Short-arc joining process, and joining device

Abstract

The invention relates to a robust method for performing a short-arc joining process having arc phases (LP) and short-circuit phases (KP), wherein weld spatter is reliably reduced and expensive measuring equipment is avoided, in which method: during a first sub-phase (KP-1) of the short-circuit phases (KP), a short-circuit current (IK) is increased from an initial current (IK1) present at the beginning of the first sub-phase (KP-1) to a predefined melting current (IKm); during a second sub-phase (KP-2) of the short-circuit phases (KP), which follows the first sub-phase (KP-1), the short-circuit current (IK) is reduced in accordance with a predefined target short-circuit current profile (Isoll), and an actual droplet value (Tist), which represents an actual value of a droplet parameter (RT) of the welding droplet (T), is determined and used in order to correct the target short-circuit current profile (Isoll) depending on the actual droplet value (Tist).

Description

[0001] Short arc joining process and joining device

[0002] The present invention relates to a short-arc joining method for producing a weld seam on a workpiece, comprising arc phases in which an arc burns between a consumable joining electrode and a workpiece, which melts the joining electrode in an end region of the joining electrode facing the workpiece, forming a weld droplet, and short-circuit phases that alternate cyclically with the arc phases, in which the joining electrode touches the workpiece and a short-circuit current flows through the weld droplet into the workpiece to melt the weld droplet.

[0003] The invention further relates to a joining device for carrying out a short-arc joining process, the joining device comprising a burner, a feed unit for supplying a melting joining electrode, a power source for supplying the joining electrode electrically and a control unit for controlling the joining device.

[0004] Interval arc welding and interval arc brazing are versatile joining processes, particularly suitable for regulating temperature distributions in joined workpieces or for controlling the heat input during arc welding or brazing. The following discussion focuses on the use of short arcs in these joining processes, with the generic term "short arc joining" encompassing both short arc (interval) welding and short arc (interval) brazing.

[0005] Arc welding processes based on short circuits have two fundamental phases: arc phases and short-circuit phases. These phases alternate cyclically. An arc phase typically follows immediately after a short-circuit phase, and vice versa. In consumable wire arc welding processes, the wire is melted during an arc phase, forming a weld droplet that grows as it melts. The weld droplet is fed to the workpiece by a continuous wire feed. At the end of an arc phase, the weld droplet touches the workpiece, causing the arc to extinguish (or "break") and initiating a short-circuit phase.As a result of the weld droplet contacting the workpiece, a short circuit forms between the wire electrode and the workpiece (a "short-circuit bridge"). A short-circuit current then flows through the weld droplet, further heating both the wire electrode and the weld droplet. This continuous heating causes the weld droplet to constrict, particularly in the liquid-solid transition zone of the wire electrode. The weld droplet gradually narrows. When the cross-section of the weld droplet becomes so small at at least one point that it can no longer be held to the wire electrode, the weld droplet detaches from the electrode and flows into a molten pool on the workpiece, where a weld bead subsequently forms. This process of weld droplet detachment is also referred to as "breaking the short circuit" or "breaking the short-circuit bridge."The droplet transfer associated with the breaking of the short circuit occurs exclusively during short-circuit phases in the short-arc welding process. With other arc welding methods, droplet transfer is also possible outside of short-circuit phases, i.e., partially even during arc phases. Breaking the short circuit results in the loss of the low-resistance short-circuit bridge to the workpiece, which formed above the weld droplet, and the end of the short-circuit phase. The detachment of the weld droplet creates a gap between the wire electrode and the workpiece, in which the arc reignites in the next process step. The reignition of the arc marks both the end of the short-circuit phases and the beginning of the arc phases.

[0006] As is known from the prior art, e.g., from EP 1 949 997 A1, unwanted weld spatter can occur during the breakdown of a short circuit, i.e., at the end of the short circuit. One reason for this is that a potentially high short-circuit current must flow through a much smaller cross-section towards the end of a short-circuit phase, requiring the transfer of high (electrical) energy through a very small volume. This small volume is heated intensely and can consequently detach uncontrollably from the unmelted part of the wire electrode, as well as from the rest of the weld droplet. Another cause of weld spatter during short-arc welding can be that part of the weld droplet bursts open due to excessive energy and ends up as weld spatter next to the weld pool.

[0007] Weld spatter occurs particularly when, as is often the case in practice, the short-circuit current is continuously increased during the short-circuit phases, e.g., linearly, exponentially, quadratically, or even stepwise, to ensure rapid heating and detachment of weld droplets. A significant disadvantage of this approach is that the short-circuit current is usually very high when the short circuit breaks (arc reignition), which can lead to significant weld spatter. Furthermore, the current can fluctuate considerably, resulting in unpredictable behavior of the joining parameters. Various methods have been developed to address this problem. One widely used short-circuit treatment method involves monitoring the time-dependent electrical resistance of the short-circuit bridge, i.e., the electrical resistance of the weld droplet.It is known that this resistance increases at least approximately exponentially shortly before the short circuit breaks, since a change in cross-section according to d occurs when the droplet constricts.

