WO1996038259A1

WO1996038259A1 - Consumable welding electrode and method

Info

Publication number: WO1996038259A1
Application number: PCT/GB1996/001299
Authority: WO
Inventors: Wayne Morris Thomas; James Christopher Needham
Original assignee: The Welding Institute
Priority date: 1995-06-01
Filing date: 1996-05-31
Publication date: 1996-12-05
Also published as: AU5840796A; GB9511108D0

Abstract

A consumable welding electrode (1) for use in arc welding comprises at least one elongate, metal core (10). An electrically insulating material (11) is in contact with the or each core, one or more parts (12) of the core not being covered by the insulating material on one side of the electrode to enable electrical contact to be made with the core between the ends of the electrode.

Description

CONSUMABLE WELDING ELECTRODE AND METHOD

This invention relates to in-situ welding with an insulating or flux covered electrode set in place on the work, or in the joint between workpieces.

In the well known fire cracker or Hafergut process, a short length of a fully covered electrode is laid in a joint and electrical contact made at the remote end as in manual metal arc welding. The other electrical contact is made to the workpiece or structure. An arc is struck between the core wire and the workpiece, usually by means of a fuse or bunch of fine iron wire at the operating end, and left to burn back along the electrode length leaving a deposit from the core wire on the work. Although this process has been known for over 50 years, it is not widely practised. One limitation is in the length of electrode that can be used in one operation, and another is in the maximum current that can be used. Both these limitations are governed by excessive resistive heating of the core wire with either too great a duration (longer length) or too great a resistive heating (with increased current or current density) . Variations of the process are described in GB-A-1319518 and GB-A-1282461.

In accordance with one aspect of the present invention, a consumable electrode for use in arc welding comprises at least one elongate, metal core; and an electrically insulating material in contact with the or each core, one or more parts of the core not being covered by the insulating material on one side of the electrode to enable electrical contact to be made with the core between the ends of the electrode. The electrode may conveniently be one or two metres in length, or depending on the application, the electrode may be up to 10 metres in length or even more. In accordance with a second aspect of the present invention, a method of continuous arc welding comprises laying a consumable welding electrode on a workpiece so that it is electrically insulated from the workpiece; placing an electrical contact on the welding electrode in contact with an uncovered part of a metal core at a position between one end and the other; and supplying power to an electrical circuit including the workpiece and the contact whereby an electrical arc is struck between the workpiece and the consumable electrode; and moving the electrical contact along the consumable electrode as the arc progresses whereby the or each metal core is melted and laid down on or in the workpiece.

The invention provides a new electrode which can carry greater currents or current densities up to the limit permitted by the flux covering used or by the arc behaviour including spatter generation, or enables greater lengths to be utilized in one operation, or both. Typically, a moving or sliding electrical contact with a core wire is used, such that the length of core between the contact point and the arc does not exceed a finite value, say 150mm. Preferably, the length of core wire subject to resistive heating is significantly less than 150mm, such as down to 50mm or less. For this the core wire must be exposed, at least intermittently along its length, on the surface or the side remote from the workpiece to enable contact to be made externally other than at the end of the core wire remote from the arc.

Preferably, the contact is moved continuously in a manner maintaining electrical continuity so as to avoid the complexity of make and break contacts, lift off and sensing devices. The invention also enables electrodes both longer and larger in cross-section than hereto to be used which is advantageous in providing greater productivity than available in the conventional fire cracker process.

The electrode could be tubular and optionally contain metal powder, flux, or mixtures of both metal and flux powder. In preferred arrangements, the arc can be directed as desired with respect to the work rather than allow the random or irregular behaviour found in the prior art. This involves arranging the return connection to be downstream of the arc, viz away from the moving contact.

By using an electrode which is fixed in position relative to the workpiece, and using a moving electrical contact an improved process for in-situ deposition is achieved which can be mechanised or automated in operation and hence reduce the dependence upon labour, which cost can in some advantageous circumstances be shared between more than one system according to the invention, operating together at the same time.

