EP3743537A1

EP3743537A1 - Welding electrode for sheets of aluminium or steel, and method for producing the electrode

Info

Publication number: EP3743537A1
Application number: EP19704851.5A
Authority: EP
Inventors: François PRIMAUX; Thierry SOREAU; Samuel DETREZ; Alain BOUYER
Original assignee: Lebronze Alloys
Current assignee: Lebronze Alloys
Priority date: 2018-01-18
Filing date: 2019-01-10
Publication date: 2020-12-02
Also published as: WO2019141916A8; WO2019141916A1; CN111615565B; JP2021511218A; JP7325446B2; CN111615565A; CA3087845C; FR3076751A1; US20230141080A1; FR3076751B1; CA3087845A1; MX2020007594A; KR20200106900A

Abstract

The invention relates to an electrode for welding sheets of steel or aluminium, with a conductivity greater than or equal to 90% IACS and made of an alloy consisting, by weight based on the total weight of the alloy, of chromium in a proportion higher than or equal to 0.1% and lower than 0.4%, between 0.02 and 0.04% of zirconium, lower than 0.015% of phosphorus, the remainder being copper and less than 0.1% of unavoidable impurities, the electrode structure advantageously comprising incoherent chromium precipitates, more than 90% of which have a projected surface area of less than 1 um2, the precipitates having a size of between 10 and 50 nm, the electrode having a fibre structure consisting of radial fibres having a thickness of less than 1 mm and a substantially central fibreless region that has a diameter of less than 5 mm. The invention also relates to a method for producing the electrode.

Description

DESCRIPTION

Title: WELDING ELECTRODE FOR ALUMINUM OR STEEL SHEETS AND PROCESS FOR OBTAINING THE ELECTRODE

The present invention relates to the field of welding electrodes.

The invention is more particularly related to copper resistance welding electrodes.

The electrodes according to the invention will be particularly interesting especially for welding aluminum sheets together.

The electrodes according to the invention can also be used for welding steel sheets.

By way of a preliminary remark, the term "aluminum sheets" in the rest of the description refers to sheets made from alloys comprising aluminum, in particular AlMgSi (aluminum-magnesium-silicon) alloy sheets, or AlMgMn alloy (aluminum-magnesium-manganese).

These aluminum sheets, once welded and assembled, have an application particularly in the field of the automotive industry.

Traditionally, the welding of two sheets is achieved by the combination of a high electrical intensity and a point pressure, also called "clamping force".

More particularly, in a first step, the clamping force between said two sheets to be assembled is increased. Then, during a second phase, and once the two sheets are tightened, current is passed between two electrodes which are arranged on either side of said sheets.

The passage of the current between the two electrodes causes an increase in the temperature in the area concerned of the sheets, to the melting point between the two sheets, which creates, after solidification, a welding point at the sheet metal interface. -Sheet metal.

In the case of aluminum welding, the clamping force reduces the contact resistance between the sheet and the electrode. The pressure maintains contact between the electrode and the sheet assembly. To weld, a clamp plate the assembly with copper electrodes, a material both excellent conductor of electricity and heat. This choice reduces the heated area, which is limited to the contact area between the two sheets to be welded.

Once the melting point is reached, the pressure is maintained, and the electrical current is stopped to cool the welding point before separating the electrodes from the assembled plates and then proceed to the next welding point.

The welding parameters are therefore dependent, in particular, on the electrical resistance of the sheets, the interface resistance between the sheets and the electrode, the total thickness of the assembly and the diameter of the electrodes.

Such a method is for example commonly used in the assembly of steel sheets having a small thickness.

This method can also be implemented, but less commonly, for aluminum sheets.

With regard to the electrode itself, it is known from WO 2016/203122 of the state of the art, a welding electrode for steel sheets, including sheets having an anticorrosion coating, and whose base composition consists of an alloy of copper, chromium and zirconium, and further comprising phosphorus and / or magnesium.

The proportion of chromium in the alloy is between 0.4 and 0.8% by weight, that of zirconium is between 0.02 and 0.09% and the total proportion of phosphorus and magnesium is greater than 0.005% in bulk, with a magnesium content of less than 0.1% by weight and a proportion of phosphorus of less than 0.03% by weight. The remainder of the composition consists of copper.

The metallurgical structure of this electrode is particular and comprises incoherent chromium precipitates of which more than 90% have a projected area less than Imth ² , said incoherent chromium precipitates having dimensions included between 10 and 50 nm. In addition, said electrode has a fiber structure.

The electrical conductivity of such an electrode for welding steel sheets is greater than 85% IACS.

Such electrodes are particularly advantageous for welding steel sheets, in particular because they are more resistant to the phenomenon of corrosion than the usual electrodes. This corrosion phenomenon results from the chemical reaction of the copper of the electrode and the zinc of the coating with the iron of the steel sheet, and leads to a degradation of the surface layer of the electrode, requiring a regular withdrawal of the corrosion layer, or even change electrode.

In any case, in the case of welding of steel sheets, the temperature at the welding point reaches a value of 1560 ° C and, when in contact with the surface of the steel sheet, the surface of the electrode goes reach a temperature above 700 ° C.

However, at such temperature levels, on the one hand, the chemical reaction that leads to the corrosion of the surface of the electrode is accelerated but, in addition, the material itself of the electrode will deform by a phenomenon of called "hot creep" resulting in a lateral detachment of the surface layer of the electrode and therefore to a widening of its ends.

As a result, the contact area between the electrode and the sheet is enlarged, and it is then necessary to increase the current density to maintain a quality of the spot welding point. However, a surface increase and an increase in current mean even greater corrosion.

The electrode as described in the international application WO 2016/203122 thus allows an improvement in the creep resistance at temperatures above 700 ° C, and can in some cases reach 800 ° C, when welding steel sheets together. .

However, the conductivity of such electrodes can be further improved. In addition, it should also be noted that, in order to reduce the weight of automobile bodies, in order to limit fuel consumption, more and more car manufacturers are now replacing steel sheets with aluminum sheets. especially aluminum alloy type AlMgSi alloy (aluminum-magnesium-silicon) or AlMgMn alloy (aluminum-magnesium-manganese).

