SHORT-CIRCUIT WELDING PROCESS AND WELDING DEVICE

Description

[0001] The invention relates to a short-circuit welding method with successive welding cycles, each comprising an arc phase and a short-circuit phase, with the following steps during a welding cycle: Conveying a consumable welding wire away from a workpiece during the short-circuit phase at a reverse speed so that material is transferred from the welding wire to the workpiece and an arc is ignited, wherein the welding wire is brought to a final reverse speed during the short-circuit phase in a first reverse conveying phase with a first time duration and is conveyed at this speed until the start of a second reverse conveying phase in which the reverse speed of the welding wire is reduced again, wherein the second reverse conveying phase immediately follows the end of the first reverse conveying phase;and conveying the welding wire towards the workpiece during the arc phase at a forward speed to bring the welding wire into contact with the workpiece and to create a short circuit between the welding wire and the workpiece, wherein during the arc phase the welding wire is brought to a final forward speed in a first forward conveying phase with a second time duration and is conveyed at this speed until the start of a second forward conveying phase in which the forward speed of the welding wire is reduced again, wherein the second forward conveying phase immediately follows the end of the first forward conveying phase. ;

[0002] Furthermore, the invention relates to a welding device for carrying out an above-mentioned short-circuit welding method with successive welding cycles, each comprising an arc phase and a short-circuit phase, wherein the welding device comprises a welding torch.

[0003] Short-circuit welding processes of the type described above include, in particular, so-called CMT (Cold Metal Transfer) welding processes. In a CMT welding process, a welding wire made of fusible material, which is subjected to a welding current, is alternately moved towards the workpiece and away from it again to create a weld seam or coating. During the short-circuit phase, a short circuit exists between the workpiece and the welding wire, which supports the transfer of material from the welding wire to the workpiece. When the short circuit is broken after the material has been transferred and the arc is (re-)ignited, the short-circuit phase ends and the arc phase begins. During the arc phase, heat energy is introduced into the welding wire and the workpiece.

[0004] In the prior art, the melting welding wire is conveyed to the workpiece (arc phase) or away from it (short-circuit phase) according to a predetermined sequence at fixed speeds and for fixed times. This is known, for example, from EP 3 292 936 A1. In the short-circuit phase, the welding wire is brought to a final reverse speed, which is then reduced again to convey the welding wire toward the workpiece at a final forward speed. The times during which the welding wire is conveyed at these final speeds in the respective phases are set in advance. The times are triggered by the onset and the breaking of the short circuit.

[0005] Further welding processes are known from JP 2021 079427 A (which forms the basis for the preamble of the independent claims), EP 3 815 828 A1 and US 2018 / 0264576 A1.

[0006] In short-circuit welding processes of the type described above, it is common for the onset and breakup of short circuits to vary from welding cycle to welding cycle, meaning that the welding frequency is not constant. This leads to a reduction in the quality of the weld seam. With many materials, for example viscous materials such as chromium-nickel alloys or titanium, unwanted weld spatter can also occur when the material is dispensed onto the workpiece, which also negatively affects the welding frequency and weld quality. Weld spatter is usually caused by the welding wire speed being too high at the time of material dispensing. If, on the other hand, the welding wire speed is too low, the welding frequency or the droplet dispensing frequency, and thus the welding process, is unnecessarily slowed, and further irregularities in the short-circuit times can occur.Even at the moment of short circuit occurrence, the welding wire speed should not be set too high to avoid further irregularities in the welding frequency and undesirable effects, such as contact of the welding wire with the unmelted base material, vibrations of the weld pool, and temporary "sticking" of the welding wire. It would therefore be desirable to select the welding wire speed profile so that the highest and, above all, most stable welding frequency is ensured, resulting in high weld quality and avoiding weld spatter.

[0007] The ideal speeds and durations at which the welding wire should be fed forward and backward in the respective phases to stabilize the welding frequency and avoid weld spatter depend, among other things, on the weld seam shape, temperature, contaminants, the workpiece material, and other parameters and are therefore difficult to determine. Therefore, given the current state of technology, it is not easy to select or achieve the ideal durations or speed profiles for a welding wire in a short-circuit welding process.

[0008] In view of these statements, it is therefore an object of the present invention to mitigate or even completely eliminate the disadvantages of the prior art. Preferably, the object of the invention is to keep the welding frequency of a short-circuit welding process as stable as possible in order to improve welding quality.

[0009] This object is achieved by a short-circuit welding method according to claim 1 and by a welding device according to claim 14. Preferred embodiments are specified in the dependent claims.

