece, held fixed is called the

target plate and the other called flyer plate. While a variety of procedures have been

successfully employed, the main techniques of explosive welding can be divided into

contact techniques and impact techniques. In critical space and nuclear application,

explosive welding permits fabrication of structures that cannot be made by any other

means and, in some commercial applications, explosive joining is the least costly method.

The main advantage of explosive welding includes the simplicity of the process, and the

extremely large surface that can be welded. Incompatible materials can also be bonded, and

thin foils can be bonded to heavier plates.



RESISTANCE WELDING- TYPES

In resistance welding the metal parts to be joined are heated by their resistance to the flow

of an electrical current. Usually this is the only source of heat, but a few of the welding

operations combine resistance heating with arc heating, and possibly with combustion of

metal in the arc. The process applies to practically all metals and most combinations of

pure metals and those alloys, which have only a limited plastic range, are welded by

heating the parts to fusion (melting). Some alloys, however, may welded without fusion;

instead, the parts are heated to a plastic state at which the applied pressure causes their

crystalline structures to grow together. The welding of dissimilar metals may be

accomplished by melting both metals frequently only the metal with the lower melting

point is melted, and an alloy bond is formed at the surface of the unmelted metal.

In resistance welding processes no fluxes are employed, the filler metal is rarely used and

the joints are usually of the lap type. The amount of heat generated in the workpiece

depend on the following factors:

(1) Magnitude of the current,

(2) Resistance of the current conducting path, and

Mathematically, H = IVt

= I(IR)t

= I2Rt

Where H = heat generated in joules

I = current in Amp.



R = resistance in ohms

t = time of current flow in seconds.

Types of Resistance welding

The major types of resistance welding are given as under:

(1) Spot Welding

(2) Seam Welding

(3) Projection Welding

(4) Resistance Butt Welding

(5) Flash Butt Welding

(6) Percussion Welding

(7) High Frequency Resistance Welding

(8) High Frequency Induction Welding


Spot Welding

In this process overlapping sheets are joined by local fusion at one or more spots, by the

concentration of current flowing between two electrodes. This is the most widely used

resistance welding process. A typical resistance spot welding machine is shown in Fig. It

essentially consists of two electrodes, out of which one is fixed. The other electrode is fixed

to a rocker arm (to provide mechanical advantage) for transmitting the mechanical force

from a pneumatic cylinder. This is the simplest type of arrangement. The other possibility

is that of a pneumatic or hydraulic cylinder being directly connected to the electrode

without any rocker arm. For welding large assemblies such as car bodies, portable spot

welding machines are used. Here the electrode holders and the pneumatic pressurizing

system are present in the form of a portable assembly which is taken to the place, where

the spot is to be made. The electric current, compressed air and the cooling water needed

for the electrodes is supplied through cables and hoses from the main welding machine to

the portable unit. In spot welding, a satisfactory weld is obtained when a proper current

density is maintained. The current density depends on the contact area between the

electrode and the work-piece. With the continuous use, if the tip becomes upset and- the

contact area increases, the current density will be lowered and consequently the weld is



obtained over a large area. This would not be able to melt the metal and hence there would

be no proper fusion. A resistance welding schedule is the sequence of events that normally

take place in each of the welds. The events are:

1. The squeeze time is the time required for the electrodes to align and clamp the two

work-pieces together under them and provide the necessary electrical contact.

2. The weld time is the time of the current flow through the work-pieces till they are heated

to the melting temperature.

3. The hold time is the time when the pressure is to be maintained on the molten metal

without the electric current. During this time, the pieces are expected to be forged welded.

4. The off time is time during which, the pressure on the electrode is taken off so that the

plates can be positioned for the next spot.

Before spot welding one must make sure that

(i) The job is clean, i.e., free from grease, dirt, paint, scale, oxide etc.

(ii) Electrode tip surface is clean, since it has to conduct the current into the work with as

little loss as possible. Very fine emery cloth may be used for routine cleaning.

