METHOD AND APPARATUS FOR ARC WELDING WITH MELTING ELECTRODE

Abstract: The invention comprises a method and an arrangement for pulsed gas metal-arc welding with an electrode fed to the welding site continuously at a given rate, the fusion conditions being adapted in such a manner that the material transfer from the electrode (7) to the workpiece (8) takes place essentially without short-circuiting drops by the welding current being periodically increased to a pulse current of such size and duration that the current density in the electrode causes sufficient electromagnetic forces in order to separate one drop for each pulse, and where the arc length is stabilized by regulating the power supply for the arc by means of a regulator mainly on the basis of a measured arc voltage, characterized in that short-circuiting of the arc is detected as arc-voltage decreases and/or current increases, and in that the fusion power supplied is increased by the arc-length regulator being given an additional increase for each short-circuit detected.

Description: METHOD AND APPARATUS FOR ARC WELDING WITH MELTING ELECTRODE Technical field The present invention relates to a method and an arrangement for correcting, during pulsed gas metal-arc welding with a continuously fed electrode and material transfer from the electrode to the workpiece essentially in the form of non- short -circuiting drops, the power supply for the welding process on the basis of the occurrence of brief short circuits between the electrode and the workpiece. The correction is carried out in a regulating process in which the power supply is in the main regulated depending on the arc voltage so that a constant arc length is obtained. The correction facilitates welding using a very short arc. Background In gas metal-arc welding using a continuously fed fusible electrode, usually called MIG/MAG welding, the workpiece is heated principally by the arc. The electrode is heated on the one hand by the power generated when the welding current flows through the electrode projection, that is to say the free electrode end between the contact nozzle, where the current transfer to the electrode takes place, and the arc, and on the other hand by the arc itself. Basic regulation of the welding process is aimed at obtaining a fusion rate of the electrode that corresponds to the electrode feed rate. Further aims of regulation may be, for example, to influence the quantity of heat supplied to the workpiece. MIG/MAG welding is carried out in one of three states. In the case of short-arc welding, the material transfer from the electrode to the workpiece takes place by means of large short-circuiting drops, as shown in principle in Fig. 2. By virtue of the fact that the process consists of alternating arcs and short-circuiting drop transfers, the average voltage between the electrode and the workpiece is low and the heat transfer to the basic material is thus moderate. When the power supplied is increased, the mixed-arc range is entered, in which the material transfer is carried out by means of a mixture of short-circuiting and non-short-circuiting drops . The result is an unstable arc with a great deal of welding splash and welding smoke. Welding in this range is usually avoided. When the power supply is sufficiently great, the process enters the spray range, in which the material transfer is carried out by small finely- distributed drops without short-circuits, as shown in Fig. 3. The quantity of splash is clearly lower than in the case of short-arc welding. The heat supply to the basic material is greater in this case and the method is suitable principally for thicker material to be welded. The third state is called pulsed welding and involves using advanced regulation to control the separation itself of the drops with the aid of a suitable current pulse. Each pulse separates a drop and the drops are sufficiently small so as not to short-circuit. This method, which is often called synergic pulsing, affords advantages from the spraying range in the form of low welding splash without the disadvantages of the great heat transfer. So as to obtain stable conditions with uniform welding results even in the case of varying gun-guidance, the arc length is regulated. A method of doing this is described in EP 387 223. In this known method, the arc voltage is measured and compared with a reference value, and the current for the welding process is regulated on the basis of the calculated voltage difference so that the fusion rate increases when the arc voltage is lower than the reference value and decreases in the opposite case . In the welding of thin plate, a low heat supply to the basic material and thus a short arc length are required in order to reduce the risk of burning through. At the same time, freedom from splashing is as always highly desirable. Pulsed welding gives freedom from splashing but greater heat than short-arc welding. It is therefore desirable to weld using the shortest possible arc length without the material transfer taking place via short-circuiting drops which explode easily and thus cause splashing. Another reason for keeping the arc short is that a long arc destroys the gas protection in the case of, for example, aluminium welding. EP 0 170 248 describes a welding machine for pulsed MIG/MAG welding, in which the equipment is regulated depending on the difference between a preset desired value for the number of drop short-circuits per unit of time and the actual number of short-circuits taking place. This regulating method gives a short arc and thus low heat transfer to the workpiece. One disadvantage, however, is that it involves