GB2365810A

GB2365810A - Reverse polarity dc plasma arc welding

Info

Publication number: GB2365810A
Application number: GB0115118A
Authority: GB
Inventors: Gerhard Schwankhart
Original assignee: Inocon Technologie GmbH
Current assignee: Inocon Technologie GmbH
Priority date: 2000-06-21
Filing date: 2001-06-20
Publication date: 2002-02-27
Anticipated expiration: 2021-06-20
Also published as: FR2810570B1; AT4668U1; DE10129965A1; FR2810570A1; GB2365810B; GB0115118D0

Abstract

A method for welding parts, in particular those made of light metal alloys, by means of a plasma, in which an arc is ignited between a substantially non-consumable electrode and a region on the workpiece to be welded, which are both connected to a direct current power supply, and a plasma gas such as argon, helium or the like is blown into said region. In order to enable the rapid and high-quality welding of workpieces which are difficult to weld, in particular those made of light metal alloys, the power supply unit is arranged as a direct current source with the positive pole being applied to the electrode (9, figure 1), and the negative pole to the workpiece 138. The workpiece 138 is held electrically insulated against electric ground, with the immediate ambience of the end of the rod-shaped electrode (9, figure 1) facing the workpiece 138 being held at the potential of the workpiece 138.

Description

2365810 WELDING METHOD AND APPARATUS The invention relates to a method for

welding.

A rod-shaped electrode may be used for welding light metal and light metal alloys. In order to achieve a 5 high welding speed with deep fusion penetration and narrow weld seams, the rod-shaped electrode is switched as a cathode and the subject is electrically connected to earth, with helium being used as plasma gas. The intention is that very hot plasma is thus 10 produced which evaporates thin oxide layers. However this is not the case in all light metal alloys.

In order to enable the welding of such alloys, welding is performed in such a method of the kind mentioned above with alternating current instead of 15 direct current, with the subject being connected with electric earth. Although a complete elimination of the oxide layers is ensured and a weld joint is enabled which is free from any cavities or holes because the oxide skin is continually torn open, this 20 advantage is offset by the disadvantage of a welding speed which is reduced by approx. 2/3 as compared with a DC/helium welding and a considerable increase of the width of the weld joints with an enlarged heat influence zone.

25 It is the object of preferred embodiments 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 30 are removed.

The present invention provides method and apparatus as defined in claims 1, 5, 8 and/or 9 respectively.

In preferred embodiments of the present invention, the proposed measures ensure that even in the case of alloys which are difficult to weld such as light metal alloys in particular, the formed oxide layers 5 are torn open continually and therefore high-quality weld joints are obtained. The easy striking of the arc and a high stability of the arc spot are ensured by the fixing device of the subject which is insulated from electric ground and the application of 10 the potential of the subject to the ambience of the end of the electrode facing the subject, thus ensuring that the weld seams can be kept narrow and the zone influenced by the heat can be kept small. Moreover, a relatively high welding speed can be 15 achieved in this manner.

Due to the easy striking ability it is also possible to work with a sequence of short plasma pulses, as is ensured by the features of claim 2. The wear and tear of the electrode is thus substantially reduced.

20 In order to arrange a device in accordance with the preamble of claim 3 in such a way that it is suitable for performing the method in accordance with the invention, the characterising features of claim 3 are proposed.

25 In preferred embodiments of the present invention, the proposed measures ensure in a very simple way that the ambience of the end of the electrode facing the subject lies at the potential of the subject, thus ensuring the easy striking capability of the arc 30 between the electrode and the subject. Moreover, the stability of the arc spot on the subject is substantially increased by the fixing device of the subject which is electrically insulated from ground and thus the quality of the welding is improved and 35 the thermal stresses on the subject are simultaneously reduced. At the same time, any impairing influence of electric and electronic devices in the ambient environment is substantially reduced.

