WELDING

Welding is a materials joining process

which produces coalescence of materials
by heating them to suitable temperatures
with or without the application of pressure
or by the application of pressure alone, and
with or without the use of filler material.
 Classification of welding processes

(i). Arc welding (iv)Thermit Welding

Carbon arc
Metal arc (v)Solid State Welding
Metal inert gas Friction
Tungsten inert gas
Plasma arc Ultrasonic
Submerged arc Diffusion
Electro-slag
(ii). Gas Welding Explosive
Oxy-acetylene (vi)Newer Welding
Air-acetylene Electron-beam
Oxy-hydrogen
(iii). Resistance Welding Laser
Butt (vii)Related Process
Spot Oxy-acetylene
Seam
Projection cutting
Percussion Arc cutting
Hard facing
 Welding Positions
INCREASING DIFFICULTY

FLAT

HORIZONTAL

OVERHEAD

VERTICAL
 FILLER METAL
A filler metal is a metal added in the making of a
joint through welding, brazing, or soldering. Four
types of filler metals exist covered electrodes,
bare electrode wire or rod, tubular electrode wire
and welding fluxes. Sometimes non-consumable
electrodes are included as well, but since these
metals are not consumed by the welding process,
they are normally excluded.
 GAS WELDING

• Sound weld is obtained by selecting proper size of

flame, filler material and method of moving torch
• The temperature generated during the process is
33000c
• When the metal is fused, oxygen from the atmosphere
and the torch combines with molten metal and forms
oxides, results defective weld
• Fluxes are added to the welded metal to remove oxides
• Common fluxes used are made of sodium, potassium.
Lithium and borax.
• Flux can be applied as paste, powder, liquid, solid
coating or gas.
GAS WELDING PRINCIPLE
• Base metal is melted
• Filler metal may be added
• Heat is supplied by various
means as oxy-acetylene gas
 GAS WELDING
EQUIPMENT
1. Gas Cylinders
Pressure
Oxygen – 125 kg/cm2
Acetylene – 16 kg/cm2
2. Regulators
Working pressure of oxygen 1 kg/cm2
Working pressure of acetylene 0.15 kg/cm2
3. Pressure Gauges
4. Hoses
5. Welding torch
 TYPES OF FLAMES
• Reducing flame is used to melt low-melting-point
metals and alloys because it does not oxidize or
corrode the metals.
• Neutral flame is the hottest one possible and is
properly adjusted for welding.
• Oxidizing flame that can cause corrosion in the
metal. It is only used for cutting flames or burning
pieces of metal from a piece of stock.
 GAS CUTTING
• Ferrous metal is heated in to red hot condition and a jet of pure

oxygen is projected onto the surface, which rapidly oxidizes

• Oxides having lower melting point than the metal, melt and are

blown away by the force of the jet, to make a cut

• Fast and efficient method of cutting steel to a high degree of

accuracy
• Torch is different from welding
• Cutting torch has preheat orifice and one central orifice for

oxygen jet
GAS CUTTING (Contd.)
 Oxygen-Hydrogen Welding

• The oxygen-hydrogen torch can reach

temperatures much higher than the oxy-
acetylene torch.

• More expensive than oxy-acetylene welding and

involves the flammability risk with hydrogen.

• It is normally used to weld thin sheets and alloys

with low melting temperatures.
 Basics of Arc Welding
• The arc is struck between the electrode
and the metal. It then heats the metal to
a melting point. The electrode is then
removed, breaking the arc between the
electrode and the metal. This allows the
molten metal to “freeze” or solidify.
 How an arc is formed?
The arc is like a flame
of intense heat that is
generated as electrical
current passes through
a highly resistant air
gap.
 Arc Welding Principle
DC - DC
+

Polarity
DC- (Direct Current
Electrode Negative)

DC+ (Direct Current

AC Electrode Positive)

