ARC WELDING METHOD COMPRISING A CONSUMABLE WELDING WIRE

Abstract

The invention relates to an arc welding method using a consumable welding wire (1), wherein in successive welding cycles (SZ) during a welding process (P.sub.i) a certain welding current (I(t)) is applied to the welding wire (1) and the welding wire is moved with a certain wire conveying speed (v(t)) towards a workpiece (2) to be processed. The aim of the invention is to further improve the stability of the welding method. The aim is achieved, according to the invention, in that in the event of a change of the welding process (P.sub.1) to a welding process (P.sub.2) with an increased mean wire conveying speed (v.sub.mean) during a welding cycle (SZ) and/or with an increased mean welding current (I.sub.mean) during a welding cycle (SZ), a lowering phase (AP) is initiated, wherein in the lowering phase (AP) the welding current (I.sub.A(t)) is lowered for a specified duration (Δt.sub.A).

Claims

1-13. (canceled)

14. An arc welding method using a consumable welding wire (1), wherein in successive welding cycles (SZ) during a welding process (P.sub.i) a certain welding current (I(t)) is applied to the welding wire (1) and the welding wire is moved with a certain wire conveying speed (v(t)) towards a workpiece (2) to be processed, wherein when the welding process (P.sub.1), in particular a short-circuit-based welding process, is changed to a welding process (P.sub.2) with an increased mean wire conveying speed (v.sub.mean) during a welding cycle (SZ) and/or with an increased mean welding current (I.sub.mean) during a welding cycle (SZ), in particular a pulsed welding process, a lowering phase (AP) is initiated, wherein in the lowering phase (AP) the welding current (I.sub.A(t)) is lowered for a specified duration (Δt.sub.A), by the mean welding current (I.sub.mean,A) in the lowering phase (AP) being reduced by 10% to 50% compared to the previous mean welding current (I.sub.mean) during a welding cycle (SZ).

15. The arc welding method according to claim 14, wherein-the lowering phase (AP) is initiated after a specified delay time (t.sub.d) has elapsed.

16. The arc welding method according to claim 14, wherein the welding current (I.sub.A(t)) in the lowering phase (AP) is reduced to a constant welding current value (I.sub.mean,A) for the specified duration (Δt.sub.A).

17. The arc welding method according to claim 14, wherein the welding current (I.sub.A(t)) in the lowering phase (AP) is reduced according to a specified function for the specified duration (Δt.sub.A).

18. The arc welding method according to claim 14, wherein the duration (Δt.sub.A) of the lowering phase (AP), the welding current (I.sub.A(t)) in the lowering phase (AP) and/or the specified delay time (t.sub.d) are specified depending on the welding wire (1) used.

19. The arc welding method according to claim 14, wherein the duration (Δt.sub.A) of the lowering phase (AP) is one to 30 welding cycles (SZ).

20. The arc welding method according to any claim 15, wherein the specified delay time (t.sub.d) is set such that the welding wire (1) is moved towards the workpiece (2) by 5 mm to 25 mm during the delay time (t.sub.d).

21. The arc welding method according to any claim 14, wherein the free length (l.sub.so) of the welding wire (1) is determined indirectly and the duration (Δt.sub.A) of the lowering phase (AP), the mean welding current (I.sub.A(t)) in the lowering phase (AP) and/or the delay time (t.sub.d) are modified depending on the determined free length (l.sub.so) of the welding wire (1).

22. The arc welding method according to claim 21, wherein the free welding wire length (l.sub.so) is determined indirectly by a measurement of the welding voltage (U(t)), the welding current (I(t)) between the welding wire (1) and the workpiece (2), and the wire conveying speed (v(t)).

23. The arc welding method according to claim 21, wherein during a welding cycle (SZ), one pulsed-arc phase (LB) with at least one pulse-shaped welding current (I(t)) and one short-arc phase with one or more alternating short circuits between the welding wire (1) and the workpiece (2) and arc phases occur consecutively.

24. The arc welding method according to claim 23, wherein the free welding wire length (l.sub.so) in the short-arc phase is determined by means of a resistance measurement during a short-circuit between the welding wire (1) and the workpiece (2).

25. The arc welding method according to claim 14, wherein in the event of a further change in the welding process (P.sub.1) to a welding process (P.sub.2) with an increased mean wire conveying speed (v.sub.mean) during a welding cycle (SZ) and/or an increased mean welding current (I.sub.mean) during a welding cycle (SZ), the duration (Δt.sub.A) of the lowering phase (AP) is corrected with regard to the specified duration (Δt.sub.A).

