Overview of joining methods

• Mechanical methods
 Screwed fasteners, bolts, rivets,

• Adhesive bonding

• Welding

• Brazing and Soldering

 Base metal does not fuse.
 Molten filler drawn into close-fit joints by capillary action
(surface tension forces).
 Brazing filler melts >4500 C, solder <4500 C
1
Welding
Welding is a process by which two materials, usually metals, are
permanently joined together by coalescence, which is induced by a
combination of temperature, pressure, and metallurgical conditions.

The particular combination of these variables can range from high

temperature with no pressure to high pressure with no increase in
temperature.
Welding (positive process)
Machining (negative process)
Forming, Casting (zero process)
Applications of welding

General applications

 The welding is widely used for fabrication of pressure vessels, bridges, building
structures, aircraft and space crafts, railway coaches and general applications
besides shipbuilding, automobile, electrical, electronic and defense industries, laying
of pipe lines and railway tracks and nuclear installations.

Specific components need welding for fabrication includes

1. Transport tankers for transporting oil, water, milk etc.

2. Welding of tubes and pipes, chains, LPG cylinders and other items.
3. Fabrication of Steel furniture, gates, doors and door frames, and body
4. Manufacturing goods such as refrigerators, washing machines, microwave
ovens and many other items of general applications
The requirement of the welding for specific area of the industry

Oil & Gas

1. Welding is used for joining of pipes, during laying of crude oil and gas pipelines, construction of
tankers for their storage and transportation. Offshore structures, dockyards, loading and unloading
cranes are also produced by welding.

Nuclear Industry
2. Spheres for nuclear reactor, pipe line bends, joining of pipes carrying heavy water require welding
for safe and reliable operations.

Defense industry
3. Tank body fabrication, joining of turret mounting to main body of tanks are typical examples of
applications of welding in defense industry.
Electronic industry
4. Electronic industry uses welding to limited extent e.g. joining leads of special transistors but other
joining processes such as brazing and soldering are widely used.
5. Soldering is used for joining electronic components to printed circuit boards (PCBs).
6. Robotic soldering is very common for joining of parts to printed circuit boards of computers,
television, communication equipment and other control equipment etc.
Electrical Industry
7. Components of both hydro and steam power generation system, such as penstocks, water control gates,
condensers, electrical transmission towers and distribution system equipment are fabricated by welding. Turbine
blades and cooling fins are also joined by welding.

Surface transport
8. Railway: Railway uses welding extensively for fabrication of coaches and wagons, repair of wheel, laying of new
railway tracks by mobile flash butt welding machines and repair of cracked/damaged tracks by thermite welding.
9. Automobiles: Production of automobile components like chassis, body and its structure, fuel tanks and joining of
door hinges require welding.

Aerospace Industry
10. Aircraft and Spacecraft: Similar to ships, aircrafts were produced by riveting in early days but with the
introduction of jet engines welding is widely used for aircraft structure and for joining of skin sheet to body.
11. Space vehicles which have to encounter frictional heat as well as low temperatures require outer skin and other
parts of special materials. These materials are welded with full success for achieving safety and reliability.

Ship Industry
12. Ships were produced earlier by riveting. Welding found its place in ship building around 1920 and presently all
welded ships are widely used. Similarly submarines are also produced by welding.

Construction industry
13. Arc welding is used for construction of steel building structures leading to considerable savings in steel and
money.
14. In addition to building, huge structures such as steel towers also require welding for fabrication.
Advantages and Limitation of Welding as a Fabrication Technique
Welding is mainly used for the production of comparatively simple shape components. It is the process of joining the
metallic components with or without application of heat, pressure and filler metal. Application of welding in
fabrication offers many advantages, however; it suffers from few limitations also.

Advantages of welding are enlisted below:

1. Permanent joint is produced, which becomes an integral part of work piece.

2. Joints can be stronger than the base metal if good quality filler metal is used.
3. Economical method of joining.
4. It is not restricted to the factory environment.

Disadvantages of welding are enlisted also below:

1. Labour cost is high as only skilled welder can produce sound and quality weld joint.
2. It produces a permanent joint which in turn creates the problem in dissembling if of sub-component required.
3. Hazardous fumes and vapours are generated during welding. This demands proper ventilation of welding area.
4. Weld joint itself is considered as a discontinuity owing to variation in its structure, composition and mechanical
properties; therefore welding is not commonly recommended for critical application where there is a danger of life.
Requirement for a high quality welding
1. A source of satisfactory heat and/or pressure,

2. A means of protecting or cleaning the metal, and

3. Caution to avoid, or compensate for, harmful

metallurgical effects.
Classification of welding processes
(i) Arc welding
• Carbon arc (iv) Thermit Welding
• Metal arc
(v) Solid State Welding
• Metal inert gas Friction
• Tungsten inert gas Ultrasonic
• Plasma arc Diffusion
• Submerged arc Explosive
(vi) Newer Welding
• Electro-slag Electron-beam
(ii) Gas Welding Laser
(vii) Related Process
• Oxy-acetylene
Oxy-acetylene cutting
• Air-acetylene Arc cutting
• Oxy-hydrogen Hard facing
Brazing
(iii) Resistance Welding
Soldering
• Butt
• Spot
• Seam
• Projection
Types of Weld Joints
• Butt joint
• Lap joint
• Corner joint
• Tee joint
• Edge joint

Welding positions
• Flat
• Vertical
• Horizontal
• Overhead

Butt joint edge preparation

• Straight
• Single V
• Double V
• Single U
• Double U
• Single J
• Double J
• Single bevel
• Double bevel
FUSION WELDING
• Melting together and coalescing of materials by means of heat usually
supplied by chemical or electrical means.
• Filler materials may or may not be used.
• Gas welding, Arc welding and High-energy-beam welding
Gas Welding
• Gas welding is a welding process that melts and joins metals by
heating them with a flame caused by a reaction of fuel gas and
oxygen.
• The most commonly used method is Oxyacetylene welding, due to its
high flame temperature.
• The flux may be used to deoxidize and cleanse the weld metal.
• The flux melts, solidifies and forms a slag skin on the resultant weld
metal.
(Red)
• Combustion of oxygen and acetylene (C2H2) in a welding torch produces a
temp. in a two stage reaction.
• In the first stage
C2 H 2 + O2 → 2CO + H 2+ Heat
This reaction occurs near the tip of the torch.
• In the second stage combustion of the CO and H2 and occurs just beyond the
first combustion zone.
2CO + O2 → 2CO2 + Heat
H2 + 1O2 → H2O + Heat
2
Oxygen for secondary reactions is obtained from the atmosphere.
• Exposer of the heated and molten metal to the various gases in the flame and
atmosphere makes it difficult to prevent contamination.

• Heat source is not concentrated, a large area of the metal is heated and
distortion is likely to occur.

• Flame welding is still quite common in field work, in maintenance and repairs,
and in fabricating small quantities of specialized products.
 Gas Flame Processes

Welding, Cutting and Straightening

• Oxy-fuel gas Welding (OFW): Heat source is the flame produced by the
combustion of a fuel gas and oxygen.

• OFW has largely been replaced by other processes but it is still popular
because of its portability and the low capital investment.

• Acetylene is the principal fuel gas employed.

