WO2011058265A1

WO2011058265A1 - Method for treating a welding stud with an ion beam and welding method implementing such a welding stud

Info

Publication number: WO2011058265A1
Application number: PCT/FR2010/052365
Authority: WO
Inventors: Denis Busardo
Original assignee: Quertech Ingenierie
Priority date: 2009-11-13
Filing date: 2010-11-04
Publication date: 2011-05-19
Also published as: FR2952562B1; FR2952562A1

Abstract

The invention relates to a method for treating a metal welding stud (30) with an ion beam (100) in which: the ions of the beam are nitrogen ions; the ions have an acceleration voltage of 10 kV to 1000 kV; the temperature of the metal welding stud (30) is no higher than T_f; and the load of ions per surface unit is selected in a range of 10¹⁴ ions/ cm² to 10¹⁹ ions/ cm² in order to create a treatment thickness of 10 to 10000 nm. Said method advantageously provides a treatment for reducing welding stud erosion in the context of a method for welding sheet steel to which a zinc-based anti-corrosion treatment has been pre-applied.

Description

METHOD FOR TREATING AN ION BEAM OF A WELDING PLOT AND WELDING METHOD USING SUCH A PLOT OF

The subject of the invention is an ion beam treatment method of a metal welding pad to increase its resistance to the metallurgical degradations induced by the welding operation. The invention also relates to a welding method using the weld pad thus treated to spot weld plates, for example steel having in particular previously undergone anti-corrosion treatment based on zinc. It is thus able to greatly increase the service life of the welding pad and significantly reduce the maintenance costs incurred by stopping the robot chains necessary for its replacement.

The term "welding pad" means the tips of resistance welding electrodes by points (carried for example by a clamp operated by a robot). These pads are subjected to mechanical and thermal stresses, and consequently to significant wear, because of the repetitive and pressure contacting of their end with the sheets to be welded, and of the importance of the intensities of the electric current. crosses, which can be of the order of 10,000 A under low voltage. This current causes a point melting of the metal of these sheets, and the end of the electrode can therefore be subjected to very large temperature rises, despite the cooling provided by the circulating water in the aforementioned circuit.

The reproducibility of the quality of a weld point in time is very dependent on the quality of the contact between the welding pads and the sheets. Now this contact is degrades very quickly over time according to different processes;

• first, there is a geometrical degradation, caused by the repeated shocks due to the closure of the welding clamp and the fact that the welding pads are not always perfectly perpendicular to the sheets (robot programming). The strong heat generated during welding, which causes an alteration of the mechanical and metallurgical properties of the copper, causes the surface of contact to tend to increase;

• Since the sheet surface in which the welding current passes becomes larger, the same welding current no longer makes it possible to obtain the fusion in the same time;

• Then, there is a metallurgical degradation, due to the oxidation of the contact surfaces, due to the temperature rise and the transfer of metal oxides between sheets and welding pads. During the welding of coated sheets (galvanized, aluminized, etc.), the coating metal is transferred to the electrode and a layer of zinc or aluminum is formed which considerably modifies the contact resistances of the welding pads / sheets , and may even cause collages or welds between solder pads and sheets.

