METHOD AND DEVICE FOR WELDING BY MEANS OF A NON-CONSUMABLE ELECTRODE

Abstract

The invention relates to a method and a device (1) for welding by means of a non-consumable electrode (2), wherein a welding current (I) alternating in polarity at a welding frequency (f.sub.s) is applied by a current source (3) between the electrode (2) and a workpiece (4) in order to form an arc (5), and the polarity is changed back to the polarity before the polarity change if the voltage (U) is above the voltage threshold value (U.sub.s+, U.sub.s−) and the welding current (I) is below the current threshold value (I.sub.s+, I.sub.s−). According to the invention, the welding voltage (U) and the welding current (I) after a preset duration (Δt) after the polarity change are compared with the voltage threshold value (U.sub.s+, U.sub.s−) and the current threshold value (I.sub.s+, I.sub.s−), and in addition the power (P) in the arc (5) is determined, and the polarity is changed back if the welding voltage (U) is greater than the voltage threshold value (U.sub.s+, U.sub.s−), and/or if the welding current (I) is less than the current threshold value (I.sub.s+, I.sub.s−), and/or the determined power (P) is less than a preset power threshold value (P.sub.s).

Claims

1. A method for welding by means of a non-consumable electrode (2), in particular a tungsten electrode, wherein a welding current (I) alternating in polarity at a welding frequency (f.sub.s) is applied by a current source (3) between the electrode (2) and a workpiece (4) in order to form an arc (5), and after a change in polarity the welding voltage (U) and the welding current (I) between the electrode (2) and the workpiece (4) are measured and the voltage (U) is compared with a preset voltage threshold value (U.sub.s+, U.sub.s−) and the welding current (I) is compared with a preset current threshold value (I.sub.s+, I.sub.s−) and the polarity is changed back to the polarity before the polarity change if the voltage (U) is above the voltage threshold value (U.sub.s+, U.sub.s−) and the welding current (I) is below the current threshold value (I.sub.s+, I.sub.s−), wherein the welding voltage (U) and the welding current (I) after a preset duration (Δt) after the polarity change are compared with the voltage threshold value (U.sub.s+, U.sub.s−) and the current threshold value (I.sub.s+, I.sub.s−), and in addition the power (P) in the arc (5) is determined, and the polarity is changed back to the polarity before the polarity change if the welding voltage (U) is greater than the voltage threshold value (U.sub.s+, U.sub.s−), and/or if the welding current (I) is less than the current threshold value (I.sub.s+, I.sub.s−), and/or the determined power (P) is less than a preset power threshold value (P.sub.s).

2. The method according to claim 1, wherein after each change in the polarity of the welding current (I) from negative polarity (−) to positive polarity (+), the welding voltage (U) and the welding current (I) after the preset duration (Δt) after the polarity change are compared with the voltage threshold value (U.sub.s+) and the current threshold value (I.sub.s+), and in addition the power (P) in the arc (5) is determined, and the polarity is changed back to the negative polarity (−) if the welding voltage (U) is greater than the preset voltage threshold value (U.sub.s+), and/or the welding current (I) is less than the current threshold value (I.sub.s+), and/or the determined power (P) is less than the preset power threshold value (P.sub.s).

3. The method according to claim 1, wherein after each change in the polarity of the welding current (I) from positive polarity (+) to negative polarity (−), the welding voltage (U) and the welding current (I) after the preset duration (Δt) after the polarity change are compared with the voltage threshold value (U.sub.s−) and the current threshold value (I.sub.s−), and in addition the power (P) in the arc (5) is determined, and the polarity is changed back to the positive polarity (+) if the welding voltage (U) is greater than the preset voltage threshold value (U.sub.s−), and/or if the welding current (I) is less than the current threshold value (I.sub.s−), and/or the determined power (P) is less than the preset power threshold value (P.sub.s).

4. The method according to claim 1, wherein a duration (Δt) from 5 μs to 500 ms is preset.

5. The method according to claim 1, wherein the power (P) in the arc (5) is determined from the product of the welding voltage (U) and the welding current (I).

6. The method according to claim 1, wherein the measured power (P) in the arc (5) is compared with a preset power threshold value (P.sub.s) of 3 W to 3000 W.

7. The method according to claim 1, wherein the welding current (I) is changed with a welding frequency (f.sub.s) from 30 Hz to 2000 Hz.

8. The method according to claim 1, wherein the welding current (I) is alternated between a time (t.sub.p) of the positive polarity which is 30% to 40% of the period (T), and a time (t.sub.n) of the negative polarity which is equal to 60% to 70% of the period (T).

