UNIT 3

Electric Welding

INTRODUCTION

Welding is the process of joining two pieces of metal or non-metal together by heating them to
their melting point. Filler metal may or may not be used to join two pieces. The physical and
mechanical properties of a material to be welded such as melting temperature, density, thermal
conductivity, and tensile strength take an important role in welding. Depending upon how the
heat applied is created; we get different types of welding such as thermal welding, gas welding,
and electric welding. Here in this chapter, we will discuss only about the electric welding and
some introduction to other modern welding techniques. Welding is nowadays extensively used in
automobile industry, pipe-line fabrication in thermal power plants, machine repair work,
machine frames, etc.

ADVANTAGES AND DISADVANTAGES OF WELDING

Some of the advantages of welding are:

o Welding is the most economical method to permanently join two metal parts.
o It provides design flexibility.
o Welding equipment is not so costly.
o It joins all the commercial metals.
o Both similar and dissimilar metals can be joined by welding.
o Portable welding equipment are available.

Some of the disadvantages of welding are:

o Welding gives out harmful radiations and fumes.

o Welding needs internal inspection.
o If welding is not done carefully, it may result in the distortion of workpiece.
o Skilled welding is necessary to produce good welding.

ELECTRIC WELDING

It is defined as the process of joining two metal pieces, in which the electrical energy is used to
generate heat at the point of welding in order to melt the joint.

The classification of electric welding process is shown in fig.

Fig. Classification of electric welding

The selection of proper welding process depends on the following factors.

o The type of metal to be joined.

o The techniques of welding adopted.
o The cost of equipment used.
o The nature of products to be fabricated.

RESISTANCE WELDING

Resistance welding is the process of joining two metals together by the heat produced due to the
resistance offered to the flow of electric current at the junctions of two metals. The heat
produced by the resistance to the flow of current is given by:

H = I2Rt,

where I is the current through the electrodes, R is the contact resistance of the interface, and tis
the time for which current flows.

Here, the total resistance offered to the flow of current is made up of:
 1. The resistance of current path in the work.
2. The resistance between the contact surfaces of the parts being welded.
3. The resistance between electrodes and the surface of parts being welded.

In this process of welding, the heat developed at the contact area between the pieces to be welded
reduces the metal to plastic state or liquid state, then the pieces are pressed under high
mechanical pressure to complete the weld. The electrical voltage input to the welding varies in
between 4 and 12 V depending upon area, thickness, composition, etc. and usually power ranges
from about 60 to 180 W for each sq. mm of area.

Any desired combination of voltage and current can be obtained by means of a suitable
transformer in AC; hence, AC is found to be most suitable for the resistance welding. The
magnitude of current is controlled by changing the primary voltage of the welding transformer,
which can be done by using an auto-transformer or a tap-changing transformer. Automatic
arrangements are provided to switch off the supply after a pre-determined time from applying the
pressure, why because the duration of the current flow through the work is very important in the
resistance welding.

The electrical circuit diagram for the resistance welding is shown in Fig. 5.2. This method of
welding consists of a tap-changing transformer, a clamping device for holding the metal pieces,
and some sort of mechanical arrangement for forcing the pieces to form a complete weld.

Fig. Electric circuit for resistance welding

Advantages
 o Welding process is rapid and simple.
o Localized heating is possible, if required.
o No need of using filler metal.
o Both similar and dissimilar metals can be welded.
o Comparatively lesser skill is required.
o Maintenance cost is less.
o It can be employed for mass production.

However, the resistance welding has got some drawbacks and they are:

o Initial cost is very high.

o High maintenance cost.
o The workpiece with heavier thickness cannot be welded, since it requires high input current.

Applications

o It is used by many industries manufacturing products made up of thinner gauge metals.

o It is used for the manufacturing of tubes and smaller structural sections.

Types of resistance welding

Depending upon the method of weld obtained and the type of electrodes used, the resistance
welding is classified as:

1. Spot welding.
2. Seam welding.
3. Projection welding.
4. Butt welding.

(i) Spot welding

Spot welding means the joining of two metal sheets and fusing them together between copper
electrode tips at suitably spaced intervals by means of heavy electric current passed through the
electrodes as shown in Fig. 5.3.
Fig. 5.3 Spot welding

This type of joint formed by the spot welding provides mechanical strength and not air or
water tight, for such welding it is necessary to localize the welding current and to apply
sufficient pressure on the sheet to be welded. The electrodes are made up of copper or copper
alloy and are water cooled. The welding current varies widely depending upon the thickness and
composition of the plates. It varies from 1,000 to 10,000 A, and voltage between the electrodes is
usually less than 2 V. The period of the flow of current varies widely depending upon the
thickness of sheets to be joined. A step-down transformer is used to reduce a high-voltage and
low-current supply to low-voltage and high-current supply required. Since the heat developed
being proportional to the product of welding time and square of the current. Good weld can be
obtained by low currents for longer duration and high currents for shorter duration; longer
welding time usually produces stronger weld but it involves high energy expenditure, electrode
maintenance, and lot of distortion of workpiece.