Types of Flames
There are three types of flames in oxyacetylene welding:

Reducing flame –
• The excess amount of acetylene is used, giving a reducing flame. The combustion of acetylene is incomplete
(greenish) between the inner cone and the outer envelope.
• Good for welding aluminium alloys, high carbon steels.
• Oxygen is turned on, flame immediately changes into a long white inner area (Feather) surrounded by a transparent
blue envelope is called Carburizing flame (30000c)

Neutral flame –
• Acetylene (C2H2) and O2 are mixed in equal amounts and burn at the tip of the welding torch. The inner cone gives 2/3
of heat whereas the outer envelope provides 1/3 of the energy.
• Addition of little more oxygen give a bright whitish cone surrounded by the transparent blue envelope is called Neutral
flame (It has a balance of fuel gas and oxygen) (32000c)
• Used for welding steels, aluminum, copper and cast iron

Oxidizing flame –
• If more oxygen is added, the cone becomes darker and more pointed, while the envelope becomes shorter and more
fierce is called Oxidizing flame
• Has the highest temperature about 34000c
• Used for welding brass and brazing operation
Three types of flame in oxyacetylene welding
 Three types of flames can be obtained by varying the oxygen/acetylene (or oxygen/fuel gas)
ratio.
• If the ratio is about 1 : 1 to 1.15 : 1, all reactions are carried to completion and a neutral flame is
produced.
• Most welding is done with a neutral flame. It is chemically neutral and neither oxidizes or carburizes
the metal being welded.

• A higher ratio, such as 1.5 : 1, produces an oxidizing flame, hotter than the neutral flame (about
3300oC) but similar in appearance.
• Used when welding copper and copper alloys but harmful when welding steel because the excess
oxygen reacts with the carbon, decarburizing the region around the weld.

• Excess fuel, on the other hand, produces a carburizing flame. Carburizing flame can carburize metal
also.
• The excess fuel decomposes to carbon and hydrogen, and the flame temperature is not as great
(about 3000oC).
• Flames of this type are used in welding Monel (a nickel-copper alloy), high-carbon steels, and some
alloy steels, and for applying some types of hard-facing material.
Oxidising Flame

Carburising Flame

Neutral Flame
Types of Gas Welding
• Leftward Welding
• Rightward Welding
Metal Flame
MS N
High carbon steel R
Grey cast iron N, slightly oxidizing
Alloy steel N
Aluminium Slightly carburizing
Brass Slightly oxidizing
Copper, Bronze N, slightly oxidizing
Nickel alloys Slightly carburizing
Lead N
GAS WELDING EQUIPMENTS
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 depending upon the thickness of the work pieces welded.
3. Pressure Gauges
4. Hoses
5. Welding torch
6. Check valve
7. Non-return valve
Diagram
• Oxygen is stored in a cylinder at a pressure ranging from 13.8 MPa to 18.2 MPa
• Due to high explosiveness of free acetylene it is stored in a cylinder with 80-
85% porous calcium silicate and then filled with acetone which absorb upto
420 times by its volume at a pressure 1.75 MPa .
• At the time of acetylene release if acetone comes with acetylene the flame
would give a purple colour.
• Another option is acetylene generator.
 Uses, Advantages, and Limitations
• OFW is fusion welding.
• No pressure is involved.
• Filler metal can be added in the form of a wire or rod.
• Fluxes may be used to clean the surfaces and remove contaminating oxide.
The gaseous shield produced by vaporizing flux can prevent oxidation during
welding, and the slag produced by solidifying flux can protect the weld pool.
Flux can be added as a powder, the welding rod can be dipped in a flux paste,
or the rods can be pre-coated.

Contd…
Disadvantages

• Limited power density

• Very low welding speed
• High total heat input per unit length
• Large heat affected zone
• Severe distortion
• Not recommended for welding reactive metals such as titanium and
zirconium.
Pressure Gas Welding

• Pressure gas welding (PGW) or Oxyacetylene Pressure Welding is a process

used to make butt joints between the ends of objects such as pipe and-
railroad rail.
• The ends are heated with a gas flame to a temperature below the melting
point, and the soft metal is then forced together under considerable pressure.
• This process, therefore, is actually a 'form of solid-state welding.
 Oxygen Torch Cutting (Gas Cutting)
• Iron and steel oxidize (burn) when heated to a temperature between 8000C to
10000C.
• High-pressure oxygen jet (300 KPa) is directed against a heated steel plate, the
oxygen jet burns the metal and blows it away causing the cut (kerf).
• For cutting metallic plates shears are used. These are useful for straight-line
cuts and also for cuts up to 40 mm thickness.
• For thicker plates with specified contour, shearing cannot be used and oxy-fuel
gas cutting (OFC) is useful.
• Gas-cutting is similar to gas welding except torch tip.

Fig- differences in torch tips for gas welding and gas cutting
• Larger size orifice produces kerf width wider and larger oxygen consumed.
• At kindling temperature (about 870oC), iron form iron oxide.
• Reaction:
3Fe + 2O2 → Fe3O4 +6.67 MJ/kg of iron
The other reactions:
2Fe + O2 → 2FeO + 3.18 MJ/kg of iron
4Fe + 3O2 → 2Fe2O3 + 4.9 MJ/kg of iron
• All exothermic reactions preheat the steel.
• For complete oxidation 0.287 m3 oxygen/kg of iron is required
• Due to unoxidized metal blown away the actual requirement is much less.
• Torch tip held vertically or slightly inclined in the direction of travel.
• Torch position is about 1.5 to 3 mm vertical from plate.
• The drag lines shows the characteristics of the movement of the oxygen
stream.

Fig- positioning of cutting torch in oxy- fuel gas cutting

• Drag is the amount by which the lower edge of the drag line trails from the top
edge.
• Good cut means negligible drag.
Contd…
• If torch moved too rapidly, the bottom does not get sufficient heat and
produces large drag so very rough and irregular-shaped-cut edges.
• If torch moved slowly a large amount of slag is generated and produces
irregular cut.
• Gas cutting is more useful with thick plates.

• For thin sheets (less than 3 mm thick) tip size should be small. If small tips are
not available then the tip is inclined at an angle of 15 to 20 degrees.

Fig. Recommended torch position for cutting thin steel

 Application
• Useful only for materials which readily get oxidized and the oxides have lower
melting points than the metals.

• Widely used for ferrous materials.

• Cannot be used for aluminum, bronze, stainless steel and like metals since
they resist oxidation.
 Difficulties
• Metal temperature goes beyond lower critical temperature and structural
transformations occur.

• Final microstructure depends on cooling rate.

• Steels with less than 0.3 % carbon cause no problem.

Contd…
• For high carbon steel material around the cut should be preheated (about 250
to 300oC) and may post heat also necessary.

• Cutting CI is difficult, since its melting temp. is lower than iron oxide.

• If chromium and nickel etc are present in ferrous alloys oxidation and cutting is
difficult.
Arc welding is a method of permanently joining two or more
metal parts.
It consists of combination of different welding processes
wherein coalescence is produced by heating with an
electric arc, (mostly without the application of pressure)
and with or without the use of filler metals depending upon
the base plate thickness.

A homogeneous joint is achieved by melting and fusing the

adjacent portions of the separate parts. The final welded joint
has unit strength approximately equal to that of the base
material. The arc temperature is maintained approximately
4400°C.

A flux material is used to prevent oxidation, which decomposes

under the heat of welding and releases a gas that shields the
arc and the hot metal.
Shielded-Metal Arc (SMAW) or Stick Welding

This is an arc welding process wherein coalescence is produced by heating the

workpiece with an electric arc setup between a flux-coated electrode and the
workpiece. The electrode is in a rod form coated with flux. Figure illustrates the
process.
Submerged Arc Welding (SAW)

This is another type of arc welding process, in which coalescence is produced by

heating the workpiece with an electric arc setup between the bare electrode and the
work piece. Molten pool remains completely hidden under a blanket of granular material
called flux. The electrode is in a wire form and is continuously fed from a reel. Movement
of the weld gun, dispensing of the flux and picking up of surplus flux granules behind the
gun are usually automatic.
Flux-Cored Arc Welding (FCAW)

This process is similar to the shielded-arc stick welding process with the main
difference being the flux is inside the welding rod. Tubular, coiled and continuously fed
electrode containing flux inside the electrode is used, thereby, saving the cost of
changing the welding. Sometimes, externally supplied gas is used to assist in
shielding the arc.
Gas-Metal Arc Welding (GMAW)

In this process an inert gas such as argon, helium, carbon dioxide or a mixture of
them are used to prevent atmospheric contamination of the weld. The shielding gas is
allowed to flow through the weld gun. The electrode used here is in a wire form, fed
continuously at a fixed rate. The wire is consumed during the process and thereby
provides filler metal. This process is illustrated in Figure.
Gas-Tungsten Arc Welding (GTAW)

This process is also known as tungsten–inert gas (TIG) welding. This is similar to the
Gas-Metal Arc Welding process. Difference being the electrode is non consumable and
does not provide filler metal in this case. A gas shield (usually inert gas) is used as in the
GMAW process. If the filler metal is required, an auxiliary rod is used.