AC (Alternating Current)
 Arc Welding Equipment
• A welding generator (D.C.) or Transformer
(A.C.)
• Two cables- one for work and one for electrode
• Electrode holder
• Electrode
• Protective shield
• Gloves
• Wire brush
• Chipping hammer
• Goggles
 Gas Metal Arc Welding (GMAW)

• In Gas Metal Arc Welding (GMAW), also known as

Metal Inert Gas (MIG) welding, an electric arc is
established between the work piece and a
consumable bare wire electrode. The arc
continuously melts the wire as it is fed to the weld
puddle. The weld metal is shielded from the
atmosphere by a flow of an inert gas, or gas mixture.
 Gas metal arc welding (MIG)

• GMAW is a metal inert gas welding (MIG)

• Weld area shielded by an effectively inert
atmosphere of argon, helium, carbon
dioxide, various other gas mixtures
Process capabilities
• GMAV process is suitable for welding a
variety of ferrous and non-ferrous metals
• Process is versatile, rapid, economical,
welding productivity is double that of SMAW
Gas metal arc welding (MIG)
 Metal Transfer in MIG Welding

The basic MIG process includes three distinctive

process techniques:
• Short circuiting metal transfer
• Globular transfer
• Spray arc.
These techniques describe the manner in which
metal is transferred from the wire to the weld pool.
Factors that determine the manner of metal transfer
are the welding current, wire size, arc
length(voltage), power supply characteristics, and
shielding gas.
Metal Transfer in MIG Welding (Contd.)
 Metal Transfer in MIG Welding (Contd.)

Short circuiting metal transfer occurs when an

electrical short circuit is established. This occurs as the
molten metal at the end of the wire touches the molten
weld pool.

Globular transfer occurs when the drops of metal are

quite large and move toward the weld pool under the
influence of gravity.

In Spray arc welding, small molten drops of metal are

detached from the tip of the wire and projected by
electromagnetic forces towards the weld pool.
Equipment used in MIG Welding
Operations
 Advantages of MIG welding

• Large gaps filled or bridged easily

• Welding can be done in all
positions
• No slag removal required
• High welding speeds
• High weld quality
• Less distortion of work piece
 GTAW Welding (TIG)
• In TIG (Tungsten Inert Gas) welding, an arc is
drawn between a non-consumable tungsten
electrode and the work piece. The electrode, the
arc and the weld pool are protected from the
atmosphere with an inert shielding gas.
• TIG welding is suitable for metal thickness
between 1 and 6 mm.
 Principle of TIG

• Tungsten electrode acts as a

cathode
• A plasma is produced
between the tungsten cathode
and the base metal which
heats the base metal to its
melting point
• Filler metal can be added to
 Advantages of TIG
• No flux is used.
• Clear visibility of arc and job.
• Can weld in all positions.
• High quality welding of thin materials (as
thin as 0.125 mm).
• Can weld nonferrous metals and stainless
steel.
 Dis-advantages of TIG

• Tungsten can contaminate the molten weld

pool if dis-integrate from electrode.
Tungsten inclusion is hard and brittle.

• Filler rod end if it by chance comes out of

the inert gas shield can cause weld metal
It is an arc welding process wherein coalescence is produced by
heating with an electric arc or arcs set up between a bare metal
electrode/ electrodes and the job.

The arc, end of the electrode and molten pool remain completely
hidden and are invisible being submerged under a blanket of
granular material (flux). The continuously fed bare metal electrode
melts and acts are filler rod. No pressure is applied for welding
purposes.
Submerged Arc Welding
 Principle of Submerged Arc Welding

• In submerged arc welding process, instead of a flux

covered electrode, granular flux and a bare (or copper
coated) electrode is used. Arc between the electrode
and job is the heat source and remains burried under
the flux. The flux serves as a shield and protects the
molten weld pool from atmospheric contamination.
The process may be semiautomatic or automatic.
 Equipment