When voltage applied across the electrode, the flow of current will generate heat at the three
junctions, i.e., heat developed, between the two electrode tips and workpiece, between the two
workpieces to be joined as shown in Fig. 3.3. The generation of heat at junctions 1 and 3 will
effect electrode sticking and melt through holes, the prevention of electrode striking is achieved
by:

1. Using water-cooled electrodes shown in Fig. 5.4. By avoiding the heating of junctions 1 and 3 electrodes
in which cold water circulated continuously as shown in Fig. 5.3.
2. The material used for electrode should have high electrical and thermal conductivity. Spot welding is
widely used for automatic welding process, for joining automobile parts, joining and fabricating sheet
metal structure, etc.
 Fig. Water cooled electrode

(ii) Seam welding

Seam welding is nothing but the series of continuous spot welding. If number spots obtained by
spot welding are placed very closely that they can overlap, it gives rise to seam welding.

In this welding, continuous spot welds can be formed by using wheel type or roller electrodes
instead of tipped electrodes as shown in Fig. 5.5.

Fig. 5.5 Seam welding

Seam welding is obtained by keeping the job under electrodes. When these wheel type
electrodes travel over the metal pieces which are under pressure, the current passing between
them heats the two metal pieces to the plastic state and results into continuous spot welds.
 In this welding, the contact area of electrodes should be small, which will localize the current
pressure to the welding point. After forming weld at one point, the weld so obtained can be
cooled by splashing water over the job by using cooling jets.

In general, it is not satisfactory to make a continuous weld, for which the flow of continuous
current build up high heat that causes burning and wrapping of the metal piece. To avoid this
difficulty, an interrupter is provided on the circuit which turns on supply for a period sufficient to
heat the welding point. The series of weld spots depends upon the number of welding current
pulses.

The two forms of welding currents are shown in Fig. 5.6(a) and (b).

Fig. 5.6 Welding current

Welding cannot be made satisfactorily by using uninterrupted or un-modulated current, which

builds up high heat as the welding progress; this will over heat the workpiece and cause
distortion.

Seam welding is very important, as it provides leak proof joints. It is usually employed in
welding of pressure tanks, transformers, condensers, evaporators, air craft tanks, refrigerators,
varnish containers, etc.

(iii) Projection welding

It is a modified form of the spot welding. In the projection welding, both current and pressure are
localized to the welding points as in the spot welding. But the only difference in the projection
welding is the high mechanical pressure applied on the metal pieces to be welded, after the
formation of weld. The electrodes used for such welding are flat metal plates known as platens.
The two pieces of base metal to be weld are held together in between the two platens, one is
movable and the other is fixed, as shown in Fig. 5.7.

Fig. 5.7 Projection welding

One of the two pieces of metal is run through a machine that makes the bumps or projections
of required shape and size in the metal. As current flows through the two metal parts to be
welded, which heat up and melt. These weld points soon reach the plastic state, and the
projection touches the metal then force applied by the two flat electrodes forms the complete
weld.

The projection welding needs no protective atmosphere as in the spot welding to produce
successful results. This welding process reduces the amount of current and pressure in order to
join two metal surfaces, so that there is less chance of distortion of the surrounding areas of the
weld zone. Due to this reason, it has been incorporated into many manufacturing process.

The projection welding has the following advantages over the spot welding.

o Simplicity in welding process.

o It is easy to weld some of the parts where the spot welding is not possible.
o It is possible to join several welding points.
o Welds are located automatically by the position of projection.
o As the electrodes used in the projection welding are flat type, the contact area over the projection is
sufficient.

This type of welding is usually employed on punched, formed, or stamped parts where the
projection automatically exists. The projection welding is particularly employed for mass
production work, i.e., welding of refrigerators, condensers, crossed wire welding, refrigerator
racks, grills, etc.
 (iv) Butt welding

Butt welding is similar to the spot welding; however, the only difference is, in butt welding,
instead of electrodes the metal parts that are to be joined or butted together are connected to the
supply.

The three basic types of the butt welding process are:

1. Upset butt welding.

2. Flash butt welding.
3. Percussion butt welding.

(a) Upset butt welding

In upset welding, the two metal parts to be welded are joined end to end and are connected
across the secondary of a welding transformer as shown in Fig. 5.8.

