No.

of the Experiment : 03
Name of the Experiment : Study on Resistance Spot Welding (RSW).

Objectives :
1. To understand the principles of Resistance Spot Welding.
2. To study the welding parameters.
3. To learn the advantages and limitations of this process.

Theory :
Resistance spot welding is a type of electric resistance welding used to weld
various sheet metal products, through a process in which contacting metal surface
points are joined by the heat obtained from resistance to electric current. This
type of welding uses a very high current and a very low voltage. It can be used on
many kinds of metals, such as different types of steel, aluminum, magnesium,
titanium, copper, and their alloys. Usually, thin sheets of the same or different
metals are joined by resistance spot welding in a lap joint arrangement.
Resistance spot welding is a fusion welding process that works on the principle of
Joule’s law of heating, which states that: Q = I2Rt
In this formula, 'Q' is the heat generated during welding, 'I' is the welding
current, 'R' is the resistance at the contact point of the metal sheets, and 't' is
the welding time.
The electrodes, which are perpendicular to the metal sheets, do two things at once.
One, they concentrate the welding current together in one small spot. Two, they
clamp the metal sheets together which prevents misalignment. The thickness of these
metal sheets usually vary from 0.5 mm to 3 mm. The metal sheets or the workpieces
should be a good conductor of electricity. The width of the metal sheets is limited
by the throat length of the welding equipment and ranges typically from 5 to 50
inches (13–130 cm). This type of welding concentrates a large amount of current in
a small spot, which creates nuggets on the weld spot. Welding time usually ranges
between 0.01 seconds to 0.8 seconds. There are three stages in the RSW process.
They are :
1. The electrodes are brought to the surface of the metal sheets being welded and a
short amount of pressure is applied.
2. Weld current is applied for a very short time, which generates heat.
3. Lastly, current is removed but the pressure is retained, which allows the weld
spot to cool and solidify.
Filler metal is not needed in resistance spot welding. No flux is used either.
However, a coolant is used in order to maintain stable temperature of the machine
and for safety.
In this experiment, we have studied the working principle and the process of
resistance spot welding practically.

Schematic Diagram :

Discussion :
Resistance spot welding is widely used in the automotive and aerospace industries
due to its many advantages. It can weld a variety of metals, including both pure
metals and alloys. This process uses high current and low voltage, making it cost-
effective. Copper electrodes, which are readily available, allow a large current to
pass through the weld. The heat is concentrated at the spot, and because the
welding time is short, the entire workpiece does not heat up, making the process
safer than many other welding methods. The force applied by the electrodes also
helps prevent misalignment, reducing the need for additional machining.
Additionally, the short welding time and use of coolant extend the life of the
electrodes and result in fewer welding defects, such as cracks and voids. There are
no risks of electric shocks as well.
However, there are limitations of this welding process also. Resistance spot
welding cannot be done on thick metal plates. The principle of spot welding works
by concentrating a large amount of heat in a very small spot on the workpiece. This
allows that spot to melt and fuse in a very short amount of time, almost
immediately. However, if this process is done on thick metal plates, the entire
thickness of the plates will not be concentrated. Instead, the heat will be
distributed throughout the workpiece. This will not allow the metal plates to melt
and fuse together. Therefore, the welding will not take place properly. So, RSW
process is limited to thin metal sheets only.
Resistance spot welding is often used to join sheet metal, wire mesh, and other
similar materials. Thicker sheets are harder to weld this way because they lose
heat quickly. Welding aluminum alloys is possible but needs more current and
stronger transformers due to their high heat and electrical conduction. This method
is very common in making cars, where it joins the sheet metals of car frames. Spot
welders can be automated, and many assembly line robots are spot welders. In
orthodontics, spot welding resizes metal molar bands. For making batteries, it
attaches straps to battery cells like nickel-cadmium, nickel-metal hydride, or
lithium-ion. This prevents overheating, which could happen with soldering. For good
welds, the surfaces must be clean and free of dirt, oil, and scale.

Report Questions :
1. What is the significant aspect of Resistance Spot Welding process?
Ans : The most important aspect of resistance spot welding is the welding current.
The amount of welding current determines the heat generated. As the welding current
goes up, so does the proportional size of the welding nugget.
2. Explain briefly about the type of electrodes which can be used in the Resistance
Spot Welding process.
Ans : The electrodes used in Resistance Spot Welding machine are usually made of
pure copper of copper alloys. These alloys include chromium-copper, zirconium-
copper, and tungsten-copper. Copper alloys are used when higher strength and better
wear resistance are needed. Copper is chosen due to its high electrical and thermal
conductivity.
3. What are the influencing factors which are affecting RSW?
Ans : There are many factors which affect the Resistance Spot Welding process. The
most important ones are welding current, weld time, electrode material, material of
the workpiece and electrical resistance. Welding current controls how much heat
will be generated at the welding spot. Weld time also contributes to it. Electrode
material controls how much current will pass through the weld. In this case, high
electrical and thermal conductivity are preferred. Material of the workpiece
determines its resistance, which also control generated heat.
4. Describe the heat affected zone in RSW.
Ans : The heat affected zone (HAZ) in resistance spot welding (RSW) is the area
around the weld nugget where heat alters the material's microstructure and
properties. This zone can experience changes in hardness, strength, and potential
defects. Controlling welding parameters and cooling rates is crucial to maintaining
weld quality.