Overview of Permanent Joining Process

1. GAS TUNGSTEN ARC WELDING
 is the arc welding process in which arc is generated between
non consumable tungsten electrode and work piece.
 The tungsten electrode and the weld pool are shielded by an
inert gas normally argon and helium.
Figures below show the principle of tungsten inert gas welding
process.

2. Figure : Some tungsten will erode and be transferred across the arc

3. TIG torch the electrode is extended beyond the shielding gas
nozzle. The arc is ignited by high voltage, high frequency (HF)
pulses, or by touching the electrode to the work piece and
withdrawing to initiate the arc at a present level of current.
Selection of Electrode
D.C.Welding : 1 or 2 % of thoria
Thoria helps to improve electron emission which
facilitates easy arc ignition
A.C.Welding : Pure tungsten or tungsten-zirconia
Tungsten electrodes are commonly available from 0.5 mm to 6.4 mm
diameter and 150 - 200 mm length.
The current carrying capacity of each size of electrode depends on whether it
is connected to negative or positive terminal of DC power source.

4. AC is used only in case of welding of aluminum and magnesium and
their alloys.
Shielding Gases
 Argon
 Argon + Hydrogen
 Argon/Helium
Helium is generally added to increase heat input (increase
welding speed or weld penetration). Hydrogen will result in
cleaner looking welds and also increase heat input, however,
Hydrogen may promote porosity or hydrogen cracking.

5. Argon or helium may be used successfully for most
applications, with the possible exception of the welding of
extremely thin material for which argon is essential.
Argon generally provides an arc which operates more smoothly
and quietly, is handled more easily and is less penetrating than
the arc obtained by the use of helium.
Pure argon can be used for welding of structural steels, low
alloyed steels, stainless steels, aluminum, copper, titanium and
magnesium. Argon hydrogen mixture is used for welding of
some grades of stainless steels and nickel alloys. Pure helium
may be used for aluminum and copper. Helium argon mixtures
may be used for low alloy steels, aluminum and copper.

6. Application
used in all positions. It is normally used for root pass(es) during
welding of thick pipes but is widely being used for welding of
thin walled pipes and tubes.
This process can be easily mechanised i.e. movement of torch
and feeding of filler wire, so it can be used for precision
welding in nuclear, aircraft, chemical, petroleum, automobile
and space craft industries.
Aircraft frames and its skin, rocket body and engine casing are
few examples where TIG welding is very popular.

7. Advantages
 Superior quality welds
 Welds can be made with or without filler metal
 Precise control of welding variables (heat)
 Free of spatter
 Low distortion
Limitations/ Disadvantages
 Requires greater welder dexterity than MIG or stick welding
 Lower deposition rates
 More costly for welding thick sections

8. GAS METALARC WELDING (MIG WELDING)
Also known as Shielded Inert Gas Metal Arc (SIGMA) welding,
Metal Inert Gas (MIG) welding or Gas Metal Arc Welding
(GMAW) uses a shielded arc struck between a bare metal
electrode and the work piece. The metal electrode is provided in
the form of a wire reel.
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.

9. Effectiveness of shielding in MIG and TIG 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.
Consumption of the electrode during welding slightly decreases
the stability of the arc.

10. 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 .
Characteristics of the MIG welding process
 Uses a consumable wire electrode during the welding process that is fed from a
spool,
 Provides a uniform weld bead,
 Produces a slag-free weld bead,
 Uses a shielding gas, usually – argon, argon - 1 to 5% oxygen, argon - 3 to 25%
CO2 and a combination argon/helium gas,
 Is considered a semi-automatic welding process,
 Allows welding in all positions,
 Requires less operator skill than TIG welding,
 Allows long welds to be made without starts or stops,
 Needs little clean up.

