EP2376255A1

EP2376255A1 - Resistance welding method and device

Info

Publication number: EP2376255A1
Application number: EP09801934A
Authority: EP
Inventors: Erdogan Karakas
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-12-12
Filing date: 2009-12-09
Publication date: 2011-10-19
Also published as: WO2010066414A1

Abstract

In a resistance welding method for welding at least two workpieces (4, 6, 8) to one another, wherein a welding current is introduced into the workpieces by means of a first electrode (14) and a second electrode (16), at least one of the workpieces is contacted by way of at least one third electrode (18) in such a way that two of the electrodes are located in a first welding circuit for introducing a first partial welding current into the workpieces, and the other electrode or at least one of the other electrodes is located in a second welding circuit for introducing a second partial welding current into the workpieces. According to the invention, the first partial welding current and the second partial welding current are measured separately from one another (21, 23), wherein at least one characteristic of the welding process is influenced in accordance with at least one of the measured partial welding currents.

Description

Resistance welding process and device

The invention relates to a resistance welding method referred to in the preamble of claim 1 and a resistance welding device referred to in the preamble of claim 18 for welding at least two workpieces together. Climate change and rising energy prices, especially in the automotive industry, are creating a need to save fuel by reducing the weight of automobiles. One possibility for weight reduction is the use of thinner high-alloy austenitic steels with high strength and / or aluminum. However, the welding properties of these materials in a resistance welding process deviate significantly from those of conventionally used workpieces, so that the quality of the welded joints deteriorates, especially in mixed joints, for example, joining a normal strength or a high strength steel sheet. The different physical properties and chemical compositions of these sheets have a decisive influence on their weldability and thus on the quality of each weld obtained. For example, in a blended joint, the high strength sheet melts rather than the normal strength because the electrical resistance of a high strength steel is higher than that of a normal strength steel. To one It is desirable, during the welding process, to form a weld nugget which is symmetrical to the interface at an interface between two workpieces to be welded. For this purpose, it is desirable that both plates melt at the same time.

The described problems occur in particular in multi-sheet welded joints, in which more than two sheets are welded together, which consist of different materials and / or have different workpiece thicknesses and / or different surface refinement.

DE 199 15 121 A1 and DE 199 23 172 A1 disclose a resistance welding method for welding together at least two workpieces, in which a welding current is introduced into the workpieces via a first electrode and a second electrode. The methods known from these documents are particularly suitable for welding aluminum sheets as well as arrangements consisting of two or three sheets. In DE 199 15 121 A1, the position of the weld nugget is controllable relative to an interface between the workpieces to be welded, so that, in particular, a nugget that is symmetrical to the boundary surface can be formed during the welding process. In the method described in the document in this case a voltage applied to the electrodes welding voltage is reversed. In this way, in particular, the formation of an asymmetrical to an interface between the workpieces to be welded

Welding lens can be avoided, the quality of the welded joint is increased. In addition, the service life or stagnation of the electrodes or electrocapsules is ciuCut. Resistance welding methods are further described by DE 20 2007 000 009 U1, WO 2004/004961 A1, US 5 552 573 A, DE 197 54 546 C1, JP 10 029 071 A, JP 08 118 037 A and JP 05 192 274 A, US 3 462 577 and DE 39 23 433 Al.

DE 10 2007, DE 196 11 597 A1, DE-AS 2 005 583, DD 259 585, DE 10 2005 014 969 A1 and DE 35 07 012 C2 disclose processes for arc welding.

DE-PS 46 70 57 a resistance welding method of the type in question for welding at least two workpieces is known to each other, in which a welding current is introduced into the workpieces via a first electrode and a second electrode and in which at least one of the workpieces is contacted via at least one third electrode that two of the electrodes in a first welding circuit for introducing a first partial welding current are in the workpieces and that the remaining electrode or at least one of the remaining electrodes in a second welding circuit for introducing a second partial

Welding current is in the workpieces. A similar method is also known from DE-PS 11 67 466.