Indeed, aluminum has a density at 35% of the density of steel sheets used until then.

It should also be noted that the tendency to replace steel sheets with those made of aluminum alloy is further amplified by the development of electric cars and the need to improve the battery life of the latter.

Another advantage in the use of aluminum sheets is improved resistance to corrosion, rendering unnecessary the presence of zinc-based anticorrosion coating required for steel sheets.

In addition, it has been found that it is quite possible for car manufacturers to use steel sheet body assembly lines with resistance welding robots to assemble aluminum sheet bodies. This is a real benefit for companies as there is no need to invest in a dedicated assembly technique (gluing, clinching, riveting, laser, etc.).

At the present time, automobile manufacturers use, for example, copper-zirconium alloy electrodes (0.15%) for the resistive welding of aluminum sheets for automobile bodies, these electrodes being also implemented. commonly used in the welding of steel sheets.

In the case of aluminum sheets, the welding point requires reaching a contact temperature between the two sheets of 660 ° C, substantially lower than the temperature of 1560 ° C reached at the point of welding of two steel sheets. The surface temperature of the sheet in contact with the electrode will be therefore also lower than that observed when welding steel.

Indeed, the much better electrical conductivity of aluminum compared to that of steel (4 to 5 times higher), considerably reduces the resistive heating that achieves fusion at the welding point.

Therefore, under similar welding conditions, it is necessary, in order to weld two aluminum sheets, to substantially increase the applied intensity, typically 120% in comparison with the intensity used for the welding of steel sheets, in particular. simultaneously reducing the welding time, the latter typically having to be divided by two compared to that of steel.

The energy dissipated in the electrode is proportional to the square of the intensity, the electrical resistance of the electrode, and the welding time. Specifically, this dissipated energy is 2.4 times higher in an electrode used for welding aluminum sheets compared to an electrode for steel.

The electrical resistance being inversely proportional to the electrical conductivity, it is necessary, for welding aluminum, to have an electrode having an electrical conductivity greater than 90% IACS (for International Annealed Copper Standard), while a higher conductivity at 75% IACS is required for welding steel.

In addition, in order to have an acceptable service life of the welding electrodes of the aluminum sheets, it is also necessary to take into account the reactions, chemical and thermomechanical, which take place during this welding, when the surface of the electrode is contacted with the aluminum sheet.

The chemical reaction is the result of hot contact between the aluminum of the sheet and the copper of the electrode, which forms a layer of oxygen alloy, aluminum and copper. This layer is substantially more resistive than the layer of copper and zinc alloy that forms on the surface of the electrode during the welding of two steel sheets coated with corrosion protection of zinc. The surface layer of the electrode, during the welding of aluminum sheets, is therefore much more likely to heat up under the effect of the resistance and the applied intensity than the matrix of this same electrode. to promote the fusion bonding of oxidized aluminum to the surface of the electrode, which should be avoided.

Typically, the surface temperature of an electrode during the welding of aluminum is between 500 and 550 ° C., whereas this same temperature is greater than 700 ° C. during steel welding.

Thus, the temperature difference between the surface of the electrode and the temperature of the metal to be welded is much greater in the case of welding of the steel, compared to the welding of aluminum sheets.

Indeed, as already mentioned above in the description, the contact temperature between the two sheets when they are steel must reach 1550 - 1560 ° C for there to be melting, while the surface temperature of the The electrode is greater than 700 ° C, which results in a temperature difference of the order of 750-850 ° C.

In the case of aluminum welding, the contact temperature between the two sheets must reach 660 ° C. while the surface of the electrode has a temperature of the order of 500 to 550 ° C., which results in a maximum temperature difference of the order of 160 ° C.

Especially since, in the case of welding of steel sheets, the zinc surface layer protects the steel from the sheet during hot welding against corrosion. The zinc layer blocks the heating of the sheet by the effect of the latent heat of melting of the zinc and prevents direct contact of the iron of the steel with the air.

Such a zinc surface layer does not exist on aluminum sheets. Therefore, no protection is provided in the case of welding of aluminum sheets. Thus, the alloy layer comprising oxygen, aluminum and highly resistive copper, which accumulates on the surface of the electrode at each welding of two aluminum sheets together, will increase this resistive effect and increase the contact temperature between the electrode and the aluminum sheet, until the melting temperature of aluminum.

At this point, the welded spot is ejected, that is, ejection of molten metal at the outer face of the sheets, and the quality of the welded spot is degraded.

With regard to the thermodynamic reaction when the surface of the electrode is contacted with an aluminum or steel sheet, this results, on the one hand, from the hot flow of the surface of the electrode during welding, under the effect of the clamping force exerted by the welding clamp, and, on the other hand, the surface tearing of the electrode under the effect of the force of opening the clamp at the end welding.

Under the clamping force, the contact surface of the electrode will expand, resulting in a reduction in current density and less and less localized heating at equal welding intensity. The diameter of the welded point is reduced, and becomes insufficient to ensure the assembly of the two sheets.

In the case of welding the steel, under the opening force, the more the electrode is stuck to the sheet, more micro-stripping occurs and degrade the contact surface of the electrode.

To return to the welding of aluminum, it is imperative to avoid the expulsion of the welded spot when a surface temperature of the electrode close to the melting temperature of the aluminum is reached.

For this purpose, it may be interesting to increase the clamping force. Indeed, the higher the clamping force is, the better the contact between the sheet and the electrode, the lower the contact resistance is and the less there is heating to the contact surface of the electrode, and, the lower the temperature, the less oxidation of aluminum and transfer of aluminum oxide to the surface of the electrode.

However, with the widening of the contact surface, resulting from the clamping force in particular, it is necessary to increase the welding current in order to maintain a satisfactory weld point quality, which results in even greater degradation of the electrode.

When the degradation of the surface of the electrode is too great, a mechanical etching of this surface is then essential, in order to guarantee the quality of the welded point.

