Gas Tungsten Arc Welding (GTAW) Process OR Tungsten Inert Gas Welding (TIG):

INTRODUCTION: Gas Tungsten Arc Welding (GTAW) also known as Tungsten Inert Gas
welding (TIG) is an electric arc welding process that produces an arc between a non-consumable
electrode (tungsten which does not melt due to its high melting point) and the work piece to
be welded. The weld is shielded from the atmosphere by a shielding gas that forms an envelope
around the weld area. However, a filler metal is usually used in the process. The weld area is
protected from atmospheric contamination by an inert shielding gas (argon or helium), and a
filler metal is normally used.
 Working Principle:
 An arc is established between the end of a tungsten electrode and the parent metal at the joint line. The electrode
is not melted and the welder keeps the arc gap constant. The current is controlled by the power-supply unit.

 A filler metal, usually available in 1 m lengths of wire, can be added to the leading edge of the pool as required.
The molten pool is shielded by an inert gas which replaces the air in the arc area.

 Argon and helium are the most commonly used shielding gases.

 The process may use direct current electrode positive, direct current electrode negative or alternating current.

 DCEN or “straight polarity” is used for welding most materials other than aluminium. The electrode tip
geometry is generally a sharp point with a small blunted end since most of heat balance is on melting of the base
material.

 DCEP or “reverse polarity” is rarely used since it results in low penetration. Also the constant bombardment of
the tungsten electrode by electrons in the DCEP mode degrades the electrode.
 Alternating current (AC) is used primarily to weld aluminium, which has a tenacious oxide surface layer.
Process:
i) To start the process, welding current, water and inert gas supply are turned on.

ii) The arc is struck by touching the electrode with a scrap metal tungsten piece.

iii) Then the welding torch is brought near to the workpiece.

iv) When electrode tip reaches within a distance of 2 to 3 mm from the workpiece, spark jumps across the air gap
between the electrode and the workpiece. Then the air path gets ionised and arc is established.

v) The welding continues by moving the torch along the joint and at the far end of the workpiece arc is broken by
increasing the air gap.

vi) To avoid atmospheric contamination, a shielding gas is impinged on the solidifying part for a few seconds.

vii)The welding torch the filler rod are generally kept inclined at an angle of 70° to 80° with weld plane.

viii)For welding, a leftward technique is used.

Equipment:

The equipment required for the gas tungsten arc welding operation includes a welding torch utilizing a non-
consumable tungsten electrode, a constant-current welding power supply, and a shielding gas source.

a. Welding torch

b. Power supply

c. Electrode

d. Shielding gas

e. Filler Rod
Advantages:
1. Works on almost all types of metals with higher melting points. Especially for reactive metals like titanium and
zirconium, which dissolve oxygen and nitrogen and become embrittled.
2. Accuracy and control. The process provides more precise control of the weld than any other arc welding process,
because the arc heat and filler metal are independently controlled.
3. Good looking weld beads
4. Very good for joining thin base metals because of excellent control of heat input.
5. Creates strong joints. It produces top quality welds in almost all metals and alloys used by industry.
6. Clean process with minimal amount of fumes, sparks, spatter and smoke i.e. minimal finishing required.
7. Works in any position.
8. Because the electrode is non-consumable, the process can be used to weld by fusion alone without the addition
of filler metal.
Disadvantages:
1. Lower filler metal deposition rates.
2. Good hand-eye coordination a required skill.
3. Brighter UV rays than other processes.
4. Slower travel speeds than other processes.
5. Equipment costs tend to be higher than other processes.
Areas of application:
i) Specially useful for welding reactive & refractory metals.
ii) It is highly used in carbon and alloy steels, stainless steel, heat resisting alloy, Al alloys, Mg-alloys, Cu-alloys,
Nickel alloys etc.
iii) Welding stainless steels, argon is recommended for manual welding of thickness up to 12mm.
iv) For thick sections, argon-helium mixtures or pure helium can be used to obtain increased weld penetration.
v) With AC, generally argon shielding gas is used for welding aluminum because it provides better arc starting,
better cleaning action, and superior weld quality than that with helium as shielding gas.
vi) TIG welding is often used for jobs that demand high quality welding such as for instance:
a. The offshore industry
b. Combined heat and power plants
c. The petrochemical industry
d. The food industry
e. The chemical industry
f. The nuclear industry
Gas Metal Arc Welding (GMAW) OR Metal Inert Gas welding (MIG)
INTRODUCTION:
Gas Metal Arc Welding (GMAW) is commonly referred to as MIG welding (Metal Inert Gas welding).
It is also referred to as MAG welding (Manual Metal Arc Welding). Metal inert gas welding or gas metal arc
welding (GMAW) is a group of arc welding process in which the workpieces are joined by the heat obtained
from an electric arc struck between a bare (uncoated) consumable electrode and the workpiece in the
presence of an inert gas atmosphere. The consumable electrode acts as a filler metal to fill the gap between the
two workpieces.
 Basic Principle
The basic principle of MIG Welding is, an arc is maintained between the end of the bare wire electrode and the work
piece where the heat source required to melt the parent metal is obtained. The arc melts the end of the electrode
wire, which is transferred to the molten weld pool. For a given wire material and diameter, the arc current is
determined by the wire feed rate. The arc and the weld pool is shielded from the atmospheric contamination by an
externally supplied shield gas. Metal Inert Gas (MIG) welding is a 'flat' arc process (constant) voltage. The required
voltage is selected by adjusting the voltage control knobs provided at the power source. The process itself can be
manual, partly mechanized, fully mechanized or automatic.
Description:
 The equipment consists of a welding torch in which a bare consumable electrode in the form of a wire is held and
guided by a guide tube.