Plasma Arc Welding (PAW)

This process is similar to TIG. A non-

consumable electrode is used in this
process. Arc plasma is a temporary state
of gas. The gas gets ionized after the
passage of electric current and becomes
a conductor of electricity. The plasma
consists of free electrons, positive ions,
and neutral particles. Plasma arc welding
differs from GTAW welding in the amount
of ionized gas which is greatly increased
in plasma arc welding, and it is this
ionized gas that provides the heat of
welding.
Shielded metal arc welding (SMAW), also known as
manual metal arc (MMA) welding or informally as stick welding, is a
manual arc welding process that uses a consumable electrode coated in
flux to lay the weld.

• An electric current, in the form of either alternating current or direct current from a
welding power supply, is used to form an electric arc between the electrode and the
metals to be joined.

• As the weld is laid, the flux coating of the electrode disintegrates, giving off vapors
that serve as a shielding gas and providing a layer of slag, both of which protect the
weld area from atmospheric contamination.

• Because of the versatility of the process and the simplicity of its equipment and
operation, shielded metal arc welding is one of the world's most popular welding
processes. It dominates other welding processes in the maintenance and repair
industry, and though flux-cored arc welding is growing in popularity,

• SMAW continues to be used extensively in the construction of steel structures and

in industrial fabrication. The process is used primarily to weld iron and steels
(including stainless steel) but aluminum, nickel and copper alloys can also be
welded with this method
OPERATION
To strike the electric arc, the electrode is brought into contact with the
workpiece in a short sweeping motion and then pulled away slightly. This
initiates the arc and thus the melting of the workpiece and the
consumable electrode, and causes droplets of the electrode to be
passed from the electrode to the weld pool.

As the electrode melts, the flux covering disintegrates, giving off vapors
that protect the weld area from oxygen and other atmospheric gases. In
addition, the flux provides molten slag which covers the filler metal as it
travels from the electrode to the weld pool. Once part of the weld pool,
the slag floats to the surface and protects the weld from contamination
as it solidifies.
Once hardened, it must be chipped away to reveal the finished weld. As
welding progresses and the electrode melts, the welder must periodically
stop welding to remove the remaining electrode stub and insert a new
electrode into the electrode holder. This activity, combined with chipping
away the slag, reduce the amount of time that the welder can spend laying
the weld, making SMAW one of the least efficient welding processes.

SMAW weld area

The actual welding technique utilized depends on
• the electrode
• the composition of the workpiece
• the position of the joint being welded.

• The choice of electrode and welding position also determine the welding
speed.
• Flat welds require the least operator skill, and can be done with electrodes
that melt quickly but solidify slowly. This permits higher welding speeds.

• Sloped, vertical or upside-down welding requires more operator skill, and

often necessitates the use of an electrode that solidifies quickly to prevent
the molten metal from flowing out of the weld pool. However, this
generally means that the electrode melts less quickly, thus increasing the
time required to lay the weld.
 APPLICATIONS
Shielded metal arc welding is one of world's most popular welding processes. Because of its
versatility and simplicity, it is particularly dominant in the maintenance and repair industry,
and is heavily used in the construction of steel structures and in industrial fabrication.

In recent years its use has declined as flux-cored arc welding has expanded in the construction
industry and gas metal arc welding has become more popular in industrial environments.
However, because of the low equipment cost and wide applicability, the process will likely
remain popular, especially among amateurs and small businesses where specialized welding
processes are uneconomical and unnecessary.

SMAW is often used to weld carbon steel, low and high alloy steel, stainless steel, cast
iron, and ductile iron. While less popular for nonferrous materials, it can be used on nickel
and copper and their alloys and, in rare cases, on aluminum. The thickness of the material
being welded is bounded on the low end primarily by the skill of the welder, but rarely
does it drop below 0.05 in (1.5 mm). No upper bound exists: with proper joint preparation
and use of multiple passes, materials of virtually unlimited thicknesses can be joined.
Furthermore, depending on the electrode used and the skill of the welder, SMAW can be used
in any position.
EQUIPMENT
Shielded metal arc welding equipment typically consists of a constant current
welding power supply and an electrode, with an electrode holder, a work clamp,
and welding cables (also known as welding leads) connecting the two.

Schematic illustration of
SMAW.
Also known as stick
welding, because the
electrode is in the shape
of a stick.
 ELECTRODES

The choice of electrode for SMAW depends on a number of factors,

including the weld material, welding position and the desired weld
properties. The electrode is coated in a metal mixture called flux.

Electrode coating has the following basic functions:

- to improve the arc stability.

- to generate gases to act as a shield against the surrounding
atmosphere in order to prevent weld contamination.
- to control the rate at which the electrode melts.
- to act as a flux to protect the weld against the formation of oxides,
nitrides and other inclusion and to protect molten–weld pool.
- to add alloying elements to the weld zone to enhance the properties of
the joint.
 Manual Metal Arc Welding
AWS A5.1 classification

E XXXX ‐ H
Hydrogen level (HmR)
Tensile Strength H = 5 ml / 100g of WM
in KPSI R = low moisture pick‐up

Useable positions Flux type

1=all positions 20 = Acidic (iron oxide)
2=flat + horizontal 10, 11 = Cellulosic
4=vertical down 12, 13 = Rutile
24 = Rutile + iron powder
27 = Acidic + iron powder
16 = basic
18, 28 = basic + iron powder
Classification of Electrodes as per Indian Standard:

Structural steel electrodes were classified as per IS 814:1974 and this code was
revised and the revised code is IS 814:1991.
The corresponding code is given on each packet of electrode.

IS 815:1974
As per IS 815 electrodes are designated with letters and digits.
PXXXXXXS
• Prefix (P) is either E or R which indicates solid extruded (E) or reinforced extruded
(R) Electrode.
• 1 st digit – Indicates type of coating.
• 2 nd digit – Indicates weld positions in which electrode can be used.
• 3 rd digit – Indicates welding current conditions.
• 4 th and 5 th digit – Indicate UTS and YS of all weld metal.
• 6 th digit – Requirement of minimum % elongation and absorbed energy in
charpy V- notch impact test of weld metal.
Suffix (s) – P – Deep penetration electrode
H – Hydrogen controlled electrode
J, K and L – Amount of metal recovery in case of iron powder electrode
Suffix (s) are optional and may or may not be given if not applicable.
IS 814:1991
As per IS 814 electrodes are designated with letters and digits as given below:
ELXXXXS
In this code
• E indicates extruded solid electrode,
• L is a letter to designate type of coating,
• first digit indicates UTS and YS of deposited weld metal,
• second digit gives percentage elongation and impact values of weld metal
deposited,
• third digit gives welding positions in which electrode can be used and fourth digit
gives the current conditions for the use of electrode.
• Suffix(s) are optional and indicate special characteristics of electrode such as H1,
H2, and H3 indicate hydrogen controlled electrodes with different amount of
diffusible hydrogen J, K, L indicate different amount of metal recovery in weld pool
in case of iron powder electrodes and X means radiographic weld quality.

Note: For details see the above codes published by Bureau of Indian Standards (BIS),
Manak Bhawan, Bahadur Shah Jafar Marg, New Delhi .
Electrodes can be divided into three groups—
• those designed to melt quickly are called "fast-fill“ electrodes
• those designed to solidify quickly are called "fast-freeze" electrodes
• intermediate electrodes go by the name "fill-freeze" or "fast-follow"
electrodes.

