3. Joining two or more elements to make a single part is termed as a
fabrication process.
Various fabrication process are
1. Mechanical joints- bolts, screws & rivets
2. Adhesive bonding- synthetic glue
3. Welding, Brazing & Soldering
Selection depends on….
1. Type of assemble-permanent, semi-permanent or temporary
2. Material being joined
3. Economy
4. Service conditions
4. Advantages and disadvantages of welding
compared to other types of assembly operations
Advantages:
(1) It provides a permanent joint,
(2) Joint strength is as high as the strength of base metals,
(3) It is most economical in terms of material usage, and
(4) It is versatile in terms of where it can be accomplished.
Disadvantages:
1) It is usually performed manually, so labor cost is high and the
skilled labor to perform it is sometimes scarce,
2) Welding is inherently dangerous,
3) A welded joint is difficult to disassemble, and
4) Quality defects are sometimes difficult to detect.
5. Introduction - Welding
● Welded Joints are permanent joints.
● Welding joints are obtained by localized heating and
are based on molecular attraction.
● The properties of the welded components are closest to
those of a solid member.
● Members of unlimited size can be fabricated.
● Strength of welded joins in terms of static
and impact loads is equal to the base metal.
● All Structural steel's including high alloy grades,
non ferrous alloys can be efficiently welded.
6. ARC WELDING
Name AWS Characteristics Applications
Carbon arc welding CAW Carbon electrode, historical Copper, repair (limited)
Flux cored arc FCAW Continuous consumable electrode filled with
Industry, construction
welding FCAW-S flux
Gas metal arc Continuous consumable electrode and
GMAW Industry
welding shielding gas
Gas tungsten arc Nonconsumable electrode, slow, high quality Aerospace,Construction(
GTAW
welding welds piping),Tool and Die
Plasma arc welding PAW Nonconsumable electrode, constricted arc Tubing, instrumentation
Consumable electrode covered in flux, can
Shielded metal arc Construction, outdoors,
SMAW weld any metal as long as they have the right
welding maintenance
electrode
Submerged arc
SAW Automatic, arc submerged in granular flux
welding
7. Oxy fuel gas welding
Name AWS Characteristics Applications
Combustion of acetylene
Oxy
with oxygen produces high- Maintenance,
acetylene OAW
temperature flame, repair
welding
inexpensive equipment
8. Resistance welding
Name AWS Characteristics Applications
Two pointed electrodes apply pressure
Resistance spot Automobile industry,
RSW and current to two or more thin
welding Aerospace industry
workpieces
Two wheel-shaped electrodes roll along
Resistance seam Aerospace industry,
ERW workpieces, applying pressure and
welding steel drums, tubing
current
Projection welding PW
Flash welding FW
Butt joint surfaces heated and brought
Upset welding RSEW
together by force
9. Solid-state welding
Name AWS Characteristics Applications
Transition joints for chemical
Joining of dissimilar materials, e.g. corrosion resistant
Explosion welding EXW industry and shipbuilding.
alloys to structural steels
Bimetal pipelines
Electromagnetic pulse Tubes or sheets are accelerated by electromagnetic forces. Automotive industry, pressure
welding Oxides are expelled during impact vessels, dissimilar material joints
The oldest welding process in the world. Oxides must be
Forge welding FOW Damascus steel
removed by flux or flames.
Thin heat affected zone, oxides disrupted by friction, needs Aerospace industry, railway, land
Friction welding FRW
sufficient pressure transport
Shipbuilding, aerospace, railway
Friction stir welding FSW A rotating consumable tool is traversed along the joint line rolling stock, automotive
industry
Metals are pressed together at elevated temperatures
Hot pressure welding HPW below the melting point in vacuum or an inert gas Aerospace components
atmosphere
Hot isostatic pressure A hot inert gas applies the pressure inside a pressure
HPW Aerospace components
welding vessel, i.e. an autoclave
Bimetallic materials are joined by forcing them between
Roll welding ROW Dissimilar materials
two rotating wheels
High-frequency vibratory energy is applied to foils, thin Solar industry. Electronics. Rear
Ultrasonic welding USW
metal sheets or plastics. lights of cars.
10. Other welding Processes
Name AWS Characteristics Applications
Electron beam
EBW Deep penetration, fast, high equipment cost
welding
Heavy plate
Welds thick workpieces quickly, vertical fabrication,
Electroslag
ESW position, steel only, construction
welding
continuous consumable electrode. Construction,
shipbuilding.