[0008] For a cross-sectional area A approaching zero, the droplet diameter d follows the increasingly steep branch of the preceding square root function near the value A=0. Measuring the resistance curve, or even the voltage curve of a voltage drop across at least part of the weld droplet (from which the resistance can be determined), consequently provides information about the diameter and thus the degree of constriction of the weld droplet. Upon detection of progressive droplet constriction, the short-circuit current is reduced, resulting in less weld spatter and increased process stability. However, a disadvantage of this approach is that the aforementioned process parameters must be measured with high precision.

[0009] Another method for short-circuit treatment involves active wire retraction, which reduces the current during the short circuit and consequently also reduces weld spatter. A disadvantage of this approach is that complex and expensive actuators are required to implement the active wire retraction with sufficient precision and dynamics.

[0010] Therefore, one of the objectives of the present invention is to provide a robust method for short-circuit treatment that reliably reduces weld spatter and does not require expensive measuring technology.

[0011] This problem is solved for a short-arc joining process mentioned above and for a joining device mentioned above by the features of the characterizing elements of the independent claims.

[0012] According to the invention, each short-circuit phase includes a first sub-phase and a second sub-phase following the first. In the first sub-phase, the short-circuit current is increased from an initial current present at the beginning of the first sub-phase to a predetermined melting current, which melting current is preferably more than 0.5 times, 1 time, 5 times, or 10 times higher than the initial current. In the second sub-phase, which in a preferred embodiment may itself contain sub-phases, the short-circuit current is reduced according to a predetermined target short-circuit current profile. An actual droplet value, which describes an actual value of a droplet parameter of the welding droplet, is determined and used to correct the target short-circuit current profile as a function of the actual droplet value.

[0013] The inventive method achieves and combines a multitude of advantageous effects. For example, the melting current is preferably selected to be high, particularly preferably as the highest current occurring during the short-circuit phase, i.e., as the maximum current of the short-circuit phase, so that rapid heating of the weld droplet is ensured in the first sub-phase. However, the high short-circuit current is not maintained or even further increased throughout the entire short-circuit phase, but is reduced again and furthermore selectively controlled in such a way that an advantageous short-circuit current-time profile is established for the overall behavior of the joining process. This profile is low towards the end of the short-circuit phases compared to the melting current and thus results in only minimal spatter.The inventive method makes it possible to precisely define a short-circuit current time profile adapted to the specific joining task, in order to achieve a low short-circuit current at the end of the short-circuit phase, resulting in minimal weld spatter. It should be noted that the inventive increase of the short-circuit current in the first partial phase is not to be understood as an exclusively monotonous or strictly monotonous increase. A phased or short-term reduction of the short-circuit current on the path from the initial current to the melting current can also be provided, whereby a reduction starting from the initial current can be particularly advantageous at the beginning of a first short-circuit phase, as will be explained in detail later.

[0014] The droplet parameters can be geometric parameters of the weld droplet, such as its diameter, circumference, or constriction, or they can be electrical parameters, such as electrical resistance, conductance, impedance, admittance, power, or voltage drop across the droplet, depending on the specific requirements of a given application. The invention offers considerable flexibility in this respect.

[0015] In a particularly advantageous manner, the actual drop value can be compared with a predetermined drop setpoint, preferably by subtracting the actual drop value from the drop setpoint, especially to determine a drop comparison result which, upon subtraction, corresponds to a control error, and the short-circuit current setpoint profile, to which the short-circuit current is set or regulated, can be corrected in such a way as to reduce the deviation between the actual drop value and the predetermined drop setpoint. For this purpose, for example, a predetermined relationship between the drop comparison result and the short-circuit current setpoint profile can be used, or a control law can be applied that determines a suitable change in the short-circuit current setpoint profile from the drop comparison result. In this way, the invention opens up the possibility of using modern control techniques (sliding mode, MPC, flatness-based control, etc.).) in the area of ​​specifying / setting / regulating short-circuit currents. According to the above explanations, a target short-circuit current profile can be selectively and controllably increased if, for example, the electrical resistance of a droplet is too low, the droplet diameter is too large, or the voltage drop across the droplet is too low; conversely, a target short-circuit current profile can be selectively decreased if, for example, the electrical resistance of a droplet is too high, the droplet diameter is too small, or the voltage drop across the droplet is too high.

[0016] In principle, the target short-circuit current profile can be empirically determined, e.g., based on measurements acquired during previous joining processes, or it can be obtained from a database, or determined using numerical simulations of the short-arc joining process. The possibility of incorporating numerical simulations into the inventive method enables the realization of extensive improvement potential. Although numerical simulation is known for supporting the planning of joining processes, it has not yet been used to optimize droplet detachment during short-arc joining.In a preferred manner, the numerical simulation can be performed as an offline simulation before the start of the short-arc welding process, which allows the use of complex and detailed models and leads to particularly precise simulation results, or the numerical simulation can be performed as an online simulation during the short-arc welding process, where typically simpler models are used, but it is possible to react to changes in the welding process during operation using simulations.