11. Shielding Gas
 forms the arc plasma, stabilizes the arc on the metal being welded, shields the arc
and molten weld pool, and allows smooth transfer of metal from the weld wire to
the molten weld pool.
The primary shielding gasses used are:
 Argon
 Argon - 1 to 5% Oxygen
 Argon - 3 to 25% CO2
 Argon/Helium

12. Benefits/ Advantages
 All position capability
 Higher deposition rates than SMAW
 Less operator skill required
 Long welds can be made without starts and stops
 Minimal post weld cleaning is required
 MIG weld is not considered as clean as TIG weld
 The MIG arc is relatively longer and less stable than TIG arc
MIG Welding Problems/Disadvantages
 Heavily oxidized weld deposit
 Irregular wire feed
 Burn Back
 Porosity
 Unstable arc
 Difficult arc starting

13. SUBMERGED ARC WELDING
is an arc welding process that uses a continuous, consumable bare wire electrode.
The arc shielding is provided by a cover of granular flux consisting of lime, silica,
manganese oxide, calcium fluoride and other compounds.
The flux is fed into the weld zone from a hopper by gravity flow through a nozzle.
The thick layer of flux completely covers the molten metal.
The electrode wire is fed automatically from a coil into the arc. The flux is
introduced into the joint slightly ahead of the weld arc by gravity from a hopper, as
shown in the figure.

14. The blanket of granular flux completely submerges the arc welding operation,
preventing sparks, spatter and radiation that are so hazardous in other arc welding
processes.
The portion of the flux closest to the arc is melted, mixing with the molten weld
metal to remove impurities and then solidifying on top of the weld joint to form a
glasslike slag.
The slag and infused flux granules on top provide good protection from the
atmosphere and good thermal insulation for the weld area. This result in relatively
slow cooling and a high-quality weld joint.
The infused flux remaining after welding can be recovered and reused. The solid
slag covering the weld must be chipped away usually by manual means.

15. Characteristics of submerged-arc welding
 The flux is fed into the weld zone from a hopper by gravity through a nozzle
 Prevents spatter and sparks;
 Suppresses the intense ultraviolet radiation and fumes characteristics of the
SMAW.
 It acts as a thermal insulator by promoting deep penetration of heat into the work
piece.
 The unused flux can be recovered, treated and reused.

16. Advantages:
 Smooth welds of high strength and ductility with low H2 and N2 content.
 Because of high current, high metal deposition, high welding speeds and good
penetration are achieved.
 Due to high speeds less distortion will occur.
 Elimination of fumes and spatter.
 Absence of visible arc and ease of penetration.
Limitations
 During welding process arc is not visible, judging the welding progress is
difficult and so tools like jigs, fixtures and guides are required.
 Pre-placing of flux may not always possible.
 This welding process is limited to flat position.
 Flux is subjected to contamination that may cause weld porosity.
 Chlorine, Aluminium, Magnesium, Lead, Zinc cannot be welded.

17. Shielded metal arc welding (SMAW):
One of the oldest, simplest, and most versatile joining
processes. About 50% of all industrial and maintenance
welding currently is performed by this process.
 In this process an electric arc is generated by touching
the tip of a coated electrode against the work piece
and withdrawing it quickly to a distance sufficient
to maintain the arc.
 The heat generated melts a portion of the electrode
tip, its coating, and the base metal in the immediate
arc area.
 In this process, a consumable electrode consisting of a
filler metal rod which is coated with deoxidizes
chemicals that provide flux and shielding, is used to
protect it from oxygen in the environment. Generally
the filler metal has chemical composition very close to

19. The process has the advantages of being relatively simple, versatile, and
requiring a smaller variety of electrodes.
SMAW is best suited for work piece thicknesses of 3 to19 mm, although this
range can be extended easily by skilled operators using multiple- pass
techniques.
Disadvantages: The multiple-pass approach requires that the slag be removed
after each weld bead. Repeated change of electrodes, current maintained in typical
range. Both labor costs and material costs are high.
Application: The SMAW process commonly is used in general construction,
shipbuilding,pipelines, Machine structures and maintenance work.