Spot welding is the main technique for assembling automotive body panels. The increased use in this sector of zinc-coated steel sheets against corrosion has led to a 10 to 100-fold increase in the metallurgical degradation rate of the solder pads, especially in alloys. copper that are in contact with the sheets. This has been demonstrated during a life test on solder pads carried out for different types of sheet metal coatings: electrogalvanized and hot-dip galvanized, both made of zinc, and "galvannealed", made of iron-zinc alloy. at higher melting point. The results have revealed common metallurgical degradation mechanisms, such as the "cratering" (digging) of the welds, their widening by erosion of their frustoconical profile and not by epilation of the copper alloy as we could have done. expect it, or the dispersion of the electric welding energy related to this enlargement, which reduces the quality of the welds. On the other hand, depending on the coating, significant differences appeared in terms of kinetics of cratering and erosion of the electrodes. These kinetics are significantly linear in relation to the number of welded points, and are much faster for a hot dip galvanized coating than for a "galvannealed" coating. The kinetics for an electrozinated coating are intermediate, close to that of a hot galvanized coating for cratering and that of a "galvannealed" coating for electrode erosion. The cratering and erosion of the weld pads is due to the formation and diffusion growth of copper-iron-zinc intermetallic layers, which are eliminated simultaneously by four mechanisms: dissolution, extrusion, peeling or "brassing". The first mechanism explains cratering, which is particularly rapid in the case of hot-dip galvanized and electrogalvanized coatings, because they consist of zinc, which is therefore liquid at an early stage during welding in order to activate the dissolution. The erosion of the plots is due to the other three mechanisms. Extrusion and peeling explain the kinetics quite slow in the case of galvannealed and electrogalvanized coatings. The behaviors of these two coatings are close because an electrogalvanized coating turns into a "galvannealed" coating during the reaction welding between the steel substrate and the zinc of the coating. On the other hand, in the case of a hot-dip galvanized coating, this reaction is inhibited by a stable layer of Fe2Al5 type, inherited from the zinc deposition process. The entire coating is melted during welding, and discharged in liquid form from the sheet metal contact / welding pad, leaving the steel substrate in direct contact with the welding pad. This activates the "brassing" mechanism, which is severe for the erosion of welding pads, and explains the particularly fast kinetics observed in the case of hot-dip galvanized coating.

With regard to the "galvannealed" treatment, its ability to protect against corrosion and the quality of its surface make it a widely used coating for many applications in the automotive industry, in visible or non-visible parts. The presence of iron in the coating gives it a particular aptitude for spot welding, which leads to recommend especially the "galvannealed" when the assembly conditions are problematic with other coatings. The "galvannealed" coating offers excellent protection against corrosion, even in case of damage (shock, scratch, grit), thanks to the electrochemical behavior of the iron / zinc couple (sacrificial anode effect). In the event of damage, the "galvannealed" coating has a low coefficient of friction, which facilitates the flow of metal between the die and the punch. The hardness and melting point of this coating makes it a preferred product for its weldability which is similar to that of a bare sheet metal, with particular life expectancy of acceptable electrodes. For example according to the ISO 18278-2 standard for a 45/45 "galvannealed" steel of 0.7 mm thickness, the weldability range is 8.6 to 9.6 kA and the service life of the welding pads of 1200 points without registration.

A single automotive production site consumes an average of 200,000 welding pads per year. The industry seeks to increase the service life of welding pads to reduce the number of maintenance cycles, extremely expensive given the immobilization of the robot chains that results from the change of these solder pads.

To solve these problems it was first thought to perform a cleaning of the solder pads with a given periodicity, but in no case break-in does recover the geometric quality of the new welding pad. In addition, the cost of this operation, including the automation of the process, the purchase and installation of the equipment and in some cases the loss of productivity, is relatively expensive. It has also been thought to gradually increase the welding current to take into account the degradation of the welding pads, but it is an extremely empirical method, which makes it possible to improve the situation, but without ensuring a regularity of quality. satisfactory because the electrode degradation processes are extremely complex and not very repetitive.

We must therefore resolve in a traditional way, especially when the welding rate is high, to replace regularly and frequently the electrode body, and especially the welding pad. Thus, the welding pad may have to be changed several times a day, and the electrode body once for every ten or twenty changes of welding pads. This results in a very strong need for treatment of welding pads, preferably according to easily industrializable methods, so as to maintain as long as possible the geometry of the welding pads, by inhibiting as much as possible the metallurgical degradation processes caused in particular by the atomic diffusion of zinc, aluminum ... or oxidation at reasonable costs.

The object of the invention is to overcome the above disadvantages, and in particular to make it possible to create an atomic diffusion barrier on the surface of the welding pad.