(iii) Water is running through the electrodes in order to

(a) Avoid them from getting overheated and thus damaged,

(b) Cool the weld.

(iv) Proper welding current has been set on the current selector switch.

(v) Proper time has been set on the weld-timer.

Spot welding electrodes

Spot welding electrodes are made of materials which have

(1) Higher electrical and thermal resistivities, and



(2) Sufficient strength to withstand high pressure at elevated temperatures. Copper base

alloys such as copper beryllium and copper tungsten are commonly used materials for spot

welding electrodes. For achieving the desired current density, It is important to have

proper electrode shape for which three main types of spot welding electrodes are used

which are pointed, domed and flat electrodes.

Applications of Spot Welding

(i) It has applications in automobile and aircraft industries

(ii) The attachment of braces, brackets, pads or clips to formed sheet-metal parts such as

cases, covers or trays is another application of spot welding.

(iii) Spot welding of two 12.5 mm thick steel plates has been done satisfactorily as a

replacement for riveting.

(iv) Many assemblies of two or more sheet metal stampings that do not require gas tight or

liquid tight joints can be more economically joined by spot welding than by mechanical

methods.

(v) Containers and boxes frequently are spot welded.

Resistance Seam Welding

It is a continuous type of spot welding wherein spot welds overlap each other to the

desired extent. In this process coalescence at the faying surfaces is produced by the heat

obtained from the resistance to electric current (flow) through the work pieces held

together under pressure by circular electrodes. The resulting weld is a series of

overlapping resistance-spots welds made progressively along a joint by rotating the

circular electrodes. The principle of seam welding is shown in Fig. (a) and resistance seam

welding process set up is shown in Fig. (b). The seam welding is similar to spot welding,

except that circular rolling electrodes are used to produce a continuous air-tight seam of

overlapping welds. Overlapping continuous spot welds seams are produced by the rotating

electrodes and a regularly interrupted current.



Applications

1. It is used for making leak proof joints in fuel tanks of automobiles.

2. Except for copper and high copper alloys, most other metals can be seam welded.

3. It is also used for making flange welds for use in watertight tanks.

Resistance Projection Welding

Fig. shows the projection welding. This process is a resistance welding process in which

two or more than two spot welds are made simultaneously by making raised portions or

projections on predetermined locations on one of the workpiece. These projections act to

localize the heat of the welding circuit. The pieces to be welded are held in position under

pressure being maintained by electrodes. The projected contact spot for welding should be

approximately equal to the weld metal thickness. The welding of a nut on the automotive

chasis is an example of projection welding.

Resistance Upset Butt and Flash Butt Welding



This welding is also used for joining metal pieces end to end but it has largely replaced the

butt-welding method for weld articles small cross-sections. It can be used for thick sections

also. Initially the current is switched on and then one end the moveable part to be welded

is brought gently closer to the fixed end of the other part to localize heat at the ends and

thus raises the temperature of the ends quickly to the welding heat. On acquiring contact of

fixed end and moveable end with each other, the moveable end is then pressed against one

another by applying mechanical pressure. Thus the molten metal and slag to be squeezed

out in the form of sparks enabling the pure metal to form the joint and disallowing the heat

to spread back. The principle of upset butt welding and flash butt welding are depicted in

Fig. (a, b). In this resistance welding single phase A.C. machines are commonly employed.

The merits and demerit of flash welding over simple butt-welding are follows:

Merits

1. It is comparatively much faster than butt welding.

2. This method utilizes less current in comparison to butt welding as the small portion of

the metal is only being heated for getting a good weld

3. Created joint by this welding is much stronger than the butt welding joint. Also the

strength of the weld produced is high even more than that of the base metal. The end of the

metal pieces to be welded in this welding need not be squared as it is the basic requirement

in butt-welding.

4. A high degree of accuracy can be easily achieved in terms of length alignment of weld.

Demerits



1. The periodic maintenance of machine and replacement of insulation is needed as flashing

particles of molten metal are thrown out during welding which may enter into the slide

ways and insulation of the set up.