20. Plasma Arc Welding (PAW)
is a variety of gas tungsten arc welding in which a constricted plasma arc is used for
welding.
Plasma is an ionized hot gas composed of nearly equal numbers of electrons and
ions. The plasma arc is concentrated by forcing it through a relatively small orifice.
In PAW, a tungsten electrode is kept in a nozzle that focuses a high velocity stream
of inert gas into the region of the arc to form a high velocity, intensely hot plasma
arc stream.
Temperatures in plasma arc welding reach 17,000°C. This is
mainly due to the constriction of the arc. The input power is
highly concentrated to produce a plasma jet of small diameter
and very high power density.
The process can be used to weld almost any material, including
tungsten.

21. • In the transferred-arc method, the work piece being
welded is part of the electrical circuit. The arc transfers
from the electrode to the work piece hence the term
transferred.
• In the non-transferred method, the arc occurs between
the electrode and the nozzle, and the heat is carried to the
work piece by the plasma gas. This thermal-transfer
mechanism is similar to that for an oxy fuel flame.

22. Two types of plasma-arc welding processes: (a) transferred and (b) non
transferred;
 Deep and narrow welds can be made by these processes at high
welding speeds.Compared with other arc-welding processes,
 Plasma-arc welding has better arc stability,
 Less thermal distortion, and
 Higher energy concentration, permitting deeper and narrower welds.
 In addition, higher welding speeds, from 120 to1000 mm/min, can
be achieved.
 A variety of metals can be welded with part thicknesses generally
less than 6 mm.

23. Resistance welding
is a fusion welding processes that uses a combined effect
of heat and pressure to accomplish joining. This heat is
generated by electrical resistance to current flow at the
location to be welded. Weld nugget is generated by this
process.
RW uses no shielding gases, flux, or filler metal.
Electrodes that conduct electrical power to the process are
non-consumable.
The heat energy supplied by RW depends on current flow,
resistance of the circuit, and length of time the current is
applied.

24. This is expressed as, H = I2RT
Current: 5000 to 20,000 A
Voltage: < 10V Duration of current: 0.1 to
0.4 s(in spot-welding operation)
Resistance in the welding circuit is the
sum of (1) resistance of the electrodes, (2)
resistances of the sheet parts, (3) contact
resistances between electrodes and sheets,
and (4) contact resistance of the faying
surfaces.
Resistance at the faying surfaces depends
on surface finish, cleanliness, contact area,
force. No paint, oil, dirt, and other
contaminants should be present to separate
the contacting surfaces.
Advantages: no filler rod required, high production rates, automation and
mechanization are possible.
Disadvantages: restricted to lap joint, costly equipment.

25. Projection Welding
as per the name, different projections are formed for effective
welding. Projection Welding is one of the types of resistance
welding and its working principle is quite the same as the
resistance welding.
The only difference here is that projection or embossed joints
are used for the welding purpose.
Working Principle
As per the definition, different projections are formed in
this welding technique. Here, the metal pieces that are to
be joined are kept in between the two electrodes. A larger
pressure force is applied to the electrodes.
As current is passed through the system, the heat
formation takes place due to the internal resistance of the
metal work pieces.

26. The heat generation takes place due to the internal
resistance of the metal work pieces rather than an electric
arc.
Those projections concentrate the heat. As the pressure
applied to the electrodes increases, this projection
collapses and the formation of the fused weld nugget
takes place. Thus, a quality weld is formed.

27. Thermit welding: (Exothermic Welding)
is the process of igniting a mix of high energy materials, also called thermite,
that produce a metallic slag that is poured between the working pieces of
metal to form a joint. Commonly utilizing the composition of iron oxide red
powder (Fe3O4) with aluminium powder (Al) giving aluminium oxide
powder (Al2O3) and iron (Fe).
Procedure:
1. Wax is poured in the joint and wax pattern is formed where the weld is to be
obtained.
2. A molding flask is kept around the joint and sand is rammed carefully
around thewax pattern
3. Pouring basin, sprue and riser are made
4. A bottom opening is provided to run off the molten wax
5. The wax is melted through the opening at the bottom, which is used to
preheat thejoint and make it ready for welding

28. 6.The igniting mixture (barium peroxide or magnesium) is placed at
the top of the thermit mixture and is ignited by means of a heated rod
of acetylene gas
7.Complete reaction takes place & molten metal is produced.
8.Strength of thermit welded joint is same as forged metal without any
defects.
Applications
Thermit welding Applications: Joining of railway lines, repair of cracks in
large steelcastings and forgings like ingot molds, large diameter shafts, frames
for machinery etc.