(a) Welding head. It feeds flux and filler metal to

the welding joint.
(b) Flux hopper. It stores the flux and controls the
rate of flux deposition on the welding joint.
(c) Welding power source. Any AC transformer or
a DC generator rated up to 1500 Amps may
be used for submerged arc welding.
(d) Flux. The granulated flux shields and thus
protects molten weld metal from atmospheric
contamination.
 WELDING ELECTRODES

An electrode is a piece of wire

or a rod (of a metal or alloy),
with or without flux covering,
which carries current for
welding. At one end it is
gripped in a holder and an arc
is set up at the other.
 Classification of Electrodes

Non-consumable (refractory)
• Carbon or graphite electrodes
• Tungsten electrodes
Consumable (Metallic)
• Bare electrodes
• Flux covered electrodes
Selection of Welding Electrodes
The choice of an electrode depends on the following factors:

1. Chemical composition of the base metal.

2. Thickness of work piece.

3. Nature of electrode coating.

4. Positions (flat, horizontal, vertical, overhead), in which welding is to be carried

out.

5. Type of joint ( lap, butt etc.)

6. Type of polarity (DCSP, DCRP).

7. Surface finish and quality of weld metal.

 WELDING ARC
Welding arc has been defined as a sustained electrical discharge

through an ionized gas. The discharge is initiated by an avalanche of

electrons emitted from the hot cathode and maintained by thermal

ionization of the hot gas.

This electrical discharge through an ionized gas (or a high

temperature conducting plasma) produces a good amount of heat

energy which is employed for joining various metals and alloys by

fusion.

A welding arc is a high current (up to 2000 amps.) and low voltage
 ARC STABILITY

• Arc is said to be stable if it is uniform and

steady. A stable arc will produce good
weld bead and a defect free weld nugget.
Defects commonly introduced by unstable
arc are slag entrapment, porosity, blow
holes and lack of proper fusion.
 Factors on which Arc Stability
depends
1.Suitable matching of arc and power source characteristics. A little variation in arc length, i.e.,

arc voltage should not extinguish the arc.

2. Continuous and proper emission of electrons from the electrode (say cathode) and thermal

ionization in the arc column.

3. Electrode tip geometry in TIG welding.

4. Presence of dampness, oil, grease, etc. on the surface of work piece.

5. Limited practice on the part of the welder.

 ARC BLOW

• The unwanted deflection or the wandering of a

welding arc from its intended path is termed as
arc blow or arc bow. Arc blow is the result of
magnetic disturbances which unbalance the
symmetry of the self induced magnetic field
around the electrode, arc and work piece.
 ARC BLOW (Contd.)
• Under arc blow, an arc may distort, deflect or
rotate. Arc blow becomes severe when welding is
carried out in confined spaces and corners on
heavy metal plates, using a DC power source. AC
arcs are less susceptible to arc blow than DC arcs;
because the alternating current reverses direction
which in turn reverses the magnetic field.
 ARC BLOW (Contd.)
• The magnetic field builds up, collapses and rebuilds as

current reverses from positive to negative. This phenomenon

does not permit the magnetic field strength to build to a value

so as to cause arc blow. On the other hand in DC welding, the

magnetic field set up in the work piece (adjacent to the arc)

continuously builds up and the arc blow occurs.

 EFFECTS OF ARC BLOW

(a) Increased arc blow results in an unstable arc and presents

considerable difficulty to a welder to carry out welding

properly.

(b) Poor weld bead appearance.

(c) Irregular and erratic weld deposition.

(d) Undercutting and lack of fusion.

