Spot welding process

Abstract

The invention relates to a spot welding method, comprising steps consisting in: arranging two electrically conductive parts (P1, P2) to be joined between two electrodes (E123, E22), each of the two parts having an interface zone (S3) between the two parts and a contact zone (S1, S2) with one of the two electrodes, establishing a first electric current between the two electrodes through the two parts, the first electric current producing thermal energy capable of forming a welding nugget (WN1-WN8) within the two parts, and adjusting a distribution of thermal energy density produced by the first electric current according to intrinsic characteristics of each of the two parts, to generate a welding nugget initiation zone, at a chosen depth in the parts to be joined. Figure 3

Description

Title of the invention: Spot welding method Technical field

[0001] The present invention relates to the assembly of metal parts, and in particular the assembly of sheets by the spot welding technique. Spot welding is used in many sectors, mainly in the automotive sector, and in particular in the aeronautical and railway sectors, to assemble steel parts, and increasingly, aluminum parts. State of the art

[0002] Spot welding generally consists of placing the parts to be welded between two electrodes with low thermal and electrical resistance, which maintain them under high pressure, of the order of a few kN. By establishing a strong electric current, of the order of ten kA, between the two electrodes, electrical energy is supplied to the parts to be welded, which heat up locally by the Joule effect. If the electric current is sufficient, this supply of thermal energy locally brings the parts to be welded to fusion in the vicinity of the interface between them, to form a weld core. Spot welding therefore has the advantage of requiring neither shielding gas nor filler material.

[0003] However, the distribution of thermal energy in the parts to be welded, between the electrodes, depends on many factors, such as the thickness and the intrinsic resistivity of each of the parts to be welded, as well as the surface conditions (contact resistivity) of the contact zones of the two parts to be welded with the electrodes and at the interface between the two parts to be welded. In addition, the two parts to be welded may have different melting temperatures, so that the input of thermal energy should be greater in the part to be welded having the higher melting temperature. The distribution of thermal energy favorable to the formation of a welding nugget centered on the interface between the two parts to be welded can therefore vary significantly depending on the intrinsic characteristics of the parts to be welded.

[0004] Under these conditions, the welding nugget is initiated in an unselected area, and the duration of the electrical energy supply is increased so that the welding nugget extends across the interface between the two parts to be welded. This can therefore result in excessive electrical consumption. Furthermore, the welding nugget may be just tangent to the interface between the two parts due to an insufficient duration of electrical energy supply. The parts to be joined may apparently be fixed to each other by a sticking phenomenon, without the parts being welded to each other. Furthermore, an excessive electrical energy supply may lead to the formation of cavities in the parts to be assembled and a deterioration in their mechanical strength. These disadvantages can reduce the range of possible applications of the spot welding technique.

[0005] In some applications, an insert is used to assemble a part, for example a non-metallic part, with a metal part. The insert is engaged in an orifice formed in the non-metallic part, then is welded to the metal part so as to hold the non-metallic part against the metal part. If the thermal energy is not properly distributed, the end of the insert may collapse under the effect of this thermal energy without the metal part melting.

[0006] It may therefore be desirable to be able to adapt the distribution of thermal energy provided by the electrodes according to the intrinsic characteristics of the parts to be welded, in particular in order to obtain a spot weld having the required quality while reducing the electrical energy required. It may also be desirable to extend the possible uses of the spot welding technique to materials other than steel and aluminum and to other shapes of parts than thin sheets. Summary of the invention

[0007] Embodiments relate to a spot welding method, comprising steps consisting of: arranging two electrically conductive parts to be joined between two electrodes, each of the two parts having an interface zone between the two parts and a contact zone with one of the two electrodes, and establishing a first electric current between the two electrodes through the two parts, the first electric current producing thermal energy capable of forming a welding nugget within the two parts, characterized in that it comprises a step of adjusting a distribution of thermal energy density produced by the first electric current as a function of intrinsic characteristics of each of the two parts, to generate a welding nugget initiation zone, at a chosen depth in the parts to be joined.

[0008] By adapting the distribution of thermal energy provided by the electric current flowing between the electrodes, depending on the intrinsic characteristics of the parts to be welded, it is possible to generate a welding nugget initiation zone at a depth corresponding to the contact surface between the parts to be joined and therefore achieve a robust spot weld, without damaging the parts, while reducing the electrical energy required. In the following, the term "depth" is used to designate the distance between a point in the parts to be joined on the common longitudinal axis of the electrodes and the contact surface of one of the electrodes,

[0009] According to one embodiment, the adjustment of the thermal energy density distribution comprises a step of selecting electrodes having different surfaces contact, to adjust a differential of contact surfaces of the electrodes with the two parts.

