Welding device for electric resistance spot welding and process

Abstract

Welding device for electric resistance spot welding with two electrode holders (2, 3) for two electrodes (14, 15) which can be supplied with a welding current, wherein at least one of the two electrode holders (2, 3) is movable via a feed device in such a way that the electrodes (14, 15) can be brought together in a closing movement with the components (4, 5) to be welded interposed, characterized by the fact that the electrode holders (2, 3) have contact surfaces (12, 13) which can be brought into contact with the components (4, 5) by the closing movement, the electrode holders (2, 3) are at least temporarily magnetizable in such a way that the closing force (F1) can be applied to the components (4, 5) by magnetic attraction between the contact surfaces (12, 13) and the electrodes (14, 15) can each be moved relative to the corresponding electrode holder (2, 3) in the direction of the components by means of a delivery device (20, 21).

Description

[0001] The invention relates to a welding device for resistance spot welding according to the preamble of claim 1, comprising two electrode holders for two electrodes to which a welding current can be applied, wherein at least one of the two electrode holders is movable via a feed device such that the electrodes can be brought together in a closing movement with the components to be welded interposed. Furthermore, the invention relates to a method for resistance spot welding.

[0002] Such welding devices are well known and are usually designed as movable welding tongs, e.g., in an X-shape or C-shape. In the X-shape, the electrode holders are attached to the ends of articulated arms. For the closing movement, the arms or one arm pivots around the joint, causing the electrode holders to move towards each other. In the C-shape, the electrode holders are arranged at opposite ends of a C-shaped frame. To achieve the closing movement, in this design one electrode is moved linearly towards the other, fixed electrode. The tong movement is usually implemented by an adjustment device in the form of a linear drive, which can be designed as a servo-electric, hydraulic or pneumatic drive, for example. For the prior art, reference is made to the documents DE 20 2008 013 883 U1, for example.

[0003] The electrodes first apply a closing force to the components, which serves to press the components together, eliminate any existing gap and ensure a defined component contact and resistance.

[0004] To form the weld spot, the electrodes are subjected to a welding current that flows through the electrode caps into the workpiece. Due to the resistance heating, the material between the electrodes becomes pasty and malleable, allowing the electrodes to penetrate the component material. This changes the resistance and the welding force exerted by the electrodes on the components. Therefore, a follow-up movement is performed during welding to compensate for this effect and maintain the welding force.

[0005] Due to the large number of spot welds, the clamp's closing movement must be as fast as possible. At the same time, however, the clamp design must also be able to generate the required high closing and welding forces, which places very high demands on the robotics required to transmit the process forces. Therefore, existing solutions are often heavy, complex, and expensive, and slow to follow-up.

[0006] In order to improve the generation of the welding force, it is known, for example from the document DE 10 2012 112 547 A1, to realize the required follow-up movement by means of an electromagnet which, in addition to the closing movement of the tongs, carries out a feed movement of the electrode in the direction of the components.

[0007] Furthermore, a system for monitoring a welding process with an ultrasonic welding device is known from the joint application. US 1 354 293 A describes a welding device in which an electrode is surrounded by a magnetic device to hold a stud in place. Further welding devices are known from CN 2 01 030 471 Y and CN 1 03 170 725 A. DE 10 2008 051 289 A1 shows a welding gun for electric spot welding with simplified disassembly and assembly options.

[0008] The object of the invention is to provide a welding device and a method with which the generation of the welding force can be further improved and the flexibility can be increased.

[0009] The object is achieved by a welding device according to patent claim 1 and a method according to patent claim 7. Further advantageous embodiments of the invention emerge from the subclaims.

[0010] In the welding device according to the invention, the electrode holders have contact surfaces that can be brought into contact with the components by the closing movement. The electrode holders are magnetizable, at least temporarily, such that they attract each other and the contact surfaces transfer the required closing force to the components. To regulate the welding force, the electrodes can be advanced toward the components relative to the corresponding electrode holder by means of a feed device.

[0011] Furthermore, a method for electrical resistance spot welding is specified, in which two electrodes, with components to be welded in between, are brought together in a closing movement and subjected to an electric welding current. According to the invention, the electrode holders have contact surfaces that are brought into contact with the components by the closing movement. The electrode holders are magnetized in such a way that they attract one another, and the contact surfaces transfer the required closing force to the components. The welding force is adjusted by moving the electrodes relative to the corresponding electrode holder, e.g., toward or away from the components.

[0012] According to the invention, the closing force is generated by the magnetic attraction between the electrode holders, for which the components are clamped between the contact surfaces of the opposing electrode holders. This minimizes external influences that must be compensated for by the control system during welding. While in conventional welding devices, the magnitude and consistency of the welding force are always determined by the stability of the tong construction and the resulting fluctuations in the closing force, the welding force is now free from such influences.

[0013] In addition, the welding device can be designed to be particularly lightweight. A tong construction that is designed to be so stable that it can withstand the closing forces is no longer necessary. Instead, a tong construction can be optimized for particularly fast closing movements or flexible use. If the electrode holders are attached to a tong construction, only one tong element with the electrode holder can be designed to be movable, or both tong elements can be movable. Completely new designs that deviate from known tong designs are also possible, such as the arrangement of the electrode holders on each robot arm, which can then assume the function of the feed device for implementing the closing movement. The invention thus also enables significantly faster cycle times and more flexible design concepts.

