GB2115993A - Initiation of welding arcs - Google Patents

Abstract

Inert gas arc welding apparatus for initiating and maintaining an arc between a workpiece 20 and an electrode 5 without reducing the arc gap 23 includes a primary arc current source 18 connected across the arc gap 23 a high impedance low powder dc source, preferably a piezoelectric spark generator 6, connected across the arc gap for providing a high voltage spark to initiate an arc, and means to provide a supplementary capacitive discharge across the arc gap substantially simultaneously with the high voltage spark in order to maintain the arc for a time sufficient to allow an adequate current build-up from the primary arc current source 18. At least one spark gap 22 is provided in sensor with the piezoelectric device 6 and the arc gap 23, the gap 22 being provided in one of the leads 15, 16 from the device 6 or between the workpiece 20 and a nozzle (17), (Figure 6b). The supplementary discharge may be produced by discharge of a capacitor 25 via a damping resistor 29, a second piezoelectric device 30 being used to charge the capacitor 25. The capacitor 25 and piezoelectric device 6 are located close to the arc gap 23. Alternatively the supplementary discharge may be produced by using a second piezoelectric device (35), (Figure 11b), to charge the welding cables 27, 28, to a voltage greater than the open circuit voltage of the primary source 18. <IMAGE>

Description

SPECIFICATION Initiation of welding arcs This invention relates to initiation of welding arcs across an open gap and is particularly applicable to the initiation of a tungsten arc operating in an inert gas, argon or helium, or mixtures thereof, or in substantially inert gases as, for example, argon 5% hydrogen.

The most common method of initiating welding arcs (apart from starting by short-circuit and withdrawing the electrode) is to apply high voltage, high frequency, sparks to the welding circuit. Typically the HF spark is several kV in magnitude with fundamental frequencies between 0.2 and 2MHz, and the sparking is semi-continuous with streams of sparks occurring at rates up to 1,000/sec. Since the high frequency spark is applied via the main welding cables, the source energy required is high in order to provide sufficient voltage at the arc gap. This gives rise to extensive radio interference, particularly as arc starting by high frequency spark is, in this context, not efficient and sparking may extend for some seconds before the arc is initiated.

An alternative method is to use high voltage d.c.

sparking, which is much more efficient, and normally a single spark is all that is required to initiate the arc. This reduces the quantity of interference, but the instantaneous magnitude is still great since the high voltage spark is applied via the welding cables. On breakdown of the arc gap, the cables still give rise to radio interference, since prior to breakdown the cables are charged to several kV and discharge oscillatorily into the arc gap. Also high voltage dc introduces a significant degree of shock hazard, whereas the HF spark as normally employed causes a tingling, burning sensation but without severe convulsive effects.

In accordance with one aspect of the present invention, inert gas arc welding apparatus for initiating and maintaining an arc between a workpiece and an electrode without reducing the arc gap comprises a primary arc current source connected across the arc gap; a high impedance low power dc source connected via at least one series spark gap across the arc gap for providing a high voltage spark to initiate an arc; and means for providing a supplementary capacitive discharge across the arc gap substantially simultaneously with the high voltage spark in order to maintain the arc for a time sufficient to allow an adequate current build up from the primary arc current source.

In this case, the high impedance, low power dc source may be provided by an electronic circuit associated with the primary source.

By providing at least one spark gap between the high impedance, low power to dc source and the arc gap a convenient method of isolating the dc source from other parts of the welding equipment is achieved. With the invention, the problems of arc initiation mentioned above are overcome.

Preferably, however, and in accordance with a second aspect of the present invention inert gas welding apparatus for initiating and maintaining an arc between a workpiece and an electrode without reducing the arc gap comprises a primary arc current source connected across the arc gap; a piezoelectric spark'generator connected across the arc gap for providing a high voltage spark to initiate an arc; and means for providing a supplementary capacitive discharge across the gap substantially simultaneously with a high voltage spark in order to maintain the arc for a time sufficient to allow an adequate current build up from the primary arc current source.

We have found that the provision of a piezoelectric spark generator provides a particularly convenient high voltage source which provides a high voltage for the desired short duration.

The piezoelectric spark generator is conveniently mounted close to the electrode so as to minimise voltage losses and may be actuated in any one of a number of known ways. For example, actuation could be achieved by means of a hammer action or alternatively a squeeze action. In the case of a squeeze action, at least one spark gap should be provided in series with the piezoelectric spark generator and the workpiece in order to isolate the spark generator from other parts of the welding equipment. We have found that if there is no spark gap in this situation the additional loading presented by the welding equipment prevents the generation of a sufficiently high voltage.

Preferably, the magnitude of the total series spark gap is substantially twice the magnitude of the arc gap.