2. Welder has to take enough care against possible fire hazard due to flashing during

welding.

3. Additional stock has to be provided for compensating loss of metal during f1ashing and

upsetting. This increases to the cost of weld.

4. Cost of removal of flash weld metal by trimming, chipping, grinding, etc. will increase to

the welded product.

5. Surface of the jobs where they come in contact with the gripping surfaces should be clean

otherwise they will restrict the flow of electric current.

6. The available power, opening between the jaws of the gripping clamps and upsetting

pressure of the welding set limit the size and cross sectional area of the jobs to be welded.

Applications

All conducting forged metals can be easily be flash welded. A number of dissimilar metals

can also be welded by controlling the welding conditions carefully. Metals generally welded

metal by the process involves lead, tin, antimony, zinc, bismuth and their alloys, low carbon

steels, stainless steel, alloy steels, tool steels, copper alloys, aluminium alloys, magnesium

alloys nickel alloys, molybdenum alloys, and titanium alloys. This process is used in

automobile industry, welding of solid and tubular structural assemblies, etc. in air-craft

industry, welding of band saw blades, welding of tool steel drills, reamers and taps etc. to

mild steel or alloy steel shanks, welding of pipes and tubes.

Common Advantages of Resistance Welding

Some common advantages of resistance welding include:

(a) It is well suited for mass production.

(b) It is economical in operation, since nothing is consumed except electrical power.

(c) Skilled welders are not required.

(d) Welds are quickly made.

(e) It is possible to weld dissimilar metals.



Some disadvantages of resistance welding include:

(a) High initial cost of the resistance welding equipment

(b) Certain resistance welding processes are limited to lap joints.

(c) A lap joint has an inherent service between the two metal pieces, which causes stress

concentrations in applications where fatigue is present. This service may also cause trouble

when corrosion is present.

RESISTANCE WELDING- TYPES

In resistance welding the metal parts to be joined are heated by their resistance to the flow

of an electrical current. Usually this is the only source of heat, but a few of the welding

operations combine resistance heating with arc heating, and possibly with combustion of

metal in the arc. The process applies to practically all metals and most combinations of

pure metals and those alloys, which have only a limited plastic range, are welded by

heating the parts to fusion (melting). Some alloys, however, may welded without fusion;

instead, the parts are heated to a plastic state at which the applied pressure causes their

crystalline structures to grow together. The welding of dissimilar metals may be

accomplished by melting both metals frequently only the metal with the lower melting

point is melted, and an alloy bond is formed at the surface of the unmelted metal.

In resistance welding processes no fluxes are employed, the filler metal is rarely used and

the joints are usually of the lap type. The amount of heat generated in the workpiece

depend on the following factors:

(1) Magnitude of the current,

(2) Resistance of the current conducting path, and

Mathematically, H = IVt

= I(IR)t

= I2Rt

Where H = heat generated in joules

I = current in Amp.

R = resistance in ohms

t = time of current flow in seconds.

Types of Resistance welding



The major types of resistance welding are given as under:

(1) Spot Welding

(2) Seam Welding

(3) Projection Welding

(4) Resistance Butt Welding

(5) Flash Butt Welding

(6) Percussion Welding

(7) High Frequency Resistance Welding

(8) High Frequency Induction Welding


Spot Welding

In this process overlapping sheets are joined by local fusion at one or more spots, by the

concentration of current flowing between two electrodes. This is the most widely used

resistance welding process. A typical resistance spot welding machine is shown in Fig. It

essentially consists of two electrodes, out of which one is fixed. The other electrode is fixed

to a rocker arm (to provide mechanical advantage) for transmitting the mechanical force

from a pneumatic cylinder. This is the simplest type of arrangement. The other possibility

is that of a pneumatic or hydraulic cylinder being directly connected to the electrode

