Blog 10: How are Automobile Bodies Welded?


Spot welding or resistance welding is one of the most commonly used welding process in the sheet metal industry. It is a type of welding process in which two or more sheets of metal are welded together, which are held together with a huge force between the copper electrodes. Heat is created through the resistance created by the electric current flowing through the electrode.

Resistance welding is one of the oldest forms of welding and is known as the most efficient form of welding. It was first introduced in 1885 by Elihu Thomson when he accidentally fused two copper wires together.

Its process can be applied to a combination of two or more sheet metals having a width of 0.5mm to 5mm depending on the usage. To calculate the amount of heat generation during the flow of current we can use the following formula


Where Q is the heat, I is current, R is the resistance and t is the duration of current flow or we can say weld time.

It can be used in a number of applications preferably automotive sector, sheet metal fabrication industry, etc. The process consists of two electrodes that grab the metal using huge force and when the current passes create heat which causes the metal to weld. This creates a nugget called a spot nugget. Spot nugget is the portion where the two metals have fused into one

Equipment of Spot Welding:

In a general distribution spot welding equipment is distributed into two major types:

  1. Stationary Spot Welder
  2. Non-stationary spot welder

Stationary Spot Welding Machine:

A stationary spot welder as the name suggests is a fixed spot welding machine in which the part is rotated or moved as per the welding location. It consists of a big casing that has the timer and transformer built inside it.

At the bottom, it has a pedal that is used to move the copper electrode in vertical direction closing and opening them and creating force. On the machine frame, they have two ends coming out, one end is fixed and has the electrode at end of it called as Lower electrode. The second end has an air cylinder at the end of it in which the electrode is connected called as an upper electrode.

This electrode is movable. Both the electrode is connected with a water supply that cools the electrode from inside and an electricity supply that helps in the welding process.

The picture below will demonstrate the basic structure of a Stationary Spot-Welding Machine

Non-stationary Spot Welding Machine:

The non-stationary spot-welding machine has multiple names. It is also called a portable spot-welding machine or hanging spot welding machine. In this machine, the part or sheet metal that needs to be welded is fixed while the machine moves to the location of the spot and completes the welding process.

Non-stationary spot weld is distributed into 3 portions.

First is the timer portion. The timer is placed at a fixed location that is usually overhead.

The second is the transformer which is hung using strong metal wires.

The third is the portable gun that is used to apply the welding process.

The gun has an air cylinder from which one side of the electrode is connected which is also the movable side of the electrode. While the other side of the gun is fixed hence having the fixed electrode. The gun has air connections, water connections coming from the pipes, and electric cables going inside it from the transformer.

There are two types of non-stationary spot-welding machines based on their design:

  1. C-type portable spot-welding machine.
  • X-type portable spot-welding machine.

Process of Spot Welding:

The process of spot welding is common for both stationary spot-welding machines and non-stationary spot-welding machines. Following are the steps for the process of spot welding

  1. The process starts with the metal that is needed to be welded are brought between the electrode and the force is applied using the button or paddle
  2. Pressing the button or the paddle triggers a signal in the timer which starts the Hold Time countdown.

Note: Hold time is basically the time between the grabbing of metal and the passing of current. This time is necessary so that if by mistake the wrong location is grabbed it can be corrected before welding.

  • After completion of hold, time starts the weld time. During weld time the timer sends the signal to the transformer that steps down from high voltage to low voltage while increasing the current. That passes through the electrode creating heat and completing the welding process. 

Note: Weld time is the duration of flow of current and is basically the time during which the welding process takes place.  

  • After completion of the welding process and weld time than starts the cool time. During the cool time, the electrode still remains closed while water circulates at the back of the electrode cooling the electrode and the welded metal.

Note: Cool time is the duration for the cooling process to be completed.

  • After completion of cool time, the electrodes open, and the spot process is completed.

In terms of the graph the timers can be explained as:

Testing of Spot weld:

Spot welds are tested to check if the strength of the spot weld is good enough. Following are some methods that can be used in industries to test the strength of Spot weld

  1. Quantitative test: Qualitative test is a long-term resulting test that gives us a concrete idea about the strength of the spot weld. These tests are basically expensive and require major types of equipment to conduct them. There are two types of quantitative testing methods:
    1. Tensile Shear test
    2. Cross tensions test:
  2. Simple test: Simple test is a quick resulting test that gives us the basic idea about the strength of the spot weld. These tests are basically inexpensive and do not require any major equipment to conduct them. There are three types of simple testing methods:
    1. Peel test
    2. Torsion test
    3. Chisel test

This test shows us the weld nugget structure which we can measure and compare with the standard to check the quality of spot weld.


Following are some of the applications:

  1. Joining of Vehicle Parts.
  2. Fuel tanks.
  3. Railway tracks.
  4. Turbine blades.
  5. Home appliances. (Fridge, washing machine, AC, etc.)


Spot welding is very beneficial for different industries. Some of its advantages are:

  1. Easy automation.
  2. Low fumes.
  3. Cost-effective.
  4. No filler metal is required.


Some of the disadvantages are:

  1. Low strength in case of a discontinuous weld.
  2. High equipment cost.
  3. The thickness of not more than 6mm can be welded.

Research on Spot Welding:

Some of the research on the topic are:

  1. Science Direct: Spot welding
  2. Research Gate: Spot welding

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