Device with a plasma torch

Abstract

A device with a plasma torch with a rod-shaped non-consumable electrode (3, 4) which is held in a receiver (1), is connected with an electric connection and penetrates a nozzle (9) which is in connection with a gas connection. In order to also enable the rapid and secure welding of difficult alloys it is provided that a further rod-shaped non-consumable electrode (3, 4) is disposed in the receiver (1) which is made from an electrically non-conducting material, which further electrode also penetrates a nozzle (9′) which is in connection with a gas connection, with the two electrodes (3, 4) enclosing an acute angle and each being in connection with a separate voltage source (31, 32) supplying direct voltage pulses, the level of which exceeds at least the arc voltage of an arc between one of the electrodes (3, 4) and a counterelectrode which is associated with the same and connected with the same voltage source (31, 32).

Description

[0001] The invention relates to a device according to the preamble of claim 1.

[0002] A device of the kind mentioned above with merely one rod-shaped electrode is used for the welding of light metal and light metal alloys. In order to achieve a high welding speed at deep fusion penetration and narrow seams, the rod-shaped electrode is switched as a cathode and helium is used as a plasma gas. A very hot plasma is obtained which evaporates thin oxide layers. This is not the case in all light metal alloys, however.

[0003] In order to also enable the welding of such alloys, welding is performed with direct current instead of alternating current, or the electrode is applied to the plus pole of the voltage source. Although a continual removal of the oxide layers is ensured and a welded joint is ensured which is free from cavities because the oxide skin is continuously torn open, this advantage is offset by the disadvantage of a welding speed which is reduced by approximately two-thirds as compared with a d.c. helium welding and a considerable increase of the width of the weld seams with an increased heat influence zone.

[0004] It is the object of the present invention to avoid such disadvantages and to provide a method of the kind mentioned above which allows a high welding speed also in difficult alloys and which also ensures that any arising oxide layers are removed.

[0005] This is achieved by a method of the kind mentioned above by the characterizing features of claim 1.

[0006] As a result of the proposed measures it is possible to connect the two rod-shaped electrodes with different poles of the voltage sources. As a result, plasma pulses which are produced with an electrode connected to the plus pole of a voltage source can be used to tear open the oxide layers and with the subsequent plasma pulses which are connected with the minus pole of a voltage source and are therefore produced by an electrode switched as a cathode it is possible to weld the basic material in a clean manner and with a high penetration depth, with very narrow and smooth weld seams being obtained. By locking the switching devices which each only allow voltage pulses of approx. 1 to 5 milliseconds, it is ensured that only one electrode can be charged.

[0007] The workpiece to be worked can appropriately be switched as the counterelectrode. It is also possible to make the nozzle or the respective nozzle body from an electrically conductive material and to switch the same as a counterelectrode.

[0008] In the case of alloys that can be welded more easily, both electrodes can also be switched as cathodes. This leads to the advantage that the required welding energy can be divided among both electrodes and they can therefore be provided with a thinner arrangement. This allows the production of very narrow receiving means of 9 mm width for example. With such devices it is therefore also possible to weld in corner zones of workpieces which are difficult to access, which substantially facilitates the constructional design of such pieces.

[0009] As a result of the two separate voltage sources they can also be controlled with respect to the pulse length and pulse power, thus enabling a highly substantial adaptation to the respective requirements.

[0010] The ignition of each plasma arc can be made by means of a high-frequency pulse when the level of the voltage of the individual voltage pulses does not exceed the respective breakdown voltage of the path between the electrode and the respective counterelectrode. The ignition can also be initiated per se by respectively high voltage pulses which exceed the respective breakdown voltage.

[0011] Very favorable conditions for the welding of very difficult alloys are obtained by the features of claim 3. It has also proven to be advantageous to also provide the features of claim 2, with the electrode switched as a cathode preferably standing perpendicularly to the workpiece.

[0012] The features of claim 4 allow keeping the wear and tear of the electrode, which is connected with the plus pole and is subject to higher stresses, at a very low level.

[0013] The features of claim 5 provide a simple arrangement of the receiving means. It must be ensured however that the higher stressed electrode is cooled sufficiently well.

[0014] The measures according to claim 6 allow achieving an ionization of the plasma gas flowing from the nozzle in the zone between the electrode and the nozzle as a result of a high-frequency arc-over and, as a result, the ignition of an arc between the electrode and the workpiece as a result of the applied direct voltage. This leads to a substantial protection of the plasma torch, because the same is not encumbered by the otherwise common pilot arc.