Such stripping operation however has the disadvantage of requiring a stop of the resistance welding robot on the assembly line of the sheets, inevitably causing a decrease in productivity, particularly if the stripping frequency is important.

It therefore appears necessary to propose an electrode that meets the needs of the resistance welding process of aluminum sheets, having an optimal electrical conductivity and an improved welding performance in comparison with the CuZr electrodes at 0.15% zirconium usually used for this process .

More generally, it is advisable to propose an electrode having in all cases an improved electrical conductivity, in particular for the welding of aluminum-based sheets, but also for the welding of steel sheets, and which makes it possible to reduce the resistance contact between the sheet and the electrode thus avoiding heating to the contact surface of the electrode and the disadvantages thereof.

For this purpose, the present invention relates to an electrode made of copper, chromium, zirconium and phosphor alloy for welding metal sheets made of steel and aluminum or aluminum alloys, characterized in that the alloy consists of chromium in a proportion greater than or equal to 0.1% and less than 0.4% by mass, of zirconium in a proportion of between 0.02 and 0.04% by mass, of phosphorus in a proportion of less than 0.015% in bulk, the remainder of the composition being copper and unavoidable impurities in a proportion of less than 0.1% by weight, and the electrical conductivity of said electrode being greater than or equal to 90% IACS (International Annealed Copper Standard). Advantageously, the structure of the electrode comprises incoherent chromium precipitates of which more than 90% have a projected area of less than 1 μm ² , said incoherent chromium precipitates having dimensions of at least between 10 and 50 nm, said electrode having in addition, a fiber-reinforced structure, visible in a transverse section of the active face of said electrode after surfacing and etching, said structure being constituted, on the one hand, of a plurality of radial fibers, said fibers having a thickness of less than 1 mm and, secondly, a substantially central zone without fiberization having a diameter less than 5 mm.

Most preferably, said electrode, when it is used in the case of welding of aluminum or aluminum alloy sheets, is capable of allowing the maintenance of a specific pressure greater than or equal to 120 MPa when welding between they two aluminum sheets, to limit the contact resistance between said electrode and the outer surface of one of two aluminum sheets.

The reduction of the chromium content in the starting alloy, in comparison with the CuCrZr alloy further comprising phosphorus and / or magnesium used for the manufacture of welding electrodes for steel sheets in the application WO 2016/203122 , allows a substantial improvement in the conductivity, which is then systematically greater than or equal to 90% IACS, as will be demonstrated in the examples given below.

In addition, such a reduced chromium content makes it possible, against all odds, to preserve the incoherent chromium precipitates which were already at the origin of the improvement in the welding performance of the electrode for steel sheets described in the international application. WO 2016/203122, in particular by increasing the resistance of this electrode to creep when hot.

Thus, the electrode according to the present invention is particularly interesting and suitable in particular for use in the welding of aluminum or aluminum alloy sheets, but also for the welding of steel sheets, in particular particular because of the particularly high electrical conductivity it has.

Advantageously, the proportion of chromium is between 0.2 and 0.3% by weight.

According to another particularity of the invention, the proportion of zirconium is between 0.03 and 0.04% by weight.

Interestingly, the proportion of phosphorus is less than 0.01% by weight.

Preferably, the proportion of unavoidable impurities is less than 0.05% by weight.

In particular, a weighting coefficient is assigned to each chemical element likely to be present as an impurity in the alloy, as a function of the effect of said chemical element on the electrical conductivity; the sum of the weighted proportions of each of said chemical elements, in part per million, being less than 5000.

Even more preferably, the sum of the weighted proportions of each of said chemical elements, in parts per million, is less than 2000.

The present invention also relates to a method for manufacturing a welding electrode according to the invention, by continuous casting, from an alloy consisting of chromium in a proportion greater than or equal to 0.1% and less than 0.4. % by weight, of zirconium in a proportion of between 0.02 and 0.04% by weight, of phosphorus in a proportion of less than 0.015% by weight, the remainder of the composition being copper and unavoidable impurities in a lower proportion; at 0.1% by weight, said process comprising at least the following steps: a) melting the various components of the alloy, namely copper, chromium, zirconium and phosphorus and / or magnesium at a higher temperature or equal to 1200 ° C;

b) continuous casting through a cylindrical die having a diameter d to obtain a bar of a diameter close to the diameter d of the die with the maintenance of the liquid metal in the casting furnace at a temperature between 1100 and 1300 ° C; c) solidification of said bar and cooling to a temperature below 100 ° C, the cooling rate being at least 10 ° C / s until a bar temperature of 1060 ° C, then at least equal to 15 ° C / s between 1060 and 1040 ° C, then at least 20 ° C / s between 1040 and 1030 ° C, then at least 25 ° C / s between 1030 and 1000 ° C, then at less than 30 ° C between 1000 and 900 ° C, then at least 20 ° C / s for temperatures below 900 ° C, until the bar is cooled to a temperature of not more than 100 ° C ; d) cold deformation to obtain a bar with a diameter less than 20 mm;

e) shearing said rod to obtain plots and punching or machining by material removal to give said electrode its final shape,

said method comprising at least one aging treatment step or income before and / or after the step e) shaping the electrode, and wherein the metallurgical structure of the active surface of said electrode process comprises precipitates incoherent chromium of which more than 90% have a projected area of less than 1 μm ² , said incoherent chromium precipitates having dimensions of at least 10 to 50 nm, said electrode further having a fiber structure, visible in a cross-section of the active face of said electrode after surfacing and etching, said structure being constituted, on the one hand, of a plurality of radial fibers, said fibers having a thickness of less than 1 mm, and, on the other hand, of a zone substantially fiber-free core having a diameter of less than 3 mm, and the electrical conductivity of said electrode being greater than or equal to 90% IACS (for International Annealed Copper Standard).

Preferably, the melting of the various components of the alloy of step a) is carried out at a temperature of between 1200 ° C. and 1300 ° C.

With regard to the continuous casting of step b), this is advantageously carried out with the maintenance of a temperature of the liquid metal in the casting furnace between 1150 and 1250 ° C.