[0010] Thus, the distribution of thermal energy can be easily adjusted in the parts to be joined to form the spot weld.

[0011] According to one embodiment, the adjustment of the thermal energy density distribution comprises a step of selecting electrodes having different contact surface resistivities to adjust a differential of contact surface resistivities between the electrodes.

[0012] According to one embodiment, the adjustment of the thermal energy density distribution comprises a step of selecting electrodes having different resistivities to adjust a resistivity differential between the electrodes.

[0013] Adjusting the differential resistivity of the electrodes makes it possible to obtain quality spot welding with a wide range of parts having intrinsic characteristics that can be very different.

[0014] According to one embodiment, the adjustment of the resistivity differential of the electrodes is carried out by at least one of the following methods: by using electrodes made of materials having the adjusted resistivity differential, by covering the contact surface of one of the electrodes E21, E23-E25) with a fixed or removable layer, made of a material having a resistivity making it possible to obtain the adjusted resistivity differential, and by inserting into one of the electrodes a layer of a material having a resistivity making it possible to obtain the adjusted resistivity differential.

[0015] The diversity of methods for adjusting the resistivity differential between the electrodes makes it possible to adapt the spot welding system to numerous applications, while reducing the costs of this adaptation.

[0016] According to one embodiment, the intrinsic characteristics of each of the two parts comprise at least one of the following characteristics: thickness, resistivity, and melting temperature.

[0017] According to one embodiment, the adjustment of the thermal energy density distribution comprises steps consisting of: using for one of the two electrodes a multiple electrode formed of at least two electrode parts electrically insulated from one another, the first electric current being established between one of the electrode parts and the other of the two electrodes, and establishing a second electric current between the two electrode parts via the parts to be assembled to form a first welding core opposite one of the two electrode parts.

[0018] The use of such an electrode with two parts of insulated electrodes has the advantage of allowing the distribution of the thermal energy density to be dynamically adjusted, without having to change an electrode or to associate with it a layer of more resistive.

[0019] According to one embodiment, the method further comprises a step of reversing the second current between the two electrode parts to extend the welding core towards an area opposite the other of the two electrode parts.

[0020] The combination of the steps of applying the second current and reversing this current makes it possible to obtain a welding core extending over a surface corresponding to the contact surface of the electrodes, without increasing the welding time.

[0021] According to one embodiment, the step of reversing the second current between the two electrode parts is carried out several times.

[0022] According to one embodiment, the method further comprises a step of adjusting the electrical energy supplied by each of the first and second electrical currents to form the initiation zone of the welding nugget at a depth in the parts depending on a differential of electrical energies supplied by the first and second electrical currents.

[0023] Thanks to this arrangement, it is possible to dynamically adapt the depth of the welding core in the stack of parts to be assembled.

[0024] According to one embodiment, adjusting the thermal energy density distribution comprises steps of: using for the other of the two electrodes a multiple electrode formed of at least two electrode parts electrically insulated from each other, selecting an electrode part from among the two electrodes, and applying a voltage polarity to the selected electrode part and applying a reverse voltage polarity to the non-selected electrode parts.

[0025] The use of a second double electrode makes it possible to reach any depth in the stack of parts to be assembled to form the welding nugget.

[0026] According to one embodiment, the two parts to be assembled belong to a stack of more than two parts, the method comprising several successive steps of adjusting the electrical energy supplied by each of the first and second electrical currents to adjust the depth of the ignition zone of the welding core in the stack, in order to extend the welding core to the interfaces between the parts of the stack.

[0027] The use of a second double electrode and the implementation of several spot welding steps by readjusting the depth of formation of the welding nugget at each step makes it possible to quickly perform a spot weld through a stack of more than two parts to be assembled.

[0028] According to one embodiment, one of the two parts to be assembled is an insert arranged in an orifice of a third part to be assembled to assemble the third part to the other of the two parts to be assembled.

[0029] Thus, it is possible to assemble a non-electrically conductive part with a conductive part, by the spot welding technique, while ensuring good quality of the weld, and without risk of damaging the non-conductive part due to excess heat or pressure.

[0030] Embodiments may also relate to a spot welding system comprising two electrodes, to be applied against two opposite faces of a stack of parts to be assembled, the system being configured to implement the method as previously defined.