[0017] In a further advantageous embodiment of the invention, each of the second sub-phases can include a first sub-phase and a second sub-phase following the first sub-phase, wherein no correction of the short-circuit current target curve is provided in the first sub-phase, and a correction of the short-circuit current target curve is provided only in the second sub-phase. This ensures that complex measurements and / or calculations are limited to a small portion of the second sub-phase KP-2. In a preferred manner, the first sub-phase can be chosen to be at least as long as the second sub-phase; that is, the two sub-phases can be chosen to be of equal length, or the first sub-phase can be chosen to be longer than the second sub-phase. This minimizes the computational effort, which is higher in the second sub-phase.If the droplet does not detach in the second subphase, further corrections can be made and another subphase can follow.

[0018] To minimize complexity when structuring the short-circuit phases, it is possible to divide them into first and second sub-phases. This means that the sum of the duration of a first sub-phase within a short-circuit phase and the duration of a second sub-phase within that phase equals the duration of the short-circuit phase itself. However, as will be explained in detail later, it is also conceivable to include further phases within the short-circuit phases.

[0019] It should be noted that the inventive method can be combined with the aforementioned approaches known from the prior art, so that, for example, it can also be provided within the framework of an inventive joining method to move the joining electrode away from the workpiece during the short-circuit phases in order to further support the resolution of the short circuit by means of the wire's return movement.

[0020] The present invention is explained in more detail below with reference to Figures 1 to 5, which show exemplary, schematic, and non-limiting advantageous embodiments of the invention.

[0021] Fig. 1 shows a standardized setup of a joining device,

[0022] Fig. 2a-d shows a real process during the formation and detachment of a sweat droplet,

[0023] Fig. 3a-c shows three variants for short-circuit treatment according to the state of the art.

[0024] Fig. 4 Signal waveforms during a short-circuit treatment according to the invention,

[0025] Fig. 5 shows a coupling diagram for the control engineering implementation of the short-circuit treatment according to the invention.

[0026] The present invention is explained in more detail below with reference to a joining device 1, which can be used in particular for metal inert gas welding (MIG welding). MIG welding includes, in particular, the known methods of metal inert gas welding (MIG welding) and metal active gas welding (MAG welding), which can be carried out, for example, with a short arc. The joining device 1 considered here can also be used for arc brazing. As will be explained in detail below, to carry out brazing operations on a joining device 1, it is only necessary to use a brazing-suitable shielding gas (SG) and a brazing-suitable filler material, i.e., a consumable joining electrode 7. Otherwise, no changes are required compared to carrying out a welding operation on the joining device 1, e.g., changes to the torch 4, etc.This is necessary so that, in particular, the control of a joining device 1 as shown in Fig. 1 does not differ between soldering and welding. Accordingly, the present invention can also be used in the context of soldering carried out on the joining device 1 according to Fig. 1, so that the following descriptions apply, mutatis mutandis, to both welding and soldering. To emphasize this fact, the general terms joining current I, joining voltage U, joining electrode 7, etc., are used below instead of the designations otherwise customary in welding such as welding current I, welding voltage U, welding electrode 7, welding wire 7a, etc., or instead of the designations otherwise customary in soldering such as soldering current I, soldering voltage U, soldering electrode 7, etc.

[0027] The joining device 1 comprises a power source 2, a hose assembly 3, a torch 4, and a shielding gas cylinder 5 containing a shielding gas SG. The shielding gas cylinder 5 is connected to the torch 4 via a shielding gas line 8. A pressure regulator (not shown), for example in the form of a known cylinder fitting, may be provided on the shielding gas cylinder 5 or in the shielding gas line 8. This regulator typically serves to control the flow of the shielding gas SG. Depending on the material of the base material G, either inert, low-reactivity 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. For MSG brazing, mixtures of argon or carbon dioxide are commonly used as the shielding gas SG.

[0028] In the power source 2, a joining electrode 7 in the form of a wire electrode 7a can be arranged, which is usually wound on a joining wire reel 13. For unwinding the joining wire 7a and, in particular, for feeding the joining wire 7a to a joining point, i.e., at a weld point during welding and at a solder joint during brazing, a feed unit 12 is arranged, which is driven by a feed drive unit 12a.

[0029] To solder on the joining device 1, a hard solder, such as silver, brass, or copper-based solder, can be used instead of a joining wire 7a. This hard solder can also be unwound from a joining wire spool 13. Those skilled in the field of joining technology are well aware of these relationships. The joining wire 7a on the joining wire spool 13, and also the feed unit 12, can also be arranged outside the power source 2 in a separate unit. It is known that embodiments of welding devices 1 with several feed units 12 exist, which advantageously coordinate to produce the required wire feed. A feed unit 12 can also be arranged in the area of ​​the torch 4. This does not result in any limitations for the applicability of the present invention.

[0030] The feed drive unit 12a is controlled by a control unit 14, which in turn typically communicates with a user interface 17. Via the user interface 17, a user can specify certain joining parameters as needed, such as a joining voltage II, a joining current I, a wire feed speed vd at which the joining wire 7a is advanced to the joining point, etc. Predefined programs with specific 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 system, such as a welding robot controller.

[0031] A power unit 15 is also arranged in the power source 2. This power unit is controlled (or regulated) by the control unit 14 and is connected to an external power supply 16. The control unit 14 can therefore be equipped to control and / or regulate the entire joining device 1 or specific components of the joining device 1, such as the feed unit 12, the control of the shielding gas supply, and / or the power unit 15, and / or the movement of the torch 4. A control unit 14 can be implemented as microprocessor-based hardware, a microcontroller, or an integrated circuit (ASIC, FPGA), and can, of course, also be arranged outside the power source 2.

