UNIT4-Welding processmechanical welding.ppt

Welding processes
Welding is a process of joining similar or dissimilar metals by application
of heat with or without application of pressure and addition of filler material
OR
Welding is defined as an localized coalescence of metals, where in
coalescence is obtained by heating to suitable temperature, with or without the
application of pressure and with or without the use of filler metal.

• Until the end of the 19th century,
the only welding process was
forge welding, which blacksmiths
had used for centuries to join
iron and steel by heating and
hammering them.
• Arc welding and oxyfuel welding
were among the first processes
to develop late in the century,
and resistance welding followed
soon after.
History of welding

Welding, was transformed during the 19th century. In 1802,
Russian scientist Vasily Perov discovered the electric arc
and subsequently proposed its possible practical
applications, including welding.
From this many other forms, including current forms, have
been born including:
 Carbon arc welding
 Alternating current welding
 Resistance welding
 Oxyfuel welding
History of welding

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.
Pressure sometimes
used in conjunction
with heat, or by itself, to
produce the weld.
How is it done?

TYPES OF WELDING :
 Fusion Welding or Non-Pressure Welding:
The material at the joint is heated to a molten state and
allowed to solidify
(Ex)- Gas welding, Arc welding
• Plastic Welding or Pressure Welding:
The piece of metal to be joined are heated to a plastic
state and forced together by external pressure
(Ex) -Friction

Classification of Welding Process

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2 TYPES OF WELDING JOINTS
Welded joints

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Butt Joint
A connection between the ends or edges of two parts making
an angle to one another of 135-180° inclusive in the region of
the joint.
Common Joint Configurations

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T Joint
A connection between the end or edge of one part and the
face of the other part, the parts making an angle to one
another of more than 5 up to and including 90° in the region
of the joint.

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Corner Joint
A connection between the ends or edges of two parts making
an angle to one another of more than 30 but less than 135° in
the region of the joint.

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Edge Joint
A connection between the edges of two parts making an angle
to one another of 0 to 30° inclusive in the region of the joint.

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Cruciform Joint
A connection in which two flat plates or two bars are welded
to another flat plate at right angles and on the same axis.

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Lap Joint
A connection between two overlapping parts making an angle
to one another of 0-5° inclusive in the region of the weld or
welds.

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Welds Based on Configuration
Slot weld
Joint between two overlapping components made by
depositing a fillet weld around the periphery of a hole in one
component so as to join it to the surface of the other
component exposed through the hole.
Types of Welding Joints

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Welds Based on Configuration
Plug weld
Weld made by filling a hole in one component of a workpiece
with filler metal so as to join it to the surface of an
overlapping component exposed through the hole (the hole
can be circular or oval).

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Based on Penetration
Full Penetration weld
Welded joint where the weld metal fully penetrates
the joint with complete root fusion.

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Based on Penetration
Partial Penetration weld
Weld in which the fusion penetration is intentionally
less than full penetration.

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Based on Accessibility

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Features of Completed Welds

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Parent Metal
Metal to be joined or surfaced by welding, braze welding or
brazing.
Filler Metal
Metal added during welding, braze welding, brazing or surfacing.
Weld Metal
All metal melted during the making of a weld and retained in the
weld.
Heat Affected Zone(HAZ)
The part of the parent metal metallurgically affected by the weld
or thermal cutting heat, but not melted.
Fusion Line
Boundary between the weld metal and the HAZ in a fusion weld.
This is a non-standard term for weld junction.

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Weld Zone
Zone containing the weld metal and the HAZ.
Weld Face
The surface of a fusion weld exposed on the side from which the
weld has been made.
Weld Root
Zone on the side of the first run furthest from the welder.
Weld Toe
Boundary between a weld face and the parent metal or between
runs. This is a very important feature of a weld since toes are
points of high stress concentration and often they are initiation
points for different types of cracks (eg fatigue cracks, cold
cracks).
In order to reduce the stress concentration, toes must blend
smoothly into the parent metal surface.

