Control of interval joining process

Abstract

In order to more accurately control the heat input that is introduced into a workpiece (6) to be joined as part of an arc interval joining process, a first pause duration (TP1) of a first pause phase (PP1) is determined during the arc interval joining process and, in the event that the determined first pause duration (TP1) deviates from a specified target pause duration (TPsoll), at least one joining parameter of the arc interval joining process, which influences the pause durations of the pause phases (PP), is changed in order to reduce a deviation between a second pause duration (TP2) of a second pause phase (PP2), which follows the first pause phase (PP1) in time, and the specified target pause duration (TPsoll) compared to the deviation of the first pause duration (TP1) from the target pause duration (TPsoll).

Description

[0001] Regulation of interval joining process

[0002] The present invention relates to an interval arc joining method comprising a sequence of arc phases and pause phases, wherein the arc phases have a predetermined arc duration and alternate with the pause phases, and wherein, during the arc phases, a joining current flows between a joining electrode and a workpiece to join the workpiece, and no joining current flows during the pause phases. Furthermore, the invention relates to a welding device for carrying out an interval arc joining method, comprising a welding torch for introducing energy into a welding or welding electrode.Soldering point on a workpiece for producing a molten pool, comprising a feed unit for feeding a welding wire to the molten pool, wherein the welding wire can be melted in the region of the molten pool by the energy introduced by the welding torch in order to produce a weld seam on the workpiece and comprising a control unit for controlling the welding device.

[0003] Intermittent arc welding and intermittent arc brazing are two widely used joining processes that are particularly suitable for regulating temperature distributions in joined workpieces or for controlling the heat input introduced into a workpiece during arc welding or arc brazing. The following sections deal with intermittent arc welding and intermittent arc brazing, with the generic term "intermittent arc joining" being used to refer to both of these joining processes.

[0004] Intermittent arc joining processes feature arc phases and pause phases. During arc phases, an arc burns (at least temporarily) between a joining electrode and a workpiece. In contrast, during pause phases, no arc burns to reduce heat input. During an arc phase, a joining current flows, which introduces heat into the workpiece to be joined. During a pause phase, the workpiece experiences no heat input due to the lack of a burning arc and consequently the lack of flowing joining current, and can therefore cool down and release heat.

[0005] Due to the fact that the temperature fields occurring in a workpiece during joining influence a number of quality factors, such as thermal distortion occurring in a joined workpiece or various metallurgical properties developing in a joined workpiece, such as grain structure or weld quality, regulating the heat input into a workpiece to be joined is of key importance, particularly when welding or brazing thin sheets. According to the above, the amount of heat introduced into a workpiece during arc intermittent joining can be influenced by adjusting the duration of the arc and pause phases, which can subsequently regulate the temperature distributions occurring in a joined workpiece.Due to its direct impact on the quality factors mentioned, the possibility of regulating temperature fields is an important reason for using an arc interval joining process instead of an alternative joining process.

[0006] In the patent literature, arc interval welding is discussed as a significant variant of interval joining. Specifically, DE 2214192 A1 describes interval welding in which interruptions in the welding phases are provided to achieve cooling of the welded workpiece. In EP 3 744 460 A1, in contrast, the duration of so-called "arc ON" and "arc OFF" phases is varied in order to impose a predetermined shape on a developing weld seam. EP 2 810 732 A2 further teaches recording the time from the start of a short circuit until the first ignition of an arc and then restarting the arc after a defined reference time.

[0007] For arc interval joining in general, but of course also for the aforementioned approaches from the state of the art, both the duration of arc phases (hereinafter "arc duration") and the duration of pause phases (hereinafter "pause duration") depend on two types of time points. On the one hand, the times at which an arc is extinguished (hereinafter "extinguishing times") are decisive, while on the other hand, the times at which the arc is reignited (hereinafter "ignition times") play a crucial role. For the aforementioned pause phases, an arc is extinguished at an extinguishing time point at the beginning of a pause phase and reignited at an ignition time point at the end of a pause phase. The pause duration of the corresponding pause phase thus results from the time between an extinguishing time point and an ignition time point immediately following this extinguishing time point.Conversely, the time period between an ignition point and an extinguishing point immediately following this ignition point defines the arc duration of a corresponding arc phase. The durations of arc and pause phases typically range from 300 milliseconds to 500 milliseconds. Arc durations and pause durations can therefore have values ​​of 250 milliseconds, 300 milliseconds, 350 milliseconds, 400 milliseconds, 450 milliseconds, or 500 milliseconds. Depending on the application, however, longer or shorter arc phases and / or pause phases can also occur (e.g., even shorter than 100 milliseconds).

[0008] In order to determine the durations of arc phases and the durations of pause phases, it is necessary to define the quenching and ignition times that occur during an interval arc joining process. In this context, it was recognized that determining quenching times is a problem that is usually easy to solve in practice. Since maintaining an arc requires a voltage supply to the joining electrodes used for welding or brazing, an arc can be extinguished simply by interrupting this voltage supply and / or by a short circuit, which is possible with high temporal precision even with minimal technical effort.

[0009] Determining ignition points, however, often proves complex and, depending on the situation, associated with various difficulties. In gas-shielded metal arc welding, such as MIG welding or MAG welding, but also in MIG brazing or MAG brazing, arc re-ignition can occur through electrical contact between a joining electrode, such as a consumable welding wire electrode, that melts during welding and a workpiece (short-circuit re-ignition). If the joining electrode touches the workpiece for re-ignition, re-ignition can be induced either by a high ignition current, or by lifting the welding or joining electrode from the workpiece during arc ignition (drawn arc re-ignition), thus "pulling" the arc.

[0010] The question of when an arc will actually reignite in such a situation is influenced by a number of factors, such as the distance the joining electrode must travel during a pause phase before making contact with the workpiece again (hereinafter "creep path"), the so-called creep speed at which the joining electrode is moved toward the workpiece during the pause phase, the surface condition of a workpiece to be welded, or the level of the welding voltage applied for reignition. If these factors fluctuate, the resulting ignition times will also fluctuate as a direct consequence. The actual ignition time may therefore deviate from a specified target ignition time.

