Resistance welding involves heating metal workpieces to welding temperature using electrical resistance. There are several types including spot welding, seam welding, and flash butt welding. Spot welding uses electrodes to create small welds at contact points by passing current through overlapping metal sheets. Seam welding produces a continuous weld seam using rotating electrodes. Flash butt welding flash burns the ends of metal pieces together before pressing them to form a weld as the pieces cool. Each method uses electrical current and pressure to generate heat and join metal workpieces.

2. Resistance Welding
Heat is obtained from the resistance of the work to the flow of electric
current in a circuit of which the work is a part with the application of
pressure.
No filler metal is needed

3. Two factors/variables responsible for resistance welding:
(1) Heat (2) pressure
Fundamentals of electric resistance welding
Heat
H ∞ I2 R T
H = heat generated (J), I = Current (A), R = Resistance (Ω), T =Time (s)
Current:
H ∞ I2
 Heat increase with increase of current
 Low current does not give proper fusion.
 High current heated the entire thickness of the work metal between the
electrodes to the plastic state; the entire thickness reaches the fusion
temperature and the electrodes themselves deeply into the metal.

4. Resistance
R1: Resistance of the workpieces
R2: Contact resistance between the electrodes and the work.
R3: Resistance between the faying surfaces of the two metal
pieces to be welded.
For sound weld, R1 and R2 should be kept as low as possible with respect to R3.
R1, resistance depends on the nature of the workpiece material and thickness. If
w/p material has low electric resistance (Al), high current are needed.
R2
R3
R1

5. Time, T
Four definite segments or periods of timing during one welding cycle
1. Squeeze time
2. Weld time
3. Hold time
4. Off time
Squeeze time: Time between the initial application of the electrode
pressure on the work and the initial application of current to make the
weld. At the end of the squeeze time, the welding current is applied.
Weld time: During this period the welding current flows through the
circuit, i.e., it enters from one electrode, passes through the work pieces
and goes out from the second electrode.
Hold time: Time during which force acts at the point of welding after
the last impulse of welding current ceases. The electrode pressure is
maintained until the metal has somewhat cooled.

6. Off time: Interval from the end of the hold time to the beginning of
the squeeze time for the next (resistance) welding cycle.

7. (i) It brings the various interfaces into intimate contact and thus affects
the contact resistance between the two workpieces.
(ii) It ensures the completion of the electrical circuit between the
electrodes and through the work.
(iii) It permits the weld to be made at lower temperatures.
(iv) It provides a forging action and thus reduces weld porosity.
 If too little electrode force is used, the contact resistance between
the two workpieces is high and surface burning and pitting of the
electrodes may result.
If too high electrode force is used, it decreases the contact resistance
of the work metal and therefore reduces the total heat generated
between the faying surfaces of the workpieces by the welding current.
Too high electrode force may, also, squeeze softened hot metal
between the surfaces or the work may be indented by the electrodes.
Pressure on the workpieces is exerted by the electrodes extending
from the arms of the welding machine.
Pressure or Electrode Force

8. 1. Current.
2. Electrode pressure.
3. Welding time.
4. Human element.
5. Welding machine characteristics.
6. Type and condition of machines.
7. Conditions of electrodes and arms.
8. Condition of the material and surfaces of material.
9. Throat depth.
10. Throat height.
Variables in Resistance Welding-

9. (i) Fast rate of production.
(ii) No filler rod is needed.
(iii) Semi automatic equipments.
(iv) Less skilled workers can do the job.
(v) Both similar and dissimilar metals can be welded.
(vi) High reliability and reproducibility are obtained.
(vii) Elimination of warping or distortion of parts.
Advantages of Resistance Welding

10. Disadvantages of Resistance Welding
(i) The initial cost of equipment is high.
(ii) Skilled persons are needed for the maintenance of equipment and
its controls.
(iii) In some materials, special surface preparation is required.
(iv) Bigger job thicknesses cannot be welded.
Applications of Resistance Welding
(i) Joining sheets, bars, rods and tubes.
(ii) Making tubes and metal furniture.
(iii) Welding aircraft and automobile parts.

11. Spot Welding
Resistance welding process in which overlapping sheets are
joined by local fusion at one or more spots by the heat
generated, by resistance to the flow of electric current through
workpieces that are held together under force by two
electrodes, one above and the other below the two overlapping
sheets.

12. Procedure
Step 1: Electrodes are brought together against the overlapping work
pieces and pressure applied so that the surfaces of the two workpieces
under the electrodes come in physical contact after breaking any unwanted
film existing on the workpieces.
Step 2: Welding current is switched on for a definite period of time. As
the current passes through one electrode and the workpieces to the other
electrode, a small area where the workpieces are in contact is heated. The
temperature of this weld zone is approximately 815°C to 930°C.

