UNIT-IV

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❖ METAL INERT GAS WELDING:
• This is also known as Gas Metal arc Welding.
• The GMAW process was developed in the 1950s
• In gas metal-arc welding (GMAW), the weld area is shielded by an external source, such as argon,
helium, carbon dioxide, or various other gas mixtures.
• The consumable bare wire is fed automatically through a nozzle into the weld arc.
• In addition to the use of inert shielding gases, deoxidizers are usually present in the electrode metal
itself, in order to prevent oxidation of the molten weld puddle.
• . Multiple weld layers can be deposited at the joint.
• The electrode and the work piece are connected to the welding power supply. The power supplies
are of constant voltage type only.
• Normally, DC arc-welding machines are used for GMAW with electrode positive. This increases the
metal deposition rate and also provides for stable arc and smooth electrode metal transfer.

fig: schematic illustration of MIG welding

• Metal can be transferred three ways in the GMAW process: a) spray b) globular c) short circuiting.
➢ Spray transfer:
• In spray transfer, small droplets of molten metal from the electrode are transferred to the weld area
at rates of several hundred droplets per second.
•The transfer is spatter-free and very stable.

• High dc current and voltages and large-diameter electrodes are used, with argon or argon-rich gas
mixtures used as the shielding gas.

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 • The average current required in this process can be reduced by pulsed arcs, which are high-
amplitude pulses superimposed over a low, steady current, and the process can be used in all
welding positions.
➢ Globular transfer:

• In globular transfer, carbon-dioxide rich gases are utilized, and globules propelled by the forces of
the electric arc transfer the metal, resulting in considerable spatter.

• High welding currents are used, with greater weld penetration and welding speed than in spray
transfer. Heavier sections are commonly joined by this method.

➢ Short circuiting:

• In short circuiting, the metal is transferred in individual droplets, at rates of more than 50 per
second, as the electrode tip touches the molten weld metal and short circuits.
• Low currents and voltages are utilized, with carbon-dioxide rich gases and electrodes made of
small- diameter wire.
• The power required is about 2kW. The temperatures involved are relatively low.

• Thus this method is suitable only for thin sheets and sections (less than 6 mm; 0.25 in.); otherwise,
incomplete fusion may occur.
• This process is very easy to use and may be the most popular for welding ferrous metals in thin
sections.
• However, pulsed-arc systems are gaining wide usage for thin ferrous and nonferrous metals.

➢ Advantages:

• It is suitable for welding a variety of ferrous and nonferrous metals and is used extensively in the
metal-fabrication industry.
• Relatively simple nature of the process.
• Training operators is easy.
• This process is rapid, versatile, and economical; welding productivity is double that of the SMAW
process.
• The GMAW process can easily be automated and lends itself readily to robotics and flexible
manufacturing systems.

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 ➢ Disadvantages:
• The metallurgical and mechanical properties of the joint may be affected due to high cooling rate.
• It is difficult to weld in small corners.
• Process variables are more.

➢ Applications:

• It is suitable for welding variety of ferrous and non-ferrous metals.

• Metal fabrication industry, shipbuilding, automobiles, pressure vessel industries etc.
• Welding tool steels and dies.
❖ Tungsten Inert Gas Arc Welding (Gas Tungsten Arc Welding):

• TIG welding set utilizes suitable power sources, a cylinder of inert gas, a welding torch having
connections of cable for current, tubing for shielding gas supply, and tubing for water-cooling for
cooling the torch. The shape of torch is characteristic having a cap at the back end to protect the
rather long tungsten electrode against accidental breakage.
• It is the welding process, in which heat is generated by an electric arc struck between a tungsten
non-consumable electrode and the work piece.
• The weld pool is shielded by an inert gas (Argon, helium, Nitrogen) protecting the molten metal
from atmospheric contamination.
• The heat produced by the arc melts the work pieces edges and joins them. Filler rod may be used,
if required.
• Tungsten Inert Gas Arc Welding produces a high quality weld of most of metals. Flux is not used
in the process.
• In this, a filler wire supplies filler metal.
• Because, the tungsten electrode is not consumed in this process, a constant and stable arc gap is
maintained at a constant arc level.
• The filler metals are similar to the metals to be welded, and the flux is not used.
• The shielding gas used is usually argon or helium.
• Depending on the metals to be welded, the power supply is either AC or Dc.