[0011] Even with contactless ignition of an arc between a joining electrode of a welding or soldering device and a workpiece, it is often difficult to precisely determine or predict the final ignition point in time. For contactless ignition, an ignition voltage pulse can be applied which ionizes the area between a joining electrode and a workpiece, so that the arc ignites in the ionized area between the joining electrode and the workpiece. This can lead to ignition errors, i.e. it can happen that insufficient ionization and therefore no arc is formed, in which case another ignition attempt with another ignition voltage pulse must be made after a pause. In such cases, there is a time shift and thus, as in the case of contact ignition, a fluctuation in the ignition point.As stated above, fluctuations in ignition timing can occur during arc intermittent joining. Fluctuations in ignition timing result in fluctuations in pause durations, with deviations of 1 ms to 400 ms between the desired and actual pause durations being observed in practice. This, in turn, results in fluctuating heat quantities introduced into a welded or brazed workpiece and can, for example, deviate from a specified target heat quantity, which in turn results in undesirable temperature distributions. As mentioned at the beginning, the temperature distributions occurring in a workpiece are directly related to various quality factors, meaning that undesirable temperature distributions can lead to weld seam defects or brazing defects, undesirable thermal distortion, or undesirable metallurgical properties.Although these interrelationships and difficulties are well known, the state of the art does not offer any suitable solutions.

[0012] It is therefore an object of the present invention to regulate the heat input into a joined workpiece more precisely during arc interval joining.

[0013] This problem is solved by the features of the independent claims. Specifically, the independent claims provide for an initially mentioned arc-interval joining method and for an initially mentioned welding device, determining a first pause duration of a first pause phase during the arc-interval joining method and, if the determined first pause duration deviates from a predetermined target pause duration, changing at least one joining parameter of the arc-interval joining method that influences the pause durations of the pause phases in order to reduce a deviation between a second pause duration of a second pause phase, which follows the first pause phase in time, and the predetermined target pause duration compared to the deviation of the first pause duration from the target pause duration.

[0014] The inventive procedure makes it possible to specifically adjust arc durations and pause durations to a predetermined, desired, or required heat input to be introduced into a workpiece during the arc intermittent joining process. The predetermined target pause duration is advantageously adjusted to the heat input to be introduced and can, for example, be determined from the heat input to be introduced using a predetermined mathematical relationship. In this way, the joining quality achievable with an arc intermittent joining process can be significantly improved.

[0015] It should be noted that the first pause phase and the second pause phase can, in principle, be any pause phases selected during the interval arc joining process, provided that the second pause phase occurs after the first pause phase. The first pause phase and the second pause phase are therefore by no means necessarily the absolute first and the absolute second pause phases of the interval arc joining process, but can just as easily be pause phases occurring later in the course of an interval arc joining process. Furthermore, the second pause phase does not necessarily have to be the pause phase that immediately follows the first pause phase. One or more additional pause phases can also occur between the first and second pause phases.

[0016] Within the scope of the invention, it was recognized that the joining current occurring during the arc interval joining process is particularly suitable for identifying arc phases and pause phases, since a joining current typically flows continuously during the arc phases, even when, for example, a pulsed arc is used (for example, a joining current also flows during an arc phase, even if an arc is interrupted by a short circuit between the welding wire and the workpiece, particularly since a joining voltage is typically still present in arc phases even during a short circuit). In an advantageous manner, however, an arc can also burn continuously during the arc phases. In any case, no joining current flows during the pause phases, and no arc is burning either. An arc phase can therefore be understood as a phase in which a joining current flows.By considering the joining current, arc phases and pause phases can be reliably distinguished.

[0017] It should be noted that special arc welding processes exist, comparatively uncommon in practice, in which the joining current can be interrupted for particularly short periods of time, even during the arc phases. These particularly short periods of time are usually shorter than 10 milliseconds, or shorter than 5 milliseconds, or shorter than 0.5 milliseconds. Phases in which a joining current always flows and which is therefore only interrupted for an extremely short time, i.e., shorter than 10 milliseconds, advantageously shorter than 5 milliseconds or shorter than 0.5 milliseconds, are also considered arc phases in the context of these explanations.

[0018] A pause phase can therefore be interpreted as a phase in which no joining current flows and which is preferably longer than 10 milliseconds or longer than 20 milliseconds or longer than 50 milliseconds.

[0019] Advantageously, the arc-interval joining process, according to the introductory explanations, can be an arc-interval welding process or an arc-interval brazing process, wherein an arc burns at least temporarily, preferably continuously, between the joining electrode and the workpiece during the arc phases, wherein the joining current for joining flows from the joining electrode through the arc between the joining electrode and the workpiece. According to the usual implementation of controls for technical systems, the determination of a first pause duration can occur continuously. This means that a first pause duration is not determined just at one point in time during the arc-interval joining process, but that this occurs multiple times, preferably at points in time spaced apart by a predetermined time interval.Each pause duration determined in this way can be a starting point for changing a joining parameter in order to bring a subsequent pause duration closer to the specified target pause duration. These relationships are, of course, sufficiently known to a specialist in the field of control engineering. In order to be able to react quickly and promptly to a deviation between a determined pause duration and a specified target pause duration using this procedure, it can be provided that between a determination time at which a pause duration is determined and a control time at which at least one joining parameter is changed, there are fewer than 100 pause phases, or fewer than 50 pause phases, or fewer than 10 pause phases PP, or fewer than five pause phases, or no pause phases at all.

[0020] Within the scope of the invention, it was further recognized that it is often advantageous not to directly determine or directly measure the pause durations in question, but instead to determine the first pause duration by determining at least one parameter value of at least one descriptive parameter of the arc interval joining process that describes the first pause duration, and to determine the first pause duration from this at least one determined parameter value, i.e., to perform an indirect determination or indirect measurement of the pause duration according to the invention. In this way, the invention can also be used in scenarios in which direct measurement of pause durations is not possible, for example, because only limited sensor technology is available.

[0021] It was also recognized that, as a description parameter for an indirect determination, in particular a time period between an activation time, at which during a pause phase in particular a wire feed is reactivated and / or a voltage for supplying a joining electrode is increased again, and an ignition time immediately following the activation time can be determined as a parameter value of the description parameter, or a creeping path that a welding wire travels between such an activation time and an ignition time immediately following the activation time.

