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Principle of resistance welding | LearnMech

FUNDAMENTALS OF RESISTANCE WELDING

In ancient times, metal welding was done in the form of forge welding (metals heated up to melting point are pressed together) and brazing (weld using alloy of low melting point). Resistance welding processes are pressure welding processes in which heavy current is passed for short time through the area of interface of metals to be joined with the application of pressure. In other way, it may be defined as “Resistance welding is a thermo-electric process in which heat is generated at the interface of the parts to be joined by passing an electrical current through the parts for a precisely controlled time and under a pressure”.

The various types of resistance welding are as follows:
(a) Spot welding –
(b) Seam welding
(c) Projection welding
(d) Resistance Butt welding
(e) Flash Butt welding .
(f) Percussion welding.

Principle of Resistance Welding

The name “resistance” welding derives from the fact that the resistance of the workpieces and electrodes are used in combination or contrast to generate the heat at their interface. Heat is generated by the passage of electrical current through a resistance circuit. Heat is generated in localized area which is enough to heat the metal to sufficient temperature so that the parts can be joined with the application of pressure. The force applied before, during and after the current flow forces the heated parts together so that coalescence will occur. Pressure is required throughout the entire welding cycle to assure a continuous electrical circuit through the work. Pressure is applied through electrodes. The pressure is applied by mechanical, hydraulic or pneumatic systems.

In resistance welding, the parts to be joined are heated to plastic state by their resistance to the flow of electric current and mechanical pressure is applied to complete the weld. In this process, there are two copper electrodes in a circuit of low resistance as shown in Figure 2.1. When the current is passed through electrodes, the electrical resistance at the metal joints becomes very high. So, the metals are brought to red-hot plastic condition. Now, the mechanical pressure is applied to complete the weld. The heat developed by the current is proportional to the electric resistance of the weld.

AC with suitable transformer is used for the power supply. Usually, 4 V to 12 V is used dependent on the composition, area and thickness of the metal to be welded. The power supply ranges from 6 to 18 kw per cm3 area used.

Resistance welding processes differ from other welding processes in the aspect that no fluxes are used and filler metal rarely used. All resistance welding operations are automatic. Therefore, all process variables are preset and maintained constant. Resistance welding equipment utilizes programmers for controlling current, time cycles, pressure and movement. Welding programs for resistance welding can become quite complex. In view of this, quality welds do not depend on welding operator skill but more on the proper set up and adjustment of the equipment and adherence to weld schedules. Three major factors involved in weld quality are as follows:

The amount of current that passes through the work
The pressure that the electrodes transfer to the work
The time the current flows through the work.

The important characteristics of the resistance welding process is the transfer of heat to two parts being welded for obtaining a proper fusion even if the plates are dissimilar from the stand point of material or thickness.

If the proper heat balance is existed only, the proper fusion can be obtained by providing an electrode with a smaller contact area at the thinner sheet and a thicker electrode at the thicker sheet together with very high current densities for short times. If two dissimilar metals with different electrical conductivities or thermal conductivities are to be joined,

1. Large contact area electrode should be used for the one which has higher electrical conductivity, and
2. Small contact area electrode should be used for the one which has higher thermal conductivity.