Some examples of consumable electrodes and methods according to the invention will now be described and contrasted with a prior art example with reference to the accompanying drawings, in which:-

Figure l illustrates a conventional fire cracker welding process; Figures 2A and 2B are a schematic, plan view and a section taken on the line B-B respectively of one example of a consumable electrode according to the invention;

Figure 3 illustrates the consumable electrode shown in

Figures 2A and 2B positioned on the substrate or workpiece; Figures 4A-4G are cross-sections through seven different examples of consumable electrodes according to the invention;

Figure 5 is a schematic, perspective view illustrating one example of a method of arc welding using a consumable electrode of the type shown in Figure 4E;

Figure 6 is a view similar to Figure 5 but showing a second example;

Figure 7 is a view similar to Figure 6 but showing how the Figure 6 example could be mechanised; Figure 8 is a view similar to Figure 7 showing a modified example; Figures 9 and 10 are views similar to Figure 5 but showing further examples;

Figure 11 shows a clad rail;

Figure 12 illustrates the application of the method according to the invention in a workpiece presenting restricted access;

Figure 13 illustrates a consumable with rail saddle supports; and,

Figure 14 shows a typical macrosection for a deposit made according to an example of the invention.

Figure l illustrates a conventional fire cracker arc welding process in which a flux coated consumable electrode 1 is laid on a workpiece 2 which is earthed. A welding current is supplied to the bared end of the core wire 3 of the consumable electrode 1 and an arc 5 is struck at the far end of the welding electrode l between the core wire 3 and the workpiece 2. This causes a deposit 6 of material to be formed on the substrate 2 and gradually the arc will progress towards the contact point of the welding supply source on the core wire 3. The problems with this arrangement have been outlined above and in particular its limitations of restricted electrode length and/or operating current.

Figures 2A and 2B illustrate one example of a nominally 3m length large section consumable electrode 9 according to the invention. In this example, the electrode comprises a metal core 10 having a generally trapezoidal cross-section to suit the joint profile, the core 10 being covered by a flux coating 11 which also acts as an electrical insulator. At regular intervals along the length of the electrode small, rectangular sections 12 of the core 10 are exposed through the flux coating 11. As shown in Figure 2B, in use, the consumable electrode is laid in a groove 13 defined at the junction between a pair of workpieces 14,15 with the sections 12 accessible to an external contact arrangement. Figure 3 shows a copper contact in the form of a skid bar 16 which is pressed against the upper surface of the consumable (as seen in Figure 2A) while one or both of the workpieces 14,15 is earthed at 17 and comprises the return to the welding supply. It is noted that the preferred position for the earth return is upstream of the arc, viz away from the moving contact bar 16. An electrical current is supplied to the skid bar 16 which passes from the bar 16 through one or more of the portions 12 of the metal core 10 into the consumable electrode and then via an arc 18 to the workpiece. This causes the flux coating to be melted off in a conventional manner to form a protective layer on the deposit or weld 19. Typically, the portions 12 are spaced apart by about 50mm or less with the contact bar 16 of greater length to ensure continuous electrical contact and hence avoid make and break of the electrical circuit.

Figures 4A-4G illustrate various alternative forms for the consumable electrode. Figure 4A illustrates a metal core 20 having an elliptical cross-section which is partially covered by a flux coating 21 which melts at a suitable temperature with respect to the metal core 20. The flux coating 21 defines a groove 22 through which contact may be made with the metal core 20 by the skid bar 16. Instead of the skid bar 16, the contact, with a continuous exposed region in the electrode coating 21, can be in the form of a wheel such as shown in Figure 5.

In the Figure 4B example, a metal core 20 is provided as before but in this case the metal core has a pair of continuously extending ridges 23,24 extending along its length which protrude or are exposed through a flux coating 25. In addition, the flux coating 25 is formed with a pair of continuous, elongate ridges 26 which define stabilizing feet and which contact the workpieces in use. The feet 26 also serve to space the core 20 a desired distance from the work.

