Short-circuit welding method and welding device

Abstract

A welding device and a short-circuit welding method uses successive welding cycles having respective arc and short-circuit phases. The method includes: conveying a welding wire from a workpiece, bringing the wire to a final rearward speed in a first rearward conveying phase and then conveying it at that speed until a second rearward conveying phase where the rearward speed is reduced; and conveying the wire toward the workpiece, the welding wire being brought to a final forward speed in a first forward conveying phase and being conveyed at that speed until the beginning of a second forward conveying phase where the forward speed is reduced. The first duration is adapted using a feedforward control and/or feedback control such that the welding wire does not exceed a specified reduced rearward speed in the second rearward conveying phase at the point in time at which the short-circuit is interrupted.

Claims

1. A short-circuit welding method with successive welding cycles, each of which having an arc phase (6) and a short-circuit phase (7), with the following steps during a welding cycle: conveying a melting welding wire (3) away from a workpiece (5) during the short-circuit phase (7) at a rearward speed (?v.sub.d) so that material is discharged from the welding wire (3) to the workpiece (5) and an arc is ignited, wherein the welding wire (3) is brought to a final rearward speed (v.sub.re_max) during the short-circuit phase (7) in a first rearward conveying phase (13) with a first duration (T.sub.1) and is conveyed therewith until the beginning of a second rearward conveying phase (15) in which the rearward speed (?v.sub.d) of the welding wire (3) is reduced again, wherein the second rearward conveying phase (15) immediately follows the end of the first rearward conveying phase (13); and conveying the welding wire (3) in the direction of the workpiece (5) during the arc phase (6) at a forward speed (v.sub.d) in order to bring the welding wire (3) into contact with the workpiece (5) and to generate a short-circuit between the welding wire (3) and the workpiece (5), wherein the welding wire (3) is brought to a final forward speed (v.sub.v_max) during the arc phase (6) in a first forward conveying phase (16) with a second duration (T.sub.2) and is conveyed therewith until the beginning of a second forward conveying phase (18), in which the forward speed (v.sub.d) of the welding wire (3) is reduced again, wherein the second forward conveying phase (18) immediately follows the end of the first forward conveying phase (16), wherein the first duration (T.sub.1) of the first rearward conveying phase (13) is adjusted by means of feedforward control and/or feedback control in such a way that the welding wire (3) in the second rearward conveying phase (15) does not exceed a predetermined reduced rearward speed (v.sub.re_K) at the point in time of interruption (12) of the short-circuit, wherein the first rearward conveying phase (13) is shortened or lengthened by adjusting the first duration (T.sub.1), wherein the absolute amount of the predetermined reduced rearward speed (v.sub.re_K) is less than the absolute amount of the final rearward speed (v.sub.re_max).

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

3. The short-circuit welding method according to claim 2, wherein the first duration (T.sub.1) is adapted by the feedforward control and/or feedback control in such a way that the interruption (12) of the short-circuit takes place in the first intermediate plateau phase (21).

4. The short-circuit welding method according to claim 3, wherein a feedback control for adapting the first duration (T.sub.1) is provided, wherein the feedback control is carried out by a control circuit (22) with a controller (23), in particular a P or PI controller, wherein the feedback control adapts the first duration (T.sub.1) so that the first intermediate plateau phase (21) corresponds to a first target duration (T.sub.min_1).

5. The short-circuit welding method according to claim 1, wherein a feedback control for adapting the first duration (T.sub.1) is provided, wherein the feedback control is carried out by a control circuit (22) having a controller (23), in particular a P or PI controller, wherein a control variable (S) of the control circuit (22) is formed by the first duration (T.sub.1), a reference variable (F) of the control circuit (22) is formed by a target rearward speed of the welding wire (3), in particular the predetermined reduced rearward speed (v.sub.re_K), and a measured control variable (Y) of the control circuit (22) is formed by a measured rearward speed (?v.sub.d) of the welding wire (3) at the point in time of the interruption (12) of the short-circuit.

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

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

8. The short-circuit welding method according to claim 1, wherein the first rearward conveying phase (13) has a first final plateau phase (14), in which the welding wire (3) is conveyed at a substantially constant final rearward speed (v.sub.re_max) and a duration (T.sub.3) of the first final plateau phase (14) is adapted by the feedforward control and/or feedback control.