The means for providing a supplementary capacitive discharge may also be built into the primary source but in one preferred form of the invention it comprises a capacitor connected in series with a damping resistor and at least one isolation diode across the arc gap. The capacitor/resistor combination is preferably positioned as close to the arc as possible, in order to reduce voltage leaks.

Alternatively, the means for providing a supplementary capacitive discharge may comprise means for isolating the welding cables from the primary source before the arc is to be produced; and a supplementary current source and a capacitance connected across the cables for charging the cables to a voltage greater than that of the primary source open circuit voltage. The charge accumulated in the capacitance in the cables serves to supplement the initial are current following breakdown of the arc gap. The supplementary current source is preferably a piezoelectric device. A particularly suitable form of a piezoelectric device for momentarily charging the cables, in synchronism with the supply of the higher voltage spark initiating the arc, is a spring-loaded piezoelectric device operating by the impact of a hammer on a piezoelectric crystal.

In one example, a metal nozzle piece is mounted to a nozzle of a welding torch, the high impedance, low power dc source or the piezoelectric spark generator being connected in series with the nozzle piece. This construction provides a convenient method by which the initial high voltage can be applied across the arc gap. In one case, the low power dc source or the piezoelectric spark generator is connected direct ly to the metal nozzle piece and the series spark gap is provided between the nozzle piece and the workpiece.

The invention is particularly useful for manual welding, the voltages used for initiating the arc being completely safe.

Some examples of inert gas arc welding apparatus in accordance with the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic, perspective, partially cut away, exploded view of an air-cooled welding torch; Figure 2 is a diagramatic section through the piezoelectric device shown in Figure 1; Figure 3 illustrates a typical voltage output as a function of time, from a piezoelectric crystal; Figure 4 illustrates the probability of a successful breakdown between tungsten electrodes in argon, as a function of breakdown voltage; Figure 5 is a schematic circuit diagram of one example of the invention; Figures 6a and 6b illustrate two arrangements for providing the additional spark gap;; Figure 7is a graph showing the relationship between the arc gap and the minimum gap for an additional spark gap in series with the piezoelectric device shown in Figure 2; Figure 8 is a graph showing the spark current as a function of time, due to a high voltage spark from a peizoelectric device connected across an arc gap; Figure 9a illustrates the current build up with a conventional welding power supply, once initiated; Figure 9b shows the improvement in current build up rate with the addition of a capacitor discharge; Figure 10 is a graph showing the current obtained from a typical supplementary capacitor discharge through a resistor;; Figure 11a shows one preferred form of the invention in which the supplementary capacitive discharge is provided by a capacitor connected in series with a damping resistor and isolating diodes across the arc gap, the capacitor being situated close to the arc gap and charged from a high impedance source similar to the piezoelectric source which provides the initiating spark; Figure lib illustrates an alternative form of the invention in which the cables connecting the primary source to the workpiece and electrode are isolated by means of isolating diodes, and are precharged by means of a supplementary current source; and, Figure 12 is a graph showing the variation of voltage against time obtained using apparatus such as is illustrated in Figure 11 b, the supplementary power source for charging the cables being a loaded piezoelectric device.

An example of an air cooled welding torch for use in tungsten inert gas (TIG) welding is illustrated in Figure 1. The torch comprises a main body portion 1 (shown exploded in Figure 1) which is conventional and an integral handle 2. The handle 2 is connected to a pipe 3 for supplying inert gas and a pipe 4 which carries electrical wiring for supplying current to a tungsten electrode 5. Mounted within the handle 2 is a piezoelectric device 6 which is illustrated in more detail in Figure 2.

The pièzoelectric device 6 comprises a metal housing 7 to which is pivoted at 8 a lever 9. A projecting part 10 of the lever 9 abuts against a metal stop 11 mounted in an end of an insulated housing 12. The opposite end of the housing 12 is mounted within the housing 7 by means of a support pin 13 which is connected to another metal stop mounted in the insulated housing 12. Two piezoelectric cylinders 14 are also mounted within the housing 12 on either side of a high voltage lead 15. When the lever 9 is pressed or squeezed in the direction of the arrow 9' the projecting part 10 will push the metal stop 11 along the housing 12 and press the two piezoelectric cylinders 14 together. The two cylinders are electrically connected to the housing 7 via the pin 13 and the projecting part 10 and the pivot 8 and are thus electrically in parallel.Thus squeezing the lever 9 will cause the generation of a current which will flow through the high voltage lead 15 and a return lead 16 connected to the housing 7.

As may be seen in Figure 1, the high voltage lead 15 is connected to a metal nozzle ring 17 mounted on the nozzle of the welding torch while the return voltage lead 16 is connected to the electrical connection for the tungsten electrode 5.