The invention thus proposes a method of treatment with an ion beam of a welding pad or:

the thickness of the treatment on the surface of the welding pad is between 10 nm and 10,000 nm (nanometer);

the ions of the ion beam are nitrogen (N);

the ion acceleration voltage is greater than or equal to 10 kV and less than or equal to 1000 kV;

the temperature of the solder pad is less than or equal to T _f , where T _f is the melting temperature of the metal of said metal layer;

the ion dose per unit area is selected in a range of between 10 ¹⁴ ions / cm "and 10" ions / cm "so as to produce at the surface of the welding pad an atomic diffusion barrier of which the thickness is between 10 nm and 10000 nm.

The welding pad is generally made of a copper alloy with a very high copper content.

The choice of the bombardment conditions of these ions according to the invention advantageously makes it possible to obtain a diffusion barrier on the contact surface of the welding pad. The inventors have found that the ranges chosen according to the invention for the acceleration voltage and the ion dose per unit area make it possible to select experimental conditions where the production of an effective atomic diffusion barrier is possible thanks to ion bombardment.

Despite the weakness of the implantation thickness, the inventors were able to note, to their astonishment, a considerable increase in the service life of the treated solder pads by a factor of 100 compared to those which were not. . These welding pads had for initial composition in% mass according to EN12163: Cu balance; Cr (min 0.5, max 1.2); Zr (min 0.03, max 0.3); Fe (min 0, max 0.08); If (min 0, max 0.1); others 0.2. These welding tests were carried out on steel sheets previously treated according to the galvannealed process. The welding pads were processed by simultaneous implantation of multi-energy nitrogen ions at 45, 90 and 135 Kev. The relative proportions of these ions were respectively 50%, 40%, 10%. The welding tests were evaluated with two total doses of nitrogen ions implanted one at 4.5 * 10 ¹⁷ ions / cm ² , the other at 9 * 10 ¹⁷ ions / cm ² . The first dose showed an increase in the life of the solder pad by a factor of 100, the second was slightly worse. The implantation treatment of the welding pads was carried out at a temperature below 100 ° C, using a N +, N2 +, N3 + multicharged ion beam emitted by an electron cyclotron resonance (ECR) source and extracted with a voltage of 45 KV extraction. The treatment thickness, therefore that of the atomic diffusion barrier, at the surface of the solder pad is estimated at about 300 nm.

The implantation of nitrogen into copper induces an increase in surface hardness. Observation of treated areas can be performed by microscopic observation techniques, such as microhardness at low load (preferably less than 10 g).

Without wishing to be bound by any scientific theory, it may be thought that the bombardment with ions under the conditions according to the invention has the effect of a forced insertion of nitrogen at the nanometric scale, in other words a compression of the network, thanks to which the atomic rearrangements allow the placement of nitrogen in interstitial insertion or in more or less stable metal nitrides, which considerably increases the displacement energy of the zinc atoms in the copper matrix. Contrary to what one might think, the microhardness induced by the treatment does not appear to be at the origin of such an effect on the extension of the service life of the welding pads. The atomic diffusion barrier created by the process of the invention seems much more effective in inhibiting metallurgical degradation due to migration in the welding pad of external species such as zinc, aluminum, etc.

According to different embodiments that can be combined with one another:

the dose of ions per unit area is between 10 ¹⁵ ions / cm ² and 10 ¹⁹ ions / cm ² ;

the treatment thickness is 10 nm and 10000 nm;

the ion acceleration voltage is between 20 kV and 200 kV;

the ions are produced by an electron cyclotron resonance (ECR) source which has the advantage of being compact and energy efficient; the production of the beam by the electron cyclotron resonance (ECR) source requires treatment in a vacuum chamber of less than 10 ^-2 mbar.