The cooling of said bar of step c) can be carried out at a cooling rate of at least 30 ° C./s for temperatures below 900 ° C., until the bar is cooled to a temperature at more than 100 ° C.

With regard to the aging treatment, it may, in a first embodiment of the method, be carried out before the step e) of shaping the electrode and consist of a precipitation treatment carried out at a temperature between 450 and 480 ° C for a period of 1 to 2 hours.

In a second embodiment, the precipitation treatment is carried out at a temperature of between 450 and 480 ° C. for a period of 1 to 2 hours, according to the step e) of shaping the electrode.

The diameter d of the die is preferably between 20 and 70 mm, preferably between 20 and 40 mm.

During step d) cold deformation, an external machining operation, less than 0.5 mm thick is advantageously carried out to eliminate the surface defects generated in step c) solidification.

The present invention has many advantages.

First, because of the composition of the base alloy used for the manufacture of the electrode according to the invention, the electrical conductivity of the latter is particularly high, typically greater than or equal to 90% IACS. This improved conductivity makes it possible to overcome the decrease in electrical resistance of aluminum, compared to that of steel.

Second, the electrode of the invention has a substantially improved resistance to the creep phenomenon, and this compared to the CuZr electrodes currently used for welding aluminum sheets in the automotive industry. This improved creep resistance results from high hardness preserved despite the heating generated in the electrode and on its surface during welding.

As a result, the contact surface of the electrode with the sheet will be less subject to expansion under the effect of the clamping force exerted by the welding clamp and thus the bonding of the electrode on the sheet will be limited . Therefore, when opening the clamp, less micro-surface tearing will occur at the electrode.

This creep resistance makes it possible to reduce the effect of broadening the contact surface which is usually capable of causing a decrease in the current density and a reduction in the diameter of the welded spot, which would become insufficient to guarantee the assembly of the two sheets.

Thirdly, this creep resistance makes it possible to maintain a high specific pressure and to reduce the contact resistance. In the case of aluminum or aluminum alloy sheet welding, poor contact resistance favors diffusion of aluminum into the copper at the surface of the electrode and transfer of aluminum oxide to the surface of the electrode. the electrode. The contact resistance results from the formation of a highly resistive oxygen, aluminum and copper alloy layer which accumulates on the surface of the electrode at each welding.

In the case of steel welding the specific pressure is of the order of 80 MPa, in the case of aluminum this pressure must remain above 120 MPa to avoid a too high contact resistance.

The electrode of the invention makes it possible to maintain a specific pressure greater than 120 MPa during the welding of aluminum sheets without generating a rapid widening of the surface of the electrode by a large creep.

Finally, it follows from the foregoing that, compared to current CuZr electrodes, the electrode of the invention can be used for a higher number of cycles before the mechanical stripping operation is necessary to restore the quality. of the surface of said electrode, resulting in a significant gain in terms of productivity. Other features and advantages of the invention will emerge from the following detailed description of non-limiting embodiments of the invention, with reference to the single appended figure illustrating, on the left, an electrode according to the invention and, on the right, an electrode made of copper and zirconium alloy, containing 0.15% by weight of zirconium, and currently used by car manufacturers for welding aluminum sheets.

The gray portion visible at the rounded end of each of the two electrodes represents the amount of material to be removed, by mechanical etching, to maintain an optimum quality of the welded spot, after welding having applied identical parameters to the two electrodes in terms of, for example, the number of soldered points, the applied electrical current, the welding time, etc.

The subject of the present invention is in particular an electrode made of an alloy consisting of:

chromium in a proportion greater than or equal to 0.1% and less than 0.4% by weight, advantageously between 0.2 and 0.3% by weight,

zirconium in a proportion of between 0.02 and 0.04% by weight, more preferably between 0.03 and 0.04% (ie between 300 and 400 ppm, 1 ppm corresponding to 1 mg / kg),

phosphorus in a proportion of less than 0.015% by weight, advantageously less than 0.01% (less than 100 ppm),

the remainder of the composition being copper and unavoidable impurities in a proportion of less than 0.1% by weight, given that, more preferably still, the proportion of impurities is less than 0.05%, ie less than 500 ppm.

The presence of impurities in an alloy is inherent in the process of producing this alloy. The total proportion of all the impurities in the alloy used for the manufacture of the electrode of the invention must not, however, exceed 0.1% by weight in order not to have a negative effect on the characteristics of said electrode, especially on her particularly high electrical conductivity, greater than or equal to 90% IACS (International Annealed Copper Standard).

The unavoidable impurities result from the elaboration of the alloy and include all the elements, other than those coming into the composition of the alloy, which are likely to affect the conductivity, with the exception, however, of silver.

Indeed, an addition up to 0.05% by weight (500 ppm) of silver is possible without harming the performance of the electrode.

The silver will therefore not be counted in the impurities and may be added up to a proportion of 500 ppm without affecting the characteristics of the electrode according to the invention.

As mentioned above, it is important that the impurities present do not reduce the electrical conductivity. However, some elements considered here as impurities act more on reducing the electrical conductivity than others.

This fact should be taken into account by assigning to each impurity a weighting factor, as shown in Table 1 below:

Table 1: Weighting coefficient value according to the chemical element The sum of the proportion of each impurity in weighted ppm of the coefficient shall not exceed the value of 5000.

Advantageously, the weighted sum of the impurities does not exceed 2000.

Thus, for example, if in the alloy are found, as impurities, in the proportions indicated, 100 ppm of silicon (Si), 100 ppm iron (Fe), 50 ppm tin (Sn), 50 ppm aluminum (Al), 50 ppm zinc (Zn), 20 ppm sulfur (S) and 100 ppm d Other impurities, the total proportion of impurities is 470 ppm.

For the weighted sum of impurities, it is calculated as follows by multiplying the proportions, in ppm, of each impurity present by their respective weighting coefficients, and summing the weighted proportions.

By taking up the impurities given in the example above, their weighted sum is calculated as follows

100x10 + 50x2 + 50x2 + 50x1 + 20x20 = 2650.