The invention has for its object to provide a resistance welding method or a resistance welding device referred to in the preamble of claim 1 or 18, with the type or with which can achieve a high quality of the welded joint.

This object is achieved with regard to the method by the invention specified in claim 1 and with respect to the device by the invention specified in claim 20.

Starting from an arrangement in which two electrodes are provided to a welding current in the to initiate welding workpieces, the invention is based on the idea that the welding process can then be particularly well influenced, for example with regard to the formation of a weld nugget, if the welding current can be influenced separately from each other at the electrodes.

For this purpose, the inventive method provides that at least one of the workpieces is contacted in such a way via at least one third electrode that two of the electrodes in a first welding circuit for

Introducing a first partial welding current into the workpieces and that the remaining electrode or at least one of the remaining electrodes lies in a second welding circuit for introducing a second partial welding current into the workpieces. According to the invention, the welding current is thus introduced into the workpieces via at least two welding circuits. In welding tongs, for example, and in particular, one of the electrodes may be in the first welding circuit and the other electrode may be in the second welding circuit, so that the part welding current flowing to the electrodes of the tongs may be separated from each other. In this way, the welding process can be influenced particularly precisely, in particular with regard to the supply of welding energy to the electrodes, for example a welding tongs. For this purpose, it is provided according to the invention that the first part of the welding current and the second part of the welding current are measured separately and that at least one parameter of the welding process is influenced as a function of at least one of the measured welding currents. For example, and in particular according to the invention, it is thereby possible to control or supply the partial welding currents separately from each other regulate. In this way, particularly high-quality welded joints can be produced by means of the method according to the invention. Instead of the partial welding currents, it is also possible to measure according to the invention dependent thereon, in particular electrical variables, for example electrical voltages, resistances or powers.

The influencing of at least one parameter of the welding process may consist in an adjustment and / or a control and / or a regulation and be carried out before or after the welding process or during the welding process.

According to the invention, it is basically sufficient if at least one third electrode is present to form at least two welding circuits.

However, more than three electrodes may be provided according to the respective requirements.

A development of the invention provides that the first and the second electrode to each other and the third electrode to the first and second

Electrode is oppositely poled, such that the first part-welding current flows through the first electrode and the third electrode and the second part-welding current via the second and the third electrode. If the first and the second electrode are, for example, electrodes of a welding tongs, they can be poled negatively according to the invention, while the third electrode is positively poled. A stronger warming of the positively poled electrode occurring due to the Peltier effect thus occurs at the third electrode and thus not at the electrodes of the welding tongs. In this way, the service life or stagnation of the relatively expensive electrodes of the welding gun is increased. The wear of the third elec- In addition, the electrode can be reduced in that it is designed as a flat electrode, since it serves exclusively for introducing the welding current and not for pressing the workpieces to be welded together, as is the case with the electrodes of a welding tongs.

An extraordinarily advantageous development of the invention provides that the first partial welding current and the second partial welding current are set and / or controlled separately from one another with regard to at least one parameter in each case.

Another advantageous development of the invention provides that the adjustment and / or control and / or regulation is carried out during the welding process. In this embodiment, the welding process can thus be influenced online.

It is also possible according to the invention that the adjustment and / or control and / or regulation is carried out before or after a welding operation, as provided for by another development of the invention.

In the embodiments in which the partial welding currents are influenced with regard to at least one characteristic, advantageous developments provide that the parameter is the amplitude and / or the time average and / or the effective value of at least one welding current and / or that the characteristic quantity is the amplitude and / or the time average and / or the rms value of at least one welding voltage and / or that the characteristic is the amplitude and / or the time average and / or the effective value of a welding power and / or that the characteristic is a time period and / or a temporal Course of a welding current or a welding voltage and / or a Force with which one of the electrodes is pressed onto the associated workpiece is.

According to the invention, the first part of the welding current and the second partial welding current can be generated by the same current source or by different current sources according to the respective requirements, as provided by other developments of the method according to the invention.