completed drop short-circuits with the attendant risk of welding splash. A further disadvantage is that, with this method, no arc- length regulation is possible if it is desired to weld using a longer arc and thus a greater heat supply to the workpiece takes place. A further problem with pulsed welding is the requirement for very accurate parameter settings. This may cause great problems when access is poor and it is necessary to turn the welding gun during welding in confined spaces. It is then very difficult to maintain a constant height above the workpiece. In this case, it is important to use a regulating method that can handle great deviations. The object of the invention The object of the present invention is thus to make available a method and an arrangement for pulsed welding that completely or partly eliminate the problems of the known art . The invention comprises a method and an arrangement for pulsed gas metal-arc welding with an electrode fed to the welding site continuously at a given rate, the fusion conditions being adapted in such a manner that the material transfer from the electrode to the workpiece takes place essentially without short- circuiting drops by the welding current being periodically increased to a pulse current of such size and duration that the current density in the electrode causes sufficient electromagnetic forces in order to separate one drop for each pulse, and where the arc length is stabilized by regulating the power supply for the arc by means of a regulator mainly on the basis of a measured arc voltage, characterized in that short - circuiting of the arc is detected as arc-voltage decreases and/or current increases, and in that the fusion power supplied is increased by the arc-length regulator being given an additional increase for each short-circuit detected. As a result of the arc pressure, the molten pool is always in motion. When the arc length is shortened, the electrode end will briefly come into contact with the moving molten pool. These brief contacts do not normally lead to any material transfer, however, as the drop is retained on the electrode by forces directed towards the electrode. These repellant forces may, for example, be connected with the vaporization of principally oxidized alloying materials from the surface of the fusible electrode end. By detecting these brief short-circuits and, on the basis of their occurrence, increasing the power for the arc, the fusion rate and thus the arc length are increased. In this way, the procedure and the arrangement according to the invention make it possible to weld with minimum arc length without ending up in the short-arc range. The method facilitates considerably synergic pulsing with a very short arc and thus makes possible virtually splash-free welding, even of thin plate. The method also offers considerably reduced sensitivity for parameter-setting accuracy and thus makes possible welding in situations that are very difficult to deal with . Further advantageous features of the invention are indicated in the subclaims. Brief description of the drawings The invention will now be described in greater detail with the aid of exemplary embodiments and with reference to the attached drawings, in which: Fig. 1 shows diagrammatically an arrangement for MIG/MAG welding; Fig. 2 shows how the current and the voltage change when a drop is transferred between the welding electrode and the workpiece in the case of short-arc welding; Fig. 3 shows a cross-sectional view of the lower part of the welding nozzle and a workpiece in the case of welding in the spray range; Fig. 4 shows a basic diagram for pulsed welding; and Fig. 5 shows a flow diagram for regulation according to the invention. Description of preferred embodiments Fig. 1 shows an arrangement for MIG/MAG welding. This arrangement comprises a pulsed welding-current source 1 and a wire-feed mechanism 2. The arrangement also comprises a welding gun and a gas container 4 connected thereto. The welding gun comprises a front nozzle comprising an outer pipe 5 through which the gas is guided out, and, arranged centrally in this pipe, an inner pipe, the contact nozzle 6, through which the electrode 7 is guided. The electrode 7 and the workpiece 8 are connected in a conventional manner to the welding- current source 1 so that a potential difference is formed between them. The arrangement is controlled so that the material transfer from the electrode to the workpiece takes place essentially without short-circuiting drops (Fig. 4) . This is effected by the welding current being periodically increased to a pulse current of such size and duration that the current density in the electrode causes sufficient electromagnetic forces in order to separate one drop for each pulse. In Fig. 4, the principle of pulsed welding is shown, in which the current strength increases in pulses, which leads to separation of drops from the electrode end. Current level 1 corresponds in this connection to the peak value of the pulses, current level 2 to the average current level and current level 3 to a background current level . The arrangement also comprises a regulator according to Fig. 5 for stabilizing the arc length so that the fusion rate is adapted to the actual electrode feed rate. This is carried out mainly by the power supply being controlled on the basis of measured arc voltage. Fig. 5 shows how regulation by the regulator according to the invention takes place. First, the regulator reads a pulse-duration timer so as to start together with a new period of the pulse cycles. As a result of the arc pressure, the molten pool is always in motion. When the arc