Fig. 5.8 Upset butt welding

Due to the contact resistance of the metals to be welded, heating effect is generated in this
welding. When current is made to flow through the two electrodes, heat will develop due to the
contact resistance of the two pieces and then melts. By applying high mechanical pressure either
manually or by toggle mechanism, the two metal pieces are pressed. When jaw-type electrodes
are used that introduce the high currents without treating any hot spot on the job.
 This type of welding is usually employed for welding of rods, pipes, and wires and for joining
metal parts end to end.

(b) Flash butt welding

Flash butt welding is a combination of resistance, arc, and pressure welding. This method of
welding is mainly used in the production welding. A simple flash butt welding arrangement is
shown in Fig. 5.9.

Fig. 5.9 Flash butt welding

In this method of welding, the two pieces to be welded are brought very nearer to each other
under light mechanical pressure. These two pieces are placed in a conducting movable clamps.
When high current is passed through the two metal pieces and they are separated by some
distance, then arc established between them. This arc or flashing is allowed till the ends of the
workpieces reach melting temperature, the supply will be switched off and the pieces are rapidly
brought together under light pressure. As the pieces are moved together, the fused metal and slag
come out of the joint making a good solid joint.

Following are the advantages of the flash butt welding over the upset welding.

o Less requirement of power.

o When the surfaces being joined, it requires only less attention.
o Weld obtained is so clean and pure; due to the foreign metals appearing on the surfaces will burn due to
flash or arc.
 (c) Percussion welding

It is a form of the flash butt welding, where high current of short duration is employed using
stored energy principle. This is a self-timing spot welding method.

Percussion welding arrangement consists of one fixed holder and the other one is movable.
The pieces to be welded are held apart, with the help of two holders, when the movable clamp is
released, it moves rapidly carrying the piece to be welded. There is a sudden discharge of
electrical energy, which establishes an arc between the two surfaces and heating them to their
melting temperature, when the two pieces are separated by a distance of 1.5 mm apart. As the
pieces come in contact with each other under heavy pressure, the arc is extinguished due to the
percussion blow of the two parts and the force between them affects the weld. The percussion
welding can be obtained in two methods; one is capacitor energy storage system and the other is
magnetic energy storage system. The capacitor discharge circuit for percussion welding is shown
in Fig. 5.10.

Fig. 5.10 Capacitor discharge circuit for percussion welding

connected to the primary of welding transformer through the switch and will discharge. This
discharge will produce high transient current in the secondary to join the two metal pieces.

Percussion welding is difficult to obtain uniform flashing of the metal part areas of the cross-
section grater than 3 sq. cm. Advantage of this welding is so fast, extremely shallow of heating is
obtained with a span of about 0.1 sec. It can be used for welding a large number of dissimilar
metals.

Applications

o It is useful for welding satellite tips to tools, sliver contact tips to copper, cast iron to steel, etc.
o Commonly used for electrical contacts.
o The metals such as copper alloys, aluminum alloys, and nickel alloys are percussion welded.

CHOICE OF WELDING TIME

The successful welding operation mainly depends upon three factors and they are:

1. Welding time.
2. Welding current.
3. Welding pressure.

Figure 5.11 shows how the energy input to the welding process, welding strength, and welding
current vary with welding time.

Fig. 5.11 Performance characteristics of electric welding

The heat developed during welding process is given by H = I2Rt. Here both welding current
and welding time are critical variables.
 Greater the welding current, the shorter the welding time required is; usually longer welding
time produces stronger weld but there is lot of distortion of workpiece and high energy
expenditure. From Fig. 5.11, it is to be noted that, from 0 to t1 sec, there is appreciable increase
in welding strength, but after t2 sec, the increase in the welding time does not appreciably result
in t2
with the thickness of the material. The optimum times of material (sheet steel) with different
thickness are given as:

Dimensions of material Optimum time

2 × 24 SWG 8 cycles

2 × 14 SWG 20 cycles

2 sec

Therefore, from the above discussion, it is observed that shorter welding times with strength and
economy are always preferable.

Electromagnetic storage welding circuit is shown in Fig. 5.12. In this type of welding, the
energy stored in the magnetic circuit is used in the welding operation.

Fig. 5.12 Magnetic energy storage welding circuit

In this system, rectifier is fed from AC supply, which is converted to DC, the DC voltage of
rectifier is controlled in such a way that, voltage induced in the primary without causing large
current in the secondary of transformer on opening the contactor switch, DC on longer flows,
there is rapid collapse of magnetic field, which induces very high current in the secondary of a
transformer. Induced currents in the secondary of the transformer flow through the electrodes
that develop heat at the surface of the metal and so forming the complete weld.