9. The short-circuit welding method according to claim 1, wherein the second duration (T.sub.2) of the first forward conveying phase (16) is adjusted by means of an additional feedforward control and/or feedback control in such a way that the welding wire (3) in the second forward conveying phase (18) does not exceed a predetermined reduced forward speed (v.sub.v_K) at the point in time of generation (11) of the short-circuit, wherein the first forward conveying phase (16) is shortened or lengthened by adjusting the second duration (T.sub.2).

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

11. The short-circuit welding method according to claim 10, wherein the second duration (T.sub.2) is adapted by the additional feedforward control and/or feedback control in such a way that the generation (11) of the short-circuit takes place in the second intermediate plateau phase (25).

12. The short-circuit welding method according to claim 11, wherein an additional feedback control for adapting the second duration (T.sub.2) is provided, wherein the additional feedback control is carried out by an additional control circuit (22) with an additional controller (23), in particular a P or PI controller, wherein the additional feedback control adapts the second duration (T.sub.2) so that the second intermediate plateau phase (25) corresponds to a second target duration (T.sub.min_2).

13. The short-circuit welding method according to claim 9, wherein an additional feedback control for adapting the second duration (T.sub.2) is provided, wherein the additional feedback control is carried out by an additional control circuit (22) with an additional controller (23), in particular a P or PI controller, wherein a control variable (S) of the additional control circuit (22) is formed by the second duration (T.sub.2), a reference variable (F) of the additional control circuit (22) is formed by a target forward speed of the welding wire (3), in particular the predetermined reduced forward speed (v.sub.v_K), and a measured control variable (Y) of the additional control circuit (22) is formed by a measured forward speed (v.sub.d) of the welding wire (3) at the point in time of the generation (11) of the short-circuit.

14. A welding device (1) for carrying out the short-circuit welding method according to a claim 1 with successive welding cycles, each comprising an arc phase (6) and a short-circuit phase (7), comprising a welding torch (2), a feedforward control and/or feedback control unit (8) and a conveying device (4) for conveying a melting welding wire (3), wherein the welding apparatus (1) is configured to convey the melting welding wire (3) away from the workpiece (5) during the short-circuit phase (7) at a rearward speed (?v.sub.d) so that material is discharged from the welding wire (3) to the workpiece (5) and an arc is ignited, wherein the welding device (1) is further configured to bring the welding wire (3) to a final rearward speed (v.sub.re_max) during the short-circuit phase (7) in a first rearward conveying phase (13) with a first duration (T.sub.1) and to convey it therewith until the beginning of a second rearward conveying phase (15) in which the rearward speed (?v.sub.d) of the welding wire (3) is reduced again, wherein the second rearward conveying phase (15) immediately follows the end of the first rearward conveying phase (13), and the welding device (1) is further configured to convey the welding wire (3) in the direction of the workpiece (5) during the arc phase (6) at a forward speed (v.sub.d) in order to generate a short-circuit between the welding wire (3) and the workpiece (5), wherein the welding device (1) is further configured to bring the welding wire (3) to a final forward speed (v.sub.v_max) during the arc phase (6) in a first forward conveying phase (16) with a second duration (T.sub.2) and to convey it with the final forward speed (v.sub.v_max) until the beginning of a second forward conveying phase (18), in which the forward speed (v.sub.d) of the welding wire (3) is reduced again, wherein the second forward conveying phase (18) immediately follows the end of the first forward conveying phase (16), wherein the feedforward control and/or feedback control unit (8) is configured so that the first duration (T.sub.1) of the first rearward conveying phase (13) is adjusted by means of feedforward and/or feedback control in such a way that the welding wire (3) in the second rearward conveying phase (15) does not exceed a predetermined reduced rearward speed (v.sub.re_K) at the point in time of interruption (12) of the short-circuit, wherein the first rearward conveying phase (13) is shortened or lengthened by adjusting the first duration (T.sub.1), wherein the absolute amount of the predetermined reduced rearward speed (v.sub.re_K) is less than the absolute amount of the final rearward speed (v.sub.re_max).