Fast-fill electrodes are designed to melt quickly so that the welding speed
can be maximized, while fast-freeze electrodes supply filler metal that
solidifies quickly, making welding in a variety of positions possible by
preventing the weld pool from shifting significantly before solidifying.
Power sources for welding
• AC power sources
• DC power sources
Welding Power Source Characteristic

33
Power Sources for Welding
AC Arc Welding Power Source
• Shielded (Manual ) Metal Arc Welding
• Electro-Slag Welding
• TIG for Aluminum Alloys (cleaning action)
• Submerged Arc Welding

DC Arc Welding Power Source

• MIG/MAG Welding
• Electro-Gas Arc Welding
• CO2 Gas Arc Welding with Flux Cored Wire
• Self Shielded Arc Welding
• TIG for Steel • Plasma Welding and Cutting
• Stud Welding
• Submerged Arc Welding with small diameter wire
Electromagnetic attractive force causes the cross section of the arc to shrink –
Electromagnetic Pinch Effect.
Arc also shrinks to reduce its surface area to suppress heat loss when the arc
is cooled from ambient – Thermal Pinch Effect
ARC BLOW
SHIELDED/MANUAL METAL ARC WELDING

SMAW or MMAW can use both AC and DC.

• The constant current DC power source is invariably used with all types of
electrode (basic, rutile and cellulosic) irrespective of base metal (ferrous and
non-ferrous).
• However, AC can be unsuitable for certain types of electrodes and base
materials. Therefore, AC should be used in light of manufacturer’s
recommendations for the electrode application.
In case of DC welding, heat liberated at anode is generally greater than the arc
column and cathode side. The amount of heat generated at the anode and
cathode may differ appreciably depending upon the flux composition of coating,
base metal, polarity and the nature of arc plasma.
In case of DC welding, polarity determines the distribution of the heat generated
at the cathode and anode and accordingly the melting rate of electrode and
penetration into the base metal are affected.

Heat generated by a welding arc (J) = Arc voltage (V) X Arc current (A) X Welding time (s)

If arc is moving at speed S (mm/min) then net heat input is calculated as:

Hnet= VI (60)/(S X 1000) kJ/mm

• SMAW normally uses constant current type of power source with welding
current 50-600A and voltage 20-80V at 60% duty cycle.

• Welding transformer (AC welding) and generator or rectifiers (DC welding)

are commonly used as welding power sources.

• In case of AC welding, open circuit voltage (OCV) is usually kept 10- 20%
higher than that for DC welding to overcome the arc un-stability related
problems due to fact that in case AC both current magnitude and direction
changes in every half cycle while those remain constant in DC.

• OCV setting is primarily determined by factors like type of welding current

and electrode composition which significantly affect the arc stability.

• Presence of low ionization potential elements (Ca, K) in coating and reduce

the OCV required for stable arc.
• SMAW or MMAW – simplest of all
• Portable equipment, low cost
• Large no. of welding as electrodes availability
• Mostly all position welding (FSMAW)

Limitations:
o Mechanization difficult as length of each electrode
limited, brittle flux coating.
o Changing of electrode time taking.
o Slower than other arc weldings
o Chances of slag entrapment
SUBMERGED ARC WELDING (SAW)
• Submerged arc welding (SAW) process uses heat generated by an
electric arc established between a bare consumable electrode wire and
the work piece.
• Since in this process, welding arc and the weld pool are completely
submerged under cover of granular fusible and molten flux therefore it is
called so.
• During welding, granular flux is melted using heat generated by arc and
forms cover of molten flux layer which in turn avoids spatter tendency
and prevents accessibility of atmospheric gases to the arc zone and the
weld pool.
• The molten flux reacts with the impurities in the molten weld metal to
form slag which floats over the surface of the weld metal.
• Layer of slag over the molten weld metal results:

(i) Increased protection of weld metal from atmospheric gas contamination

and so improved properties of weld joint
(ii) Reduced cooling rate of weld metal and HAZ owing to shielding of the
weld pool by molten flux and solidified slag in turn leads to
a) smoother weld bead and
b) reduced the cracking tendency of hardenable steel
SAW is known to be a high current (sometimes even greater 1000A) welding process that is
mostly used for joining of heavy sections and thick plates as it offers deep penetration with high
deposition rate and so high welding speed.

High welding current can be applied in this process owing to three reason a) absence of spatter,
b) reduced possibility of air entrainment in arc zone as molten flux and slag form shield the weld
metal and c) large diameter electrode.

Continuous feeding of granular flux around the weld arc from flux hopper provides shielding to the
weld pool from atmospheric gases and control of weld metal composition through presence of
alloying element in flux. Complete cover of the molten flux around electrode tip and the welding
pool during the actual welding operation produces weld joint without spatter and smoke.
Components of SAW System

 Power source

Generally, submerged arc welding process uses power source at 100 %

duty cycle; which means that the welding is done continuously for minimum
5 min without a break or more.

Depending upon the electrode diameter, type of flux and electrical resistivity
submerged arc welding can work with both AC and DC.

Alternating current and DCEN polarity are generally used with large
diameter electrode (>4mm).

DC with constant voltage power source provides good control over bead
shape, penetration, and welding speed. However, DC can cause arc blow
under some welding conditions. Polarity affects weld bead geometry,
penetration and deposition rate. DCEP offers advantage of self regulating
arc in case of small diameter electrodes (< 2.4mm) and high deposition rate
while DCEN produces shallow penetration.
 Welding Electrode

The diameter of electrodes used in submerged arc welding

generally ranges from 1–5 mm.

The electrode wire is fed from the spool through a contact tube
connected to the power source.

Electrode wire of steel is generally copper coated for two reasons

a) to protect it from atmospheric corrosion and b) to increase
their current carrying capacity. However, stainless steel wires are
not coated with copper.
 SAW Flux
Role of fluxes in SAW is largely similar that of coating in stick electrodes of
SMAW i.e. protection of weld pool from inactive shielding gases generated
by thermal decomposition of coating material.

SAW fluxes can influence the weld metal composition appreciably in the form
of addition or loss of alloying elements through gas metal and slag metal
reactions. Few hygroscopic fluxes are baked (at 250–3000C for 1-2 hours) to
remove the moisture.

There are four types of common SAW fluxes namely fused flux,
agglomerated flux, bonded flux and mechanical fluxes. Manufacturing
steps of these fluxes are given below.

• Fused fluxes: raw constituents-mixed-melted-quenched-crushed screened-

graded
• Bonded fluxes: raw constituents-powdered-dry mixed-bonded using K/Na
silicates-wet mixed-pelletized-crushed-screened
• Agglomerated fluxes: made in similar way to bonded fluxes but ceramic
binder replaces silicate binder
• Mechanically mixed fluxes: mix any two or three type of above fluxes in
desired ratios
• Fused fluxes

Positives
– Uniformity of chemical composition
– No effect of removal of fine particles on flux composition
– Non-hygroscopic: easy handling and storage
– Easy recycling without much change in particle size and composition

Limitation is related with difficulty in – incorporating deoxidizers and ferro alloys

– melting due to need of high temperature

• Bonded fluxes

Positives
– Easy to add deoxidizers and alloying elements
– Allows thicker layer of flux during welding
Limitation
– Hygroscopic
– Gas evolution tendency
– Possibility of change in flux composition due to removal of fine particles
Agglomerated fluxes
These are similar to that of bonded fluxes except that these use ceramic binders

Mechanical fluxes

Positives
– Several commercial fluxes can be easily mixed & made to suit critical application
to get desired results

Limitations
– Segregation of various fluxes during storage / handling in feeder and recovery
system
- inconsistency in flux from mix to mix
The fused and agglomerated types of fluxes usually consist of different
types of halides and oxides such as MnO, SiO2, CaO, MgO, Al2O3, TiO2,
FeO, and CaF2 and sodium/potassium silicate. Halide fluxes are used for
high quality weld joints of high strength steel to be used for critical
applications while oxide fluxes are used for developing weld joints of
non-critical applications.

