What is Spot Welding?
Definition: Spot Welding (also known as resistance spot welding) is a fusion welding process in which the two metals sheets (or metal plates) are joined by applying heat and pressure that forms small nuggets at the interface of two metal plates.
The heat required for joining the metals is obtained by the heating effect of electrical resistance across the joint.
Having discussed the definition of spot welding, further I have discussed the basics of spot welding process, principle of spot welding process, electrodes used for spot welding, spot welding cycle and many more.
So let’s dive right into it.
Principle of spot welding
Spot welding principle: In spot 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 spot welding process, a very high amount of heat is generated at the region of the metal joint.
Heat generated in spot 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 spot welding (H) is mostly dependent on the value of current (I).
The heat generated in spot welding process 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.
During the spot welding process, the higher current is required to get more heat at the workpiece interface.
Because of this reason, the electrodes of spot welding are made up of copper (as copper has less electrical resistance).
During the spot welding procedure, the pressure is applied either mechanically, hydraulically or pneumatically.
Parts of spot welding machine
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 arms of the spot welding machine contain electrodes which hold the metal plates at the desired position.
Spot welding electrodes
The electrodes used in the resistance spot welding process are shown in the above image.
But the question is;
What are the functions of electrodes in spot welding?
The electrode is used for following purposes during the resistance spot 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.
Spot welding electrodes are made of which material?
The spot 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 spot 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 spot 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 spot 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.
Spot 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 a spot welding process, 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 spot welding the plates having different thickness, the proper fusion can be obtained by providing the smaller diameter electrode at the thinner metal sheet and a bigger diameter electrode at the thicker metal 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 spot welding process, the proper current density should be maintained and this current density depends on the contact area between the workpieces and electrode.
Spot welding cycles
There are various cycles of operations that are followed to get perfect weld using spot welding (or resistance welding). These are known as spot welding cycles.
Spot welding cycle has four main stages.
- Squeeze time
- Weld time
- Hold time
- Off time
Let’s discuss these spot 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 spot 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.
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 parameters
Now the main question is;
On which Parameters does the quality of spot welding depend?
The quality of Spot welding depends on following parameters.
- Electric current
- Pressure on electrode
- Welding time
- Characteristics of spot welder machine
- Condition of machine
- Type of material to be welded
- Thickness of material, etc.
Advantages of spot welding
Advantages of spot welding process are mentioned below.
- Less skilled welder can also operate the spot welding machine.
- Most of the spot 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 a spot welding process.
Disadvantages of spot welding
Disadvantages of spot welding process are mentioned below.
- Machines used for spot 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).
Spot welding: Image by Himanshu19s, CC BY-SA 4.0, via Wikimedia Commons
Spot welding electrode tips: Image credit India Mart
Spot welding electrodes: Image by Shreekant26, CC BY-SA 4.0, via Wikimedia Commons