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Excess Weld Metal
Weld metal lying outside the plane joining the toes. Other non-
standard terms for this feature: reinforcement, overfill.
Note: The term reinforcement, although commonly used, is
inappropriate because any excess weld metal over and above the
surface of the parent metal does not make the joint stronger.
In fact, the thickness considered when designing a welded
component is the design throat thickness, which does not include
the excess weld metal.

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Run (pass)
The metal melted or deposited during one passage of an
electrode, torch or blowpipe.
Layer
Stratum of weld metal consisting of one or more runs.

CLASSIFICATION OF WELDING
PROCESSES:
 Gas welding(Oxy- Acetylene)
 Arc welding(Metal Arc)
 Resistance welding
 Solid state welding
 Thermo-chemical welding

Gas Welding:
Gas Welding is a fusion welding
process, in which the heat for welding is
obtained by the combustion of oxygen
and fuel the gas may be acetylene
,hydrogen or propene .
Types:
• Oxy- Acetylene
• Air-Acetylene
• Oxy-Hydrogen
• Oxy-Fuel

Oxy-Acetylene Welding:
When a combination of
Oxygen and acetylene is
used in correct proportions
to produce an Intense gas
flame, the process is known
as oxy-acetylene welding.

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
• Working pressure varies depends upon the thickness of the work pieces
welded.
3. Pressure Gauges
4. Hoses
5. Welding torch
6. Check valve
7. Non return valve

This flame directly strikes the weld area and melts the weld surface and filler material.
The melted part of welding plates diffused in one another and create a weld joint after
cooling.
This welding method can be used to join most of common metals used in daily life.
Types of gases (Fuels):
 Acetylene
 hydrogen
 propane
 natural gas etc.
Types of gas welding :
Based on the combination of the gases used :
 Oxy acetylene gas welding (most common type)
 Air- acetylene gas welding
 Oxy-hydrogen gas welding

Neutral Flame:
•Carburizing Flame: •Oxidizing Flame:
There are three basic flame types:
1. Neutral Flame (balanced)
2. Oxidizing (excess oxygen) and
3. Carburizing (excess acetylene)
Types of flames in gas welding

Types of flames in gas welding ….
Commonly used to weld:
Mild steel
Stainless steel
Cast Iron
Copper
Aluminum
There are two clearly defined zones in the neutral
flame.
 The inner zone consists of a luminous cone that is
bluish-white.
 Surrounding this is a light blue flame envelope or
sheath.
Neutral Flame:
-Equal volume of acetylene and oxygen.
-Obtains additional oxygen from the air and
provides complete combustion.
The oxygen to acetylene ratio is around 1.1 to 1.0.
Generally preferred flame.
The neutral flame has a clear, well-defined, or
luminous cone indicating that combustion is
complete

Oxidizing Flame:
 Excess oxygen.
 The oxygen to acetylene ratio in the case of Oxidizing flame is 1.15 to 1.5.
 When the flame is properly adjusted, the inner cone is pointed and slightly purple.
 An oxidizing flame can also be recognized by its distinct hissing sound.
 The temperature of this flame is approximately 3482ºC at the inner cone tip.
Oxidizing welding flames are commonly
used to weld these metals:
•Zinc
•Copper
•Manganese steel
•Cast iron

1. Clearly defined bluish-white inner cone,
2. White intermediate cone indicating the
amount of excess acetylene, and
3. A light blue outer flare envelope.
Carburizing Flame:
 Excess acetylene, the inner cone has a feathery edge extending beyond it.
 Oxygen to acetylene ratio in case of reducing flame varies from 0.85 to 0.95.
 The reducing or carburizing flame can always be recognized by the presence of three
distinct flame zones.
 It has a temperature of approximately 3149ºC at the inner cone tips.

FLAMES
ADJUSTMENT
FOR OAW
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Advantages:
 Portable and most versatile process.
 Better control over the temperature.
 Suitable to weld dissimilar matter.
 Low cost & maintenance.
Disadvantages:
 Not suitable for heavy section.
 Less working temperature of gas
flame.
 Slow rate of heating.