In theory, compressing a piezoelectric crystal will give rise to a momentary voltage surge of the order 10kV which lasts only for a few milliseconds due to the low source energy and self-leakage. A typical output from a piezoelectric crystal is shown in Figure 3 where the build-up time to peak voltage is of the order of 30ms. This build-up would be similar to that achieved by squeezing the lever 9.

Although the voltage produced in itself is sufficiently high to break down the arc gap with a tungsten electrode in inert gas as normally used, such high impedance sources of high voltage cannot readily be applied to a welding system. This is due to the excessive leakage from the welding circuit which attenuates the momentary voltage to well below that necessary for spark breakdown of the arc gap. In this connection, the breakdown voltage with dc surges is typically around 4kV for pointed tungsten electrodes in argon, and about half this value in helium, with clean electrodes. High voltage breakdown, as is well known, is a statistical phenomenon and many factors contribute to variability, such that the probability of breakdown increases with the applied voltage. Typical results for tungsten electrodes in argon with a two millimetre gap are shown in Figure 4, where it is seen that voltages as high as 7 kV are necessary to ensure virtually 99% probability of breakdown from a single applied surge. On the other hand, breakdown can occur from voltages as low as 2 or3 kV under favourable circumstances, especially if the electrode is contaminated or the surface work function is reduced and particularly when other ionizing radiation is present.

In orderto achieve successful breakdown of a tungsten gap with a high impedance high voltage source actuated in this way (by squeeze action) we have found that it is necessary to introduce a local series spark gap and apply the voltage close to the electrode/workpiece combination. This is illustraed in Figure 5.

Figure 5 illustrates a conventional welding set 18 having a negative terminal connected to the tung sten electrode 5 by a cable 19 and a positive terminal connected to a work piece 20 by a cable 21. The piezoelectric device 6 is shown diagramatically in Figure 5 and is connected to the tungsten electrode 5 by the cable 16 as previously described. The cable 15 is broken at 22 to form a spark gap. Thus, in use, on squeezing the lever 9, a high voltage charge is produced which, in turn, breaks down the series spark gap 22 and thus an instantaneous voltage is suddenly applied to the arc gap 23. It should be appreciated that the time for spark breakdown at the arc gap 23 when an over voltage is applied can be very short, less than 0.1 micro seconds.

In this example, the spark gap is shown positioned in the cable 15. This arrangement is also shown in Figure 6a which illustrates in more detail the metal nozzle ring 17. As may be seen in Figure 6a, the nozzle ring 17 has a number of projections 24 which rest in use on the workpiece 20. This provides a convenient way of accurately setting the size of the arc gap 23. In a modified example, (Figure 6b), the spark gap 22 is formed instead between the projections 24 and the workpiece 20 with the nozzle ring spaced from the workpiece 20. In this case, the tungsten electrode 5 projects beyond the end of the projections 24. Furthermore, the spark gap 22 could be provided in the cable 16 and additionally, more than one spark gap could be provided.

We have found that the magnitude of the spark gap 22 or the magnitude of the sum of the spark gaps if more than one are provided, should exceed that of the arc gap 23 and to a first approximation the spark gap 22 in air should be about double that of the spark gap 23 in argon. (Figure 7) An example of a typical output from the piezoelectric device 6 is illustrated in Figure 8 which indicates that the duration of the spark discharge is very short, of the order of a few microseconds only. Normally, the current from the welding power supply does not instantly rise to the nominal operating value but, due to source inductance, the current builds up at a finite rate. Typically, in commercial welding power supplies intended for tungsten arc welding, the overall build-up rate is in the order of 10# A/sec. This is illustrated in Figure 9a.Thus, in the time associated with the discharge from the piezoelectric device, that is less than 10 micro seconds, the current normally available from the welding power supply is less than the 0.1 A required to sustain an arc.

One way to overcome this problem is to add a series resistor/capacitor combination in parallel with the welding leads. On open circuit the capacitor is charged such that, on subsequent breakdown of the arc gap, the capacitor discharges through the welding cables, but the discharge current is ensured to be non-oscillatory by the damping resistor associated with capacitor. Atypical Rc combination could be 1 OQ/1 FF, but a wide range of values have been employed extending from as little as 0.11lFto 100#F and more. Equally, the damping resistor may range from just critically under damped to heavily over damped such as, for example, 20of/88 giving nominal time constants of over 150 micro seconds.

This is illustrated in Figure 10. Such a uni-polarity discharge superimposed on the welding current build-up from the main power source gives an initial boost, such as is shown in Figure 9b, which aids arc formation following spark breakdown of the arc gap 23. It should be noted that, even with the addition of the damped capacitor discharge, there is a finite limit to the initial rate of rise of current for the first few amperes. This occurs due to the inherent inductance of the welding leads, such that there is a finite rate of rise of current even from the capacitor at the welding set (see Figure 9b).