5 The features of claim 6 help to ensure that the end of the electrode facing the subject will rapidly assume a spherical shape and even in the case of a new electrode there are no substantial changes to the electrode geometry which negatively influence the 10 welding as is the case in conical electrode ends for example.

The features of claim 7 allow simply producing respectively short voltage pulses for producing a pulse plasma.

15 The invention is now explained in closer detail by reference to the enclosed drawings, wherein:

Fig. 1 shows a sectional view through a plasma torch for a device in accordance with the invention; Fig. 2 shows a sectional view along line II-II in 20 fig. 1; Fig. 3 shows a sectional view along line III-III in fig. 1; Fig. 4 shows a sectional view along line IV-IV in fig. 1; 25 Fig. 5 a partial sectional view through the plasma torch according to fig. 1, Fig. 6 shows a sleeve on which helically shaped ducts are shaped; Fig. 7 schematically shows a circuit diagram of a 30 power supply for a device in accordance with the invention including a connection for a subject to the power supply.

A device in accordance with the invention is provided with a basic body 1 made of an electrically wellconducting material. It is covered at the top by means of a cover which is made of electrically 5 insulating material and is fastened by means of screws 3.

Said basic body 1 is provided with a central.bore 4 which ends in the uppermost zone of the basic body and converges into a radial bore 5 which ends in a 10 connection 5 for a cooling water inlet.

A tube 7 which is provided at its upper end with a flange is inserted in an extension of said central bore 4. In the uppermost zone of said sleeve 7 the same is pressed into a hollow holder 8 for an is electrode 9, which holder 8 is made of an electrically and thermally well-conducting material such as copper. Said holder 8 is connected with the basic body 1 by means of screws 10 which are inserted on the face side into the basic body and is in an 20 electrically well-conducting contact with the same. A seal 11 is inserted into the basic body 1, which seal seals the holder 8.

An annular intermediate element 12 made of an electrically insulating material rests on the basic 25 body 1, which intermediate element is provided with a radial bore 13 which ends on the outside of the intermediate element 12 in a gas connection 14.

An annular chamber 15 remains between the interior wall of the annular intermediate element 12 and the 30 holder 8, which chamber is penetrated by holder 8. The intermediate element 12 rests on the axial shoulders of basic body 1 and a carrier body 16, with seals 17, 18 being inserted into said shoulders.

The intermediate element 12 is connected with the carrier body 16 and the basic body 1 by means of screws 20, 21 which engage in threaded bores of non metallic inserts 28, with the carrier body 16 and the 5 basic body 1 being penetrated by said screws 20, 21.

The inserts 28 are embedded in the intermediate element 12 which is produced of insulating material and therefore do not produce any electrically conducting connection between the basic body 1 and 10 the carrier body 1.

The carrier body 16 is provided with a central axial bore which is penetrated by the holder 8, with an annular gap 19 remaining between the holder 8 and the interior wall of the carrier body 16 through which 15 the gas can flow from the chamber 13 in order to form a plasma.

The carrier 16 is further provided with a chamber 22 which is formed by turning out and into which opens a cooling water outlet 23. An annular overflow body 24 20 is inserted into the chamber 22, which overflow body 24 projects with its two ends into the annular gap 19 between the interior wall of the carrier body 16 and the holder 8 and is sealed up in said annular gap 19 by means of seals 25, 26 which are inserted into 25 grooves 261 of the holder 8 (fig. 6).

The overflow body 24 is provided with a plurality of axially continuous conduits 27 and is provided with two radial bores 29. The zones of said bores 29 are kept free from any conduits 27, so that the gas can 30 flow through the overflow body 24 only outside of the zones of the projections of said bores 29.

The holder 8 is provided in the zone of the radial bores 29 of the overflow body 24 with radial bores 30 which produce a connection between the chamber 22 of the carrier body 16 and an annular gap 31 which remains between an extension of the central bore 32 of the hollow holder 8 and the sleeve 7. An inner thread 33 is provided in the lower zone of the holder 5 8 into which a cap 34 is screwed which holds the electrode 9, is a part of the holder 8 and overlaps the end of sleeve 7 with play.