[0015] As a result of the ionization by the high-frequency arc-over, which imposes only very low thermal stresses on the nozzle, it is also possible when using helium as a plasma gas to easily ignite over larger distances between the electrode and workpiece of 10 mm for example.

[0016] The use of a nozzle which is made from an electrically well-conducting material and its connection via a high-resistance electric resistor with the pole of the voltage source which is connected with the workpiece is also of advantage in devices in accordance with the invention in which the plus pole of the voltage source is connected with the electrode penetrating the nozzle.

[0017] The features of claim 8 allow using the device in a very universal manner.

[0018] The features of claim 9 and 11 produce a constriction of the plasma and avoid a divergence of the same due to the friction of the plasma in the air which emerges with a high speed, so that a very high concentration of the energy is achieved.

[0019] The invention is now explained in closer detail by reference to the enclosed drawing, wherein:

[0020] FIG. 1 shows a sectional view through a first embodiment of the device in accordance with the invention;

[0021] FIG. 2 shows a cross-sectional view through the device according to FIG. 1;

[0022] FIG. 3 shows a sectional view through a second embodiment of the device in accordance with the invention;

[0023] FIG. 4 shows a top view of the device according to FIG. 3;

[0024] FIGS. 5 and 6 show a device according to FIGS. 3 and 4 with a power supply unit, shown in a partial sectional view, in a projection and top view;

[0025] FIG. 7 shows a detail of the nozzle area;

[0026] FIG. 8 schematically shows the electric power supply of the device;

[0027] FIG. 9 shows a diagram of the progress over time of the voltage charging of the electrodes of a device in accordance with the invention;

[0028] FIG. 10 shows a variant of the embodiment according to FIGS. 1 and 2 in a sectional view.

[0029] A receiver 1 is provided in the embodiment according to FIGS. 1 and 2, which receiver is made from an electrically insulating material. Two holding devices 2 are inserted in said receiver 1, at the end of which there are two electrodes 3, 4 made of a thermally stable material such as tungsten for example.

[0030] The holding devices 2 are made of an electrically well-conducting material and are provided with a central bore 5 which are connected in the upper and lower range via radial bores 6 with chambers 7, 8, of which the chambers 7 are each connected with a gas conduit 109, 109′ through which plasma gas can be supplied separately, and the chambers 9 are each connected with an ejection nozzle 9, 9′.

[0031] Said nozzles 9, 9′ are provided with conical inner walls, with the inner wall of nozzle 9 extending substantially parallel to the conical end zone of electrode 3, whereby the free end of the electrode 3 can be flattened. The electrode 5 is provided with a substantially blunt arrangement in contrast to electrode 3.

[0032] Furthermore, a cooling conduit 10 is further provided in the receiver 1, which conduit leads from an inlet 11 to an annular chamber 12 which is penetrated by holding device 2 of the electrode 4, and from the same, divided into two branch conduits (FIG. 2), to a further annular chamber 13 which is penetrated by the holding device 2 of the electrode 3 and from the same to an outlet 14.

[0033] The electric connection of the two electrodes 3, 4 or their holding devices 2 can be provided through screw caps 15, or if the gas conduits 109, 109′ are provided with electrically conducting walls, via the same. In the latter case the connection can be made through connecting nipples through which gas is supplied.

[0034] In the embodiment according to FIGS. 1 and 2, a tubular guide means 16 is provided between the nozzles 9, 9′, which guide means is provided for guiding a wire used as an additional material. The guide means 16 is offset.

[0035] As can be seen from FIG. 2, the receiver 1 can be provided with a very narrow arrangement.

[0036] In the embodiment according to FIGS. 1 and 2, the electrode 3 extends in the position for use of the receiver 1 in a substantially vertical manner and the electrode 4 encloses with the same an acute angle which can usually be 20° to 70°.

[0037] Two similar electrodes 3 are provided in the embodiment according to FIGS. 3 and 4, which electrodes both enclose an angle with the perpendicular.

[0038] As is shown in FIG. 5, the receiver 1 is provided with flange-like projections 16 which are penetrated by screws 17 with which the receiver 1 can be fastened to a connecting head 18, with the screws 17 engaging in threaded bores 19 of the connecting head 18.