• GTAW uses three polarities.

a) DCSP (direct current straight polarity):
Tungsten electrode –ve and work +ve, is used for applications requiring deep penetration and a
narrow bead as in mild steel, stainless steel, copper and titanium.
b) DCRP (direct current reverse polarity):
Tungsten electrode +ve and work -ve, is used for welding aluminium and heavily oxidized
aluminum castings.
c) ACHF (Alternating current high frequency):
It is used for welding aluminium and magnesium, which have oxide coatings that contaminate the
molten Al and Mg while welding and ACHF help to remove these oxides.

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 .

➢ GTAW Tungsten electrodes:

5 types of electrodes are given below and the colours by which these are identified are also indicated
in bracket.
i) Pure tungsten (W) (green) ii) 1% Thoriated W (yellow) iii) 2% Thoriated W (red)
iv) Striped W (blue) v) Zirconium W (brown)
• Pure tungsten electrode is used only on ACHF and designed for Al and Mg welding only.
• 1% Thoriated tungsten electrode is used for copper and copper alloys.
• 2% Thoriated tungsten electrode is used for almost any metal.
• Striped tungsten electrode combines pure tungsten and a stripe of 2% thoriated tungsten. Thoriat
helps to keep a stabilized arc and increases melting temperature.
• Zirconium tungsten electrode reduces the contamination effects of dipping the tungsten in to the
molten puddle while welding Al and Mg.

➢ Advantages of Tungsten Inert Gas Arc Welding (TIG, GTAW):

• Weld composition is close to that of the parent metal;

• High quality weld structure
• Slag removal is not required (no slag);
• Thermal distortions of work pieces are minimal due to concentration of heat in small zone.

➢ Disadvantages of Tungsten Inert Gas Arc Welding (TIG, GTAW):

• Low welding rate;

• Relatively expensive;

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 • Requres high level of operators skill.

➢ Applications

• The aerospace industry is one of the primary users of gas tungsten arc welding.
• Many industries use GTAW for welding thin workpieces, especially nonferrous metals.
• It is used extensively in the manufacture of space vehicles, and is also frequently employed to
weld small-diameter, thin-wall tubing such as those used in the bicycle industry.
• In addition, GTAW is often used to make root or first pass welds for piping of various sizes.
• In maintenance and repair work, the process is commonly used to repair tools and dies, especially
components made of aluminum and magnesium.

❖ FRICTION WELDING:

• In friction welding, the heat required for welding is generated through, as the name implies, friction
at the interface of the two components being joined.
• In this process one of the components remain stationary while the other is placed in a chuck or
collect and rotated at a high constant speed.
• The two members to be joined are brought in to contact under an axial force.
• After sufficient contact is established, the rotating member is brought in to a quick stop, while the
axial force is increased.
• Oxides and other contaminants at the interface are removed by the radially outward movement of
the hot metal at the interface.
• The rotating member must be clamped securely to the chuck or collect ,to resist both torque and
axial forces with out slipping.

• The pressure at the interface and the resulting friction produce sufficient heat for a strong joint to
form.
• The weld is usually confined to a narrow region whose size depends on following parameters.
a) The amount of heat generated.
b) The thermal conductivity of the materials.
c) The mechanical properties of the materials at elevated temperatures.
• The shape of the welded joint depends on the rotational speed and on the axial pressure applied.
• These factors must be controlled to obtain a uniformly strong joint.
• Because of combined heat and pressure, the interface at FRW develops a flash by plastic
deformation of the heated zone.

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 Advantages:

• Wide variety of metals can be joined.

• Friction welding machines are fully automated
• The operator skill required is minimum.
• No distortion and warping.
• Dissimilar metals can often be easily joined, even some considered incompatible or unweldable

Disadvantages:

• Because of combined heat and pressure, the interface at FRW develops a flash by plastic
deformation of the heated zone.
• Flash is to be removed by machining and grinding.

Applications:

• Solid or tubular parts can be joined by this method.

• It is used in aerospace industry and automobile industries.
• In the production of cutting tools.

❖ INDUCTION WELDING PROCESS:

• Induction welding of tubes is similar to resistance welding except that heat generated in the work
material is by the current induced in it.
• Because there is no electrical contact with the work this process can be used only where there is a
complete current path or closed loop wholly with in the work.
• The induced current flows not only through the weld area but also through the other portions of the
work.

