Part. 8: FACTORS AFFECTING THE WELDING PROCESS
8a: Current intensity
The modern evolution of resistance leading to shorter welding times has resulted in the use of very strong currents with very low voltages. The amount of heat needed to obtain spot welding is given by Joule’s law, as shown above: a reduction of time (t) corresponds to an appropriate increase in (I).
- pressure ,which influences the contact resistances : lower resistances correspond to higher pressure and so higher current intensity;
- The surface condition, the presence of oxides, impurity, etc..increase much the contact resistance and decrease so the intensity, making the welding time longer whenever they make welding even impossible; if, because of too high voltage currents, the approach doesn’t attain correctly, it can occur a twinkle produced by the creation of a voltaic arc from which results a surface oxidation. Through a braking action this obstacle could be removed before making a 2nd welding.
- The piece shape can come from the currents by removing them from the welding area;
- The length and distance of the electrode arms, that is the surface embraced by the secondary, that increase the secondary circuit reactance and decrease so the intensity.
- The presence of magnetic masses around the secondary, they increase the secondary circuit’s reactance and decrease so the intensity.
Pressure is applied for a longer period than current is transmitted: the pressure cycle starts before the current cycle and ends after it. The association of a strong pressure makes sintering effect through the surface crystals break.
We can identify three stages in the overall welding cycle:
- Squeeze time: when pressure alone is applied without the transmission of current;
- Welding phase: involving contemporary application of pressure and of current;
- Cooling phase: when current is turned off but pressure continues to be applied.
8c: Squeeze time
In this phase the squeeze pressure generated by compression brings together the surfaces to be welded until they touch in the welding point ;if the parts touch badly, the effort should be able to deform that in an elastic way until when they perfectly match.
You so understand that in this first phase it is useful to use redundant compression efforts; the effort must be higher if the surfaces are not perfectly clean.
8d: Welding phase
In this second phase the compression effort performs in different functions:
- The parts remain in contact: this turns out to be easier than realizing the match itself, as when the current runs along the parts, these get warm and become so better deformable;
- Act on the contact resistances: in order to enable the passage of a correct welding current and to locate the heating in the area of most contact of the joining surfaces.
In relation to the second function please note that the resistance R3 between the parts and the R1 and R5 between the electrodes show opposite pressure exigencies: R3 should be decreased as little as possible to help the local warming of the surfaces in the part to be welded; this would require low pressure values; but so the resistances assume too strong values and the electrode tips quickly wear out and the piece warms too much. Three is therefore a lower pressure limit under which you have not to go, if you do not want to damage the electrode, the piece and the welding. By increasing the pressure you realize on the other hand that R1 and R5 loose the resistance welding: you can so find a pressure value that, leaving R3 quite high to produce a necessary temperature for welding in a short time, reduces R1 and R5 to compatible values with warming limited to the electrode tips.
8e: Cooling phase
During this last phase the pressure must keep the pieces well matched during the whole period of crystallization.
The 3rd generation machine is based on this technology. It is necessary to remind that through the copper electrodes, thanks to the higher thermal conductivity the heat, harmless for the tissues, both in short and long term, is being dissipated.
You find the process also in the Argon’s saturated atmosphere that is being directed to the interested point with a precise flux and controlled by a microprocessor in compliance with the procedures formulated by the author.