Arc Welding:
“Arc welding is a fusion welding process in which the
heat required to fuse the metal is obtain from the
electric arc between the base metal and an electrode.
Types:
1. Metal Arc Welding
2. Submerged Arc Welding
3. Tungsten Inert Gas Welding
4. Metal Inert Gas Welding

ARC WELDING
 The arc welding is a fusion welding process in which the heat required to fuse the
metal is obtained from an electric arc between the base metal and an electrode.
 The electric arc is produced when two conductors are touches together and then
separated by a small gap of 2 to 4 mm, such that the current continues to flow,
through the air. The temperature produced by the electric arc is about 4000°C to
6000°C.
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ARC WELDING
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 A metal electrode is used which supplies the filler metal. The electrode may be
flux coated or bare. In case of bare electrode, extra flux material is supplied. Both
direct current (D.C.) and alternating current (A.C.) are used for arc welding.
 The alternating current for arc is obtained from a step down transformer. The
transformer receives current from the main supply at 220 to 440 volts and step
down to required voltage i.e., 80 to 100 volts. The direct current for arc is usually
obtained from a generator driven by either an electric motor, or patrol or diesel
engine.
 An open circuit voltage (for striking of arc) in case of D.C. welding is 60 to 80 volts
while a closed circuit voltage (for maintaining the arc) is 15 to 25 volts

PROCEDURE OF ELECTRIC ARC WELDING
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 First of all, metal pieces to be weld are thoroughly cleaned to remove the dust,
dirt, grease, oil, etc. Then the work piece should be firmly held in suitable
fixtures. Insert a suitable electrode in the electrode holder at an angle of 60 to
80° with the work piece.
 Select the proper current and polarity. The spot are marked by the arc at the
places where welding is to be done. The welding is done by making contact of
the electrode with the work and then separating the electrode to a proper
distance to produce an arc.

PROCEDURE OF ELECTRIC
ARC WELDING
 When the arc is obtained, intense heat so produced, melts the work below the
arc, and forming a molten metal pool. A small depression is formed in the work
and the molten metal is deposited around the edge of this depression. It is
called arc crator. The slag is brushed off easily after the joint has cooled. After
welding is over, the electrode holder should be taken out quickly to break the
arc and the supply of current is switched off.
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ELECTRIC CURRENT FOR WELDING
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Both D.C. (direct current) and A.C. (alternating current) are used to produce an arc in
electric arc welding. Both have their own advantages and applications.
The D.C. welding machine obtains their power from an A.C. motor or diesel/petrol
generator or from a solid state rectifier.
The capacities of D.C. machine are:
Current:
Up to 600 amperes.
Open Circuit Voltage:
50 to 90 volts, (to produce arc).
Closed Circuit Voltage:
18 to 25 volts, (to maintain arc
The A.C. welding machine has a step down transformer which receives current from
main A.C. supply. This transformer step down the voltage from 220 V-440V to normal
open circuit voltage of 80 to 100 volts. The current range available up to 400
amperes in the steps of 50 ampere.

The capacities of A.C. welding machine are:
Current Range:
Up to 400 ampere in steps of 50 ampere.
Input Voltage:
220V- 440V
Actual Required Voltage:
80 – 100 volts. Frequency: 50/60 HZ.
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ELECTRIC CURRENT FOR WELDING

SIGNIFICANCE OF POLARITY
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When D.C. current is used for welding, the following two types of polarity are
available:
(i)Straight or positive polarity.
(ii)Reverse or negative polarity.
When the work is made positive and electrode as negative then polarity is called
straight or positive polarity.
In straight polarity, about 67% of heat is distributed at the work (positive terminal)
and 33% on the electrode (negative terminal). The straight polarity is used where
more heat is required at the work. The ferrous metal such as mild steel, with faster
speed and sound weld, uses this polarity.