without any rocker arm. For welding large assemblies such as car bodies, portable spot

welding machines are used. Here the electrode holders and the pneumatic pressurizing

system are present in the form of a portable assembly which is taken to the place, where

the spot is to be made. The electric current, compressed air and the cooling water needed

for the electrodes is supplied through cables and hoses from the main welding machine to

the portable unit. In spot welding, a satisfactory weld is obtained when a proper current

density is maintained. The current density depends on the contact area between the

electrode and the work-piece. With the continuous use, if the tip becomes upset and- the

contact area increases, the current density will be lowered and consequently the weld is

obtained over a large area. This would not be able to melt the metal and hence there would

be no proper fusion. A resistance welding schedule is the sequence of events that normally

take place in each of the welds. The events are:



1. The squeeze time is the time required for the electrodes to align and clamp the two

work-pieces together under them and provide the necessary electrical contact.

2. The weld time is the time of the current flow through the work-pieces till they are heated

to the melting temperature.

3. The hold time is the time when the pressure is to be maintained on the molten metal

without the electric current. During this time, the pieces are expected to be forged welded.

4. The off time is time during which, the pressure on the electrode is taken off so that the

plates can be positioned for the next spot.

Before spot welding one must make sure that

(i) The job is clean, i.e., free from grease, dirt, paint, scale, oxide etc.

(ii) Electrode tip surface is clean, since it has to conduct the current into the work with as

little loss as possible. Very fine emery cloth may be used for routine cleaning.

(iii) Water is running through the electrodes in order to

(a) Avoid them from getting overheated and thus damaged,

(b) Cool the weld.

(iv) Proper welding current has been set on the current selector switch.

(v) Proper time has been set on the weld-timer.

Spot welding electrodes

Spot welding electrodes are made of materials which have

(1) Higher electrical and thermal resistivities, and

(2) Sufficient strength to withstand high pressure at elevated temperatures. Copper base

alloys such as copper beryllium and copper tungsten are commonly used materials for spot

welding electrodes. For achieving the desired current density, It is important to have



proper electrode shape for which three main types of spot welding electrodes are used

which are pointed, domed and flat electrodes.

Applications of Spot Welding

(i) It has applications in automobile and aircraft industries

(ii) The attachment of braces, brackets, pads or clips to formed sheet-metal parts such as

cases, covers or trays is another application of spot welding.

(iii) Spot welding of two 12.5 mm thick steel plates has been done satisfactorily as a

replacement for riveting.

(iv) Many assemblies of two or more sheet metal stampings that do not require gas tight or

liquid tight joints can be more economically joined by spot welding than by mechanical

methods.

(v) Containers and boxes frequently are spot welded.

Resistance Seam Welding

It is a continuous type of spot welding wherein spot welds overlap each other to the

desired extent. In this process coalescence at the faying surfaces is produced by the heat

obtained from the resistance to electric current (flow) through the work pieces held

together under pressure by circular electrodes. The resulting weld is a series of

overlapping resistance-spots welds made progressively along a joint by rotating the

circular electrodes. The principle of seam welding is shown in Fig. (a) and resistance seam

welding process set up is shown in Fig. (b). The seam welding is similar to spot welding,

except that circular rolling electrodes are used to produce a continuous air-tight seam of

overlapping welds. Overlapping continuous spot welds seams are produced by the rotating

electrodes and a regularly interrupted current.



Applications

1. It is used for making leak proof joints in fuel tanks of automobiles.

2. Except for copper and high copper alloys, most other metals can be seam welded.

3. It is also used for making flange welds for use in watertight tanks.

Resistance Projection Welding

Fig. shows the projection welding. This process is a resistance welding process in which

two or more than two spot welds are made simultaneously by making raised portions or

projections on predetermined locations on one of the workpiece. These projections act to

localize the heat of the welding circuit. The pieces to be welded are held in position under

pressure being maintained by electrodes. The projected contact spot for welding should be

approximately equal to the weld metal thickness. The welding of a nut on the automotive

chasis is an example of projection welding.