[0022] As mentioned earlier, the inventive arc intermittent joining process can advantageously be a MIG welding process or a MAG welding process in which a consumable welding wire is provided as the joining electrode during the arc phases, or it can be a MIG brazing process or a MAG brazing process in which a consumable solder is provided as the joining electrode. If a consumable solder is provided as part of a MIG or MAG brazing process, this is referred to as "melt brazing with temporarily liquid solders", whereby molten solder forms a liquid phase and thus a molten pool. In all cases, a molten pool is created on the workpiece in the area of ​​a joint by the arc burning at least temporarily between the joining electrode and the workpiece, both during welding and brazing.Although there are differences in the details of how the respective molten pool is formed: in welding, the base material and the filler material are melted, i.e., the workpiece and the welding electrode. In brazing, only the filler material and not the workpiece are melted. In both welding and brazing, a joining electrode is applied, and the joining electrode is melted by the applied energy to create a weld or braze seam and thus a molten pool. A molten pool thus corresponds to a liquid phase of filler material and / or material in the area of ​​the joint.

[0023] When carrying out the joining method according to the invention as a welding or soldering method, at the beginning of the pause phases during both welding and soldering, the arc is extinguished and the wire feed is stopped, the wire feed is reactivated at an activation time after a rest interval in the pause phases, and a time duration between the activation time and an ignition time immediately following the activation time is determined as a parameter value of the description parameter and / or a creeping path that the welding wire travels between the activation time and an ignition time immediately following the activation time is determined as a parameter value of the description parameter.

[0024] As joining parameters, a wire feed speed can be used and thus changed in an advantageous manner, and / or a joining current in an arc phase, and / or an activation time, and / or a distance between the welding wire and the workpiece at an activation time, and / or a time period between an extinguishing time and an activation time, and / or a current-time area in an arc phase.

[0025] In particular, a combination of determining a creep path as a parameter value of the description parameter and changing a joining current in an arc phase as a joining parameter of the arc interval joining method for adjusting a pause duration to a target pause duration was recognized as advantageous, since a creep path can often be determined precisely and without great sensory effort, and, for example, the level of a joining current in an arc phase can in many cases be changed easily and without great effort. The present invention is explained in more detail below with reference to Figures 1 to 4b, which show exemplary, schematic and non-limiting advantageous embodiments of the invention.

[0026] Fig.1 a standardized structure of a welding device,

[0027] Fig.2 shows a schematic sequence of an arc interval joining process according to the state of the art,

[0028] Fig.3a shows a schematic sequence of an arc interval joining method according to the invention,

[0029] Fig.3b shows an inventive arc interval joining method comprising a change of the joining current and the wire feed speed,

[0030] Fig.4a, Fig.4b each show a coupling plan for implementing the joining method according to the invention.

[0031] The present invention is explained in more detail below using a MSG welding device 1 for gas metal arc welding (GMA welding), which particularly includes the known methods of metal inert gas welding (MIG welding) and metal active gas welding (MAG welding). However, it should be noted that the invention is by no means limited to the field of gas metal arc welding and can be used, among other things, in the field of tungsten inert gas welding (TIG welding) or any other welding technique.

[0032] The MSG welding device 1 considered in the present case can, as is known, also be used for arc brazing. As will be explained in detail below, in order to carry out brazing processes on a MSG welding device 1, it is only necessary to use a shielding gas SG suitable for brazing and a solder suitable for brazing as the filler material, i.e. as the consumable joining electrode 7. Otherwise, no changes are required, e.g. changes to the welding torch 4, etc., so that in particular the control of a MSG welding device 1 as shown in Fig. 1 does not differ during brazing and welding. The present invention can accordingly also be used in the context of interval brazing, which is carried out on the MSG welding device 1 according to Fig. 1, so that the following explanations, mutatis mutandis, apply to both welding and brazing.To emphasize this fact, the general terms joining current I, joining voltage U, joining electrode 7, etc. are used below, instead of the terms otherwise common in welding such as welding current I, welding voltage U, welding electrode 7, welding wire 7a, etc. and instead of the terms otherwise common in brazing such as brazing current I, brazing voltage U, brazing electrode 7, etc. The MSG welding device 1 has a power source 2, a hose package 3, a welding torch 4 and a shielding gas container 5 with a shielding gas SG. The shielding gas container 5 is connected to the welding torch 4 by means of a shielding gas line 8. A pressure regulator (not shown), for example in the form of a known bottle fitting, can be provided on the shielding gas container 5 or in the shielding gas line 8, which pressure regulator generally serves to regulate the flow of the shielding gas SG.Depending on the base material G, either low-reactivity inert shielding gases (SGi), such as argon (Ar) or helium (He), are used, or active shielding gases (SGa), such as oxygen (O) or carbon dioxide (CO2), are used. In a welding process without shielding gas, the shielding gas container 5 and the shielding gas line 8 can, of course, also be omitted. For MSG brazing, mixtures of argon, carbon dioxide, and hydrogen are typically used as the shielding gas (SG).

[0033] A joining electrode 7 in the form of a welding wire 7a, which is usually wound on a welding wire reel 13, can be arranged in the power source 2. For unwinding the welding wire 7a and in particular for feeding the welding wire 7a to a joining point, ie, during welding at a welding point and during soldering at a soldering point, a feed unit 12 is arranged, which is driven by a feed drive unit 12a.

[0034] To solder on the MSG welding device 1, a hard solder, such as a silver solder, a brass solder, or a copper-based solder, can be used instead of a welding wire 7a, or a soft solder, such as a tin solder, can be used, which can also be unwound from a welding wire reel 13. These relationships are well known to a person skilled in the field of joining technology.

[0035] The welding wire 7a on the welding wire reel 13, as well as the feed unit 12, can also be arranged outside the power source 2 in a separate unit. As is known, embodiments of welding devices 1 with multiple feed units 12 also exist, which advantageously coordinate with one another to achieve the required wire feed. However, a feed unit 12 can also be arranged in the area of ​​the joining electrode 7. This does not impose any restrictions on the applicability of the present invention.

[0036] The feed drive unit 12a is controlled by a control unit 14, which in turn generally communicates with a user interface 17. Via a user interface 17, a user can specify certain joining parameters as required, such as a joining voltage U, a joining current I, a wire feed speed vd at which the welding wire 7a is advanced to the joining point, etc. For example, predefined welding programs with certain preset joining parameters can also be stored in the control unit 14, which can be selected by the user via the user interface 17, or by a higher-level control, such as a control of a welding robot.

