Influences on the resistances in the welding circuit
The contact resistances in resistance pressure welding are characterised and can be influenced by the electrode force and the condition of the material surfaces. In addition to their basic dependence on material, conductive cross-section and conductor length, the material resistances are also dependent on temperature. This is clearly noticeable in the course of the welding. This can be seen particularly well in the graphical progression of the resistance during welding. In systems with visualisation of the welding current, welding voltage and resistance, this can be viewed in the user interface (e.g. HWI2xxx IQR and GeniusMFI IQR with Pegasus or XPegasus user interface).
For the commonly used steel materials (materials with positive temperature coefficients), the resistance increases with temperature. While the transition resistances still dominate at the beginning of the weld, they recede into the background during the further course of the weld compared to the material resistances. The material resistances increase with temperature, as already described above. As the electrode force increases, the contact resistance decreases due to the increasing contact area. In particular, the influence of the electrode force on the contact resistance must be taken into account in practice. Often the welding force is increased and it is forgotten to increase the welding current as well.
Since the contact resistance decreases with increasing electrode force, a higher welding current must flow for the same energy input.