Apart from the recesses 35 which allow the application of a tool for tightening and loosening 10 the cap 34, said cap 34 is provided with a conical outside surface which extends substantially parallel to the inner wall of a conical nozzle 36.

Moreover, the holder 8 is provided with an enlargement 37 in the zone directly adjacent to its 15 cap 34 into which helically extending conduits 38 are incorporated, as can be seen in particular in figs. 5 and 6.

The conical nozzle 36 is shaped on a sleeve 39 which is screwed into the carrier body 16 and is sealed 20 with a seal 40. The sleeve 39 is provided in the zone of its cylindrical section on the inner side with a coating 41 made of insulating material.

The basic and carrier body 1, 16 are each provided with an electric insulation 42.

25 The basic body 1 and the holder 8 which is in electrically conducting connection, and thus also the electrode 9 which is mostly produced of a tungsten alloy, can be connected to a cooling water connection (not shown) which can be inserted into connector 6 30 and is used simultaneously as an electric connection to a source of electric power. The second pole of the power source is connected to a subject (not shown).

During operation cooling water flows in through connection 6, flows through the bores 5 and 6 and subsequently through sleeve 7 and reaches the cap 34 of holder 8 and is deflected in the same and rises in 5 the annular gap 19 between the sleeve 7 and the inner wall of the holder 8 into chamber 22. From there the heated cooling water reaches a cooling water discharge means (not shown) via connection 23.

The gas required for forming a desired plasma is 10 supplied via the gas connection 14 and reaches chamber 1S via bore 13, which chamber is penetrated by holder 8. The gas flows downwardly from said chamber, with the same flowing through the conduits 27 of the overflow body 24 and reaching the zone of 15 the helically extending conduits 38 which are obtained between the enlargement 37 and the coating 41 of sleeve 39.

when said conduits are flowed through by the gas, the flowing gas is given a respective twist and with this 20 twist it flows through the conical nozzle 36 which is defined by the conical section of the sleeve 39 and the cap 34 of holder 8.

The gas such as argon, helium or the like flows at high speed and with a respective twist from the 25 orifice of said nozzle 36 from which the electrode 9 projects and forms a highly stable gas column which is ionised by an arc burning between the electrode, which is connected with the plus pole of a DC source, and the subject. The gas column then becomes plasma.

30 As a result of the gas column which is highly stable due to the twist, the arc spot produced on the subject which is switched as cathode is held in a stable way and hindered from any continual migration. As a result of the conical shape of the nozzle 36, a considerable constriction of the plasma column is obtained, thus also keeping the arc spot small and producing a high energy density within the same.

A power supply for a plasma torch 100 according to 5 the figs. 1 to 6 is shown in fig. 7, with the power supply 200 being capable of producing both a pulse plasma as well as a flow plasma.

A capacitor battery 130 is connected via a charging resistor 131 with the connections X1 of a 10 controllable direct voltage source 132. The capacitor battery is provided with a fixedly connected capacitor 1C1 and a capacitor 1C2 which can be cut in parallel via a switch 1S1. In both cases groups of capacitors can also be involved.

15 Said capacitor battery 130 is connected via connecting lines 133, 134 with the plasma torch 100 or its electrode 9 (not shown in fig. 7) and the subject 138, with the plus pole lying on electrode 9 and the minus pole on subject 138.

20 An RC-el ement is switched parallel to the capacitor battery 130, which RC-element is formed by a capacitor 1C3 and a resistor 1R1. Said RC-element forms in connection with the throttle 1L1 which is switched in the connecting line 133 an RF blocking 25 circuit which is provided for protecting the capacitor battery 130 and the power supply from RF signals.