[0037] Furthermore, a power unit 15 is arranged in the power source 2, which is controlled (or regulated) by the control unit 14 and is connected to an external voltage supply 16. The control unit 14 can therefore be equipped to control and / or regulate the entire MSG welding device 1 or specific components of the MSG welding device 1, such as the feed unit 12, the control of the shielding gas feed and / or the power unit 15 and / or the movement of the welding torch 4. A control unit 14 can be implemented in the form of microprocessor-based hardware, a microcontroller, or an integrated circuit (ASIC, FPGA), and can of course also be arranged outside the power source 2.

[0038] The power unit 15 of the power source 2 supplies the welding torch 4 with the required joining current I and a joining voltage U via a power line 19 arranged in the hose package 3. The joining voltage U is applied to the joining electrode 7 for welding, so that the joining current I flows when an arc is formed. In addition, the welding wire 7a (by means of the feed unit 12) and the shielding gas SG, and optionally also a cooling medium for cooling the welding torch 4, are usually also supplied to the welding torch 4 via the hose package 3. Control lines can also be provided in the hose package. However, several individual lines for the respective media, controls, and energies can be provided as the hose package 3.

[0039] In order to carry out a welding or soldering process, a first electrical potential is applied to a workpiece 6 made of a base material e by means of an electrical connection 18, and a second electrical potential is applied to the welding wire 7a as the joining electrode 7, as a result of which a joining current I flows after an arc 11 is ignited between the welding wire 7a and the workpiece 6. Various types of arc can be used here, such as short arcs, transition arcs, spray arcs, pulsed arcs, or cold metal transfer (“CMT”) arcs, which are well known to those skilled in the art in the field of joining technology. Specifically, during welding, the welding wire 7a and a region of the base material G are melted by means of the arc 11, resulting in a material-to-material connection between the melted welding wire 7a and the base material G.In the example shown, a weld seam 10 is welded onto the workpiece 6; this is referred to as build-up welding. However, two workpieces 6, 6a could also be joined, as symbolized by the dashed line; this is then referred to as joint welding. The shielding gas SG flowing from the welding torch 4 flows around the arc 11 in the form of a shielding gas bell 9 in order to shield the molten material in the area of ​​the weld seam 10 from the environment. It should be noted, however, that welding or brazing could in principle also be carried out without shielding gas SG. In this case, a shielding gas container 5 and a shielding gas line 8 could be dispensed with.

[0040] In a welding process with a non-consumable joining electrode, such as TIG welding, the welding wire 7a is fed into the arc and melted in the arc, with the arc burning between the joining electrode and the workpiece.

[0041] The welding wire 7a, i.e. a consumable joining electrode 7, is fed to the joining point at a specific wire feed speed Vd, which can depend on several influencing factors. In manual welding, in which the welding torch 4 is guided by hand by a person, a constant wire feed speed Vd is generally selected, for example, depending on the set joining current I. In automated welding processes, for example, when the welding torch 4 is guided by a welding robot, the wire feed speed Vd can, for example, additionally be set depending on a welding speed v s can be selected with which the welding torch 4 is moved relative to the workpiece 6.

[0042] The interval joining method according to the invention can be used in an advantageous manner for welding or soldering on a MSG welding device 1 as shown in Fig.1, wherein welding can be carried out automatically or manually.

[0043] In order to generate the required arc phases LP, in which joining, i.e. welding or soldering, takes place, and the pause phases PP, in which no joining, i.e. no welding or soldering, takes place, which cyclically alternate with the arc phases LP (see Fig. 2), it is necessary, as stated above, to first extinguish a burning arc 11 at the beginning of a pause phase PP and to stop the wire feed. Stopping the wire feed means that the wire feed speed Vd is reduced and brought to zero, for which purpose the rotational movement of the welding wire reel 13 must be braked in the welding device shown in Fig. 1. Since a welding wire reel 13 has a physical inertia, a transition of a wire feed speed to the value zero does not occur instantaneously.

[0044] As mentioned earlier, the extinguishing of an arc 11 is possible conveniently and with precise timing, in particular by interrupting the joining voltage U. Stopping or reducing the speed of the wire feed is also generally problem-free, although it should be noted that the reduction in speed of the wire feed can begin exactly at the beginning of the pause phases PP, or slightly before the start of a pause phase PP, or only slightly after the start of a pause phase PP. These relationships will be discussed in detail later. A welding wire 7a often continues to burn for a short time during a so-called burn-off period even after the arc 11 has been extinguished, in particular since high heat is typically stored in the area of ​​the end of the welding wire 7a facing the workpiece 6.This burning, in addition to any movement of the welding wire 7a by the feed unit 12, influences the remaining distance, the so-called creeping path Sd, between the axial end of the welding wire 7a and the workpiece 6 at the beginning of a pause phase PP, specifically the distance between that end of the welding wire 7a which is facing the workpiece 6 and the workpiece 6.

[0045] As also explained earlier, reigniting an arc 11, in contrast to extinguishing it, proves to be difficult in many cases. Specifically, reigniting an arc in the case of a short-circuit ignition requires electrical contact between the welding wire 7a and the workpiece 6. However, since the wire feed is initially reduced to zero in the pause phases PP and the welding wire 7a is away from the workpiece 6 by the creeping distance Sd, it is necessary to first reactivate the wire feed in the pause phase PP in order to move the welding wire 7a back towards the workpiece 6. In order to enable stable reignition of the arc 11, a wire feed speed Vd is typically used that is lower than that used in the arc phases LP; this speed is referred to in welding technology as the creeping speed.

[0046] A point in time in a pause phase PP at which the wire feed is reactivated and, typically, the voltage U applied to the joining electrode 7 is increased again for reignition, is referred to in this patent application as the activation point in time TA. An activation point in time TA divides a pause phase PP into two intervals. The interval between a quenching point in time TL at the beginning of a pause phase PP and an activation point in time TA is referred to below as the rest interval AR, and the interval following the activation point in time TA until the arc 11 is reignited at an ignition point in time TZ is referred to as the activation interval AA. As with quenching points in time TL, the same applies to activation points in time TA: these points in time can be specified with high temporal precision (the activation of a power unit 15 or a feed drive unit 12a is usually subject to only negligible disturbances).Target values ​​can also be provided for the duration of rest intervals AR, which can be realized in practice with less effort and high accuracy.

[0047] Based on the activation times TA considered, it can be seen that the creeping distance Sd, which the welding wire 7a must travel in a pause phase PP after the wire feed is reactivated at an activation time TA in the activation interval AA, represents a decisive factor influencing the time of re-ignition, the ignition time. An obvious reason for this is that, at a constant wire feed speed Vd, it takes different amounts of time to travel different creeping distances Sd.