 The electrode material used in MIG welding is of the same material or nearly the same chemical composition as
that of the base metal. Its diameter varies from 0.7 -2.4 mm.

 The electrode is fed continuously at a constant rate through feed rollers driven by an electric motor.

 MIG makes use of shielding gas to prevent atmospheric contamination of the molten weld pool. Mixture of argon
and carbon dioxide in a order of 75% to 25% or 80% to 20% is commonly used.

 The shielding gas flow from the cylinder, through the passage in the electrode holder and then impinges on the
workpiece.

 AC is rarely used with MIG welding; instead DC is employed and the electrode is positively charged. This results
in faster melting of the electrode which increases weld penetration and welding speed.
MIG/MAG WELDING EQUIPMENT:
Welding equipment for MIG/MAG welding consists of
the following components:

 A shielding gas system with control

 A power source
 A wire feed unit
 A complete welding torch
 A reel of welding wire
 Gas cylinders with pressure-reducing valve and flow
meter
Advantages:

 MIG welding is fast and economical.

 The electrode and inert gas are automatically fed, and this makes the operator easy and to concentrate on the arc.
 Weld deposition rate is high due to the continuous wire feed
 Weld cleaning and preparation time is less for MIG welding than for stick electrode welds.
 Since the gaseous shield protects the molten metal from the atmospheric gases, there is no flux or slag, and spatter is
minimal.
 No flux is used. Hence, no slag formation. This results in clean welds.
 Thin and thick metals can be welded.
 Process can be automated.
 Because of the fast travel speed at which MIG welding can be done, there is a smaller heat-affected zone than with the
shielded metal arc welding process.
 The weld visibility is generally good.
 There is less smoke and fumes so operator environment is improved.
Disadvantages:
 Equipment is costlier
 Porosity (gas entrapment in weld pool) is the most common quality problem in this process. However,
extensive edge preparation can eliminate this defect.
 The lower heat input characteristic of the short-circuiting mode of metal transfer restricts its use to thin
materials.
 The higher heat input axial spray transfer generally restricts its use to thicker base materials.
 The higher heat input mode of axial spray is restricted to flat or horizontal welding positions.
 The use of argon based shielding gas for axial spray and pulsed spray transfer modes is more expensive than
100% carbon dioxide (CO2).
APPLICATIONS

 Structural steel.

 Aluminum sections.

 Stainless steel and nickel alloys.

 Some offshore applications (flux core only).

 Automotive repair

 Rebuilding equipment

 Weld pipes

 To reinforce the surface of a worn-out railroad track

THERMIT WELDING:

 Thermite welding is a type of Thermo-Chemical fusion welding process.

 It is also known as Exothermic welding or bonding.

 This process is used to welding a heavy and large structures.

 It requires no electrode and power supply.

 Aluminum has greater affinity to react with oxygen; it reacts with ferric oxide to liberate pure iron and slag of
aluminum oxide.
 Aluminum oxide floats on top of molten metal pool in the form of slag and pure iron (steel) settled below,
because of large difference in densities.
 Liquid Metal acts as filler metal too.
 Thermit welding process is essentially a casting and foundry process, where the metal obtained by the Thermit
reaction is poured into the refractory cavity made around the joint
What is Thermite / Thermit?