13. Step 3: At this stage, the welding current is cut off. Extra electrode force
is then applied or the original force is prolonged. This electrode force or
pressure forges the weld and holds it together while the metal cools
down and gains strength.
Step 4: The electrode pressure is released to remove the spot welded
workpieces.

14. Heat Shrinkage in Spot Welding
Spot welding usually leaves slight depressions or indentations on the workpieces
and these are often undesirable on the show side of the finished products such as
refrigerators.
Electrodes being under pressure prevent any vertical movement of material,
which, then, naturally expands in the horizontal plane and causes a slight ridge.
After welding, as the work cools, contraction takes place in the vertical plane,
i.e., along the line of the least resistance, thereby resulting in the concave surface
or the depression.
These depressions can be
minimized by the following
techniques.
(a) Use larger sized
electrode tips on the show
side.
Show
Side

15. (b) On the show side use the electrode
having depression. The hot metal conforms
to this depression and, since it is above the
surface of the surrounding material in the
job, it is more easily removed. This method
however produces a depression around the
periphery of the weld.
Show
Side
Insulatio
n
(c) Arrangement helps in
obtaining a minimum of
marking on show side by
affecting current distribution
in that part.
Sho
w
Side

16. Heat Balance in Spot Welding
To obtain a proper spot weld between two workpieces, the fusion zones in the two
pieces should experience the same degree of heat and electrode force.
(b) When welding two dissimilar metal sheets of varying conductivity, e.g. those of a
high copper content alloy and stainless steel, use a smaller (electrode) tip area on the
side of the high conductivity alloy. This helps obtaining equal degrees of fusion by
varying current density.
(a) When welding two different thicknesses of the same material, e.g. mild steel, use a
smaller (electrode) tip area on the side of bigger thickness. This will increase current
density on the side of bigger thickness and thus help in obtaining equal degrees of
fusion in the two pieces to be welded.

17. Spot Weldable Metals
Ferrous Metals
(i) Low carbon steel (mild steel)
(ii) Hardenable steels
(iii) High speed steel
(iv) Stainless steels
(v) Coated steel sheets
Non-ferrous Metals
(i)Aluminium
(ii) Aluminium Magnesium Alloys
(iii) Aluminium Manganese Alloys
(iv) Copper
(v) Nickel, Nickel alloys and Monel
Metal
Spot Welding Methods
(i) Direct
(ii) Indirect (or series)
(iii) Push pull

18. (i) Direct Welds
It is a welding method in which one or more electrodes oppose each other,
contacting both sides of the work and with the current passing from the
electrodes on one side directly through the work into the electrodes on the
other side and back to the welding transformer.
(ii) Series Welds
It is a welding method in which two or more spots are produced
simultaneously with only one common but indirect current path. In series
welding, a portion of the secondary current by passes (shunts) any weld
nugget being formed. This shunt current passes through one of the panels
being welded.

19. (iii) Push Pull Welds
A push pull system employs
transformers with an
electrically reversed polarity
arrangement wherein two
transformers complement each
other to form circulating
welding current circuit.
Opposing electrodes are
connected to different
transformers and are of
opposite polarity. Two spot
welds may be obtained
simultaneously.

20. (i) Low cost
(ii) High speed of welding
(iii) Dependability
(iv) Less skilled worker will do
(v) More general elimination of warping or distortion of parts
(vi) High uniformity of products
(vii) Operation may be made automatic or semiautomatic
(viii) No edge preparation is needed
Advantages of Spot Welding

21. (i) Spot welding of two 12.5 mm thick steel plates has been done
satisfactorily as a replacement for riveting.
(ii) Many assemblies of two or more sheet metal stampings that do
not require gas tight or liquid tight joints can be more economically
joined by spot welding than by mechanical methods.
(iii) Containers such as receptacles and tote boxes frequently are spot
welded.
(iv) The attachment of braces, brackets, pads or clips to formed sheet
metal parts such as cases, covers, bases or trays is another application
of spot welding.
(v) Spot welding finds application in automobile and aircraft
industries.
Applications of Spot Welding

22. Seam Welding
Heat obtained from resistance to electric current (flow) through the
work parts held together under pressure by electrodes.
Resulting weld is a series of overlapping resistance-spot welds made
progressively along a joint by rotating the circular electrodes.

23. Principle of Operation
 Seam welding is similar to spot welding, except that circular rolling
electrodes are used to produce a continuous air-tight seam of overlapping
welds. Overlapping (spot) welds are produced by the rotating electrodes
and a regularly interrupted current.
 Workpieces cleaned, overlapped suitably and placed between the two
circular electrodes, which clamp the workpieces together by the electrode
force.
 Current impulse is applied through the rollers to the material in contact
with them, heat generated and pressure from the electrodes completes the
weld
 As the first current impulse is applied, the power driven circular
electrodes are set in rotation and the workpieces steadily move forward.
 Throughout the welding period, the electrodes revolve and the work
passes through them at a specific speed.
 Intermittent current: On for a definite length of time and off for
another definite and short period.

