The modern evolution of resistance welding towards very short welding times has resulted in the adoption of very strong currents; the amount of heat to obtain a welding point is in fact given by the Joule law reported above, in which evidently the decrease of the term (t) must correspond to an adequate increase of (I).
The intensity value therefore varies primarily with the speed of welding, but it is also a function of other parameters:
- the pressure, which greatly affects the contact resistances: at higher pressure there are lower resistances and therefore higher current intensities;
- the condition of the surfaces, the presence of oxides, impurities, etc. they greatly increase the contact resistance and therefore decrease the intensity, making the welding time longer when they do not even make welding impossible. When the combination is not correctly applied due to excessive tension, sparking may occur due to the formation of an electric arc which leads to the formation of superficial oxidation. This obstacle must be eliminated with a braking action that removes the obstacle before performing a second welding;
- the shape of the piece can derive currents by subtracting them from the welding area;
- the length and distance of the electrode-holder arms, that is the surface embraced by the secondary: the greater this is, the greater the impedance becomes and consequently the lower the current intensity for the same machine setting;
- the presence of magnetic masses embraced to the secondary, which increase the reactance of the secondary circuit and therefore decrease the intensity.
The pressure is applied for a longer period than the current flow: the pressure cycle in fact starts before and ends after the current cycle. The association of a strong pressure leads to the execution of the sintering process due to the breaking of the surface crystals.
In the total welding cycle we could then distinguish three phases:
1. approach: during which there is only the application of pressure without current;
2. welding: with simultaneous action of current and pressure;
3. cooling: in which the current is removed while the pressure is maintained.
The compressive force must in this phase bring the faces close to the point to be welded up to the point of contact; if the pieces are put together badly the effort must be able to deform them elastically or even plastically until they fit together.
It is thus seen that there is an interest in using, in this first phase, overabundant compression efforts; the effort must be even greater if the surfaces are not perfectly clean.
In this second phase the compression effort performs different functions:
· maintain the juxtaposition of the parts to be welded: which is generally easier than the combination itself, because when the current passes the pieces heat up and are better deformed;
· act on the contact resistances: so as to allow the passage of a suitable welding current and to locate the heating in the most intimate contact area of the mating surfaces.
With regard to the second function, note that the resistance R3 between the pieces and those R1 and R5 between pieces and electrodes have opposite pressure requirements: R3 should be decreased as little as possible to favor the local heating of the surfaces in the point to be welded, ie it would require low pressure values; but then the resistances R1 and R5 assume too strong values and the electrode tips wear out quickly and the piece heats up too much. There is therefore a lower pressure limit below which one must not descend, if one does not want to deteriorate electrodes, pieces and welding, triggering a voltage arc that is highlighted with a spark accompanied by a considerable temperature rise that can reach harmful values for mucous membranes and bone. By increasing the pressure it is in fact possible to reach a value which, leaving a sufficiently high R3 to produce a temperature necessary for welding in a short time, reduces R1 and R5 to values compatible with a limited heating of the electrode tips.
During this last phase the pressure must keep the pieces well together during the crystallization time.
The third generation welder is based on this technology. It should be remembered that through the copper electrodes, thanks to the greater thermal conductivity, the heat produced is dissipated which is harmless for peri-implant tissues both short and long term.
The process then takes place in an Argon-saturated atmosphere that is directed to the point concerned with targeted flow and controlled by a microprocessor in accordance with the procedures developed by the author.