To provide an even faster build-up of current following spark breakdown of the arc gap 23, a precharged capacitor 25 can be placed close to the welding head, as illustrated in Figure 11 a, the capacitor 25 being isolated from the welding set 18 by high voltage series diodes 26. Figure 1 la illustrates the welding set 18 connected to the workpiece 20 by a welding cable 27 and to the tungsten electrode 5 by a welding cable 28. The series resistance of the diodes 26 in the forward direction is supplemented to provide sufficient damping by a resistor 29. The capacitance 25 can be charged from a piezoelectric device 30 similar to the device 6 with which it can be actuated in conjunction.Thus a second piezoelectric crystal (not shown in Figure 1) can be squeezed simultaneously with the crystal of the device 6 to provide a local charge at the welding torch head which is isolated from the high voltage spark by the series diodes 26.

In an alternative arrangement shown in Figure 11 b, a supplementary capacitive discharge can be obtained from the welding cables 27, 28 themselves by charging these appropriately and providing suitable isolation from the main power supply 18. Thus, adding high current series diodes 31,32 to the output from the welding power supply 18 allows the output cables 27, 28 to be charged to a voltage greater than that of the power supply open circuit voltage. This charge serves to supplement the initial arc current following breakdown of the arc gap 23. In this case supplementary charging of the welding cables is shown with a series resistance/capacitance 33, 34 connected in parallel (as previously described) but this is not essential. Conveniently, charging of the welding cables 27,28 is synchronized with the energizing of the piezoelectric device 6. In this example, a piezoelectric device 35 is used to charge the welding cables. In the absence of the resistor/ capacitance combination 33,34 and with highly efficient isolating diodes 31,32 between cables 27, 28 and the welding set 18, the cables can be momentarily charged by a loaded pieozelectric crystal to voltages in excess of 2 kV (see Figure 12).

As may be appreciated, the supplementary current provided by one of the means outlined above serves to maintain the arc until the current build-up from the main welding power supply 18 alone is sufficient to maintain the arc.

Claims (11)

1. Inert gas arc welding apparatus for initiating and maintaining an arc between a workpiece and an electrode without reducing the arc gap, the apparatus comprising a primary arc current source connected across the arc gap; a high impedance, low power dc source connected via at least one series spark gap across the arc gap for providing a high voltage sparkto initiate an arc; and means for providing a supplementary capacitive discharge across the arc gap substantially simultaneously with the high voltage spark in order to maintain the arc for a time sufficient to allow an adequate current built up from the primary arc current source.

2. Inert gas arc welding apparatus for initiating and maintaining an arc between a workpiece and an electrode without reducing the arc gap, the apparatus comprising a primary arc current source connected across the arc gap; a piezoelectric spark generator connected across the arc gap for providing a high voltage spark to initiate an arc; and means for providing a supplementary capacitive discharge across the arc gap substantially simultaneously with the high voltage spark in order to maintain the arc for a time sufficient to allow an adequate current build up from the primary arc current source.

3. Apparatus according to claim 2, wherein at least one spark gap is provided in series with the piezoelectric spark generator and the workpiece.

4. Apparatus according to claim 1 or claim 3, wherein the magnitude of the total series spark gap is substantially twice the magnitude of the arc gap.

5. Apparatus according to any of the preceding claims, wherein the means for providing a supplementary capacitive discharge comprises a capacitor connected in series with a damping resistor and at least one isolation diode across the arc gap.

6. Apparatus according to any of claims 1 to 4, wherein the means for providing a supplementary capacitive discharge comprises means for isolating the welding cables from the primary source before the arc is to be produced; and a supplementary current source and a capacitance connected across the cables for charging the cables to a voltage greater than that of the primary source open circuit voltage.

7. Apparatus according to claim 6, wherein the supplementary current source comprises a piezoelectric device.

8. Apparatus according to claim 7, wherein the piezoelectric device providing the supplementary current source comprises a spring-loaded piezoelectric device operated by the impact of a hammer on a piezoelectric crystal.

9. Apparatus according to any of the preceding claims, wherein a metal nozzle piece is mounted to a nozzle of the welding torch, the high impedance low power dc source or the piezoelectric spark generator being connected in series with the nozzle piece.

10. Apparatus according to claim 9, wherein the high impedance low power dc source is connected directly to the metal nozzle piece and the series spark gap is provided between the nozzle piece and the workpiece in use.

11. Apparatus according to claim 1, substantially as described with reference to any of the examples shown in the accompanying drawings.