According to the respective requirements, the welding current may be a direct current or an alternating current, as provided by other developments of the invention.

According to another advantageous embodiment of the method according to the invention, the polarity of the electrodes is changed during the welding process. In this way, there are additional possibilities with regard to influencing the welding process. In particular, it can be changed in this embodiment, which are the same polarity of the electrodes. For example, when using a welding gun to

At the beginning of the welding operation, first of all the two electrodes are poled identically, while the third electrode is polarized in opposite directions, the polarity during the welding process can be changed so that one of the electrodes of the welding gun is the same polarity as the other electrode of the welding gun is poled.

Another development of the invention provides that the position and / or size of a welding lens which forms at an interface between two workpieces during the welding process is influenced by adjusting or controlling at least one parameter of the welding process, as another to provide advantageous development of the invention. INS In particular, the position and size of the weld nugget can be influenced so as to form a nipples symmetrical to the interface between the workpieces to be welded. If more than two workpieces are welded together by means of the method according to the invention, then according to the invention the partial welding currents can be chosen such that a weld nugget symmetrical to the boundary surface forms at each interface between two workpieces to be welded.

A development of the aforementioned embodiment provides that the parameters of the welding process at least one welding current and / or at least a welding voltage and / or a temperature of at least one electrode and / or at least one force with which an electrode is pressed onto the associated workpiece, and / or an electrical resistance and / or one of one of these parameters. The welding current may be at least one of the partial welding currents or the total welding current flowing through the workpieces to be welded. According to the invention, the welding process is understood to mean the respective resistance welding process with its parameters and boundary conditions. In principle, the method according to the invention is suitable for welding workpieces which are identical in terms of material and workpiece thickness. However, the method according to the invention is particularly well suited if at least two of the workpieces differ with regard to material and / or workpiece thickness, as provided by another advantageous development. Since, by means of the method according to the invention, the partial welding currents flowing at the electrodes can be influenced separately from one another, can workpieces are welded together by means of the method according to the invention, which differ greatly from each other in terms of their material and their workpiece thickness. This is especially true for welded joints of more than two workpieces. Since it may be important for certain welding tasks, which of the electrodes is electrically connected to ground, another embodiment of the invention provides that the ground connection of the electrodes can be switched.

The electrodes can be configured in any suitable manner when carrying out the method according to the invention. An advantageous development of the method according to the invention is when two of the electrodes are formed by the electrodes of a welding tongs.

According to the invention, it is basically sufficient if two workpieces are welded together by means of the method according to the invention. An extremely advantageous development of the invention, however, provides that at least three workpieces are welded together. In this case, it is possible according to the invention that each of the workpieces is contacted by means of an electrode, that is, for example, three workpieces are each contacted by one of three electrodes. According to the invention, however, it is also possible that the number of electrodes corresponds to the number of workpieces or is greater than the number of workpieces, wherein at least one of the workpieces is contacted by at least two electrodes. Combinations of the two aforementioned constellations are possible.

Further developments of the device according to the invention are specified in claims iy to 34. The invention will be explained in more detail with reference to the attached highly schematic drawing, in the embodiments of a resistance welding device according to the invention for carrying out embodiments of the invention

Resistance welding process are shown. All the features described, illustrated in the drawings and claimed in the claims, taken alone and in any combination, constitute the subject matter of the invention, regardless of their summary in the patent claims and their dependency and independently of their description or representation in the Drawing.

1 shows a schematic illustration of a first exemplary embodiment of a resistance welding device according to the invention, which is also referred to below as a device, FIG. 2 shows in the same representation as FIG. 1 a second embodiment of a device according to the invention together with an electrical equivalent circuit diagram, Fig. 3 shows the embodiment of FIG.

2 with changed polarity,

FIG. 4 shows in the same representation as FIG. 2 a third exemplary embodiment of a device according to the invention, FIG.