Description

[0023] The present invention will be explained in further detail by reference to the attached drawings. In the figures:

[0024] FIG. 1 shows a schematic drawing of a welding torch with a consumable welding wire;

[0025] FIG. 2 shows time waveforms of the welding current I(t), the welding voltage U(t) and the welding wire speed v(t) for a short-circuit-based welding process;

[0026] FIG. 3 shows the temporal waveform of the welding current I(t) in the event of a change in the welding process in a design variant of the lowering phase according to the invention;

[0027] FIG. 4 shows the waveform of the welding wire corresponding to the current waveform according to FIG. 3 from the time of the process change;

[0028] FIG. 5 shows the temporal waveform of the welding current I(t) in the event of a change in the welding process in a second design variant of the lowering phase according to the invention;

[0029] FIG. 6 shows the waveform of the welding wire corresponding to the current waveform according to FIG. 5 from the time of the process change; and

[0030] FIG. 7 shows a block diagram of a device for implementing the method according to the invention.

[0031] FIG. 1 shows a schematic drawing of a welding torch 6 with a consumable welding wire 1. The consumable welding wire 1 is transported with a suitable conveying speed v(t) through a contact tube 3 of the welding torch 6 and contacted in the contact tube 3, so that the specified welding voltage U(t) and the specified welding current I(t) can be applied to the welding wire 1. If the welding parameters are appropriately matched, an arc 4 results between the end of the welding wire 1 and the workpiece 2, which should remain as constant as possible during the welding process. The free welding wire length or the so-called stickout is the length l.sub.so of the welding wire 1 from the end of the contact tube 3 to the beginning of the arc 4. The length l.sub.so of the stickout should remain as constant as possible in a stable welding process. The welding current I flows from the contact point on the contact tube 3 through the welding wire 1 and contributes to its heating. The schematically drawn sub-segments of the welding wire 1 require a certain time to travel from the contact point on the contact tube 3 to the end of the welding wire 1. In a stable welding process, only a small sub-segment or a small length section of the welding wire 1 is melted and this molten part of the welding wire 1 passes into the melting bath.

[0032] If the welding process P.sub.i is changed to a welding process P.sub.i+1, which is accompanied by an increased mean wire conveying speed v.sub.mean during a welding cycle SZ and/or an increased mean welding current I.sub.mean during a welding cycle SZ, there is a risk that parts of the free welding wire end become overheated, resulting in a sudden melting of multiple sub-segments or a longer portion of the welding wire 1, which also abruptly increases the length of the arc 4 and thus disrupts the welding process P.sub.i and may even lead to the formation of welding spatter. Such instabilities during the welding lead to a reduction of the weld quality. Each sub-segment of the stickout with length l.sub.so is heated differently with an increased mean welding current I.sub.mean from the time the change in the welding process P.sub.i occurs, since the wire conveying speed v can only increase gradually in the form of a ramp but the welding current I must be increased abruptly, as the droplet release cannot otherwise take place. This momentary imbalance between wire conveying speed v and welding current I causes the temperature increase in that region (sub-segments) of the free wire length through which the increased welding current I passes the longest. These are the sub-segments that are located at the point of contact or current transition point in the contact tube 3 at the time of the increase in the welding current I and/or the wire conveyance v. If the welding current I is not changed, some of these segments of the welding wire 1 will melt abruptly when they arrive at the front part of the free wire end, i.e. when they are positioned in the immediate vicinity of the onset of the arc 4. The longer the length l.sub.so of the free wire end, the longer the increased welding current I flows through the sub-segments and the longer their temperature increases, so that above a certain free wire end (stickout), this sudden melting of a longer part of the wire end can occur if no countermeasures are implemented.

[0033] FIG. 2 shows the temporal waveforms of the welding current I(t), the welding voltage U(t) and the welding wire speed v(t) in a short-circuit-based welding process P.sub.i. The welding process consists of welding cycles SZ which are repeated with the welding frequency and in which the welding parameters follow a specific time profile. In the example profile of the welding current I(t) shown, a current pulse is applied during the arc phase LB and during the short-circuit phase KS a further current pulse is applied with a lower amplitude than during the arc phase LB. The welding voltage U(t) has a profile that is essentially constant during the arc phase LB and breaks down as expected during the short-circuit phase KS. During the arc phase LB, the wire conveying speed v(t) has an essentially positive course, i.e. a movement in the forward direction and during the short-circuit phase a reverse movement, i.e. away from the workpiece 2. On average, a certain mean wire conveying speed v.sub.mean in the direction of the workpiece 2 occurs during a welding cycle SZ, because during the welding process P.sub.i the welding wire 1 melts and material is transferred to the workpiece 2. Of course, the welding process P.sub.i can also include a wire conveying speed in the direction of the workpiece only, without reverse movement.