According to various embodiments of the method according to the present invention, which can be combined with one another:

the welding pad is movable relative to the ion beam at a speed, V _D , of between 0.1 mm / s and 1000 mm / s. It is thus possible to move the sample to treat areas whose size is greater than that of the beam. The scrolling speed V _D can be constant or variable. According to one embodiment, the welding pad moves and the ion beam is fixed. According to another embodiment, the ion beam sweeps the solder pad to be treated. It is also possible that the welding pad moves when the ion beam is movable. According to one embodiment, the same area of the welding pad (for example the frustoconical head) is moved under the ion beam in a plurality, N, of passages at the speed V _D. It is thus possible to treat the same area of the welding pad with a dose of ions corresponding to the sum of the doses of ions received by this zone at the end of the N passes. It is also noted that if the size of the welding stud head to be treated allows it, the treatment can be static and result from one or more "flash" of ions;

the welding stud consists of copper or an alloy of this metal;

The invention also relates to the method of welding galvanized sheet which uses the welding pad treated according to the method of the invention.

According to different embodiments of the method of welding a galvanized sheet, which can be combined with one another: the sheet to be welded is made of a material chosen from the list consisting of all the grades of steel;

- The sheet has been the subject of an anticorrosion treatment selected from the list of treatments based on zinc; for example, the galvannealed, electro-galvanized, hot-dip galvanized process will be included in this list.

According to one embodiment, the welding process using the solder pad treated by the method of the invention makes it possible to weld at a lower economic, energy and environmental cost, with a better quality of welding, especially in terms of stability and reproducibility of the Thermal conditions, new sheet metal composition, lighter with highly corrected corrosion protection. Maintenance cycles are much longer, reducing the cost of stopping robots.

Other features and advantages of the present invention will emerge in the following description of nonlimiting exemplary embodiments, in particular with reference to the accompanying drawings in which:

FIGS. 1a to 1d illustrate various stages of degradation of a welding pad;

FIG. 2 illustrates the atomic diffusion barrier produced on the surface of the welding pad according to the method of the invention;

FIGS. 3a and 3b show the nitrogen concentration profiles characterizing the diffusion barrier obtained according to the process of the invention for two doses which made it possible to establish its effectiveness.

FIG. 4 illustrates a welding pad treatment device according to the method of the invention; For the sake of clarity, the dimensions of the various elements shown in these figures are not necessarily in proportion to their actual dimensions. The same numerical references in the different figures correspond to the same parts.

FIGS. 1a-1d illustrate various steps that may lead to degradation of a solder pad: the welding operation, "fouling" of the surface of the solder pad with zinc, the diffusion of zinc deep into the welding stud, the erosion of the welding stud by tearing material that gives rise to a crater.

Figure l.a illustrates a welding operation. A zinc layer 10 which has been previously deposited on a steel sheet 20 to provide protection against ambient corrosion. The frustoconical head of a welding stud 30 is applied against the galvanized steel sheet, to allow the circulation of an electric current of about 10000 A (Amperes). A peak of heat of about 700 ° C. is thus created in the contact zone 40 (delimited in dashed lines) allowing the formation of a zone allowing the soldering, referred to as a soldering point 60 between the sheet 20 and the sheet 50.

Fig. 1b illustrates the fouling of a welding pad head 30 by the liquid zinc residues 70 produced at the point of contact 80 of the welding operation. Upon removal of the welding pad, a zinc deposit 10 remains stuck and "dirty" the head of the welding pad 30, thus changing its contact resistance. This has a direct effect on the reproducibility and quality of the next weld point.

FIG. 1c illustrates the dissolution by atomic diffusion of the zinc present on the sheet in the pad of welding. The temperature of the welding operation is sufficient to cause the atomic diffusion of the zinc of the layer 10 in the welding pad 30. A bonding or welding 90 connects the galvanized sheet and the solder pad, the latter being weakened by a zone underlying the creation of intermetallic compounds 100 consisting of copper and zinc.

Figure 1.d illustrates the tearing of material in the form of a crater. During the removal of the welding pad, the bonding or welding zone 90 remains integral with the galvanized sheet 10, driving with it the tearing of material 100 on the surface of the welding pad 30, giving rise to a crater 110.