5 shows an electrical equivalent circuit diagram according to FIG. 1, Fig. 6A in the same representation as Fig. 1, a fourth ÄuaJlühxur-ysbeisp; ^'■ ' - ^' * a device according to the invention,

6B in the same representation as FIG. 6A shows a modification of the exemplary embodiment according to FIG. 6A, FIG.

7 shows, in the same representation as FIG. 1, a fifth exemplary embodiment of a device according to the invention in which tyrist holes are used,

8 to 10, the device of FIG. 7 in different switching states of the thyristors,

Fig. 11 in the same representation as Fig. 7, the embodiment of FIG.

7 in combination with measuring means and control and / or regulating means,

12 to 14 different possibilities of a ground connection of electrodes of the device according to the invention,

15 shows an example of the time course of partial welding currents in carrying out the method according to the invention and

Fig. 16 in the same representation as Fig. 1, a sixth embodiment of a device according to the invention.

In the figures of the drawing, the same or corresponding components are provided with the same reference numerals.

In Fig. 1 is a first embodiment of a Inventive device 2 for performing an embodiment of a method according to the invention shown, which serves to weld workpieces together. In the illustrated exemplary embodiment, by means of the method according to the invention, three workpieces 4, 6, 8 are welded, which in this embodiment are sheets which consist of different materials and have different workpiece thicknesses. The device 2 according to the invention comprises means 10 for generating a

Welding current, which have a current source 12 in this embodiment. For introducing the welding current into the workpieces 4, 6, 8, a first electrode 14 and a second electrode 16 are provided, which are formed in this embodiment by electrodes of a welding tongs.

In a generally known manner, the workpieces 4, 6, 8 are pressed together by means of the electrodes 14, 16 and welded together when the welding current flows. As can be seen from FIG. 1, the first electrode 14 contacts the workpiece 4, while the second electrode 16 contacts the workpiece 8.

According to the invention, the device 2 has a third electrode 18. As shown in FIG. 1, the current source 12 in this embodiment has two negative poles 20, 22 connected to the first electrode 14 and the second electrode 16, respectively, and a positive pole 24 connected to the third electrode 18 is. As a result of this interconnection, a first part of the welding current flows via the first electrode 14, the first workpiece 4, the second workpiece 6 and the third electrode 18 during a welding operation, while a second part of the welding current i2 passes through the second electrode 16, the third workpiece 8, that second workpiece 6 and the third electrode 18 flows. Thus, in the illustrated embodiment, the electrodes 14, 18 are in a first welding circuit while the second electrode 16 is in a second welding circuit together with the third electrode 18.

According to the invention, the first part of the welding current and the second part of the welding current are measured separately, wherein a measuring device for measuring the first part of welding current in Fig. 1 schematically at reference numeral 21 and a measuring device for measuring the second partial welding current in Fig. 1st at the reference numeral 23 is indicated. According to the invention, at least one parameter of the welding process is influenced as a function of at least one of the measured partial flows. In the illustrated embodiment, the first part welding current and the second partial welding current are controlled separately. Corresponding control or regulating means are schematically indicated in FIG. 1 and designated by the reference numeral 25.

When carrying out the method according to the invention flows in the first welding circuit and thus by the workpieces 4, 6, the first part welding current il, while in the second welding circuit and thus by the workpieces 6, 8, the second part -Welding current i2 flows. During the welding process, that is, during the formation of a welded joint between the workpieces 4, 6, 8, the first part-welding current and the second part-welding current are measured by the measuring devices 21 and 23 separately from each other and the measured values via not shown signal lines the Steuerungsbzw , Control means 25 supplied. If required according to the respective requirements, too Further parameters of the welding process the Steuerungsbzw. By selecting the potentials at the poles 20, 22, 24 of the current source 12, the partial welding currents il and i2 are independent of each other by the control or regulating means 25 adjustable or controllable or adjustable. In particular, the partial welding currents il and i2 can be chosen such that, despite the different materials, during the welding process

Sheet thicknesses of the workpieces 4, 6, 8 both between the workpieces 4, 6 and between the workpieces 6, 8 forms a weld joint of high quality.