Induction welding IW
Laser beam Automotive
LBW Deep penetration, fast, high equipment cost
welding industry
Combines LBW with GMAW in the same Automotive,
Laser-hybrid welding head, able to bridge gaps up to Shipbuilding,
welding 2mm (between plates), previously not Steelwork
possible with LBW alone. industries
Percussion Following an electrical discharge, pressure is Components of
PEW
welding applied which forges the materials together switch gear devices
Exothermic reaction between alumnium
Thermite welding TW Railway tracks
powder and iron oxide powder
11. Oxy Acetylene
welding
• OAW is a manual process in which the welder must
personally control the torch movement and filler rod.
• Cylinders contain oxygen and acetylene gas at extremely
high pressure.
• Acetylene is stored in the form of dissolved acetone under a
pressure of 16 to 22 atm gauges.
13. Regulator Hoses
• Hoses are fabricated from
rubber
• Oxygen hoses are green in
color and have right hand
thread.
• Acetylene hoses are red in
color with left hand thread.
• Left hand threads can be
identified by a grove in the
body of the nut and it may
have “ACET” stamped on it
14. Flame Settings
• There are three distinct types of oxy-acetylene
flames, usually termed:
– Neutral
– Carburizing (or “excess acetylene”)
– Oxidizing (or “excess oxygen” )
• The type of flame produced depends upon the ratio
of oxygen to acetylene in the gas mixture which
leaves the torch tip.
15. • The neutral flame is produced when the ratio of oxygen to
acetylene, in the mixture leaving the torch, is almost exactly one-
to-one. It’s termed ”neutral” because it will usually have no
chemical effect on the metal being welded. It will not oxidize the
weld metal; it will not cause an increase in the carbon content of
the weld metal.
• The excess acetylene flame, as its name implies, is created when
the proportion of acetylene in the mixture is higher than that
required to produce the neutral flame. Used on steel, it will cause
an increase in the carbon content of the weld metal.
• The oxidizing flame results from burning a mixture which contains
more oxygen than required for a neutral flame. It will oxidize or
”burn” some of the metal being welded.
18. Type of flame Application
1.Neutral flame Steel, Cast-Iron ,Copper, Aluminum
2.Carburizing flame Low carbon steels, alloy steels, non ferrous
metals
3.Oxidizing flame Brass, Bronze
19. Filler rod
• Used to supply additional metal to the weld zone during
welding
• Available as filler rods or wire and may be bare or
coated with flux
• Purpose of the flux is to retard oxidation of the surfaces
of the parts being welded by generating a gaseous
shield around the weld zone
20. Advantages of Gas welding
1.Temp.of flame cane be controlled easily.
2.The amount of filler metal deposits can be controlled easily.
3.Flame can be used for welding and cutting.
4.All types of metal can be welded.
5.Cost of equipment is less.
6.Portable equipment.
7.Low maintenance cost.
Limitations of Gas welding
1.Not suitable for thick plates.
2.Slow process.
3.Handling and storing of gas cylinders needs more care.
4.Strength of weld is not so good as arc welding.
5.Gas flame takes up a longer time to heat up the metal than
arc welding.
21. Gas cutting
Ferrous metal is heated in to red hot condition and a jet of
pure oxygen is projected onto the surface, which rapidly
oxidizes
Melt are blown away by the force of the jet, to make a cut
Fast and efficient method of cutting steel to a high degree
of accuracy
Torch is different from welding
Cutting torch has preheat orifice and one central orifice for
oxygen jet
22. Flame Cutting
Fig. (a) Flame cutting of steel plate with an oxyacetylene torch, and a
cross-section of the torch nozzle.
(b) Cross-section of a flame-cut plate, showing drag lines.
23. Arc welding
• Equipments:
• A welding generator (D.C.) or Transformer (A.C.)
• Two cables- one for work and one for electrode
• Electrode holder
• Electrode
• Protective shield
• Gloves
• Wire brush
• Chipping hammer
• Goggles
25. Arc Shielding
• At high temperatures in AW, metals are
chemically reactive to oxygen, nitrogen, and
hydrogen in air
– Mechanical properties of joint can be degraded by
these reactions
– To protect operation, arc must be shielded from
surrounding air in AW processes
• Arc shielding is accomplished by:
– Shielding gases, e.g., argon, helium, CO2
– Flux
26. Flux
A substance that prevents formation of oxides
and other contaminants in welding, or dissolves
them and facilitates removal
Provides protective atmosphere for welding
Stabilizes arc
Reduces spattering
27. Power Source in Arc Welding
DC arc welding is more expensive than AC welding.