[0031] According to one embodiment, the electrodes have a contact surface differential, and / or the electrodes have a contact surface resistivity differential with the parts to be assembled, and / or the electrodes have a resistivity differential, and / or at least one of the electrodes comprises two electrode parts electrically insulated from each other and connected so as to each be able to receive a respective voltage.

[0032] According to one embodiment, one of the two electrodes has one of the following characteristics: is made of a material having a resistivity greater than the resistivity of the other electrode, has a contact surface covered with a fixed or removable layer, made of a material having a resistivity greater than the resistivity of the electrode, and comprises a housing in which is inserted a material having a resistivity greater than the resistivity of the electrode. Brief description of the figures

[0033] The present invention will be better understood with the aid of the following description of exemplary embodiments with reference to the appended figures, in which identical reference signs correspond to structurally and / or functionally identical or similar elements.

[0034] [Fig-1] Figure 1 shows a schematic section of two parts to be assembled arranged one against the other between two spot welding electrodes, according to the prior art,

[0035] [Fig.2][Fig.3][Fig.4][Fig.5][Fig.6][Fig.7][Fig.8] Figures 2 to 8 represent diagram matically in section two parts to be assembled arranged one against the other between two spot welding electrodes, according to various embodiments,

[0036] [Fig.9] Figures 9A, 9B, 9C schematically represent in section two parts to assemble arranged against each other between two spot welding electrodes, according to another embodiment,

[0037] [Fig. 10] Figure 10 shows schematically in section two parts to be assembled arranged against each other between two spot welding electrodes, according to another embodiment,

[0038] [Fig. 11] Figures 11 A, 1 IB, 1 IC, schematically represent in section two parts to be assembled together at different stages of an assembly process, according to another embodiment,

[0039] [Fig. 12] Figures 12A, 12B schematically represent in section two parts to assemble one against the other at different stages of an assembly process, according to another embodiment. Detailed description

[0040] Figures 1 to 8 show two parts to be assembled PI, P2 arranged against each other between two spot welding electrodes E1, E2, E11, E12, E21-E26 aligned along a Z axis. [Fig.l] illustrates a spot welding method according to the prior art. In the examples of Figures 1 to 8, the parts PI, P2 have the shape of plates shown horizontally, with a contact surface S3 at the interface between the two parts. The part PI shown in the figures in the upper position, has a thickness less than the part P2 shown in the lower position. The upper electrode E1, E11, E21-E26 is therefore placed against the upper part PI and has a contact surface S1 with the part PL. The lower electrode E2, E12 is placed against the lower part P2 and has a contact surface S2 with the part P2.

[0041] To assemble the two parts PI, P2 to each other by the spot welding technique, the parts are electrically conductive, and the electrodes E1, E2, E11, E12, E21-E26 are subjected to a voltage, so as to cause an electric current to flow from one to the other of the electrodes. The passage of the current through the parts PI, P2 produces a more or less high heating locally, depending on the resistivity of the regions crossed by the current. These resistive regions include: - the electrodes E1, E2, E11, E12, E21-E26 which are made of materials with low resistivities RI, R7, for example copper, - the contact surfaces SI, S2 between the electrodes and the parts PI, P2 to be assembled, the respective resistivities R2, R6 of which depend on the pressure exerted by the electrodes and the state of the surfaces in contact, - the parts PI, P2 to be assembled having respectively resistivities R3, R5, and - the contact surface at the interface S3 between the parts to be assembled, the resistivity R4 of which depends on the pressure exerted by the electrodes and the state of the surfaces in contact.

[0042] The respective resistivities R2, R4, R6 of the contact surfaces SI, S2, S3 are partly linked to the pressure exerted by the electrodes El, E2, which can reach several kN.

[0043] Heating produced by the Joule effect by the passage of current between the electrodes El, E2 Eli, E12, E21-E26 and in the parts PI, P2, forms a thermal energy density distribution in the parts PI, P2, promoting the formation of a welding nugget initiation zone in the parts to be joined. When the temperature of this initiation zone reaches the melting temperature of the part in which it is located, a welding nugget WN develops in and around this zone, in which the materials constituting the parts melt. The joining of the two parts PI, P2 is achieved when the welding nugget WN extends to both parts through the contact surface S3, which implies that the initiation zone has reached or exceeded the melting temperatures of the two parts.

[0044] Figures 1 to 8 represent the relative values ​​of the resistivities R1-R7, and the values ​​of the current densities II, 12, 13 through the contact surfaces SI, S2 and the interface S3.