[0032] The power unit 15 of the power source 2 provides the required welding current I and the required welding voltage U via a power line 19 located in the hose assembly 3. The welding voltage U is applied to the welding electrode 7 for welding, so that when an arc is lit, the welding current I flows, and during a short circuit, a short-circuit current IK flows. In addition, the welding wire 7a (via the feed unit 12) and the shielding gas SG, and optionally a cooling medium for the torch 4, are typically supplied to the torch 4 via the hose assembly 3. Control lines can also be provided in the hose assembly. However, the hose assembly 3 can also consist of several individual lines for the respective media, controls, and power supplies.To carry out a welding or soldering process, a first electrical potential is applied to a workpiece 6 made of a base material G by means of an electrical connection 18, and a second electrical potential is applied to the joining wire 7a as joining electrode 7, whereby after ignition of an arc 11 in arc phases LP a joining current I flows between the joining wire 7a and the workpiece 6 and in short-circuit phases KP a short-circuit current IK flows.

[0033] When joining on a joining device 1 according to Fig. 1, various types of arcs can be used, such as short arcs, transition arcs, spray arcs, or pulsed arcs, which are well known to anyone skilled in the field of joining technology. Specifically, in welding, the arc 11 melts the joining wire 7a at the end facing the workpiece 6 and a region of the base material G. In the example shown, a weld 10 is welded onto the workpiece 6; this is called build-up welding. However, two workpieces 6 and 6a could also be joined, as symbolized by the dashed line; this is called joining welding. The arc 11 is surrounded by the shielding gas SG flowing from the torch 4 in the form of a shielding gas bell 9 to protect the molten material in the area of ​​the weld 10 from the surroundings.

[0034] The joining wire 7a, i.e., the consumable joining electrode 7, is fed to the joint at a specific wire feed rate Vd, which can depend on several influencing factors. In manual welding, where the torch 4 is guided by hand, a wire feed rate Vd that is at least approximately constant is usually selected, for example, depending on the set joining current I. In automated welding processes, for example, when the torch 4 is guided by a welding robot, the wire feed rate Vd can also be adjusted, for example, depending on a welding speed v. s The speed at which the torch 4 is moved relative to the workpiece e can be selected. As is known, speeds such as Vd and v can be used. s It can also be adjusted when adding manually.

[0035] As mentioned, different types of arcs, in particular short-circuit arcs, can be realized with the joining devices shown in Fig. 1. As explained at the beginning, arc joining processes based on short circuits have two basic temporal phases: arc phases (LP) and short-circuit phases (KP). Arc phases (LP) and short-circuit phases (KP) typically alternate cyclically, with an arc phase (LP) usually following directly after a short-circuit phase (KP), and vice versa. As shown in Figures 2a-d based on the sequence of a real short-circuit joining process (Figures 2a-d are based on images of a real short-circuit joining process taken with a high-speed camera), in arc joining processes with a consumable wire electrode 7a, an arc 11 is first ignited at the beginning of an arc phase (LP) (Fig.2a), the wire electrode 7a is melted in the arc phase LP (Fig. 2b), so that a weld droplet T forms, growing during the melting process, and the weld droplet T is brought towards the workpiece 6 to be joined until the weld droplet T touches the workpiece 6. The arc 11 then extinguishes and a short-circuit phase KP begins. As a result of the weld droplet T touching the workpiece 6, a short circuit forms between the wire electrode 7a and the workpiece e (Fig. 2c). A short-circuit current IK then flows through the weld droplet T, which further heats the wire electrode 7a and consequently the weld droplet T. Due to the continuous heating, the weld droplet is constricted in the region of the transition to the unmelted part of the wire electrode, i.e., there is a continuous reduction in the cross-sectional area d of the weld droplet T.If the cross-sectional area d of the weld droplet T is so small at at least one point that the weld droplet T can no longer be held on the wire electrode 7a, the weld droplet T detaches from the wire electrode 7a and enters a melt pool on the workpiece 6, after which a gap forms between the wire electrode 7a and the workpiece 6, and a new arc 11 ignites (Fig.2d).

[0036] As can be seen from the preceding explanations, the joining voltage U, the joining current I, and the wire feed rate Vd are essential joining parameters during a short-arc welding process. Figures 3a-c illustrate how the time profiles of these joining parameters can be specifically designed using three established methods for short-circuit treatment.

[0037] Fig. 3a shows the procedure commonly used in practice, in which the short-circuit current IK is continuously increased during the short-circuit phases KP, e.g., linearly, exponentially, quadratically, or even stepwise, to ensure rapid heating and subsequent melting and detachment of weld droplets T. An advantage of this approach is that it largely eliminates the need for complex sensors, actuators, and control technology. However, a significant disadvantage of this method is that the short-circuit current IK is high when the short circuit breaks, in the case shown at the indicated time U, which can sometimes result in significant weld spatter.

