Resistance Spot Welding is a fascinating topic to both welders as well as researchers. It is a type of Resistance Welding technique, which itself falls under the Pressure Welding Category. Traditionally, Spot Welding is an integral part of automotive manufacturing. You might have seen large robots swiftly moving around and welding the chassis of a car. This process is actually Spot Welding. In recent times, another simple application of Spot Welding is lithium-ion battery packs.
If you are learning to be a welder or looking for more information on Spot Welding, then you have come to the right place. In this guide, we will try to explain everything about Spot Welding from a beginner’s perspective.
What is Resistance Welding?
Pressure Welding is a technique where metal surfaces are joined under pressure or frictional heat. There are several categories of Pressure Welding methods. In some types, the welding involves heating and melting the metals to join them while in other cases, there is no heat but just pressure (frictional heat).
Here is a list of different types of Pressure Welding (partial list):
- Friction Welding
- Resistance Welding
- Ultrasonic Welding
- Cold Pressure Welding
- High-Frequency Welding
Resistance Welding is a family of welding processes that use the concept of Joule Heating to weld metals. In this, the combination of heat and pressure will cause the fusion between the parts.
The heat is produced by the electric resistance between surfaces of the metal to the passage of high current through them as they are held under high pressure. Due to high current density (large current in a tiny area), the metals melt and join.
Electrodes, which are responsible to pass the current will also apply constant pressure on the two parts. The heat depends on the current passing from the electrodes to the metal surfaces, the time for which we apply this current, and the total resistance between the electrodes.
Mathematically, we can calculate the Thermal Energy when an electric current passes through two sheets of metals as:
J = I2×R×t
Here, J is the quantity of Thermal Energy with units Ws (Watt Second), I is the current in Amps, R is the resistance in Ohms (Ω) and t is the time in seconds.
Importance of Resistance of Material
An important characteristic here is the contact resistance between the metals. We know that the resistance of a material is a physical property that is directly proportional to the length and inversely proportional to the cross-sectional area.
The proportionality constant is the Resistivity of the material, which we represent using ρ (Rho). If the resistivity of a material is high, it means it is not a good conductor of electricity. This is an important factor in deciding whether you can use Resistance Welding on that material or not.
For example, Steel has a high resistivity (more than six times that of copper) and is not a good conductor of electricity. Hence, when we pass a high current, it generates heat and thus melts and welds.
On the other hand, aluminum and its alloys have low resistivity (high conductivity). In order to produce the heat for welding, we need to pass three to four times the current than what we did for steel. Hence, Resistance Welding is usually not the preferred method for welding aluminum.
Resistance Welding is one of the earliest forms of welding that is still being used to weld several types of materials. It is fast, efficient, and doesn’t need any consumables. But the downsides are the high cost of the equipment and a relatively small set of applications.
We can further divide Resistance Welding into five types. They are:
- Spot Welding
- Seam Welding
- Projection Welding
- Resistance Butt Welding
- Flash Welding
- Upset Welding
- Percussion Welding
As the main topic of this discussion is Spot Welding, we will cover more about that in the next section. If future guides, we will explore more about the other types of Resistance Welding.
What is Spot Welding?
When we say “Resistance Welding”, we usually mean Resistance Spot Welding (RSW) or simply Spot Welding. It is the most popular and common type of Resistance welding.
Using Spot Welding, you can weld metal sheets with thicknesses up to 25mm each. The automotive industry is a major implementor of Spot Welding. You can see YouTube videos of assembly plants where large Robotic Spot Welders weld the chassis of the car. In fact, you can have an average of 4,500 to 5,000 Spot Weld Joints in a regular-sized car’s chassis.
A typical Spot Welding system consists of three parts. They are the main structure, the electrical system, and the force/pressure application system.
The frame or structure of a Spot Welding system provides strength and rigidity to the whole system. It also has the mechanism to hold and move the electrodes. The electrical system consists of a transformer (step down transformer), capacitor, current regulator, timing circuit, and control circuit.
Finally, we have the mechanical system that is responsible to hold the metal sheets and also to apply force through electrodes during the welding process.
There are a couple of ways to produce this force. Medium and large Spot Welding systems use a pneumatic (compressed air) sub-system. Smaller spot welding units are using high-quality servo motors.
Apart from these two, we also have hydraulic and spring-loaded mechanisms to produce the clamping force.
Another important part of the Spot Welding system is the electrodes. They are responsible for two functions. The first one is to clamp the workpieces with force and apply pressure on them. Second, they carry the high amperage and low voltage current that produces heat and essentially welds the pieces together.
Principle of Operation
The working of a Spot Welding unit is very simple. It involves the precise application of electric current and mechanical force with the necessary extent and period. The electrodes, that hold the workpieces together, also carry the current.
First, the passage of electric current at the contact point of electrodes and metals causes a significant amount of heat that will melt the metal. Next, the two pieces are held under pressure for the metal to fuse and cool off.
Finally, you can release the pressure from electrodes on the sheets. Depending on the shape of the tip of the electrode, the weld will be an elliptical-shaped nugget (or any other shape).
As it is a time-sensitive process, we can divide the operation of a typical spot weld cycle into four “time” steps. They are:
- Squeeze Time
- Weld Time
- Hold Time
- Off Time
Overlap the two pieces of sheet metal and place them between the electrodes. Now, it is time to hold these workpieces with the electrodes and apply pressure on them. The amount of time required for the electrodes to build or raise the force and properly clamp the metals is Squeeze Time.
It is also the time between the first application of force on the workpieces and just before applying the electric current.