[0010] According to claim 1, in a short-circuit welding method of the type mentioned above, it is initially provided that the first time period of the first reverse feed phase is adjusted by means of a control and / or regulation system such that the welding wire in the second reverse feed phase does not exceed a predetermined reduced reverse speed at the time the short circuit is broken. Preferably, the first time period is adjusted by the control and / or regulation system such that the welding wire in the second reverse feed phase essentially has the predetermined reduced reverse speed at the time the short circuit is broken.By adjusting the first time period, the first reverse feed phase and thus the feed of the welding wire at the final reverse speed is ended in good time before the short circuit is broken, and the reduction of the reverse speed of the welding wire begins. This means that the welding wire is not fed too quickly when the material is delivered to the workpiece, thus avoiding weld spatter. The control and / or regulation therefore ensures that the reverse speed of the welding wire is reduced if it is too high. In other words, the control and / or regulation ensures that the welding wire in the second reverse feed phase is set to a lower reverse speed value at the time the short circuit is broken, which does not exceed the specified reduced reverse speed value. This stabilizes the welding frequency.The absolute value of the predetermined reduced reverse speed is less than the absolute value of the final reverse speed. The first time period of the first reverse feed phase is variably adjustable and is adapted by the control and / or regulation. The breaking of the short circuit, as well as the onset of the short circuit, is preferably detected by measuring the welding voltage. Typical values ​​for the first time period are, for example, between 1 ms and 10 ms, preferably between 2 and 3 ms, for welding frequency values ​​between 5 Hz and 200 Hz, preferably between 80 Hz and 150 Hz. The adjustment of the first time period by the control and / or regulation can take place from welding cycle to welding cycle or, in some embodiments of the invention, even within a welding cycle. By adjusting the first time period, the first reverse feed phase is shortened or lengthened.If, for example, it is determined that the welding wire is being fed too quickly at the time the short circuit is broken, the first time period of the first reverse feed phase can be shortened accordingly. If, for example, it is determined that the welding wire is being fed too slowly at the time the short circuit is broken, the first time period of the first reverse feed phase can be extended accordingly. The first reverse feed phase begins when the welding wire changes from a forward speed to a reverse speed. In the first reverse feed phase, the welding wire is accelerated to the final reverse speed. Once the final reverse speed is reached, the acceleration of the welding wire can be stopped and the speed of the welding wire can be kept essentially constant.Preferably, a substantially constant acceleration occurs in the first reverse feed phase. The first reverse feed phase is immediately followed by the second reverse feed phase, in which the reverse speed is reduced again by accelerating ("braking") the welding wire (now in the opposite direction). Reducing the reverse speed means a reduction in the absolute value and thus a slowing down of the reverse movement of the welding wire. Breaking the short circuit preferably takes place in the second reverse feed phase, with the control and / or regulation ensuring that the welding wire does not exceed the specified reduced reverse speed at the time the short circuit is broken.It is preferred if the control and / or regulation ensures that the welding wire corresponds to the predetermined reduced reverse speed at the time the short circuit is broken. When the short circuit is broken, the acceleration of the welding wire can be stopped for a holding time. The acceleration can then be continued in the second reverse feed phase. The second reverse feed phase can be followed by the first forward feed phase, in which the welding wire is brought to a final forward speed. The first forward feed phase follows the second forward feed phase, in which the forward speed is reduced again by accelerating the welding wire. The second forward feed phase is again followed by the first reverse feed phase. A welding cycle therefore consists of the first and second reverse feed phases and the first and second forward feed phases in this order.The absolute value of the reverse final speed and / or the absolute value of the forward final speed can, for example, be between 10 m / min and 70 m / min, in particular between 20 m / min and 60 m / min. In this disclosure, forward speeds have a positive or no sign. Reverse speeds are given a negative sign to distinguish them. The forward speeds and the reverse speeds are selected such that, on average, a forward speed in the direction of the workpiece results over several welding cycles. In other words, the integral over the speed curve is positive over several welding cycles. This is necessary because the consumable welding wire releases material to the workpiece to form a weld seam or a coating. Typical acceleration values ​​for the welding wire are, for example, in the range between 30,000 m / min / s and 60,000 m / min / s.The welding wire is preferably fed by an electric motor, in particular a gearless direct drive.

[0011] In a preferred embodiment, the second reverse feed phase has a first intermediate plateau phase with a substantially constant reverse speed, wherein the welding wire is preferably conveyed away from the workpiece at the predetermined reduced reverse speed in the first intermediate plateau phase. The intermediate plateau phase interrupts the acceleration with which the reverse speed of the welding wire is reduced. The advantage of the first intermediate plateau phase is that the welding wire is conveyed during this time at a reduced and substantially constant reverse speed compared to the final reverse speed, and the material can be transferred to the workpiece without acceleration, so that no welding spatter is caused. The reverse speed is substantially constant throughout the entire first intermediate plateau phase.

[0012] It is therefore preferred if the first time period is adjusted by the control and / or regulation system such that the short circuit is broken in the first intermediate plateau phase. After the short circuit is broken, the acceleration of the welding wire can be suspended for a certain holding period before the acceleration of the welding wire is continued in the second reverse feed phase. After the holding period, the welding wire can be further accelerated until the welding wire is finally fed at a forward (final) speed.

[0013] In one embodiment of the invention, a control system for adapting the first time period is provided, wherein the control is carried out by a control loop with a controller, in particular a P or PI controller, wherein the control system adapts the first time period so that the first intermediate plateau phase corresponds to a first target duration. It is preferred if a manipulated variable of the control loop is formed by the first time period and a reference variable of the control loop is formed by the first target duration of the first intermediate plateau phase. The duration of the first intermediate plateau phase can serve as the measured controlled variable. The controller adapts the first time period such that the short circuit is broken in the first intermediate plateau phase. The first target duration specifies how long the first intermediate plateau phase should last. The first target duration can, for example, be between 0.5 ms and 1.5 ms. A reference variable can also be referred to as the target variable for the control loop.The controller adapts the first time period from welding cycle to welding cycle. If there is a deviation between the first target duration and the actual duration of the first intermediate plateau phase, the first time period of the first reverse feed phase is adjusted in the next welding cycle. If the actual duration of the first intermediate plateau phase is too long, the first time period of the first reverse feed phase is shortened. If the actual duration of the first intermediate plateau phase is too long, the first time period of the first reverse feed phase is extended. To avoid any oscillations, the controller can preferably have parameterizable filters. A filter can, for example, specify the maximum rate of change or the slew rate. A filter can, for example, be formed by a PT1 element.