[0038] Fig. 3b shows a common form of short-circuit treatment in which the time course of the electrical resistance of the short-circuit bridge is monitored. Since the short-circuit current IK is known in most practically relevant cases, the resistance can be easily deduced if the voltage drop across the weld droplet T is also known, e.g., by applying Ohm's law R = U / l. As explained earlier, the resistance of a weld droplet T is related to its shape and constriction, so that a short-circuit breakup can be predicted in the immediate future based solely on the resistance curve. Upon detection of progressive droplet constriction, the short-circuit current IK can be reduced, as can be seen in the circled sections of Fig. 3b, resulting in less weld spatter and increased process stability.One disadvantage of this approach is that at least the welding voltage U and the welding current I must be measured with high precision and high resolution to obtain meaningful resistance values. It is evident that a variety of disturbances can affect the measuring equipment used in a welding process, such as electromagnetic interference, measurement noise, temperature effects, etc., making high-quality measurements difficult. With the established method of RL adjustment, which compensates for the influence of, for example, a hose assembly and / or ground cable on measurement results, precise, model-based descriptions of the hose assembly and ground cable are essential.

[0039] In the procedure shown in Fig. 3c, active wire retraction is provided, as can be seen from the time course of the wire feed rate vd, which, unlike the scenarios in Fig. 3a and Fig. 3b, also exhibits negative rate values. Retracting the wire supports droplet detachment due to surface tension, thereby further reducing the current during the short-circuit phases KP, which in turn leads to lower currents IK at the time of short-circuit breakup. However, a disadvantage of this variant is that complex actuators are required to implement the active wire retraction.

[0040] The present invention aims to at least mitigate the problems of the procedures shown in Figs. 3a-c. To this end, in a short-arc joining process according to the preceding descriptions, a first sub-phase KP-1 and a second sub-phase KP-2 following the first sub-phase KP-1 are provided in each of the short-circuit phases KP, as explained below, particularly with reference to Fig. 4.

[0041] The sum of the durations of a first sub-phase KP-1 and a second sub-phase KP-2, which lie within the same short-circuit phase KP, preferably corresponds to the duration of the short-circuit phase KP. This means that, besides sub-phases KP-1 and KP-2, there are preferably no further phases present in the short-circuit phases KP. This advantageous configuration results in droplet detachment at the end of the second sub-phase KP-2, the arc reignites, and the overarching short-circuit phase KP ends together with the second sub-phase KP-2. Sub-phases KP-1 and KP-2 can be of equal or different durations. It has been shown that the time lengths of the sub-phases KP-1, KP-2 of a short-circuit phase KP are best in a ratio between 0.3 and 1.7, or between 0.5 and 1.5, or between 0.75 and 1.25, or between 0.85 and 1.15.It should be noted, however, that in a further embodiment, the invention can also be applied in scenarios where, in addition to the first sub-phase KP-1 and the second sub-phase KP-2, further sub-phases of a short-circuit phase KP are possible, e.g., before the first sub-phase KP-1 or between the first sub-phase KP-1 and the second sub-phase KP-2. Since controlling a corresponding joining process is particularly simple with only two sub-phases, dividing a short-circuit phase KP into only two sub-phases is advantageous.

[0042] According to the invention, in the first sub-phase KP-1 the short-circuit current IK is reduced from an initial current I Ki present at the beginning of the first sub-phase KP-1 to a predetermined melting current IK mThe initial current IKi can vary from one short-circuit phase KP to the next due to a wide range of influences that can occur during an arc phase. Furthermore, the increase of the short-circuit current IK in the first sub-phase KP-1 according to the invention should not be understood as a strictly monotonical increase. While a monotonical increase is advantageous, for various reasons the short-circuit current IK can also be held constant for a short period during its increase or even temporarily reduced, for example, at the beginning of a first sub-phase KP-1, starting from the initial current IKi.

[0043] In the second sub-phase KP-2, the short-circuit current IK is adjusted according to a predetermined short-circuit current target curve l so n is reduced, and a drop-time value Tj is obtained. St, which describes an actual value of a droplet parameter RT of the welding droplet T, is determined and used to calculate the short-circuit current target curve l so to correct n. As already explained earlier, it is particularly advantageous if the actual drop value Tj s t with a predetermined drop setpoint T so ii is compared, preferably by subtracting the actual drop value Tj s t from drop setpoint T S0 H, in particular to determine a drop comparison result 6T, and the change in the short-circuit current target curve depending on the drop comparison result e T to undertake.

[0044] The inventive method achieves a multitude of advantageous effects. On the one hand, providing a high current in the first phase KP-1 ensures rapid heating of the weld droplet. However, this high short-circuit current IK is not maintained throughout the entire short-circuit phase KP. On the other hand, it becomes possible to specify a time profile precisely adapted to the conditions of a specific joining task for the subsequent reduction of the short-circuit current, in order to have only a low short-circuit current at the end of the short-circuit phase, resulting in minimal weld spatter.To specify a suitable short-circuit current target profile for reducing the short-circuit current, to which the short-circuit current IK is regulated or set, a number of influencing factors can be considered, such as an expected, specified, or optimized duration of a short-circuit phase KP and / or an expected, specified, or optimized temperature distribution in the workpiece 6 or in the wire electrode 7a and / or an expected, specified, or optimized time profile of the feed rate vd, etc. As explained in detail below, numerical simulations can be used for this purpose.