29. Electron beam welding:
 is a fusion welding process in which a beam of high-velocity
electrons is applied to two materials to be joined. The work pieces
melt and flow together as the kinetic energy of the electrons is
transformed into heat upon impact.
 EBW is often performed under vacuum conditions to prevent
dissipation of the electron beam. The electron beam gun has a
tungsten filament which is heated, freeing electrons.

30. EBW Benefits are Single pass welding of thick joints, Hermetic seals of
components retaining a vacuum, Low distortion, Low contamination in vacuum,
Weld zone is narrow, Heat affected zone is narrow, dissimilar metal welds of
some metals and Uses no filler metal
EBW Limitations: High equipment cost, Work chamber size constraints, Time
delay when welding in vacuum, high weld preparation costs, X-rays produced
during welding and rapid solidification rates can cause cracking in some
materials.

31. Laser beam welding (LBW)
 is a welding technique used to join multiple pieces of metal
through the use of a laser. The beam provides a
concentrated heat source, allowing for narrow, deep welds
and high welding rates. The process is frequently used in
high volume applications, such as in the automotive
industry.
The term laser is an acronym for Light Amplification by
Stimulated Emission of Radiation.
A laser beam is a powerful, narrow, monochromatic and
directional beam of electromagnetic radiation.
Like electron beam welding (EBW), laser beam welding has
high power density (on the order of 1 MW/cm2) resulting in
small heat-affected zones and high heating and cooling rates.
The spot size of the laser can vary between 0.2 mm and 13 mm,
though only smaller sizes are used for welding.

32. Some of the advantages of LBW in comparison to EBW are as follows:
The laser beam can be transmitted through air rather than requiring a
vacuum, the process is easily automated with robotic machinery, x-rays are
not generated and LBW results in higher quality welds.

33. Laser Welding is used in electronics, communication
and aerospace industry, for manufacture of medical and
scientific instruments, for joining miniature components.
 Laser beam produced by a CO2 or YAG Laser
 High penetration, high-speed process
 Concentrated heat = low distortion
 Laser can be shaped/focused & pulsed on/off
 Typically automated & high speed (up to 250 fpm)
 Work pieces up to 1” thick

34. Advantages of Laser Welding: Works with high alloy metals without
difficulty, Can be used in open air, Can be transmitted over long distances
with a minimal loss of power and Narrow heat affected zone, Low total
thermal input, Welds dissimilar metals, No filler metals necessary, No
secondary finishing necessary, Extremely accurate, Produces deep and narrow
welds, Low distortion in welds, High quality welds, Can weld small, thin
components, No contact with materials
Disadvantages: Rapid cooling rate may cause cracking in some metals, High
capital cost for equipment, Optical surfaces of the laser are easily damaged,
high maintenance costs, and the maximum joint thickness that can be welded
by laser beam is somewhat limited. Thus weld penetrations of larger than 19
mms are difficult to weld.

35. Weld joint configurations

37. A welded joint may develop various discontinuities which can be caused by an
inadequate or careless application of proper welding technologies or by poor
operator training.
Porosity: Caused by gases released during melting of the weld area, chemical
reactionsand contaminants.
Porosity can be reduced by: Proper selection of electrodes and filler metals,
Improved welding techniques, proper cleaning and the prevention of
contaminants, reduced welding speeds.
Slag Inclusions: Slag inclusions are compounds and electrode coating
materials that are trapped in the weld zone
Slag Can be prevented by: Cleaning the weld-bead surface, Providing
sufficient shielding gas, Redesigning the joint.
Incomplete Fusion and Penetration: Produces poor weld beads
Better weld can be obtained by: Raising the temperature of the base metal, Cleaning
the weld area before welding, Modifying the joint design, Providing sufficient
shielding gas.