[0045] In [Fig.l], the contact surfaces of the electrodes E1, E2 are identical. As a result, the density of the thermal energy generated by the Joule effect can be distributed so as to form a welding nugget initiation zone located on the median plane PM at equal distances from the contact surfaces SI, S2 between the parts PI, P2 and the electrodes E1, E2. If the parts PI, P2 have different thicknesses, identical resistivities and identical melting temperatures, the distribution of the thermal energy can generate a welding nugget WN in an area far from the interface zone S3 between the two parts PI, P2, where it is desirable to form the welding nugget to ensure the attachment of the parts PI, P2 to each other. As a result, the current between the electrodes will have to be supplied for a longer time to allow the welding nugget to extend and reach the other part PI through the interface zone S3.

[0046] In the example of [Fig.l], the resistivities RI, R7 of the electrodes E1, E2 are low, the resistivities R2, R6 of the contact surfaces SI, S2 are a little higher, the resistivity R3 of the part PI is substantially identical to the resistivity R2 of the contact surface SI, the resistivity R5 of the part P2 is greater than that of the part PI, and the resistivity of the interface S3 is substantially identical to that of the part P2. The current densities II, I2, I3 are substantially identical to the interfaces SI, S2, S3.

[0047] According to an embodiment illustrated by [Fig. 2], the electrodes E11, E12 differ from the electrodes E1, E2 in that they have different respective contact surfaces SI1, S12 with the parts PI, P2. In the example of [Fig. 2], the electrode E11 has the smallest contact surface SI1 and is in contact with the part PI having the smallest thickness. As a result, the current density II, and therefore the thermal energy density, is higher in the vicinity of the electrode E11 having the smallest contact surface S11. The current density I3 is higher. low in the vicinity of the electrode E12 having the largest contact surface S12. On the other hand, the resistivities R1-R7 are not modified compared to the embodiment of [Fig.l]. The ignition zone where the thermal energy is most concentrated is therefore moved towards the electrode El 1, which makes it possible to form a welding nugget WN1 at a depth closer to the interface S3 between the two parts PI, P2.

[0048] By adjusting the respective contact surfaces of the electrodes E11, E12, it is therefore possible to adjust the depth of the ignition zone in the parts P1, P2, on the common longitudinal axis Z of the electrodes, and therefore the position of the welding core WN1 within the parts P1, P2 to be assembled.

[0049] However, it turns out that the possible displacement of the area receiving the most thermal energy is limited and may not be sufficient to correctly place the ignition point of the welding nugget, taking into account the thicknesses and melting temperatures of the parts PI, P2, and the resistivities R2-R6. Furthermore, due to the contact pressure of the two electrodes, necessary to perform a spot weld, an electrode having too small a contact surface can deform the part to be assembled with which it is brought into contact, and therefore damage it.

[0050] Figure 3 shows the two parts to be assembled PI, P2 arranged against each other between two spot welding electrodes E21, E2. According to one embodiment, electrode E21 differs from electrode E1 in that its resistivity RI is greater than that (R7) of electrode E2. Current densities II, 12, 13 are not modified. In this way, the more resistive electrode E21 heats up more by Joule effect than the less resistive electrode E2. As a result, the distribution of thermal energy has a zone receiving the most thermal energy at a depth even closer to electrode E21 than to electrode E2. By adjusting the respective resistivities of the electrodes, it is therefore possible to adjust the ignition zone of the welding core WN2 within the parts PI, P2 to be assembled so that it is generated on or in the vicinity of the interface S3 between the parts PI, P2.

[0051] Such a result can also be obtained in the embodiments illustrated by Figures 4 to 8.

[0052] In [Fig.4], the resistivity R2 of the contact surface SI of the upper electrode E22 is increased for example by increasing the roughness of the electrode surface so that only part of the surface of the electrode E22 is in contact with the part PL

[0053] In [Fig.5], the resistivity RI of the upper electrode E23 is increased in placing a resistive layer RL on the contact surface SI of the electrode E23. The resistive layer RL is formed of a material more resistive than the material constituting the electrodes E23, E2. The resistivity of the resistive layer RL can be adjusted by the choice of the material which constitutes it and its dimensions, so that the ignition zone of the welding nugget WN2 within the parts PI, P2 to be assembled is located on or in the vicinity of the interface S3 between the parts PI, P2. The presence of the resistive layer at the interface between the electrode E23 and the part PI can have the effect of increasing the resistivity R2 of the contact surface SI.