[0048] The creeping path Sd to be covered depends, among other things, on the aforementioned burn-up of the welding wire 7a after the end of an arc phase LP, i.e. on the length of the welding wire 7a that is still burned off due to the heat stored in the welding wire 7a. The end of the welding wire 7a facing the workpiece e therefore moves further or closer away from the workpiece 6 depending on the burn-up, which increases or decreases the creeping path Sd to be subsequently covered. The creeping path Sd is therefore the distance between the end of the welding wire 7a facing the workpiece 6 and the workpiece 6 at an activation time TA. Of course, a changing surface profile of the workpiece 6, over which the welding torch 4 moves, can also influence the creeping path Sd. Likewise, the movement of the welding wire 7a by the feed unit 12 can influence the creeping path Sd.

[0049] Since the increase in the voltage U applied to the welding wire 7a typically begins immediately at the activation time TA, and thus a high voltage U is already present at the time of renewed contact, reignition usually occurs immediately after the time of renewed contact between the welding wire 7a and the workpiece 6, specifically when the welding wire 7a is lifted again and the arc 11 is drawn up. The times of renewed contact between a welding wire 7a and a workpiece 6 and the ignition times TZ occurring after a pause phase are therefore very close to one another and are often considered to be identical in practice as a first approximation. Fluctuations in the times of renewed contact between the welding wire 7a and the workpiece 6 therefore inevitably lead to fluctuations in the ignition times TZ, at which the arc 11 is reignited at the end of the pause phases PP.With high-voltage ignition, too, depending on the applied (high) ignition voltage U, breakdown and thus re-ignition only occur when the remaining creeping distance Sd has fallen below a certain lower limit. As with short-circuit ignition, the point in time at which this lower limit is exceeded depends on the creeping distance Sd given at the beginning of an activation interval AA. Thus, the same problem arises here.

[0050] To visualize these relationships, Fig. 2 shows schematic time profiles of the joining parameters joining current I, joining voltage U and wire feed speed Vd, which can occur in an interval joining process according to the prior art, for example on a MSG welding device 1 according to Fig. 1. It can be seen that a pulsed joining voltage U and a pulsed joining current I are used in the arc phases LPi, LP2, LP3, although this is of course not necessary and is only an example of welding during the arc phases. The wire feed speed Vd has a value VdL during the arc phases LPi, LP2, LP3 and, in this exemplary embodiment, is initially reduced to the value zero during the pause phases PP1, PP2. The joining voltage U and, as a consequence, the joining current I are also initially reduced to the value zero at the beginning of the pause phases PP1, PP2.In order to ensure reignition of the arc 11 at the end of the pause phases PP1, PP2, the wire feed in the direction of the workpiece 6 is restarted during the pause phases PP1, PP2 at an activation time TA1, TA2, respectively, initially using a lower wire feed speed Vdp as the creep speed. In this exemplary embodiment, the voltage U is also increased again at the activation times TA1, TA2 in order to immediately reignite the arc 11 upon renewed contact between the welding wire 7a and the workpiece 6.

[0051] Furthermore, the curves in Fig. 2 show that the arc phases LP are each located between an ignition time TZ, at which the arc 11 is ignited, and an extinguishing time TL, at which the arc 11 is extinguished, and that the pause phases PP are each located between an extinguishing time TL and an ignition time TZ. Furthermore, a target pause duration TP is specified. S0 H, whereby it is desired that the actual durations of the pause phases PP1, PP2 of this target pause duration TP S0 H as accurately as possible.

[0052] However, the fact that this requirement is not always met in practice can be deduced, in particular, from the second pause phase PP2 shown. The duration of the second pause phase PP2 is noticeably longer than the duration of the first pause phase PP1 and deviates significantly from the specified target pause duration TP. S0H. In the present case, the reason is, for example, a changed (increased) burn-off of the welding wire 7a, requiring a longer creep path Sd to be covered. Due to the causal chain explained above: pause durations - heat input - weld seam defects, such deviations are highly undesirable.

[0053] In order to at least mitigate the problems described in Fig.2, it is provided within the scope of the invention to determine a first pause duration TP1 of a first pause phase PP1 during the arc interval joining process, and in the event that the determined first pause duration TP1 differs from a predetermined target pause duration TP S0H, to change at least one joining parameter of the arc interval joining process, which influences the pause durations of the pause phases PP, in such a way that a deviation between a second pause duration TP2 of a second pause phase PP2, which follows the first pause phase PP1, and a predetermined target pause duration TP S0 H compared to the deviation of the first break duration TP1 from the target break duration TP S0 H is reduced. By changing the at least one joining parameter according to the invention, further pause durations TP3, TP4, etc. of further pause phases PP3, PP4 following the second pause phase PP2 are of course also adjusted to the specified target pause duration TP S0 H. Ideally, the deviation between the specified target break duration TP S0 H and the occurring pause durations TP2, TP3, etc. are brought to zero.

[0054] This is shown in Fig.3a using an embodiment of the invention, as will be explained in more detail below.

[0055] An efficient way of implementing the invention is to directly measure the pause durations TP of the pause phases PP. As mentioned, the extinguishing times TL are usually known, and the ignition times TZ are also often automatically monitored in welding devices 1, such as the one shown in Fig. 1, for example, by monitoring the joining current I. The pause duration TP of a pause phase can be determined from the time difference between an ignition time TZ and the preceding extinguishing time TL.

[0056] In a welding process in which the arc 11 is extinguished at the beginning of the pause phases PP, the wire feed is reduced and the wire feed speed is consequently brought to zero, the duration of an activation interval AA between an activation time TA and a subsequent ignition time TZ can also be measured in order to determine the pause duration of a pause phase PP. As mentioned, activation times TA can be determined with high precision, so that the duration of a rest interval AR is usually known. If the durations of both the rest interval AR and the activation interval AA in a pause phase PP are known, the pause duration TP of the pause phase PP can be determined by simple summation.