SIGNIFICANCE OF POLARITY
On the other hand, when the work is made negative and electrode as positive then
polarity is known as reverse or negative polarity, as shown in Fig. 7.16 (b).
In reverse polarity, about 67% of heat is liberated at the electrode (positive terminal)
and 33% on the work (negative terminal).
The reverse polarity is used where less heat is required at the work as in case of thin
sheet metal weld. The non-ferrous metals such as aluminum, brass, and bronze
nickel are welded with reverse polarity.
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Equipments Required for Electric Arc Welding
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The various equipment's required for electric arc welding are:
1. Welding Machine:
 The welding machine used can be A.C. or D.C. welding machine. The A.C. welding
machine has a step-down transformer to reduce the input voltage of 220- 440V to
80-100V.
 The D.C. welding machine consists of an A.C. motor-generator set or diesel/petrol
engine-generator set or a transformer-rectifier welding set.
 A.C. machine usually works with 50 hertz or 60 hertz power supply.
 The efficiency of A.C. welding transformer varies from 80% to 85%. The energy
consumed per Kg. of deposited metal is 3 to 4 kWh for A.C. welding while 6 to 10
kWh for D.C. welding.
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2. Electrode Holders:
The function of electrode holder is to hold the electrode at desired angle. These are
available in different sizes, according to the ampere rating from 50 to 500 amperes.
3. Cables or Leads:
The function of cables or leads is to carry the current from machine to the work.
These are flexible and made of copper or aluminum. The cables are made of 900 to
2000 very fine wires twisted together so as to provide flexibility and greater
strength.
The wires are insulated by a rubber covering, a reinforced fibre covering and further
with a heavy rubber coating.
4. Cable Connectors and Lugs:
The functions of cable connectors are to make a connection between machine
switches and welding electrode holder. Mechanical type connectors are used; as
they can he assembled and removed very easily. Connectors are designed
according to the current capacity of the cables used.
5. Chipping Hammer:
The function of chipping hammer is to remove the slag after the weld metal has
solidified. It has chisel shape and is pointed at one end.
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6. Wire Brush, Power Wire Wheel:
The function of wire brush is to remove the slag particles after chipping by chipping
hammer. Sometimes, if available a power wire wheel is used in place manual wire
brush.
7. Protective Clothing:
The functions of protective clothing's used are to protect the hands and clothes of
the welder from the heat, spark, ultraviolet and infrared rays. Protective clothing
used are leather apron, cap, leather hand gloves, leather sleeves, etc. The high
ankle leather shoes must be wear by the welder.
8. Screen or Face Shield:
The function of screen and face shield is to protect the eyes and face of the welder
from the harmful ultraviolet and infrared radiations produced during welding. The
shielding may be achieved from head helmet or hand helmet

ARC WELDING ELECTRODES
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Arc welding electrodes can be classified into two broad categories:
1.Non-Consumable electrodes.
2.Consumable electrodes.
1.Non-Consumable Electrodes:
These electrodes do not consumed during the welding operation, hence they named,
non-consumable electrodes. They are generally made of carbon, graphite or
tungsten. Carbon electrodes are softer while tungsten and graphite electrodes are
hard and brittle.
Carbon and graphite electrodes can be used only for D.C. welding, while tungston
electrodes can be used for both D.C. and A.C. welding. The filler material is added
separately when these types of electrodes are used. Since, the electrodes do not
consumed, the arc obtained is stable.
2.Consumable Electrodes:
These electrodes get melted during welding operation, and supply the filler material.
They are generally made with similar composition as the metal to be welded.

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Arc Welding
Consumable Electrodes
Mild Steel Electrodes Copper Electrodes

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Arc Welding
Non Consumable Electrodes
Carbon Electrodes Tungsten Electrodes

Advantages of Arc Welding
Some of the chief advantages of the electric arc welding are given as
follows −
The electric arc welding is the suitable welding process for high speed
welds.
Apparatus required for arc welding is very simple and portable.
The electric arc welding gives superior temperature at the point of welding.
Electric arc welding can work on both AC and DC supply.
It is inexpensive to install.
Disadvantages of Electric Arc Welding
The disadvantages of electric arc welding are as follows −
The welding process with electric arc welding requires skilled operators.
Electric arc welding cannot be used for welding of reactive metals such as
aluminium, titanium, etc.
Electric arc welding is not suitable for welding thin metals.