FIG. 2 represents a section of a frustoconical solder pad having been treated according to the method of the invention. An atomic diffusion barrier 302 is created at the contact surface of a welding pad 30. In this highly compressed zone, there is a high concentration of nitrogen placed in interstitial insertion or present in the form of metal nitrides. This atomic diffusion barrier has a thickness 303 of a hundred nanometers which varies according to the implantation energies. It is effective enough to significantly slow the progression of zinc and other external elements inside the weld pad. It is very likely that this is reflected during the first welding operations, a difficulty of adhesion of liquid zinc to the surface of the welding pad thus reducing its fouling. The upper zone 301 is not treated.

Figure 3.a shows the atomic concentration profile of nitrogen in a copper welding pad. It was calculated for a nitrogen ion beam composed of 50% nitrogen ions at 45 Kev, 40% ions of 90 Kev and finally 10% ion of 135 Kev. The total dose of implanted ions is 4.5 * 10 ¹⁷ ions / cm 2. The lifetime of the pad has been multiplied by 100. The abscissa axis expressed in Angstr m represents the depth of penetration of the nitrogen ions in the welding pad by taking its surface contact with the sheet as a reference. The y-axis represents the atomic concentration of nitrogen. It can be seen that an atomic barrier with a thickness of about 3000 Å (Angström) has been created on the surface of the pad. The maximum atomic concentration of nitrogen, at about 500 Å (Angstrom), is 30%, which makes it sufficiently close to the stoichiometric composition of copper nitride Cu ₃ N (3 atoms of copper for a nitrogen atom) for to promote their production. It is highly likely that the presence of relatively stable metal nitrides may explain the effectiveness of the atomic diffusion barrier to significantly slow the migration of zinc. Fig. 3.b represents the nitrogen concentration profile, calculated this time, for a dose of 9 * 10 ¹⁷ ions / cm ² . The depth of the diffusion barrier remains at about 3000 Å (Angstrom). On the other hand, the surface concentration is closer to 40%, a value that is too high to favor the formation of CU ₃ . This could explain the slight decrease in the service life of the weld pad compared to that observed for solder pads treated with aose of 4.5 * 10 ¹⁷ ions / cm 2.

Figure 4 shows a device for processing metal welding pads. The frustoconical head of the studs is turned towards the beam. The welding pads 30 placed side by side move under a beam 100 produced by a compact source RCE (electronic cyclotron resonance) 110. These welding pads can for example scroll along an axis at a speed Vd under the beam 100 or for example on two axes on a plateau.

Claims

1. A process for treating an ion beam (100) with a metal welding pad (30), characterized in that:

the ions of the ion beam are nitrogen (N);

the ion acceleration voltage is greater than or equal to 10 kV and less than or equal to 1000 kV, preferably between 20 kV and 200 kV;

the temperature of the welding pad (30) is less than or equal to T _f , where T _f is the metal melting temperature of said welding pad (30);

the dose of ions per unit area is chosen in a range of between 10 ¹⁴ ions / cm ² and 10 ²⁰ ions / cm ² , preferably between 10 ¹⁵ ions / cm ² and 10 ¹⁹ ions / cm ² so as to treat superficially the welding pad in a thickness between 10 and 10000 nm.

2. Processing method according to the preceding claim characterized in that the metal welding pad (30) is movable relative to the ion beam (100) at a speed, V _D , between 0.1 mm / s and 1000 mm / s.

3. Treatment method according to the preceding claim characterized in that a same region of the welding pad (30) is moved under the ion beam (100) in a plurality, N, of passages at the speed V _D.

4. Treatment method according to any one of the preceding claims characterized in that the beam is produced by a source RCE (electronic cyclotron resonance) (110).

5. Treatment process according to any one of the preceding claims characterized in that the metal welding pad (30) consists of a metal or an alloy of this metal, where the metal is copper (Cu).

6. A method of welding sheets (20, 50) characterized in that it uses at least one welding pad (30) treated according to any one of the preceding claims.

7. Sheet welding process according to the preceding claim characterized in that the sheet (20, 50) is made of a steel grade.

8. Sheet welding process according to the preceding claim characterized in that the sheet (20) has been subjected to a corrosion treatment based on zinc (10).