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.
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
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;
Pressure welding
Thermo chemical welding
(a) Fusion welding
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
fall under solid state
(cold) welding
(a)
Pressure welding
Friction welding
Ultrasonic welding
Explosive welding
Forge and diffusive welding
(c)
Thermo chemical welding
Thermit welding
Atomic hydrogen welding
hydrogen welding are not commonly used in industries these days.
Fusion welding
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
coating)
Control of weld metallurgy (e.g. preheat, post welding heat treatment)
Gas Welding
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)
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-
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:-
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
flame:-
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
flame:-
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.
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
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
Metal arc welding
Metal arc welding
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.
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.
produces a gas shield around the arc to protect it from atmospheric
contamination of molten weld metal
Metal arc welding |
Metal arc welding |
Arc
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 |
Types
of electrode
Coated Electrodes
Non- consumable electrodes
Resistance welding
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.
by
RT
Joules
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
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.
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
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 projection
welding
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 |
Advantages:-
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.
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
present.
Flash Welding
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.
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
are produced by Flash Welding.
Flash Welding |
Resistance Butt Welding
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.
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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