Applications of Electric Arc Welding
The important applications of electric arc welding are as
follows −
Electric arc welding is used in repairing of broken parts of
machines.
It is used for welding of cast iron or steel housings and
frames.
Electric arc welding is used in various industries such as
automotive industries, construction industries, mechanical
industries, etc.
Electric welding is also used for welding process in
shipbuilding.

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Resistance Welding
The welding process studied so far are fusion-welding processes
where only heat is applied in the joint. In contrast, resistance
welding process is a fusion-welding process where both heat
and pressure applied on the joint but no filler metal or flux is
added.
The heat necessary for the melting of the joint is obtained by the
heating effect of the electrical resistance of the joint and hence,
the name resistance welding.

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Resistance Welding
Principle
In resistance welding (RW), a low voltage (typically 1 V) and very
high current (typically 15000 A) is passed through the joint for a
very short time (typically 0.25 Sec). This high amperage heats the
joint, due to the contact resistance at the joint and melts it.
The pressure on the joint is continuously maintained and the
metal fuses together under this pressure.
The heat generated in resistance welding can be expressed as:
H = k I² R t

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Resistance Welding
H = k I² R t
Where,
H = the total heat generated in the work, J
I = electric current, A
R = the resistance of the joint, ohms
t = time for which the electric current is passing through the joint, Sec
k = a constant to account for the heat losses from the weld joint.
The resistance of the joint, R, is a complex factor to know because it is
composed of the
a) Resistance of the electrode,
b) Contact resistance between the electrode and the workpiece,
c) Contact resistance between the two workpiece plates, and
d) Resistance of the workpiece plates.

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Resistance Welding
1. The schematic representation of the resistance welding is shown
and the main requirement of the process is the low voltage and
high current power supply.
2. This is obtained by means of a step down transformer with a
provision to have different tappings on the primary side as
required for different materials.
3. The secondary windings are connected to the electrodes, which
are made of copper to reduce their electrical resistance.
4. The time of the electric supply needs to be closely controlled so
that the heat released is just enough to melt the joint and the
subsequent fusion takes place due to the force on the joint.

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Resistance Welding
5. The force required can be provided either mechanically,
hydraulically or pneumatically.
6. To precisely control the time, sophisticated electronic timers are
available.
7. The critical variable in a resistance welding process is the contact
resistance between the two workpiece plates and their
resistances themselves.
8. The contact resistance is affected by the surface finish on the
plates, since the rougher surfaces have higher contact resistance.

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Resistance Welding
9. The contact resistance also will be affected by the cleanliness of
the surface.
10. Oxides or other contaminants if present, should be removed
before attempting resistance welding.
11. The lower resistance of the joint requires very high currents to
provide enough heat to melt it.
12. The average resistance may be of the order of 100 micro ohms,
as a result, the current required would be of the order of tens of
thousands of amperes. With a 10 000 A current passing for 0.1
sec, the heat liberated is
H= (10 000)²(0.0001) (0.1) = 1000 J

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Resistance Welding
13. This is typical for the welding of 1-mm thick sheets.
14. The actual heat required for melting would be the order of 339 J.
15. The rest of the heat is actually utilized in heating the surrounding
areas and lost at other points.
16. The welding force used has the effect of decreasing the contact
resistance and consequently, an increase in the welding current
for the proper fusion.

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8 Types of Resistance Welding
Following are the 4 different types of resistance
welding:
1. Spot resistance welding
2. Projection resistance welding
3. Seam resistance welding
4. Flash or Butt resistance welding

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Resistance Welding
The Process of Resistance Welding

THERMIT WELDING
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Is a process that uses heat from an exothermic reaction to produce
coalescence between metals. The name is derived from 'thermite' the
generic name given to reactions between metal oxides and reducing
agents.