DC W is generally preferred because of the control of heat
input offered by it.
70 % of heat is liberated near the anode ,30% cathode.
If more heat is required at w/p ( thicker w/p, high thermal
conductivity metals such as Al, Copper ) w/p can be
connected to anode –Straight polarity or DCEN ( Direct
current Electrode negative)
It produces welds that are narrow and deep.
28. If less heat is required at w/p, (thinner w/p)
w/p can be connected to negative. This is
referred as reverse polarity, or DCEP(direct
current Electrode positive )
The weld zone is shallower and wider
DCEN DCEP
Fig. The effect of polarity on weld beads:
(a) dc current straight polarity; (b) dc current reverse polarity; (c) ac
current.
29. Comparison of AC & DC welding machines
AC machine ( Transformer) DC machine (Generator)
1.Efficency is more (80 to 85 %) Efficiency is less (30 to 60 %)
2.Power consumption is less Power consumption is more
3.Cost of equipment is less Cost of equipment is more
4. Any terminal can be connected Polarity is significant
to work or electrode
5.Voltage is higher, not safe Voltage is low, safer operation
6.Not suitable for welding Very much suitable for both
nonferrous metals ferrous & nonferrous metals
7.Not preferred for welding thin Preferred for welding thin
sections sections
30. Arc welding
Advantages Limitations
– Most efficient way to join • Manually applied, therefore
metals high labor cost.
– Lowest-cost joining • Need high energy causing
method danger
– Affords lighter weight • Not convenient for
through better utilization disassembly.
of materials • Defects are hard to detect at
– Joins all commercial joints.
metals
– Provides design flexibility
31. Heat transfer in arc welding
H VI H = heat input l = weld length
• Heat input is l
e
v
V = voltage applied I = current
v = welding speed e = efficiency
• Heat input to melt a certain volume of
material is
u = specific energy required for melting
H uV m uAl Vm = volume of material melted
A = cross section of the weld
• Welding speed is
VI
v e
uA
32.
33. Consider the situation where a welding operation is being
performed with V = 20volts, I = 200A and the cross-sectional area
of the weld bead is 30 mm2. Estimate the welding speed if the work
piece and electrode are made of (a) aluminium, (b) carbon steel,
and (c) titanium. Use an efficiency of 75%.
Solution VI 20 200
v e 0 . 75 34 . 5 mm/s
a)For aluminium, uA 2 . 9 30
b)For carbon steel, v 8 . 1 mm/s
c)For titanium, v 7 . 0 mm/s
34. Consumable Electrode Non consumable
AW Processes Electrode Processes
• Shielded Metal Arc • Gas Tungsten Arc
Welding Welding (TIG)
• Gas Metal Arc • Plasma Arc Welding
Welding(MIG) • Carbon Arc Welding
• Flux-Cored Arc Welding • Stud Welding
• Electrogas Welding
• Submerged Arc Welding
37. MIG or GMAW
Consumable electrode is in the form of a wire reel which is fed
at constant rate.
Weld area is shielded by an external source of gas.
Deoxidizers are present in the electrode to prevent oxidation.
Process is rapid, versatile and economical.
Shielding gas :
Argon
Nitrogen
Helium
38. TIG (Tungsten Inert Gas)
or
Gas Tungsten Arc Welding ( GTAW)
As the tungsten electrode is not consumed, a constant
and stable arc gap is maintained at a constant current
level
GTAW process is used for applications with aluminium,
magnesium, titanium and the refractory metals
Cost of the inert gas is more expensive but provides
high quality welds and surface finish
40. Gas Tungsten Arc Welding
Advantages:
• High quality welds for suitable applications
• No spatter because no filler metal through arc
• Little or no post-weld cleaning because no flux
Disadvantages:
• Generally slower and more costly than consumable
electrode AW processes
41. Shielded Metal Arc Weld (SMAW):
Most popular welding technique (stick welding).
The electrode coating performs the following:
A – Produce gaseous shield to exclude oxygen.
B – Introduce dioxider material to improve grain.
C – Produce a blanket of slag to retard cooling and
prevent oxidation.