Furthermore, an igniter 135 is connected to the connecting lines 133, 134, with the same also being 30 conducted through the igniter 135. Said igniter 135 is connected at the input side with an alternating voltage source X2 and provided with a trigger switch 1S2, through the actuation of which an ignition pulse can be initiated.

The connecting line 134 is connected with the subject 138. The connecting line 133 is connected with the 5 electrode 9. The nozzle 36 is connected via a highresistance resistor 1R2 with the subject 36.

The nozzle 36 of plasma torch 100 lies at the potential of the subject 138 via the resistor 1R2. This leads to the formation of an electric field

10 between the nozzle 36 and the electrode 9 which is switched as an anode and to the ionisation of the gap between said parts of the plasma torch 100, thus facilitating the ignition of an arc between the electrode 9 and the subject 138. As a result of the 15 high-resistance resistor 1R2, there is no formation of an arc between nozzle 36 and electrode 9, because no adequate current flow is allowed to form over this path.

The ionised plasma gas is pushed by the following 20 plasma gas in the zone between the electrode 9, which can project from the face side of the plasma torch 100 facing the subject 138, and the subject 138 and causes a rapid ignition of an arc, thus forming a plasma. The ignition is substantially facilitated by 25 the ionisation of the air between the nozzle 36 and the electrode 9.

If pulse plasma is desired, i.e. only individual short plasma pulses in a more or less rapid sequence are to be produced, e.g. for producing a spot weld 30 seam with very small distances between the individual weld points, the system only works with the capacitor battery. The charging of the capacitor battery 130 occurs according to the set voltage of the direct voltage source 132 which is adjustable between 50V and 300V for example and the time constant as codefined by the capacity of the capacitor battery 130 and the specific resistances and the charging resistor 131.

5 Once the capacitor 130 reaches a voltage which, considering the ionisation of the zone between the nozzle 36 and the subject 138, in which zone the ionised gas cloud forming between the nozzle 36 and the electrode 9 is blown by the flowing plasma gas, corresponds to the arc-over voltage of the eletrode to-subject path 9, 138, the ignition of an arc occurs and thus to the formation of plasma in the zone between the electrode 9 and the subject 138.

At the same time, the capacitor battery 130 is discharges according to the time constant as defined by its capacity and the resistivity and the resistance of the arc. If as a result of this discharge the voltage of the capacitor battery 130 falls below the arc voltage, the arc will extinguish and the capacitor battery 130 will recharge. As a result, the described process repeats itself and a frequency is obtained which is defined by the charging and discharging time constants. The operation of the igniter is not required.

It may be desirable for certain applications to precisely define the ignition time of the arc or to initiate the same prior to reaching the arc-over voltage of the electrode-to-subject path 9, 138 in order to enable the production of particularly short plasma pulses.

In this case an ignition pulse is initiated by actuating the trigger switch 1S2 which leads to a rapid and substantial ionisation of the zone between the electrode 9 and the subject 138 and thus to the ignition of an arc without the capacitor battery 130 having reached a voltage corresponding to the arc over voltage of this path. In this way it is possible to accordingly change the pulse duty factor, which 5 can be chosen between 1:10 and 1:100 and beyond this for example, and the ratio between the arc duration and its interruption during a cycle within the terms of an extension of the interruption because although the energy of the ignition pulses of the igniter 135 10 is sufficient to ignite the arc, it is not sufficient to maintain the same once the voltage of the capacitor battery 130 has dropped below the arc voltage.

For applications in which a very high pulse duty 15 factor or a continuous plasma, a so-called flow plasma, is desired, the power supply is provided with a power supply unit 136 which can be connected to an a.c. mains and is provided with a rectifier circuit.

The connecting line 1331 which is connected to the 20 positive pole of the output of the power supply unit is connected to the connecting line 133, which is connected to the positive pole of the capacitor battery 130, and the electrode 9 of the plasma torch 100. The connecting line 1341 which is connected to 25 the negative pole of the power supply unit 136 is connected to the connecting line 134 which is connected to the negative pole of the capacitor battery 130 and to the workpiece 138 and, via the high-resistance resistor 1R2, with the nozzle 36 of 30 the plasma torch 100.