FIG. 2 shows a second exemplary embodiment of a device 2 according to the invention, which differs from the exemplary embodiment according to FIG. 1 in that, instead of the common current source 12, two separate current sources 12 ', 12 "are provided, of which the current source 12 'for generating the partial welding current il and the current source 12' 'for generating the partial welding current i2 is used.

For the following, let it be assumed that the first workpiece 4 and the second workpiece 6 are normal-strength steel sheet, while the third workpiece 8 is high-strength steel sheet. In this case, the specific electrical resistance of the third workpiece 8 is greater than the electrical resistance of the workpieces 4, 6.

FIG. 2 shows on the right the electrical equivalent circuit diagram of the device 2, Rül the contact resistance between the first electrode 14 and the first workpiece 4, RwI the resistance of the first workpiece 4, Rü2 the contact resistance between the workpieces 4, 6, κw2 the resistance of Werkalüukö 6, RÜ3 represents the contact resistance between the workpieces 6, 8, Rw3 the resistance of the workpiece 8 and RÜ4 the contact resistance between the workpiece 8 and the third electrode 16. This results in the electrical resistance R _A in the first welding circuit and the electrical resistance R _B in the second circuit as follows:

R _A = R ₀₁ + R _-1 + R _ü2 + 1/2 * R _w2 R _B = R _ü3 + R _w3 + R _ü4 + 1/2 * R _w2

If the partial welding currents il and i2 flow in the circuits, the power P _A dissipated between the workpieces 4, 6 and the power P _B dissipated between the workpieces 6, 8 results as follows:

P _A = il '* (R _ül + R _wl + R _ü2 + 1/2 * R _w2 ) P _B = i2 * * (R _ü3 + R _w3 + R _ü4 + 1/2 * R _w2 )

By appropriate selection of the potentials at the poles 20, 22, 24 'and 24' 'of the current sources 12' and 12 '', the partial welding currents il and i2 can be adjusted or controlled or regulated so that both during the welding process forms at the interface between the workpieces 4 and 6 as well as at the interface between the workpieces 6 and 8 a nugget, which leads to a secure welding of the workpieces 4, 6, 8 together.

In the exemplary embodiment illustrated in FIG. 2, the first electrode 14 is negatively polarized by connection to a corresponding pole 20 'of the voltage source 12', while the third electrode 18 is connected by connection to the corresponding poles 24 'and 24''of the current sources 12 ', 12''posiciv and the second Ξlek 16 is negatively polarized by connection to a corresponding pole 22 'of the current source 12''.

In Fig. 3, the embodiment of FIG. 2 with changed polarity of the poles 20 ', 22', 24 ', 24' 'is shown. Here, the pole 20 'of the power source

12 'and the pole 24' 'of the current source 12' 'positive, while the pole 24' of the current source 12 'and the pole 22' of the current source 12 '' are negative. As a result, in this embodiment, the first electrode 14 is positive and the second electrode 16 is negatively polarized. Which polarity the third electrode 18 has depends on the potentials at the poles 24 ', 24' 'of the current sources 12', 12 '', so that the partial welding currents il and i2 can be controlled by appropriate adjustment of these polarities ,

In Fig. 4, a third embodiment of a device 2 according to the invention is shown, which differs from the embodiment of FIG. 3 in that the current sources 12 ', 12' 'are connected to the electrodes 14, 16, 18, that the first Electrode 14 and the third electrode 18 are positively poled, while the second electrode 16 is negatively polarized.

By virtue of the fact that the currents il and i2 can be adjusted or controlled separately from one another, it is possible for the currents at the interface between the workpieces 4, 6 to be on the one hand and to independently adjust the energy dissipated at the interface between the workpieces 6, 8, on the other hand. This makes it possible to selectively control the formation of the weld nugget at the respective interface and by avoiding excessive thermal stress on the electrical exhaust To increase service life or level of the electrodes 14, 16.