The SMAW process is
designated by AWS as
“E6OXX” or “E7OXX”
e.g. (E 6013)
42. Submerged Arc Weld (SAW):
In this process the automatically fed arc (spool) is
protected by a blanket of granular material called “flux”.
This flux material acts to improve weld quality and to protect it
from the air.
Submerged arc welding (SAW)
43. Welded Joints
Fig. Examples of
welded joints and
their terminology.
44. Resistance Welding
• RW- heat required is produced by electrical
resistance across the two components to be
joined H = Heat
• Heat generated is H I Rt
2
I = Current
R = Resistance
t = Time of current flow
• By including a factor K, which denotes energy
losses through conduction and radiation, we
have H I RtK
2
45. The desirable properties of a metal that
would provide good weld ability for resistance
welding are
High resistivity,
Low electrical conductivity
Thermal conductivity and
Low melting point.
46. Resistance Welding
• Total resistance is the sum of:
1. Resistances of the electrodes
2. Electrode–workpiece contact resistance
3. Resistances of the individual parts to be welded
4. Contact resistance between the two workpieces
to be joined (faying surfaces)
• Temperature rise at the joint depends on the
specific heat and the thermal conductivity of
the metals to be joined
47. Resistance Spot Welding
• Tips of 2 opposing solid, cylindrical electrodes
touch a lap joint of two sheet metals, and
resistance heating produces a spot weld
• To obtain a strong bond in the weld nugget,
pressure is applied until the current is turned
off and the weld has solidified
• Surface of the spot weld has a slightly
discolored indentation
• Current level depends on the materials
thicknesses
49. Heat generated = I2Rt
Heat required for melting uV m
Volume of nugget Vm = (3.14*D2 )/4 *d
Where D =dia of nugget
d= thickness of nugget
Heat distribution through conduction and radiation is =
Heat generated-heat required for melting.
50. Resistance Spot Welding
• Simplest and most commonly used
• May be performed by means of single or
multiple pairs of electrodes
• Required pressure is supplied through
mechanical or pneumatic means
• Variety of electrode shapes are used to spot-
weld areas that are difficult to reach
51. An RSW operation is used to make a series of spot welds between two
pieces of aluminum, each 2.0 mm thick. The unit melting energy for
aluminum = 2.90 J/mm3. Welding current = 6,000 amps, and time duration
= 0.15 sec. Resistance = 75 micro-ohms. The resulting weld nugget
measures 5.0 mm in diameter by 2.5 mm thick. How much of the total
energy generated is used to form the weld nugget? How much of heat is
dissipated into the surroundings?
Solution: H = I2Rt = (6000)2(75 x 10-6)(0.15) = 405 W-sec = 405 J
Weld nugget volume V = πD2d/4 = π(5)2(2.5)/4 = 49.1 mm3
Heat required for melting = UmV = (2.9 J/mm3)(49.1 mm3) =142.4 J
Proportion of heat for welding =142.4/405 = 0.351 = 35.1%
The remaining heat 405 J-142.4 J= 262.6 J is dissipated into the
metal surrounding through conduction and radiation.
54. Advantages &Drawbacks of RW
Advantages:
• No filler metal required
• High production rates possible
• Lends itself to mechanization and automation
• Lower operator skill level than for arc welding
• Good repeatability and reliability
Disadvantages:
• High initial equipment cost
• Limited to lap joints for most RW processes
55. Resistance Seam Welding
• Electrodes are replaced by rotating wheels
or rollers
• Using a continuous AC power supply to
rollers
• In roll spot welding, current is applied
intermittently and a series of spot welds at
specified intervals
• In mash seam welding, overlapping welds
are about one to two times the sheet
thickness
57. High-frequency RW
• High-frequency current (up to 450 kHz) is used
• Used for production of butt-welded tubing or
pipe
• For high-frequency induction welding (HFIW),
the roll-formed tube is subjected to high-
frequency induction heating
58. Resistance Projection Welding
• High electrical resistance is developed by
embossing one or more projections on one of
the surfaces to be welded
• Used for resistance projection welding by
modifying the electrodes
59. RW:Flash Welding
• Heat is generated from the arc as the ends of
the two members begin to make contact and
develop an electrical resistance at the joint
• Quality of the weld is good
• Suitable for end-to-end or edge-to-edge joining
of sheets of similar or dissimilar metals
• Can be automated
• Can be used in operating rolling mills and
feeding of wire-drawing equipment
