Part. 7: ELECTRIC RESISTANCE WELDING OF TITANIUM
Electric resistance welding is an autogenous welding method using pressure where the heat required to bring the immediate surfaces to the right sintering and syncrystallization temperature , is provided by the electrical resistance created by the passage of an electric current through the joint zone. Spot welding is performed without additional metal. And below, the description of the processes occurring in the III° generation machines.
Syncrystallization is the term used to describe the union of two metallic surfaces through the sharing of atoms constituting the crystal lattice in the joint zone .
Sintering is a heat treatment of materials in the form of fractionate or crystal particles.
It’s a densification process of a dusts close with interstitial porosity removal, coalescence and development of strong bonds among contiguous particles.
According Heat treatment, heat level must be included between a percentage of 70 and 90 % of the melting temperature. First, material undergoes a pressure treatment , that brings a “densification”, resulting from the decrease of the ratio surface/volume. With a consequent reduction in the system’s free energy.
You need to remember that atoms of the surface layer have a not completely chemical balanced bond and so, energy must be supplied to move atoms from the inside of a solid, where the bonds are balanced, to the surface. The compacting of spherical particles with the same diameter isn’t higher than 74% and so, in concrete terms, we use materials with a different granulometry. The positive effect is that the little particles can insert in the interstitial spaces resulting from the closing up of the bigger particles with a better compaction of the “green”, that is the handiwork before the heat treatment.
The very nature of Titanium surface together with surface treatments, directed to increase the bone interface, makes us consider the possibility to use sintering like a method associated to the deep syncrystallization, in the case of a surface irregularity increase.
The resulting advantage is the formation of a very resistant muff, peripherally placed, as a protection of the joint. We can therefore define two different phases in the process of intra-oral welding.
it corresponds to the 1st pulse applied to the joint zone. The application of electrodes associated to the strong pressure (acted by the preloaded) supplied by a pincer causes a break for fragmentation of the Titanium surface crystals with a consequent “densification”. The next current passage concordant with the pre-set heat level brings sintering to the solid state, according a diffusion mechanism in which the closing
up of crystals causes the formation of a grain boundary (crystal) where material moves from the inside of the grain to the outside and produces a progressive approach of the middles of the contiguous crystals.
Densification is produced by the material displacements from the inside of the grains that is to say, motion of atoms to the boundary grains, through motion of atoms from the middle of the grains, by displacements and dislocations of crystal lattice. These last movements produce a structural change of the grains that reach in this way a high degree of “densification” (fig.1). Sintering concludes, passing from a progressive reduction of the spaces among crystal grains up to the formation of the final structure. The Heat increase, caused by the current passage, optimizes the process related to the pressure.
Densification speed’s equation:
“p” is the density
“α” is the mean radius of the grains
“C” is a constant
And so finally, the main concept of the entire procedure is the surface energy. In the case of fractioned substances, the more the volume-unit surface energy increases the less the grains contract . This means that the alloy of two metals causes an increase of the volume and a fall in the surface and a consequent fall in energy. The process is facilitated energetically. As we said, surface sintering produces a very resistant interface.
The failed succession of the right time sequence between the 1st and the 2nd pulse can determine a barrier effect which could affect negatively the consequent deep syncrystallization. Therefore, you need to respect a precise time in the operational sequence besides to the pressure and quantity of the applied energy.
The 3rd generation welding machine implies the respect of this parameters that are totally different than the previous ones. So the energetic levels are re-set respecting different emission power curves and they are graduated according to the pressure values in the joint zone. The time sequence is calculated in order to not interfere with the peripheral sintering and at the same time, to not obstruct the deep inter-crystalline bonds produced by the syncrystallization.
We must monitor it through the application of appropriate power curves which don’t cause an extreme hardening of the joint zone. In this case, the protection from yield stress is very important because of one-sided strains and breaks. Yield stress value depends on both the strain rate and the strain temperature.
We underline that implants titanium works in extremely variable temperature ambient resulting from different factors like breathing in very cold atmosphere or ingestion of food or very hot drinks.
The other advantage of peripheral sintering is the protection from coxrosion. The coxrosion is the deterioration produced by the combined action of corrosion and the load applying.