length is shortened, the electrode end will briefly come into contact with the moving molten pool . These brief contacts do not normally lead to any material transfer, however, as the drop is retained on the electrode by forces directed towards the electrode. These repellant forces may, for example, be connected with the vaporization of principally oxidized alloying materials from the surface of the fusible electrode end. These short-circuits are detected by identifying arc- voltage decreases and/or current increases, and the regulator is given an additional increase for each short- circuit detected. Then, regulator calculations are carried out according to: t λ = t. + t b • k. (ΔU - k k ) Δt b = t b • ΔU • k a + t, t- b _ t bnom + Δt b i b = m/t b where : t. = integration sum ΔU = error signal in arc length regulation k a , k. = amplification factors for the proportional and, respectively, integrating part in the arc-length regulation t b = background time i b = background current t bnom = nominal background time m = constant for current/time area during background time k k is set to a positive value if there has been a short- circuit during the previous period of time, otherwise to zero. Finally, i b is interpreted as current reference and a background-time timer is set to t b . In this way, the fusion power supplied is thus increased on detection of a short-circuit, and the integrating part of the arc- length regulator increases the fusion power as long as short-circuits occur. The power supply is influenced by regulating the size and duration of the background current. Then the regulating process is repeated. Preferably, only short-circuits shorter than 500 μs generate an additional increase for the regulator as it is principally short-circuits of short duration that do not involve material transfer that are relevant in regulation. By detecting brief short-circuits and, on the basis of their occurrence, increasing the power for the arc, the fusion rate and thus the arc length are increased. In this way, the procedure and the arrangement according to the invention make it possible to weld with minimum arc length without ending up in the short-arc range, as the regulating system according to the invention reacts before the actual short-circuit that brings about material transfer from the electrode to the workpiece. The invention therefore results in a considerable improvement on the previously known art, in particular as far as welding with short arc lengths is concerned, and with regard to the tolerance range for setting parameters. The arc-length regulator described can of course be realized in both software and hardware and in digital and/or analogue form.

Claims: PATENT CLAIMS 1. Method for pulsed gas metal-arc welding with an electrode fed to the welding site continuously at a given rate, the fusion conditions being adapted in such a manner that the material transfer from the electrode to the workpiece takes place essentially without short- circuiting drops by the welding current being periodically increased to a pulse current of such size and duration that the current density in the electrode causes sufficient electromagnetic forces in order to separate one drop for each pulse, and where the arc length is stabilized by regulating the power supply for the arc by means of a regulator mainly on the basis of a measured arc voltage, characterized in that short- circuiting of the arc is detected as arc-voltage decreases and/or current increases, and in that the fusion power supplied is increased by the arc-length regulator being given an additional increase for each short-circuit detected. 2. Method according to Claim 1, characterized in that an integrating part of the arc-length regulator increases the fusion power as long as short-circuits occur. 3. Method according to Claim 2, characterized in that only short-circuits shorter than 500 μs generate an additional increase. 4. Method according to any one of Claims 1 - 3 , characterized in that the power supply is influenced by regulating the value and time of the background current. 5. Arrangement for pulsed gas metal-arc welding comprising a welding-power unit (1) , a wire-feed mechanism (2) , adapted for in the main non-short- circuiting material transfer from the electrode (7) to the workpiece (8) by periodically increasing the welding current to a pulse current of such size and duration that the current density in the electrode causes sufficient electromagnetic forces in order to separate one drop for each pulse, and a regulator which regulates the power supply for the arc for stabilizing the arc length mainly on the basis of a measured arc voltage, characterized in that the regulator is also adapted to detect arc-voltage decreases and/or current increases indicating short- circuits of the arc, and in that the power regulator gives an additional increase of the welding power for each short-circuit detected. 6. Arrangement according to Claim 5, characterized in that the regulator comprises an integrating part adapted to increase the welding power as long as short - circuits occur. 7. Arrangement according to Claim 6, characterized in that the regulator is adapted to generate an additional increase only for short-circuits shorter than 500 μs. 8. Arrangement according to any one of Claims 5 - 7, characterized in that the regulator influences the power supply by means of the value and time of the background current .

Inventors: AABERG PER, BENGTSSON LARS, AABERG, PER, BENGTSSON, LARS
Applicants: AABERG PER, BENGTSSON LARS, ESAB AB, AABERG, PER, BENGTSSON, LARS
Application year: 1998
Main classification: B23K9
IPC codes: B23K9