ABSTRACT

In resistance welding, the heat needed to create the coherence is generated by applying an electric current through the stack-up of sheets between the electrodes. Therefore, the formation of a welded joint, including the nugget and the heat-affected zone (HAZ), strongly depends on the electrical and thermal properties of the sheet and coating materials. With the knowledge of phases and their transformations, as discussed in Chapter 1, a weld’s formation can be linked to the electrical and thermal processes of welding. Controlling the parameters of these processes is a common practice in resistance welding. Governed by the principle of Joule heating, the general expression of heat generated in an electric circuit can be expressed as

Q = I2Rτ (2.1)

where Q denotes heat, I is the current, R is the electrical resistance of the circuit, and τ is the time the current is allowed to ˜ow in the circuit. When the current or resistance is not constant, integrating the above expression will result in the heat generated in a time interval τ. For resistance welding, the heat generation at all locations in a weldment, rather than the total heat generated, is more relevant, as heating is not and should not be uniform in the weldment. In addition, the heating rate is more important than the amount of heat, as how fast the heat is applied during welding determines the temperature history and, in turn, the microstructure. This can be easily understood by considering an aluminum welding. If the welding current is low, melting may not be possible no matter how long the heating is, because of the low electrical resistivity of aluminum, and the fact that the heat generated is conducted out quickly through the water-cooled electrodes and the sheets due to the high thermal conductivity of aluminum. In general, the electrical and thermal processes should be considered together in welding, and such a consideration is essential in understanding resistance welding process and choosing correct process parameters.