Spot Welding: What It Is and How Does It Work

Spot welding is a simple, cost-effective, and quick welding process used in specific applications, such as joining metal sheets and thin metal pieces.

For some applications and scenarios, it’s an ideal option because of its speed, low cost, automation, and minimal HAZ (heat-affected zone).

Let’s understand what spot welding is and how it works. In this detailed guide, we will go through its working principle, process, parameters, applications, and everything you must know before getting started. 

What Is Spot Welding?

Spot welding refers to welding two metal pieces at a small spot. The energy is concentrated at a tiny spot where both metal pieces melt and fuse. Only one spot joins both metal pieces and creates a strong bond.

Usually, when people say Spot Welding, they mean Resistance Spot Welding. In this welding process, two metal pieces are joined by applying pressure and generating heat at the same time. The heat is generated by passing a heavy current through an electrical resistor.

It can join two metal sheets in minimal time. It is commonly used in the automotive industry for joining car body parts. Other than that, it is used in joining metal sheets in home appliances (refrigerators and microwave ovens), attaching terminals on batteries, metal fabrication, etc.

It creates a localized weld through heat generated by electrical resistance. It does not affect the surrounding area and joins metals at specific points. The best part is that it’s an economical process and does not require any filler material, gas, welding rod, etc.

How Does Spot Welding Work?

The basic principle behind resistance spot welding is generating immense heat using electrical resistance. The heat is enough to melt metal pieces and create a joint between them.

For this purpose, heavy electrical current is passed through the resistance, which converts electrical energy into heat. The higher the current, the more heat it will generate.

The heat generated by an electrical resistance is shown by the following equation.

Where,

Q = heat generated

I = current passed through the resistor

R = resistance of the resistor

t = time or duration for which the current is passed

A spot welding machine has a transformer inside. When the machine is connected to the AC supply, the transformer steps down the voltage and increases the current. The voltage becomes very low, and the current becomes very high.

The heat generated is directly proportional to the current, as indicated in the equation. Immense heat is generated when a heavy current is passed through the resistor.

The machine has two electrodes. They are arranged in the vertical plane. The metal pieces are placed between them. Some parts of both pieces overlap each other, and that’s where the joint is created.

The electrodes are pointy or cone-shaped, so they target a small point or spot. They exert pressure on the metal pieces and also serve as the conductor for carrying the current. They clamp both pieces and keep them in place during the process.

When the current is passed, it goes from one electrode to the other while passing through the metal pieces. Due to the resistance of the electrodes, the metal pieces heat up quickly.

The temperature could reach several thousand centigrade, which is enough to fuse both metals. It’s an instant process, and within seconds or milliseconds, the heat is generated and the metal pieces are joined.

The electrodes exert pressure and help in joining the materials. When the current is stopped, the metal pieces are cooled down, and the weld joint is created.

Variables and Parameters in Spot Welding

Spot welding depends on several variables and parameters that welders need to master. It helps in selecting the right parameters for the right materials and achieving perfection.

Here are the primary variables and parameters.

Current

The current plays a crucial role in spot welding. By varying the magnitude of the current, welders can control the heat generated. If they want to weld material with a high melting point, it will require more current compared to materials with a lower melting point.

If the current is low, the weld won’t be strong enough. It will break with minimal pressure or stress. The reason behind this is the incomplete fusion of the materials. And if the current is too high, it could create a hole in the material and pass through.

Thus, adjusting the current is extremely important.

Resistance

Resistance is the basis of resistance spot welding. It does not change during the process because it depends on the electrode you have used.

In spot welding, two types of electrodes are used. If the metal pieces are conductive, the electrodes should have the resistive property. For instance, when joining metals like copper and gold, tungsten electrodes are used.

Similarly, when the metal pieces offer some resistance to the current, the electrode is conductive. While joining metals like stainless steel and nickel, copper electrodes are used, and these are the most common electrodes in spot welding machines.

An optimal electrode with suitable resistance is chosen according to the applications.

Pressure

In spot welding, there are two important things; the heat generated and the pressure exerted. The heat melts the material and is responsible for the fusion.

However, the pressure exerted by the electrode holds the metal pieces. It helps in the fusion and offers a stronger weld joint.

