Welding: Classification of welding process, Fusion welding, Gas flame welding, Electric arc welding, Pressure welding, Thermo chemical welding, Oxy-acetylene welding, Oxy-hydrogen welding, Carbon arc welding,


 Welding is a fabrication process that
joins materials, usually metals or thermoplastics, by causing coalescence. This
is often done by melting the work
pieces and adding a filler material to form  a 
pool  of  molten  material 
(the weld  pool)  that 
cools  to  become   a  
strong   joint, with pressure
sometimes used in conjunction with
heat, or by itself, to produce the weld.
Welding is used in every industry large or small, as a principle means of fabrication and repairing metal product.
The process is efficient, economical
and dependable. This is the only
process which has been tried in the
space and finds applications for fabricating products in air, underwater and in space

Classification of welding process

There are many ways in which the welding processes can be
classified. A simple and logical way is to satisfy the welding processes
according to the sources of energy used for achieving coalescence. The welding
processes can thus be broadly classified as;

(a)  Fusion welding

Pressure welding

Thermo chemical welding

(a)     Fusion welding

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1.      Gas flame welding

Oxy-acetylene welding

Oxy-hydrogen welding

2.      Electric arc welding

Carbon arc welding

Shielded metal arc welding

Submerged arc welding

Tungsten arc welding

Metal inert gas (MIG) welding

Plasma arc welding

Atomic hydrogen welding

3.      Radiant energy welding

Electron beam welding

Laser beam welding

4.      Electric resistance welding

Spot welding

Seam welding

Projection welding

Resistance butt welding

Flash welding

Percussion welding

Electroslag welding

The following processes
fall under solid state
(cold) welding

Pressure welding

Friction welding

Ultrasonic welding

Explosive welding

Forge and diffusive welding

Thermo chemical welding

Thermit welding

Atomic hydrogen welding

Out of the above welding processes carbon arc and atomic
hydrogen welding are not commonly used in industries these days.

Fusion welding

Fusion welding is a group of processes that bond metals
together by heating a portion of each piece above the melting point and causing
them to flow and fuse together. Usually filler materials are used to insure the
joint is filled. All fusion welding processes have four requirements for
obtaining satisfactory welds.

A source of energy to create
union (e.g. gas flame, electric arc, electric resistance etc.)

Removing surface contaminants (e.g. organic and oxide films etc.)
Protecting metal from atmospheric contamination ( shielding
gas atmosphere like argon, helium, carbon dioxide, smoke of burning flux
Control of weld metallurgy (e.g. preheat, post welding heat treatment)

Gas Welding

It is fusion welding process that joins the metal by using
heat of combustion of a strong gas flame. The intense heat raises the
temperature of ends, melts and fuses together the edges of the plate to be
welded. Filler metal may be added the plate to flowing molten to fill the
cavity between the plates. Different oxy-acetylene combinations are used to
produce different types heating flames i.e. O2 and C2H2,
O2 and H2 with coal gas etc. Oxy –acetylene welding is
the most commonly used gas welding process. This process is explained in detail
in the following paragraphs.

Oxyacetylene welding(O2, C2H2)

 is a gas welding process. In this process
coalescence (or bond) is  produced by heating with the gas flame
obtained from the combustion of acetylene with oxygen. A welding torch is used to mix the gases in the
proper proportions and to direct the
flame against the parts to be welded.
The molten edges of the parts then
literally fuse together and after cooling form a strong joint. Usually, it is necessary
to add extra material to the joint. The correct material in the rod form of a rod is dipped in a flux powder and is fused with the puddle of and fuses with
the puddle of molten metal obtained from the parent metal parts. Acetylene is widely used as the combustible gas
because of its high flame temperature when mixed with oxygen. The temperature,
which ranges from approximately21000– 35000, is far above the melting point of all
commercial metals thus it provides a
means for the rapid, localized melting essential in welding. The oxyacetylene flame is also used in cutting
ferrous metals. The oxyacetylene welding and cutting methods are widely used by
all  types  of 
maintenance activities because the flame is easy to regulate, the gases
may be produced inexpensively, and the equipment can be  transported easily and safely.