[0014] In an alternative embodiment, a control system for adapting the first time period is provided, wherein the control is carried out by a control loop with a controller, in particular a P or PI controller, wherein a manipulated variable of the control loop is formed by the first time period, a reference variable of the control loop is formed by a desired reverse speed of the welding wire, in particular the predetermined reduced reverse speed, and a measured controlled variable of the control loop is formed by a measured reverse speed of the welding wire at the time the short circuit is broken. If the measured reverse speed of the welding wire deviates from the desired reverse speed at the time the short circuit is broken, the first time period is adapted in the next welding cycle.Therefore, if the welding wire's reverse speed is too high or too low at the time the short circuit is broken, the initial time of the first reverse feed phase in the next welding cycle will be shortened or lengthened, respectively. The welding wire's speed at the time the short circuit is broken can be measured.

[0015] In terms of control, it may be advantageous to add an offset value to the reference variable.

[0016] In a further alternative embodiment, a resistance value is continuously determined from a welding current and a welding voltage, and a prediction of the time at which the short circuit will break is made from the resistance value. The first time period is adjusted based on the prediction by starting the second reverse feed phase. The continuously determined resistance value allows conclusions to be drawn about the impending break of the short circuit. When the short circuit is imminent, the first reverse feed phase is terminated, and the reverse feed speed of the welding wire is reduced so that the welding wire does not exceed the predetermined reduced reverse speed at the time the short circuit breaks or essentially corresponds to it.Preferably, the first reverse feed phase is terminated in such a timely manner that the short circuit is broken during the first intermediate plateau phase. In this embodiment, the adjustment of the first time period therefore takes place within one welding cycle.

[0017] To determine the time at which the short circuit breaks, the prediction can be based on a predefined resistance threshold or on a resistance change rate. For example, the first reverse feed phase can be terminated if the continuously determined resistance value exceeds the predefined resistance threshold or the resistance change rate exceeds a predefined resistance change rate threshold.

[0018] A particularly efficient embodiment of the invention results when the first reverse conveying phase has a first final plateau phase in which the welding wire is conveyed at a substantially constant final reverse speed, and a duration of the first final plateau phase is adjusted by the control and / or regulation system. The first duration of the reverse conveying phase is adjusted by adjusting the duration of the first final plateau phase.

[0019] The previous statements relate to the short-circuit phase of the short-circuit welding process. In order to increase the efficiency and stability of the welding process, in particular the welding frequency, the process steps described so far can be applied analogously to the arc phase and the onset of the short circuit. In one embodiment, it is therefore provided that the second time period of the first forward feed phase is adjusted by means of a further control and / or regulation such that the welding wire in the second forward feed phase does not exceed a predetermined reduced forward speed at the time the short circuit occurs. By adjusting the second time period, the first forward feed phase is shortened or lengthened so that the welding wire in the second forward feed phase does not exceed the predetermined reduced forward speed at the time the short circuit breaks or essentially corresponds to this.The second forward feed phase immediately follows the first forward feed phase. The predetermined reduced forward speed is lower than the final forward speed. Preferably, the second forward feed phase has a second intermediate plateau phase with a substantially constant forward speed, wherein the welding wire is preferably conveyed at the predetermined reduced forward speed in the second intermediate plateau phase. Furthermore, it is preferably provided that the second time period is adjusted by the further control and / or regulation such that the short circuit occurs in the first intermediate plateau phase.

[0020] The control and / or regulation embodiments described above in connection with the short-circuit phase are analogously applicable to the arc phase.

[0021] In one embodiment of the invention, therefore, a further control system for adapting the second time period is provided, wherein the further control is carried out by a further control loop with a further controller, in particular a P or PI controller, wherein the further control system adapts the second time period so that the second intermediate plateau phase has a second target duration. It is preferred if a manipulated variable of the further control loop is formed by the second time period and a reference variable of the further control loop is formed by the second target duration of the second intermediate plateau phase. The further controller adapts the second time period such that the onset of the short circuit occurs in the second intermediate plateau phase. The second target duration specifies how long the second intermediate plateau phase should last. The second target duration of the second intermediate plateau phase can, for example, be between 0.5 ms and 2 ms.A reference variable can also be referred to as a target variable for the control loop. The additional controller adapts the second time period from welding cycle to welding cycle. If there is a deviation between the second target duration and the actual duration of the second intermediate plateau phase, the second time period of the second forward feed phase is adapted in the next welding cycle. In this embodiment, the second target duration is therefore specified and the second time period is adapted such that the second intermediate plateau phase has the second target duration. If, for example, the actual duration of the second intermediate plateau phase is too short, the second time period of the first forward feed phase is shortened. If, for example, the actual duration of the second intermediate plateau phase is too long, the second time period of the second forward feed phase is extended.