Adequate pressure is needed to create a strong and perfect joint. If the pressure is low, the metal pieces won’t join properly. Moreover, if the pressure is high, it could deform the pieces and might create a hole.

Welders can adjust the pressure on the spot welding machine. It can be adjusted according to the thickness of the metal pieces.

Duration

The duration for which the current flows is an important parameter. Welders can control the heat generated and the melting of the metal pieces by adjusting the time.

Spot welding is a fast process. It can be done in a fraction of a second to several seconds, depending on the machine.

It is extremely fast compared to other welding methods. Thus, it is difficult to control the time precisely. But automation and computerized control have made it easier.

According to the metal type, thickness, current, and other factors, a suitable duration is selected. It’s often selected after tests and trials.

Size of the Electrode

The size of the weld depends on the size of the electrode, specifically the diameter of the electrode or the area of the pointy surface.

If it is large, you’ll have a large spot weld, and if it’s small, you can weld tiny objects with extreme precision.

When Do You Need to Spot Weld?

Spot welding is preferred where you need to weld materials, especially metal sheets, without affecting a larger area. It’s a localized welding technique that welds metal pieces in no time.

Automotive

Spot welding is widely used in automotive manufacturing. A car usually has around 4,000-6,000 spot weld joints.

It is used to join metal sheets to create the body of the vehicle, such as the roof, doors, and side panels. It joins metal sheets quickly and is suitable for mass manufacturing processes.

The metal sheets overlap each other. Spot welding can join both pieces offering excellent strength.

The primary reason for using spot welding in the automotive industry is that it does not affect the aesthetics. It looks clean, and if done properly, it’s easier to cover up.

Metal Fabrication

In metal fabrication, spot welding is a useful technique. It is used in buildings and construction.

Metal doors and windows require spot welding for joining parts and frames.

In HVAC systems, spot welding aesthetically joins metal sheets for ducts, air vents, pipes, etc.

Spot welding is also useful for making wire meshes that are used in fencing, concrete reinforcement, etc.

Home Appliances

In many home appliances, metal bodies are joined using spot welding.

For instance, the body of a microwave oven and washing machine requires spot welding to join metal sheets.

Similarly, for metal sheets in other appliances, such as dishwashers, refrigerators, freezer cabinets, and dryers, spot welding is the best method for manufacturing the outer body.

Electrical Connections

Spot welding is also used in electrical and electronics applications. For joining terminals or wires to the batteries, spot welding is preferred. The Li-ion cells in laptop batteries are connected with each other through spot welding.

Similarly, some electronic components are also joined using spot welding. Sensitive components and sensors are installed on PCBs through this procedure. Other than that, it’s used in solar panels, sensors, electrical assemblies, etc.

Aerospace Industry

The aerospace industry also uses spot welding to join different types of metal sheets. It can work on a variety of materials.

It’s used to make the body of the spacecraft, wire mesh, shielding, etc. It’s also used to assemble battery packs. 

Advantages of Spot Welding

High Speed

Spot welding can be done at an incredible speed. It requires a fraction of a second or several seconds to make a spot weld. That said, it’s an extremely fast process. It can be done in no time. That’s the reason it’s used in mass manufacturing processes, such as automotive manufacturing.

Easy Process

The process is quite simple, and beginners can also master it quickly. It is not as complex as other welding processes and does not need extensive experience or expertise. It does not require heavy pre-processing; minimal cleaning is enough.

Aesthetics

When you create spot welding, there is no disposition of the material or any elevated surface that gives a clue of welding. There is just a small spot that can be easily covered. Therefore, it’s a process that does not affect the aesthetics of the product. It looks clean.

Low Cost

The cost of spot welding is minimal. There is no consumable electrode or gas. It does not require any consumable material. It creates a weld with electrical resistance, and all it needs is electricity. Moreover, the energy consumption is also low. It’s an extremely low-cost process, which is why it’s preferred in several applications over other welding processes.

Automation

Spot welding can be automated. In the automotive industry, robots are used to make spot weldings on the car frame. It saves the time of the workers and allows them to focus on more important things. On top of that, it ensures consistent results at an incredible speed.

