GB2039462A - Controlling arc welding - Google Patents

Abstract

In the automatic control of welding by monitoring the radiation from the back face of a weld being produced by an electric arc directed onto the front face so thereby to detect incipient breakthrough, the sensed radiation providing a signal for controlling the power into the arc and/or the rate of traverse, the control operation is improved by providing a solenoid 30 or annular permanent magnet around the welding head 11 to produce a magnetic field aligned with the arc; this field, in conjunction with the arc current flowing into the workpiece through molten metal in the weld pool causes stirring of that metal so giving a more uniform heat distribution and thereby eliminating or reducing random spikes in the electrical output signal from the radiation sensor. <IMAGE>

Description

SPECIFICATION Methods of and apparatus for electric arc welding This invention relates to the control of welding and is concerned more particularly with the automatic control of weld penetration, that is to say the depth of weld, in electric arc welding, by sensing the radiation from the rear surface of the workpiece to detect incipient penetration.

In welding two workpieces together, a weld pool is created by an arc struck between an electrode, typically a tungsten electrode and the workpiece to be joined. The weld pool is normally constituted by approximately equal amounts of material melted from the two workpieces and this weld pool must be deep enough to penetrate the full thickness of the plates. Good welding requires consistent full penetration and adequate mixing of the metal of the two workpieces, particularly if joining dissimilar materials. If two abutting plates are to be welded together, the weld penetration has to be matched to the local thickness of the material. If too little material is melted, the joint is weak whereas, if too much is melted, the weldpool may be distorted in profile or may cause too much stress in the workpieces.With the developments in mechanised welding, it has become possible to effect automatic control using a radiation sensor which is placed a small distance behind the weld; incipient penetration can be detected by sensing the thermal radiation from the weld and the output from the sensor can be used to maintain this condition despite changes in thickness of the material.

In particular, in Specification No. 1521846 there is described a method of controlling penetration through a workpiece in welding, wherein radiation from the rear surface of the weld is sensed, at a wavelength less than the wavelength ho of maximum emittance as predicted by Wien's law applied to the melting point of the material to be welded, by passing the radiation througha filter to a photoelectric sensor and amplifying the sensor output, and filter having a peak transmissivity at a wavelength Ax between 0.6 Xw and 0.1 Ax and a passband to pass radiation of wavelengths within the range Aw + 0.1 Ax to zero, and using the amplified output to control the power in the beam and/or the rate of relative movement between the beam and the workpiece so as to increase the power and/or decrease the rate of relative movement as the sensor output falls and vice versa.

In referring to the rear or back surface of the weld, this term is used to mean the surface of the workpiece in the region of the weld but on the opposite side to the heat source used for effecting the weld.

The heat source is typically a TIG torch.

The present invention is concerned with improving such a feedback control system utilising monitoring of the radiation from the rear face of the weld.

The radiation signal is a sensitive function of workpiece temperature and the emissive intensity of any area is proportional to the fourth power of its temperature (Stefan's law). Hence turbulent convective flow of heated liquid to the back face causing random local temperature variations can seriously affect the magnitude of the radiated signal.

According to one aspect of the present invention, in a method of welding two workpieces together utilising an electric arc to apply heat to a front su rface of the weld thereby to form a weld pool of molten metal through which the arc current flows, and in which the heat source and/or the rate of movement of the heat source with respect to the workpieces is/are controlled in response to the magnitude of radiation from the rear surface of the weld, the weld pool is stirred during the welding operation by an applied magnetic field. The stirring arises from the action of the magnetic field in conjunction with the welding current flowing through the metal of the weld pool. It is found that, by the application of magnetic stirring, the dispersal of heat in the weld pool can be controlled thereby reducing the fluctuations in the monitored radiation signal from the rear face of the weld.

The invention is applicable to any welding techno; que using monitored radiation from the rear surface for feedback control of a heat source providing an electric current through the metal in the weld pool, e.g. tungsten inert gas (TIG), submerged arc, and metal inert gas (MIG) welding processes. As one example, without magnetic stirring, it is found that, when using a pulsed arc current supply to a TIG torch, the radiated signal contains random fluctuations which are indicative of the random nature in which the heat is dispersed in the weld pool. These random fluctuations can seriously disguise the true condition of weld pool breakthrough to the rear face.

The magnetic stirring results in a more regular waveform in the monitoring sensor and hence gives an improved control of the overall operation. The magnetic field may be a unidirectional field. This, however, gives stirring in one direction and may give asymmetrical heat distribution about the line of the weld. Although it is often acceptable for the horizontal/vertical configuration of welding, it is preferred periodically to reverse the direction of the field in the downhand configuration, e.g. by applying an alternating current to a solenoid which provides the magnetic field thereby maintaining centre-line symmetry.

If a unidirectional magnetic field is required, a permanent magnet may be used instead of a solenoid.

For some welding operations, e.g. welding aluminium, the arc must carry an alternating current (which is typically at 50 Hz) and, in this case, the magnetic field must be synchronised with the arc current in order to maintain the same direction of stirring. If, as is explained later, periodic reversal of the direction of stirring is required, this phase relation between the magnetic field and arc current may be periodically reversed.