[0057] However, it is by no means mandatory that the duration of an activation interval AA be measured directly. Within the scope of the invention, it was recognized that the creeping distance Sd traveled by the welding wire 7a in a previous activation interval AA can also be determined. In cases relevant to practice, this information is directly available in a control unit 14 for controlling a feed drive unit 12a. Since the wire feed speed Vdp used in the pause phases PP is of course known, the associated duration of an activation interval AA can easily be deduced from the distance traveled by the welding wire 7a after an activation time TA, in the simplest case by division. The total duration of a pause phase PP is also obtained in this case by summing the durations of the rest interval AR and the activation interval AA.Since the welding wire typically does not move during a rest interval AR, the distance traveled by the welding wire 7a during the entire pause phase PP could also be determined. Several options are available as joining parameters for influencing and correcting the pause durations TP of the pause phases PP. Specifically, within the scope of the invention, an activation time TA can be shifted in time, e.g., closer to the previous extinguishing time TL or further away from the previous extinguishing time TL, whereby the desired correction of the pause durations TP can be achieved.

[0058] Likewise, the wire feed speed Vdp specified after an activation time TA can be increased or decreased in order to increase or decrease the duration of the activation interval AA and thus correct the pause durations TP.

[0059] Furthermore, it was recognized that the creeping distance Sd, which determines the duration of an activation interval AA, is influenced by the level of the joining current I used in the previous arc phase LP. The reason for this is that a higher joining current I leads to more heat stored in the welding wire 7a, which leads to greater burn-off and thus to a longer creeping distance Sd. With a constant creeping speed Vd but a changing creeping distance Sd, the duration of an activation interval AA and thus the resulting pause durations PP change directly for obvious reasons.

[0060] In a particularly advantageous manner, the joining parameters just mentioned can also be changed in combination to influence an ignition point TZ.

[0061] How the procedure according to the invention can affect the time profiles of the joining current I, joining voltage U and wire feed speed Vd is shown in Fig. 3a. As in Fig. 2, in the case shown in Fig. 3a, a pulsed joining voltage U, which results in a pulsed joining current I, is used in the arc phases LPi, LP2, LP3. As in Fig. 2, the wire feed speed Vd in Fig. 3a also has a value VdL during the arc phases LP1, LP2, LP3 and is initially reduced to zero during the pause phases PP1, PP2. The joining voltage U and the joining current I are also initially reduced to zero at the beginning of the pause phases PP1, PP2, PP3. As in the case shown in Fig. 2, the pause duration of the first pause phases PP1 deviates noticeably from the specified target pause duration TP. S0 Have.

[0062] However, during the subsequent, second pause phase PP2, the second activation time TA2 is shifted forward, i.e., closer to the previous extinguishing time TL2. This results in a shorter rest interval AR2 compared to the first pause phase PP1, and thus a pause duration TP2 of the entire pause phase PP2 that already corresponds very precisely to the target pause duration. In the third pause phase PP3, the temporal position of the activation time TA3 is maintained compared to the second pause phase PP2, i.e., in the third pause phase PP3, a rest interval AR3 of the same length is provided as in the second pause phase PP2, so that a satisfactory correspondence with the specified target pause duration TP S0 H is maintained. The invention thus makes it possible to achieve a constant pause and arc duration after only a few interval cycles.

[0063] In general terms, the determination of the pause durations TP according to the invention can be implemented by determining a parameter value PW of a description parameter PT of the arc interval joining method, such as the duration of an activation interval AA or a creeping path Sd which describes the first pause duration TP1, in order to determine the first pause duration TP1, and from this parameter value PW the at least one pause duration TP1 required to implement the invention is determined.

[0064] To determine the parameter value PW of the description parameter PT, various methods can be used, in particular those known from control engineering or signal processing, wherein the parameter value PW can be determined, among other things, by means of a filter, such as in particular a Kalman filter, or by means of an observer, such as in particular a Luenberger observer, or by means of an adaptive system or by means of a neural network from measured values ​​of one or more joining parameters of the arc interval joining process.

[0065] As explained earlier, the joining current I flowing during the arc phases LP influences the burn-off of a welding wire 7a or a solder, and thus has a direct effect on the creeping distance Sd that occurs during the pause phases PP. As is well known, the energy transported by an electric current over time results from the current-time area covered by this time course. If the current-time area is larger, more electrical energy and thus more heat is transferred, in this case into a welding wire 7a or into a solder, which leads to greater burn-off. If the current-time area is smaller, there is less heat input and therefore less burn-off. Similarly, by appropriately changing the joining current I, it is possible to control the heat remaining in a welding wire 7a at the end of an arc phase LP, and thus the burn-off and consequently the creeping distance Sd to be covered.

[0066] In addition to the joining current I, as mentioned, it is primarily the wire feed speed Vd that influences the creeping path Sd to be covered starting from an activation time TA, so that in a particularly advantageous embodiment the joining current I and the wire feed speed Vd can be changed in combination in order to regulate the creeping path Sd and thus ultimately the pause durations TP of the respective pause phases PP.

[0067] It should be noted, however, that a change in the joining current I can of course also be made alone, without additional change in the wire feed speed Vd, and that a change in the wire feed speed Vd can of course also be made alone, independently of a change in the joining current I.

[0068] The effect that a change in the joining current I can have within the scope of this invention is shown in Fig. 3b. In the curves shown in Fig. 3b, the amplitude of the last joining current pulse is reduced starting from the first arc phase LPi, which leads to a reduction in the heat transferred and thus to a reduction in the burn-off that occurs. In order to combine the change in the joining current I with a change in the wire feed speed Vd, a so-called "lag" of the wire feed speed Vd is provided in the situation shown in Fig. 3b. This means that the reduction in the wire feed speed Vd does not begin immediately at the quenching time TL2, TL3, but only slightly after the start of the subsequent pause phases PP2, PP3.

[0069] In welding or brazing processes without joining current pulses, a current-time area can advantageously be directly specified, for example, an entire current-time area swept in an arc phase LP, or a current-time area swept in the second half of an arc phase LP. By changing the joining current I described, it is possible to implement a so-called "burn-up program" and thus achieve a predetermined melting rate of a welding wire 7a or a solder. In general, various attributes of a joining current I can be changed to regulate the burn-up, such as a peak value of the joining current I or an effective value of the joining current I or the frequency of pulses of a joining current I, or other attributes of a joining current I.