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Forge Welding
1. Forge welding (FOW) is a solid-state welding process that joins
two pieces of metal by heating them to a high temperature and
then hammering them together.
2. It may also consist of heating and forcing the metals together
with presses or other means, creating enough pressure to
cause plastic deformation at the weld surfaces.
3. The process is one of the simplest methods of joining metals and
has been used since ancient times.
4. Forge welding is versatile, being able to join a host of similar and
dissimilar metals.
5. With the invention of electrical and gas welding methods during
the industrial revolution, manual forge-welding has been largely
replaced, although automated forge-welding is a common
manufacturing process.

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Applications Forge Welding
The significant applications of weld forging in blacksmithing
include;
1. It is used to create a more substantial metal from smaller
pieces by allowing blacksmiths to join metal and steel.
2. It is particularly useful in the welding process of weapons like
swords.
3. It is crucial in creating architectural structures such as gates
and prison cells.
4. It is useful in the welding barrels of shotguns.
5. Forge welding is usually employed in the production of
various cookware.

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Advantages Forge Welding
The advantages of forge welding include;
1. It is relatively straightforward and less complicated.
2. It can easily be carried out by most blacksmiths because it
doesn’t cost much and requires only small pieces of metal.
3. Forge melting is sufficient to join both dissimilar and similar
metals.
4. The weld joint usually takes most of its properties from the
base material.
5. Forge welding of metals does not require any filler material
to be reliable.

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Disadvantages Forge Welding
The disadvantages of blacksmithing include;
1. It is not useful for mass production of materials.
2. It is preferable for steel and iron.
3. The forge welding process is relatively slow.

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Heat Affected Zone
The heat affected zone (HAZ) is a non-melted area of metal that has undergone
changes in material properties as a result of being exposed to high
temperatures. These changes in material property are usually as a result of
welding or high-heat cutting. The HAZ is the area between the weld or cut and
the base (unaffected), parent metal.
The HAZ area can vary in severity and size depending on the properties of the
materials, the concentration and intensity of the heat, and the welding or cutting
process used.

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Heat Affected Zone
What are the Causes of Heat-Affected Zones?
1. The heating associated with welding and/or cutting generally
uses temperatures up to and often exceeding the
temperature of melting of the material.
2. The heating and cooling thermal cycle associated with these
processes is different to whatever processing has occurred
with the parent material previously. This leads to a change in
microstructure associated with the heating and cooling
process.

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Heat Affected Zone
3. The size of a heat affected zone is influenced by the level of
thermal diffusivity, which is dependent on the thermal
conductivity, density and specific heat of a substance as well
as the amount of heat going in to the material.
4. Those materials with a high level of thermal diffusivity are
able to transfer variations of heat faster, meaning they cool
quicker and, as a result, the HAZ width is reduced.
5. On the other hand, those materials with a lower coefficient
retain the heat, meaning that that the HAZ is wider.
6. Generally speaking, the extension of the HAZ is dependent on
the amount of heat applied, the duration of exposure to heat
and the properties of the material itself. When a material is
exposed to greater amounts of energy for longer periods the
HAZ is larger.

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Heat Affected Zone
7. With regard to welding procedures, those processes with low
heat input will cool faster, leading to a smaller HAZ, whereas
high heat input will have a slower rate of cooling, leading to a
larger HAZ in the same material.
8. In addition, the size of the HAZ also grows as the speed of the
welding process decreases. Weld geometry is another factor
that plays a role in the HAZ size, as it affects the heat sink, and a
larger heat sink generally leads to faster cooling.
9. High temperature cutting operations can also cause a HAZ and,
similarly to welding procedures, those processes that operate at
higher temperatures and slow speeds tend to create a larger
HAZ, while lower temperature or higher speed cutting processes
tend to reduce the HAZ size.

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Heat Affected Zone
10. Different cutting processes have differing effects on the HAZ,
regardless of the material being cut.
11. For example, shearing and water jet cutting do not create a HAZ,
as they do not heat the material, while laser cutting creates a
small HAZ due to the heat only being applied to a small area.
12. Meanwhile, plasma cutting leads to an intermediate HAZ, with
the higher currents allowing for an increased cutting speed and
thereby a narrower HAZ, while oxyacetylene cutting creates the
widest HAZ due to the high heat, slow speed and flame width.
13. Arc welding falls between the two extremes, with individual
processes varying in heat input.