[0039] Spring-biased connecting nipples 30 are held axially displaceable in said connecting head 18, to which a water supply line 21 and a water discharge line 22 for supplying and discharging cooling water are connected, with said spring-biased connecting nipples 20 engaging, when the receiver 1 is closed, in the inlet and outlet 11, 14 of the same. Fixed connecting nipples 23 are further provided in said connecting head 18 to which gas lines 24 are connected which convey helium for example. The fixed connecting nipples 23 engage in the inlets 25 of the gas conduits 109, 109′ when the receiver 1 is closed. O-rings are used for sealing in the inlets 25, as in the inlet and outlet 11, 14.

[0040] Furthermore, a pin 26 which is arranged off-center is held in the connecting head 18, which pin engages in a respective bore 27 of a receiver 1. This ensures that a connection of a receiver 1 to the connecting head is only possible in a certain position in which the correct flow of the gas and cooling conduits is given.

[0041] Receivers 1 which are equipped with different electrodes 3, 4 can be connected to the connecting head 18. Such an exchange can be performed very simply.

[0042] FIG. 7 shows a detail of the nozzle body 9 for an electrode 3 which is provided with a conical or substantially tapered end. The inner wall 27 of the nozzle body 9 extends substantially parallel to the conical end of electrode 3. This measure ensures that the plasma gas emerges directed in an inclined manner against the axis of nozzle 9, therefore counteracting the tendency of the emerging plasma to diverge with increasing distance from the orifice of nozzle 9 owing to friction in the ambient air. As a result, only a small arc spot is obtained in a desired manner on the workpiece to be processed.

[0043] Cold gas conduits 29 are provided in the nozzle body 9 and enclose its conical nozzle bore 28. Said conduits are evenly distributed concentrically about the nozzle bore 28. The axes of said cold gas conduits 29, most of which are provided in odd numbers such as 3, 5 or 7, form a generatrix of a conical surface whose axis lies concentrically to the axis of the nozzle bore 28. Said cold gas conduits are open towards the chamber 8 and open at the face side of the nozzle body 9.

[0044] The plasma gas flowing through these cold gas conduits produces a cooling of the nozzle body 9 on the one hand and a further constriction of the plasma emerging from the nozzle 9 on the other hand and thus a reduction of the arc spot and thus an increase in the energy concentration in the same. The supply of the chamber 8 with plasma gas is performed through a gas conduit 109, 109′, the upper radial bores 6 of the holder 2, its central bore 5 and the lower radial bores 6.

[0045] FIG. 8 schematically shows the connection of the device in accordance with the invention. The electrodes 3, 4 are each connected with a pole of a voltage source 31, 32 each whose respective second pole is connected via a switching device 33, 34 each to a workpiece 30.

[0046] The two switching devices 33, 34 are mutually locked, so that only one switching device 33 or 34 each can be switched through. Only short switch-through times are provided for the two switching devices 33, 34, so that the electrodes 3, 4 can only be charged in pulses.

[0047] For numerous applications the electrode 3, which is disposed at the back as seen in the welding direction, is switched as a cathode and connected to the minus pole of the voltage source 32.

[0048] Typical values are a current application of approx. 170 A for a time of approx. 15 ms each and a break of approx. 3 ms. During this time the switching device 33 switches through and the electrode 4 connecting the plus pole of the current source 31 is charged with approx. 250 A for approx. 3 ms.

[0049] With such a mode of operation it is also possible to favorably and rapidly weld even alloys which are difficult to weld, because by charging the electrode 4 the plasma pulses thus produced will securely tear open any oxide skins and the basic material can be welded very favorably with the subsequent plasma pulses which are produced by charging the electrode 3.

[0050] For certain applications it is also possible to insert two electrodes 3 in the receiver 1 and to connect both with the minus pole of one direct voltage source 32 each and to charge the same substantially alternatingly. Overlap periods can also be provided, however. Since in this way the stress on each electrode 3 is respectively low, electrodes 3 with a small diameter can be used, thus enabling the construction of the receiver with a narrow design.

[0051] The embodiment according to FIG. 10 differs from the one of FIGS. 1 and 2 in such a way that helical ribs 35 are arranged in the chambers 8 which are in connection with the gas connections through the gas conduits 109, 109′, with helically extending conduits remaining between said helical ribs through which the plasma gas flows to the nozzles 9, 9′.

[0052] The same is subjected to a twist which leads to a stabilization of the plasma emerging at a high speed from the nozzles 9, 9′, thus substantially preventing any divergence of the plasma due to friction in the substantially static air and thus leading to a very small arc spot with high energy density on the workpiece 30 to be processed.