In the following explanation, it should be assumed that between the workpieces 4, 6 (referred to as "A-side" in FIG. 5) and between the workpieces 6, 8 (in FIG. 5) to achieve the desired welding result "B" side) is to disperse substantially the same electrical energy. During the welding process must be in such a constellation of the part-welding current in the

Circuit are reduced with the larger resistance, so that at different total resistance in the circuit, the dissipated electrical power is equal (P _B = P _A ). If, for example, R _B = 1, 2 * R _A and P _A = P _B , then the following results:

il ² * R _A = i2 ² R _n

It follows

il ² * R _A = i2 ² * 1,2 * R _A

such as

il = i2 * SQR (1,2)

If the partial welding currents il and i2 are set accordingly, the desired welding result is obtained during the welding process. The partial welding current il can be selected according to the equivalent circuit diagram in FIG. 5 by the potential difference UOl between the poles 20 ', 24' of the current source 12 'and by an adjustable resistor 26, during the time course of the partial welding current il can be influenced by a switch 28. In a corresponding manner, the partial welding current il can be selected by the potential difference U02 between the poles 22 ', 24 "of the current source 12" and by an adjustable resistor 30, while its temporal progression can be influenced by means of a switch 32 ,

In Fig. 6A, a fourth embodiment of a device 2 according to the invention is shown, in which the power source 12 is formed as an AC power source and consists of a MF inverter and a transformer 34 with center tap and two rectifier diodes 36, 38. As can be seen from FIG. 6A, the electrodes 14, 16 are negatively poled, while the electrode 18 is positively poled.

FIG. 6B shows a modification of the fourth exemplary embodiment according to FIG. 6A, which differs from the exemplary embodiment according to FIG. 6A in that, instead of a single transformer, two transformers 34 ', 34 "are provided with respective rectifier diodes 36'. In this way, both half-waves of the current flowing on the primary side of the respective transformer 34 'or 34 "current on the secondary side of the respective transformer 34' and 34" in a In addition, in this embodiment, one of the transformers 34 'and 34 "is prevented from saturating.

FIG. 7 shows a fifth exemplary embodiment of a device 2 according to the invention, in which the current source 12 is connected to the electrodes 14, 16, 18 via four thyristors 40, 42, 44, 46. In this case, a current source 12 provided with two negative poles 20, 22 and one positive pole 24 is used. det, as already shown in Fig. 1. The thyristor 40 is connected in a connection line between the negative pole 20 and the first electrode 14, while the thyristor 46 is connected in a connecting line between the negative pole 22 and the second electrode 16. The thyristors 40, 42 are connected in parallel to each other in opposite directions to the electrodes 14, 16.

Fig. 8 shows the device 2 of FIG. 7 in an operating condition in which the thyristors 40, 42 lock while the thyristors 44, 46 conduct, so that both in the extending across the electrodes 14 and 18 welding circuit and in the over the electrodes 16 and 18 extending welding circuit a partial welding current flows.

FIG. 9 shows the device 2 according to FIG. 7 in an operating state in which the thyristors 40 and 44 conduct, while the thyristors 42 and 46 block. In this operating state, a welding current flows exclusively in the welding circuit running across the electrodes 14, 18, while the welding circuit running across the electrodes 16, 18 is interrupted.

In contrast, FIG. 10 shows the device 2 according to FIG. 7 in an operating state in which the thyristors 40, 46 conduct while the thyristors 42, 44 are blocking, in this operating state a welding current flows exclusively in the welding circuit extending across the electrodes 16, 18 while the welding circuit running across the electrodes 14, 18 is interrupted. Fig. 11 shows the embodiment of FIG. 7 in connection with additional measuring means for measuring at least one parameter of the welding process during the welding process. For example, and in particular, the measuring means 48 may have a characteristic of the sectional Welding currents il and i2, the welding voltages Ul and U2, a force with which the electrodes 14, 16 are placed on the workpieces 4, 8, or measure other parameters of the welding process. The measuring means 48 are in data transmission connection with control and / or regulating means 50, by the current sources 12 ', 12''can be controlled or regulated, for example, to influence the welding currents il and i2 according to the respective requirements. By the control and / or regulating means 50, the thyristors 40, 42, 44 and 46 are further controlled.