[0045] Furthermore, the invention allows for the additional monitoring of the shape and geometry of the weld droplet T. Consequently, geometric parameters of the weld droplet T, e.g., diameter, circumference, or constriction, can be defined as droplet parameters RT. Electrical parameters, such as electrical resistance, conductance, impedance, admittance, voltage, or power, can also be defined as droplet parameters RT. Since the short-circuit treatment according to the invention is not based solely on (measurement-wise complex) monitoring of the weld droplet T, but rather the monitoring of the weld droplet T is merely additive to the specification of the target short-circuit current profile, the monitoring of the actual droplet value can be simplified, and in particular, only within a short time interval during the second sub-phase KP-2.In comparison to the approaches discussed above with reference to Fig. 3, the invention achieves at least an equivalent reduction in weld spatter, even though it eliminates the need for complex actuators for wire retraction and sensitive measuring technology. In particular, the invention allows, for example, the measurement of only the voltage drop directly across the power section 15 in the power source 2 to monitor the weld droplet T, while still achieving satisfactory results. In the prior art, the voltage drop directly across the weld droplet T must be measured or calculated in a complex manner. The invention therefore allows the determination of an actual electrical voltage U, which only partially drops across the weld droplet T, as the actual droplet value Tactual, and a target electrical voltage as the target droplet value Ttarget. S0to specify H, or to calculate an actual electrical resistance RT as a drop value Tj from the actual electrical voltage U and the short-circuit current IK. s to determine t and a target electrical resistance as the drop setpoint T S0to specify H, and thus already ensure an effective reduction of weld spatter. Of course, it is also conceivable within the scope of the present invention to provide extensions as described above, and, for example, to calculate a voltage drop across the weld droplet T from a voltage measured at the power section 15 in the power source 2, and to use such a voltage to assess the weld droplet T and the constriction. For this purpose, methods for compensating resistances and inductances in the electrical connection between the power section 15 and the weld droplet T can be used ("RL compensation"). However, this is not necessary. Similarly, it is conceivable to combine the invention with the short-circuit treatment shown in Fig. 3c, i.e., to provide active wire retraction, preferably during the entire short circuit, and to move the joining electrode 7 away from the workpiece 6 during the second partial phase KP-2.In this respect as well, satisfactory results can be achieved with the invention even without such an additional measure.

[0046] Possible scenarios that can occur within the framework of a procedure according to the invention are shown in Fig. 4. It can be seen that the melting current IK m The melting current IK is higher than the initial current IK. This ensures efficient heating and ultimately melting of the weld droplet T. In this context, it has been observed that particularly good results are achieved when the melting current IK is higher than the initial current IK. mThe initial current IKi is more than 0.5 times higher than the initial current IKi, or more than 1 time higher than the initial current IKi, or more than 5 times higher than the initial current IKi, or more than 10 times higher than the initial current IKi. As mentioned earlier, the current waveform at the beginning of a short-circuit phase KP can vary from one short-circuit phase KP to the next, as indicated in Fig. 4 by the dashed lines to the left of the first short-circuit phase KP-1.

[0047] As can also be seen in Fig. 4, the short-circuit current target curve l so n in the case shown, the melting current IK m always with a value below the melting current IK mThe final current IK2 is fixed. This is not mandatory, so variable final currents IK2 can also be used. Regardless of whether a constant or variable final current IK2 is used, in practice it proves advantageous to choose the final current IK2 between 20 and 200 A, or between 20 A and 150 A, or between 30 A and 130 A, or between 40 A and 120 A, or between 50 A and 110 A. This ensures a low short-circuit current IK at the end of the short-circuit phases KP.

[0048] In order to make the adaptation according to the invention, i.e. the correction, of the short-circuit current target curve Isoii robust against disturbances, it can be provided that a change in the short-circuit current target curve l only occurs if so n occurs when the comparison result is 6T or, more generally, a deviation between the actual drop value Tj. S t and the drop setpoint T soii exceeds a predefined threshold. Specifically, the short-circuit current target curve Isoii can be reduced for this purpose if the actual drop value Tist exceeds the predefined drop value T. so ii is located, preferably when the actual drop value Tist exceeds the predetermined target drop value T by more than a predetermined deviation threshold. so ii, which deviation threshold can be, for example, 5% or 10% or 20% or 50% of the target drop value Tsoii, and that the target short-circuit current Isoii is increased when the actual drop value Tj S t is below the specified drop setpoint Tsoii, preferably if the actual drop value Tist is more than a specified deviation threshold below the specified drop setpoint T. S0H is the deviation threshold, which can be 5%, 10%, 20%, or 50% of the target drop value Tsoii. However, depending on the drop parameter RT under consideration, a reverse approach may be necessary. For example, when considering electrical resistance, the procedure corresponds to the above description, as the short-circuit current IK must be reduced if the resistance is too high. Conversely, if the diameter of the drop's constriction is considered, the short-circuit current must be increased if the diameter is too large, in order to accelerate melting and reduce the diameter to the specified target diameter.