The present invention also relates to a method for manufacturing a resistance welding electrode from an alloy whose composition consists of copper, chromium, zirconium and phosphorus, in the proportions indicated in particular below. above.

The method of manufacturing the electrode is a continuous casting process and it comprises at least the following steps: a) the various components of the alloy are fused at a temperature above 1200.degree. C., preferably between 1200.degree. 1300 ° C;

b) a continuous casting is carried out through a cylindrical die, or a cylindrical mold having a diameter d to obtain a bar;

This casting may be performed at a holding temperature of the liquid metal in the casting furnace of between 1100 and 1300 ° C, preferably between 1150 and 1250 ° C.

c) said bar is solidified and cooled, preferably at a defined cooling rate to a temperature below 100 ° C, the cooling rate being at least 10 ° C / s until a temperature is reached of the bar of 1060 ° C, then at least 15 ° C / s between 1060 and 1040 ° C, then at least 20 ° C / s between 1040 and 1030 ° C, then at least equal to 25 ° C / s between 1030 and 1000 ° C, then at least equal to 30 ° C between 1000 and 900 ° C, then at least equal to 20 ° C / s for temperatures below 900 ° C, until the bar is cooled to a temperature not higher than 100 ° C.

The cooling rate is therefore at least 20 ° C / s until it reaches at least a bar temperature of 100 ° C.

Preferably, the cooling rate is at least 30 ° C / s for temperatures below 900 ° C, until the bar is cooled to a temperature of at most 100 ° C.

Advantageously, the cooling of said bar of step c) is carried out at a cooling rate always at least equal to 30 ° C / s for temperatures below 700 ° C.

This solidification and cooling step does not include a specific heat treatment, the dissolution can be done from the end of the solidification at 1060 ° C.

d) cold deformation of said bar to obtain a bar with a diameter less than 20 mm, preferably between 12 and 19 mm; optionally, an outer machining operation, preferably less than 0.5 mm thick, may be performed so as to eliminate surface defects possibly generated by the previous step; e) the electrode is shaped by shearing said bar to obtain slugs and punching or machining by material removal to give said electrode its final shape.

During the process, at least one aging treatment, or income treatment, is carried out. This step takes place before and / or after the step e) of shaping the electrode.

This aging treatment consists of a heat treatment that can be carried out in different ways.

Preferably, it is a precipitation treatment carried out at a temperature of between 450 and 480 ° C. for a period of between 1 h and 2 h.

It is therefore possible to perform this precipitation treatment at a temperature between 450 and 480 ° C, for a period of between 1h and 2h between the cold deformation step d) and the step e) of implementation. shape of the electrode. According to another embodiment, the precipitation treatment is carried out according to step e) of shaping the electrode, as the only aging treatment of the process.

The implementation of a precipitation treatment at the end of the process, after step e), has the advantage of providing greater stability of the mechanical characteristics of the electrode.

Two precipitation treatments under the aforementioned conditions of time and temperature can also be conducted, one before step e), and the second following this step e) shaping the electrode.

Particularly advantageously, in step b) of the process of the invention, the diameter d of the continuous casting cylindrical die is less than 70 mm.

Preferably, said diameter d is between 20 and 70 mm and, more preferably still, this diameter is between 20 and 40 mm.

In addition, the cooling rate applied during step c) of the process and allowing the solidification of the bar and solid cooling is particularly important, resulting in rapid solidification and extremely powerful peripheral cooling.

Preferentially, the cooling rate is also variable as a function of the temperature of said bar.

More specifically, said cooling rate is advantageously at least 10 ° C./s when the bar has a temperature greater than 1060 ° C. and then at least 15 ° C./s when the temperature is between 1060 and 10 ° C. 1040 ° C, then at least 20 ° C / s when the temperature is between 1040 and 1030 ° C, then at least equal to 25 ° C / s when the temperature is between 1030 and 1000 ° C, then at less than 30 ° C / s between 900 and 1000 ° C. For bar temperatures below 900 ° C, the cooling is preferably carried out at a speed of at least 20 ° C / s. The cooling rate can be at least 30 ° C / s for temperatures below 900 ° C.

Preferably, in the process according to the invention, the cooling is not applied to a solid but to a liquid and starts from the solidus, that is to say at a temperature of the order of 1070 ° C. In particular, it has been demonstrated a temperature range, between 1060 and 900 ° C, for an improvement of the dissolution with a minimum cooling rate which was repeated above in the definition of the process.

Below 900 ° C dissolution is impossible, it will be ensured for temperatures below 900 ° C to continue cooling with a minimum of 20 ° C / s to not generate uncontrolled aging.

More precisely, the very fast solidification and cooling up to a temperature where the diffusion of chromium atoms is limited allows a homogeneous distribution of coherent and incoherent chromium precipitates.

These cooling conditions, which are furthermore applied to a cylindrical mold having a reduced diameter of between 20 and 70 mm, preferably between 20 and 40 mm, contribute to obtaining a bar with a columnar solidification texture orientated in such a way radial. This texture is visible by making a cross section of said bar, and on 1 'the entire volume of the latter.

The die or mold, having a cylindrical shape, is preferably surrounded by an envelope in which circulates either an oil or a refrigerant gas, or water, so as to allow solidification and cooling.

Another advantage of the process according to the invention lies in the fact that it makes it possible to avoid dynamic recrystallization when hot, due to reheating and simultaneous deformation. As a result, the interesting precipitates and textures resulting from the implementation of the process of the invention are preserved. Within the base alloy used for the manufacture of the innovative welding electrodes, there is preferably a chromium content in a proportion of greater than or equal to 0.1% and less than 0.4% by weight. proportion being preferably between 0.2 and 0.3%.

By means of the process according to the invention, incoherent precipitates of chromium, that is to say particles without crystallographic relationship with the matrix, are in excess of solubility limit.

Indeed, in the process of the invention, the application of the quenching treatment from the solidification of the alloy, which is complete at a temperature of the order of 1070 ° C., makes it possible to maximize the solubility of the chromium in the copper and maintain the eutectic copper chromium at the grain boundaries.