24. 24
Different types of seams produced
V
Electrode
wheel
Overlapping weld
nuggets
Sheet metal
parts
V
Individual weld
nuggets
V
Continuous weld
seam
Conventional resistance seam welding Roll spot welding
Continuous resistance seam

25. Current flow control
Stitch welding: Current is put off and on quickly, a continuous
fusion zone made up of overlapping nuggets is obtained.
Roll welding: Individual spot welds (or nuggets) are obtained
by constant and regularly timed interruptions of the welding
current.
Roll welding simply joins two workpieces whereas stitch welding
produces gas tight and liquid tight joints.

26. Roller motion control
Continuous motion method:
Electrodes rotate at a constant speed
Current flows continuously or are interrupted
Intermittent motion method:
Electrodes travel the distance necessary for each successive
weld and then stop.
The current is then switched on and the weld made

27. Seam welding machines
(i) Circular: Faces of the electrode wheels are at right angles to the
throat of the machine. Used for circular work, such as welding the
heads on containers and for flat work requiring long seams.
(ii) Longitudinal: Faces of the electrode wheels are parallel to the
throat of the machine. Used for welding short seams in containers,
etc.
(iii) Universal: Electrode wheels may be-set in either the circular or
longitudinal position by the use of a swivel type upper head in which
the upper wheel and its bearing can be swiveled 90 degrees.

28. Metals Welded
(i) Low-carbon, high carbon and low-alloy steels.
(ii) Stainless and many coated steels.
(iii) Aluminium and its alloys.
(iv) Nickel and its alloys.
(v) Magnesium alloys.
Seam Welding Equipments
1. Power Supply
2. Electrode Force and Support
3. Electrode or workpiece drive
4. Controls

29. Advantages of Seam Welding
(i) It can produce gas tight or liquid-tight joints.
(ii) Overlap can be less than for spot or projection welds.
(iii) A single seam weld or several parallel seams may be produced
simultaneously.
Disadvantages of Seam Welding
(i) Welding can be done only along a straight or uniformly curved line.
(ii) It is difficult to weld thicknesses greater than 3 mm.
(iii) A change in the design of electrode wheels is required to avoid
obstructions along the path of the wheels during welding.
Applications of Seam Welding
(i) Girth welds can be made in round, square or
rectangular parts.
(ii) Except for copper and high copper alloys, most
other metals of common industrial use can be seam
welded.
(iii) Besides lap welds, seam-welding can be used
for making butt seam welds too.

30. Flash Butt Welding
 Flash produced during the process.
 Probably, flash welding process was
developed from resistance butt welding
by accident in attempts to increase the
capacity of the butt welding machines
by raising the voltage and applying
pressure intermittently.
 Heat obtained from resistance to
electric current between the two
surfaces, and by the application of
pressure after heating is substantially
substantially completed.

31. Principle of Operation
(i) Flash butt welds are
made on a machine
having one stationary and
one opposing movable
platen, on which are
mounted the flash-
welding dies or clamps.
These clamps securely
hold the two workpieces
to be welded while
simultaneously serving to
conduct the welding
current through these
workpieces.

32. (ii) Movable platen is brought
towards the one gripped in the
stationary platen until the two
come in light contact. Welding
current (with voltage sufficiently
high) is turned on, flashing is
established. Movable platen keeps
move with a constant speed. The
end of the workpieces burn off as
gradually the temperature raised
into welding temperature.
(iii) Pressure of the moving clamp
is quickly increased to forge the
parts together and expel the molten
metal later.
(iv)Welding current is cut off and
the workpieces are unclamped as
the weld cools.

33. Weldable metal:
(i) Low carbon steels.
(ii) Medium strength and high strength low alloy steels.
(iii) Tool steels.
(iv) Stainless steels.
(v) Aluminium alloys (with thickness greater than 1.25
mm).
(vi) Copper alloys (with high zinc content).
(vii) Magnesium alloys.
(viii) Molybdenum alloys.
(ix) Nickel alloys.
(x) Titanium alloys.

34. Flash Butt Welding Equipment
(i) A main frame,
(ii) A stationary platen,
(iii) A movable platen,
(iv) Watercooled clamps,
(v) Transformer, as used on other resistance welding machines,
(vi) Tap switch,
(vii) Electrical controls,
(viii) A flashing and upsetting mechanism.