• A water-cooled induction coil or inductor made of copper encircles the tube at the open end of the
vee.
• High frequency current flown through the coil induces the electric current around the outside surface
of the tube and along the edges of the vee, heating them to a welding temperature.
• Pressure is applied to accomplish the weld.
Advantages:
• The process can be advantageously used for manufacturing of tubing from coated material, small or
thin walled tubing and it eliminates surface marking by electrical contacts.
Disadvantages:
• It is not suitable for welding high conductivity metals or those, which form refractory oxides, as there
is no effective mechanism for oxide disposal.

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 Applications:
• Suitable for tubing made of any metal with in a diameter range of 15-150 mm with a wall thickness
of 0.15 – 10 mm at a welding speed ranging from 5 and 300 m/min.
• To make circumferential welds for welding cap to tube.

❖ EXPLOSIVE WELDING:
• In this, strong metallurgical bonds can be produced between metal combinations, which cannot be
welded by other methods or processes.
• For example, tantalum can be explosively welded to steel although the welding point of tantalum is
higher than the vaporization temperature of steel.
• It is carried out by bringing together properly paired metal surfaces with high relative velocity at high
pressure and a proper orientation to each other so that a large amount of plastic interaction occurs
between the surfaces.

• The work piece, held fixed is called the target plate and the other is called flyer plate.
• While a variety of procedures have been successfully employed, the major techniques of explosive
welding can be divided in to contact techniques and impact techniques.
Dissimilar metals may be joined by Explosive Welding:
Copper to steel;Nickel to steel;Aluminum to steel;Tungsten to steel;Titanium to steel;Copper to aluminum.

Advantages of Explosive Welding :

• Large surfaces may be welded;

• High quality bonding: high strength, no distortions, no porosity, no change of the metal
microstructure;
• Low cost and simple process;
• Surface preparation is not required.

Disadvantages of Explosive Welding:

• Brittle materials (low ductility and low impact toughness) cannot be processed;
• Only simple shape parts may be bonded: plates, cylinders;
• Thickness of flyer plate is limited - less than 2.5” (63 mm);
• Safety and security aspects of storage and using explosives.

Applications

• Explosive Welding is used for manufacturing clad tubes and pipes, pressure vessels,
aerospacestructures, heat exchangers, bi-metal sliding bearings, ship structures, weld transitions,
corrosion resistant chemical process tanks.

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❖ LASER WELDING:
• Word LASER stands for Light Amplification by stimulated emission of radiation
• Laser welding is performed by focusing the coherent monochromatic light beam emitted by the
LASER source on to the metal parts, which are welded by surface heating and thermal conduction
through the metal.
• There are two types of LASERs: solid lasers (ruby-Al2 O3 crystal with Cr ions) and Gas LASERS.
• The laser crystal is in the form of a cylinder, the ends being flat and parallel to high degree of
accuracy and silvered to give mirror-reflecting surfaces.

• There is a small aperture on the axis of the crystal, through the mirror at the output end.
• When the system is pumped with high intensity white light from xenon or krypton lamp, the Cr ions
in the crystal gets exited.
• The exited ions possess more energy and some of it is given as a red florescent light.
• This light is reflected backward and forward in the crystal between the two ends, striking the Cr ions
on the path.
• These ions affected by collisions are each caused to emit their quota of red light exiting more and
more Cr ions, until the number of collisions is high enough to cause a burst of red light through the
small aperture in the mirror at the output end.
• The beam produced is extremely narrow and can be focused to a pinpoint area by an optical lens.
Advantages:
• Heat effected zone is very less.
• This process reduces roughness of the weld surface.
• Welds can be made inside transparent glass of plastic housings.
• Because it is light, it can be focused to microscopic dimensions and directed with great accuracy.
Disadvantages:
• Slow welding speed.
• It is dangerous for operator’s eyes.
• Laser welding is limited to a depth of approximately 1.5 mm only.

Applications:
• It can be used for joining multilayered materials with different thermal properties.

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• It can be used for cutting as well as welding.
• It can join wire-to-wire, sheet-to-sheet, wire to sheet etc.
• It can be used to join copper, nickel, aluminum, titanium etc.