38. Weld Profile: It is important because its effects on the strength and
appearance of the weld, indicate incomplete fusion or the presence of slag
inclusions in multiple-layer welds

39. Cracks: Occur in various locations and directions in the weld area. It
results from a combination of: Temperature gradients, Variations in the
composition of the weld zone, Embrittlement of grain boundaries,
Hydrogen embrittlement, Inability of the weld metal to contract during
cooling.

40. Basic crack-prevention measures: Modify the joint design to minimize
stresses, Change the parameters, procedures, and sequence of the welding
operation, preheat the components to be welded, avoid rapid cooling of the
welded components.
Lamellar Tears
Work piece is weaker when tested in its thickness direction because of the alignment
ofnon-metallic impurities and inclusions. Lamellar tears may develop because of
shrinkage of the restrained components of thestructure during cooling

41. Surface Damage: Metal will spatter during welding and be deposited as
small droplets on adjacent surfaces. Surface discontinuities will cause
appearance or subsequent use of the welded part disapproval. Discontinuities
will also affect the properties of the welded structure
Residual Stresses: Residual stresses can cause: Distortion, warping and buckling,
Stress-corrosion cracking, further distortion, reduced fatigue life

42. Oxy fuel–gas Cutting
The heat source is now used to remove a narrow zone
from a metal plate or sheet. It is suitable for steels; basic
reactions with steel are greatest heat is generated by the
second reaction.
Temperature is not high to cut steels and the Work piece
need to preheat with fuel gas. The process generates a
kerf. Maximum thickness that can be cut by OFC
depends mainly on the gases used.

43. Arc Cutting
Based on the same principles as arc-welding processes, a variety of
materials can be cut, leave a heat affected zone that needs to be taken into
account In air carbon-arc cutting (CAC-A), a carbon electrode is used and
the molten metal is blown away by a high-velocity air jet.
Plasma-arc cutting (PAC) used for rapid cutting of nonferrous and stainless
steel plates.

44. It is a joining process in which a filler metal is melted and distributed by
capillary action between the faying (contact) surfaces of the metal parts
being joined.
Base material does not melt in brazing; only the filler melts. In brazing, the
filler metal has a melting temperature (liquidus) above 450°C, but below the
melting point (solidus) of base metals to be joined.
Advantages
can be used to join a large variety of dissimilar metals. Pieces of different
thickness can be easily joined by brazing.
Thin-walled tubes & light gauge sheet metal assemblies not joinable by
welding can be joined by brazing. Complex & multicomponent assemblies
can be economically fabricated with the help of brazing.
Inaccessible joint areas which could not be welded by gas metal or gas
tungsten arc spot or seam welding can be formed by brazing.
Brazing

45. Brazing fluxes
Characteristics of a good flux include,
(1) Low melting temperature,
(2) low viscosity so that it can be displaced by the filler metal,
(3) facilitates wetting, and
(4) protects the joint until solidification of the filler metal.
Other types:
Furnace brazing, Induction brazing, Resistance brazing, Dip brazing, infrared
brazing.
Applications:
Automotive - joining tubes, Pipe/Tubing joining (HVAC), Electrical equipment
- joining wires, Jewelry Making, Joint can possess significant strength.

46. SOLDERING:
Joining similar or dissimilar metals by means of a filler metal whose
liquidus temperature is below 4500C.
Used for obtaining a neat leak-proof joint or a low resistance electrical joint.
Not suitable for high-temperature service because of the low melting
temperatures of the filler metals used.
Similar to brazing as filler metal enters the joint by capillary action. Soldered
joint is weaker compared to a brazed joint. To remove oxides from the joint
surfaces, fluxes are used.
For electrical soldering work, Rosin and Rosin plus alcohol based fluxes
are used
Applications:
Printed Circuit Board (PCB) manufacture, Pipe joining (copper pipe),
Jewelry manufacture (typically non-load bearing).