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Defects are common in any type of manufacturing, welding
including. In the process, there can be deviations in the
shape and size of the metal structure. It can be caused by the
use of the incorrect welding process or wrong welding
technique.
Welding Defects, Causes and Remedies

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Types of defects
Slag Inclusion
Undercut
Porosity
Incomplete fusion
Spatter
Underfill
Hot cracks
Cold Cracks

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Slag Inclusion
Slag inclusion is one of the welding defects that are usually easily
visible in the weld. Slag is a vitreous material that occurs as a
byproduct of stick welding, flux-cored arc welding and submerged
arc welding. It can occur when the flux, which is the solid shielding
material used when welding, melts in the weld or on the surface of
the weld zone.

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Causes:
1. Improper cleaning.
2. The weld speed is too fast.
3. Not cleaning the weld pass before starting a new one.
4. Incorrect welding angle.
5. The weld pool cools down too fast.
6. Welding current is too low.

98
Remedies:
1. Increase current density.
2. Reduce rapid cooling.
3. Adjust the electrode angle.
4. Remove any slag from the previous bead.
5. Adjust the welding speed.

99
Undercut
This welding imperfection is the groove formation at the weld toe,
reducing the cross-sectional thickness of the base metal. The
result is the weakened weld and workpiece.

100
Causes:
1. Too high weld current.
2. Too fast weld speed.
3. The use of an incorrect angle, which will direct more heat to
free edges.
4. The electrode is too large.
5. Incorrect usage of gas shielding.
6. Incorrect filler metal.
7. Poor weld technique.

101
Remedies:
1. Use proper electrode angle.
2. Reduce the arc length.
3. Reduce the electrode’s travel speed, but it also shouldn’t be
too slow.
4. Choose shielding gas with the correct composition for the
material type while welding.
5. Use of proper electrode angle, with more heat directed
towards thicker components.
6. Use of proper current, reducing it when approaching thinner
areas and free edges.
7. Choose a correct welding technique that doesn’t involve
excessive weaving.
8. Use the multipass technique

102
Porosity
Porosity occurs as a result of weld metal contamination. The
trapped gases create a bubble-filled weld that becomes weak and
can with time collapse.

103
Causes of porosity:
1. Inadequate electrode deoxidant.
2. Using a longer arc.
3. The presence of moisture.
4. Improper gas shield.
5. Incorrect surface treatment.
6. Use of too high gas flow.
7. Contaminated surface.
8. Presence of rust, paint, grease or oil

104
Remedies:
1. Clean the materials before you begin welding.
2. Use dry electrodes and materials.
3. Use correct arc distance.
4. Check the gas flow meter and make sure that it’s optimized as
required with proper with pressure and flow settings.
5. Reduce arc travel speed, which will allow the gases to escape.
6. Use the right electrodes.
7. Use a proper weld technique

105
Incomplete Fusion
This type of welding defect occurs when there’s a lack of proper
fusion between the base metal and the weld metal. It can also
appear between adjoining weld beads. This creates a gap in the
joint that is not filled with molten metal.

106
Causes:
1. Low heat input.
2. Surface contamination.
3. Electrode angle is incorrect.
4. The electrode diameter is incorrect for the material thickness
welding.
5. Travel speed is too fast.
6. The weld pool is too large and it runs ahead of the arc.

107
Remedies:
1. Use a sufficiently high welding current with the appropriate arc
voltage.
2. Before begin welding, clean the metal.
3. Avoid molten pool from flooding the arc.
4. Use correct electrode diameter and angle.
5. Reduce deposition rate.

108
Spatter
Spatter occurs when small particles from the weld attach
themselves to the surrounding surface. It’s an especially common
occurrence in gas metal arc welding. No matter how hard you try,
it can’t be completely eliminated. However, there are a few ways
you can keep it to a minimum.