It has been possible to determine that, in a particularly surprising manner, a proportion of chromium greater than or equal to 0.1% and less than 0.4% makes it possible to produce the desired chromium precipitation.

Thus, contrary to the idea commonly established in the state of the art, despite a decrease in the proportion of chromium within the alloy, the combination of the process steps used on the composition of the alloy described here allows the incoherent chromium precipitates to be conserved, and this without generating chromium precipitates of too large a size, which would be likely to cause the appearance of decohesions during step d) of cold transformation.

The very fine columnar solidification texture, obtained by the implementation of the process of the invention, makes it particularly advantageous to distribute the heterogeneity of chromium composition (chromium in solid solution, eutectic chromium and chromium metal) homogeneously in the entire volume of the welding electrode obtained by said method.

These chromium precipitates are at the origin of the improvement of the welding performance of the electrode, increasing the resistance of the latter to hot creep. As note, with regard to the welding of steel sheets with a zinc coating, these precipitates have the function of delaying or blocking the diffusion of iron and zinc which are at the origin of the chemical corrosion of the active face of said electrode.

The method of the invention, and in particular the preferential application of cooling from the solidus, also promotes a homogeneous distribution of coherent precipitates of chromium, that is to say the precipitates having a continuity with the crystallographic structure of the matrix.

By implementing the method of the invention, the electrode obtained also has a fiber structure, due to the presence of copper precipitates, or grains, which themselves have a very fiber-like shape.

According to a longitudinal section of an electrode of the invention after punching (results not shown), it appears that the fiberization is symmetrical right left, the fibers starting from the active face, and close to the internal cooling face of the electrode and moving closer to the skirt of the electrode.

According to a cross section of the same electrode, the fibers are comparable to the spokes of a wheel whose hub, corresponding to the central zone of the electrode without distinctive fiber has a diameter less than 5 mm, preferably less than 3 mm. The fine radial fibers have a thickness advantageously less than 1 mm, and more preferably less than 0.5 mm.

This fiber texture, very characteristic of the electrode obtained by implementing the method of the invention, is a direct consequence of the metallurgical structure obtained after step c) of said process, and is very different from the fine and homogeneous structure some traditional electrodes.

The fiberization of the electrode obtained by the present method, in particular due to the presence of needle-like copper grains having a large length, makes it possible to improve the resistance to thermomechanical stress fields, including the field of deformation and the temperature field of the active face of said electrode during welding.

More particularly, the fibering of the electrode of the invention promotes, during the welding of the steel or aluminum sheets, a heat dissipation radially and longitudinally, from the central zone of the electrode, where the temperature is maximum, to the cold areas, that is to say the inner face and the periphery of the electrode. Consequently, the electrode of the invention is more resistant in particular to the creep phenomenon.

It has already been mentioned, above, the composition of the base alloy to obtain said electrode of the invention. This alloy comprises copper and chromium, the latter component being present in the alloy in a proportion greater than or equal to 0.1% and less than 0.4%.

In addition to these two constituents, the alloy according to the invention also comprises zirconium in a proportion of preferably between 0.02 and 0.04% by weight. Such a proportion advantageously makes it possible to avoid generating precipitates which could promote cold cracking of the material.

The proportion of zirconium is, still more advantageously, between 300 and 400 ppm, ie between 0.03 and 0.04%.

It is also advantageous for the base alloy to contain phosphorus in a proportion of less than 0.015% by weight, this proportion preferably being less than 100 ppm.

This element, which is both more deoxidizing than chromium and less than zirconium, facilitates a good control of the residual zirconium content when large quantities of production are envisaged.

The present invention also relates to an electrode capable of being obtained by the method described above.

As already mentioned above, said electrodes of the invention have original microscopic properties compared to traditional electrodes. Transmission microscopy analyzes of the structure of the material of the electrodes of the invention, before and after welding, made it possible to demonstrate differences with respect to the microscopic structure of the traditional CuZr electrodes, and in particular on the morphology of the crystalline grains. as well as the dimensions and distribution of chromium precipitates.

In particular, it is observed on a microscopic scale that the material of the electrode according to the invention comprises more than 90% of incoherent chromium precipitates which have a projected area of less than lpm ² .

In addition, at the nanoscale, in addition to coherent chromium precipitates having dimensions of the order of 2 to 5 nm, a population of incoherent chromium precipitates with dimensions of between 10 and 50 nm is observed, and more precisely between 10 and 20 nm.

These incoherent chromium precipitates are characteristic of the electrodes of the invention and are not visible at the material of conventional CuZr electrodes.

In addition, it should be noted that the analyzes carried out also demonstrated a dimensional evolution of these incoherent chromium precipitates, during the sheet-welding step, in this case zinc-coated steel sheets, by means of the electrode of the invention.

Indeed, during the welding of zinc-coated steel sheets, a coalescence of the precipitates is observed at the approach of the active face of the electrode and, more precisely, incoherent nanometric precipitates of 30 to 50 nm in the b-layer. and from 100 to 150 nm in the y layer.

Typically, the layer b of the chemical reaction layer is furthest from the surface of the electrode. It is a yellow zinc diffusion layer in copper, at 40% zinc. At the surface, the chemical reaction layer comprises an iron-rich layer, typically 25%, which forms when the steel sheet is bonded to the surface of the electrode at a temperature above 850 ° C. Finally, between layer b and the iron-rich layer is the 55% zinc layer. Other analyzes carried out on the electrodes of the invention have shown that the incoherent chromium precipitates present in the layer g become rich in iron and, as a result, make it possible to block the diffusion of iron.

Finally, mechanical hot characterization tests were also conducted on electrodes obtained by the method of the invention. The results of these tests have shown that the creep temperature is increased by 100 ° C with the present electrodes, with respect to the creep temperature of some traditional electrodes.