[0054] The upper electrode E24 of [Fig. 6] differs from that (E23) of [Fig. 5] in that its contact surface S21 is larger than the contact surface S2 of the lower electrode E2. The resistive layer RL3 can also be enlarged to cover the entire contact surface SI of the electrode E24. This arrangement makes it possible to obtain an enlarged welding nugget WN3. It should be noted here that the increase in resistivity RI obtained by the resistive layer RL3 largely compensates for the increase in the contact surface S21 and makes it possible to form the welding nugget WN3 in the vicinity of the interface between the two parts PI, P2, even if the lower part P2 has a greater thickness than the upper part PL

[0055] In [Fig.7], the resistivity RI of the upper electrode E25 is increased by inserting a resistive material RL1 into a housing located in the vicinity of the contact surface SI of the electrode E25. In this way, the surface condition of the electrode E25 can be identical to that of the electrode E2. The resistivity of the resistive insert RL1 can also be adjusted for the same purpose as in the embodiment of [Fig.5].

[0056] The upper electrode E25 of [Fig. 8] differs from that (E23) of [Fig. 5] in that the resistive layer RL of the upper electrode is replaced by a removable part RL2 capable of being fixed so as to cover the contact surface SI of the electrode E25. The part RL2 is formed from a more resistive material than the material constituting the electrodes E25, E22. The part RL2 is for example fixed by clips on the electrode E25.

[0057] The resistive layer RL, RL1, RL2 can be made for example of steel, aluminum, bronze or nickel.

[0058] Figures 9A, 9B, 9C illustrate a method of assembling the parts PI, P2 by spot welding, according to another embodiment. Figures 9A, 9B, 9C represent the two parts PI, P2 arranged against each other and between two electrodes E31 / E32 and E2. In particular, these figures represent the parts PI, P2 to be assembled in the form of horizontal plates, the upper part PI having a thickness less than the lower part P2. The electrodes therefore comprise an upper electrode E31 / E32 arranged above the parts to be assembled and a lower electrode E2 arranged below the parts to be assembled. According to one embodiment, the electrode E31 / E32 is a double electrode comprising two electrodes (or two electrode parts) E31 and E32 separated by an electrically insulating layer DL. In this way, the electrodes E31 and E32 can be subjected to different voltages. different.

[0059] Figure 9A illustrates a step of the assembly method, in which the electrode E31 is subjected to a positive voltage and the electrodes E32 and E2 are subjected to a negative voltage. In this way, a first current 111 is established between the electrodes E2 and E31 through the parts PI, P2, and a second current 112 is established between the electrodes E32 and E31 mainly through the part PI in contact with the double electrode E31 / E32. The two currents 111, 112 form a zone of higher current density in the vicinity of the contact surface between the part PI and the single electrode (E31) subjected to a positive voltage, this zone of higher current density being conducive to the formation of a strike zone SZ of a welding nugget WN4.

[0060] Figure 9B illustrates a step of the assembly method, in which the electrode E32 is subjected to a positive voltage and the electrodes E31 and E2 are subjected to a negative voltage. In this way, the first current 111 is established between the electrodes E2 and E32 through the parts PI, P2, and the second current 112 is established between the electrodes E31 and E32 mainly through the part PI in contact with the double electrode E31 / E32. The combination of the two currents 111, 112 therefore promotes the formation of a priming zone SZ of a welding nugget in the vicinity of the electrode E32.

[0061] Figure 9C illustrates a final state of an assembly process linking the steps illustrated by Figures 9A and 9B, exploiting thermal inertia. During the linking of these steps, the ignition zone SZ is moved laterally from the electrode E31 to the electrode E32. As a result, the welding core WN4 is extended in the direction of the electrode E32 to form a welding core WN5 extending to the interface S3 and having the width of the double electrode E31, E32 or of the electrode E2. Since the current density obtained in each of these steps is higher than with a single electrode, the duration of the two-step welding operation (Figures 9A, 9B) can be shorter at equal voltages than in a single step with two single electrodes with the same contact surfaces. Thus, the method illustrated by figures 9A, 9B makes it possible to control the direction of extension of the welding core.In contrast, in the prior art spot welding technique, the duration of the welding operation is extended to allow the welding nugget to expand sufficiently in all directions, without being certain that the welding nugget extends sufficiently across the interface between the parts to be joined. Such an extension of the welding time results in inefficient energy expenditure and a risk of damaging the parts to be joined, without guaranteeing the strength of the assembly.

[0062] It can be observed that the double electrode E31 / E32 can be used as a single electrode by subjecting electrodes E31 and E32 to the same positive voltage, while electrode E2 is subjected to a negative voltage. In this way, a current is established between electrodes E2 and E31 / E32 through parts PI, P2.