(e) Spatter.
 Heat-affected Zone (HAZ)
• Adjacent to the weld metal zone is
the heat-affected zone that is
composed of parent metal that did
not melt but was heated to a high
enough temperature for a sufficient
period that grain growth occurred.
Heat affected zone is that portion of
the base metal whose mechanical
properties and microstructure have
been altered by the heat of welding.
 Heat-affected Zone (HAZ)

• The heat affected zone is subjected to a complex

thermal cycle (sudden heating followed by rapid
cooling) in which all temperatures from the melting
range of the steel down to comparatively much lower
temperatures are involved.
• HAZ, usually contains a variety of microstructures.
Heat-affected Zone (HAZ)

Zone-I Coarse grains

Zone-II Fine grains
Zone-III Fine grains
mixed with
original structure
Zone-IV Transition zone
Zone-V Original
structure
 Introduction to Resistance Spot Welding

Top Electrode

Weld
Nugget

Distance

Resistance
Bottom Electrode
Typical Equipment of Resistance
Spot Welding
Process Operation of Resistance Spot Welding
 Basic Single Impulse Welding Cycle
Electrode Force

Welding Current

Squeeze Time Weld Time Hold Off

Time Time
Welding Cycle
Advantages of Resistance Spot Welding

 Adaptability for Automation in High-Rate Production

of Sheet Metal Assemblies

 High Speed

 Economical

 Dimensional Accuracy
 Limitations of Resistance Spot Welding

 Difficulty for maintenance or repair

 Adds weight and material cost to the
product, compared with a butt joint
 Generally have higher cost than most arc
welding equipment's
 Low tensile and fatigue strength
 Eccentric loading condition
 Resistance Seam Welding
• RSEM is modification of spot welding wherein the electrodes are
replaced by rotating wheels or rollers
• The electrically conducting rollers produce a spot weld
• RSEM can produce a continuous seam & joint that is liquid and gas tight
RESISTANCE SEAM WELDING
 Introduction to Brazing and Soldering

•Brazing and soldering –A filler metal is melted and distributed

by capillary action but no melting of parent metals occurs.

•Brazing & soldering instead of fusion welding

–Join the metals with poor weldability.
–Join dissimilar metals.
–No heat damage on the surfaces.
–Geometry requirement is more relaxed than welding.
–No high strength requirement
 Brazing

• It is a low temperature joining process. It is

performed at temperatures above 840°F and it
generally affords strengths comparable to those
of the metal which it joins. It is low temperature in
that it is done below the melting point of the base
metal. It is achieved by diffusion without fusion
(melting) of the base.
 Advantages & Disadvantages

Advantages
• Dissimilar metals which can not be welded can be joined by
brazing.
• Very thin metals can be joined.
• Metals with different thickness can be joined easily.
• In brazing thermal stresses are not produced in the work piece.
Hence there is no distortion.
• Using this process, carbides tips are brazed on the steel tool
holders.

Disadvantages
• Brazed joints have lesser strength compared to welding.
• Joint preparation cost is more.
 Brazed Joints

• Clearance between mating surface for capillary action (0.025 and

0.25mm)

• Cleanliness of the joint –chemical (solvent cleaning & vapor

degreasing) and mechanical (wire brushing & sand blasting)

treatments

• Fluxes are used during brazing to clean surfaces and to promote

wetting.

• Common filler metals

–Compatible melting temperature compatible with base metal.

–Low surface tension for wetting.

 Soldering

It is a low temperature joining process. It is performed at

temperatures below 840ºF for joining.

Soldering is used for,

• Sealing, as in automotive radiators
• Electrical Connections
• Joining thermally sensitive components
• Joining dissimilar metals
• Electric Soldering • Non-Electric Soldering
Iron Iron
 Most commonly used Solders

%Tin %Lead Use

60 40 Good for all electrical and mechanical
work
45 55 Very liquid used in plumbing
50 32 Low melt solder for white-metal castings
 Electron Beam Welding (EBW)

Electron Beam Welding

• Is a welding process utilizing a heat generated by
a beam of high energy electrons. The electrons
strike the work piece and their kinetic energy
converts into thermal energy heating the metal
so that the edges of work piece are fused and
joined together forming a weld after Solidification.
Electron Beam Welding (EBW)
 Electron Beam Welding (EBW)