More precisely in the case of welding steel sheets, generally, the creep of the active face of a traditional electrode becomes sensitive, during the welding operation, at a temperature of the order of 700 ° C. Indeed, with the surface softening of the electrode, there is creep of the surface and cracking of the layer y, which promotes a diffusion of iron in the layer y and in the layer b in the form of FeZn precipitates. The layer b becomes resistive, and heats up above 850 ° C, causing the layer g to disappear. As a result, the material of said conventional electrode will begin to tear as welding points are made, resulting in rapid degradation of the weld spot.

On the contrary, for an electrode according to the invention, in the case of welding of steel sheets, this creep temperature is of the order of 800 ° C., which makes it possible to delay the mechanical stressing of the layer g, thus favoring the protective maintenance of said layer g at the active face of said electrode.

As a result, the electrodes obtained by implementing the present method have in particular an increased life and improved welding performance.

In order to illustrate the interest and the technical characteristics of the electrode according to the invention for the resistive welding of aluminum sheets, three examples comparing the performance of said electrode with copper-zirconium electrodes (0.15%), currently used by car manufacturers with aluminum body, are given below. EXAMPLE 1 Comparative Characterization Tests of the 3 mm Layer on the Surface of the Electrode Before and After Heat Treatment

The Brinell hardness (HB hardness) was measured at the surface and at least 3 mm from the surface of a CuZr electrode currently used by car manufacturers and an electrode according to the invention, before and after a heat treatment of 500 ° C applied for a period of 8 hours.

In addition, the% IACS conductivity was also measured for these two electrodes, before and after heat treatment.

The composition of the alloy that was used to make the electrode that was tested is as follows:

Cr: 0.2 to 0.3%;

Zr: 300 to 400 ppm;

P: 80 to 120 ppm;

Remaining: copper and unavoidable impurities in a proportion of less than 300 ppm with a weighted sum <2000.

The results obtained in these comparative tests are summarized in Table 2 below.

Table 2: Comparison of "hardness" and

"Conductivity" between a conventional CuZr electrode and an electrode of the invention before and after heat treatment

The results presented in Table 2 show that, in comparison with the traditional CuZr electrode, the "HB hardness" and "% IACS conductivity" characteristics of the electrode according to the invention are more constant between before and after the heat treatment. has been applied. Indeed, the surface of a new CuZr electrode, before heat treatment, is less conductive than the surface of a new electrode according to the invention, with a% IACS conductivity of 86 against 91.

Consequently, the traditional CuZr electrode will heat up more significantly and withstand thermal softening less well, which results in a drop in hardness after heat treatment at 100 HB against 140 HB for the electrode of 1 invention.

Such a difference in conductivity ultimately leads to greater surface creep for the CuZr electrode than for the electrode according to the present invention.

Example 2 Comparative Tests for Characterization of the Surface Layer after Welding

The Brinell hardness (HB hardness) was measured at the surface and at least 3 mm from the surface of a CuZr electrode currently used by car manufacturers and an electrode according to the invention, before welding ("new" electrode ) and after welding ("end of welding"). For the electrode according to the invention only, the HB hardness was also measured after 30 welding points.

In addition, the% IACS conductivity was also measured for these two electrodes, before and after welding, and after 30 welding points for the electrode of the invention.

The results obtained in these comparative tests are summarized in Table 3 below.

Table 3: Comparison of the "Hardness" and "Conductivity" Characteristics Between a Usual CuZr Electrode and an Electrode of the Invention Before and After Welding The results summarized in this table also demonstrate that the electrode according to the invention is much more constant between before and after welding.

The electrode of the invention operates, throughout its operating cycle, on the one hand with a higher conductivity (between 90 and 92 against 86-88) and on the other hand with a better resistance to softening. Indeed, the electrode according to the invention still has an HB surface hardness of 150 at the end of welding while the usual electrode has, at the end of welding, a hardness of 125 HB.

The results obtained also show that the loss of softening on CuZr electrodes is localized on the surface. Indeed, the hardness at least 3 mm from the surface remains substantially constant, of the order of 140-150HB, and the conductivity is not raised to 94. Despite this, the surface creep of the CuZr electrode leads to widening of the contact face and an insufficient weld point diameter.

The electrode of the invention operates in a field where it retains its mechanical characteristics.

In particular, the electrode of the invention retains a hardness at a high level, despite the heating generated in the electrode during welding, and the creep resistance is thus increased.

Therefore, said electrode deforms less during welding, allowing the user to gain productivity because the frequency of mechanical stripping decreases.

Example 3: Comparative tests of welding performance

The third test, with reference to the single appended figure, is to compare the welding performance between a CuZr electrode usually implemented by the manufacturers and an electrode of the invention.

Due to the better resistance to creep during welding, and all other parameters (in terms of welding parameters: intensity, clamping time, cooling in particular), during mechanical etching to return the surface of the electrode to the initial state, 15% less material is removed with the electrode of the invention.

The amount of material that is removed during the mechanical etching operation of the electrode of the invention 1 corresponds to the gray portion of Figure 1 attached. This amount of material that must be removed is lower for the electrode of the invention 1, compared with the conventional electrode CuZr 2, the latter undergoing a large creep resulting in an enlargement of its end, as shown in FIG. figure 1.

One cycle corresponds to the number of welded points before carrying out the mechanical stripping operation.

It is possible, with the electrode of the invention, and without changing the welding parameters, on the one hand to increase the number of cycles by 15% and, on the other hand, to increase the number of points by cycle of 10%, relative to the number of cycles and the number of points per cycle can be performed with a CuZr electrode currently used, before it is necessary to carry out the mechanical etching of said electrode of the invention to maintain a optimal quality of the welded point.

The electrode of the invention therefore makes it possible to improve the productivity of the order of 27%, without changing the welding parameters.

The electrode of the invention has a very high stability during welding cycles on aluminum sheet, by implementing the welding parameters defined specifically for optimal use of CuZr electrodes.

This implies that the welding parameters defined for these CuZr electrodes do not degrade the surface of the electrode of the invention, despite a number of weld points increased by 27% with the latter.

It therefore appears obvious to those skilled in the art that a definition of the electrode-specific welding parameters of the invention will provide further improvement in the number of soldered points.

