Arc Welding

Abstract

A weld is formed in a workpiece such as a pipeline by first activating a melting device, such as a laser, to form a molten weld pool in the workpiece and then activating a welding device, such as a GMAW torch, to initiate a weld in the weld pool. The weld therefore incorporates the weld pool homogeneously. Relative movement between the activated welding device and the workpiece continues and completes the weld while the melting device remains deactivated.

Claims

1-35. (canceled)

36. A method of welding a workpiece comprises: activating a melting device to form a molten weld pool in the workpiece; activating a welding device to initiate a weld in the weld pool; effecting relative movement between the activated welding device and the workpiece to continue the initiated weld while the melting device is deactivated; and protecting the deactivated melting device from radiated heat emitted by the activated welding device by moving a barrier into a blocking position between the deactivated melting device and the activated welding device; wherein the welding device and the melting device are in fixed relation during said relative movement between the activated welding device and the workpiece.

37. The method of claim 36, comprising completing the initiated weld while the melting device is deactivated throughout.

38. The method of claim 36, comprising moving the barrier into the blocking position in response to deactivation of the melting device.

39. The method of claim 36, wherein the welding device and the melting device are both supported by a welding bug or robot that is moved relative to the workpiece.

40. The method of claim 36, comprising: directing a flow of shielding gas at the workpiece; and activating the melting device to form the weld pool in a shielding atmosphere defined by the flow of shielding gas.

41. The method of claim 40, comprising initiating and continuing the weld in a shielding atmosphere defined by a continuing flow of the shielding gas.

42. The method of claim 40, comprising forming the weld pool in a shielding atmosphere of shielding gas flowing from the welding device.

43. The method of claim 40, comprising forming the weld pool in a shielding atmosphere of shielding gas flowing from the melting device.

44. The method of claim 40, comprising initiating and continuing the weld in a shielding atmosphere of substantially the same composition as the shielding atmosphere used when forming the weld pool.

45. The method of claim 40, comprising initiating and continuing the weld in a shielding atmosphere of a substantially different composition to the shielding atmosphere used when forming the weld pool.

46. The method of claim 36, comprising: monitoring the weld pool; and activating the welding device in response to monitored characteristics of the weld pool.

47. The method of claim 36, comprising applying welding heat from the welding device to the workpiece before deactivating the melting device.

48. The method of claim 47, comprising: determining activation of the welding device; and deactivating the melting device in response to activation of the welding device.

49. The method of claim 36, comprising: directing melting heat along a first axis from the melting device to the workpiece; and directing welding heat along a second axis from the welding device to the workpiece; wherein the first and second axes converge and intersect the weld pool.

50. The method of claim 36, wherein the welding device is, individually, capable of making an effective weld in the workpiece.

51. The method of claim 36, wherein the melting device is, individually, incapable of making an effective weld in the workpiece.

52. The method of claim 36, wherein the welding device comprises a GMAW torch that forms the weld by a shielded arc.

53. The method of claim 36, wherein the melting device comprises a laser or a plasma torch.

54. The method of claim 36, wherein the melting device comprises a tungsten electrode that forms the weld pool by an arc.

55. The method of claim 54, wherein the tungsten electrode is retracted upon deactivation of the melting device.

56. A welding apparatus comprises: a melting device for forming a weld pool at a weld initiation point; a welding device for forming a weld to be initiated at the weld pool; a controller that is programmed to deactivate the melting device and to activate the welding device to complete the weld with the melting device remaining deactivated; and a barrier that is movable, on deactivation of the melting device, into a blocking position between the deactivated melting device and the activated welding device; wherein the welding device and the melting device are mounted together on a common supporting structure for movement relative to a workpiece.

57. The apparatus of claim 56, wherein: the melting device is arranged to direct melting heat to a workpiece along a first axis; and the welding device is arranged to direct welding heat to the workpiece along a second axis that converges with the first axis.

58. The apparatus of claim 57, wherein the welding device is arranged to direct a flow of shielding gas toward the workpiece to intersect the first axis.

59. The apparatus of claim 56, wherein the controller is responsive to a weld pool sensor to activate the welding device in response to monitored characteristics of the weld pool.