❖ BRAZING:
• Definition
Brazing is a method of joining two metal work pieces by means of a filler material at a temperature
above its melting point but below the melting point of either of the materials being joined.
Flow of the molten filler material into the gap between the work pieces is driven by the capillary force.
The filler material cools down and solidifies forming a strong metallurgical joint, which is usually
stronger than the parent (work piece) materials. The parent materials are not fused in the process.
• There are two main types of brazing processes.
a) Ordinary brazing
b) Braze welding

• Braze welding is a process in which filler metal is deposited at the joint with a technique similar to
oxy-fuel gas welding.
• Brazing is similar to Soldering. The difference is in the melting point of the filler alloy: brazing filler
materials melt at temperatures above 450°C; soldering filler materials (solders) melt at temperatures
below this point.
Filler metals:
• Several filler metals are available with a range of brazing temperature.
• They come in variety of shapes such as wires, rings, shims, and filings.
• Yhe choice of filler metal and its coposition are important, inorder to avoid embrittlement of the joint
,formation of brittle,inter metallic compounds at the joint.
• Because of diffusion between the filler metal and base metal, the mechanical and metullurgical
properties at the joint can change in subsequent processing or during the service life life of brazed
component.

Base Metal Filler Metal

Aluminum and its alloys Aluminum-silicon
Magnesium alloys Magnesium-Aluminum
Ferrous & non-Ferrous (Al & Mg) Silver and Copper alloys
Stainless Steels Nickel-Silver

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 Fluxes:
• A brazing flux has a melting point below the melting point of the filler metal, it melts during the
heating stage and spreads over the joint area, wetting it and protecting the surface from oxidation.
• It also cleans the surface, dissolving the metal oxides.
• It is important that the surface tension of the flux is: 1. Low enough for wetting the work piece
surface; 2. Higher than the surface tension of the molten filler metal in order to provide displacement
of the flux by the fused brazing filler.
• The latter eliminates the flux entrapment in the joint.
• The flux is applied onto the metal surface by brushing, dipping or spraying.
• Brazing fluxes are generally made of borax, boric acid,borates, fluorides,chlorides.
• Wetting agents may also be added, to improve both the wetting charecteristics of the molten filler
metaland the capillary action.

Brazing methods

1) Torch brazing

• utilizes a heat of the flame from a torch.

• The torch mixes a fuel gas with Oxygen or air in the proper ratio and flow rate, providing
combustion process at a required temperature.
• The torch flame is directed to the work pieces with a flux applied on their surfaces. When the work
pieces are heated to a required temperature, filler alloy is fed into the flame.
• The filler material melts and flows to the gap between the joined parts.
• Torch brazing is the most popular brazing method.
• Torch brazing equipment:
- Fuel gas cylinder with pressure regulator;
- Oxygen cylinder with pressure regulator;
- Welding torch;
- Blue oxygen hose;
- Red fuel gas hose;
- Trolley for transportation of the gas cylinders.
• Suitable part thickness es are usually in the range of from 0.25 to 6 mm.

2) Furnace brazing:

In this the .parts are first pre cleaned and then pre loaded with brazing metal in appropriate
configerations before being placed in a furnace.

Furnaces may be batch type ,for complex shapes, or continuos type for high production runs-
espicially for small parts with simple joint designes.

Skilled labour is not required and complex shapes can be brazed, because the whole assembly is
heated uniformly in the furnace.

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 3) Vacuum brazing is a type of furnace brazing, in which heating is performed in vacuum.
4) Induction brazing

The source of heat in induction brazing is induction heating by high frequency AC current.parts are
pre loaded with filler metal and are placed near the induction coils for rapid heating.unless the
protective atmosphere is utilised, fluxes are generally used. Part thicknesses are used generally less
than 3mm. Induction brazing is particularly suitable for brazing parts continuously.

5) Resistance brazing :

The source of heat is electrical resistance of the components to be brazed.Electeodes are utilised for
this method.Either the parts are preloaded with filler metal or it is supplied externally during brazing.
Parts that are brazed by this process comonly have thicknesses of 0.1mm to 12mm.

6) Dip brazing is a brazing method, in which the work pieces together with the filler metal are
immersed into a bath with a molten salt. The filler material melts and flows into the joint. This method is
used for only small parts such as sheet, wire, fittings usually of less than 5mm in thickness or diameter.

7) Infrared brazing: The source of heat is a high intensity quartz lamp. This process is particularly
suitable for brazing very thin components, usually less than 1mm thick, including honey comb
structures.

Braze Welding : The joint in braze welding is prepared as it is in the fusion welding. While an oxy
acetylene torch with an oxidizing flame is used, filler metal is deposited at the joint, rather than by
capillary action. As a result considerably more filler metal is used than in brazing. However,
temperatures in braze welding are generally lower than in Fusion welding, and part distortion is minimal.
The use of flux is essential in this process. The use of this process is for maintenance and repair work,
such as on Ferrous Castings and on Steel components.