[0022] In an alternative embodiment, a further control system is provided for adapting the second time period, wherein the further control system is carried out by a further control loop with a further controller, in particular a P or PI controller, wherein a manipulated variable of the further control loop is formed by the second time period, a reference variable of the further control loop is formed by a desired forward speed of the welding wire, in particular the predetermined reduced forward speed, and a measured controlled variable of the further control loop is formed by the forward speed of the welding wire at the time the short circuit occurs. If the forward speed of the welding wire deviates from the desired forward speed, the second time period is adapted in the next welding cycle.Therefore, if the welding wire's forward speed is too high or too low at the time the short circuit occurs, the second time period of the second forward feed phase in the next welding cycle will be shortened or lengthened, respectively. The welding wire's speed at the time the short circuit occurs can be measured.

[0023] In terms of control, it may be advantageous to add an offset value to the reference variable.

[0024] A particularly efficient embodiment of the invention results when the first forward conveying phase has a second final plateau phase in which the welding wire is conveyed at a substantially constant forward final speed, and a duration of the second final plateau phase is adjusted by the further control and / or regulation. The adjustment of the second duration of the forward conveying phase is achieved by adjusting the duration of the second final plateau phase.

[0025] The object mentioned above is also achieved by a welding device according to claim 14. The welding device is configured to perform a short-circuit welding process and has a welding torch. The welding device is designed to to convey a consumable welding wire away from a workpiece during the short-circuit phase at a reverse speed so that material is transferred from the welding wire to the workpiece and an arc is ignited, wherein the welding device is further configured to bring the welding wire to a final reverse speed during the short-circuit phase in a first reverse conveying phase with a first time duration and to convey it at the final reverse speed until the start of a second reverse conveying phase in which the reverse speed of the welding wire is reduced again, wherein the second reverse conveying phase immediately follows the end of the first reverse conveying phase, and the welding device is further configured to convey the welding wire towards the workpiece during the arc phase at a forward speed in order to create a short circuit between the welding wire and the workpiece,wherein the welding device is further configured to bring the welding wire to a forward final speed during the arc phase in a first forward feed phase with a second time duration and to feed it at the forward final speed until the start of a second forward feed phase in which the forward speed of the welding wire is reduced again, wherein the second forward feed phase immediately follows the end of the first forward feed phase, wherein a control / regulation unit is provided which is configured to adapt the first time duration of the first reverse feed phase by means of a control and / or regulation such that the welding wire does not exceed a predetermined reduced reverse speed in the second reverse feed phase at the time the short circuit is broken.

[0026] With regard to the advantages and further features of the device, reference is made to the short-circuit welding method described above. The features described in connection with the short-circuit welding method can also be applied to the welding device. The welding device can have a wire feed unit configured to feed the welding wire. The welding device can have an electric motor, in particular a gearless direct drive.

[0027] Furthermore, a further short-circuit welding method of the type described below is disclosed. Accordingly, in a short-circuit welding method of the type mentioned above, the second time period of the first forward feed phase is adjusted by means of a further control and / or regulation such that the welding wire does not exceed a predetermined reduced forward speed in the second forward feed phase at the time the short circuit occurs. The method can be described as follows: A short-circuit welding method with successive welding cycles, each comprising an arc phase and a short-circuit phase, with the following steps during a welding cycle: Conveying a consumable welding wire away from a workpiece during the short-circuit phase at a reverse speed so that material is transferred from the welding wire to the workpiece and an arc is ignited, wherein the welding wire is brought to a final reverse speed during the short-circuit phase in a first reverse conveying phase with a first time duration and is conveyed at this speed until the start of a second reverse conveying phase in which the reverse speed of the welding wire is reduced again, Conveying the welding wire towards the workpiece during the arc phase at a forward speed in order to bring the welding wire into contact with the workpiece and create a short circuit between the welding wire and the workpiece,wherein the welding wire is brought to a final forward speed during the arc phase in a first forward feed phase with a second time period and is fed with this speed until the start of a second forward feed phase in which the forward speed of the welding wire is reduced again, wherein the second time period of the first forward feed phase is adjusted by means of a further control and / or regulation such that the welding wire does not exceed a predetermined reduced forward speed in the second forward feed phase at the time the short circuit occurs.

[0028] The control and / or regulation ensures that the forward speed of the welding wire is reduced if it is too high. In other words, the control and / or regulation ensures that the welding wire is set to a lower forward speed value in the second forward feed phase at the time the short circuit occurs, which does not exceed the specified reduced forward speed value.

[0029] The further control and / or regulation has already been described in connection with the short-circuit method according to claim 1. The above features and explanations also apply to the short-circuit method described here.

