Resistance Welding : Principle, Types, Application, Advantages and Disadvantages

Resistance welding is a liquid state welding process in which the metal to metal joint created in a liquid or molten state. It is a thermo-electric process in which heat is generated at the interface surfaces of welding plates due to electric resistance and controlled low pressure is applied to these plates to create a weld joint. It is named as resistance welding because it uses electric resistance to produce heat. It is a very efficient pollution-free welding process but its applications are limited due to its high equipment cost and limited material thickness.

Resistance Welding – Spot, Seam, Projection, and Flash Welding:

Resistance Welding : Principle, Types, Application, Advantages and Disadvantages

Principle:

All resistance welding like spot welding, seam welding, projection welding, etc. are worked on the same principle of heat generation due to electric resistance. When a current passes through electric resistance, it produces heat. This is the same principle that is used in the electric coil. The amount of heat produced depends on the resistance of the material, surface conditions, the current supplied, the time duration of current supplied, etc. This heat generation takes place due to the conversion of electric energy into thermal energy. The heat generation formula is

H = I2RT

Where

H = Heat generated in joule

I = Electric current in ampere

R = Electric resistance in Ohm

T = Time of current flow in the second

This heat is used to melt the interface metal to form a strong weld joint by fusion. This process produces weld without application of any filler material, flux, and shielding gases.

Types:

There are four main types of resistance welding. These are

Spot Welding:

It is the simplest type of resistance welding in which the workpieces are held together under the pressure of anvil face. The copper electrodes are brought in contact with the workpiece and current start to flow through it. The workpiece material applies some resistance inflow of current which causes local heat generation. At the interface surfaces the resistance is high due to the air gap. The current starts to flow through it which meltdown the interface surface. The amount of current supply and time should be sufficient for the proper melting of interface surfaces. Now the current stopped to flow but the pressure applied by electrode maintained for a fraction of second, while the weld rapidly cooled. After it, the electrodes remove and brought to contact at another spot. It will create a circular nugget. The nugget size depends on the size of the electrode. It is generally about diameter 4-7 mm.

Seam Welding:

Seam welding is also called continuous spot welding in which a roller type electrode is used to flow current through workpieces. First, the rollers are brought in contact with the workpiece. A high ampere current is passed through these rollers. This will melt the interface surfaces and form a weld joint. Now the rollers start rolling at work plates. This will create a continuous weld joint. The timing of the weld and movement of the electrode is controlled to assure that the weld overlap and workpiece do not get too hot. The welding speed is about 60 in/min in seam welding. It is used to create airtight joints.

Projection Welding:

Projection welding is the same as spot welding except a dimple is produced on workpieces at the location where the weld is desired. Now the workpieces held between the electrode and a large amount of current pass through it. A small amount of pressure is applied through an electrode on welding plates. The current pass through dimple which melts down it and the pressure causes the dimple to flatten and form a weld.

Flash butt Welding:

It is another type of resistance welding which is used to weld tubes and rods in steel industries. In this process, two workpieces that are to be welded will be clamped in the electrode holders and high pulsed current in the range of 100000 amperes is supplied to the workpiece material. In this two-electrode holders are used in which one is fixed and the other is movable. Initially, the current is supplied and the movable clamp is forced against the fixed clamp due to contact of these two workpieces at high current, the flash will be produced. When the interface surface comes into plastic form, the current is stopped and axial pressure is increased to make joint. In this process, the weld is formed due to plastic deformation.

Application:

• Resistance welding is widely used in the automotive industries.
• Projection welding is widely used in the production of nut and bolt.
• Seam welding is used to produce leak prove joint required in small tanks, boilers, etc.
• Flash welding is used for welding pipes and tubes.

Advantages and Disadvantages:

Advantages:

• It can weld thin (0.1 mm) as well as thick (20mm) metals.
• High welding speed.
• Easily automated.
• Both similar and dissimilar metals can be weld.
• The process is simple and fully automated so does not require highly skilled labor.
• High production rate.
• It is an environment-friendly process.
• It does not require any filler metal, flux, and shielding gases.

Disadvantages:

• High equipment cost.
• The thickness of the workpiece is limited due to the current requirement.
• It is less efficient for high conductive materials.
• High electric power required.
• Weld joints have low tensile and fatigue strength.