9. A device (1) for welding by means of a non-consumable electrode (2), in particular a tungsten electrode, having a current source (3) for applying a welding current (I) alternating in polarity at a welding frequency (f.sub.s) between the electrode (2) and a workpiece (4) to form an arc (5), and having a control device (6) which is designed to measure the welding voltage (U) between the non-consumable electrode (2) and the workpiece (4) and the welding current (I) after the change in polarity and to compare the measured voltage (U) with a preset voltage threshold value (U.sub.s+, U.sub.s−) and the measured welding current (I) with a preset current threshold value (I.sub.s+, I.sub.s−), wherein the control device (6) is designed for carrying out a method for welding by means of the non-consumable electrode (2), in particular the tungsten electrode, wherein the welding current (I) alternating in polarity at the welding frequency (f.sub.s) is applied by the current source (3) between the electrode (2) and the workpiece (4) in order to form the arc (5), and after a change in polarity the welding voltage (U) and the welding current (I) between the electrode (2) and the workpiece (4) are measured and the voltage (U) is compared with a preset voltage threshold value (U.sub.s+, U.sub.s−) and the welding current (I) is compared with the preset current threshold value (I.sub.s+, I.sub.s−) and the polarity is changed back to the polarity before the polarity change if the voltage (U) is above the voltage threshold value (U.sub.s+, U.sub.s−) and the welding current (I) is below the current threshold value (I.sub.s+, I.sub.s−), wherein the welding voltage (U) and the welding current (I) after a preset duration (Δt) after the polarity change are compared with the voltage threshold value (U.sub.s+, U.sub.s−) and the current threshold value (I.sub.s+, I.sub.s−), and in addition the power (P) in the arc (5) is determined, and the polarity is changed back to the polarity before the polarity change if the welding voltage (U) is greater than the voltage threshold value (U.sub.s+, U.sub.s−), and/or if the welding current (I) is less than the current threshold value (I.sub.s+, I.sub.s−), and/or the determined power (P) is less than a preset power threshold value (P.sub.s).

Description

[0021] The present invention will be explained in further detail by reference to the attached drawings. Shown are:

[0022] FIG. 1 a schematic block diagram of a device for welding by means of a non-consumable electrode;

[0023] FIG. 2 the temporal waveforms of the welding voltage, the welding current and the power in the arc during normal operation of a device for welding by means of a non-consumable electrode;

[0024] FIG. 3 the temporal waveforms of the welding voltage, the welding current and the power in the arc at the onset of a polarity reversal according to the invention to the negative polarity;

[0025] FIG. 4 the temporal waveforms of the welding voltage, the welding current and the power in the arc at the onset of a polarity reversal according to the invention to the positive polarity; and

[0026] FIG. 5 a flowchart illustrating the method according to the invention for welding with a non-consumable electrode.

[0027] FIG. 1 shows a schematic block diagram of a device 1 for welding by means of a non-consumable electrode 2, in particular a device 1 for TIG (tungsten inert-gas) welding with a tungsten electrode. A current source 3 is connected to both the non-consumable electrode 2 and the workpiece 4 made of electrically conductive material. The current source 3 applies a welding current I that alternates in polarity with a welding frequency f.sub.s between the non-consumable electrode 2 and the workpiece 4. This causes an arc 5 between the end of the non-consumable electrode 2 and the workpiece 4 to be ignited both in the positive polarity + phase and in the negative polarity − phase. A control device 6, which is usually located in the current source 3, is used to control the timing sequences and control the respective values of the welding current I and the welding voltage U. In addition, the control device 6 is designed to measure the welding voltage U between the non-consumable electrode 2 and the workpiece 4 and to measure the welding current I after the change in polarity.

[0028] FIG. 2 shows the temporal waveforms of the welding voltage U, the welding current I, and the power P in the arc during normal operation of a device 1 for welding by means of a non-consumable electrode 2. After a start phase, the welding current I is applied with negative polarity − with a specific negative welding current. At the end of the time t.sub.n of the negative polarity −, the polarity is changed to the positive polarity +. When the time t.sub.p of the positive polarity has elapsed, the polarity is changed again. In contrast to the practice, here the times t.sub.n of the negative polarity − and the times t.sub.p of the positive polarity are essentially equal in length. The resulting welding voltage U as a function of the time t is shown in the second diagram. In the third time diagram, the power P in the arc 5 is shown as a function of the time t. According to the invention, voltage threshold values U.sub.s− and U.sub.s+ and current threshold values I.sub.s− and I.sub.s+ are defined for the negative polarity − and the positive polarity +, with which the measured values of the welding voltage U and the welding current I are compared, as conditions on whether a polarity reversal should take place. According to the invention, a power threshold value P.sub.s is also introduced, with which the power P in the arc 5, either measured or determined from the welding voltage U and the welding current I, is compared as a further condition on reversing the polarity. FIG. 2 shows the case in which in either case or with either polarity a reignition of the arc 5 occurs and thus no polarity reversal is necessary, but the polarity is always changed to the next polarity at the specified times.