Some of oxides such as CaO, MgO, BaO, CaF2, Na2O, K2O, MnO etc. are
basic in nature (donors of oxygen) and few others such as SiO2, TiO2,
Al2O3 are acidic (acceptors of oxygen). Depending upon relative amount
of these acidic and basic fluxes, the basicity index of flux is decided.

The basicity index of flux is ratio of sum of (wt. %) all basic oxides to all
non-basic oxides. Basicity of flux affects the slag detachability, bead
geometry, mechanical properties and current carrying capacity as
welding with low basicity fluxes results in high current carrying
capacity, good slag detachability, good bead appearance and poor
mechanical properties and poor crack resistance of the weld metal
while high basicity fluxes produce opposite effects on above
characteristics of the weld.
General Methods:

The SAW can be applied in 3 different modes: Semi-automatic,

Automatic and Machine.

Each method requires the work be positioned so that flux and

the molten weld pool will remain in place until they have
solidified. Fixtures and positioning equipment can be used for
typical requirements.
• Welding parameters

Electrode wire size

Welding voltage
Welding current
welding speed

4 most important parameters (apart from flux) that play a

major role on soundness and performance of the weld
therefore these must be selected carefully before welding.
an increase in size of the electrode decreases the depth of penetration and
increases width of weld bead for a given welding current (Fig). Large
diameter electrodes are primarily selected to take two advantages a) higher
deposition rate owing to their higher current carrying capacity and b) good
gap bridging capability under poor fit-up conditions of the plates to be
welded due to wider weld bead.
Limitations
There are three main limitations of SAW

a) invisibility of welding arc during welding,

b) difficulty in maintaining mound of the flux cover around the arc in odd positions of
welding and cylindrical components of small diameter and
c) increased tendency of melt through when welding thin sheet.

Invisibility of welding arc submerged under un‐melted and melted flux cover in SAW makes
it difficult to ensure the location where weld metal is being deposited during welding.
Therefore, it becomes mandatory to use an automatic device (like welding tractors) for
accurate and guided movement of the welding arc in line with weld groove so that weld
metal is deposited correctly along weld line only.

Applications of SAW process are mainly limited to flat position only as developing a mound
of flux in odd position to cover the welding arc becomes difficult which is a requisite for
SAW. Similarly, circumferential welds are difficult to develop on small diameter components
due to flux falling tendency away from weld zone. Plates of thickness less than 5 mm are
generally not welded due to risk of burn through.
SAW Applications:

The process is very commonly used in joining the two deep drawn vessels of the
liquefied gas cylinder bodies.

Some other applications of SAW are in the welding of:

• High strength low alloy steels

• Low carbon steels, Stainless steels, Aluminum and Titanium alloys
• Other non-ferrous alloys
• Fabrication of thick plates and thick pipes
• Pressure vessels and heat exchangers,
• Rail road tanks and ship body fabrication
• GAS TUNGSTEN ARC WELDING (GTAW)

Gas Tungsten Arc welding uses

a) electrode primarily made of tungsten and
b) inert gas for shielding the weld pool to prevent
its contamination from atmospheric gases
especially when joining high strength reactive
metals and alloys such as stainless steel,
aluminum and magnesium alloys, wherever high
quality weld joints need to be developed for
critical applications like nuclear reactors, aircraft
etc.
Invention of this process in middle of twentieth century gave a big boost to
fabricators of these reactive metals as none of the processes (SMAW and
Gas welding) available at that time were able to weld them successfully
primarily due to two limitations a) contamination of weld from atmospheric
gases and b) poor control over the heat input required for melting
There are four basic components of TIG welding system

a) DC/AC power source to deliver the welding current as per

needs,

b) welding torch (air/water cooled) with tungsten electrode and

gas nozzle,

c) inert shielding gas (He, Ar or their mixture) for protecting the

molten weld pool contamination from atmospheric gases and

d) controls for moving the welding torch as per mode of

operation (manual, semi-automatic and automatic).
Power source

• TIG welding normally uses constant current type of power source with welding
current ranging from 3-200A or 5-300A or higher and welding voltage ranging from
10-35V at 60% duty cycle.

• Pure tungsten electrode of ball tip shape with DCEN provides good arc stability.

• Moreover, thorium, zirconium and lanthanum modified tungsten electrodes can be

used with AC and DCEP as coating of these elements on pure tungsten electrodes
improves the electron emission capability which in turn enhances the arc stability.

• TIG welding with DCEP is preferred for welding of reactive metals like aluminum to
take advantage of cleaning action due to development of mobile cathode spots in
work piece side during welding which loosens the tenacious alumina oxide layer.
This helps to clean the weld pool. DCEN polarity is used for welding of metal such
as carbon steel that don’t require much cleaning.
TIG Arc Initiation

Direct work piece touch start method of initiating TIGW arc is not considered
as a good approach because it generally leads to many undesirable effects

a) contamination of tungsten electrode,

b) partial melting of electrode tip (due to short circuiting) so reduction in life
of the electrode and
c) formation of tungsten inclusions which deteriorate the mechanical
performance of weld joint.

Therefore, alternative methods of TIG arc initiation have been developed

over the years so as to avoid undesirable effects of touch start method.
Three methods are commonly used for initiating TIG welding arc a) use of
carbon block as scrap material, b) use of high frequency high voltage unit
and c) use of low current pilot arc.
Pilot arc method

Pilot arc method is based on the principle of using low current for initiating
the arc to reduce adverse effects of high heat generation in form of electrode
contamination and electrode melting during the arc initiation. For this
purpose, an additional power source can be used to strike the arc between
the tungsten electrode and auxiliary anode (fitted in nozzle) using low current
called pilot arc. This pilot arc is then brought close to base metal to be
welded so as to ignite the main arc between electrode and work piece. Once
the main arc is established auxiliary power source is taken off.
Maintenance of TIG welding arc

Arc maintenance in TIG welding with DC power supply does not create any problem.
However, in case of AC TIG welding, to have smooth and stable welding arc,
methods like use of high OCV, imposing the high frequency and high voltage pulse at
the moment when current is zero can be used so that arc is not extinguished.

Pulse TIG Welding

Pulse TIG is a variant of tungsten inert gas welding. In this process, welding current
is varied between a high and a low level at regular time intervals. This variation in
welding current between high and low level is called pulsation of welding current.
High level current is termed as peak current and is primarily used for melting of faying
surfaces of the base metal while low current is generally called background current
and it performs two functions
1) maintenance of the welding arc while generating very low heat and
2) allows time for solidification of the weld pool by dissipating the heat to base metal.

This feature of current pulsation associated with this process effectively reduces net
heat input to the base metal during welding which in turn facilitates a) easy welding
especially of thin sheets and b) refinement of grain structure of the weld. Reduction in
net heat input using arc pulsation decreases undesirable effects of comparatively
high heat input of conventional TIG welding such as melt through, wrapping/buckling
and fit-up.
Welding Torch

• TIG welding torch includes three main parts namely non-consumable tungsten
electrode, collets and nozzle.
• A collet is primarily used to hold the tungsten electrodes of varying diameters in
position.
• Nozzle helps to form a firm jet of inert gas around the arc, weld pool and the tungsten
electrode. The diameter of the gas nozzle must be selected in light of expected size
of weld pool so that proper shielding of the weld pool can be obtained by forming
cover of inert gas.
• The gas nozzle needs to be replaced at regular interval as it is damaged by wear and
tear under the influence of intense heat of the welding arc. Damaged nozzle does not
form uniform jet of inert gas around the weld pool for protection from the atmospheric
gases.
• Typical flow rate of shielding inert gas may vary from 5-50 liters/min.