of gas and heat liberated first stage-

C2H2 + O2 → 2 CO + H2  + 448 kj/mol

Second stage        2
CO + H2 + 3O → 2 CO2 + H2O + 812 kj/mol Total heat liberated by
combustion= 448+812= 1260 kj/mol of acetylene 

Gas welding equipments

Oxygen gas cylinder

Acetylene gas cylinder

Welding torch

Pressure regulator

Hose pipe

Stop valve

Pressure gauge


Types of flame:-

To achieve successful and efficient
welding, proper mixing of gas in appropriate proportion is necessary. According
to the volume of O2 and C2H2 flame can be
classified as

1.      Neutral flame:-

Neutral Flame is produced when equal volume of O2 and
C2H2 are mixed. It
has well defined inner cone, which has light blue color. Neutral Flame doesn’t
create any change in the molten metal and doesn’t oxidize or carburize the metal. This flame is used for welding
of mild steel, cast iron, aluminum
and stainless steel. Temperature of neutral
flame is 32320 C.

2.      Oxidizing

Oxidizing flame is obtained by increasing the supply of oxygen in mixture. It has small white cone which is shorter and
more pointed than that of neutral flame. It creates high
temperature of 3482̊ C. It
is normally not used except in the case of brass.

3.      Reducing


  It is produced
when supply of C2H2 is
higher than supply of oxygen
and can create an approximate temperature of 31500 C its outer envelope is longer
than that of neutral
flame. It is used for the
welding of lead.

In these
flames the temperature is maximum at the tip of the inner cone. Feather or brush exists next to the cone and occurs in carburizing
flame only.

Electric arc Welding

1.      In electric arc welding
metal pieces to be joined are heated to melting point by
creating an electric arc between them to form
a pool of molten metal and then allowing it
solidify to form welded joint. Sometimes additional material is added to
form the weld by melting the wire known as filler metal. Electric arc
welding is classified as

1.      Carbon arc welding (Obsolete)

Metal arc welding

Metal arc welding

In metal arc welding arc is maintained between electrode and
work piece and work piece which form the two terminals. In this welding
electrode used may be bare or coated. Bare electrode has same composition as
that of parent metal whereas coated electrode have some material or flux that
prevents the oxidation of surface.

To obtain the required heat arc is struck by making light 
contact of electrode with work piece and then electrode is withdrawn to a proper distance. This current arc produces large amount of
heat and melts the electrode end and the work piece. Due to melting of
electrode material droplets are transferred to the work piece through
the arc and deposited along the joint.

Metal electrode is coated with a flux which burns and
produces a gas shield around the arc to protect it from atmospheric
contamination of molten weld metal

Metal arc welding

                     Metal arc welding

Metal arc welding

                          Metal arc welding                

welding Power source

In arc welding both A.C and D.C.
power sources could be used. In D.C. welding

Polarity in arc welding 

When A.C is used polarity
is not fixed at any terminal and it interchanges in every cycle thus the heat
generated at each pole is same. But in D.C welding polarity is fixed. Job acts
as one terminal and electrode acts as another terminal. Heat developed at +ve
terminal is 2/3 rd and at –ve terminal is 1/3 rd of the total heat. In D.C
welding polarity is of two types.

Straight polarity

In straight polarity
electrode forms the –ve terminal and work piece forms the +ve terminal. This
polarity is used in welding of thick materials due to large requirement of heat
on the plate

Reverse polarity

In Reverse polarity
electrode forms the +ve terminal and work piece form the -ve terminal. This
polarity is used in welding of thin materials due to less requirement of heat
in welding zone.