[0029] FIG. 3 shows the temporal waveforms of the welding voltage U, the welding current I and the power P in the arc at the onset of a polarity reversal according to the invention to the negative polarity −. During the change in polarity from the negative polarity − to the positive polarity + the preset duration Δt is allowed to elapse and the conditions

[0030] 1. U>U.sub.s+

[0031] 2. I<I.sub.s+

[0032] 3. P<P.sub.s

[0033] are tested. If at least one of these conditions is met, this is an indication of a failure of the ignition of the arc, and therefore the polarity is changed back to the previous polarity, in the example shown, the negative polarity −. In the exemplary embodiment shown, at the second and third polarity change from negative polarity − to positive polarity + the polarity of the welding current I is changed back again to the negative polarity − each time. After one or two polarity reversals, it is finally possible to break open the oxide layer of the workpiece so that the arc can be reignited. In this case, the polarity is changed to the next polarity again as normal. If it has not been possible to break open the oxide layer, then very long phases of the negative polarity − can result.

[0034] FIG. 4 shows the temporal waveforms of the welding voltage U, the welding current I, and the power P in the arc at the onset of a polarity reversal according to the invention to the positive polarity +. In this case, the preset duration Δt is allowed to elapse when reversing the polarity from positive polarity + to negative polarity − and the conditions

[0035] 1. U>U.sub.s−

[0036] 2. I<I.sub.s−

[0037] 3. P<P.sub.s

[0038] are tested. If at least one of these conditions is met, this is an indication of a failure of the ignition of the arc and the polarity is therefore changed back to the previous polarity, in the example shown, the positive polarity +. In the exemplary embodiment shown, at the first polarity change from positive polarity + to negative polarity − the polarity of the welding current I is changed back again to the positive polarity +. After a polarity reversal, the arc is finally successfully re-ignited during the positive polarity +. Therefore, the polarity is then changed to the next polarity again as normal. If no oxide layer is formed, this can also result in long phases of positive polarity.

[0039] Finally, FIG. 5 shows a flowchart to illustrate the method according to the invention for welding with a non-consumable electrode. At block 100, the method for welding with a non-consumable electrode is started. After the start, a welding current I with negative polarity − is applied (block 101), whereupon the preset duration Δt is allowed to elapse (block 102) before the measured values of the welding current I are compared with the preset current threshold value I.sub.s− (block 103). If the welding current I is less than this preset current threshold value I.sub.s−, the polarity is changed back to the positive polarity + by jumping to block 107. If the welding current I is greater than or equal to the preset current threshold value I.sub.s−, processing continues with query 104. Here, the welding voltage U is compared with the preset voltage threshold value U.sub.s−. If the welding voltage U is greater than this preset voltage threshold U.sub.s−, the polarity is changed back to the positive polarity + by jumping to block 107. If the welding voltage U is less than or equal to the preset voltage threshold value U.sub.s−, processing continues with query 105. Here, for example, the power P is determined from the welding voltage U and the welding current I and compared with the preset power threshold value P.sub.s. If the power P is less than this preset power threshold value P.sub.s, the polarity is changed back to the positive polarity + by jumping to block 107. If the power P is greater than or equal to the preset power threshold value P.sub.s, the time t.sub.n of the negative polarity − is allowed to elapse (block 106) and then the polarity is changed to the positive polarity + (block 107).

[0040] After the polarity change to the positive polarity +, the preset duration Δt (which can theoretically differ from the preset duration Δt when changing to the negative polarity − according to query 102) is allowed to elapse (block 108) before the measured values of the welding current I are compared with the preset current threshold value I.sub.s+ (block 109). If the welding current I is less than this preset current threshold value I.sub.s+, the polarity is changed back to the negative polarity − by jumping to block 101. If the welding current I is greater than or equal to the preset current threshold value I.sub.s+, processing continues with query 110. Here, the welding voltage U is compared with the preset voltage threshold value U.sub.s+. If the welding voltage U is greater than this preset voltage threshold U.sub.s+, the polarity is changed back to the negative polarity − by jumping to block 101. If the welding voltage U is less than or equal to the preset voltage threshold value U.sub.s+, processing continues with query 111. Here the power P is compared with the preset power threshold value P.sub.s. If the power P is less than this preset power threshold value P.sub.s, the polarity is changed back to the negative polarity − by jumping to block 101. If the power P is greater than or equal to the preset power threshold value P.sub.s, the time t.sub.p of the negative polarity − is allowed to elapse (block 112) and then processing proceeds to the query to determine whether the method should be continued (block 113). If appropriate, the polarity is changed to the negative polarity − by returning to block 101. If the method is to be terminated according to query 113, the sequence is terminated (block 114).

[0041] The flowchart according to FIG. 5 is intended to illustrate a variant of the method according to the invention. Of course, the comparisons of the measured values with the threshold values (blocks 103, 104, 105 or 109, 110, 111) can be carried out in any order. The flow diagram illustrated shows the comparisons of the measured values with the threshold values in the form of logical “Or” operations. Other operations for the comparisons according to the blocks 103, 104, 105 or 109, 110, 111 in the form of logical “And” or “And/Or” operations would also be possible.