• Electron Beam is capable to weld work pieces with thickness

from 0.01 mm up to 150 mm of steel and up to 500 mm
of aluminum.
• Electron Beam Welding may be used for joining any metals
including metals, which are hardly weldable by
other welding methods: refractory metals (tungsten,
molybdenum, niobium) and chemically active metals (titanium,
zirconium, beryllium).
• Electron Beam Welding is also able to join dissimilar metals.
 EBW Benefits

• Single pass welding of thick joints

• Hermetic seals of components retaining a vacuum
• Low distortion
• Low contamination in vacuum
• Weld zone is narrow
• Heat affected zone is narrow
• Can weld dissimilar metals
• Uses no filler metal
 EBW Limitations

• High equipment cost

• Work chamber size constraints
• High weld preparation costs
• Rapid solidification rates can cause cracking in
some materials
 Plasma Arc Welding

Plasma arc welding is an arc welding process wherein coalescence

is produced by the heat obtained from an arc set up between a
tungsten/alloy tungsten electrode and the water cooled nozzle (non
transferred arc) or between a tungsten/alloy tungsten electrode and
the job (transferred arc).

The process employs two inert gases, one forms the arc plasma and
the second shields the arc plasma. Filler metal may or may not be
added.
 Plasma Arc Welding

• Arc plasma is the temporary state of a gas.

The gas gets ionized after the passage of
electric current through it and it becomes a
conductor of electricity. In ionized state gas
atoms break into electrons (-) and ions (+) and
the system contains a mixture of ions,
electrons and highly excited atoms.
 Non-transferred arc process
The arc is formed between the electrode (-)
and the water cooled constricting nozzle (+).
Arc plasma comes out of the nozzle as a
flame. The arc is independent of the work
piece and the work piece does not form a
part of the electrical circuit.
 Transferred arc process
The arc is formed between the electrode (-) and the work piece (+).

In other words, arc is transferred from the electrode to the work

piece. A transferred arc possesses high energy density and plasma

jet velocity. For this reason it is employed to cut and melt metals.

Besides carbon steels, this process can cut stainless steel and

nonferrous metals also, where oxyacetylene torch does not

succeed. Transferred arc can also be used for welding at high arc

travel speeds.
 Electro-slag Welding (ESW)

• It is a welding process, in which the heat is generated by an electric current passing

between the consumable electrode (filler metal) and the work piece through a molten

slag covering the weld surface.

• Prior to welding the gap between the two work pieces is filled with

a welding flux. Electro-slag Welding is initiated by an arc between the electrode and the

work piece (or starting plate). Heat, generated by the arc, melts the fluxing powder and

forms molten slag. The slag, having low electric conductivity, is maintained in liquid

state due to heat produced by the electric current.

• The slag reaches a temperature of about 3500°F (1930°C). This temperature is

sufficient for melting the consumable electrode and work piece edges. Metal droplets
 Friction Welding

Electrical

Solid
State Chemical
Welding
Pressure & Friction
Friction
Deformation Weld

Mechanical
 Friction Welding
Friction welding is a
solid state joining
process that produces
coalescence by the heat
developed between two
surfaces by mechanically
induced surface motion.
 Thermit welding principle

• The necessary heat for joining metal of

thermit welding is obtained from chemical reaction
of metal oxide and metal reducing agent. Usually
iron oxide is used as a metal oxide and aliminium
or magnesium is used as metal reducing agent.
The strong chemical attraction of aluminium for
oxygen is the basis for thermit process.
Thermit Welding
Thermit Welding
 Refrences
1R.S.Parmar Welding Technology,
Khanna Publishers
2.R.K. Rajput, A Text book of
Manufacturing Technology ( Laxmi
Publications )
3. S. Kalpakjian& Steven R. Schmid,
Manufacturing Engineering & Technology
(Pearson)
4.Nptel notes