[0014] To magnetize the electrode holders, they are preferably designed as electromagnets. This has the advantage that the duration and strength of the magnetization can be easily influenced by the duration and strength of the electric current flowing through the electromagnet. A portion of the welding current, such as a phase of the welding transformer, can be used to excite the electromagnets. The polarity of the electromagnets is such that the electrode holders attract each other.

[0015] In one embodiment, the electrode holders designed as electromagnets comprise a ferromagnetic body wound with a copper winding. The current is conducted through the copper winding, generating a magnetic field that magnetizes the ferromagnetic body. To prevent excessive heating of the electrode holder, a fluid line through which a cooling fluid, such as water, can be passed can be provided in the electrode holder and, in particular, in the ferromagnetic body.

[0016] According to the invention, a repositioning movement of the electrodes required during welding can be realized by the feed devices, which can each advance an electrode relative to the associated electrode holder further in the direction of the components. This relative movement enables the generation or control of the actual welding force. In a preferred embodiment, the feed device is formed by at least one piezo actuator. The piezo actuator can be, for example, a single piezoelectric element, piezohydraulic, or even a stack of piezoelectric elements. Furthermore, the effective length of the piezo crystals can be extended by suitable levers and reduction ratios. By applying a voltage, the piezoelectric elements experience a change in thickness, which causes a movement of the electrode relative to the electrode holder.Preferably, the piezo actuator is arranged in the electrode holder in such a way that the generated thickness change (longitudinal change) acts directly on the electrode and moves it. If particularly large feed movements or welding forces are desirable or necessary, two or more piezo actuators can be arranged in series.

[0017] To control and regulate the welding process, a suitable control and regulation device known to those skilled in the art can be provided. The welding force can be regulated, for example, by known control systems that determine the welding force and compare it with a specified target value, by specifically controlling the feed devices or piezo actuators. The piezo actuators advantageously enable very rapid adjustment of the welding force.

[0018] In one embodiment, the control and regulating device is configured to operate the piezo actuator in a pulsed manner during welding. During pulsed operation, the voltage acting on the piezo actuator varies repeatedly and very quickly, whereby the welding force has several peaks, e.g. several tens or hundreds of peaks, during a welding cycle. Such a dynamic variation of the welding force can be used, for example, in combination with a resistance measurement and the welding current to vary the weld nugget. Advantageously, the closing force acting on the components via the contact surfaces of the electrode holder remains unaffected by this variation in the welding force, so that component contact is maintained despite the dynamic welding force.

[0019] Furthermore, the welding device can additionally comprise an evaluation device configured to evaluate the welding parameters, such as welding force, welding current, electrical resistance, etc., to assess the welding quality. This can be done, for example, by comparison with reference values ​​or by simulation.

[0020] The device and method can be used to weld both metal components, in particular steel and aluminum components, and plastic components made of weldable plastics.

[0021] The above-described properties, features, and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more readily understood from the drawings and in conjunction with the following description of the exemplary embodiments. Where the term "may" is used in this application, it refers to both the technical possibility and the actual technical implementation.

[0022] The following examples are explained using a single drawing. It shows: Fig. 1 a schematic representation of the electrode holders of a welding device according to the invention in a sectional view.

[0023] Fig.1 shows the electrode holders of a welding device 1 according to the invention for electric resistance spot welding. The electrode holders 2, 3 are brought towards one another by a closing movement with two components 4, 5 to be welded interposed on opposite sides. The closing movement can be carried out, for example, by a tong construction (not shown), as is known from conventional welding devices. Likewise, the closing movement can also be implemented, for example, by a robot, for which purpose either only one electrode holder or both electrode holders are moved. The electrode holders 2, 3 are brought towards the components 4, 5 during the closing movement and positioned at the joint.

[0024] Each electrode holder 2, 3 has a base body 6, 7 made of a ferromagnetic material and is wrapped with a copper winding 8, 9. To apply the closing force, a current is passed through the copper windings 8, 9 of the electrode holders 2, 3, thereby magnetizing the base body 6, 7 like an electromagnet. For the power supply, for example, one phase of the welding transformer (not shown) of the welding device 1 can be used. To dissipate the generated heat, fluid lines 10, 11 are provided in the base bodies, through which a coolant is conducted.

[0025] The current direction or the winding direction of the copper windings 8, 9 is selected such that the electrode holders 2, 3 have opposite polarity and thus attract each other. As a result, the components 4, 5 are clamped between the contact surfaces 12, 13 with which the electrode holders 2, 3 rest on the components 4, 5. The attractive force between the electrode holders acts as a closing force F1 on the components 4, 5, pressing them together.

