2. 2
IMPORTANCE OF WELDING
• Welding is the most efficient way to join metals.
• Only way to join two or more pieces of metals to make
them act as one piece.
• Welding is the most economical method to
permanently join metal parts.
• All metals can be joined by one welding process or
another.
• There is saying “if it is metal, weld it” and it is true.
• Look around, almost everything made of metal is
welded, the world’s tallest building, moon rocket
engines, nuclear reactors, home appliances and
automobile, barely start the list.
• The use of welding is still increasing.
3. 3
Historical Development of Welding
• Welding, one of the newer metalworking trades, can
trace its historic development back to ancient times.
The earliest example comes from the Bronze Age.
Small gold circular boxes were made by pressure
welding lap joints together. It is estimated that
these boxes were made more than 2000 years ago
and are presently on exhibit at the National Museum
in Dublin.
• Items of iron and bronze that exhibit intricate
forging and forge welding operations have been
found in the pyramids of Egypt.
4. 4
Historical Development of Welding..contd
• During the Middle Ages, the art of blacksmithing
was developed and many items of iron were
produced which were welded by hammering. One
of the largest welds from this period was the Iron
Pillar of Delhi in India, which was erected about the
year A.D. 310. It was made from iron billets welded
together. It is approx. 25 ft (7.6m) tall with a
diameter of 12 in.(300 mm) at the top and 16 in
(400 mm) at the bottom. Its total weight is 12,000
lb (5.4 metric tons). It was not until the nineteenth
century that welding as we know it today was
invented.
7. 7
Historical Development of Welding..contd.
• The production of an arc between two carbon
electrodes using a battery is credited to Sir Humphry
Davy in 1800. In the mid nineteenth century, the
electric generator was invented and arc lighting
became popular.
• In 1890, C.L. Coffin of Detroit was awarded the first
U.S. patent for an arc welding process using a metal
electrode.
• In about 1900, Strohmenger introduced a coated
metal electrode in Great Britain.
8. 8
Historical Development of Welding..contd.
• Resistance welding processes were
developed, including spot welding, seam
welding, projection welding, and flash butt
welding. Elihu Thompson originated
resistance welding. His patents began in
1885. He originated a company, the
Thompson Electric Welding Company and
developed the different resistance welding
processes that time in 1900.
9. 9
Historical Development of Welding..contd.
• Stud welding was developed in 1930 at the New
York Navy Yard, specifically for attaching wood
decking over a metal surface.
• Gas tungsten arc welding had its beginnings from
an idea by C.L. Coffin to weld in a non-oxidizing gas
atmosphere, which he patented in 1890.
• The gas shielded metal arc welding process was
successfully developed at Battelle Memorial Institute
in 1948 under the sponsorship of the Air Reduction
Company.
10. 10
Historical Development of Welding..contd.
• In 1953, Lyubavskii and Novoshilov announced the
use of welding with consumable electrodes in an
atmosphere of CO2 gas.
• Soon after the introduction of CO2 welding a
variation utilizing a special electrode wire was
developed.
• Flux core wire process invented by Bernard was
announced in 1954 but was patented in 1957, when
it was reintroduced by the National Cylinder Gas
Company.
11. 11
Historical Development of Welding..contd.
• Plasma arc welding was invented by Gage in 1957.
Plasma arc welding uses a constricted are or an arc
through an orifice, which creates an arc plasma that
has a higher temperature than the tungsten arc.
• The electron beam welding process, which uses a
focused beam of electrons as a heat source in a
vacuum chamber, was developed in France. J.A.
Stohr of the French Atomic Energy Commission
made the first public disclosure.
12. 12
Historical Development of Welding..contd.
• The newest welding process is laser welding . The
laser originally developed at the Bell Telephone
Laboratories was used as a communication device.
Because of the tremendous concentration of energy
in a small space it proved to be powerful heat
source. It has been used for cutting metals and
non-metals.
13. 13
The Welding Industry and its future
• Welding is now the universally accepted method of
permanently joining metals. It is considered a
mature industry but it is still a growing industry on a
world wide basis.
14. 14
Welding Defined
• “Welding as defined by American Welding Society”
• Welding is defined as “a joining process that produces
coalescence (Fusion) of materials by heating them to
the welding temperature, with or without the application
of pressure or by the application of the pressure alone,
and with or without the use of filler metal”.
• In less technical language; A weld is made when
separate pieces of material to be joined combine and
form one piece when heated to a temperature high
enough to cause softening or melting and flow together.