109
Causes:
1. The running amperage is too high.
2. Voltage setting is too low.
3. The work angle of the electrode is too steep.
4. The surface is contaminated.
5. The arc is too long.
6. Incorrect polarity.
7. Erratic wire feeding.

110
Remedies:
1. Clean surfaces prior to welding.
2. Reduce the arc length.
3. Adjust the weld current.
4. Increase the electrode angle.
5. Use proper polarity.
6. Make sure you don’t have any feeding issues.

111
Incomplete Penetration
Incomplete penetration occurs when the groove of the metal is
not filled completely, meaning the weld metal doesn’t fully extend
through the joint thickness.

112
Causes:
1. There was too much space between the metal welding
together.
2. Moving the bead too quickly, which doesn’t allow enough
metal to be deposited in the joint.
3. Using a too low amperage setting, which results in the current
not being strong enough to properly melt the metal.
4. Large electrode diameter.
5. Misalignment.
6. Improper joint.

113
Remedies:
1. Use proper joint geometry.
2. Use a properly sized electrode.
3. Reduce arc travel speed.
4. Choose proper welding current.
5. Check for proper alignment.

114
1. Hot cracks: These can occur during the welding process or
during the crystallization process of the weld joint.
2. Cold cracks: These cracks appear after the weld has been
completed and the temperature of the metal has gone down.
They can form hours or even days after welding. It mostly
happens when welding steel. The cause of this defect is usually
deformities in the structure of steel.
3. Crater cracks: These occur at the end of the welding process
before the operator finishes a pass on the weld joint. They
usually form near the end of the weld. When the weld pool
cools and solidifies, it needs to have enough volume to
overcome shrinkage of the weld metal. Otherwise, it will form
a crater crack.

115
Causes of cracks:
1. Use of hydrogen when welding ferrous metals.
2. Residual stress caused by the solidification shrinkage.
3. Base metal contamination.
4. High welding speed but low current.
5. No preheat before starting welding.
6. Poor joint design.
7. A high content of sulfur and carbon in the metal.

116
Remedies:
1. Preheat the metal as required.
2. Provide proper cooling of the weld area.
3. Use proper joint design.
4. Remove impurities.
5. Use appropriate metal.
6. Make sure to weld a sufficient sectional area.
7. Use proper welding speed and amperage current.
8. To prevent crater cracks make sure that the crater is properly
filled.

Arc welding process that
produces coalescence of
metals by heating them
with a constricted arc
between an electrode and
the work piece
Plasma: A gaseous mixture
of positive ions, electrons
and neutral gas molecules.
Plasma ARC Welding

How Plasma Welding Works
Plasma
• Gas which is heated to an extremely high temperature and ionized so that it
becomes electrically conductive
• PAW process uses this plasma to transfer an electric arc to the work piece
• The metal to be welded is melted by the intense heat of the arc and fuses
together
Objective of PAW
• To increase the energy level of the arc plasma in a controlled manner
• This is achieved by providing a gas nozzle around a tungsten electrode
operating on direct current electrode negativity

Equipment
Power Supply
• A DC power source (generator or rectifier) having drooping characteristics
and open circuit voltage of 70 volts or above is suitable for PAW
• Rectifiers are generally preferred over DC generators
• High frequency generator and current limiting resistors used for arc ignition

Shielding gases
• Shields the molten weld from the atmosphere.
• Two inert gases or gas mixtures are employed.
• Argon(commonly used), Helium, Argon+Hydrogen and Argon+Helium, as inert
gases or gas mixtures.
• Helium is preferred where a broad heat input pattern and flatter cover pass is
desired.
• A mixture of argon and hydrogen supplies heat energy higher than when only
argon is used and thus permits higher arc alloys and stainless steels.
• Hydrogen, because of its dissociation into atomic form and thereafter
recombination generates temperatures above those attained by using argon or
helium alone.

7 - Applications
• Aerospace Industries
• Foodstuff and Chemical Industries
• Machine and Plant Construction
• Automobiles and Railways
• Ship Construction
• Tank Equipment and Pipeline Construction etc.

UNIT4-Welding processmechanical welding.ppt

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