Advantages of brazing

• Low thermal distortions and residual stresses in the joint parts;

• Microstructure is not affected by heat;
• Easily automated process;
• Dissimilar materials may be joined;
• High variety of materials may be joined;
• Thin wall parts may be joined;
• Moderate skill of the operator is required.

Disadvantages of brazing

• Careful removal of the flux residuals is required in order to prevent corrosion;

• No gas shielding may cause porosity of the joint;

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 • Large sections cannot be joined;
• Fluxes and filler materials may contain toxic components;
• Relatively expensive filler materials.

Applications:

• To join dissimilar metals.

• Fastining of pipe fittings, tanks,carbide tips,heat exchangers.

Soldering:

In soldering the filler material called solder melts at relatively low temperature. As in brazing, the solder
fills the joint by capillary action between closely fitting or closely placed components. Thus, important
characteristics for solders are high wetting capability and low surface tension. Heat sources for soldering are
usually Soldering Irons, Torches or Ovens.

Some of the soldering techniques are similar to brazing methods. Among them are the following :
1) Torch Soldering
2) Furnace Soldering
3) Induction Soldering
4) Resistance Soldering
5) Dip Soldering
6) Infrared Soldering
7) Ultrasonic Soldering in which a transducer subjects the molten solder to Ultrasonic cavitation and
thereby removes the oxide films from the surfaces to be joined and so eliminates the need for a flux.
8) Reflow Soldering:
Solder pastes are solder metal particles held together by flux and by binding and wetting agents. The
pastes are semi-solid in consistency. The following are sequence of events that occurs in reflow
soldering as the product is heated in the controlled manner :

a) Solvents present in the paste are evaporated

b) The flux in the paste is activated, and fluxing action occurs
c) The components are carefully preheated
d) The solder particles are melted and wet the joint
e) The assembly is cooled at low rate to prevent thermal shock and fracture of the solder joint.

Basic Operations in Soldering Process

• Shaping and fitting of metal parts together
• Cleaning of surfaces
• Flux application
• Application of heat and solder

Solder:
Most solders are alloys of Lead and Tin. Solders may also contain certain other elements like Cadmium and
Antimony in small quantities. The percentage composition of Tin and Lead determines the physical and
mechanical properties of the solder and the joint made. Solder is available in many forms such as Bar, Stick,
Fill, Wire, Strip and so on. It can be obtained in circular or semi-circular rings or any other desired shape.
Sometimes flux is included with the solder. For eg: a cored solder wire is a tube of solder filled with flux.

Flux in Soldering:
The function of flux is to remove the non-metallic oxide film from the metal surface during the heating and
soldering operation so that the clean metals may make mutual metallic contact. The flux does not constitute
a part of the soldered joint. Zinc Chloride, Ammonium Chloride and Hydrochloric acid are the examples of
fluxes commonly used in Soldering.

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➢ HEAT –AFFECTED ZONE:
It is with in the base metal itself. it has a micro structure different from that of the base metal prior to
welding, because it has been subjected to elevated temperatures for a period of time during welding. The
portions of the base metal that are far enough away from the heat source do not undergo any structural
changes during welding.
The properties and microstructure of HAZ depends on (1) the rate of heat input and cooling
(2) the temperature to which the zone is raised. In addition to metallurgical factors (such as original grain
size, grain orientation and degree of prior cold work), physical properties (such as specific heat and thermal
conductivity of the metals) influence the size and characteristics of the zone.
The strength and hardness of the HAZ depend partly on how the original strength and
hardness of the base metal was developed prior to the welding. They may have been developed by cold
working, by solid solution strengthening, by precipitation hardening or by various heat treatments. The
effects of these strengthing methods are complex.
The heat applied during the welding recrystallizes the elongated grains of the cold worked
metal .the grains which are away from the weld metal will recrystalize in to fine equiaxed grains. Grains
close to the weld metal on the other hand, have been subjected to elevated temperatures for a longer period o
time consequently, they will grow. This growth will cause their region softer and have less strength.Such a
joint will be weakest in the HAZ.

Sub divisions of HAZ are

• partly molten zone,

• Underbead zone,
• Grain Growth Zone,
• Grain refined zone
• Partially transformed zone
• Zone of spherodised carbides

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