Furthermore, an automatic current controller 137 is connected to the power supply unit 136.

In operation the power supply unit 136 also supplies current to the plasma torch 100 once an arc has been ignited in the aforementioned manner, with the electric circuit for the power supply unit 136 being closed via the electrode 9 of the plasma torch, the plasma and the workpiece 138 as well as the 5 connecting lines 1331, 133, 1341.

Once the arc extinguishes in the plasma torch 100 due to the decrease of the voltage of the capacitor battery 130 below the arc voltage, the electric current for the power supply unit 136 is also 10 interrupted whose output voltage is insufficient to maintain an arc between the electrode and the workpiece 138.

By making a respective choice of the supply voltage of the capacitor battery 130 and the power supply 15 unit, which is supplied via a variable voltage source such as a variable transformer with a plurality of voltage taps with alternating voltage, a smooth transition of pulse plasma with an adjustable pulse duty factor and a continually burning flow plasma is 20 possible, whereby an arc needs to be ignited only once for a work operation, which arc is supplied continually with sufficient energy by the power supply unit 136 in order to avoid being extinguished.

As is shown further in fig. 7, the workpiece 138 is 25 situated on a base 301 of an electrically wellinsulating material which rests on a grounded carrier. The housing of the welding apparatus 300, which contains the entire power supply unit, is connected to ground.

30 The workpiece is connected with the minus pole of the welding apparatus 300, but not with electric ground. The same electric potential lies at the nozzle 36 of the plasma torch 100 as at the workpiece 138.

Claims

CLAIMS:

1. A method for welding parts, by means of a plasma in which an arc is ignited between a substantially non-consumable electrode and the workpiece to be 5 welded, which are both connected to a power supply, and a plasma gas such as argon, helium or the like is blown into said region, wherein the power supply unit is arranged as a direct current source and the plus pole of the same is applied to 10 the, electrode and the minus pole thereof to the workpiece and the workpiece is held electrically insulated against electric ground, with the immediate ambience of the end of the electrode facing the workpiece being held at the potential 15 of the workpiece.

2. A method according to claim 1 wherein the electrode is rod-shaped.

3. A method according to any preceding claim for 20 welding light metal alloys

4. A method as claimed in any preceding claim, wherein merely voltage pulses are applied to the electrode and the workpiece.

25

5. A device for performing the method according to claim 1 with a plasma torch with a nozzle which can be flowed through by a plasma gas, which nozzle is made from an electrically well conducting material and is disposed concentrically 30 to a rod-shaped non-consumable electrode which is electrically insulated against the nozzle and is connected to a pole of the direct current source, and with a connection which is connected to the second pole of the direct current source for a workpiece to be welded, with an RF igniter being in connection with the rod-shaped electrode and the workpiece, wherein the plus pole of the direct current source is connected with the rod-shaped 5 electrode which is axially aligned to the nozzle and the same is connected by way of a high resistance resistor to the workpiece, with said resistance value being in the range of 10' to 10' ohms, preferably 105 ohms and with the workpiece 10 being insulated from electric ground.

6. A device as claimed in claim 5, wherein the electrode which is connected with a plus pole projects with its free end from the nozzle and is provided with a substantially blunt arrangement.

is

7. A device as claimed in any of claims 5 or 6, wherein a capacitor battery is provided for power supply, which battery is connected with a charging circuit and is connected on the output side with the electrode of the plasma torch and the 20 workpiece, with a power supply unit preferably additionally being provided which is provided with a rectifier circuit and is connected with the analogous poles of the capacitor batteries.

8. A method substantially as hereinbefore described 25 with reference to the accompanying figures.

9. A device substantially as hereinbefore described with reference to the accompanying figures.