Polarity in arc welding

                 Polarity in arc welding

of electrode

1.      Consumable electrodes

a.      Bare Electrodes

Coated Electrodes

Non- consumable electrodes

Resistance welding

 It is the process of
joining two metal pieces by application of mechanical pressure and heat. The
heat is generated due the resistance offered by the two pieces to the flow of
current. In resistance welding two metal pieces to be joined are held together
and high electric current is passed through it. Due to flow of current across
the resistance the temperature at the junction reaches the fusion point. At
this time slight mechanical pressure is applied to complete the weld.

The magnitude of current in resistance welding varies from 3000 to 100,000 ampere for a fraction of a second with voltage of 1 to 25 volts.

Heat generated in resistance welding is given


Where H= Heat generated in

I= Current in ampere

T= Time of
current flow in second Resistance welding can be further classified as

Spot welding

Seam welding

Projection welding

Butt welding

Flash welding

Percussion welding

Resistance spot welding

Spot welding is a resistance welding process in which
overlapping metal plates are held between two copper electrodes, which
concentrate welding current to melt the interface over a spot and apply
pressure to complete the weld.

In this welding

Current is order of 3000-
10000 ampere

Temperature of weld
zone is 8150 C-9300C

Electrode- Copper base alloy

Weld nugget- 6-10 mm in diameter

Advantages Resistance spot welding
High welding rates;

Low fumes;

Cost effectiveness;

Easy automation;

No filler materials
are required

Low distortions.


High equipment cost;

Low strength of discontinuous welds;

Thickness of welded sheets is
limited – up to 1/4” (6 mm);

Resistance Seam welding

Seam welding is similar to spot welding except that continuous circular rotating
electrodes are used in place of tip electrode. The weld produced is continuous  air tight
seam. It can also be defined as
continuous series of spot welds.

Resistance Seam welding

Resistance Seam welding

Resistance Seam welding

Resistance Seam welding

Resistance projection

welding is a development of resistance spot welding. In spot welding, the size
and position of the welds are determined by the size of the electrode tip and
the contact point on the work pieces, whereas in projection welding the size
and position of the weld or welds are determined by the design of the component
to be welded. The force and current are concentrated in a small contact area
which occurs naturally, as in cross wire welding or is deliberately introduced
by machining or forming.

Resistance projection welding

Resistance projection welding

Resistance projection welding

Resistance projection welding


The advantages of projection welding include its versatility,
the speed and ability to automate, the
ability to make a number of welds simultaneously and minimization of
marking on one side of joints in sheet
materials. Capacitor discharge supplies used with machined annular projections
can compete with power beam welding, as the weld is completed in a
single shot within milliseconds.


There are some limitations on
material weldability but attention to correct setting up and good process
control can solve most production problems. The main safety factors are
trapping hazards and splash metal. Little fume is produced but may need
attention when welding coated steels or when oils or organic materials are

Flash Welding

Flash Welding is a Resistance Welding (RW) process, in which
ends of rods (tubes, sheets) are heated and fused by an arc struck between them
and then forged (brought into a contact under a pressure) producing a weld.

The welded parts are held in electrode clamps, one of which
is stationary and the second is movable. Flash Welding method permits fast
(about 1 min.) joining of large and complex parts. Welded parts are often
annealed for improvement of Toughness of the weld. Steels, Aluminum alloys,
Copper alloys, Magnesium alloys, Copper alloys and Nickel alloys may be welded
by Flash. Thick pipes, ends of band saws, frames, and aircraft landing gears
produced by Flash Welding.


Flash Welding

                                   Flash Welding           

Resistance Butt Welding

 is a Resistance
Welding (RW) process, in which ends of wires or rods are held under a pressure
and heated by an electric current passing through the contact area and
producing a weld.

Butt welding is used for welding small parts. The process is
highly productive and clean. In contrast to Flash Welding, Butt Welding
provides joining with no loss of the welded materials.

Percussion Welding 

 It is a resistance welding process wherein
coalescence is produced
simultaneously over the entire area of abutting surfaces by heat obtained from
an arc produced by a rapid discharge of electrical
energy, with pressure percussively (rapidly) applied during or immediately
following the electrical discharge.

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