Definition
Electric resistance welding is an autogenous pressure welding process, in which the heat required to locally bring the surfaces to be welded to the sintering and syncrystallization temperature is provided by electric resistance by the passage of a current through the area to be joined. Welding is accomplished without filler metal. We now describe the two processes that take place in 3rd generation machines.
By syncrystallization we mean the joining of two metal surfaces by pooling the atoms in the constitution of the crystal lattice of the junction area.
Sintering is a thermal treatment of particulate materials in fractionated or crystalline form. It is a process of densification of a compact of powders with removal of interstitial porosity, coalescence and development of strong bonds between adjacent particles. The heat treatment involves reaching a thermal level between 0.7 and 0.9 times the melting temperature. The material is first subjected to a pressure treatment which leads to obtaining the “densification” result of the reduction in the surface / volume ratio. With consequent reduction of the free energy of the system. It should be kept in mind that the atoms of the surface layer have a chemical bond that is not completely balanced, so it is necessary to supply energy to move atoms from within a solid, where the bonds are balanced, to the surface. The compacting of spherical particles of the same diameter does not exceed 74%, so that in reality materials with different grain sizes are used simultaneously. The positive effect is given by the fact that small particles can be inserted in the interstices left by the combination of larger particles with an improvement in the degree of compaction of the “green” or of the product before the heat treatment. The very nature of the surface of the Ti associated with surface treatments aimed at increasing the surface of contact with the bone with the increase in surface irregularity leads us to consider the possibility of using sintering as a procedure associated with that of deep syncrystallization. The resulting advantage is the formation of an extremely resistant sleeve positioned peripherally to protect the joint. We can therefore define two distinct phases in the intraoral welding process.
Phase 1. Corresponds to the first pulse applied to the joint area. The application of the electrodes associated with the great pressure exerted by the preload provided by the clamp causes a break due to fragmentation of the surface crystals of the Ti with consequent “densification”
The subsequent passage of current in accordance with the preset thermal level brings the sintering to the solid state according to a diffusive mechanism in which the juxtaposition of the crystals involves the formation of a grain edge (crystal) seat of movement of matter from the inside of the grain towards the outside with progressive approach of the adjacent crystal centers. Densification is produced by shifts of matter from within the grains or the migration of atoms and grain boundary vacations, from migration of atoms from the grain center, to displacements and dislocations of the crystal lattice. These last movements involve a structural variation of the grains which thus reach a high degree of “densification” (fig. 6-1). The sintering reaches its final point passing through a progressive reduction of the spaces between the crystalline grains until reaching the definitive structure. The thermal rise caused by the passage of current optimizes the procedure associated with pressure. The densification speed is regulated by the equation:
Where is it
“p” represents the density
“α” represents the average radius of the grains
“C” represents a constant
To conclude, therefore, we must stress that the guiding concept of the whole process is surface energy. For fractionated substances the surface energy per unit of volume is much greater the smaller the grains. This means that if one metal grain joins another there is an increase in volume but a decrease in surface area and a consequent decrease in energy. The process is therefore favored from an energy point of view. As said the surface sintering leads to the formation of a very resistant interface. This fact, if the correct temporal succession between the first and second impulse is not respected, can determine a barrier effect that can negatively influence the subsequent in-depth syncrystallization. For this reason, in addition to the pressure and amount of energy applied, it is necessary to respect a precise temporality in the operating sequence.
The third generation welder therefore requires compliance with these parameters which are completely different from the previous ones. The energy levels reached are reset according to different power emission curves and calibrated to the pressure values applied in the area of the joint. The temporal sequence is calculated so as not to interfere with the peripheral sintering process and at the same time not to impede the deep intercrystalline bonds produced by syncrystallization. This must be checked accurately with the application of curves of adequate power that do not lead to excessive work hardening of the joint area. Here it is essential to protect against the yield stress which can cause irreversible deformations and subsequent breaks. The value of the yield stress depends on both the strain rate and the temperature at which the deformation takes place. We remind you that the Ti of the plants works in an extremely variable temperature environment dependent on many factors such as breathing in a very cold atmosphere or ingestion of very hot food or drinks.
Another advantage of peripheral sintering is the protection of stress corrosion cracking. Stress-corrosion means the degradation due to the combined action of corrosion and load application.
This action can cause sudden and unexpected breakages. The speed of propagation of the fracture is remarkable precisely because of the combined action of the two factors.
