Arc welding, Inert Gas welding, TIG Welding, MIG Welding, Advantages, Limitations, applications of each Welding.

1. Arc welding

2. Principle of Arc welding:
• The principle of arc welding is based on the formation of an electric arc
between consumable electrode (bare or coated) or non-consumable
electrode and the base metal.
• The heat of the arc is concentrated at the point of welding, and as a result,
it melts the electrode (consumable electrode) and base metal. When the
weld metal solidifies, a sound joint is formed.
• When the metal solidifies, the slag gets deposited on its surface as it is
lighter than metal, and the weld metal is allowed to cool gradually and
slowly. The slag deposited over the weld is removed by chipping.
• The electric arc is produced when current flows across the airgap between
the end of metal electrode and the work surface. This arc is strong stable
electric discharge occurring in the air gap between an electrode and the
work.
• The temperature of this arc is about 3600°C which can melt and fuse the
metal very quickly to produce joint. The temperature of the arc at the
centre is around 6500°C. Only 60 to 70 percent of the heat is utilized in arc
welding to heatup and melt the metal. The remaining heat is dissipated
into surroundings.

3. ARC WELDING EQUIPMENT AND ACCESSORIES
• In arc welding process, the source of heat is electricity. The required
electrical (high ampere-low voltage) is obtained by an arc welding
equipment. The functions of equipment are
-To provide AC or DC welding supply for arc welding.
-To change the high voltage of the main supply (AC) to low voltage and heavy
curent (AC or DC) suitable for arc welding.
-To control and adjust the required welding current during arc welding.
The equipment and accessories required for electric are welding usually
1. Equipment to provide the welding current
(a) Transformer (for A.C)
(b) Generator or Rectifier (for D.C)
2. Accessories
(a) Electrodes
(b) Electrode holder
(c) Cables
(d) Safety devices, and
(e) Tools

4. (a) Transformer: The purpose of transformer is to change the high input
voltage and low amperage to a low voltage (2-80 V) and high amperage (80-
500 amps). Its cost is low, and A.C gives a smoother arc when using high
currents. However, its use is confined mostly to ferrous metals. It is not
suitable for bare electrodes and welding special jobs where fine setting of
current is required.
(b) Generator: It is driven by a motor or an engine. It generates and supply
D.C. for electric arc welding.
(c) Rectifier: The purpose of rectifier is to change the output A.C. of a
transformer into D.C. which is required for electric arc welding. The output of
stepdown transformer is connected to the rectifier unit which converts A.C. to
D.C. The D.C output is connected to positive and negative terminals from
where it is tapped for welding purpose.

5. • A.C. and D.C. plants are used in manual metal arc welding. Both plants are
suitable for the welding of all metals except for some non-ferrous metals which
require a D.C. plant.
A.C. Equipment - Advantages:
-Equipment is simple, and costs only about 60 percent as much as D.C equipment.
- Low maintenance cost, as it has no moving parts.
-No change of polarity when working with various types of electrodes.
Disadvantages:
-Not suitable for non-ferrous metals and thin sheets.
-Electric shock is more intense.
D.C. Equipment - Advantages:
-Can be used for ferrous and non-ferrous metals.
-Stable arc, smoother welding facilitates welding of thin sheets.
-Easy of operation, and suitable for overhead weldings.
-Safer to use where the risk of an electric shock is great.
Disadvantages:
-More expensive to purchase.
-High maintenance cost because of the moving parts.
-Troubles from arc blow i.e., the arc is forced away from the weld point. This
condition is encountered only with D.C. welding equipment.

6. D.C. Polarity: Polarity indicates the direction of current flow in the D.C. welding circuit
In D.C. welding 2/3 of the heat is liberated from the positive end and 1/3 from the
negative end. Due to unequal heat distribution in the electrode and base metal, the
polarity plays an important role for successful welding.
Kinds of polarity: There are two ways by which an electrode and work metal are
connected to power source.
1. Straight polarity (Electrode negative)
2. 2. Reverse polarity (Electrode positive)
Straight polarity: In straight polarity the electrode is connected to the negative and
work to the positive terminal of the power source. It is used for welding thicker
sections with light and medium coated electrodes.
Reverse polarity: In reverse polarity the electrode is connected to the positive and the
work to the negative terminal of the power source. It is used for welding of non-
ferrous metals and cast iron with heavy coated electrodes. It is also used sheet
metal welding.