Resistance Upset Butt and Flash Butt Welding



This welding is also used for joining metal pieces end to end but it has largely replaced the

butt-welding method for weld articles small cross-sections. It can be used for thick sections

also. Initially the current is switched on and then one end the moveable part to be welded

is brought gently closer to the fixed end of the other part to localize heat at the ends and

thus raises the temperature of the ends quickly to the welding heat. On acquiring contact of

fixed end and moveable end with each other, the moveable end is then pressed against one

another by applying mechanical pressure. Thus the molten metal and slag to be squeezed

out in the form of sparks enabling the pure metal to form the joint and disallowing the heat

to spread back. The principle of upset butt welding and flash butt welding are depicted in

Fig. (a, b). In this resistance welding single phase A.C. machines are commonly employed.

The merits and demerit of flash welding over simple butt-welding are follows:

Merits

1. It is comparatively much faster than butt welding.

2. This method utilizes less current in comparison to butt welding as the small portion of

the metal is only being heated for getting a good weld

3. Created joint by this welding is much stronger than the butt welding joint. Also the

strength of the weld produced is high even more than that of the base metal. The end of the

metal pieces to be welded in this welding need not be squared as it is the basic requirement

in butt-welding.

4. A high degree of accuracy can be easily achieved in terms of length alignment of weld.

Demerits



1. The periodic maintenance of machine and replacement of insulation is needed as flashing

particles of molten metal are thrown out during welding which may enter into the slide

ways and insulation of the set up.

2. Welder has to take enough care against possible fire hazard due to flashing during

welding.

3. Additional stock has to be provided for compensating loss of metal during f1ashing and

upsetting. This increases to the cost of weld.

4. Cost of removal of flash weld metal by trimming, chipping, grinding, etc. will increase to

the welded product.

5. Surface of the jobs where they come in contact with the gripping surfaces should be clean

otherwise they will restrict the flow of electric current.

6. The available power, opening between the jaws of the gripping clamps and upsetting

pressure of the welding set limit the size and cross sectional area of the jobs to be welded.

Applications

All conducting forged metals can be easily be flash welded. A number of dissimilar metals

can also be welded by controlling the welding conditions carefully. Metals generally welded

metal by the process involves lead, tin, antimony, zinc, bismuth and their alloys, low carbon

steels, stainless steel, alloy steels, tool steels, copper alloys, aluminium alloys, magnesium

alloys nickel alloys, molybdenum alloys, and titanium alloys. This process is used in

automobile industry, welding of solid and tubular structural assemblies, etc. in air-craft

industry, welding of band saw blades, welding of tool steel drills, reamers and taps etc. to

mild steel or alloy steel shanks, welding of pipes and tubes.

Common Advantages of Resistance Welding

Some common advantages of resistance welding include:

(a) It is well suited for mass production.

(b) It is economical in operation, since nothing is consumed except electrical power.

(c) Skilled welders are not required.

(d) Welds are quickly made.

(e) It is possible to weld dissimilar metals.



Some disadvantages of resistance welding include:

(a) High initial cost of the resistance welding equipment

(b) Certain resistance welding processes are limited to lap joints.

(c) A lap joint has an inherent service between the two metal pieces, which causes stress

concentrations in applications where fatigue is present. This service may also cause trouble

when corrosion is present.

Plasma-arc welding (PAW)

Plasma-arc Welding. In plasma-arc welding (PAW), developed in the 1960s, a concentrated

plasma arc is produced and directed towards the weld area. The arc is stable and reaches

temperatures as high as 33,000°C. A plasma is an ionized hot gas composed of nearly equal

numbers of electrons and ions. The plasma is initiated between the tungsten electrode and

the orifice by a low-current pilot arc. What makes plasma-arc welding unlike other

processes is that the plasma arc is concentrated because it is forced through a relatively

small orifice. Operating currents usually are below 100 A, but they can be higher for special

applications. When a filler metal is used, it is fed into the arc, as is done in GTAW. Arc and

weld-zone shielding is supplied

by means of an outer-shielding ring and the use of gases such as argon, helium, or mixtures.