Pressure may or may not be used to force the pieces
together. Filler material is added when needed to form a
completed weld in their joint.
19. 19
Resistance Welding
Resistance welding (RW): a group of welding processes
that produce coalescence of the faying surfaces with the
heat obtained from resistance of the workpieces to the flow
of the welding current in a circuit of which the workpieces
are a part and by the application of pressure.
20. 20
Processes of Resistance welding
There are at least eight different resistance welding processes
with many variations:
Resistance spot welding (RSW)
Projection welding (PW)
Resistance seam Welding (RSEW) Lap welding
High frequency (RSEW-HF)
Induction (RSEW-I)
Flash Welding (FW) Butt
welding
Percussion Welding (PEW)
Upset Welding (UW)
High frequency (UW-HF)
Induction (UW-I)
23. 23
Resistance Welding - Principles of Operation.
The concept of resistance welding is most easily understood
by relating it to spot welding.
Heat is generated by the
passage of electric current
through a resistance circuit
. The max, amount of heat is
generated at the point of max.
resistance
which is at the surface
between the parts being weld.
The high current, up to 1,00000 A. at low voltage generates
sufficient heat at this resistance point so that the metal
reaches a plastic stage.
The force applied before, during and after the current flow
forges the heated parts together so that fusion will occur.
24. 24
Principles of Operation ..contd.
Four factors are involved in making a resistance
weld.
1. The amount of current that passes through
the work
2. The time the current flow through the work.
3. The pressure/force that the electrode transfer
to the work
4. The area of electrode tip in contact with the
work.
Heat produced in the weld is expressed by the
formula
H=I2xRxT
Where H=Heat energy
I=current in amp.
R=resistance of the work in ohms.
T=Time of current flow in seconds.
25. 25
Weldability of metals – Another factor
Weldability of metal is controlled by three factors:
1. Resistivity 2) Thermal conductivity 3) Melting temp.
The metal with a high resistance to current flow and with a low
thermal conductivity and a relatively low melting temperature
are easily weldable.
These three properties can be combined into a formula that will
an indication of ease of welding a metal. The formula is
W=R/FKtx100
Where W=weldability
R=Resistivity
F=Melting Temp. of the metal in 0C.
Kt=Relative thermal conductivity with copper equal to1.0
If weldability is below 0.25 it is a poor rating. If W is 0.25to0.75
It is fair. Between 0.75 and 2.0 weldability is good and above 2.0
weldability is excellent.
28. 28
Resistance Spot Welding
Spot welding is the most common of various resistance welding
processes. In this process the weld is produced by the heat obtained at
the interface between the work pieces. The heat is due to the resistance
to the flow of electric current through the work-pieces, which are held
together by pressure from the electrodes. Electrodes serve to concentrate
the welding current and the pressure at weld area. The size and shape of
formed welds are controlled somewhat by the size and contour of the
electrodes.
29. 29
Spot Welding Machines
• The spot welding system needs at least the following
components
Welding Transformer for supplying power
A means of applying pressure
A controller/contactor
Electrode tips for conducting welding current to the work
piece.
• Spot welding machines are available in two categories
• Single spot welding machine (stationary,portable)
• Multiple spot welding machine
• More popular machines are stationary single spot welding
machines of either horn or rocker arm type or the press
type
30. 30
The rocker arm machines
have a pivoted on rocking
upper electrode arm which is
actuated by either the
operator’s physical power or
by air or hydraulic power.
They are used for a wide variety of work but are restricted to 50
KVA and are used for thinner sheets.
Rocker Arm Spot Welding M/c
31. 31
Press Type Spot Welding M/c
For heavier requirements, press type
machines are used. This type of
machines normally rated at 50 KVA
and above.
In this press type of machine, the
upper electrode moves in a slide.
The pressure and motion on the
upper electrode are provided by
pneumatic or hydraulic pressure, or
are motor operated.
32. 32
Portable Spot Welding Machines
When the work is too bulky to take to the welding machine, a
portable spot welding machine can be used.
The portable machine is moved from one welding location or
fixture to another, and a trigger on the gun actuates the
welding cycle.
Portable units are normally operated by air or hydraulic
pressure. There are two types of portable welding guns.
34. 34
Resistance Spot Welding Electrodes
The working part of the resistance welding machine is the
electrode. The electrode is the means for conducting welding
current to the work and for providing the force necessary to make
welds.