[0070] In contrast to the situation shown in Fig. 3b, the amplitude of the last joining current pulse I can also be increased to increase the remaining heat in the welding wire 7a and thus cause more burn-off. Furthermore, the change in the magnitude of joining current pulses is by no means limited to the last joining current pulse, so that the amplitudes of several current pulses at the end of an arc phase LP can be increased or reduced in the same way, e.g., the amplitudes of the last two joining current pulses or the amplitudes of the last three joining current pulses. Furthermore, the point in time at which the reduction in the wire feed speed Vd begins can not only be shifted to a subsequent pause phase, but also brought forward.Within the scope of the invention, a reduction in the wire feed speed Vd can therefore be started before an extinguishing time TL, within an arc phase LP, which ultimately contributes to an increase in the creeping path Sd.

[0071] Furthermore, with regard to the time delay for the start of the wire feed speed reduction Vd, it should be noted that such delays are preferably selected to be small compared to the occurring pause durations TP and arc durations TL. "Small" means that the wire feed speed reduction Vd typically begins no later than after a tenth, a quarter, or a third of a pause phase PP has elapsed, in any case such that the wire feed speed Vd can be reduced to zero within a pause phase PP.

[0072] If a change in the joining current I and / or a change in the wire feed speed Vd is used, changing the activation time TA during the pause phases can be omitted, as shown in Fig. 3b. However, it is also possible to change the activation times TA as an additional control variable in addition to the joining current I and / or the wire feed speed Vd.

[0073] The method steps according to the invention can of course be implemented in software, preferably in a control unit such as the control unit 14 shown in Fig.1. When implemented in software, the processing of the method steps to be carried out typically takes place at discrete times tk= / c • T s , which is the product of a given sampling time T sand a discrete index k. In a time-discrete, but of course also in an analogue, implementation, it can be advantageously provided to determine the pause duration of the first pause phase PPi during the arc interval joining process at a discrete determination time TE, which follows the first pause phase PPi, and to use the at least one joining parameter of the arc interval joining process to influence the pause duration TP2 of the second pause phase PP2 in the event that the determined first pause duration TP1 differs from the specified target pause duration TP S0 H, at a discrete control time TR, which follows the determination time TE.

[0074] In order to respond quickly and promptly to a deviation between a determined break duration and a specified target break duration TP S0H, it can be provided that between the determination time TE and the control time TR there are fewer than 100 pause phases PP or fewer than 50 pause phases PP or fewer than 10 pause phases PP or fewer than five pause phases PP or no pause phases PP at all. In a particularly advantageous manner, the second pause phase PP2 can only begin after the control time TR, thus ensuring that the adaptation of the joining parameter to be changed has already taken place before a new pause phase PP begins.

[0075] Two advantageous implementation variants of the method according to the invention in the form of coupling diagrams are shown in Figures 4a and 4b. Figure 4a summarizes the basic idea of ​​the present invention in the form of a standard control loop well known from control engineering. According to the preceding explanations, the welding device 1 represents the section to be controlled, from which the occurring pause durations TP of the pause phases PP are determined.

[0076] A determined break duration TP is compared with a specified target break duration TP S0 H, resulting in a pause duration control error e Tp. The pause duration control error eyp is fed to a controller Ri as an input variable, which uses it to determine a modified activation time TA as a manipulated variable. The determined activation time TA is in turn used in the operation of welding device 1, whereby the actually occurring pause durations TP adjust to the target pause duration TPsoii when using a suitably designed controller.

[0077] The controller R1 can be a PID controller, a model predictive controller, a flatness-based controller, a backstepping controller, or a sliding-mode controller, which can of course be implemented in a discrete-time formulation in software according to the above explanations.

[0078] In a particularly advantageous embodiment of the invention, the control law for the controller R1

[0079] TA k+1 = TA k + e TP kbe selected. As mentioned earlier, the variable k represents a time-discrete index, so that TAk and TAk+i represent two consecutive activation times. This simple control law, which corresponds to an integrator with a gain factor of 1 / Ts, always shifts a subsequent activation time TAk+i by exactly the difference between the pause duration and the target pause duration at the previous time.

[0080] For example, to provide an averaging of the occurring control errors eyp.k, the above control law can be

[0081] TA k+1 = TA k + B ■ e TP k be supplemented with a multiplicative setting parameter B to weight the control error eyp,k.

[0082] As mentioned, in addition to changing the activation time TA, other or additional joining parameters can also be changed to influence the pause durations of the pause phases PP. One such possibility, in which the joining current I and the wire feed speed Vd are used in addition to the activation time TA to implement the invention, is shown in Fig. 4b.

[0083] Here, too, a first control loop is provided, where, as in Fig.4a, a determined pause duration TP is compared with a predetermined target pause duration TP S0 H is compared, resulting in a pause duration control error e Tp is determined and from this a changed activation time TA is determined as the first manipulated variable. A special feature of the controller R2 shown in Fig.4b is that the controller R2 can influence two manipulated variables, on the one hand the activation time TA, and on the other hand the wire feed speed Vd used in the pause phases. As explained in detail above, the wire feed speed Vd used in the pause phases PP can be used to influence the duration between activation time TA and the subsequent ignition time TZ, so that the wire feed speed Vd can of course also be used as a manipulated variable to change the duration of an activation interval AA. As described in Fig.3b, it is also conceivable to use the wire feed speed Vd at the end of an arc phase LP as a manipulated variable to change pause durations TP.

[0084] As explained in detail earlier, from a given target break duration TP S0 H deviating pause duration TP, in particular from changing creeping paths Sd, so that in Fig.4b by means of a further controller R3 it is provided not only to regulate the pause duration TP itself, but also to determine the creeping path Sd, to compare this with a predetermined target creeping path Sd,soii, from which a creeping path control error e s to determine and correct this creeping path control error e s to the further controller R3 for control. The possibilities provided within the scope of the invention for influencing or controlling the creep path Sd, for example, by changing a joining current I or a wire feed speed Vd, were explained in detail earlier. It should be noted that the lower control circuit shown in Fig. 4b can also be provided alone.

[0085] When designing a control loop according to Fig.4b with more than one manipulated variable, it can also be advantageous to use methods from the control engineering field of control allocation in order to suitably coordinate all control loops and to prevent different controllers R2, R3 or different manipulated variables TA, Vd, I from interfering with each other.