60. The apparatus of claim 56, wherein the controller is responsive to an activation sensor to deactivate the melting device in response to activation of the welding device.

61. The apparatus of claim 56, wherein the welding device and the melting device share a common power supply.

62. The apparatus of claim 56, wherein the welding device comprises a GMAW torch.

63. The apparatus of claim 56, wherein the melting device comprises a laser or a plasma torch.

64. The apparatus of claim 56, wherein the melting device comprises a tungsten electrode.

65. The apparatus of claim 64, wherein the tungsten electrode is retractable upon deactivation of the melting device.

Description

[0039] In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings, in which:

[0040] FIGS. 1a to 1e are a sequence of schematic part-sectioned side views of a welding apparatus in accordance with a first embodiment of the invention, firstly forming a weld pool in a workpiece and then arc-welding along a joint using that weld pool as an initiation point;

[0041] FIG. 2 is a schematic part-sectioned side view of a welding apparatus in accordance with a second embodiment of the invention, corresponding to the stage of operation shown in FIG. 1c in relation to the first embodiment;

[0042] FIGS. 3a and 3b are schematic part-sectioned side views of a welding apparatus in accordance with a third embodiment of the invention, corresponding to the stages of operation shown in FIGS. 1c and 1d in relation to the first embodiment; and

[0043] FIG. 4 is a simplified system block diagram of the welding apparatus shown in the preceding drawings.

[0044] Referring firstly to the first embodiment shown in FIGS. 1a to 1e, a welding apparatus in accordance with the invention is shown here in conjunction with a workpiece 12 that comprises a joint to be welded. The welding apparatus 10 and the workpiece 12 are supported for automated relative movement to perform a weld pass automatically along the joint.

[0045] For ease of illustration, the workpiece 12 is shown here as a flat plate, such that relative movement between the welding apparatus 10 and the workpiece 12 follows a straight path. However, the workpiece 12 will often be curved, for example where pipe joints are brought together to fabricate a subsea pipeline. In that case, relative movement between the welding apparatus 10 and the workpiece 12 would follow a similarly curved path. Also, in that case, the weld will typically extend back to the initiation point following a full circuit of the abutting pipe joints.

[0046] The welding apparatus 10 comprises a primary welding device 14 and an auxiliary or secondary melting device 16 that is suitably held in fixed relation to the primary welding device 14. Conveniently, therefore, the primary welding device 14 and the secondary melting device 16 are fixed beside each other to a common support structure 18. That support structure 18 may be, or may be supported by, a welding bug or a robot arm.

[0047] The primary welding device 14 is exemplified here by a conventional GMAW torch for performing MIG or MAG welding. In that case, the workpiece 12 is typically connected electrically to ground.

[0048] FIG. 1a shows the welding apparatus 10 brought into an operational position close to the workpiece 12. The primary welding device 14 comprises a consumable electrode 20 that is advanced within a contact tube or tip 22. The welding apparatus 10 is positioned to bring the electrode 20 into alignment with a desired weld initiation point 24 on the workpiece 12.

[0049] The contact tip 22 is supported within a concentric shroud or nozzle 26 that then directs a flow of shielding gas 28 toward the workpiece 12 as shown in FIG. 1b. The flow of shielding gas 28 could, for example, emanate from holes 30 in the contact tube 22 as shown. The skilled reader will understand that the shielding gas 28 may be selected from a wide range of options depending upon the material(s) of the workpiece and the parameters of the weld.

[0050] With the flow of shielding gas 28 now creating a shielding atmosphere that surrounds and protects the initiation point 24 from ambient air, the secondary melting device 16 is activated as shown in FIG. 1c. In this example, the secondary melting device 16 is a low-power LBW device that generates a laser beam 32 from a laser 34 and directs and focuses that beam 32 through optics 36 and an aperture 38 to impinge upon the workpiece 12 at the initiation point 24. Thus, the laser beam 32 emerges from the aperture 38 of the secondary melting device 16 on a path that intersects a longitudinal axis projecting toward the workpiece from the electrode 20.

[0051] The laser beam 32 heats and locally melts the workpiece 12 around the initiation point 24 to form a weld pool 40. The shielding gas 28 surrounds and shields the weld pool 40 from ambient air.