Hydrogen may be added to argon or shielding gas The drawing(s) originally filed was/were informal and the print here reproduced is taken from a later filed formal copy.

when using TIG welding; the use of such argon/hyd rogen or helium hydrogen gas shielding mixtures in conjunction with the magnetic stirring in a feedback controlled system using monitored radiation from the rear face improves the time response of control loops for feedback control, over conventional methods, without increasing the inherent variability of the radiated signal. This is because the addition of hydrogen increases the speed of convective heat transfer in the weld pool whilst the magnetic stirring serves to distribute the heat more evenly in the body of the liquid zone.

The invention furthermore includes within its scope apparatus for welding having means for applying heat to the front face of a workpiece by an electric arc and in which the arc current flows through the workpiece, a monitor for monitoring radiation from the rear face of the weld and feedback control means for controlling the amount of heat or the rate of relative movement between the workpiece and heat source in accordance with the magnitude of the radiation so as to keep the sensed radiation at a predetermined level, means are provided to produce a magnetic field in the region of the weld pool for stirring the molten metal in the weld pool.

The means for producing the magnetic field conveniently comprises a permanent magnet or a solenoid. The solenoid may be arranged around the heat source which is typically a TIG welding torch.

The magnetic field is conveniently coaxial with the axis of the arc so as to cause the arc and pool to rotate about that axis. As explained above, the direction of stirring in some cases may be periodically reversed by periodic reversal of the field and hence conveniently an alternating current energised solenoid is employed to produce the field.

The invention furthermore includes within its scope an article welded by the above-described method.

In the following description of one embodiment of the invention reference will be made to the accompanying drawings in which: Figure 1 illustrates diagrammatically one form of weld control apparatus making use of the present invention; and Figure 2 is a graphical diagram illustrating certain waveforms.

Referring to Figure 1 there is shown diagrammatically a workpiece which is to be welded along a line or curve extending in a direction indicated by the arrow A. This workpiece might typically consist of two plates 10 abutting along the line of the weld. The weld is effected by means of a heat source 11, typically a TIG torch, producing a welding arc, indicated at 12 which extends from a tungsten electrode 13 to a weld pool 14 formed by melting of metal of the workpieces 10. A shielding gas is injected at 15 to surround the arc 12. Relative movement between the heat source and the workpiece is effected, in the embodiment diagrammatically illustrated, by a mechanism 28 moving the workpiece. Weld penetration is sensed by a sensor 16 receiving radiation from the back surface 17 of the workpiece. A fibre optic link may be provided to couple this radiation to the sensor.As is described in the aforementioned Specification No. 1521846, the radiation sensor 16 may be a photo-transducer, e.g. a photo diode or a plurality of photo-diodes, together with a narrow passband filter 18 which typically passes radiation of a passband width of 10 nm. A simple screen 19 is provided in front of the filter to exclude unwanted radiation, e.g. from other hot bodies or light sources.

The output of the sensor 16 is amplified by a high gain amplifier 20 and applied to a comparator 21 where the amplitude is compared with a preselected reference signal from a reference unit 22, typically an adjustable potential divider. The comparator produces a control signal which is applied to a control unit 23 controlling the power from the heat source 11, as shown at 24. With a pulsed arc source, such control may be of the magnitude of the current in the arc and/or of the pulse duty cycle. A closed loop con trol circuit is employed with a feedback circuit 25 feeding back to the unit 23 a signal which is proportional to the power output from heat source 11. The control unit 23 increases the power output when the radiated signal at the sensor 16 decreases and vice versa.This control unit 23 may also control the speed of movement of the workpiece as shown at 26.

The rate of movement is decreased when the radiated signal at the sensor falls and vice versa. Again a closed loop control is employed with feedback to the control unit as shown at 27.

The control system using the radiation sensor 16 as described above, operates in a known way to keep the sensed radiation at a predetermined level so that a continuous weld is formed along the required weld line with a controlled depth of penetration.

In accordance with the present invention, magnetic stirring of the metal in the weld pool is effected.

For this purpose a solenoid 30 is provided around the heat source 11, this solenoid being fed with alternating current from a power oscillator 31. The solenoid 30 produces a field coaxial with the axis of the arc from the torch. The arc current spreads out through the weld pool 14 and the effect of the magnetic field and the current is to cause stirring of the molten metal in the weld pool, this stirring being generally around the axis of the arc.

If a permanent magnetic is to be employed to provide the magnetic field, an annular magnet may be located around the heat source 11 in the same place as the solenoid 30 of Figure 1.

Referring to Figure 2 the graph shown at (a) illus- trates the amplitude of the radiation sensed by the monitor 16, in the absence of any power applied to the solenoid, when a pulsed current is applied to the TIG torch, this arc current being shown at (b) in Figure 2. It will be noted that the arc current is unidirectional with a square waveform switching alternately between high and low values. Utilising magnetic stirring by energising the solenoid 30, the monitored waveform becomes of a much more regular shape as shown at (c) in Figure 2.