Description

(1) In the following, the invention will be explained in more detail with reference to figures, to which, however, it is not intended to be limited. They show in:

(2) FIG. 1, a welding device for carrying out a short-circuit welding method;

(3) FIG. 2, a welding voltage profile, a welding current profile and a speed profile of a welding wire in a short-circuit welding method from the state of the art;

(4) FIG. 3, a welding voltage profile, a welding current profile and a speed profile of a welding wire in a short-circuit welding method according to the invention;

(5) FIG. 4A and FIG. 4B, a control circuit;

(6) FIG. 5, a welding voltage profile, a welding current profile and a further speed profile of a welding wire in a short-circuit welding method according to the invention;

(7) FIG. 6A and FIG. 6B, a further control circuit; and

(8) FIG. 7, a welding voltage profile, a welding current profile, a speed profile of a welding wire and a time profile of a resistance value.

(9) FIG. 1 shows a welding device 1 with a welding torch 2, which is designed to carry out a short-circuit welding method, in particular a CMT welding method. In a short-circuit welding method, a melting welding wire 3 (electrode) is moved alternately in the direction of a workpiece 5 and back by a conveying device 4, which can have an electric motor. As a result of this sequence, a short-circuit welding method can be subdivided into arc phases 6 and short-circuit phases 7 (cf. for example FIGS. 2 and 3). The welding device 1 has a control/regulating unit 8 for controlling and/or regulating the welding wire 3. The welding wire 3 is guided by a supply drum 9 into the region of the welding torch 2.

(10) FIG. 2 shows (below a time curve of a welding voltage U and a time curve of a welding current I) a velocity curve 10 of a welding wire 3 in a short-circuit welding method from the state of the art with fixed predetermined and preset time sequences. The speed v.sub.d is plotted on the ordinate, where +v.sub.d denotes a forward speed of the welding wire 3 in the direction of the workpiece 5 and ?v.sub.d denotes a rearward speed of the welding wire 3 away from the workpiece 5. The short-circuit welding method can be divided into arc phases 6 and short-circuit phases 7. A short-circuit phase 7 begins with the generation 11 of the short-circuit between the welding wire 3 and the workpiece 5 when the welding wire 3 touches the workpiece 5. An arc phase 6 begins with the interruption 12 of the short-circuit between the welding wire 3 and the workpiece 5 and ends again with its generation. During the arc phase 6, the welding wire 3 is conveyed mainly in the direction of the workpiece 5 at a forward speed +V.sub.d. During the short-circuit phase 7, the welding wire 3 is mainly conveyed away from the workpiece 5 at a rearward speed ?V.sub.d. A welding cycle consists of an arc phase 6 and a short-circuit phase 7.

(11) The speed curve 10 of the welding wire 3 can be further subdivided into individual welding wire speed phases, with which the welding wire 3 is conveyed. In a first rearward conveying phase 13, the welding wire 3 is brought to a final rearward speed v.sub.re_max by acceleration. The final rearward speed v.sub.re_max can, for example, be in the range between 10 m/min and 60 m/min, in particular between 20 m/min and 60 m/min. The first rearward conveying phase 13 has a total duration, which is referred to as the first duration T.sub.1 and which can be, for example, 3 ms. After the welding wire 3 has reached the final rearward speed v.sub.re_max, the acceleration is ended and the welding wire 3 is conveyed away from the workpiece 5 in a first final plateau phase 14 at substantially constant final rearward speed v.sub.re_max. The final plateau phase 14 has the duration T.sub.3. Directly following the end of the first rearward conveying phase 13 is a second rearward conveying phase 15, in which the rearward speed ?v.sub.d of the welding wire 3 is again reduced by acceleration. The rearward speed ?v.sub.d is reduced to zero in this phase. Subsequently, a first forward conveying phase 16 is provided, in which the welding wire 3 is accelerated to a final forward speed v.sub.v_max. The total duration of the first forward conveying phase 16 is referred to as the second duration T.sub.2. In a second final plateau phase 17, the duration of which is denoted by T.sub.4, the final forward speed v.sub.v_max is kept substantially constant. The first forward conveying phase 16 is immediately followed by a second forward conveying phase 18, in which the forward speed +v.sub.d is reduced again to zero. Subsequently, a first rearward conveying phase 13 is again provided. The speed curve shown is a schematic and idealised representation of a desired curve. The effective speed curves may differ physically from the curve shown.

(12) At the end of the short-circuit phase 7, at the time of the interruption 12 of the short-circuit, a delivery of material (droplet delivery) from the welding wire 3 onto the workpiece 5 occurs to produce a weld seam 19 (see FIG. 1). However, the speed of the welding wire 3 is too high at the time of the interruption 12 of the short-circuit, as is the case in FIG. 2, so that welding splashes can arise, which adversely affect the quality of the weld seam 19. This is particularly the case with viscous materials such as titanium.

