In electric resistance welding, two materials that are to be welded are joined together and the heavy current is made to flow through it.
The resistance of joint generates IR loss causing heat which melts the metal joint and causes fusion of joint.
The welding process is completed. The heat produced at the junction point is given by,
H = 0.24 ∫0t1 i2 r dt Calories
i = Current flowing through joint in Amperes.
r = Resistance of the junction in ohms.
The junction resistance is usually very small therefore, it is very essential to use very high currents 1000 A and above at a low voltage of 1V to 15V.
The amount of power supplied to the weld usually ranges from about 60 W to 180 W for each square mm of area.
For resistance welding, alternating found to be most suitable as with the help of a step-down transformer required a combination of current and voltage can be produced.
The effective pressure to weld vanes from 2.50 to 5.50 kg/mm2 .
The heat developed depends upon the size and shape of the joint to be welded.
Types Of Resistance Welding
There are four types of electric resistance welding are:
- Butt Welding
- Upset Butt Welding
- Simple Butt Welding
- Flash Butt welding
- Pressure Butt welding
- Spot Welding
- Projection Welding
- Seam Welding
Rods, pipes and wires are welded by butt welding.
As shown in Fig. there is a transformer and across secondary of the transformer two pieces of metal to be welded are held.
The two pieces are held in clamps.
One clamp is fixed and other is movable.
The pieces of rods are made to touch each other and force is applied through springs.
When supply is given to the transformer, current passes through the pieces of rods, and sufficient heat is developed at the joint causing welding at the joint.
Upset Butt Welding
The welding current flows through the area where the surfaces are in contact.
It is a resistance welding process that produces growth over the entire area of joint.
This method is used only if the parts to be welded are equal in cross-sectional area. Fig. given below shows upset butt welding.
Flash butt welding
Flash welding may be defined as a resistance welding process that produces coalescence at the faying surfaces of a butt joint by flashing action which is produced by the application of pressure after heating is substantially completed.
Fig. shows flash butt welding process. In this process of welding no special preparation of the faces to be welded is necessary.
In flash butt welding, voltage is applied across joints before welding them together. Slowly contact is made and the temperature of the material rises which causes them to melt.
The molten metal is blown out and small arc is formed which raises the temperature of adjoining parts of the abutting surfaces,
Then upsetting pressure is applied on metallic vapour. As soon as the whole or surfaces to be welded reach the forging temperature of the component, the pressure is applied and the power supply is cut off.
Spot welding is the most widely used form of resistance welding.
It is characterized by low cost, speed and dependability making it a common electrical welding process.
Spot welding is done at certain points on metallic sheets.
The pieces to be welded are held between the two electrodes. Fig. is given below shows the spot welding process. When the current passes through the electrodes, a spot weld is produced between the sheets.
In this type of welding the workpieces to be welded are pressed together by mechanical pressure exerted through electrodes.
Electrodes are made up of pure copper or alloy of copper.
The mechanical pressure may be developed by pneumatic or by hydraulic pressure,
The pressure remains applied till the weld cools and have sufficient strength.
The current required is above 4500 A and the voltage between the electrodes is usually less than 2 volts. The open-circuit voltage is less than 12 volts.
The time period of current is about 1/50 seconds for each 0.25 mm of total thickness of the two sheets to be joined.
To avoid the heating of junction faces between electrodes and work, the electrodes are made up of material having high electrical as well as thermal conductivity.
Water-cooled electrodes are used to avoid the electrode sticking.
Efficient spot welds are possible only by a correct combination of current, pressure, voltage and welding time.
Generally heat developed is given by, H = I2Rt joules.
Projection welding is a resistance welding process that produces coalescence by the heat obtained from the resistance to the now of the welding current.
The resulting welds are localized at predetermined points by projections.
Projection welding uses the same equipment as spot welding.
The electrodes used are flat and larger in diameter.
Successful projection welding depends greatly on the surface preparation of the pieces to be welded.
The weld point soon reaches the plastic state, and the applied by the electrodes finishes the weld.
Projection welding has been used by auto manufacturers for many years. Auto bodies have many areas that are projection welded.
Projection welding reduces the amount of current and pressure required in order to form a good bond between two surfaces.
Seam welding can be defined as series of continuous spot welds.
This process is employed for making a continuous joint between two overlapping pieces of sheet metal.
In this process the work to be welded is placed between the two. wheels which apply sufficient mechanical pressure and also carry sufficient current for producing continuous welds.
The mechanical pressure applied is kept constant and the current is regulated by a timer.
As the pressure is applied, the drive is started, the welding current is switched on and simultaneously the overlapping surfaces of the metal are forced together as fast as they are heated.
The heat is generated due to the now of current through resistance in the circuit.
The heat is controlled by either varying the current or pressure between the sheets.
The electrodes are made up of copper alloys and water is circulated in order to dissipate heat from them.
In high speed seam welding using continuous current. the frequency of the current acts as an interrupter.
The number of spots obtained per meter of joint are 200 and 400.
Welding speed ranges from 0.25 meter to 10 meters per minute.
Seam welding is employed for welding pipes, conduits, tanks, transformers, refrigerators, and aircraft. This process is best used for thickness from 0.25 mm to 3 mm.