After the build-up of proper force i.e., after squeeze time, we are now ready to apply the current. The time during which the weld current passes from the electrodes to the workpieces is the Weld Time.
Weld Time is usually very short. Hence, instead of specifying time, we usually mention the number of cycles of the AC Mains supply. For instance, in North America with a 60 Hz frequency supply, 1 cycle takes 1/60 = 0.0167s while in other parts of the world with a 50 Hz frequency supply, 1 cycle is 1/50 = 0.02s.
If you take a carbon sheet metal in the thickness range of 0.4 mm to 3 mm, it requires about 8 to 10 cycles of weld time per mm of thickness.
By the end of the weld time, the contact point will be in a molten state. After the weld time, the current shuts off but the electrodes continue to apply pressure on the workpieces for a certain amount of time, which is the Hold Time.
During this time, we are essentially allowing the metal to cool down and at the same time apply force. This stage will solidify the molten metal and create the weld nuggets.
After the hold time, the electrodes release the workpieces from their grip and be ready to weld the next spot. This time is the Off Time.
Parameters of Spot Welding
We have already seen the timing-related parameters of a Spot Welding process. But are there any other important variables or parameters? Yes. We have a couple of important parameters. Let us take a quick look at the same.
It is the four steps we saw earlier consists of Squeeze Time, Weld Time, Hold Time, and Off Time. The Welding Cycle varies from material to material as it is associated with physical and mechanical properties.
It is the electric current that flows through the electrodes and then to the surface of the workpieces. If you remember the equation of the Thermal Energy we mentioned before, this energy is proportional to the square of the current.
So, applying the right amount of current is crucial. In most cases, the current of high amperage and low voltage. If you take a 1mm thick uncoated steel sheet, for example, it needs anywhere between 7kA to 10kA of current. This can be either AC or DC.
Clamping Force (Pressure)
The electrode force or the clamping force is another important parameter. It is the force that the electrodes exert on the workpieces. If the force is too little, this will increase contact resistance and result in poor weld quality.
On the other hand, if the clamping force is too high, it will reduce the area or the weld which in turn reduces the strength of the weld.
The area of the electrode determines the current density of the Spot Welding System, which is the amount of current per unit area. The diameter of the electrode is proportional to the thickness of the metal.
If you remember the Thermal Energy equation, we mentioned the amount of heat generated is directly proportional to the resistance across the weld, which is the Total Resistance.
When we say Total Resistance, it is the sum of individual resistances such as contact resistance between the electrode and the metal, the resistance through the individual metal, and also the contact resistance between the metals.
Spot Welding Electrodes
The electrodes of the Spot Welding System play an important role. They are responsible to clamp the workpieces together and also apply the high amperage electric current to weld the metals.
Copper is a popular choice of electrode material due to their superior electrical conductivity, thermal conductivity, and strength. Copper Alloys with internal water cooling are commonly used electrodes for spot welding. Apart from copper alloys, we can also use tungsten and molybdenum.
The next important factor regarding electrodes is their shape. Some commonly available shapes of electrodes are: Pointed, Flat, Offset, Dome, etc. You have to choose a proper shape depending on the type of material you are welding and also its thickness.
Types of Spot Welding Machines
Spot Welding Machines are available in several configurations. One of the simplest Spot Welding Machines consists of a spring-loaded rocker arm with foot pedals. This is completely a manual machine where the operator has to do all the heavy lifting.
The operator places the workpieces between the electrodes and aligns them properly. Then, he/she depresses a foot pedal, which activates the electrode rocker-arm mechanism. Some models have a simple air compressor to control the rocker arms. These Spot Welding Machines are inexpensive and suitable for small shops.
Next, we have the Press-Type Spot Welding Machines. In this, the lower electrode arm usually stays stable while a pneumatic system controls the movement of the upper electrode arm.
These machines provide very stable operation with precise electrode alignment. Slightly larger machines use a hydraulic system to control the electrode.
Both these machines are good for small productions. If you are looking for a large production that operates at very high speed, then you should look at Robotic Spot Welder.
These types of Spot welders are very common in industries, especially automotive assembly plants. They are high-speed, precise, and accurate, and the best thing is you can automate the entire process. The clamping force is provided by a pneumatic system or servo motors.
For a high production rate, you can also get Multi-spot Welders. Instead of one weld, you can perform more than 20 spot weld joints at a time. But the limitation of such systems is they are suitable for relatively straight pieces of sheet metals.
Advantages of Spot Welding
- Spot Welding is quick. Suitable for high-quantity production
- It is faster than ARC Welding and brazing
- The operator or technician needs very little training to operate the machine
- You can easily automate the welding process with Spot Welding Machines
- No deformation of the metal as the area of the weld is pretty small and the thermal energy is confined to this area
Disadvantages of Spot Welding
- Equipment is expensive for high-rate production
- Maintenance of the Spot Welding machines is difficult
- As it involves high amperage and low voltage current, material cost is high (wires, etc.)
- Puts a lot of strain on the power supply, especially if it is a single-phase unit
- The thickness of the workpieces must be close to a 1:1 ratio (must not exceed 2:1)
Resistance Spot Welding is a fast, reliable, and accurate welding technique often used to weld sheet metals. The automotive assembly industry is one of the largest implementors of Spot Welding using Robotic Spot Welders.
Apart from the automotive industry, Spot Welding is suitable wherever there is sheet metal i.e., building, construction, electrical and electronic appliances, furniture, aerospace, etc.
In this guide, we saw the basics of Resistance Welding and then focused on one important type, Resistance Spot Welding (RSW).
We missed some elements of the Spot Welding. If you are interested, we can work around it and add more information in the future.