[0049] In the practical implementation of the invention, it has proven advantageous to provide a first subphase KP-21 and a second subphase KP-22 following the first subphase KP-21 in each of the second subphases KP-2, and not to include a correction of the short-circuit current target curve Isoii in the first subphase KP-21, but only in the second subphase KP-22. The first subphase KP-21 can advantageously be chosen to be at least as long as the second subphase KP-22, such that the first subphase KP-21 extends over at least half of the second subphase KP-2. In this way, complex measurements and / or calculations are limited to a small part of the second subphase KP-2 (the second subphase KP-22), which reduces complexity and conserves computing resources compared to known methods.

[0050] As mentioned previously, the invention makes it possible to determine the target short-circuit current profile Isoii by means of numerical simulation of the short-arc welding process. In this way, a preliminary simulation can be performed using highly precise models of the welding process, and a suitable target short-circuit current profile Isoii can be determined through optimization. This profile is then further adapted and improved using online simulations during the welding process. In this way, changing boundary conditions during the welding process can be taken into account. Thus, in a particularly advantageous embodiment, the invention allows a desired joining behavior to be defined in advance based on a short-circuit current profile determined by simulation or empirically, which is stored, for example, in the form of a matrix or in a database, and expressed in the form of the target short-circuit current profile.In the course of empirically or through simulation-based parameter determination, i.e., in particular, in the course of defining a target profile for the short-circuit current IK in the second sub-phase KP-2 of the short-circuit phases KP, an optimal relationship between short-circuit current IK and short-circuit duration, i.e., the time length of the short-circuit phases KP, can be determined. The control of the short-circuit current IK can be designed such that, initially, the short-circuit current IK is controlled and reduced purely based on the predetermined time profile to ensure a low current when the short circuit breaks. However, since a MIG / MAG welding process is very dynamic and sometimes non-deterministic, so that deviations and changes in boundary conditions, e.g., ambient temperature, workpiece surface, material properties, etc., often occur, the invention allows for the additional provision of monitoring.

[0051] As explained above, according to the invention, a resistance / voltage profile is tracked during the second sub-phase KP-2. This is based on the fact that, in the second sub-phase KP-2 with reduced short-circuit current, an increasing resistance profile is advantageous for a stable welding process. If this is not the case, the short-circuit current IK is increased, resulting in greater thermal stress on the welding wire. The current increase can, for example, be implemented by gradually increasing the current and recalculating / observing the resistance profile after a predetermined time has elapsed.

[0052] As shown in Fig. 5, to implement the invention advantageously, a control law R can also be specified, for example in the form of a PID control law, a sliding-mode control law, an MPC control law, a backstepping control law, or a flatness-based control law, with which a correction signal A / of the short-circuit current setpoint l so n is determined as a function of the drop comparison result 6T, which, as shown in Fig. 5, can be added to the short-circuit current target curve Isoii. In this way, it becomes possible to establish a control law and thus the manner in which the short-circuit current target curve l so n is modified to precisely match the requirements of a specific joining task. A short-circuit current target curve l modified in this way so n can subsequently be used for a standard current control. The formation of the drop setpoint values ​​T is also possible.S0 H can be flexibly designed. In particular, in the first sub-phase KP-1, preferably in a constant current phase or a constant voltage phase of the first sub-phase KP-1, in which the short-circuit current IK equals the melting current IK m corresponds to a first drop actual value Tj S t, preferably in the form of an electrical resistance, is determined and this first drop actual value Tj s t in the second sub-phase KP-2 as the first drop setpoint T S0 H is specified. As mentioned, the second sub-phases are designed in an advantageous way such that at their end a droplet detachment occurs and thus also an end to the higher-level short-circuit phase KP.

Claims

Patent claims 1. Short-arc joining method for producing a weld seam (10) on a workpiece (6), comprising arc phases (LP) in which an arc (11) burns between a consumable joining electrode (7) and a workpiece (6), which melts the joining electrode (7) in an end region of the joining electrode (7) facing the workpiece (6) to form a weld droplet, and in which short-circuit phases (KP) alternate cyclically with the arc phases (LP), in which the joining electrode (7) touches the workpiece (6) and a short-circuit current (IK) flows through the weld droplet (T) into the workpiece (6) to melt the weld droplet (T), characterized in that the following are provided in each of the short-circuit phases (KP): - a first sub-phase (KP-1), in which the short-circuit current (IK) is increased from an initial current (IK1) present at the beginning of the first sub-phase (KP-1) to a predetermined melting current (IK).m ), - one of the first sub-phases (KP-1) followed by a second sub-phase (KP-2), in which o the short-circuit current (IK) is reduced according to a predetermined short-circuit current target curve (Isoii), and o a drop actual value (Ti) s t) a droplet parameter (RT) of the weld droplet (T) is determined and is used to correct the short-circuit current target curve (Isoii).

2. Short-arc joining method according to claim 1, characterized in that the actual drop value (Tactual) is compared with a predetermined target drop value (Ttarget). so ii) is compared, preferably to determine a drop comparison result (er), and that the short-circuit current target profile (Isoii) is used to reduce a deviation between the drop actual value (Tist) and the predetermined drop target value (T). S0 u) is corrected, preferably depending on the drop comparison result (er).

3. Short-circuit arc joining method according to claim 1 or 2, characterized in that the short-circuit current target profile (Isoii) is determined empirically or from a database or by means of a numerical simulation of the short-circuit arc joining method.