(13) In the state of the art, the speed curve 10 is determined over fixedly predefined time durations T.sub.1, T.sub.2 or T.sub.3, T.sub.4. The first duration T.sub.1 should be set in such a way that the welding wire 3 is conveyed sufficiently slowly at the time of the interruption 12 in order to avoid welding splashes. However, the time point of the interruption 12 of the short-circuit as well as the time point of the generation 11 of the short-circuit may vary, as shown in FIG. 2, and depend on parameters such as the material, the weld seam, impurities, and the temperature, etc. The selection of the fixed durations T.sub.1 and T.sub.2 or T.sub.3, T.sub.4 is therefore not entirely simple. For example, the case may occur in which the welding wire 3 is conveyed at an excessively high rearward conveying speed ?v.sub.d at the point in time of the interruption 12 of the short-circuit and thus weld splashes are produced. This is shown in FIG. 2 where the-short-circuit is interrupted at the points in time 20 at too high a speed. The points in time 20 vary in time relative to the speed curve 10, as can also be seen in FIG. 2. The points in time of the generation 11 of the short-circuit can also vary. Due to the variation of the points in time of the generation 11 and interruption 12 of the short-circuit, the welding frequency is irregular and thus not stable.

(14) According to the invention, it is therefore provided that the first duration T.sub.1 of the first rearward conveying phase 13 is adapted by means of a feedforward control and/or feedback control in such a way that the welding wire 3 in the second rearward conveying phase 15 does not exceed, in particular does not correspond to, a predetermined reduced rearward speed v.sub.re_x at the point of time of the occurrence 12 of the short-circuit. This is illustrated in FIG. 3 in the right short-circuit phase 7. The first rearward conveying phase 13 therefore ends in time compared to the left short-circuit phase 7, and the second rearward conveying phase 15 is initiated before the interruption 12 of the short-circuit takes place. The control and/or regulating operations can be implemented in the control and/or regulating unit 8.

(15) The second rearward conveying phase 15 may have a first intermediate plateau phase 21 with a substantially constant rearward speed ?v.sub.d. In the first intermediate plateau phase 21, the welding wire 3 is preferably conveyed away from the workpiece 5 at the predetermined reduced rearward speed v.sub.re_K. The feedforward control and/or feedback control adjusts the first duration T.sub.1 in the embodiment shown such that the interruption 12 of the short-circuit takes place in the first intermediate plateau phase 21.

(16) This can be done within a welding cycle or from welding cycle to welding cycle. After the interruption 12 of the short-circuit, the predetermined reduced rearward speed v.sub.re_K of the welding wire 3 is still held for a holding duration T.sub.hold_1 before the acceleration of the welding wire 3 is continued.

(17) In FIG. 4A, a block diagram for a control system is shown.

(18) The control system is formed by a closed control circuit 22 and is configured to adapt the first duration T.sub.1, in the event of a deviation, from welding cycle to welding cycle, so that the welding wire 3 does not exceed a predetermined reduced rearward speed v.sub.re_K in the subsequent welding cycle in the second rearward conveying phase 15 at the point in time of interruption 12 of the short-circuit. FIG. 3 shows an adjustment from welding cycle to welding cycle. In particular, the first duration T.sub.1 can be adapted in such a way that the interruption 12 of the short-circuit takes place in the first intermediate plateau phase 21. The control circuit 22 contains a controller 23, which can be designed, for example, as a P or PI controller, and the controlled section 24. The first duration T.sub.1 is provided in the control circuit 22 as the control variable S. This can be adjusted by the controller 23. The target or reference variable F of the control circuit 22 is formed by a first target duration T.sub.min_1 of the first intermediate plateau phase 21. The duration T.sub.21 of the first intermediate plateau phase 21 is used as the measured control variable Y.