7. • Accessories :
(a) Electrodes: Electrode is a conductor from which current is applied (arc is struck) during arc
welding. The electrode is made in circular section from approximately the same material as
the base metal.
The functions of electrodes are :
(i) To convey welding current to work piece
(ii) To act as a filler materials
Electrodes used for arc welding are classified as
1. Consumable electrode
(a) Bare electrode (b) Coated electrode
2. Non-consumable electrode:
(b)Electrode holder: It is a clamping device for holding the electrodes. Through holder, the
welding current is conveyed from source to the electrode.
(c) Cables: The welding leads or cables (insulated) carry the electric current to the area. One of
the cables is fixed to electrode holder, and other cable is clamped to metal work table or
to the work directly. The earth clamps should have a proper grip to ensure good electrical
contact.
(d) Tools : Tools are not required for actual welding operation, but are necessary of a good
welded joint. The following tools are most widely used.
Chipping hammer: used for removal of slag and spatter
Brushes :Used for removal of loose slag and spatter that may be left on the weld after chipping.
Tongs: Used for holding the weld specimens while chipping, brushing and during visual
examination.
Magnetic blocks: Used to set up the plates for tack welding. The blocks should be removed once
the plates have been tack welded.
Welding fixtures: Used for the assembly of welded fabrication Parts are only tack welded in
fixtures, then removed and welded completely

8. • (e) Safety devices: The use of various safety devices by a welder is
most essential during arc welding operation. The following protective
(safety) devices are most common.
• Face shields: These are made of fire proof materials and fitted with a
special dark glass which is protected by clear glass. No mutual fittings
should be incorporated because of risk of electric shocks. They protect
the eyes of the welder from the radiation of electric arc. Face shields
are available as hand shield (welder holds it with his left hand) or head-
shield (helmet).
• Hand gloves: These are generally made of leather. Its function is to
protect the hands of the welder from sparks during the welding
• Apron: It is made of leather. Its function is to protect the body of
welder and clothes from the sparks during the welding
• Safety goggles. The purpose of goggles is to protect the eyes during the
chipping, welding
• Skull caps: These are made from leather and designed to protect the
head during overhead
• Safety boots and Spats: Safety boots are provided to protect the feet of
the welder from hat slag Spats are usually made with leather and
designed to protect the top of the foot by ensuring that hot metal does
not enter the boot

9. Manual Metal Arc Welding (MMAW)
• Metal arc welding is the most common type of arc welding and it is
normally a manual operation. Therefore it is usually referred as Manual
Metal Arc Welding (MMAW). It uses consumable metal electrodes. The
composition of electrode is similar to that of the parent metal. When
the bare electrode is used, the joint formed is weak and brittle. This is
due to the absorption of oxygen and nitrogen from the atmosphere. To
overcome these difficulties coated electrodes are used. Flux coating on
the electrode also melts and provides a gaseous shield around the arc
which protects the molten pool from atmospheric contaminations.
Electrode melts and provide filler metal for weld. Metal arc welding
using coated electrode that provides shield is called Shielded Metal Arc
Welding (SMAW)
• The basic setup for metal arc welding is shown. The arc is produced with
low voltage (20-80V) but with very high current (80-500 amps). The
electric circuit consists A.C or D.C power source, and from the power
source welding current is conveyed to work through electrode, electrode
holder and welding leads. The circuit is completed by striking the arc i.e.,
making the contact between electrode and work. The operator
maintains arc at required intensity.