12, 13 and 14 show various possibilities of an electrical connection of the third electrode 18. FIG. 15 shows by way of example time profiles of the currents il and i2, wherein it can be seen that the time profile of the current il of the time profile of the current i2 is independent.

Fig. 16 shows a sixth embodiment of a device according to the invention, which differs from the

Embodiment of FIG. 2 differs in that it is used to weld only two workpieces 4, 8 together. Accordingly, the third electrode 18 is formed so as to be suitable for simultaneously contacting the workpieces 4 and 8.

Claims

claims

1. resistance welding process for welding at least two workpieces together,

in which a welding current is introduced into the workpieces via a first electrode and a second electrode, and in which

at least one of the workpieces is contacted via at least one third electrode such that two of the electrodes lie in a first welding circuit for introducing a first partial welding current into the workpieces, and the remaining electrode or at least one of the remaining electrodes in a second welding circuit for initiating a second partial welding current lies in the workpieces,

characterized,

that the first part welding current and the second part welding current are measured separately from each other and

that at least one parameter of the welding process is influenced as a function of at least one of the measured partial welding currents.

2. The method according to claim 1, characterized in that the first and second electrodes are equal to each other and the third electrode to the first and second electrodes oppositely poled, such that the first partial welding current across the first electrode and the third electrode and the second part welding current flow across the second and third electrodes.

3. The method according to claim 1 or 2, characterized in that the first part of the welding current and the second part-welding current in each case at least one characteristic set separately from each other and / or controlled and / or regulated.

4. The method according to claim 3, characterized in that the adjustment and / or control and / or regulation is carried out during the welding operation.

5. The method according to claim 3, characterized in that the adjustment and / or control and / or regulation is carried out before or after a welding operation.

6. The method according to any one of claims 3 to 5, character- ized in that the characteristic is the amplitude and / or the time average and / or the effective value of at least one welding current.

7. The method according to any one of claims 3 to 6, character- ized in that the characteristic is the amplitude and / or the time average and / or the effective value of at least one welding voltage.

8. The method according to any one of claims 3 to 7, character- ized in that the characteristic is the amplitude and / or the time average and / or the effective value of a welding power.

9. The method according to any one of claims 3 to δ, da- characterized in that the characteristic is a period of time and / or a time course of a welding current or a welding voltage and / or a force with which one of the electrodes is pressed onto the associated workpiece.

10. The method according to any one of the preceding claims, characterized in that the first part-welding current and the second part-welding current are generated by the same power source.

11. The method according to any one of claims 1 to 9, characterized in that the first part welding current and the second partial welding current are generated by different current sources.

12. The method according to any one of the preceding claims, characterized in that the welding current is a direct current.

13. The method according to any one of claims 1 to 9, characterized in that the welding current is an alternating current.

14. The method according to any one of the preceding claims, characterized in that the polarity of at least one electrode is changed during the welding operation.

15. The method according to any one of the preceding claims, characterized in that by adjustment or control or regulation of at least one parameter of the welding process, the position and / or size of a while the Schweißvoiyanyeö anz ^~ Gϊθiizflache between see two workpieces forming weld nugget is affected.

16. The method according to any one of the preceding claims, characterized in that the characteristics of the welding process at least one welding current and / or at least one welding voltage and / or a temperature of at least one electrode and / or at least one force with which an electrode on the associated Werk- piece and / or comprise an electrical resistance and / or a parameter dependent on one of these parameters.

17. The method according to any one of the preceding claims, characterized in that at least two of

Differentiate workpieces with regard to material and / or workpiece thickness.