4. Short-arc joining method according to one of the preceding claims, characterized in that in the second partial phases (KP-2) a first sub-phase (KP- 21) and a second sub-phase (KP-22) following the first sub-phase (KP-21) are provided, wherein no correction of the short-circuit current target curve (Isoii) is provided in the first sub-phase (KP-21), and only in the second sub-phase (KP-22) is a correction of the short-circuit current target curve (l) provided. so n) is provided for.

5. Short-arc joining method according to one of the preceding claims, characterized in that an actual electrical voltage (II), which drops at least partially across the weld droplet (T), is referred to as the droplet actual value (Tj). S t) is determined and that an electrical target voltage is used as the drop setpoint (T) S0 H) is specified.

6. Short-arc joining method according to one of claims 1 to 4, characterized in that an actual electrical voltage (II), which drops at least partially across the weld droplet (T), is determined, and that an actual electrical resistance (RT) as the droplet actual value (Tj) is calculated from the actual electrical voltage (II) and the short-circuit current (IK). S t) is determined and that an electrical target resistance is used as the drop target value (T) so ii) is specified.

7. Short-arc joining method according to one of the preceding claims, characterized in that in the case of a droplet actual value (Tj) St), which, preferably by more than a predetermined deviation threshold, is above the predetermined drop setpoint (Tsoii), the short-circuit current setpoint (l so n) is reduced or increased, and that in the case of a drop actual value (Tj) S t), which, preferably by more than a predetermined short-circuit current (IK), is below the predetermined drop setpoint (T) so ii) is located, the short-circuit current target curve (Isoii) is increased or decreased.

8. Short-arc joining method according to one of the preceding claims, characterized in that in the first partial phase (KP-1), preferably in a constant current phase or in a constant voltage phase of the first partial phase (KP-1), in which the short-circuit current (IK) corresponds to the melting current (IK) m ) corresponds to a first drop actual value (Tj S t) is determined and the first drop actual value (Tj) St) in the second sub-phase (KP-2) when determining the target drop value (T so ii) is taken into account.

9. Short-arc joining method according to one of the preceding claims, characterized in that the melting current (IK) m ) is more than 0.5 times the initial current (I Ki), or preferably more than 1 time, 5 times or 10 times the initial current (I Ki).

10. Short-arc joining method according to one of the preceding claims, characterized in that the joining electrode (7) is exposed during the short-circuit phase KP of the workpiece (6) is moved away in order to further support the resolution of a short circuit by moving the joining electrode (7).

11. Short-arc joining method according to one of the preceding claims, characterized in that the sum of the time length of a first sub-phase (KP-1) provided in a short-circuit phase (KP) and the time length of a second sub-phase (KP-2) provided in the same short-circuit phase (KP) corresponds to the time length of the short-circuit phase (KP).

12. Short-arc joining method according to one of the preceding claims, characterized in that a ratio of a temporal length of a first partial phase (KP-1) provided in a short-circuit phase (KP) to the temporal length of a second partial phase (KP-2) provided in the short-circuit phase (KP) corresponds to a predetermined factor kt, wherein the factor kt preferably corresponds to a value greater than 0.3 and less than 1.7, or preferably to a value greater than 0.5 and less than 1.5, or preferably to a value greater than 0.75 and less than 1.25, or preferably to a value greater than 0.85 and less than 1.

15.

13. Short-arc joining method according to one of the preceding claims, characterized in that the short-arc joining method is a short-arc welding method or a short-arc brazing method.

14. Short-arc joining method according to one of the preceding claims, characterized in that a control law is specified, preferably a PID control law or a sliding-mode control law or an M PC control law or a backstepping control law or a flatness-based control law, with which a correction of the short-circuit current setpoint (l so n) depending on a deviation of the actual drop value (Tactual) from the target drop value (Ttarget) so ii), preferably depending on the drop comparison result (6T).

15. Joining device (1) for carrying out a short-arc joining process, comprising a torch (4), a feed unit (12) for supplying a consumable joining electrode (7) through the torch, a power source (2) for supplying electrical power to the joining electrode (7), and a control unit (14) for controlling the joining device (1), comprising the short-arc joining process, arc phases (LP) in which an arc (11) burns between the consumable joining electrode (7) and a workpiece (6), which melts the joining electrode (7) in an end region of the joining electrode (7) facing the workpiece (6), forming a weld droplet, and short-circuit phases (KP) that alternate cyclically with the arc phases (LP), in which the The joining electrode (7) touches the workpiece (6) and a short-circuit current (IK) flows from the power source (2) through the weld droplet (T) into the workpiece (6) to melt the weld droplet (T), characterized in that the control unit (14) is designed to provide in each of the short-circuit phases (KP): - a first partial phase (KP-1), in which the short-circuit current (IK) is increased from an initial current (I Ki) present at the beginning of the first partial phase (KP-1) to a predetermined melting current (IK). m ), - one of the first sub-phases (KP-1) followed by a second sub-phase (KP-2), in which the short-circuit current (IK) corresponds to a predetermined short-circuit current target curve (l so n) is reduced, and o a drop actual value (Tj) S t) a droplet parameter (RT) of the welding droplet (T) is determined and used to calculate the short-circuit current target curve (l so n) to correct.

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