(19) In FIG. 4B, an alternative embodiment of the control system is shown. The control variable S of the control circuit 22 is again used for the first duration T.sub.1. This can be adjusted by the controller 23. However, the predetermined reduced rearward speed v.sub.re_K of the welding wire 3, which the welding wire is to have at the time of interruption 12 of the short-circuit, is used as the target or reference variable F of the control circuit 22. The measured rearward speed ?v.sub.d of the welding wire 3 at the point in time of interruption 12 of the short-circuit is used as the measured control variable Y. If the speed of the welding wire 3 at the point in time of interruption 12 of the short-circuit deviates from the predetermined reduced rearward speed v.sub.re_K, the controller 23 adjusts the first duration T.sub.1 in the next welding cycle, so that the welding wire 3 is conveyed in the future at the predetermined reduced rearward speed v.sub.re_K in the interruption 12 of the short-circuit. An adjustment from welding cycle to welding cycle is shown in FIG. 3. In particular, the first duration T.sub.1 can be adapted in such a way that the interruption 12 of the short-circuit takes place in the first intermediate plateau phase 21.

(20) Analogous to the short-circuit phase, an adaptation of the second duration T.sub.2 of the first forward conveying phase 16 can be carried out by an additional feedforward control and/or feedback control in an additional control and/or regulating unit 8, so that the welding wire 3 does not exceed a predetermined reduced forward speed v.sub.v_K in the second forward conveying phase 18 at the point in time of generation 11 of the short-circuit. This is illustrated in FIG. 5 in the right arc phase 6. The first forward conveying phase 16 therefore ends in good time and the second forward conveying phase 18 is initiated before the generation 11 of the short-circuit takes place. FIG. 5 also shows a welding current profile and a welding voltage profile.

(21) Further controls are illustrated in FIGS. 6A and 6A.

(22) In FIG. 6A, a block diagram for an additional control system is shown. The additional control system is formed by a closed control circuit 22 and is designed to adapt the second T.sub.2, if necessary, from welding cycle to welding cycle, so that the welding wire 3 does not exceed a predetermined reduced forward speed v.sub.v_K in the subsequent welding cycle in the second rearward conveying phase 18 at the point in time of generation 11 of the short-circuit. This is shown in FIG. 5. In particular, the first duration T.sub.2 can be adapted in such a way that the generation 11 of the short-circuit takes place in a second intermediate plateau phase 25 at a constant predetermined reduced forward speed v.sub.v_K. The control circuit 22 contains a controller 23, which can be designed, for example, as a P or PI controller, and the controlled section 24. As control variable S, the additional control circuit 22 uses the second duration T.sub.2. This can be adjusted by the additional controller 23. The target or reference variable F of the additional control circuit 22 is formed by a second target duration T.sub.min_2 of the second intermediate plateau phase 25. The duration T.sub.25 of the second intermediate plateau phase 25 is used as the measured control variable R.

(23) In FIG. 6B, an alternative embodiment of an additional control circuit 22 is shown. The control variable S of the additional control circuit 22 is again used for the second duration T.sub.2. This can be adjusted by the additional controller 23. However, the predetermined reduced forward speed v.sub.v_K of the welding wire 3, which the welding wire is to have at the point in time of generation 11 of the short-circuit, is used as the target or reference variable F of the additional control circuit 22. The measured forward speed v.sub.d of the welding wire 3 at the point in time of generation 11 of the short-circuit is used as the measured control variable Y. If the speed of the welding wire 3 at the point in time of generation 11 of the short-circuit deviates from the predetermined reduced forward speed v.sub.v_K, the additional controller 23 adjusts the second duration T.sub.2 in the next welding cycle, so that the welding wire 3 is conveyed in the future at the predetermined reduced forward speed v.sub.v_K in the generation 11 of the short-circuit. An adjustment from welding cycle to welding cycle is shown in FIG. 5. In particular, the second duration T.sub.2 can be adapted in such a way that the generation 11 of the short-circuit takes place in the second intermediate plateau phase 25.

(24) FIG. 7 shows, below a velocity curve 10, a welding voltage curve, and a welding current curve, a time curve of a resistance value R on the basis of which the interruption 12 of the short-circuit can be predicted. The resistance value is calculated from the welding voltage U and from the welding current I. Based on the prediction, the first duration T.sub.1 can be adjusted. This can be done within one cycle. The adjustment of the first duration T.sub.1 can be carried out by a control system. For example, if the resistance value exceeds a resistance threshold R.sub.s or the resistance change rate dR/dt exceeds a predetermined resistance change rate threshold, the imminent interruption 12 of the short-circuit may be determined. Subsequently, the control system may terminate the first rearward conveying phase 13 and initiate the second rearward conveying phase 15. In this manner, the first time duration T.sub.1 is adjusted.