[0026] Furthermore, each electrode holder 2, 3 holds an electrode 14, 15. The electrode 14, 15 is surrounded by a ring-shaped contact surface 12, 13. The contact surface does not necessarily have to be ring-shaped, but can also be formed in another form, e.g., by individual contact surfaces interrupted from one another. At the free end, the electrodes 14, 15 each have an electrode cap 16, 17, which is placed on the components 4, 5 by the closing movement. Furthermore, the electrodes 14, 15 have a conventional electrode cooling system 18, 19.

[0027] To form the weld spot, a current from the welding transformer (not shown) is passed through the electrodes 14, 15. The material of the components 4, 5 becomes doughy at the joint, causing the electrode caps 16, 17 to sink into the components 4, 5 and the welding force F2 to drop. To implement the required adjustment movement, a feed device 20, 21 is arranged at the end of the electrode 14, 15 opposite the electrode cap 16, 17. Each feed device is in the form of a piezo actuator comprising a stack of piezo elements. By applying a voltage to the respective piezo actuator 20, 21, the thickness of the piezo actuator can be changed, whereby the electrode 14, 15 can be moved toward (or away from) the component relative to the associated electrode holder 2, 3.

[0028] A control and regulation device 22, e.g., in the form of a computer, is configured to control and regulate the piezo actuators 20, 21 and to maintain the welding force F2 at the desired value during welding. The control and regulation device 22 can also be configured to control the closing force F1 between the electrode holders 2, 3, the welding current, and the closing movement, and is connected in corresponding operative connections.

[0029] Advantageously, the control and regulation device 22 is also configured to operate the piezo actuators 20, 21 in a pulsed manner. During pulsed operation, the voltage acting on the piezo actuator is varied repeatedly and very rapidly, causing the welding force F2 to exhibit several peaks, e.g., several tens or several hundred, during a welding cycle.

[0030] Such a dynamic variation of the welding force F2 enables a previously unattainable variation of the weld nugget. Advantageously, the closing force F1, which acts on the components 4, 5 via the contact surfaces 12, 13 of the electrode holders 2, 3, remains unaffected by this variation of the welding force F2, so that component contact is maintained despite the dynamic welding force.

[0031] The welding device further comprises an evaluation device 23 configured to evaluate the welding parameters, such as the welding force, the welding current, the electrical resistance, etc., to assess the welding quality. This can be done, for example, by comparison with reference values ​​or by simulation. The evaluation device can be a computer, for example.

[0032] The exemplary embodiments are not to scale and are not limiting. Modifications within the scope of expert practice are possible. List of reference symbols 1 welding device 2, 3 Electrode holders 4, 5 components 6, 7 basic body 8, 9 copper winding 10, 11 Fluid lines 12, 13 contact surfaces 14, 15 Electrode 16, 17 Electrode cap 18, 19 Electrode cooling 20, 21 Delivery device 22 Control and regulation device 23 Evaluation device F1 closing force F2 welding force

Claims

[1] Welding device for electrical resistance spot welding with two electrode holders (2, 3) for two electrodes (14, 15) which can be supplied with a welding current, wherein at least one of the two electrode holders (2, 3) is movable via a feed device in such a way that the electrodes (14, 15) can be brought together in a closing movement with the components (4, 5) to be welded interposed, characterized by , that the electrode holders (2, 3) have contact surfaces (12, 13) which can be brought into contact with the components (4, 5) by the closing movement, the electrode holders (2, 3) are at least temporarily magnetizable in such a way that the closing force (F1) can be applied to the components (4, 5) by magnetic attraction between the contact surfaces (12, 13) and the electrodes (14, 15) can be fed in the direction of the components relative to the corresponding electrode holder (2, 3) by a respective feeding device (20, 21). [2] Device according to claim 1, wherein the electrode holders (2, 3) are designed as electromagnets. [3] Device according to one of the preceding claims, in which the electrode holders (2, 3) have a ferromagnetic body (6, 7) which is wound with a copper winding (8, 9). [4] Device according to one of the preceding claims, in which the feed device (20, 21) is formed by at least one piezo actuator. [5] Device according to claim 4, in which at least one piezo actuator (20, 21) can be controlled via a control device (22) which is designed to operate the piezo actuator in a pulsed manner during welding. [6] Device according to one of the preceding claims, further comprising an evaluation device (23) which is arranged to evaluate the welding parameters for assessing the welding quality. [7] Method for electrical resistance spot welding with a device according to one of claims 1 to 6, in which two electrodes (14, 15) are brought together in a closing movement with components (4, 5) to be welded in between and are subjected to an electric welding current, characterized by , that the electrode holders (2, 3) have contact surfaces (12, 13) which are brought into contact with the components (4, 5) by the closing movement, the electrode holders (2, 3) are at least temporarily magnetized in such a way that the closing force (F1) is applied to the components (4, 5) by magnetic attraction between the contact surfaces (12, 13) and the welding force (F2) is adjusted by a movement of the electrodes (14, 15) relative to the corresponding electrode holder (2, 3). [8] Method according to claim 7, wherein the electrode holders (2, 3) are designed as electromagnets and one phase of the welding current transformer is used to operate the electromagnets. [9] Method according to claim 7 or 8, wherein the piezo actuators (20, 21) are operated in a pulsed manner during welding, whereby a dynamic welding force (F2) is generated.

Images