• TIG welding torch is generally rated on the basis of their current carrying capacity as
it directly affects the welding speed and so the production rate. Depending upon the
current carrying capacity, the welding torch can be either water or air cooled. Air
cooled welding torch is generally used for lower range of welding current than water
cooled torches.
Filler wire

• Filler metal is generally not used for welding thin sheet by TIGW. Welding of thick steel plates
by TIG welding to produce high quality welds for critical applications such as joining of
nuclear and aero-space components, requires addition of filler metal to fill the groove.

• The filler wire can be fed manually or using some wire feed mechanism. For feeding small
diameter filler wires (0.8-2.4mm) usually push type wire feed mechanism with speed control
device is used.

• Selection of filler metal is very critical for successful welding because in some cases even
use of filler metal similar to that base metal causes cracking of weld metal especially when
their solidification temperature range is every wide (>50oC).

• Therefore, selection of filler wire should be done after giving full consideration to the following
aspects such as mechanical property requirement, metallurgical compatibility, cracking
tendency of base metal under welding conditions, fabrication conditions etc.

• For welding of aluminium alloys, Al-(5-12wt.%) Si filler is used as general purpose filler
metal. Al-5%Mg filler is also used for welding of some aluminium alloys.

• Welding of dissimilar steels namely stainless steel with carbon or alloy steels for high
temperature applications needs development of buttering layer before welding for reducing
carbon migration and residual stress development related problems.
Shielding gas

Helium, Argon and their mixtures are commonly used as inert shielding gas for
protecting the weld pool depending upon the metal to be welded, criticality of
application and economics.

Helium or hydrogen is sometimes added (1-2%) in argon for specific purposes such
as increasing the arc voltage and arc stability which in turn helps to increase the
heat of arc.

The selection of inert gases to be used as shielding gas in GTAW and GMAW
process depends upon the type of metal to be welded and criticality of their
applications.

Carbon dioxide is not used with GTAW process, at high temperature in arc
environment, the thermal decomposition of the carbon dioxide produces CO and O2.
Generation of these gases adversely affect the quality and soundness of the weld
joint and reduces the life of tungsten electrode.
Inert Gases

Argon and helium are the mostly commonly used shielding gases for developing high
quality weld joints of reactive and ferrous metals. Small amount of hydrogen or helium
is often added in argon to increase the penetration capability and welding speed.

A. Heat of welding arc

The ionization potential of He (25eV) is higher than Ar (16eV). Therefore, application
of He as shielding gas results in higher arc voltage and hence different VI arc
characteristics of arc than when argon is used as shielding gas. In general, arc voltage
generated by helium for a given arc length during welding is found higher than argon.
This results in hotter helium arc than argon arc. Hence, helium is preferred for the
welding of thick plates at high speed especially metal systems having high thermal
conductivity and high melting point.

B. Arc efficiency
Helium offers higher thermal conductivity than argon. Hence, He effectively transfers
the heat from arc to the base metal which in turn helps in increasing the welding speed
and arc efficiency.
C. Arc stability
He is found to offer more problems related with arc stability and arc initiation than Ar as
a shielding gas. This behaviour is primarily due to higher ionization potential of He than
Ar. High ionization potential of helium means it will result in presence of fewer charged
particles between electrode and work piece required for initiation and maintenance of
welding arc. Therefore, arc characteristics are found to be different for Ar and He.

With argon as shielding gas the welding current corresponding to the lowest arc
voltage is found around 50A while that for helium occurs at around 150A (Fig.).

Reduction in welding current below this critical level (up to certain range) increases the
arc voltage; which permits some flexibility in arc length to control the welding operation.
D. Flow rate of shielding gas

Argon (density 1.783g/l) is about 1.33 and 10 times heavier than the air and the helium
respectively. This difference in density of air with shielding gases determines the flow
rate of particular shielding gas required to form a blanket over the weld pool and arc
zone to provide protection against the environmental attack. Helium being lighter than
air tends to rise up immediately in turbulent manner away from the weld pool after
coming out of the nozzle. Therefore, for effective shielding of the arc zone, flow rate of
helium (12-22 l/min) must be 2-3 times higher than the argon (5-12 l/min).

Flow rate of shielding gas to be supplied for effective protection of weld pool is
determined by the size of molten weld pool, sizes of electrode and nozzle, distance
between the electrode and work piece, extent of turbulence being created ambient air
movement (above 8-10km/hr). For given welding conditions and welding torch,
flow rate of the shielding gas should be such that it produces a jet of shielding gas so
as to overcome the ambient air turbulence and provides perfect cover around the weld
pool. Unnecessarily high flow rate of the shielding gas leads to poor arc stability and
weld pool contamination from atmospheric gases due to suction effect.
Advantages of Ar over He as Shielding Gas

For general, purpose quality weld, argon offers many advantages over helium a) easy
arc initiation, b) cost effective and good availability c) good cleaning action with
(AC/DCEP in aluminium and magnesium welding) and d) shallow penetration
required for thin sheet welding of aluminium and magnesium alloys.
Plasma Arc Welding (PAW)
It is a fusion welding process wherein the coalescence is produced by
heating the work with a constricted arc established between a non
consumable tungsten electrode and work piece or between a non
consumable electrode and constricted nozzle. The shielding of the weld pool
is obtained by the hot ionized gas produced by passing inert gas through the
arc and constricted nozzle. Filler material may or may not be applied.

Low velocity plasma and diffused arc is generated in the TIG welding while in
case of PAW very high velocity and coherent plasma is generated. Large
surface area of the arc exposed to ambient air and base metal in case of TIG
welding causes greater heat losses than PAW and lowers the energy density.
Therefore, TIG arc burns at temperature lower than plasma arc.
Principles of Operation:
In the PAW process, the workpiece is cleaned and edges are prepared. An arc is
established between a non consumable tungsten electrode and workpiece or between
a non consumable electrode and constricted nozzle. An inert gas is passed through
the inner orifice surrounding the tungsten electrode and subsequently the gas is
ionized and conducts electricity. This state of ionized gas is known as plasma. The
plasma arc is allowed to pass through the constricted nozzle causing high energy and
current density. Subsequently high concentrate heat and very high temperatures are
reached. The low flow rate (0.25 to 5 l/min) of the orifice gas is maintained as
excessive flow rate may cause turbulence in the weld pool. However the orifice gas at
this flow rate is insufficient to shield the weld pool effectively. Therefore inert gas at
higher flow rate (10-30 l/min) is required to pass through outer gas nozzle surrounding
the inner gas nozzle to protect the weld pool.
Plasma arc welding is of two types: Non-transferred plasma arc welding process and
transferred arc welding process. In the former, the arc is established between the
electrode and the nozzle and in the latter process the arc is established between the
electrode and the workpiece.

Depending upon the current, plasma gas flow rate, and the orifice diameter
following variants of PAW has been developed such as:
• Micro-plasma (< 15 Amperes)
• Melt-in mode (15–400 Amperes) plasma arc
• Keyhole mode (>400 Amperes) plasma arc
Equipment and Consumables:

Power source: A conventional DC current power supply with drooping V-I

characteristics is required. Both rectifier or generator type power source may
be used; however, rectifier type power source is preferred. The general
range of the open-circuit voltage and current is 60-80V and 50-300A
respectively.

Plasma torch: It consists of non consumable tungsten electrode, inner

nozzle (constricting nozzle) and outer gas nozzle. The torch is water cooled
to avoid heating of the nozzle. It is of two types: transferred arc and non
transferred arc welding torch.