The waveforms as shown at (a) and (c) in Figure 2 are the responses detected using a photo-diode without any filtering. It will be seen that the output signal contains fluctuations which correlate in phase with the arc supply current but which, when stirring is employed, do not contain the random fluctuations which, in practice can seriously disguise the true condition of the weld pool breakthrough to the back face. This waveform is readily acceptable for use in a control loop and gives improved feedback control by elimination of the random fluctuations.

In one example of the operation of this apparatus, the solenoid 30 was fed with a direct current. The stirring of the metal in the weld pool was unidirectional which was acceptable for the welding in the horizontal/vertical regime. However, when welding using the downhand regime, this may lead to asymmetry of heat distribution about the line of the weld. In this case asymmetry may be avoided by periodically reversing the direction of stirring. The power supply frequency for the solenoid may typically be a few Hz. This power supply is preferably in a fixed phase relation with the pulsed frequency of the arc if the arc is pulsed. More generally however the supply to the solenoid is chosen in accordance with the prevailing conditions.A continuous current to the solenoid in conjunction with a pulsed arc current is, in its stirring effect, similar to a pulsed current to the solenoid and a continuous arc current.

The reversal of the direction of stirring assists in keeping the weld symmetrical about the welding centre line. A low frequency is used so that the duration of each direction of stirring enables significant motion to be induced in the weld pool in order to obtain the optimum heat distribution.

For the welding of aluminium, an alternating arc current is employed, typically at 50 Hz. Unidirectional stirring can be obtained by energising the solenoid with a synchronous alternating supply.

Periodic reversal of the direction of stirring can be effected by reversing the phase relationship of the solenoid supply with respect to the arc current.

When using a TIG welding torch, hydrogen may be included in the argon or helium shielding gas. The use of hydrogen leads to increased penetration of the weld pool. There is a particular advantage however in utilising an argon/hydrogen gas shielding mixture in conjunction with an arrangement employing magnetic stirring of the weld pool and feedback control. The use of hydrogen in the shielding gas improves the time response of the control loops without increasing the inherent variability of the radiated signal. This is because the addition of hydrogen increases the speed of convective heat transfer in the weld pool whilstthe magnetic stirring serves to distribute this heat more evenly in the body of the liquid zone.

Claims (17)

1. A method of welding two workpieces together utilising an electric arc to apply heat to a front surface of the weld thereby to form a weld pool of molten metal through which the arc current flows, and in which the heat source and/or the rate of movement of the heat source with respect to the workpieces is/are controlled in response to the magnitude of radiation from the rear surface of the weld, wherein the weld pool is stirred during the welding operation by an applied magnetic field.

2. A method as claimed in claim 1 wherein the applied magnetic field is a unidirectional field.

3. A method as claimed in claim 1 wherein the applied magnetic field is periodically reversed.

4. A method as claimed in claim 3 and in which a pulsed direct current feed to the arc is employed, wherein the periodic reversal of the magnetic field is maintained in a constant phase relationship to the arc current.

5. A method as claimed in any of the preceding claims and employing a TIG welding torch with hydrogen or helium injection into the shielding gas.

6. A method as claimed in any of the preceding claims wherein the electric arc is an alternating current arc the magnetic field is a synchronised alternating magnetic field.

7. Apparatus for welding having means for applying heat to the front face of a workpiece by an electric arc and in which the welding current flows through the workpiece, a monitor for monitoring radiation from the rear face of the weld and feedback control means for controlling the amount of heat or rate of relative movement between the workpiece and heat source in accordance with the magnitude of the radiation so as to keep the sensed radiation of a predetermined level, wherein means are provided to produce a magnetic field in the region of the weld pool for stirring the molten metal in the weld pool.

8. Apparatus as claimed in claim 7 wherein said means to produce a magnetic field comprises a permanent magnet.

9. Apparatus as claimed in claim 7 wherein said means to produce a magnetic field comprises a solenoid.

10. Apparatus as claimed in claim 9 wherein the solenoid is arranged to produce a magnetic field coaxial with the axis of the arc so as to cause the metal in the weld pool to rotate about that axis.

11. Apparatus as claimed in either claim 9 or claim 10 and having a periodically reversed current source for feeding said solenoid.

12. Apparatus as claimed in any of claims 9 to 11 and havingadirectcurrentsourceforfeedingthe solenoid.

13. Apparatus as claimed in claim 9 and having an alternating current arc and wherein the solenoid is energised with an alternating supply synchronised with the arc current.

14. Apparatus as claimed in any of claims 9 to 13 wherein the means for applying heat comprises a TIG torch with the solenoid arranged around the torch.

15. Apparatus as claimed in claim 14 wherein the TIG torch has means for injecting hydrogen and argon or hydrogen and helium to form a shielding gas mixture around the arc.

16. A method of welding two workpieces together substantially as hereinbefore described with reference to the accompanying drawings.

17. Apparatus for welding substantially as hereinbefore described with reference to the accom panying drawings.