[0063] According to one embodiment, the steps illustrated by FIGS. 9A, 9B are chained together several times by reversing the polarities of the voltages applied to the electrodes E31 and E32 several times. According to one exemplary embodiment, the polarities of the voltages applied to the electrodes E31 and E32 are reversed at a frequency of several hundred Hertz (for example 1 kHz) for a duration of several hundred milliseconds (for example 1 s).

[0064] According to one embodiment, in the cases illustrated by figures 9A, 9B where the voltages supplied to the electrodes E31 and E32 have reverse polarities, the voltage supplied to the electrodes E2, E31, E32 is modulated so as to adjust the electrical energy supplied by the first current 111 between the electrodes of reverse polarities E2 and E31 (case of figure 9A) or E2 and E32 (case of figure 9B) on the one hand, and on the other hand the electrical energy supplied by the second current 112 between the electrodes E32 and E31. This adjustment makes it possible to adjust the position of the higher energy density zone and therefore the ignition zone, between the electrode E2 and the double electrode E31 / E32. Indeed, if the first current 111 is zero, the ignition zone SZ of the welding core is generated by the second current 112 and is therefore located closest to the double electrode E31 / E32.If the second current 112 is zero, the ignition zone of the welding core is formed by the first current II 1 and is therefore substantially at equal distances from the contact surfaces SI, S2 of the electrodes E31 / E32 and E2, on the median plane PM at equal distances from the contact surfaces SI, S2. Consequently, in the presence of the two currents II 1, 112, the ignition zone SZ of the welding core is formed between these two extremes in the parts PI, P2, at a distance from the double electrode E31 / E32 depending on the ratio between the intensities of these two currents.

[0065] The adjustment of the electrical energy supplied by the currents 111, 112 between each pair of electrodes (E31, E2) and (E31, E32) or (E32, E2) and (E32, E31), of opposite polarities can be carried out by applying different modulations to these pairs of electrodes. According to an exemplary embodiment, the modulation used can be of the PWM ("Pulse Width Modulation") type, the electrical energy supplied being adjusted by adjusting the duty cycle of the modulation.

[0066] Figure 10 schematically represents in section the two parts PI, P2 to be assembled arranged one against the other between two spot welding electrodes E31 / E32 and E41 / E42, according to another embodiment. The embodiment of [Fig. 10] differs from that of Figure 9A in that the two electrodes on either side of the parts PI, P2 to be assembled are double electrodes. Thus, the lower electrode (in [Fig. 10]) is formed of two electrodes E41 and E42 separated from each other by a layer DL1 made of an electrically insulating material. The provision of two double electrodes makes it possible to generate the welding nugget ignition zone in the vicinity of any of the electrodes E31, E32, E41, E42, and therefore to adjust the position of the welding nugget WN7 to any depth in the workpieces PI, P2 between the double electrodes E31 / E32 and E41 / E42. Indeed, the welding nugget ignition zone can be located as previously described between the median plane PM located at equal distances from the contact surfaces SI, S2 and the double electrode E31 / E32 by establishing the first current between the double electrode E41 / E42 and the electrode E31 or E32, and the second current between the electrodes E31 and E32. The provision of the double electrode E41 / E42 allows the first current to be established between the double electrode E31 / E32 and the electrode E41 or E42, and the second current between the electrodes E41 and E42.Thus, the welding nugget initiation zone can be located in the thickness of the parts PI, P2, between the median plane PM of the cumulative thickness of the parts PI, P2 and the contact surface of the double electrode E41 / E42.

[0067] According to one embodiment, the double electrode E31, E32 can be replaced by a multiple electrode, divided into more than two electrodes separated by electrically insulating layers, one of the electrodes of the multiple electrode being subjected to a first voltage and the other electrodes of the multiple electrode being subjected to a second voltage of opposite polarity, during a step of the welding method. The welding method can then comprise as many steps like those illustrated by FIGS. 9A, 9B as there are electrodes in the multiple electrode, the electrode subjected to the first voltage being modified at each step. This arrangement makes it possible to further increase the current density in the vicinity of the electrode subjected to the first voltage.

[0068] In the embodiments illustrated by Figures 2 to 11, the spot welding method makes it possible to obtain a weld having the required quality while reducing the electrical energy required, with parts to be assembled which may have very different thicknesses, different resistivities and different melting temperatures. Thus, the welding method can be applied to the assembly of two electrically conductive parts, of the same composition or of different compositions, for example made of steel, aluminum, copper, titanium, nickel alloy for example of the Inconel® type.

