There are mainly 5 different types of Resistance Welding process.
- Spot resistance welding
- Seam resistance welding
- Projection resistance welding
- Upset butt resistance welding
- Flash butt resistance welding
Let’s quickly discuss each of these Types of Resistance Welding process one by one.
Types of resistance welding process
#1) Spot welding
Out of other resistance welding processes, the Spot resistance welding process is the most common type.
Spot welding process is used to weld the two metal sheets (or metal plates) in a lap joint forming a small nugget at the interface of two metal plates.
The spot welding machine has following parts as mentioned below.
- Two electrodes (one if fixed arm and other is movable arm).
- Electric power supply.
- Hydraulic or pneumatic control.
- Water circulating system.
The surface of the plates which are to be welded are first cleaned.
After cleaning the surfaces, the plates are kept one over the other as shown in the diagram above.
The arms of the spot welding machine contain electrodes which hold the metal plates at the desired position.
These electrodes are made up of materials which are hard as well as good conductors of electricity.
Most of the spot welding electrodes are made of;
- Copper metal alloyed with cadmium (0.5% to 1%)
- Copper metal alloyed with chromium (0.5% to 1%)
- Copper metal alloyed with cobalt and beryllium
Spot welding electrode tips can be of different shapes and sizes depending on the applications.
During the spot welding process, the pressure is applied to the electrodes and the pressure is maintained for some interval of time. This time interval is known as squeeze time.
A very high electric current (around 15000 amps) is passed through the electrodes.
The current is also passed through the metal plates for a short period of time and it is measured in terms of number of cycles.
It is necessary to maintain the pressure when the current is passed through the metal plates.
This high current melts the metal at the joint portion. The current is turned off and the pressure is maintained for a short time known as hold time.
During this hold time, the molten metal solidifies and weld nuggets are formed.
After the hold time, the pressure is released and the new position is adjusted for the other spot welding process.
The time duration for which the pressure is released is known as off time.
If the current is kept very high during the spot welding operation, then you may see the following defects.
- Weld expulsion
- Weld cracking
- Reduced mechanical properties
- Electrode embedding in the metal plates, etc.
On the other hand if the current value is less, then it results in a poor weld surface.
Spot welding process is applicable mostly in the aircraft industries, automobile industries and instrument industries.
Advantages of spot welding
Advantages of resistance spot welding process are mentioned below.
- Resistance spot welding is suitable for mass production.
- The speed of operations is higher.
- The operation is clean.
- Welding rods are not used.
- Less skilled welders can also operate the resistance spot welding machine.
#2) Seam welding
As shown in above diagram, the resistance seam welding process uses roller type electrodes.
As the workpiece moves further, the rollers are pressed and rotated over the workpiece.
The pressure applied in the resistance seam welding process ranges from 3 MPa to 8.5 MPa. The range of pressure depends on the thickness of the workpiece.
During the seam welding process, the current density is kept around 775 Amps/sq mm.
The rollers are required to be cooled down to protect them against excessive heat produced due to electric current.
To control the heating of rollers, the water cooling arrangement is provided.
Resistance seam welding process is used for welding of steels, aluminum, magnesium and nickel alloys.
Seam welding process is not suitable for welding of copper and its alloys.
#3) Projection welding
Projection welding is similar to the spot welding process, but the only difference is that the welding is carried out at the places where the projections are made.
Projections are created by pressing the sheet or metal plate at the particular location. (See the small “U-shaped” bend on the top portion of the upper plate in above diagram).
The electric current is passed through the metal plates and the heat is generated at the point of contact between the metal plates.
This heat is generated due to the electrical resistance between the joints of metal plates.
This causes the metal to melt and during this time, the pressure is applied to the metal plates which on cooling forms a strong welded joint.
Projection welding is mainly used for mass production work and it is suitable for the workpieces in which many spot welds are required.
#4) Upset butt welding
Upset butt welding (or resistance butt welding) is a type of resistance welding process in which the two metal pieces are welded by bringing their end to end contact under pressure and allowing the current to flow from one metal piece to the other.
For upset butt welding process, it is necessary to have a smooth contact surface.
Upset butt welding process is mainly used for welding the tools with the shank, joining the pipe from edges, etc.
#5) Flash butt welding
Flash butt welding is a type of resistance welding process in which two metal pieces are welded with each other by pressing against each other in such a way that the contact will be at points due to surface roughness and then the electric current is passed to melt and weld the metal.
When the electric current is passed through the metal pieces, the metal heats up to its molten conditions and one of the metal pieces is then slowly pushed towards the other.
Because of this, the welding of two metal pieces takes place.
The difference between flash butt welding and upset butt welding is that, in flash butt welding there are just point contacts just because of the roughness of the surface. While in upset butt welding, the joining surface is smooth and the entire surface is in contact.