In the case of implant structures it must be remembered that being placed in an environment rich in bacteria, they can be affected by biocorrosion factors caused by the production by the micro-organisms of substances that can attack the part of Al present in Titanium alloys. The passivability, understood as a property of forming autonomously for the sole contact with the air, a very thin oxide layer associated with surface treatment is a predisposing factor to the phenomenon.
The electric resistance welding is performed by applying to the contact surfaces a pressure by means of devices that are called electrodes as they also serve to bring the current to the pieces to be welded.
The welding is carried out with the sintering and syncrystallization sequence in the area of the mating surfaces, crossed by the maximum current density; since the welded area is very limited around a point, it is called a welding point.
Principle of operation
Consider the following example:
We have two titanium sheets superimposed between two copper electrodes connected to the ends of the secondary of a transformer.
The upper electrode can be vertically movable, while the lower one is fixed; we exert a certain pressure on the first and then close the switch A.
The current passing in the secondary will develop in the various sections of it a heat which is more intense the higher the resistance encountered according to the law of Joule:
Where is it:
Q = quantity of heat expressed in large calories (Cal);
J = mechanical equivalent of the large calorie expressed in Joule / Cal;
I = current intensity in Amperes;
R = electrical resistance in Ohm;
t = time in seconds
Let us consider the factors on which the development of heat depends, namely R, I and t.
The resistance R of the secondary circuit includes:
R0 = resistance of the secondary circuit between the electrode tips (ie excluding the two sheets to be welded) this circuit is made of copper;
R1 = contact resistance between upper electrode (copper) and lower sheet;
R2 = upper part resistance;
R3 = contact resistance between the two sheets;
R4 = lower sheet resistance;
R5 = contact resistance between the lower price and the lower electrode.
Since we are dealing with series resistances we can write:
In this sum:
- R0 is negligible since the electrodes are made of copper (thermal conductivity decidedly greater than that of the Titanium) and therefore it is not even to consider the temperature increase that could be harmful for the soft tissues;
- R1 and R5 contact resistances between electrodes and sheets, after R3 they are those of higher intensity
- R2 and R4 internal resistance of the sheets, are quite considerable, given the resistivity of the titanium and are naturally increasing as the temperature increases; but the heating produced by them is always lower than that of contact resistances;
- R3 is the contact resistance between the two pieces, the maximum that is encountered in the secondary circuit
The three contact resistances R1, R3 and R5 are therefore the fundamental resistances to the effects of heating: they vary depending on the nature of the metals and the state of the surfaces and also as a function of temperature and pressure. The latter has indeed a fundamental importance, as it is well known that the contact resistances of the materials is very sensitive to it; two surfaces, however well polished, always touch each other through the tips of their microscopic asperities; if the surfaces are compressed against each other, the tips are crushed and the areas of effective contact increase, so that the contact resistance decreases with increasing pressure.
It would seem from what has been said that the application of a pressure to the pieces to be welded is counterproductive for the purposes of heating the pieces themselves; it is essential because:
1.only applying a sufficient pressure the contact resistances are reduced so as to allow the passage of currents so high as to produce in a short time an intense and localized thermal effect which must be lower than the melting point at the time of the first impulse it causes sintering. The second pulse brings the interface to the melting temperature causing its syncrystallization;
2.Thanks to the application of a strong pressure, the contact resistances R1 and R5 between the pieces and the electrodes can be reduced to values so low as not to overheat the copper electrodes and not to cause a deformation due to heating and a surface contamination inadmissible.
The temperature trend through the welded joint is the one shown in figure 6 – 6.
Next to the central tip corresponding to the welding point, this diagram also has two lateral points at the electrode-piece contact. In order to contain these tips at minimum value, the electrodes are made of Copper, which is an excellent conductor of electricity and heat.
In addition to the pressure factor that regulates the contact resistance, to make resistance welding possible, it is also necessary that the other two heating factors intervene appropriately.
The current intensity intervenes in the squared Joule formula and therefore there is convenience to increase it; this reduces the execution time and consequently prevents a considerable heat dissipation from the contact areas of the two sheets. This dissipation is, in fact, as far as possible to be avoided given that, not only is it a waste of energy, but the increase in temperature in areas far from the point to be welded, could damage the soft tissue of the mouth; furthermore, the spread of heat under the electrode tips could quickly ruin them.
The appropriate combination of pressure, intensity and time therefore provides the right energy source for resistance welding without reaching damaging tissue temperatures.