Figure 4C is similar to Figure 4B except that instead of a single core 20, a pair of cores 20A,20B are provided separated by part of the flux coating at 27 and contacted in use by respective bars 16A,16B. In this case, two contacts are used which are separated electrically from one another. Figure 4D is similar to Figure 4A except that instead of a flat exposed surface to the core 20, the core has a re-entrant or groove shape. This assists in maintaining alignment with the contacting electrode wheel or skid 16 as in Figure 3. Figure 4E illustrates a consumable electrode similar to that shown in Figure 2B but which will be described below.

Figure 4F illustrates a consumable electrode suitable for welding a pair of workpieces 30,31 at right angles. The consumable electrode has a triangularly shaped cross- section metal core 32 surrounded by a flux coating 33 with a continuous, elongate ridge 34 protruding or exposed through the coating.

Figure 4G is similar to one half of Figure 4C except that provision is made for increased flux content for a single core 61 with nominally a circular outline to the electrode, the core having a flux covering 62 and contact ridge 63.

These and other shapes for particular applications are within the general field of the invention as described with access for electrical contact at least at frequent intervals along the electrode length.

Due to the presence of the flux covering on the metal core, the latter fills a more limited space than the original electrode overall. This is not a problem in surfacing as the metal can spread transversely as desired, but with a more limited thickness than that of the original core. Thus, for example, with an electrode and core wire such as illustrated in Figure 4G with a cross-section of some 20mm , metal is deposited at the rate of -2.5 milligram/second/ampere. A typical cross-section of a stainless steel deposit on to a mild steel base is shown in Figure 14 and exhibits a satisfactory penetration and profile. In a joint inevitably the thickness or depth of the melted pass laid down is less than that of the principal dimension of the metal core. This can be supplemented by feeding a relatively thin wire into the arc zone, which is melted off together with the main core rod. One convenient method is to use a hollow core 100 with the additional thin wire feed 101 passing through (Figure 4E) . This latter can amount to an appreciable fraction of the total metal melted by the arc, and can even exceed the metal volume of the core itself.

In practice, straight lengths of the additional thin wire can be passed down an electrode of any of the kinds already illustrated in Figure 4. Alternatively, a coil of the thin wire can be mechanically fed into the hollow core via feed rolls at the extremity away from the arc zone. Preferably, the feed rolls are followed by a wire straightening stage to allow the wire to feed more readily through the bore of the electrode. As the main core is being melted by the arc, the thin wire is urged forward towards the arc to be melted together with the core. The thin wire tends to protrude into the arc a limited distance, such as 3 to 5mm, in operation, but the overall burning back is controlled by the presence of the core together with its flux covering. The electrical contact with the core is maintained via a contact wheel or skid 16 as previously described.

The wire feed can be energised automatically when the arc is struck, or by the operator via a hand controlled switch mounted by the roll, or by a foot switch as convenient, and so forth.

Figure 5 illustrates an alternative method of supplying electrical current to the consumable electrode 150 which is suited to a continuous contact ridge protruding or exposed through the flux covering such as shown in Figure 4E. In this case, use is made of a copper conductor wheel 35 rotatably mounted to a shaft 36. A power cable 37 is connected by a brush system or the like (not shown) to the central region of the copper conductor wheel 35 which is also insulated from the shaft 36. In use, an operator grasps the shaft 36 and holds the conductor wheel 35 on to the continuous contact ridge of the consumable electrode shown at 38. Power is supplied to the cable 37 maintaining an arc 39 struck between the melting end of the consumable electrode and the workpiece 40 which is connected to the power supply return at 41 upstream of the arc. This produces a deposit 42. Gradually, the operator rolls the wheel 35 away in the direction shown so that a continuous deposit is laid down, and the distance to the point of arcing is approximately constant such as at around 50mm. Conveniently, the wheel is moved such that it substantially or partially obscures the arc from the operator's view. Alternatively, a suitable guide or sight (not shown) is mounted on the shaft 36, so that a substantially constant distance is maintained between the contact point of the wheel 35 and the arc 39. In all these arrangements, it is convenient although not necessary to add a further connection from the electrode supply to the extremity of the core wire away from the arcing end. This reduces the tendency for any inadvertent arcing following poor or broken contact between the skid 16 or wheel 35 and the contact ridge on the core to the electrode. This applies particularly where an intermittent contact ridge is employed as illustrated in Figure 2A. This supplementary connection does not apply to the multiple core wire arrangement of Figure 2C which has a separate restricted current supply as explained with respect to Figure 9.