This action can cause sudden and unexpected breakings. Breaking transmission speed is high because of the combined action of the two factors.
Implant structures, if put in a bacterial ambient, could undergo factors of bio corrosion produced by micro-organisms of substances that can damage the part of Al in Titanium alloys. This problem is enhanced by the power to form, through the only contact with air, a very thin oxide film, associated to the surface treatment.
Resistance electric welding is made applying a pressure to the contact surfaces through electrodes because they need also to conduct current to the elements to weld.
Welding is realized by the sequence of sintering and syncrystallization in the zone of corresponding surfaces, crossed by the maximum current density; as the welded zone is very delimited around a point, it is called welding point.
Please Consider the following example:
We place two Titanium plates one over the other between two copper electrodes connected to the ends of a transformer secondary.
The upper electrode has to be vertically movable, whereas the lower one fixed; we can press the first and then close the A switch.
The current passing to the secondary circuit generates an intense heat in its different sections that varies in relation to the resistance encountered according to the Joule’s law:
Q = quantity of heat expressed as large calories (Cal);
J = mechanical equivalent of the great calorie expressed in Joule/Cal;
I = current intensity in Ampere;
R = electric resistance in Ohm;
t = time in seconds
Now, consider single factors from which depends the heat development, that is to say R, I and t.
Resistance R of the secondary circuit includes:
R0 = secondary circuit resistance between the electrode tips (the 2 plates to weldare excluded) this circuit is made of copper;
R1 = contact resistance between the upper electrode (copper) and the lower plate;
R2 = resistance of the upper piece;
R3 = contact resistance between the two plates;
R4 = resistance of the lower plate;
R5 = contact resistance between the lower piece and the lower electrode.
Considering they are resistances in series we can write:
R = R0 + R1 + R2 + R3 + R4 + R5
In this sum:
· R0 is not so important because electrodes are made of copper (heat conductivity is higher than the one fo Titanium) and so the temperature icrease, that could damage soft tissues, is not considered;
- R1 e R5 contact resistances between electrodes and plates, after R3 are the ones with higher intensity
· R2 e R4 internal resistances of plates, quite important, considering Titan strength, and do naturally increase according to the temperature increase; but the heating they produce is always lower than the one of the contact resistances ;
· R3 is the contact resistance between the two parts, the max you met in the secondary circuit,
So the 3 contact resistances R1, R3 e R5 are very important for the heating: they change according to the nature of metals and to the surface condition, and also to the temperature and pressure. This last one is so much important because materials contact resistance is very sensitive to pressure; two surfaces, although well smooth, they always touch by the tips of their microscopic asperities; if the surfaces are compressed one against the other, the tips crack and the areas of real contact increase, so that the contact resistance decreases when the pressure increases.
In consideration of what stated above, we could deduct that the pressure we exert on the parts to be welded is negative; it is essential instead as:
- Only with a sufficient pressure you reduce the contact resistances, in order to enable the power supply passage, so high to produce an intense local thermal effect in a short while; that must be lower than the melting point on releasing of the 1st pulse that causes the sintering. Thanks to the 2nd pulse the interface will reach the melting point and so syncrystallization.
- Thanks to the big pressure we exert, the contact resistances R1 and R5 between the parts and the electrodes can be reduced to values so low, that cannot heat the copper electrodes too much and cause a heating deformation and an unacceptable surface contamination.
The course of temperature along the welded joint is shown in the following fig 6.
Near the central point that corresponds to the welding point, the diagram shows also two lateral points connected with the contact electrode-part. In order to keep them at min. value, the electrodes are made of copper, which is very good conductor of electricity and heat.
Apart from the pressure element that regulates the contact resistance, to enable the welding at resistance, it is also necessary that other two heating elements act in a proper way.
The power supply intensity is present in Joule’s formula squared and it is convenient to increase it; this enables to shorten the execution time and so to avoid a great waste of heat from the contact area of the two plates. This waste is, as a matter of fact, as much as possible to avoid, considering that, not only it represents an energy waste but also the increase of temperature in areas far from the point to be welded, it could damage the soft tissue of the mouth; moreover, the heat spread beneath the electrodes tips could quickly damage them. The correct combination of pressure, intensity and of time gives the proper energy source for welding and resistance.