Filler material and shielding gases:

Filler material used in this process is the same as those used in the TIG and
MIG welding processes. The selection of the gases depends upon the
martial to be welded. The orifice gas must be an inert gas to avoid
contamination of the electrode material. Active gas can be used for shielding
provided it does not affect the weld quality. In general, the orifice gas is the
same as the shielding gas.
Applications of PAW:
This process is comparatively new and hence the potential of the process is yet to be
understood/ accepted. This process can be used to join all the materials those can be
welded by welding TIG process. Present applications of the process include:
1) Piping and tubing of stainless and titanium,
2) Submarine, aeronautical industry and jet engine manufacturing,
3) Electronic components.
Advantages of PAW:
1) Welding speed is higher.
2) Penetration is more.
3) Higher arc stability.
4) The distance between torch and workpiece does not affect heat concentration on
the work up to some extent.
5) Addition of filler material is easier than that of TIG welding process.
6) Thicker job can be welded.
7) Higher depth to width ratio is obtained resulting in less distortion.
Disadvantages of PAW:
1) Higher radiations.
2) Noise during welding.
3) Process is complicated and requires skilled manpower.
4) Gas consumption is high.
5) Higher equipment and running cost.
6) Higher open circuit voltage requiring higher safety measures to take.
 GAS METAL ARC WELDING (GMAW)

• Metal Inert Gas (MIG) Welding

• Metal Active Gas (MAG) Welding
CO2 welding or MAG welding

• CO2 used as shielding gas

• Used for welding carbon and low-alloy steels
• Produces deeper penetration than argon
• Overcomes the restriction of using small lengths of electrodes as in SMAW and the
inability of SAW to welds in various positions.

During the welding operation when CO2 is exposed to the high temperature of the
electric arc, it decomposed into CO and O2 and molecular oxygen changes to its atomic
form. This increases the amount of oxygen in the weld zone and atomic oxygen being
very active may react with iron and other alloying elements of base metal.

• When CO2 is used as shielding gas, the electrode wire must contain deoxidizers like
Mn, Si that readily combine with oxygen and prevent it from combining with weld
metal. SiO2, MnO pass into slag.
METAL INERT GAS (MIG) WELDING
This process is based on the principle of developing weld by melting faying surfaces
of the base metal using heat produced by a welding arc established between base
metal and a consumable electrode. Welding arc and weld pool are well protected by
a jet of shielding inactive gas coming out of the nozzle and forming a shroud around
the arc and weld.

MIG weld is not considered as clean as TIG weld. Difference in cleanliness of the
weld produced by MIG and TIG welding is primarily attributed to the variation in
effectiveness of shielding gas to protect the weld pool in case of above two
processes.

Effectiveness of shielding in two processes is mainly determined by two

characteristics of the welding arc namely stability of the welding arc and
length of arc besides other welding related parameters such as type of
shielding gas, flow rate of shielding gas, distance between nozzle and work-
price.
The MIG arc is relatively longer and less stable than TIG arc. Difference in stability of
two welding arcs is primarily due to the fact that in MIG arc is established between
base metal and consumable electrode (which is consumed continuously during
welding) while TIG welding arc is established between base metal and non-
consumable tungsten electrode. Consumption of the electrode during welding slightly
decreases the stability of the arc. Therefore, shielding of the weld pool in MIG
welding is not as effective as in TIG welding.

Metal inert gas process is similar to TIG welding except that it uses the automatically
fed consumable electrode therefore it offers high deposition rate and so it suits for
good quality weld joints required for industrial fabrication. Consumable electrode is
fed automatically while torch is controlled either manual or automatically. Therefore,
this process is found more suitable for welding of comparatively thicker plates of
reactive metals (Al, Mg, Stainless steel). The quality of weld joints of these metals
otherwise is adversely affected by atmospheric gases at high temperature.
B)
Power source for MIG welding

Depending upon the electrode diameter, material and electrode extension required, MIG
welding may use either constant voltage or constant current type of the welding power
source. For small diameter electrodes (< 2.4 mm) when electrical resistive heating controls
the melting rate predominantly, constant voltage power source (DCEP) is used to take
advantage of the self regulating arc whereas in case of large diameter electrode constant
current power source is used with variable speed electrode feed drive system to maintain
the arc length
Shielding gases for MIG welding

Like TIG welding, shielding gases such as Ar, He, CO2 and their mixtures are used for
protecting the welding pool from the atmospheric gases. Effect of the shielding gases
on MIG weld joints is similar to that of TIG welding. Inert gases are normally used
with reactive metal like Al, Mg and while carbon dioxide can be used for welding of
steel for reasonably good quality of weld joints. Application of CO2 in welding of
reactive none-ferrous metal is not preferred as decomposition of CO2 in arc
environment produces oxygen. Interaction of oxygen with reactive metals like Al and
Mg (which show greater affinity to the oxygen) form refractory oxides having higher
melting point than the substrate which interferes with melting as well as increases the
inclusion formation tendency in the weld metal. Moreover, shielding gases in MIGW
also affect the mode of metal transfer from the consumable electrode to the weld pool
during welding
MIG welding with Ar as shielding gas results in significant change in the mode of metal
transfer from globular to spray and rotary transfer with maximum spatter while He
mainly produces globular mode of metal transfer. MIG welding with CO2 results in
welding with a lot of spattering. Shielding gas also affects width of weld bead and depth
of penetration owing to difference in heat generation during welding.

Effect of MIG welding process parameters

Among various welding parameters such as welding current, voltage and speed
probably welding current is most influential parameters affecting weld penetration,
deposition rate, weld bead geometry and quality of weld metal.

However, arc voltage directly affects the width of weld bead. An increase in arc
voltage in general increases the width of the weld.

Welding current is primarily used to regulate the overall size of weld bead and
penetration. Too low welding current results pilling of weld metal on the faying surface
in the form of bead instead of penetrating into the work piece. These conditions
increase the reinforcement of weld bead without enough penetration.

Excessive heating of the work piece due to too high welding current causes weld sag.
Optimum current gives optimum penetration and weld bead width.
• Stick out of the electrodes (electrode extension) affects the weld bead
penetration and metal deposition rate because it changes the electrode heating
due to electric resistance.

• Increase in stick out increases the melting rate and reduces the penetration
due to increased electrical resistive heating of the electrode itself.

• Selection of welding current is influenced by electrode stick out and electrode

diameter. In general, high welding current is preferred for large diameter
electrodes with small electrode extension in order to obtain optimal weld bead
geometry.

• Increase in welding speed reduces the penetration.

Resistance Welding is a welding process, in which work
pieces are welded due to a combination of a pressure applied to them
and a localized heat generated by a high electric current flowing
through the contact area of the weld.
Distinct advantages of Resistance Welding over other welding
processes:

There are a number of distinct advantages that account for wide use of the
resistance welding processes, particularly in mass production. These
advantages include:
• They are very rapid in operation.
• The equipment can be fully automated.
• They conserve materials as no filler material, shielding gas or flux is
required.
• Skilled operators are not required.
• Dissimilar metals can be easily joined.
• A high degree of reliability and reproducibility can be achieved.

Resistance Welding has some limitations, the principal ones being:

The equipment has a high initial cost.

There are limitations to the type of joints that can be made (mostly
suitable for lap joints).
Skilled maintenance persons are required to service the control
equipment.
Some materials require special surface preparations prior to welding.
 Overall resistance very low.
 Very high-current
 Very low-voltage is used.

The following metals may be welded by Resistance Welding:

• Low carbon steel (widest application)
• Aluminum alloy
• Medium carbon steel, high carbon steel and alloy steels may be joined but weld will be
brittle.
Types of Resistance Welding
Spot welding is probably the most
common type of resistance welding. The
material to be joined between two electrode,
pressure is applied, and the current is on.