Strength

Within seconds, the weld joint is created, but it’s strong enough. If the weld is done properly, it’s not easy to separate two metal pieces with human force. Even if you do it somehow, it will create a hole in the sheet.

Minimal HAZ

Spot welding affects a very small area. It is an instant process in that the heat does not travel to far and wide areas. Thus, there is no need to worry about metal deformation. It can even weld thin metal sheets without any hassle. Only the spot you are welding is affected.

Versatility

It’s a versatile welding process. It can be used on different materials, including steel, aluminum, nickel alloys, copper, etc. Even if you want to weld two different metal pieces, it’s also possible. It can be used in a wide range of applications where metal sheet welding is required. 

Limitations of Spot Welding

Limited Material Thickness

With spot welding, you can only weld metal sheets or metal pieces that aren’t too thick. It works on materials with thicknesses of 0.5mm to 3mm, and it depends on the machine you have. It’s suitable for thin materials and can weld them without any issues. But if you try to weld thicker pieces, it might not be able to fuse the metal pieces and make the weld.

Low Strength

Even though the strength of spot welding is good, it’s much lower compared to other welding processes. It joins metal sheets at a tiny spot, which can’t bear heavy or medium loads. It’s not a continuous welding process, so the strength is affected.

Accessibility Issues

You can’t do spot welding in every scenario. Because it does not have a handheld electrode, stinger, or torch. You have to place the metal pieces between the electrodes to create the joint. The electrodes hold both pieces and exert pressure from both sides. That said, there are limitations.

Tips for Perfect Spot Welding Results

Preparation

Spot welding does not require excessive preparation of the materials. However, it’s better if they are clean from rust, dirt, oil, grease, and other contaminants. Because it affects the weld quality. Welders can clean the metal pieces with a cleaning solvent, wire brush, sandpaper, etc.

Alignment

The metal pieces should be aligned properly on the machine. They should have a suitable overlap area to create the joint. Make sure both metals overlap each other perfectly, and there should be no gap in overlapping. It ensures proper joint formation.

Secondly, the electrodes should be properly aligned. You can adjust the positions of the electrodes on the machine. For ideal results, the tip of one electrode should be aligned with the tip of the other electrode.

Correct Parameters

Selecting the right parameters is the key to getting good results. According to the type of material, thickness, and other factors, welders need to select the appropriate current magnitude, time of the weld, exerted pressure, and other parameters.

It takes time to understand these parameters. Welders have the experience and experience to select the right values to get good results. However, it’s better to do some test welds to adjust the parameters accurately.

Electrode Cleaning

The electrodes of a spot welding machine require proper maintenance. They should be cleaned to ensure there is no dust, debris, oxidation, etc. Secondly, they should be in proper shape. Their tips often get deformed. Welders need to clean and reshape the electrode often or according to their usage. 

Laser Spot Welding

Resistance spot welding is undoubtedly the most used spot welding process. However, laser spot welding is the future.

In laser spot welding, a highly concentrated laser beam is used to make extremely precise spot welds. It’s much cleaner than the traditional spot weld and offers excellent strength.

It’s capable of heating a minimal area without creating contact. There is no need to worry about the deformation of the materials under the electrode.

It also eliminates the accessibility issues of resistance spot welding. It can be done almost anywhere. All you need to do is target the metal pieces with the laser.

Laser spot welding is suitable for intricate and sensitive jobs. It affects only the targeted area and ensures the area around it remains unharmed. That’s the reason it’s preferred in applications where there are delicate materials or components around. It can easily work on small components and sensitive objects.

Moreover, laser spot welding is even faster than the traditional method. It does not require aligning the metal pieces under electrodes. It saves time because of the non-contact process.

Laser spot welding is superior to resistance spot welding in every aspect. It’s not being adopted in different industries to achieve exceptional welds at minimal cost and high speed.

Conclusion

Spot welding is an instant welding technique that creates discrete welds on spots or points. It generates heat through electrical resistance and melts metal pieces in no time. It is used in various industries and applications for joining metal sheets quickly and at a low cost. Resistance spot welding offers incredible advantages that we have discussed but also has some limitations. It’s an easy process but requires an accurate selection of the parameters. Welders need to understand the welding process, parameters, and other factors to master the welding process and get excellent results.