Claims

1. A device with a plasma torch with a rod-shaped non-consumable electrode (3) which is held in a receiver (1), is connected with an electric connection and penetrates a nozzle (9) which is in connection with a gas connection, characterized in that a further rod-shaped non-consumable electrode (3, 4) is disposed in the receiver (1) which is made from an electrically non-conducting material, which further electrode also penetrates a nozzle (9′) which is in connection with a gas connection, with both electrodes (3, 4) enclosing an acute angle and each being in connection with a separate voltage source (31, 32) supplying direct voltage pulses, the level of which exceeds at least the arc voltage of an arc between one of the electrodes (3, 4) and a counterelectrode associated with the same and connected with the same voltage source (31, 32), with the voltage supply being performed at different polarization of the two electrodes (3, 4) via mutually locked switching devices (S1, S2) which prevent any simultaneous voltage charging of the two rod-shaped electrodes (3, 4).

2. A device as claimed in claim 1, characterized in that the two rod-shaped electrodes (3, 4) enclose an angle of 20° to 70°, preferably 30°.

3. A device as claimed in claim 1 or 2, characterized in that one pole each of each voltage source (31, 32) can be connected to a workpiece (30) to be processed and the two rod-shaped electrodes (3, 4) are connected to different poles of the two voltage sources (31, 32), with the electrode (4) which is connected to a plus pole being disposed at the front as seen in the direction of welding.

4. A device as claimed in one of the claims 1 to 3, characterized in that the electrode (4) connected to a plus pole is provided with a substantially blunt arrangement in the zone of its free end projecting from the nozzle (9′), whereas the electrode (3) connected with a minus pole being provided with a substantially conical arrangement in the zone of its free end.

5. A device as claimed in one of the claims 1 to 4, characterized in that a cooling conduit (10) is provided in the receiver (1), which conduit mutually connects chambers (8) with one another which are arranged in the receiver (1) and are penetrated by the rod-shaped electrodes (3, 4) or their holders (2) and with an inlet (11) and an outlet (14), with the chamber (8) which is penetrated by the electrode (4) which is connected to the plus pole being connected to the inlet (11).

6. A device as claimed in one of the claims 1 to 5, characterized in that at least the nozzle (9) which is penetrated by an electrode (3) which is connected to the minus pole of the respective voltage source (32) is made of an electrically well-conducting material and is connected via a high-resistance electric resistor (1R2) in the range of 103 to 106 ohms, preferably 105 ohms, with the pole of the voltage source (32) which is connected with the workpiece (30).

7. A device as claimed in one of the claims 1 to 5, characterized in that at least the nozzle (9) which is penetrated by an electrode (3) which is connected to the plus pole of the respective voltage source (32) is made from an electrically well-conducting material and is connected via a high-resistance electric resistor (1R2) in the range of 103 to 106 ohms, preferably 105 ohms, with the pole of the voltage source (32) which is connected with the workpiece (30).

8. A device as claimed in one of the claims 1 to 7, characterized in that a guide means (16) for a wire used as additional material is provided in the receiver (1) between the nozzles (9, 9′) penetrated by the rod-shaped electrodes (3, 4).

9. A device as claimed in one of the claims 1 to 8, characterized in that the electrode (3) connected with a minus pole of a voltage source (32) penetrates a conical nozzle (9) with its conical free end zone, with the conical surfaces of the nozzle (9) and the electrode (3) extending substantially parallel with respect to one another and being provided substantially with a cone angle of approx. 20°.

10. A device as claimed in one of the claims 1 to 9, characterized in that the electrode (4) connected with a plus pole of a voltage source (31) penetrates a substantially cylindrical nozzle (9′) and a substantially constant annular gap remains between its inner wall and the electrode (4).

11. A device as claimed in one of the claims 1 to 10, characterized in that the bodies of the nozzles (9) are provided with cold gas conduits (29) which are in connection with the respective gas connection and are arranged concentrically to the axis of the nozzle bore (2) and evenly distributed about the same and are open at the free face side of the nozzle (9), with the axes of said cold gas conduits (29) forming generatrices of a conical surface whose tip is disposed before the free end of the electrode (3).

12. A device as claimed in one of the claims 1 to 10, characterized in that helically extending ribs (30) which define helical conduits are arranged in a chamber (8) which is in connection with a gas connection and is penetrated by an electrode (3, 4) or its holder (2).