It is common practice in manual metal arc welding with a covered electrode to weave the electrode from side to side in simple or more complex patterns, partly to shape the bead being deposited and partly to ensure satisfactory sidewall fusion in a V joint and better bonding of the deposit with such sides especially in positional welding. Weaving is in principle not readily possible with the continuous contact electrode set in a groove and alternative arrangements are required. In one system, the arc from a single electrode is biased from side to side by electro-magnetic effects and/or the path taken by the current or both. In an alternative system two or more electrodes could be energised in turn to produce a similar effect.

One arrangement for partially deflecting the arc is to arrange for the return current to be taken from one or other side of the joint line using for example a segmented contact wheel 43 as illustrated in Figure 6. The wheel comprises two discs 44,45 each with alternate conducting 46 and insulating 47 segments. The conducting segments 46 of one disc 44 are arranged to overlap the insulating segments 47 of the other disc 45 and vice versa to maintain continuity of electrical contact as well as the alternating return path. The wheel 43 is coupled by insulated connecting bars 48 to the wheel 35 to define a working head and is connected to the power supply return via a cable

151. As the working head is traversed slowly along the joint line the discs 44,45 make contact on the workpiece in close proximity to the arc 39 on alternate sides in turn.

In practice, the conducting segments 46 cover a much smaller arc than illustrated diagrammatically such as preferably some 2 or 3mm along the periphery. For example, an all metal wheel can be cut with teeth as in a gear wheel and the spaces filled with insulating material with the whole machined smooth giving, for example, 3mm conducting segments together with 2mm insulating segments which effectively provide some ^mm overlap before and after each insulating stage. Preferably, one disc is fixed on a common shaft and the other disc is provided with a fine adjustment (not shown) for accurately setting the angular position of the second disc with respect to the first. The segments 46,47 can be scaled up or down as desired with preferably not less than lmm conducting zone and correspondingly not less than ^mm insulation as a minimum or not more than 6mm conducting or more than 4mm insulation as a maximum. The discs can be made of any suitable conducting material such as brass, a hard copper alloy such as chromium copper as used in resistance seam welding or mild steel and so forth.

Alternatively, in an arrangement not shown, in place of the contact wheels, a slide finger in contact on either side of the joint can be raised in turn with the other contact finger having been brought into contact before. This latter arrangement is suitable for the more awkward joint profiles such as a horizontal vertical joint where the current return is taken from the base plate and the upright member in turn to produce the biasing desired. Figure 7 illustrates an arrangement similar to that shown in Figure 6 but in this case having a central control bar 50 pivoted to the connecting bars 48. This enables mechanised or automatic welding to be achieved by attaching the bar 50 to a robot arm or the like. Conveniently, power supply to the wheel 35 and return from the wheel 43 will pass through the control bar 50.

The biasing of the arc path can be augmented by electro-magnets 51 positioned close to the arc zone as illustrated in Figure 8. The electro-magnets 51 are provided with replaceable pole pieces which can be shaped as desired. The electro-magnets can be energised independently of the contact wheel when the contact wheel 43 is continuous or can be energised in synchronism with a segmented contact wheel. In a particular arrangement, the welding current return 41 can be taken via the electro¬ magnet 51 with one or two turns so that each is energised in turn as required. Where the electro-magnets are separately energised preferably a multi-turn winding operated at relatively low current is used giving a total energising of at least some 100-150 ampere-turns.

Previous examples have shown the use of single core electrodes. In another arrangement, a two core wire electrode 152 is used (Figure 9) and contact made with the exposed metal ridges using a contact wheel 55 of the type already described with respect to Figure 6 (wheel 43) . To maintain continuity of arcing and allow each electrode readily to take up the supply, it is preferable to maintain a common continuous current in each of the two electrodes. For example, a current feed of some 10-50 amp in each of the two electrodes maintains a continuous arc with the full operating current being supplied to the two electrode cores in turn. The continuous electrical supply is conveniently supplied by making contact with the extremity of the electrode away from the arcing zone. This continuous current supply is also useful for initiating the arc from the operating end of the twin electrode system at the start of welding. This twin electrode arrangement can be combined if desired with the partially electro magnetic deflection described earlier from a twin segmented wheel 43 for the return current.