There are two methods of plasma-arc welding:

In the transferred-arc method (Fig.a), the workpiece being welded is part of the

electrical circuit. The arc transfers from the electrode to the workpiece hence the

term transferred.

In the nontransferred method (Fig.b), the arc occurs between the electrode and the

nozzle, and the heat is carried to the workpiece by the plasma gas. This thermal-

transfer mechanism is similar to that for an oxyfuel flame.



Compared with other arc-welding processes, plasma-arc welding has better arc

stability, less thermal distortion, and higher energy concentration, thus permitting

deeper and narrower welds. In addition, higher welding speeds, from 120 to 1000

mm/min, can be achieved. A variety of metals can be welded with part thicknesses

generally less than 6 mm.

The high heat concentration can penetrate completely through the joint (known as

the keyhole technique), with thicknesses as much as 20 mm for some titanium and

aluminum alloys. In the keyhole technique, the force of the plasma arc displaces the

molten metal and produces a hole at the leading edge of the weld pool.

Plasma-arc welding (rather than the GTAW process) often is used for butt and lap

joints because of its higher energy concentration, better arc stability, and higher

welding speeds. Proper training and skill are essential for operators who use this

equipment.

Safety considerations include protection against glare, spatter, and noise from the

plasma arc.

Gas Cutting



In addition to being cut by mechanical means, material can be cut into various contours

with the use of a heat source that melts and removes a narrow zone in the workpiece. The

sources of heat can be torches, electric arcs, or lasers. Oxyfuel-gas Cutting. Ox;/fuel-gas

cutting (OFC) is similar to oxyfuel welding, but the heat source is now used to remove a

narrow zone from a metal plate or sheet (Fig.a). This process is suitable particularly for

steels. The basic reactions with steel are

Fe + O ----> FeO + Heat,

3Fe +2O2 -> Fe3O4 + Heat,

and

4Fe + 3O2 -> 2 Fe2O3 + Heat.

The greatest heat is generated by the second reaction, and it can produce a temperature

rise to about 870°C. However, this temperature is not sufficiently high to cut steels;

therefore, the workpiece is preheated with fuel gas, and oxygen is introduced later (see the

nozzle cross section in Fig.a). The higher the carbon content of the steel, the higher is the

preheating temperature required. Cutting takes place mainly by the oxidation (burning) of

the steel; some melting also takes place. Cast irons and steel castings also can be cut by this

method. The process generates a kerf similar to that produced by sawing with a saw blade

or by wire electrical discharge machining.

The maximum thickness that can be cut by OFC depends mainly on the gases used. With

oxyacetylene gas, the maximum thickness is about 300 mm; with oxyhydrogen, it is about



600 mm. Kerf widths range from about 1.5 to 10 mm, with reasonably good control of

tolerances. The flame leaves drag lines on the cut surface (Fig.b), resulting in a rougher

surface than that produced by processes such as sawing, blanking, or other operations that

use mechanical cutting tools. Distortion caused by uneven temperature distribution can be

a problem in OFC. Although long used for salvage and repair work, OFC can be used in

manufacturing as well. Torches may be guided along specified paths either manually,

mechanically, or automatically by machines using programmable controllers and robots.

Underwater cutting is done with specially designed torches that produce a blanket of

compressed air between the flame and the surrounding water.

Arc Cutting. Arc-cutting processes are based on the same principles as arc-welding

processes. A variety of materials can be cut at high speeds by arc cutting. As in welding,

these processes also leave a heat-affected zone that needs to be taken into account,

particularly in critical applications.

In air carbon-arc cutting (CAC-A), a carbon electrode is used and the molten metal is blown

away by a high-velocity air jet; thus, the metal being cut doesn’t have to oxidize. This

process is used especially for gouging and scarfing (removal of metal from a surface).