[0030] A further welding device of the type described below is also disclosed. The welding device is configured to carry out the further short-circuit welding method and has a welding torch, wherein the welding device is adapted to to convey the welding wire away from the workpiece during the short-circuit phase at a reverse speed so that material is transferred from the welding wire to the workpiece and an arc is ignited, wherein the welding device is further configured to bring the welding wire to a reverse final speed during the short-circuit phase in a first reverse conveying phase with a first time duration and to convey it at the reverse final speed until the start of a second reverse conveying phase in which the reverse speed of the welding wire is reduced again, and the welding device is further configured to convey a consumable welding wire towards a workpiece during the arc phase at a forward speed in order to create a short circuit between the welding wire and the workpiece, wherein the welding device is further configured toto bring the welding wire to a forward final speed during the arc phase in a first forward feed phase with a second time period and to feed it at the forward final speed until the start of a second forward feed phase in which the forward speed of the welding wire is reduced again, wherein a further control and / or regulating unit is provided which is designed to adapt the second time period of the first forward feed phase by means of a further control and / or regulating unit such that the welding wire does not exceed a predetermined reduced forward speed in the second forward feed phase at the time the short circuit occurs.

[0031] The invention is explained in more detail below with reference to the figures, to which, however, it is not intended to be limited. They show: Fig. 1 a welding device for carrying out a short-circuit welding process; Fig. 2 a welding voltage curve, a welding current curve and a speed curve of a welding wire in a short-circuit welding process from the prior art; Fig. 3 a welding voltage curve, a welding current curve and a speed curve of a welding wire in a short-circuit welding method according to the invention; Fig. 4A und Fig. 4B a control loop; Fig. 5 a welding voltage curve, a welding current curve and a further speed curve of a welding wire in a short-circuit welding method according to the invention; Fig. 6A und Fig. 6B another control loop; and Fig. 7 a welding voltage curve, a welding current curve, a speed curve of a welding wire and a time curve of a resistance value.

[0032] Fig. 1 shows a welding device 1 with a welding torch 2, which is designed to carry out a short-circuit welding process, in particular a CMT welding process. In a short-circuit welding process, a melting welding wire 3 (electrode) is moved alternately toward a workpiece 5 and back by a conveyor device 4, which may have an electric motor. Through this sequence, a short-circuit welding process can be divided into arc phases 6 and short-circuit phases 7 (see, for example, Fig. 2 and 3 ). The welding device 1 has a control / regulation unit 8 for controlling and / or regulating the welding wire 3. The welding wire 3 is fed from a storage drum 9 into the area of ​​the welding torch 2.

[0033] Fig. 2 shows (below a time profile of a welding voltage U and a time profile of a welding current I) a speed profile 10 of a welding wire 3 in a short-circuit welding process from the prior art with fixed and pre-set time sequences. The speed vd is plotted on the ordinate, where +vd denotes a forward speed of the welding wire 3 in the direction of the workpiece 5 and -vd a backward speed of the welding wire 3 away from the workpiece 5. The short-circuit welding process can be divided into arc phases 6 and short-circuit phases 7. A short-circuit phase 7 begins with the occurrence 11 of the short circuit between the welding wire 3 and the workpiece 5, when the welding wire 3 touches the workpiece 5. An arc phase 6 begins with the breaking 12 of the short circuit between the welding wire 3 and the workpiece 5 and ends again with its occurrence.During the arc phase 6, the welding wire 3 is fed primarily toward the workpiece 5 at a forward speed of +vd. During the short-circuit phase 7, the welding wire 3 is fed primarily away from the workpiece 5 at a reverse speed of -vd. A welding cycle consists of an arc phase 6 and a short-circuit phase 7.

[0034] The speed curve 10 of the welding wire 3 can be further divided into individual welding wire speed phases at which the welding wire 3 is conveyed. In a first backward conveying phase 13, the welding wire 3 is accelerated to a final backward speed v re_max. The final backward speed v re_max can, for example, be in the range between 10 m / min and 60 m / min, in particular between 20 m / min and 60 m / min. The first backward conveying phase 13 has a total duration which is referred to as the first time period T 1 and which can, for example, be 3 ms. After the welding wire 3 has reached the final backward speed v re_max, the acceleration is ended and the welding wire 3 is conveyed away from the workpiece 5 in a first final plateau phase 14 at an essentially constant final backward speed v re_max. The final plateau phase 14 has the time period T 3 .Immediately following the end of the first reverse feed phase 13, a second reverse feed phase 15 follows, in which the reverse speed -vd of the welding wire 3 is reduced again by acceleration. The reverse speed -vd is reduced to zero in this phase. This is followed by a first forward feed phase 16, in which the welding wire 3 is accelerated to a final forward speed v v_max. The total duration of the first forward feed phase 16 is referred to as the second time period T 2. In a second final plateau phase 17, the duration of which is referred to as T 4, the final forward speed v v_max is kept essentially constant. The first forward feed phase 16 is followed by a second forward feed phase 18, in which the forward speed +vd is reduced again to zero. This is then followed by another first reverse feed phase 13.The speed curve shown is a schematic and idealized representation of a target curve. Actual speed curves may differ from the shown curve due to physical reasons.

[0035] At the end of the short-circuit phase 7, at the moment of breaking 12 of the short circuit, a material release (drop release) from the welding wire 3 onto the workpiece 5 takes place in order to form a weld seam 19 (see Fig. 1 However, if the speed of the welding wire 3 at the time of breaking 12 of the short circuit is too high, as shown in Fig. 2 If this is the case, welding spatter may occur, which negatively affects the quality of the weld 19. This is particularly the case with viscous materials such as titanium.