[0069] Furthermore, the number of parts to be assembled may be greater than two, the parts to be assembled forming a stack of parts. The assembly of such a stack may for example be carried out by using multiple electrodes as illustrated by [Fig. 10], and by varying the depth of formation of the welding point initiation zone to reach all the interfaces between two parts of the stack.

[0070] Figures 1 1A, 1 1B, 1 1C, represent two parts P3, P4 to be assembled against each other the other at different stages of an assembly process, according to one embodiment. The part P3 may be made of a material not compatible with the spot welding technique, such as for example a non-electrically conductive material, or a material likely to be damaged due to the high temperatures and pressures to which the parts are subjected. An insert INS is used to assemble the parts P3, P4. For this purpose, the insert INS is engaged in a through hole H formed in the part P3. The insert INS comprises a central pad CP intended to be welded onto the part P4 and a peripheral rim PE intended to form a stop during its insertion into the part P3 and to retain the part P3 after welding the insert to the part P4. The insert INS may also comprise a peripheral collar AC intended in particular to protect the part P3 from high temperatures during welding of the pad CP onto the part P4.The collar AC can also be shaped to create the hole H when engaging the insert in the part P3.

[0071] In Figure 1 1A, the insert INS is engaged in the orifice H formed in the part P3. In Figure 1 1B, the assembly formed by the parts P3, P4 and the insert INS is placed between spot welding electrodes, for example the electrodes E21, E2, the resistivity differential between the electrode E21 placed in contact with the part P4 and the electrode E2 placed in contact with the insert INS, being adjusted so that the ignition zone of the welding nugget is formed at a depth corresponding to that of the free end of the pad CP. In Figure 1 1C, a current is supplied between the electrodes E21, E2, which makes it possible to form a welding nugget WN8 including the free end of the pad CP which has deformed, and a part of the part P4 in contact with the pad. The INS insert then allows parts P3 and P4 to be held against each other.

[0072] Of course, the depth of the welding nugget initiation zone can also be adjusted by using multiple electrodes as illustrated in Figures 9 and 10.

[0073] By using such a welded insert, the method can be applied to the assembly of an electrically conductive part, for example made of steel, aluminum, copper, etc. with a part made of a non-electrically conductive material, such as for example a resin, polymer, ceramic or composite material. The INS insert can be formed in a material such as steel or aluminum.

[0074] Figures 12A, 12B show the two parts P3, P4 to be assembled together at different stages of an assembly method, according to another embodiment. The embodiment illustrated by Figures 12A, 12B differs from that of Figures 11A-1 IC in that the insert INS1 used to assemble the parts P3, P4 has a simplified shape without a peripheral collar AC. In Figure 12B, a current is supplied between the electrodes E21, E2, which makes it possible to form a welding core WN9 including the free end of the pad CP which has deformed, and a part of part P4 in contact with the stud. The INS1 insert then keeps parts P3, P4 pressed against each other.

[0075] It will be clear to those skilled in the art that the present invention is susceptible to various alternative embodiments and various applications. In particular, the invention is not limited to the embodiments presented, but also covers all technically possible combinations of these embodiments. Thus, an adjustment of the contact surface of one of the electrodes can be combined with an adjustment of the respective resistivities of the electrodes (or of the differential of these resistivities) as illustrated by Figures 3 to 8. Similarly, in the use of double electrodes illustrated by Figures 9A-9C and 10, one of the electrodes formed of at least one double electrode, can have a different contact surface resistivity or a different resistivity from that of the other of the electrodes.

Claims

Claims

1. 1. A spot welding method, comprising steps consisting of: arranging two electrically conductive parts (PI, P2, P4, INS, INS1) to be assembled between two electrodes (E11-E26, E31 / E32, E12-E22, E41 / E42), each of the two parts having an interface zone (S3) between the two parts and a contact zone (SI, S2) with one of the two electrodes, and establishing a first electric current between the two electrodes through the two parts, the first electric current producing thermal energy capable of forming a welding nugget (WN1-WN8) within the two parts, characterized in that it comprises a step of adjusting a distribution of thermal energy density produced by the first electric current as a function of intrinsic characteristics of each of the two parts, to generate a welding nugget initiation zone, at a chosen depth in the parts to be assembled (PI, P2, P4, INS, INS1),adjusting the resistivity differential of the electrodes comprising covering the contact surface (SI) of one of the electrodes (E21, E23-E25) with a fixed (RL) or removable (RL2) layer, made of a material having a resistivity making it possible to obtain the adjusted resistivity differential.,

2. 2. Method according to claim 1, wherein the adjustment of the thermal energy density distribution comprises a step of selecting electrodes (Eli, E12, E24, E22) having different contact surfaces (SI, S2, SU, S12), to adjust a differential of contact surfaces of the electrodes with the two parts (PI, P2, P4, INS).