However, the process is noisy, and the molten metal can be blown substantial distances

and cause safety hazards.

Plasma-arc cutting (PAC) produces the highest temperatures. It is used for the rapid

cutting of nonferrous and stainless-steel plates. The cutting productivity of this process is

higher than that of oxyfuel-gas methods. PAC produces a good surface finish and narrow

kerfs, and is the most popular cutting process utilizing programmable controllers

employed in manufacturing today.

Electron beams and lasers also are used for very accurately cutting a wide variety of

metals. The surface finish is better than that of other thermal cutting processes, and the

kerf is narrower.

Gas Tungusten Arc Welding (GTAW) or Tungusten Inert Gas Welding (TIG)

In this process a non-consumable tungsten electrode is used with an envelope of inert

shielding gas around it. The shielding gas protects the tungsten electrode and the molten

metal weld pool from the atmospheric contamination. The shielding gases generally used



are argon, helium or their mixtures. Typical tungsten inert gas welding setup is shown in

Fig.

Electrode materials

The electrode material may be tungsten, or tungsten alloy (thoriated tungsten or

zirconiated tungsten). Alloy-tungsten electrodes possess higher current carrying capacity;

produce a steadier arc as compared to pure tungsten electrodes and high resistance to

contamination.

Electric power source

Both AC and DC power source can be used for TIG welding. DC is preferred for welding of

copper, copper alloys, nickel and stainless steel whereas DC reverse polarity (DCRP) or AC

is used for welding aluminium, magnesium or their alloys. DCRP removes oxide film on

magnesium and aluminium.

Inert gases

The following inert gases are generally used in TIG welding:

1. Argon

2. Helium

3. Argon-helium mixtures

4. Argon-hydrogen mixtures



Tig Nozzle

The nozzle or shield size (the diameter of the opening of the shroud around the electrode)

to be chosen depends on the shape of the groove to be welded as well as the required gas

flow rate. The gas flow rate depends on the position of the weld as well as its size. Too high

a gas consumption would give rise to turbulence of the weld metal pool and consequently

porous welds. Because of the use of shielding gases, no fluxes are required to be used in

inert gas shielded arc welding. However for thicker sections, it may be desirable to protect

the root side of the joint by providing a flux. The process is generally used for welding

aluminium, magnesium and stainless steel.

Gas Metal ARC Welding (GMAW) or Metal Inert Gas Welding (MIG)

Metal inert gas arc welding (MIG) or more appropriately called as gas metal arc welding

(GMAW) utilizes a consumable electrode and hence, the term metal appears in the title.

There are other gas shielded arc welding processes utilizing the consumable electrodes,

such as flux cored arc welding (FCAW) all of which can be termed under MIG. Though gas

tungsten arc welding (GTAW) can be used to weld all types of metals, it is more suitable for

thin sheets. When thicker sheets are to be welded, the filler metal requirement makes

GTAW difficult to use. In this situation, the GMAW comes handy. The typical setup for

GMAW or MIG welding process is shown in Fig.



The consumable electrode is in the form of a wire reel which is fed at a constant rate,

through the feed rollers. The welding torch is connected to the gas supply cylinder which

provides the necessary inert gas. The electrode and the work-piece are connected to the

welding power supply. The power supplies are always of the constant voltage type only.

The current from the welding machine is changed by the rate of feeding of the electrode

wire. Normally DC arc welding machines are used for GMAW with electrode positive

(DCRP). The DCRP increases the metal deposition rate and also provides for a stable arc

and smooth electrode metal transfer. With DCSP, the arc becomes highly unstable and also

results in a large spatter. But special electrodes having calcium and titanium oxide

mixtures as coatings are found to be good for welding steel with DCSP. In the GMAW

process, the filler metal is transferred from the electrode to the joint. Depending on the

current and voltage used for a given electrode, the metal transfer is done in different ways.


production technology notes

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