10. Manual metal are welding

11. • The sequences of steps involved in arc welding operation
are:
1. Preparation of edges,
2. Holding the work piece in a fixture.
3. Striking the arc, and
4. Welding the joint.
Before welding, the edges of the work pieces are suitably
prepared, and the joint areas cleaned with the wire brush.
This ensures sound joint. The parent metals are held in a
fixture and welding leads are properly connected to power
source.
The arc is struck by scratching the tip of electrode on the
parent metal. As the electrode tip makes contact, the
current flows and then as it is drawn away an arc is formed
across the gap. Arc can also be struck by tapping down the
electrode to make contact with work piece and is then
raised to maintain a constant gap across which an arc is
formed

12. During the welding, the electrode is given the following motions.
(i) The electrode is fed downwards, and it should be controlled properly to
maintain constant gap (2 to 4 mm).
(ii) The electrode is moved slowly along the joint.
(iii) The electrode tip is given an oscillating movement across the (weaving
motion) weld to maintain proper bead width and secure good penetration of
the weld. The width of weave should not be greater than three times the
diameter of the electrode.
• In multipass welding, slag coated on the bead after the first pass should be
chipped off and cleaned by wire brush before the start of the second pass. This
procedure is followed for the subsequent passes. In most cases, after welding,
the part must be heat-treated to change the size of the grain in the weld bead
and the surrounding area.
• Selection of electrodes :Type of electrode used depends upon :
i. the type of metal to be welded (ii) the position in which the weld is to be
done(iii) the power source(iv)polarity in case of D.C(v) thickness of the base
metal(vi)expected properties of weld joint.
Bare electrodes are used for welding low-carbon steel or wrought iron. They
are also mostly used in submerged arc welding. Coated electrodes are
employed for welding high carbon steels alloy steels and non-ferrous metals
and their alloys. It is best to follow the recommendations of manufacturers
for using electrodes for different works.

13. • Advantages :
-Metal arc welding is faster and lower in cost than gas welding.
-The process is a quite versatile, and welds can be made in any position.
-Suitable for wide range of metals (ferrous and non-ferrous) and their
alloys.
-Less sensitive to weld than other processes.
• limitations.
-The process is not suitable for thin sections.
-The process is not suitable for mechanization.
Electrode replacement is necessary for long joints.
-Not suitable for heavy fabrications because less metal is deposited per
hour.
-Failure to remove the slag when run is interrupted leads to slag inclusions
in the weld.
Applications: The Manual Metal Arc Welding (MMAW) has a wider field of
applications. It is employed for fabrication of pressure vessels, ships,
structural steel work, joints in pipe work, construction and repair of
machine parts.
This processes can also be used for hard facing and repairs of the broken
parts.

14. SUBMERGED ARC WELDING
• Submerged arc welding (SAW) is used for the production of long continuous
weldings. In this case, a bare electrode (1.5-10 mm) in the form of continuous
wire is used; and the arc is struck between electrode wire and work piece under
the granular flux, consisting of lime, silica, magnesium oxide, calcium fluoride
and other elements.
• The flux is fed as a powder in front of the electrode. The flux near the arc melts
and forms a protective coating of slag, which is easily detached from finished
weld. Any unburt flux is recovered and reused. The rate of cooling of the weld
metal is slow, and it is also protected from atmosphere while cooling.
• In this case, an automatic feeding device ensures that the gap between the
electrode and the base metal is constant. Electric current usually range between
300 A and 2000 A. The power source can be AC or DC, but AC may be preferred
to avoid arc blow. Power supply is from a standard single or three phase power
lines with a primary rating up to 440 V.