A mixture of fine aluminum powder and iron oxide that produces a very high temperature on combustion, used in
welding and for incendiary bombs.

 By weight, 75% of iron oxide and 25% of fine aluminum powder (OR) Fine aluminum powder and iron oxide
should have a ratio of 1:3 (by weight).

 We can also use copper or chromium instead of aluminum.

Method of thermit welding: The heat of thermit welding may be utilized in two ways:
•Fusion welding: It may heat and fuse the metal part to be welded, thermit mixture acts as the
filler material also.
•Pressure welding: It may heat the metal parts to be welded and raise them to forging
temperature, and forging force is applied to join them.
Procedure of thermit welding:
Various steps involved in fusion thermit welding of metal part are;

 Cleaning the joint: joints to be welded should be cleaned and free from dust, grease, oxide, before welding
 Allow for contraction: A space of 1.5 to 6mm is given between in as contraction allowance.
 Construction of mould: A mould is the part in which the metal art to be joint lies and molten metal is poured in it.
It is 100mm fixed between wax pattern and moulding box and should provide necessary pouring gates, heating
gates, and riser
 Preheating the mould: Mould is preheated to remove wax pattern, dry the mould thoroughly and to bring the
parts to be welded at desired temperature 816°C to 982°C.
 Crucible and it’s charging: Crucible is a conical shape vessel. It is filled with required amount of thermit mixture
to join the metal part. The mixture of aluminum and iron oxide is placed in a crucible.
 Barium Peroxide is added to the crucible. Barium peroxide arises by the reversible reaction of O2 with barium
oxide. The peroxide forms around 500°C and oxygen is released above 820°C
 Igniting the metal mixture: Mixture is ignited using a match stick (or) small magnesium wire. So combustion
takes place in the crucible Due to combustion, thermite mixture becomes superheated liquid within max. time of
30 seconds. Ignition of thermit mixture lasts up to 66 seconds. That superheated liquid (pouring metal) have
temperature of 2500-3000°C (around 5000°F). The reduction of ferric oxide by aluminum is highly exothermic
and therefore the iron formed will be in the molten state.
 After the ignition is completed crucible is tapped to release molten metal mould where fussion takes place.
Crucible is placed directly above the joints to be welded. After preheating of joints, liquid thermite steel from the
crucible is poured into mold between the gaps to weld. The molten metal solidifies and weld is completed.
 Opening the mould: Mould is opened after 2-4 hours. The longer the mould is left unopened, the better it is.
 Finishing the weld: After removing the mould riser and gate are cut away by cutting torch and joint it prepared.
Advantages:
i) The heat necessary for welding is obtained from a chemical reaction and thus no costly power supply is
required. Therefore broken parts (rails etc.) can be welded on the site itself.

ii) It is very portable process

iii) Very large heavy section parts may be joined. For welding large fractured crankshafts.

iv) For welding broken frames of machines.

v) On site welding can be done for railways.

vi) For building up worn wobblers.

vii)For welding sections of castings where size prevents there being cast in one piece.

viii)For replacing broken teeth on large gears.

ix) Forgings and flame cut sections may be welded together to make huge parts.

x) For welding new necks to rolling mill rolls and pinions.

xi) For welding cables for electrical conductors. For end welding of reinforcing bars to be used in concrete
(building) construction.
Disadvantages:

i) Low deposition rate with operating factor.

ii) Can only be used for ferrous metals.

iii) It is uneconomical when used for welding cheap metals or light parts.

iv) Thermite mixtures cannot be stored due to safety hazards and should be used as soon as prepared.

v) It cannot weld low melting point.

vi) High temperature process may cause distortions and changes in Grain structure in the weld region.

vii)Weld may contain gas (Hydrogen) and slag contaminations.

viii)Extremely high level of fume.

ix) It is high skill factor.

Applications:

i) Thermit welding is used chiefly in the repair or assembly of large

ii) For repairing fractured rails (railway tracks).

iii) For butt welding pipes end to end.

iv) For welding large fractured crankshafts

v) For welding broken frames of machines

vi) Welding of sections of casting where size prevents there being caste in one piece

vii)Replacing broken pieces or large gears

viii)End welding of reinforcing bars used in huge concrete constructions

ix) This process is generally used for welding ferrous metals.