[0036] In the prior art, the speed curve 10 is determined over fixed time periods T 1 , T 2 or T 3 , T 4 . The first time period T 1 should be set such that the welding wire 3 is conveyed sufficiently slowly at the time of breaking 12 to avoid welding spatter. However, the time of breaking 12 of the short circuit as well as the time of occurrence 11 of the short circuit can vary, as shown in Fig. 2 shown, and depend on parameters such as the material, the weld seam, impurities and temperature, etc. The selection of the specified time periods T 1 and T 2 or T 3 , T 4 is therefore not easy. For example, it may happen that the welding wire 3 is conveyed at an excessively high reverse conveying speed -vd at the time of breaking 12 of the short circuit, thus causing welding spatter. This is explained in Fig. 2 where the short circuit breaks at times 20 with too high a speed. The times 20 vary in time relative to the speed curve 10, as shown in Fig. 2 is also visible. The times of initiation (11) of the short circuit can also vary. Due to the variation in the times of initiation (11) and breakage (12) of the short circuit, the welding frequency is irregular and therefore unstable.

[0037] According to the invention, it is therefore provided that the first time period T 1 of the first reverse conveying phase 13 is adjusted by means of a control and / or regulation such that the welding wire 3 in the second reverse conveying phase 15 at the time of breaking 12 of the short circuit does not exceed a predetermined reduced reverse speed v re_K, in particular corresponds to this. This is shown in Fig. 3 in the right short-circuit phase 7. The first reverse feed phase 13 is therefore terminated in a timely manner compared to the left short-circuit phase 7, and the second reverse feed phase 15 is initiated before the short circuit is broken 12. The control and / or regulation can be implemented in the control / regulation unit 8.

[0038] The second reverse conveying phase 15 can have a first intermediate plateau phase 21 with a substantially constant reverse speed -vd. Preferably, the welding wire 3 is conveyed away from the workpiece 5 in the first intermediate plateau phase 21 at the predetermined reduced reverse speed v re_K. In the embodiment shown, the control and / or regulation adapts the first time period T 1 such that the breaking 12 of the short circuit takes place in the first intermediate plateau phase 21. This can take place within a welding cycle or from welding cycle to welding cycle. After the breaking 12 of the short circuit, the predetermined reduced reverse speed v re_K of the welding wire 3 is maintained for a holding time period T hold_1 before the acceleration of the welding wire 3 is continued.

[0039] In Fig. 4A A block diagram of a control system is shown. The control system is formed by a closed control loop 22 and is designed to adapt the first time period T 1 , in the event of a deviation, from welding cycle to welding cycle, so that the welding wire 3 in the subsequent welding cycle does not exceed a predetermined reduced reverse speed v re_K in the second reverse feed phase 15 at the time of breaking 12 of the short circuit. Fig. 3 an adaptation from welding cycle to welding cycle is shown. In particular, the first time period T 1 can be adjusted so that the breaking 12 of the short circuit takes place in the first intermediate plateau phase 21. The control loop 22 contains a controller 23, which can be designed, for example, as a P or PI controller, and the controlled system 24. The first time period T 1 is provided as the manipulated variable S in the control loop 22. This can be adjusted by the controller 23. The setpoint or reference variable F of the control loop 22 is formed by a first setpoint duration T min_1 of the first intermediate plateau phase 21. The duration T 21 of the first intermediate plateau phase 21 is used as the measured controlled variable Y.

[0040] In Fig. 4B an alternative embodiment of the control is shown. The first time period T 1 again serves as the manipulated variable S of the control loop 22. This can be adjusted by the controller 23. However, the predetermined reduced reverse speed v re_K of the welding wire 3, which the wire should have at the time 12 of the short circuit breaking, is used as the setpoint or reference variable F of the control loop 22. The measured reverse speed -vd of the welding wire 3 at the time 12 of the short circuit breaking is used as the measured controlled variable Y. If the speed of the welding wire 3 at the time 12 of the short circuit breaking deviates from the predetermined reduced reverse speed v re_K, the controller 23 adjusts the first time period T 1 in the next welding cycle so that the welding wire 3 is fed at the predetermined reduced reverse speed v re_K when the short circuit breaking in the future.An adjustment from welding cycle to welding cycle is possible in . Fig. 3 shown. In particular, the first time period T 1 can be adjusted so that the short circuit is broken 12 in the first intermediate plateau phase 21.

[0041] Analogous to the short-circuit phase, the second time period T 2 of the first forward feed phase 16 can be adjusted by a further control and / or regulation in a further control / regulation unit 8', so that the welding wire 3 in the second forward feed phase 18 does not exceed a predetermined reduced forward speed v v_K at the time of occurrence 11 of the short circuit. This is shown in Fig. 5 shown in the right arc phase 6. The first forward-feeding phase 16 is therefore terminated in time and the second forward-feeding phase 18 is initiated before the short circuit occurs 11. Fig. 5 also shows a welding current curve and a welding voltage curve.

[0042] In Fig. 6A und Fig. 6A Further regulations are illustrated.