35. (i) Many dissimilar metals with different melting temperatures can be
flash welded.
(ii) Flash welding offers strength factors up to 100%.
(iii) Generally no special preparation of the weld surface is required.
(iv) Flash welding can be used for the welding of those highly
alloyed steels which cannot be welded satisfactorily by any other
process.
This is because, in flash welding, under correctly controlled
conditions, the heating is not only even but extremely local, so that
the cooling stresses are maintained at a minimum; this avoids
hardening and cracking in highly alloyed steels.
(v) The process is cheap, i.e., the cost of current per weld is small.
(vi) Flash welding is faster than many other methods.
(vii) Flash welding gives a smaller upset.
Advantages of Flash Welding

36. Disadvantages of Flash Welding
(i) The most undesirable feature of flash welding is the flashing
operation during which it is impossible to protect the welding
machine and the surrounding area from these particles, which can
burn into slide way bearings, insulation etc. This necessitates more
frequent maintenance.
(ii) The process presents a considerable fire hazard. Operators
need be protected from flying particles.
(iii) Concentricity and straightness of workpieces during
welding is often difficult to maintain.
(iv) Metal is lost during flashing and upsetting.
(v) Shape of the workpieces to be flash welded should be similar.
(vi) It is generally not recommended for welding zinc and its alloys,
cast iron, lead and its alloys.

37. Applications of Flash Welding
(i) Flash welding is applied primarily in the butt welding of metal
sheets, tubing, bars, rods, forgings, fittings etc.
(ii) Flash welding finds applications in automotive and aircraft
products, household appliances, refrigerators and farm
implements.
(iii) The process is also used for welding the band saw blades into
continuous loops, and joining of tool steel drill, tap and reamer
bodies to low carbon steel and alloy steel shanks.
(iv) Flash welding is used to produce assemblies that otherwise
would require more costly forgings or castings.

38. Projection Welding
 Heat obtained from resistance to electric current flow through the
work parts held together under pressure by electrodes.
 Resulting welds are localized at predetermined points by projections,
embossments or intersections.

39. Principle of Operation
Projection welding is similar to spot welding except
that
(i) The electrodes, instead of being tips as in spot
welding, are flat and relatively large in surface area.
Electrodes are cooled as in spot welding.
(ii) Since a number of welds are made at each
operation, both the electrical power (kVA) and
mechanical pressure must be correspondingly greater,
as compared with a spot welder.
Success of projection welding depends on the surface
preparation of the work-pieces to be welded.
Projections, small deformations that will touch the
surface of the material to be welded are formed on the
weld areas by embossing, stamping, casting or
machining.

40. These projections serve to concentrate (localize) the welding heat at
these areas and facilitate fusion without the necessity of employing a
large current.
(i) The projection in the upper piece is held in contact with the lower
piece under electrode pressure.
(ii) The current flows and being localized to the region around the
projection, heats the metal in that area to the plastic state.
(iii) The heated and softened projection collapses under the pressure of
the electrodes thereby forming the weld.

41. Metals Welded
(i) Low carbon (0.20% C max) steels
(ii) Coated metals such as galvanized steel, terne plate, tin plate,
etc.
(iii) Naval brass, Monel (Ni-Cu) alloys
(iv) Stainless steel
(v) Titanium alloys

42. Advantages of Projection Welding
(i) Number of welds can be made simultaneously.
(ii) Projection welds can be made in metals that are too thick to be joined
by spot welding.
(iii) Scale, rust, oil and work-metal coatings interfere less with projection
welding than with spot welding.
(iv) Projection welding electrodes possess longer life than spot welding
ones because of less wear and maintenance resulting from fusion and
overheating.
(v) Show-sides of the jobs can be produced with no electrode marking,
thus making it possible to paint or plate them without grinding.
(vi) Any two surfaces which can be brought together to give line or point
contact can be projection welded; projection welding is not limited to sheet -
sheet joints;
(vii) Projection welding locates the welds at certain desired points.
(viii) A better heat balance can be obtained in difficult-to-weld combinations
of compositions and thicknesses.
(ix) Projection welding lowers the amount of current and pressure needed to
form a good bond between two surfaces. This reduces the chances of
shrinkage and distortion around the weld zone.

43. Disadvantages of Projection Welding
(i) The projection welding is limited to combinations of metal
thickness and composition which can be embossed.
(ii) Metals that are not strong enough to support projections (e.g.,
some brasses, or copper) cannot be projection welded satisfactorily.
(iii) Forming of projection on one of the workpieces is an extra
operation.
(iv) For proper welding, all projections must be of the same height.
(v) Since there is no possibility of using post-weld current pulses for
heat-treatment (as is possible in spot welding) weld assemblies in
hardenable materials must be heat-treated in a furnace.
Applications
Automobiles
Mass production work- refrigerator, condenser, crossed wire welding etc.

44. Thank You