Apart from providing for extra flux by shaping the electrode in an appropriate manner as illustrated in Figure 4G, flux 67 can be added as a powder or granules to the electrode prior to the arcing point via for example a hopper arrangement 64 as shown diagrammatically in Figure 10. This can be used also in the trolley arrangements of Figures 6, 7 and 8. The additional flux 67 may be of the same general composition as that of the electrode, or may be of different formulation especially where the extra flux is of a composition which is not readily extrudable, or for the addition of metal powders for alloying purposes and so forth.

For particular applications, especially surfacing, the electrode shape such as in Figure 2A or 2B can be extended laterally to cover a wider area. One example is shown in Figure 11 for cladding or re-surfacing a rail section. Contact with the metal core is obtained via the protruding ridge as before. To assist even burning back of the consumable electrode the arc can be driven from side to side by electro-magnets in a manner similar to that illustrated in Figure 8. For wider excursions of the arc as desired than obtained readily by magnetic deflection, the magnet itself or pair of magnets may be mechanically traversed from side to side, driving the arc from relatively close proximity.

The maximum width of metal core that can be conveniently handled is limited partly by the length of time for the arc to return to a previous melt position before the solidification of the flux around the molten metal interferes with further arcing, and partly by the amount of molten material which can bridge between the core and the substrate.

In the limit the available current from the power supply s not sufficient to prevent such metal bridging from forming a permanent short circuit and extinguishing the arc.

In this case, a supplementary low voltage high current supply can be added in parallel with the main supply and of the same polarity. For example, with a DC positive main supply to the moving contact, the supplementary supply is also DC positive and feeds via a rectifier to the main supply. The rectifier serves to isolate the supplementary supply while an arc 18 is present, but provide current into a short circuit. The supplementary supply is typically of some 8V to 16V open-circuit with a current capability of some two or three times the average working current. In some examples, the supplementary supply can conveniently be added to the remote end of the core wire such that it parallels the main welding current supply as before.

In some cases, a wide layer can be successfully deposited using a multiple core arrangement analogous to that shown in Figure 2C. Here preferably the arc is driven transversely sufficiently to bridge the insulating gap between cores and so continue the side to side excursion. The cores may be connected in turn to the power supply in a manner similar to that illustrated in Figure 9, or adjacent cores both connected to the power supply during the transition stage, in association with the means for driving the arc from side to side.

Such arrangements for surfacing a relatively wide zone can be applied to nominally flat surfaces, or to radiused surfaces such as the rail section 60 illustrated in Figure

11, or to curved surfaces such as a cylinder or shaft with suitably shaped electrode profiles.

One particular advantage of the process according to the invention is in applications with restricted access requiring, for example, internal welding or surfacing. Some examples are shown in Figures 12A-12C. In Figures 12A,12B the contact roll 35' is urged against the consumable electrode within a pipe 70 by contact with a diametrically opposed insulation strip 71, which is of resilient material or resiliently mounted. Guide rollers

72 mounted on arms 73 (not shown in Figure 12A) which support the roll 35' provide additional alignment support.

In Figure 12C, the skid 16' is spring loaded at 65 to bear on the exposed region or ridge of the consumable electrode 66, the reaction force being borne by an insulation strip 71' which will bear on the inside of a pipe. The skid 16' and insulation strip 71' are supported by an arm 74. The mechanical support to the roll or skid can also be adapted to provide the required connection to the power supply, and, in the trolley arrangement of Figures 6, 7 and 8, the moving return connection as well. This can be used to provide, for example, an internal backing or root pass for a longitudinal seam in a tube or square section tube. One particular example is in the welding of the seam between two half sections of a tube such as used in the construction of heavy duty rear axle casings.