[0043] In Fig. 6A A block diagram for a further control is shown. The further control is formed by a closed control loop 22' and is designed to adapt the second time period T 2 , if necessary, from welding cycle to welding cycle, so that the welding wire 3 in the subsequent welding cycle in the second reverse feed phase 18 does not exceed a predetermined reduced forward speed v v_K at the time of occurrence 11 of the short circuit. This is shown in Fig. 5 shown. In particular, the first time period T 2 can be adjusted such that the onset 11 of the short circuit takes place in a second intermediate plateau phase 25 with a constant, predetermined reduced forward speed v v_K. The control loop 22' includes a controller 23, which can be designed, for example, as a P or PI controller, and the controlled system 24'. The further control loop 22' uses the second time period T 2 as the manipulated variable S. This can be adjusted by the further controller 23'. The setpoint or reference variable F of the further control loop 22' is formed by a second setpoint duration T min_2 of the second intermediate plateau phase 25. The duration T 25 of the second intermediate plateau phase 25 is used as the measured controlled variable R.

[0044] In Fig. 6B an alternative further control loop 22' is shown. The second time period T 2 again serves as the manipulated variable S of the further control loop 22'. This can be adjusted by the further controller 23'. However, the predetermined reduced forward speed v v_K of the welding wire 3, which the wire should have at the time 11 of the short circuit occurring, is used as the setpoint or reference variable F of the further control loop 22'. The measured forward speed vd of the welding wire 3 at the time 11 of the short circuit occurring is used as the measured controlled variable Y. If the speed of the welding wire 3 at the time 11 of the short circuit occurring deviates from the predetermined reduced forward speed v v_K, the further controller 23' adjusts the second time period T 2 in the next welding cycle so that the welding wire 3 is fed at the predetermined reduced forward speed v v_K when the short circuit occurring in the future.An adjustment from welding cycle to welding cycle is possible in . Fig. 5 shown. In particular, the second time period T 2 can be adjusted so that the onset 11 of the short circuit occurs in the second intermediate plateau phase 25.

[0045] Fig. 7 shows, below a speed curve 10, a welding voltage curve and a welding current curve, a time curve of a resistance value R,on the basis of which the breaking 12 of the short circuit can be predicted. The resistance value is calculated from the welding voltage U and the welding current I. Based on the prediction, the first time period T 1 can be adjusted. This can take place within one cycle. The adjustment of the first time period T 1 can be carried out by a controller. If, for example, the resistance value exceeds a resistance threshold RS or the resistance rate of change dR / dt exceeds a predetermined resistance rate of change threshold, the impending breaking 12 of the short circuit can be determined. The controller can then end the first reverse feed phase 13 and initiate the second reverse feed phase 15. In this way, the first time period T 1 is adjusted.

Claims

1. A short-circuit welding method with successive welding cycles, each of which having an arc phase (6) and a short-circuit phase (7), with the following steps during a welding cycle: conveying a melting welding wire (3) away from a workpiece (5) during the short-circuit phase (7) at a rearward speed (-vd) so that material is discharged from the welding wire (3) to the workpiece (5) and an arc is ignited, wherein the welding wire (3) is brought to a final rearward speed (vre_max) during the short-circuit phase (7) in a first rearward conveying phase (13) with a first duration (T1) and is conveyed therewith until the beginning of a second rearward conveying phase (15) in which the rearward speed (-vd) of the welding wire (3) is reduced again, wherein the second rearward conveying phase (15) immediately follows the end of the first rearward conveying phase (13); and conveying the welding wire (3) in the direction of the workpiece (5) during the arc phase (6) at a forward speed (vd) in order to bring the welding wire (3) into contact with the workpiece (5) and to generate a short-circuit between the welding wire (3) and the workpiece (5), wherein the welding wire (3) is brought to a final forward speed (vv_max) during the arc phase (6) in a first forward conveying phase (16) with a second duration (T2) and is conveyed therewith until the beginning of a second forward conveying phase (18), in which the forward speed (vd) of the welding wire (3) is reduced again, wherein the second forward conveying phase (18) immediately follows the end of the first forward conveying phase (16), characterised in that the first duration (T1) of the first rearward conveying phase (13) is adjusted by means of feedforward control and / or feedback control in such a way that the welding wire (3) in the second rearward conveying phase (15) does not exceed a predetermined reduced rearward speed (vre_K) at the point in time of interruption (12) of the short-circuit, wherein the first rearward conveying phase (13) is shortened or lengthened by adjusting the first duration (T1).

2. The short-circuit welding method according to claim 1, characterised in that the second rearward conveying phase (15) comprises a first intermediate plateau phase (21) with a substantially constant rearward speed (-vd), wherein the welding wire (3) in the first intermediate plateau phase (21) is preferably conveyed away from the workpiece (5) at the predetermined reduced rearward speed (vre_K).

3. The short-circuit welding method according to claim 2, characterised in that the first duration (T1) is adapted by the feedforward control and / or feedback control in such a way that the interruption (12) of the short-circuit takes place in the first intermediate plateau phase (21).

4. The short-circuit welding method according to claim 3, characterised in that a feedback control for adapting the first duration (T1) is provided, wherein the feedback control is carried out by a control circuit (22) with a controller (23), in particular a P or PI controller, wherein the feedback control adapts the first duration (T1) so that the first intermediate plateau phase (21) corresponds to a first target duration (Tmin_1).