3 stage are involved in welding cycle :

1. Squeeze time
2. Weld time
3. Hold time

• Spot welding may be done on material as

low as 0,0001” in thickness and in joint
having member as heavy as one inch.
• The bulk of resistance welding is confined
to metals that are lesss than ¼ “ in
thickness.
Spot welding is widely used in automotive
industry for joining vehicle body parts.
• The welding cycle starts with the upper electrode moving and contacting the work pieces
resting on lower electrode which is stationary. The work pieces are held under pressure and
only then heavy current is passed between the electrodes for preset time. The area of metals
in contact shall be rapidly raised to welding temperature, due to the flow of current through
the contacting surfaces of work pieces. The pressure between electrodes, squeezes the hot
metal together thus completing the weld. The weld nugget formed is allowed to cool under
pressure and then pressure is released. This total cycle is known as resistance spot welding
cycle and illustrated in Figure
Condition
• Smaller electrode – on high conductivity metals
• Large electrode – on low conductivity metals
• Using a high thermal resistance electrode – on high
conductivity metals
• Using a thermal resistance electrode – tungsten, molybdenum
• Increase the thickness of higher conductivity metals – better
heat balance

r2 and r4 = conductivity of metals

r1 dan r5= conductivity between
surface metals and electrode
r3 = conductivity between joined
metals
• Spot welding electrodes of different shapes are used. Pointed
tip or truncated cones with an angle of 120° ‐ 140° are used
for ferrous metal but with continuous use they may wear at
the tip. Domed electrodes are capable of withstanding
heavier loads and severe heating without damage and are
normally useful for welding of nonferrous metals. The radius
of dome generally varies from 50‐100 mm. A flat tip electrode
is used where minimum indentation or invisible welds are
desired.
Seam welding is similar to spot welding. Equipment is very similar both in terms of
welding current production, control and pressing force. however, differs from spot welding
mainly because of the rolling welding wheel. In most applications, wheels on both sides of
the workpiece produce the weld.
• Weld is made between overlapping sheets of metal.
• The seam is a series of overlapping spot welds.
• The basic equipment is the same as for spot welding. except that the
electrodes are now in the form of rotating disks.
• Timed pulses of current pass to form the overlapping welds.
• Welding current is a bit higher than spot welding, to compensate short
circuit of the adjacent weld.
• In other process a continuous seam is produced by passing a continuous
current through the rotating electrodes with a speed of 1.5 m/min for thin
sheet.

Applications:

Materials from 0.13 mm thickness to more than 19 mm thickness can be

welded up to 82 meter /min.
The combination of high frequency current and high welding speed
produces a very narrow heat affected zone.
Almost all types of materials can be welded, including dissimilar metals and
high conductivity metals, such as aluminum and copper.
Limitations of spot welding:

1. Electrode condition must be maintained continually, and only

one spot weld at a time.
2. For additional strength multiple welds needed.

Projection welding (RPW) overcomes above limitations.

Dimples are embossed on work pieces at the weld locations and then
placed between large-area electrodes, and pressure and current applied like
spot welding.
Current flows through the dimples and heats them and pressure causes
the dimples to flatten and form a weld.
 Projections are press-formed in any shape.
Multiple welds at a time.
No indentation mark on the surface.
Bolts and nuts can be attached to other metal parts.
Metals adaptable for Projection Welding

Not all metals can be projection welded. Brass and copper as a rule do not
lend themselves to projection welding because the projections collapse too
easily under pressure. Aluminium projection welding is generally limited to
extruded parts. Galvanized sheet steel, tin plate, and stainless steel as well as
most other thin gage steel can be successfully projection welded.
Flash Butt Welding

The two pieces of metal to be joined are clamped in dies which conduct the
electric current to the work the ends of the two metal pieces moved together
until an arc established

(D)
BRAZING and SOLDERING
Brazing and soldering both are solid liquid processes primarily involve three
steps a) heating of plates to be joined using suitable heat source, b) placing and
melting of solder or brazing materials followed by heating to the molten state
and c) filling of molten filler metal between the faying surfaces of the
components to be joined by capillary action and then solidification results in a
joint. These three steps are schematically shown in Fig. An attractive feature of
these processes is that a permanent joint produced without melting of parent
work pieces

a) heating of plates,
b) Placing brazing/
soldering metal and
heating and
c) filling of molten metal
by capillary action
followed by solidification
Joints for Brazing and Soldering
Lap joint is commonly developed using both the techniques. Clearance
(0.075-0.125mm) between the plates to be joined is of great importance as it
affects the capillary action and so distribution of joining metal between the
faying which in turn affects the strength of joint. Both too narrow clearance
and too wide clearance reduce sucking tendency of liquid joining metal by
capillary action. To ensure good and sound joint between the sheets, surfaces
to be joined must be free from impurities to ensure proper capillary action.
Butt joint can also be developed between the components with some edge
preparation primarily to increase the contact area between the plates to be
joined.
Melting point of filler
Soldering uses the filler metal system having low melting point (183-2750 C generally
less than 4500C) called solder (alloy of lead and tin) while brazing uses comparatively
higher melting point (450-12000C) filler metals (alloys of Al, Cu and Ni).

Strength of Joint
Strength of solder joint is limited by the strength of filler metal. In general, brazed joints
offer greater strength than solder joints. Accordingly, brazed joints are used for
somewhat higher loading conditions than solder joint.

Ability to withstand under high temperature conditions

In general, braze joints offer higher resistance to thermal load than soldered joint
primarily due to difference in melting temperature of solder and braze metal. Therefore,
solder joints are preferred mainly for low temperature applications.
• All types of carbon steels may be soldered but wettability
decreases with increasing carbon content.

• Cast irons are difficult to solder. The problem is created by the

graphitic carbon flakes which resist the wetting action of solder.
Besides carbon, the oxide film or sand particles on the surface of
cast iron interfere with satisfactory melting and bonding of the
solders. Electrolytic cleaning prior to soldering is recommended.

• Stainless steels are rather difficult to wet with solder because of

chromium oxide. Roughening is a good practice for better
adherence.

• Aluminum and its alloys need certain consideration before

soldering – difficulty of oxide removal, high thermal conductivity,
distortion problem with larger sections, poor corrosion resistance
of soldered joints.
Role of flux in brazing
Fluxes react with impurities present on the surface of base metal or those formed
during joining to form slag apart from reducing contamination of the joints from
atmospheric gases (formation of oxides and nitrides due to atmospheric gases). For
performing above role effectively fluxes should have low melting point and molten
filler should have low viscosity. Fluxes applied over the surface of work piece for
developing joint must be cleaned from the work surface after brazing/soldering as
these are corrosive in nature.

Source of Heat for Joining

Soldering can be carried out using heat from soldering iron (20-150W), dip soldering
and wave soldering. Brazing can performed using gas flame torch, furnace heating,
induction heating, and infrared heating methods.

Limitation of Brazing and Soldering

These processes have major limitation of poor strength and inability to withstand at
higher temperature with some possibility of colour mismatch with parent metals.
Brazing Methods

• Torch brazing - flux is applied to the part surfaces and a torch is

used to focus flame against the work at the joint. A reducing
flame is used to prevent the oxidation.
• Furnace brazing - used to heat the workpieces to be joined by
brazing operation. The component parts and brazing metal are
loaded into a furnace, heated to brazing temperature, and then
cooled and removed.
• Dip brazing - assembled parts are typically dipped in a heated
chemical bath which serve as both fluxing agent and heat source
to melt pre-applied filler material.
• Induction brazing – a process that uses electrical resistance of
workpiece and high frequency current induced into the same as a
source of heat generation. The parts are preloaded with filler
metal and placed in a high frequency AC field.
 Torch Brazing

Furnace Brazing
 Dip Brazing

Induction Brazing
Soldering Methods

Iron soldering - The oldest and simplest soldering method and is

still widely used today. Soldering irons have copper tips which
easily stores and transfers heat to the joint.

Wave soldering -A specific method used in the fabrication of

electronic components and printed circuit boards (PCB). In this
method, continually circulating fountains or waves of solder are
lifted into contact with the joints.
Application

Soldering is mostly used for joining electronic components where they are normally not
exposed to severe temperature and loading conditions during service.

Brazing is commonly used for joining of tubes, pipes, wires, cable, and tipped tool.