Resistance Welding, as the name indicates, uses the resistance of the parts and the electrodes to generate the heat energy required for welding. In a typical welding setup, the parts to be welded are pushed together by the welding electrodes; the electrodes also supply welding current. Heat is generated at several locations in the weld assembly, including in the parts, the electrodes, and at the interfaces. Ideally, we want to generate all the heat at the interface between the parts. However, it is inevitable that some amount of heat is generated at the other interfaces and in the parts themselves. In addition to generating heat, some heat is lost from the weld, mostly by conduction by the fixtures and the electrodes. Managing the heat generated and lost so as to create the required temperature at the weld interface is the key to successful resistance welding. There are multiple issues that are important in resistance welding, but first and foremost, it is important to understand fundamentals of resistance welding
It is of primary importance to understand the "resistance" in resistance welding. Instead of having just one value for resistance, there are seven different locations in a typical current path where the flowing current encounters resistance as can be seen in the schematic. The schematic below shows a typical welding configuration where the two electrodes (shown in red) are pinching two flat parts that are to be welded. As the current flows from one electrode to the other, it encounters seven different resistances of which four (R1, R3, R5, and R7) are bulk resistances and three (R2, R4, and R6) are contact resistances. Bulk resistance is the resistance to current flow inside the part or electrode; this is the type of resistance that one commonly thinks of when we say that copper is more conductive than steel. In addition to bulk resistances, the three contact resistance provide resistance to current as it flow across the interface from one part to the other or from one electrode to the part.
Further adding to the complication is the fact that these resistances change value throughout the duration of the weld. Contact resistances (also known as interface resistances) start out high and decrease in strength as the weld progresses. The action of heat and pressure of the welding processes brings the interfaces into intimate contact thus reducing the contact resistances to practically insignificant value towards the end of the weld. On the other hand, the bulk resistances increase in value as the parts (and electrodes) get hotter during the weld. The dynamic swing in resistances have to be understood in order to design a suitable weld profile. A schematic of the changes in resistance values is shown below.
If the energy is delivered in a short time, most of the heat will be generated at the interfaces, where as if the energy is delivered over a long period of time, most of the heat will be generated in the bulk. Since conductive metals and alloys do not have any significant bulk resistance, they have to be welded in a short time weld. Materials that are very resistive are typically welded with a longer weld cycle.