What Is Spot Welding? How Is It Done?

What Is Spot Welding.
Where Is Spot Welding Used.
Pros and Cons of Spot Welding.
Spot Welding Carbon Steel.

Spot Welding Stainless Steel.
Spot Welding Aluminum.
Spot Welding Copper.
Common Spot Welding Problems.

What Is Spot Welding

Spot welding is primarily a resistance welding process used to weld two or more metal sheets together. It is achieved by applying pressure and electric current to the spot weld area. Heat is released as a result of the metal’s internal resistance to the electric current. This heat allows the two metals to fuse at the weld point.

The electric current and pressure are applied by copper-alloy electrodes whose tips are placed on opposite sides of the metal parts. As the heat produced melts the metal, the pressure from the electrodes compresses the molten metal, forming a weld. It is also called spot welding because this welding method creates a small weld mark resembling a single point.

Working principle of spot welding. The parts placed between copper electrodes are brought into contact. (above)

A pedestal spot welding machine commonly used in small and medium-sized businesses (above)

A handheld spot welding machine that provides flexibility in non-robotic manufacturing processes

A robotic spot welding machine used intensively, especially in the automotive industry

Where Is Spot Welding Used

Spot welding is used to join electrically conductive sheet metal and wire meshes. It is typically used to weld thin metals, but thicknesses above 25 mm are only possible with special heavy-duty equipment. It has replaced rivet construction, especially for thin materials.

It is mostly used to manufacture automobiles. A single vehicle’s body panels typically have more than 1000 spot welds. This is done in a few seconds with spot welding robots. However, sheet metal shops use less complex methods when working on a car body.

Use of spot welding robots in the automotive industry (above)

Spot welding is also used in industries such as aerospace, railways, manufacturing, electronics, construction, batteries, and others. It is among the economical options among welding methods.

Spot welding is used in sheet plate construction, (for example, the red fire cabinets we especially see in official buildings), ship and aircraft manufacturing, and the production of fixtures and tools.

How spot welding is done

Pros

Relatively easy to apply.
Welds any conductive metal
Saves time and labor compared to other welding processes.
The best method for achieving adequate weld strength with thin metal without burning through
Many spot welding electrode types are available for welding different metal alloys.
Welding current usage is quite efficient.
Electrodes solve the thermal conductivity problem by drawing heat away from the weld point.

Cons

You cannot detect the weld metal if one side is inaccessible.
Can cause cracking by hardening the weld zone and the surrounding material.
Can affect the chemical and physical properties of the workpiece metal. Corrosion resistance can be compromised in stainless steel, aluminum, and other metals.
Delivers low voltages (1-20V). Therefore, any fluctuation can affect spot weld quality.
May require frequent repair depending on the metal type and thickness.

Resistance Heating Principle

Spot welding joins two metal sheets using resistance heat generated by passing a high current between them.

Short Welding Time

The spot welding process is a fast joining method usually completed in just a few seconds.

Widespread Use in the Automotive Industry

Spot welding is used intensively in body manufacturing, with thousands of spot welds performed on a single vehicle.

Spot Welding Different Materials

Can carbon steel, aluminum, stainless steel, and copper be spot welded? If so, how? Let’s take a look at which electrodes are used and under what conditions.

Carbon Steel

Low-carbon steel is used in all welding processes, including spot welding. Thanks to its high electrical resistance and low thermal conductivity, mild steel is truly excellent for spot welding.

Steels with a higher carbon content (>0.4%) are difficult to weld. They tend to form hard and brittle microstructures when exposed to high heat. Therefore, special post-weld heat treatments are required to reduce the likelihood of cracking. This makes high-carbon steel unfavorable for spot welding.

Galvanized steel is coated with zinc and requires a higher electric current than uncoated steel. This is difficult for spot welders because copper electrodes deteriorate rapidly when exposed to zinc alloys. Electrodes must be frequently replaced or cleaned by machining the contaminated surfaces.

Stainless Steel

Stainless steel is sensitive to high, concentrated heat sources. However, it can be spot welded. Copper-cobalt-beryllium electrodes are commonly used for spot welding stainless steel. They offer optimum tensile strength and electrical conductivity.

Aluminum

Aluminum is the most challenging metal for spot welding, because it is an excellent conductor of heat and electricity. Electricity flows through it without much resistance, producing less heat. In addition, heat quickly moves away from the weld, making it difficult to form a molten pool.

The only way to spot weld aluminum is to use two to three times the welding current you would use when welding steel. The spot welder must also have a capacitor system. This allows it to “charge” and store electricity, then instantly release a significant amount of power. As a result, the power produced will be higher than aluminum can conduct, and the sheets will fuse together.

Resistance spot welding of any aluminum alloy significantly degrades the electrodes after just a few welds. Therefore, high-volume aluminum spot welding is not a suitable option.

Copper

Copper materials can be spot welded. However, it’s no surprise that standard copper electrodes don’t work. Molybdenum and tungsten electrodes are the primary choices for spot welding copper. These metals have high electrical resistance and melting temperatures.

Common Spot Welding Problems

Some of the most notable spot welding problems are:

Spatter reduces aesthetics.
Electrode costs can rise rapidly.
Poor weld quality due to electrode misalignment.
Cold welds are extremely weak.
Electrode indentation on the metal surface.
Welding too close to an edge can compromise weld quality.

In conclusion, spot welding is a specialized process used primarily with sheet metal and serves certain applications well. However, unlike MIG or TIG welding, it cannot handle a wide variety of applications.

In this article, we covered what spot welding is and how it’s done. We discussed how spot welding works and its areas of application. We gave the advantages and disadvantages of spot welding. We tried to explain how spot welding is applied to aluminum, copper, stainless steel, and carbon steel. See you in new articles.