15. Advantages:
• 1. High rate of deposition
• 2. No visible arc with little fumes and spatter
• 3. Smooth weld surface for long lengths
• 4. High quality welds of excellent penetration, and
• 5. Weld area is free from surface ripples.
disadvantages:
1. Process is not flexible, and requires costly equipment.
2. Slag must be removed after each pass.
3. The process is not economical for thin plates.
4. Only suitable for making straight welds in flat positions.
Applications
Developed in the 1940s, submerged arc welding process is best
suited for the continuous welding of components having 12
to 50 mm thickness. This process is used to weld a variety of
carbon and alloy steels, and stainless steel

16. • INERT GAS SHIELDED ARC WELDING :In all arc welding processes, the
arc is shielded from the surrounding air to obtain joint. In gas shielded
arc welding, shielding is achieved by covering the weld zone with a
blanket gases (argon, helium and carbon dioxide) which prevent the
exposure of weld metal to oxyg hydrogen of the air. Argon or helium is
widely used in inert gas welding with non-consumable m and with
consumable (MIG) electrodes. For welding ferrous metals with carbon
dioxide arc welding carbon dioxide is used as shielding gas
• Tungsten Inert Gas Welding :The tungsten inert gas welding (TIG
welding) is an improvement of manual metal are welding It uses non-
consumable electrode, and the arc is struck between electrode and
work. The art and weld zone is shielded by argon or helium gas and the
weld metal is protected from the atmosphere Thus TIG welding can be
carried out without using flux material. This process can also be called
as tungsten arc welding (GTAW). In TIG welding tungsten electrode is
placed centrally in the lo The inert gas is supplied thought the annular
space between torch and electrode. Filler male supplied using a
separate rod.
• Small torches for welding thin sections are air-cooled but large torches
designed for heavy duty i.e., welding of thick sections are water
cooled.

18. • Depending on weld materials, any standard AC or DC machines may be used to
maintain electric arc. DC is normally used for TIG welding of copper, titanium,
stainless steels and other alloys which don't contain aluminium and magnesium,
AC with argon as shielding gas is generally used for TIG welding of aluminium
alloys.
Advantages
• No flux is used, and no cleaning of the joint is required after the weld is made.
• Inert gas shield prevents contamination by oxygen and nitrogen from the
atmosphere; thus produce high quality welds.
• Minimum distortion of the metal near weld zone.
• the weld is uniform as slag inclusions and trapped flux are absent.
• Due to absence of smoke and fumes, welding process can be easily observed and
operator can exercise better control on the welding process.
• Dissimilar metals can also be weided easily.
Disadvantages:
• All joints require proper cleaning before welding as the inert gas does not provide
fluxing (cleaning) action.
• The process is slow in operation.
• The cost of the process is high.
Applications:
TIG welding is used for welding all metals and alloys having various thicknesses. It is
extensively used for welding aluminium, stainless steel and titanium. The process
is employed for fabrication of missiles, air crafts and rockets. It is also used for
fabrication of chemical plant and high pressure steam pipes.

19. Metal Inert Gas Welding
In metal inert gas welding (MIG welding) consumable electrode in the form
of continuous wire is used. The arc is struck between the work piece and
the wire, which acts as electrode and filler material, and the arc and weld
pool are shielded by inert gas. Depending upon the work material, the
shielding gas may be argon, helium and carbon dioxide.
This process can also be referred as gas metal arc welding (GMAW). In this
case, the bare metal electrode (consumable electrode) in the form of
continuous wire is fed through welding torch (gun) with the help of
electrical motor and feed rolls. The electrode and work piece are
connected to electrical supply. The gas from the cylinder is supplied
through nozzle to form a protection shield around the weld area. For
MIG welding argon is generally used for non-ferrous metals, but helium is
preferred for copper. For ferrous metals, the carbon dioxide or mixture of
argon and CO₂ gas is used as shielding gas.
Direct current reverse polarity (DCRP) is generally used in MIG welding. DC
straight polarity (DCSP) is sometimes used, but AC is not used in MIG due
to unequal burn off rates of electrode wire.

21. • Advantages
-Process is very rapid than TIG and other welding
processes due to continuous feeding of electrode (filler
rod),
-Deeper penetration of weld, and weld produced is of
better quality.
-The use of bare electrode and shielding gases eliminate
the slag on the weld bead, and thus ideal process for
multi welding passes on the same joint.
-Welding can be done in all positions.
-Arc is visible to operator, and he can exercise better
control on the process.
Disadvantages
- Cost of equipment is high
-Not suitable for out door work as the strong wind may
break the shield.