In some cases it is convenient to mount the electrode a finite distance above the work as indicated by the shaped flux "feet" of Figure 4C. For similar purposes, and particularly for greater spacings, it is convenient to add suitable insulating spacers or saddles 80 to the electrode as illustrated generally in Figure 13. In this case the distance from wheel centre to arc position should be greater than the saddle supporting space. To maintain adequate flux coverage to the melting electrode, it is preferable to provide additional flux via a hopper 64 similar to the method illustrated in Figure 10.

This method can be further extended to a lightly covered core, or even a base metal core 160 (without covering) as shown in Figure 13. This has the advantage that the metal bar can be bent as desired to follow a joint line which is not straight, or within limits follow a curved path. Obviously for the non-straight situations, the hopper arrangement and its mechanical connection to the contact roll has to be adapted to suit. For a constant radius of curvature, the hopper (and return current contact roll or rolls) can be suitably offset from the nominal line of the contact roll.

Furthermore, electrodes can be fabricated according to the invention from bare metal bar together with flux segments which are closely spaced or preferably assembled like a string of beads. These may be threaded on from one end and closely stacked, or in effect clipped on from one side especially (as shown in Figure 13) where the flux does not extend much over about 180° of a circular metal rod in section.

These and other applications of the method according to the invention provide for simple seam welds with relatively unskilled labour, or with machine welding where no operator skill is involved during deposition, and in particular for providing welds or cladding in otherwise remote or restricted access situations.

Claims

1. A consumable welding electrode for use in arc welding, the electrode comprising at least one elongate, metal core; and an electrically insulating material in contact with the or each core, one or more parts of the core not being covered by the insulating material on one side of the electrode to enable electrical contact to be made with the core between the ends of the electrode.

2. An electrode according to claim 1, the electrode comprising two elongate metal cores, one or more parts of each core not being covered by the insulating material on one side of the electrode to enable electrical contact to be made with the cores between the ends of the electrodes.

3. An electrode according to claim 1 or claim 2, wherein the part(s) of the or each core which is not covered comprises an elongate ridge protruding through the insulating material.

4. An electrode according to claim 1 or claim 2, wherein the part(s) of the or each core which is not covered comprises two elongate ridges protruding through the insulating material.

5. An electrode according to claim 1 or claim 2, wherein the part(s) of the or each core which is not covered comprises a series of longitudinally spaced protrusions extending through the insulating material.

6. An electrode according to claim 5, wherein successive protrusions are spaced apart by less than 50mm.

7. An electrode according to any of the preceding claims, wherein the insulating material on the side of the electrode which contacts the workpiece in use is shaped to stabilize the electrode on the workpiece.

8. An electrode according to claim 7, wherein the insulating material is shaped to define a pair of elongate, laterally spaced apart feet.

9. An electrode according to claim 8, wherein the feet extend along the full length of the electrode.

10. An electrode according to any of the preceding claims, wherein the electrode has a substantially triangular cross- section.

11. A method of continuous arc welding comprising laying a consumable welding electrode on a workpiece so that it is electrically insulated from the workpiece; placing an electrical contact on the welding electrode in contact with an uncovered part of a metal core at a position between one end and the other; and supplying power to an electrical circuit including the workpiece and the contact whereby an electrical arc is struck between the workpiece and the consumable electrode; and moving the electrical contact along the consumable electrode as the arc progresses whereby the or each metal core is melted and laid down on or in the workpiece.

12. A method according to claim 11, wherein the contact comprises a wheel.

13. A method according to claim 11 or claim 12, wherein the electrical circuit is connected to the workpiece via an auxiliary wheel.

14. A method according to claim 13, wherein the auxiliary wheel comprises sets of metal segments spaced apart by insulating segments.

15. A method according to claim 14, wherein the auxiliary wheel comprises a pair of discs, each having alternating conducting and insulating segments, with the conducting segments of one disc aligned with the insulating segments of the other disc.

16. A method according to any of claims 13 to 15, when dependent on claim 13, wherein the wheel and auxiliary wheel are coupled together.

17. A method according to any of claims 11 to 16, wherein the electrode is constructed according to any of claims 1 to 10.

18. A method according to any of claims 11 to 16, wherein the electrode comprises an uninsulated metal conductor, the method comprising supporting the conductor away from the workpiece.