5. The short-circuit welding method according to any one of claims 1 to 3, characterised in that a feedback control for adapting the first duration (T1) is provided, wherein the feedback control is carried out by a control circuit (22) having a controller (23), in particular a P or PI controller, wherein a control variable (S) of the control circuit (22) is formed by the first duration (T1), a reference variable (F) of the control circuit (22) is formed by a target rearward speed of the welding wire (3), in particular the predetermined reduced rearward speed (vre_K), and a measured control variable (Y) of the control circuit (22) is formed by a measured rearward speed (-vd) of the welding wire (3) at the point in time of the interruption (12) of the short-circuit.

6. The short-circuit welding method according to any one of claims 1 to 3, characterised in that a resistance value (R) is continuously determined from a welding current (I) and a welding voltage (U) and a prediction is made about the point in time of interruption (12) of the short-circuit from the resistance value (R), wherein the first duration (T1) is adapted on the basis of the prediction by starting with the second rearward conveying phase (15).

7. The short-circuit welding method according to claim 6, characterised in that the prediction is generated on the basis of a predetermined resistance threshold value (RS) or on the basis of a resistance change rate (dR / dt).

8. The short-circuit welding method according to any one of claims 1 to 7, characterised in that the first rearward conveying phase (13) has a first final plateau phase (14), in which the welding wire (3) is conveyed at a substantially constant final rearward speed (vre_max) and a duration (T3) of the first final plateau phase (14) is adapted by the feedforward control and / or feedback control.

9. The short-circuit welding method according to any one of claims 1 to 8, characterised in that the second duration (T2) of the first forward conveying phase (16) is adjusted by means of an additional feedforward control and / or feedback control in such a way that the welding wire (3) in the second forward conveying phase (18) does not exceed a predetermined reduced forward speed (vv_K) at the point in time of generation (11) of the short-circuit, wherein the first forward conveying phase (16) is shortened or lengthened by adjusting the second duration (T2).

10. The short-circuit welding method according to claim 9, characterised in that the second forward conveying phase (18) comprises a second intermediate plateau phase (25) with a substantially constant forward speed (vd), wherein the welding wire (3) in the second intermediate plateau phase (25) is preferably conveyed away from the workpiece (5) at the predetermined reduced forward speed (vv_A).

11. The short-circuit welding method according to claim 10, characterised in that the second duration (T2) is adapted by the additional feedforward control and / or feedback control in such a way that the generation (11) of the short-circuit takes place in the second intermediate plateau phase (25).

12. The short-circuit welding method according to claim 11, characterised in that an additional feedback control for adapting the second duration (T2) is provided, wherein the additional feedback control is carried out by an additional control circuit (22') with an additional controller (23'), in particular a P or PI controller, wherein the additional feedback control adapts the second duration (T2) so that the second intermediate plateau phase (25) corresponds to a second target duration (Tmin_2).

13. The short-circuit welding method according to any one of claims 9 to 11, characterised in that an additional feedback control for adapting the second duration (T2) is provided, wherein the additional feedback control is carried out by an additional control circuit (22') with an additional controller (23'), in particular a P or PI controller, wherein a control variable (S) of the additional control circuit (22') is formed by the second duration (T2), a reference variable (F) of the additional control circuit (22') is formed by a target forward speed of the welding wire (3), in particular the predetermined reduced forward speed (vv_K), and a measured control variable (Y) of the additional control circuit (22') is formed by a measured forward speed (vd) of the welding wire (3) at the point in time of the occurrence (11) of the short-circuit.

14. A welding device (1) for carrying out a short-circuit welding method according to any one of the preceding claims 1 to 13 with successive welding cycles, each comprising an arc phase (6) and a short-circuit phase (7), comprising a welding torch (2), wherein the welding apparatus (1) is configured to convey a melting welding wire (3) away from the workpiece (5) during the short-circuit phase (7) at a rearward speed (-vd) so that material is discharged from the welding wire (3) to the workpiece (5) and an arc is ignited, wherein the welding device (1) is further configured to bring the welding wire (3) to a final rearward speed (vre_max) during the short-circuit phase (7) in a first rearward conveying phase (13) with a first duration (T1) and to convey it therewith until the beginning of a second rearward conveying phase (15) in which the rearward speed (-vd) of the welding wire (3) is reduced again, wherein the second rearward conveying phase (15) immediately follows the end of the first rearward conveying phase (13), and the welding device (1) is further configured to convey the welding wire (3) in the direction of the workpiece (5) during the arc phase (6) at a forward speed (vd) in order to generate a short-circuit between the welding wire (3) and the workpiece (5), wherein the welding device (1) is further configured to bring the welding wire (3) to a final forward speed (vv_max) during the arc phase (6) in a first forward conveying phase (16) with a second duration (T2) and to convey it with the final forward speed (vv_max) until the beginning of a second forward conveying phase (18), in which the forward speed (vd) of the welding wire (3) is reduced again, wherein the second forward conveying phase (18) immediately follows the end of the first forward conveying phase (16), characterised in that a feedforward control and / or feedback control unit (8) is provided, which is configured so that the first duration (T1) of the first rearward conveying phase (13) is adjusted by means of feedforward control and / or feedback control in such a way that the welding wire (3) in the second rearward conveying phase (15) does not exceed a predetermined reduced rearward speed (vre_K) at the point in time of interruption (12) of the short-circuit, wherein the first rearward conveying phase (13) is shortened or lengthened by adjusting the first duration (T1).