Singh R. Introduction to Basic Manufacturing Processes and Workshop Technology

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328 Introduction to Basic Manufacturing Processes and Workshop Technology

smoke, fumes and gases, excessive shock or vibrations, steam and corrosive

atmosphere, etc.

32. One should not work on the wiring of an arc welding machine unless qualified to

do so.

33. Welding equipment used in the open and shall be protected from weather conditions.

If it has been wetted it shall be thoroughly dried before being used.

34. Proper terminals should be used on the arc welding machines for the power line

voltage connection.

35. Neither terminal of the welding generator shall be bonded to the frame of the

welding machine.

36. Periodically clear out the accumulated dust from the welding machine with suction

cleaner as this will not blow dust into other parts of the machine.

37. Over greasing may foul the commutators of DC generator.

38. Check and, if necessary, clean commutators of DC generator periodically, using fine

sand paper.

39. Excessive sparking may result in a worn commutator of DC generator which may

be cured by skimming in a lathe.

40. Brushes should move freely and have adequate spring tension. This can be tested

by lifting and releasing them. Brushes should snap be firmly against the commutator

of DC generator.

41. Greasing points need attention periodically.

42. Switch contacts should be cleaned periodically.

17.7 RESISTANCE WELDING

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

= I

Welding 329

Where H = heat generated in joules

I = current in Amp.

R = resistance in ohms

t = time of current flow in seconds.

17.7.1 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

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

17.7.1.1 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. 17.23. 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.

330 Introduction to Basic Manufacturing Processes and Workshop Technology

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.

Movable arm

Fixed arm

Pressure

Frame

Job

ulator

Throu

h contact

to AC current suppl

Transformer

Water-cooled

electrode

Fig. 17.23 Resistance spot welding machine setup

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.

Welding 331

17.7.1.2 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. 17.24(a) and resistance seam welding process set up is shown in Fig. 17.24(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.

Force

Wheel

electrode

A.C.

power

Contactor

Force

Motor driven

Wheel electrode

Weld

Workpiece

Weldin

transformer

and controls

Foot switch

Fig. 17.24 (a) Principle of seam welding process Fig. 17.24 (b) Resistance seam welding process setup

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.

17.7.1.3 Resistance Projection Welding

Fig.17.25 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 auto-

motive chasis is an example of projection

welding.

Pressure

Projections

Fixed

electrode

Weld

spots

Moveable

electrode

Fig. 17.25 Resistance projection welding

332 Introduction to Basic Manufacturing Processes and Workshop Technology

17.7.1.4 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.

17.26 (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:

Workpiece

Jaws

Workpiece

Joint

Clamps

Workpieces

Flash

Clamps

Fig. 17.26 (a) Resistance upset butt welding Fig. 17.26 (b) Resistance flash butt welding

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.

Welding 333

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.

17.7.2 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.

(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.

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

cause trouble when corrosion is present

17.8 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,

334 Introduction to Basic Manufacturing Processes and Workshop Technology

17.8.1 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.

17.3. 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.

17.8.2 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. 17.27. 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.

Rotatin

chuck

Slidin

chuck

Thrust

linder

Brake

Motor

Belt

Fig. 17.27 Friction welding process

Welding 335

17.8.3 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. 17.28. 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.

Detonator

Explosive

Flyer

plate

Tar

plate

Anvil

Buffer

plate

Detonator

Explosive

Flyer

plate

Tar

plate

Anvil

Buffer

plate

(a) Parallel Stand Off

(b) An

ular Stand Off

Fig. 17.28 Explosive welding process

17.9 THERMIT WELDING

It may be of forge or fusion kind of welding. Fusion welding requires no pressure. Thermit

welding process is depicted in Fig. 17.29. It is a process which uses a mixture of iron oxide

and granular aluminium. This mixture in superheat liquid state is poured around the parts

to be joined. The joint is equipped with the refractory mold structure all around. In case of

thermit pressure welding, only the heat of thermit reaction is utilized to bring the surface

of metal to be welded in plastic state and pressure is the applied to complete the weld. The

temperature produced in the thermit reaction is of the order of 3000°C. Thermit welding is

used for welding pipes, cables, conductors, shafts, and broken machinery frames, rails and

repair of large gear tooth.

17.10 RADIANT ENERGY WELDING PROCESSES

In radiant energy welding processes, heat is produced at the point of welding when a stream

of electrons or a beam of electro-magnetic radiations strikes on the workpiece. This welding

can be carried out in vacuum or at low pressures. Electron beam welding (EBW) and laser

welding are two main types of radiant energy welding processes.

336 Introduction to Basic Manufacturing Processes and Workshop Technology

8Al + 3Fe O = 4Al O + 9Fe

34 23

1. Crucible,

2. Sla

basin,

3. Runner,

4. Wax pattern,

5. Sand plu

6. Preheatin

7. Workpiece,

8. Riser.

Fig. 17.29 Thermit welding process

17.10.1 Electron Beam Weldinig (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. 17.30. 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 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

Welding 337

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.

Tun

usten

filament

Cathode

Anode

Focus coil

volta

Electron beam

Work piece

Fig. 17.30 Electron beam welding set up

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.

17.11 EFFECTS OF WELDING HEAT

During the process of welding, the metal is heated over a range of temperature up to fusion

and then allowed to cool. Heating and cooling causes metallurgical and mechanical effects on

the metal pieces being welded. The region of the base metal which is affected by metallurgical

change due to the welding heat is called the heat affected zone. In this zone, the metal has

been heated and cooled through a range of temperature great enough to cause changes in the

structure of the metal. The amount of change may be controlled by using the suitable welding

processes. Sometimes, the original properties may be restored by heat treatment after welding.

Although the mechanical strength of weld metals is at least equal to that of the base metal

but the metallurgical changes can reduce greatly the associated properties of the joint

particularly for shock and fatigue.

Mechanical effects of welding heat are the distortion and the residual stress resulting

from welding. The magnitude of these effects depends on the factors of design and welding

procedure. But the effects will always be present to a certain extent in any structure fabricated

or repaired by non-pressure welding. Because of expansion and contraction of the heated

portion the distortion and residual stresses set up. The base metal heated locally will expand

and up set owing to the restraint imposed by tile surrounding cold metal. This upset portion

of the base metal will contract on cooling beyond its original dimensions and thereby setup

internal stresses causing distortion. If the pieces being welded are not tree to move, high

residual stresses will be formed which may cause cracking during welding.