Claims

Patent claims 1. An interval arc joining method comprising a sequence of arc phases (LP) and pause phases (PP), wherein the arc phases (LP) have a predetermined arc duration and alternate in time with the pause phases (PP), wherein in the arc phases (LP) a joining current (I) flows between a joining electrode (7) and a workpiece (6) for joining the workpiece (6) and in the pause phases (PP) no joining current (I) flows, characterized in that during the interval arc joining method a first pause duration (TPi) of a first pause phase (PPi) is determined and that in the event that the determined first pause duration (TPi) deviates from a predetermined target pause duration (TPsoii), at least one joining parameter of the interval arc joining method which influences the pause durations of the pause phases (PP) is changed in order to compensate for a deviation between a second pause duration (TP2) of a second pause phase (PP2),which follows the first break phase (PP1) in time, and the specified target break duration (TPsoii) compared to the deviation of the first break duration (TPi) from the target break duration (TP, S0 H).

2. Arc interval joining method according to claim 1, characterized in that the arc interval joining method is an arc interval welding method or an arc interval brazing method, wherein in the arc phases (LP) between the joining electrode (7) and the workpiece (6) an arc (11) burns at least temporarily, through which the joining current (I) for joining flows from the joining electrode (7) to the workpiece (6).

3. Arc interval joining method according to claim 2, characterized in that the arc (11) is extinguished at the beginning of a pause phase (PP) at an extinguishing time (TL) and is reignited at the end of a pause phase (PP) at an ignition time (TZ), that the pause duration (TP) of a pause phase (PP) corresponds to a time interval between the extinguishing time (TL) at the beginning of the pause phase (PP) and the ignition time (TZ) at the end of the pause phase (PP), and that the at least one joining parameter of the arc interval joining method influences the respective ignition times (TZ) at the end of the pause phases (PP).

4. Arc interval joining method according to one of the preceding claims, characterized in that the pause duration (TPi) of the first pause phase (PP1) is determined during the arc interval joining method at a determination time (TE) following the first pause phase (PP1), that the at least one joining parameter of the arc interval joining method for influencing the pause duration (TP2) of the second pause phase (PP2) in the event that the determined first pause duration (TPi) of the specified target break duration (TP SO II), is changed at a control time (TR) following the determination time (TE), and that between the determination time (TE) and the control time (TR) there are less than 100 pause phases (PP) or less than 50 pause phases (PP) or less than 10 pause phases (PP) or less than five pause phases (PP) or no pause phase (PP) at all.

5. Arc interval joining method according to claim 4, characterized in that the second pause phase (PP2) begins after the control time (TR).

6. Arc interval joining method according to one or more of the preceding claims, characterized in that to determine the first pause duration (TPi) at least one parameter value (PW) of at least one description parameter (PT) of the arc interval joining method, which describes the first pause duration (TPi), is determined, and that the first pause duration (TPi) is determined from the at least one determined parameter value (PW).

7. Arc interval joining method according to claim 6, characterized in that the arc interval joining method is a MIG welding method or a MAG welding method in which a consumable welding wire (7a) is provided as the joining electrode (7) in the arc phases (LP), or a MIG brazing method or a MAG brazing method in which a consumable solder is provided as the joining electrode (7), wherein the arc (11) burning at least temporarily between the joining electrode (7) and the workpiece (6) introduces energy on the workpiece (6) in the region of a joining point (25) in order to produce a molten pool, wherein the joining electrode (7) is fed to the molten pool (26) at a wire feed speed (Vd) and wherein the joining electrode (7) is melted by the introduced energy in the region of the molten pool (26) in order to produce a weld seam orto produce a solder seam (10) on the workpiece (6), that the arc (11) is extinguished at the beginning of the pause phases (PP) and the wire feed is stopped, that the wire feed is reactivated at an activation time (TA) after a rest interval (AR) in the pause phases (PP), and that a time period between the activation time (TA) and an ignition time (TZ) immediately following the activation time (TA) is determined as a parameter value (PW) of the description parameter (PT), and / or that a creeping path (Sd) traveled by the welding wire (8) between the activation time (TA) and an ignition time (TZ) immediately following the activation time (TA) is determined as a parameter value (PW) of the description parameter (PT).

8. Arc interval joining method according to claim 7, characterized in that a wire feed speed (Vd), and / or a joining current (I) in an arc phase (LP), and / or an activation time (TA), and / or a distance between the Welding wire (7a) and the workpiece (6) at an activation time (TA) and / or a time period between an extinguishing time (TL) and an activation time (TA) and / or a current-time area in an arc phase (LP) is changed as a joining parameter of the arc interval joining process for influencing an ignition time (TZ).

9. Arc interval joining method according to claim 8, characterized in that a wire feed speed (Vd) and / or a joining current (I) in an arc phase (LP) and / or a current-time area in an arc phase (LP) are changed in order to change a distance between the welding wire (7a) and the workpiece (6) at an activation time (TA) as a joining parameter of the arc interval joining method for influencing an ignition time (TZ).

10. Arc interval joining method according to one of claims 6 to 9, characterized in that the at least one parameter value (PW) of the at least one description parameter (PT) is determined by means of a filter or by means of an observer or by means of an adaptive system or by means of a neural network from at least one measured value of one or more joining parameters of the arc interval joining method and that the at least one joining parameter is set for influencing the second pause phase (PP2) according to a predetermined control law (R).

11. Welding device (1) for carrying out an interval arc joining process, wherein the interval arc joining process has a sequence of arc phases (LP) with a predetermined arc duration and pause phases (PP) alternating in time with the arc phases (LP), wherein in the arc phases (LP) a joining current (I) flows between a joining electrode (7) and a workpiece (6) for joining the workpiece (6) and in the pause phases (PP) no joining current (I) flows, comprising a welding torch (4) for generating an arc (11) for introducing energy at a joining point (25) on the workpiece (6) for generating a molten pool (26), comprising a feed unit (12) for feeding the joining electrode (7) to the molten pool (26), wherein the joining electrode (7) is heated in the region of the molten pool (26) by the energy introduced by the welding torch (4). is meltable,to produce a seam (10) on the workpiece (6) and comprising a control unit (14) for controlling the welding device (1), characterized in that the control unit (14) is designed to determine a first pause duration (TPi) of a first pause phase (PPi) during the arc interval joining process and in the event that the determined first pause duration (TPi) differs from a predetermined target pause duration (TP, S0 u) to change at least one joining parameter of the arc interval joining process which influences the pause durations of the pause phases (PP) in order to avoid a deviation between a second pause duration (TP2) of a second pause phase (PP2) which follows the first pause phase (PPi) in time, and the specified target break duration (TP SO II) compared to the deviation of the first break duration (TPi) from the target break duration (TP S0 u) to reduce.

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