3. 3. The method of claim 1 or 2, wherein adjusting the thermal energy density distribution comprises a step of selecting electrodes (E22, E2) having different contact surface resistivities (SI, S2) to adjust a differential in contact surface resistivities between the electrodes.

4. 4. Method according to one of claims 1 to 3, wherein the adjustment of the thermal energy density distribution comprises a step of selecting electrodes (E21-E26, E2) having different resistivities to adjust a resistivity differential between the electrodes.

5. 5. The method of claim 4, wherein adjusting the dif- differential resistivity of the electrodes further comprises one of the following steps: the selection of electrodes (E21, E22) from a set of electrodes made of different materials, the insertion into one of the electrodes of a layer (RL1) made of a material selected to obtain the adjusted resistivity differential.

6. 6. Method according to one of claims 1 to 5, wherein the adjustment of the thermal energy density distribution comprises steps consisting of: using for one of the two electrodes a multiple electrode formed of at least two electrode parts (E31, E32) electrically insulated from each other, the first electric current (111) being established between one of the electrode parts and the other (E2) of the two electrodes, and establishing a second electric current (112) between the two electrode parts via the parts to be assembled (PI, P2) to form a first welding nugget (WN4) opposite one of the two electrode parts.

7. The method of claim 6, further comprising a step of reversing the second current (112) between the two electrode portions (E31, E32) to extend the welding nugget (WN5) toward an area facing the other (E32) of the two electrode portions.

8. The method of claim 7, wherein the step of reversing the second current (112) between the two electrode portions (E31, E32) is performed several times.

9. A method according to one of claims 6 to 8, further comprising a step of adjusting the electrical energy supplied by each of the first and second electrical currents (111, 112) to form the ignition zone of the welding core at a depth in the parts (PI, P2) depending on a differential of electrical energies supplied by the first and second electrical currents.

10. The method of claim 9, wherein adjusting the thermal energy density distribution comprises steps of: using for the other of the two electrodes a multiple electrode (E41 / E42) formed of at least two electrode parts (E41, E42) electrically insulated from each other, selecting an electrode part (E31) from the two electrodes (E31 / E32, E41 / E42), and apply one voltage polarity to the selected electrode part and apply reverse voltage polarity to the unselected electrode parts (E32, E41, E42).

11. A method according to claim 10, wherein the two parts to be joined belong to a stack of more than two parts, the method comprising several successive steps of adjusting the electrical energy supplied by each of the first and second electrical currents to adjust the depth of the welding nugget initiation zone in the stack, in order to extend the welding nugget to the interfaces between the parts of the stack.

12. 12. Method according to one of claims 1 to 11, in which one of the two parts to be assembled (P4, INS) is an insert (INS) arranged in an orifice (H) of a third part (P3) to be assembled to assemble the third part to the other (P4) of the two parts to be assembled.

13. 13. Spot welding system comprising two electrodes (E11-E26, E31 / E32, EE12-E22, E41 / E42), arranged to be applied against two opposite faces of a stack of parts (PI, P2, P4, INS) to be assembled, a welding device configured to subject the electrodes to a voltage so as to cause an electric current to flow between the electrodes through the stack of parts, the system being configured to implement the method according to one of claims 1 to 12, one of the two electrodes having a contact surface covered with a fixed or removable layer (RL, RL2), made of a material having a resistivity greater than the resistivity of the electrode.

14. 14. System according to claim 13, wherein: the electrodes (E11, E12, E24, E2) have a differential contact surface (SU, S12, S21, S2), and / or the electrodes (E22, E2) have a differential resistivity of contact surfaces with the parts (PI, P2) to be assembled, and / or the electrodes (E21, E23-E25, E2) have a differential resistivity, and / or at least one of the electrodes comprises two electrode parts (E31, E32, E41, E41) electrically insulated from each other and connected so as to be able to each receive a respective voltage.

15. 15. System according to claim 13 or 14, in which one of the two electrodes has one of the following characteristics: is made of a material having a resistivity greater than the re- sistivity of the other electrode (E22), and comprises a housing into which is inserted a material (RL1) having a resistivity greater than the resistivity of the electrode.

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