18. The method according to any one of the preceding claims, characterized in that at least two of the electrodes are formed by the electrodes of a welding tongs.

19. The method according to any one of the preceding claims, characterized in that at least three workpieces are welded together.

20. resistance welding device for welding at least two workpieces (4, 6, 8) together,

with means for generating a welding current, which are connected to a first electrode (14) and a second electrode (16) for introducing the welding current into the workpieces (4, 6, 8) and a third electrode (18) connected to the means for generating a welding current, the electrodes (14, 16, 18) being connected to the means for generating a welding current such that two of the electrodes (14, 16) engage in a first welding circuit for introducing a first partial welding current into the workpieces (4, 6, 8) and the remaining electrode (18) or at least one of the remaining electrodes in a second welding circuit for introducing a second partial welding current into the workpieces (4 , 6, 8),

marked by

Measuring means (21, 23) for separate measurement of the first part of the welding current and the second partial welding current and

by means (25) for influencing at least one parameter of the welding process as a function of at least one of the measured partial welding currents.

21. The device according to claim 20, characterized in that the first and the second electrode (14, 16) equal to each other and the third electrode (18) to the first and second electrodes (14, 16) is polar oppositely, such that at Using the device (2) the first part-welding current flows through the first electrode (14) and the third electrode (18) and the second part-welding current flows through the second electrode (16) and the third electrode (18).

22. Device according to claim 20 or 21, characterized by control and / or regulating means (25) for separated setting and / or control and / or regulation of the first partial welding current (il) and the second part-welding current (i2) with respect to a respective characteristic.

23. The apparatus according to claim 22, characterized in that the adjustment and / or control and / or regulation during the welding operation is executable.

24. The apparatus according to claim 22, characterized in that the setting and / or control and / or regulation before or after a welding operation is executable.

25. Device according to one of claims 20 to 24, characterized in that the characteristic is the amplitude and / or the time average and / or the effective value of at least one welding current.

26. Device according to one of claims 22 to 25, characterized in that the characteristic is the amplitude and / or the time average and / or the effective value of at least one welding voltage.

27. Device according to one of claims 22 to 26, characterized in that the characteristic is the amplitude and / or the time average and / or the effective value of a welding power.

28. Device according to one of claims 22 to 27, characterized in that the characteristic is a time duration and / or a time course of a welding current or a welding voltage.

29. Device according to one of claims 20 to 28, characterized in that the means for generating a welding current having a common power source (12) for generating the first part-welding current and the second part-welding current.

30. Device according to one of claims 20 to 28, characterized in that the means for generating a welding current different current sources (12 ', 12' ') for generating the first part-welding current and a second part-welding current.

31. Device according to one of claims 20 to 30, characterized in that the welding current is a direct current.

32. Device according to one of claims 20 to 30, characterized in that the welding current is an alternating current.

33. Device according to one of claims 20 to 32, characterized by means for changing the polarity of the electrodes during the welding operation.

34. Device according to one of claims 20 to 33, characterized in that by adjustment or control or regulation of at least one parameter of the welding process, the position and / or size of a during the welding process of an interface between two workpieces (4, 6 6, 8) forming weld nugget is influenced.

35. The device according to any one of claims 20 to 34, characterized gekmeizt-ichnet that the FcixciiiieLei. of the welding process at least one welding current and / or at least one welding voltage and / or a temperature of at least one electrode (14, 16, 18) and / or at least one force with which an electrode (14, 16, 18) acts on the associated workpiece (4, 6, 8) is pressed.

36. Device according to one of claims 20 to 35, characterized in that differ at least two of the workpieces (4, 6, 8) in terms of material and / or workpiece thickness.

37. Device according to one of claims 20 to 36, characterized in that two of the electrodes (14, 16, 18) are formed by the electrodes of a welding tongs.

38. Device according to one of claims 20 to 37, characterized in that at least three workpieces (6, 8) are welded together.