Electrode shapes have been standardized by RWMA. Figure
given below shows the six standard. nose shapes identified by
letters A through F
36. 36
Electrode coolant parameter
• It is recommended that the water flow to the electrodes should be a
minimum of 4 litres per minute for welding two uncoated steel
sheet of thickness upto 3.00 mm.
• Higher flow rates are recommended when welding coated steels.
• The internal water cooling feed tube should be arranged to ensure
that the water impinges onto the back working face of the
electrode.
• The inlet water temperature should not exceed 20 0C and the outlet
temperature should not exceed 30 0C. To maintain these
temperature levels, the electrode cooling water supply should be
independent of transformer and thyristor water cooling circuits.
• Electrode tip dia should not be allowed to increase 30% of initial
tip dia as recommended by RWMA standards. When this diameter
has been reached, the electrode should be replaced or redressed to
its original size and contour.
37. 37
Basic periods of Spot Welding
Squeeze time is the time between the first application of the
electrode force and the first application of welding current.
Weld time is the actual time the current flows.
Hold time is the period during which the electrode force is
applied and the welding current is shut off.
Off time is the period during which the electrodes are not
contacting the work pieces.
The inbuilt timer on the machine controls these four different
steps.
42. 42
Thumb Rule for selecting spot welding parameters
for low carbon steel sheets
If t =thickness of thinnest outside sheet in mm.
Then required nugget diameter is = 4 x t
To achieve this following weld parameters can be selected.
Weld current = 9 x t KA
Weld time =10 x t cycles
Weld force = 200 x t kgf.
Electrode tip dia = 5 x t
(Tip dressing needed if dia
increases by 30%)
Note: This rule is for reference only. End results can be
achieved by trials only.
48. 48
MINIMUM WELD NUGGET DIAMETER FOR AUTOMOTIVE RESISTANCE
SPOT WELDS
Material Thickness (in mm) Weld Diameter (in mm)
0.38-0.62 3.0
0.63-0.87 3.8
0.88-1.13 4.3
1.14-1.38 5.1
1.39-1.64 5.3
1.65-1.89 5.6
1.90-2.14 6.1
2.15-2.52 6.6
2.53-2.91 7.1
2.92-3.29 7.4
3.30-3.67 7.6
3.68-4.05 8.1
4.06-4.22 8.4
Avg. diameter = D+d/2 as obtained from a peel test
Weld nugget diameter taken from AWS D8.7-88 SAE J-1188 Recommended Practices
for Automotive Weld Quality – Resistance Spot Welding.
49. 49
Spot welding thickness limitations
Parts of widely different thicknesses may be spot welded.
For example 0.4mm material can be welded to 150 mm thick
piece. The thickness of the thinner component is referred to
as the governing metal thickness because it dictates the heat
input. However, the thermal mass of the larger component
may make RSW applications impractical because the thick
section may not reach the fusion temperature before the thin
piece is melted.
The limit of welding piece of equal thickness with an
uninterrupted flow of current seems to be approximate 3.2
mm. Hence spot welded lap joints are widely used in
joining sheet steels up o about 3.2 mm thick and are used
occasionally in joining steel 6.4 mm or more in thickness
Preferred sheet thickness ratio of two un even sheet
thickness sheets should not exceed 1:3 for spot welding.
51. 51
Resistance Projection Welding
Projection welding is somewhat similar to spot welding.
To make a projection weld, projections are formed on at least one
of the work pieces at the points where welds are desired. The
projections, small raised areas, can be any shape such as round,
oval, circular, oblong or diamond. They can be formed by
embossing, casting, stamping or machining.
The work pieces that have the
projections are placed between
plain,large area electrodes in the
welding machine. The current is
turned on, and pressure is applied.
Since nearly all of the resistance is in
the projections, most of the heating
occurs at the points where welds are
desired resulting in fusion of the
work pieces.
59. 59
Quality of Projection nut welding - Test methods
Method for peel test by pressing. Method for peel test by torque
60. 60
Resistance Seam Welding
Seam welding is similar in some
way to spot welding except that the
spots are spaced so close together
that they actually overlap one
another to make a continuous seam
weld. In this process roller wheels
act as electrodes. These roller type
electrodes are usually copper alloy
disc 10mm to 16mm thick. Both the
upper and lower wheels are
powered. Pressure is applied in the
same manner as a press type spot
welder. Water cooling is not
provided internally and the weld area
is flooded with cooling water to keep
the electrode wheels cool.
This process is quite
common for making
flange welds, for
making water tight
joints for tanks, and so
on.