Claims

1. Electrode made of copper, chromium, zirconium and phosphorus alloy for welding metal sheets of steel and aluminum or aluminum alloys, characterized in that the alloy consists of chromium in a higher proportion or equal to 0.1% and less than 0.4% by mass, of zirconium in a proportion of between 0.02 and 0.04% by weight, of phosphorus in a proportion of less than 0.015% by weight, the rest of the composition being copper and unavoidable impurities in a proportion of less than 0.1% by weight, and the electrical conductivity of said electrode being greater than or equal to 90% International Annealed Copper Standard (IACS) and in that the structure of said electrode comprises incoherent chromium precipitates of which more than 90% have a projected area of less than 1 μm ² , said incoherent chromium precipitates having dimensions of at least between 10 and 50 nm, said electrode p further resenting a fiber structure, visible in a cross-section of the active face of said electrode after surfacing and etching, said structure being constituted, on the one hand, of a plurality of radial fibers, said fibers having a thickness less than 1 mm and, secondly, a substantially central zone without fiberization having a diameter of less than 5 mm.

2. Copper alloy electrode according to the preceding claim for welding metal sheets of aluminum or aluminum alloys characterized in that said electrode is able to maintain a specific pressure greater than or equal to 120 MPa when soldering them together. two aluminum sheets to limit the contact resistance between said electrode and the outer surface of one of the two sheets.

3. Copper alloy electrode according to any one of the preceding claims, characterized in that the proportion of chromium is between 0.2 and 0.3% by weight.

4. Copper alloy electrode according to any one of the preceding claims, characterized in that the proportion of zirconium is between 0.03 and 0.04% by weight.

5. Copper alloy electrode according to any one of the preceding claims, characterized in that the proportion of phosphorus is less than 0.01% by weight.

6. Copper alloy electrode according to any one of the preceding claims, characterized in that the proportion of unavoidable impurities is less than 0.05% by mass.

7. copper alloy electrode according to any one of the preceding claims, characterized in that a weighting coefficient is assigned to each chemical element that may be present as an impurity in the alloy, depending on the effect of said chemical element on electrical conductivity; the sum of the weighted proportions of each of said chemical elements, in part per million, being less than 5000.

8. Copper alloy electrode according to the preceding claim characterized in that the sum of the weighted proportions of each of said chemical elements, in part per million, is less than 2000.

9. A method of manufacturing a welding electrode according to any one of the preceding claims by continuous casting from an alloy consisting of chromium in a proportion of greater than or equal to 0.1% and less than 0.4% by weight. mass, of zirconium in a proportion of between 0.02 and 0.04% in mass, of phosphorus in a proportion of less than 0.015% by weight, the remainder of the composition being copper and unavoidable impurities in a proportion of less than 0.1% by weight, said process comprising at least the following steps: a) melting the various components of the alloy, namely copper, chromium, zirconium and phosphorus at a temperature greater than or equal to 1200 ° C;

b) continuous casting through a cylindrical die having a diameter d to obtain a bar of a diameter close to the diameter d of the die with the maintenance of the liquid metal in the casting furnace at a temperature between 1100 and 1300 ° C;

c) solidification of said bar and cooling to a temperature below 100 ° C, the cooling rate being at least 10 ° C / s until a bar temperature of 1060 ° C, then at least equal to 15 ° C / s between 1060 and 1040 ° C, then at least 20 ° C / s between 1040 and 1030 ° C, then at least 25 ° C / s between 1030 and 1000 ° C, then at less than 30 ° C between 1000 and 900 ° C, then at least 20 ° C / s for temperatures below 900 ° C, until the bar is cooled to a temperature of not more than 100 ° C ; d) cold deformation to obtain a bar with a diameter less than 20 mm;

e) shearing said rod to obtain blanks then punching or machining by material removal to give said electrode its final shape, said method comprising at least one aging treatment step or income before and / or after step e) of shaping the electrode, and wherein the metallurgical structure of the active face of said electrode comprises incoherent chromium precipitates of which more than 90% have a projected area of less than ² μm, said chromium precipitates incoherent having dimensions of at least 10 to 50 nm, said electrode further having a fiber structure visible in a cross-section of the active face of said electrode after surfacing and etching, said structure being constituted, on the one hand, of a plurality of radial fibers, said fibers having a thickness of less than 1 mm, and, on the other hand, of a substantially central zone without fiberization having a diameter of less than 3 mm, and the electrical conductivity of said electrode being greater than or equal to 90% IACS (for International Annealed Copper Standard).

10. A method of manufacturing a welding electrode according to the preceding claim wherein the melting of the various components of the alloy of step a) is carried out at a temperature between 1200 ° C and 1300 ° C.

11. A method of manufacturing a welding electrode according to any one of claims 9 or 10 wherein the continuous casting of step b) is performed with the maintenance of a temperature of the liquid metal in the casting furnace included between 1150 and 1250 ° C.

12. A method of manufacturing a welding electrode according to any one of claims 9 to 11 wherein the cooling of said bar of step c) is performed at a cooling rate of at least 30 ° C / sec. for temperatures below 900 ° C, until the bar is cooled to a temperature of not more than 100 ° C.

13. A method of manufacturing a welding electrode according to any one of claims 9 to 12 wherein the aging treatment is performed prior to step e) shaping the electrode and consists of a precipitation treatment performed at a temperature between 450 and 480 ° C for a period of 1 to 2 hours.

14. A method of manufacturing a welding electrode according to any one of claims 9 to 13 wherein, according to step e) shaping the electrode is carried out a precipitation treatment carried out at a temperature between 450 and 480 ° C for a period of 1 to 2 hours.

15. A method of manufacturing a welding electrode according to any one of claims 9 to 14 wherein said diameter d of the die is between 20 and 70 mm, preferably between 20 and 40 mm.

16. A method of manufacturing a welding electrode according to any one of claims 9 to 15 wherein, during step d) of cold deformation, an external machining operation, less than 0.5 mm thickness, is performed to eliminate the surface defects generated during the step c) of solidification.