What is Resistance Welding?
Definition: Resistance Welding (also known as electric resistance welding (ERW)) is a fusion welding process in which the metals are joined by applying heat and pressure to the joint without the use of filler metal and flux.
The heat required for joining the metals is obtained by the heating effect of electrical resistance across the joint.
Having discussed the definition of resistance welding, further I have discussed the basics of resistance welding process, principle of resistance welding process, electrodes used for resistance welding, resistance welding cycle and many more.
So let’s dive right into it.
Principle of resistance welding
Resistance welding principle: In resistance welding process, a low voltage and a high current (around 15000 ampere) is passed through the joint for a very short time. This high ampere current heats up the joint and melts it. The pressure applied to the joint fuses the metal parts with each other and final welded joint is obtained.
During the resistance welding process, a very high amount of heat is generated at the region of the metal joint.
Heat generated in resistance welding is expressed by the following equation.
H = I2 × R × T × C
H = total heat generated in Joule
I = electric current (amp)
R = resistance of the joint (ohms)
T = time for which the current is passing (seconds)
C = constant for heat losses from the joint
The total resistance of the joint includes the resistance of following parts/components.
- Resistance of workpiece
- Resistance of electrode
- Resistance of the joint
- Contact resistance between electrode and workpiece
- Contact resistance between two workpieces
As you can see in the above equation, the current (I) is squared. So the current should be selected very precisely as the heat generated in resistance welding (H) is mostly dependent on the value of current (I).
The heat generated in resistance welding is directly proportional to the resistance across the joints.
The heat is released at all the resistance mentioned above, but maximum heat is released at the interface of the joints of two workpiece plates.
In resistance welding, the higher current is required to get more heat at the workpiece interface.
Because of this reason, the electrodes of resistance welding are made up of copper (as copper has less electrical resistance).
During the resistance welding procedure, the pressure is applied either mechanically, hydraulically or pneumatically.
Resistance welding electrodes
The electrodes used in resistance welding process (or spot welding process) are shown in the above image.
But the question is;
What are the functions of electrodes in resistance welding?
The electrode is used for following purposes during resistance welding process.
- It carries high current which is required for fusion.
- It transmits the mechanical force to keep the welding plates under pressure.
- To keep the welding plates aligned during the fusion.
- Helps in removing the heat from the weld zone to prevent the overheating or excess melting.
To fulfil the above mentioned functions, the electrode should have good electrical conductivity and it should be hard enough to resist the mechanical pressure.
Resistance welding electrodes are made of which material?
The resistance welding electrode material should be such that it can conduct higher electric current as well as it should have enough hardness.
Most commonly, the alloyed copper material is used for making electrodes of resistance welding.
- Copper-cadmium (0.5% to 1%) alloy has highest electrical conductivity and has medium strength. So the electrode made of copper-cadmium alloy is used for resistance welding of non-ferrous metals.
- Copper-chromium (0.5% to 1%) alloy has little lower electrical conductivity as compared to that of copper-cadmium alloy, but it has very good mechanical strength. So the electrode made of copper-chromium alloy is used for resistance welding of low strength steel (like mild steel and low alloy steel).
- Copper metal alloyed with cobalt and beryllium have less electrical conductivity, but its strength is high. Hence this type of electrodes are used for welding of hard materials like stainless steel.
Resistance welding electrode tips
Electrodes used in spot welding (or resistance welding) have various tips as shown in the image above.
Based on different welding situations, the electrodes having particular tips are used.
How to balance heat in Spot welding (Resistance welding)?
If you want to have a perfect weld using resistance welding (or spot welding), then the fusion zone of the workpiece should have equal amount of heat as well as equal amount of electrode force.
If the two metal plate thickness are same then there will not be any issues of heat balance.
But if the plates having different thickness are to be welded, then the problem of heat balancing arises.
How to balance the heat while welding plates having different thickness?
While welding the plates having different thickness, the proper fusion can be obtained by providing the smaller diameter electrode at the thinner sheet and a bigger diameter electrode at the thicker sheet. During this process high current should be passed for a short time.
How to balance the heat while welding dissimilar metals having different thermal conductivity?
For spot welding of dissimilar metals having different thermal conductivity, you should use a larger diameter electrode near the metal piece having more thermal conductivity.
So for getting a perfect welding using resistance welding process, the proper current density should be maintained and this current density depends on the contact area between the workpieces and electrode.
Resistance welding cycles
There are various cycles of operations that are followed to get perfect weld using resistance welding (or spot welding). These are known as resistance welding cycles.
Resistance spot welding cycle has four main stages.
- Squeeze time
- Weld time
- Hold time
- Off time
Let’s discuss these resistance welding cycles one by one.
#1) Squeeze time
Squeeze time is the time which is required by the electrodes to align and clamp the workpiece and providing the electrical contact.
#2) Weld time
Weld time is the time for which the current flows through the workpieces till they heats up and reaches their melting temperature.
In a resistance welder, the weld time can be controlled by using electronic timers.
#3) Hold time
Hold time is time for which the pressure is applied and maintained on the molten metal without passing the electric current during this period.
During this time, the workpieces are expected to weld with each other due to pressure.
#4) Off time
Off time is the time for which the pressure is released so that the workpiece can be positioned for a new spot.
On which Parameters does the quality of Resistance Welding (spot welding) depend?
The quality of Resistance Welding (spot welding) depends on following parameters.
- Electric current
- Pressure on electrode
- Welding time
- Characteristics of resistance welding machine
- Condition of machine
- Type of material to be welded
- Thickness of material, etc.
Advantages of resistance welding
Advantages of resistance welding process are mentioned below.
- Less skilled welder can also operate the resistance welding machine.
- Most of the resistance welding machines are semi-automatic or automatic.
- This process is suitable for mass production as it takes less time, and it is a semi-automatic or automatic process.
- No filler material is used during the process.
- Heating of metal takes place in a very small region which saves the metal from distortion.
- Dissimilar metals can also be welded by using resistance welding processes.
Disadvantages of resistance welding
Disadvantages of resistance welding process are mentioned below.
- Machines used for resistance welding are complex and they are expensive.
- It is suitable for welding the metal plates having thickness of 3mm to 4mm (by using lap joints only).
Types of resistance welding
There are total 5 main types of resistance welding process.
- Spot welding
- Seam welding
- Projection welding
- Upset butt welding
- Flash butt welding
For more information, visit the main article on: Types of resistance welding process.
Spot welding electrode tips: Image credit India Mart