[0034] FIG. 3 shows the temporal waveform of the welding current I(t) when a change occurs from a welding process P.sub.1 to a welding process P.sub.2 for a design variant of the lowering phase AP according to the invention. For example, the welding process P.sub.1 is a short-circuit-based welding process, while the welding process P.sub.2 is a pulsed welding process. However, these could also be two pulsed processes of different power. At the time of the change from the welding process P.sub.1 to the welding process P.sub.2, a timer is started and after expiry of a specified delay time t.sub.d the lowering phase AP according to the invention is initiated. As mentioned above, instead of setting a delay time t.sub.d, a certain path length can also be set over which the welding wire 1 is moved when the welding process is changed from P.sub.1 to the welding process P.sub.2. For the duration Δt.sub.A of the lowering phase AP, the mean welding current I.sub.mean,A during the lowering phase AP is reduced compared to the mean welding current during the second welding process P.sub.2. In the example shown in FIG. 3, a lowering to a constant current value I.sub.mean,A takes place. After the end of the duration Δt.sub.A of the lowering phase AP, the welding process P.sub.2 is continued with the welding parameters that were valid before the lowering phase AP or with the welding parameters specified for the welding process P.sub.2.

[0035] FIG. 4 shows the waveform of the welding wire 1 corresponding to the current waveform according to FIG. 3 from the time of the process change. The distance travelled by the free welding wire end x(t) as a function of time t extends up to the stickout length l.sub.so, whereupon the lowering phase AP is initiated. The delay time t.sub.d until the initiation of the lowering phase AP is thus set or selected such that, from the start (time t.sub.w) of the change from the welding process P.sub.1 to the welding process P.sub.2, the welding wire 1 executes an conveyance corresponding to the desired stickout length l.sub.so during this time. The length l.sub.d which the welding wire 1 is moved forward during the delay time t.sub.d in this case is equal to the stickout length l.sub.so. The lowering phase AP is then initiated, thus effectively preventing an overheating of the free welding wire end. However, the delay time t.sub.d can also be chosen to be shorter or longer than the time required by the welding wire 1 to reach the stickout length l.sub.so (see example according to FIGS. 5 and 6).

[0036] FIG. 5 shows the temporal waveform of the welding current I(t) when a change occurs from the welding process P.sub.1 to a welding process P.sub.2 (time t.sub.w) in a second design variant of the lowering phase AP according to the invention. In contrast to the variant according to FIGS. 3 and 4, here the mean welding current during the lowering phase AP is reduced in the form of a predefined function, here in the shape of a ramp, and increased again. In this way, a further improvement in the consistency of the temperature in the free welding wire end can be achieved under certain conditions, and thus a more stable welding process can also be achieved when changing from a welding process P.sub.1 to a different welding process P.sub.2 with an increased mean wire conveying speed v(t) during a welding cycle SZ and/or an increased mean welding current I.sub.mean(t) during a welding cycle SZ.

[0037] FIG. 6 shows the waveform of the welding wire 1 corresponding to the current waveform according to FIG. 5 from the time of the process change at time t.sub.w. Here, the delay time t.sub.d until the initiation of the lowering phase AP is chosen shorter than in the exemplary embodiment according to FIG. 4. From the time when the welding process P.sub.1 changes to the welding process P.sub.2, the welding wire 1 has moved forward during this delay time t.sub.d a length l.sub.d which is shorter than the stickout length l.sub.so.

[0038] Finally, FIG. 7 shows the block diagram of a device for carrying out the method according to the invention. The welding torch 6 with the contact tube 3 and welding wire 1 is connected to the welding current source 5 and to a wire conveying device 7, which moves the welding wire 1 from a wire reel 8 with a corresponding speed v(t) through the contact tube 3 of the welding torch 6. The workpiece 2 to be processed is connected to the welding current source 5, as a result of which an arc 4 is formed when a corresponding welding current I(t) and a corresponding welding voltage U(t) are applied to the welding wire 1 via the contact tube 3. A control device 12 controls the plurality of welding parameters during the welding method.

[0039] According to the invention, specific parameters of the welding method are recorded in a processing device 11 and processed accordingly. These parameters include the wire conveying speed v(t), which is supplied from the wire conveying device 7 to the processing device 11, the welding current I(t), which is determined by a corresponding device 9 for measuring the welding current I(t) and fed to the processing device 11, and the welding voltage U(t), which is determined with an appropriate device 10 for measuring the welding voltage U(t).

[0040] When a welding process P.sub.i is changed, these parameters are then processed accordingly in the processing device 11 and used to control the control device 12. For example, the settings for the lowering phase AP according to the invention are selected from a database 13 according to the welding parameters set by the welding current source 5 and the measured parameters. By introducing the lowering phase AP with the delay time t.sub.d, duration Δt.sub.A and the lowered mean welding current I.sub.mean,A during the lowering phase AP, it is possible to effectively counteract overheating of the stickout when the welding process P.sub.i is changed, so that a stable welding process and an optimum weld quality can be achieved.