[0052] The temperature and other characteristics of the weld pool 40 are monitored by an infra-red weld pool sensor 42 that is conveniently supported by the welding apparatus 10 as shown. When the weld pool 40 has been established and stabilised, the system is ready for welding to begin by activating the primary welding device 14 as shown in FIG. 1d.

[0053] When the primary welding device 14 is activated as shown in FIG. 1d, an arc 44 is established between the electrode 20 and the workpiece 12 via the weld pool 40. Thus, the primary welding device 14 initiates welding in the already-molten and homogeneous weld pool 40. This avoids or mitigates the discontinuities that bedevil weld initiation in prior art welding techniques.

[0054] Elegantly, the shielding gas 28 flowing from the primary welding device 14 not only protects the weld pool 40 while the secondary melting device 16 is active but also surrounds the arc 44 to protect the weld itself. The same composition of shielding gas 28 could be used for both purposes, or the composition of the shielding gas 28 could be varied from one operation to the next. In any event it is advantageous for the shielding gas 28 to flow continuously and without interruption as the welding apparatus 10 switches from forming the weld pool 40 to forming the weld.

[0055] When the primary welding device 14 is activated, the secondary melting device 16 is deactivated to turn off the laser 34. For example, establishment of the arc 44 could be sensed as the trigger for deactivating the secondary melting device 16. Alternatively establishment of the arc 44 could be triggered immediately upon deactivation of the secondary melting device 16. Either way, the arc 44 is established before the weld pool 40 cools significantly. Also, the aperture 38 is closed to protect the laser 34 and the optics 36 from electromagnetic radiation emitted from the arc 44.

[0056] In the examples shown, the aperture 38 is closed by a shutter 46 that is driven by a shutter drive mechanism 48 to slide across the aperture 38. In the closed position shown in FIG. 1d, the shutter 46 bears against a stop 50. Sliding or translational movement of the shutter 46 is convenient as shown. However, the shutter 46 could instead pivot relative to the welding apparatus 10 so as to close the aperture 38 by rotating about any suitable axis.

[0057] The establishment and stability of the arc 44 may be determined by the weld pool sensor 42 and/or by monitoring fluctuations in current or voltage in the power supply of the primary welding device 14. When the arc 44 has been established and stabilised, welding can begin by effecting relative movement between the welding apparatus 10 and the workpiece 12 as shown in FIG. 1e.

[0058] The relative movement between the welding apparatus 10 and the workpiece 12 creates a weld 52 that incorporates the original weld pool 40, whose previous position is shown in FIG. 1e in dashed lines. Advantageously, therefore, the original weld pool 40 becomes a substantially homogeneous part of the weld 52 itself. The weld 52 is formed from the advancing electrode 20 and the material of the workpiece 12, which melt together into a new weld pool 54 ahead of the weld 52.

[0059] Welding continues until the weld 52 is complete and the welding apparatus 10 reaches a termination point on the workpiece 12. The primary welding device 14 is then deactivated and the flow of shielding gas 28 is turned off.

[0060] As noted above, it is possible for the weld 52 to return to the initiation point 24 and therefore to be endless, most commonly when forming a circumferential girth weld when fabricating a pipeline. In that event, there will be some re-welding at a termination point coinciding with the initiation point 24.

[0061] FIG. 2 shows a second embodiment of the invention in which like numerals are used for like features. The second embodiment is shown here at the same stage as shown in FIG. 1c for the first embodiment, that is, with the flow of shielding gas 28 turned on and the weld pool 40 being formed in the workpiece 12 by the secondary melting device 16.

[0062] In FIG. 2, the stages shown in FIGS. 1a and 1b have already happened, so that the welding apparatus 10 has been positioned and the flow of shielding gas 28 has been turned on. Conversely, the stages shown in FIGS. 1d and 1e are about to happen, in which the primary welding device 14 will be active, the secondary melting device 16 will be turned off and the shutter 46 will be closed.

[0063] The second embodiment shown in FIG. 2 differs from the first embodiment shown in FIG. 1 in that the laser 34 and optics 36 of the secondary melting device 16 are replaced by a plasma arc device 56 that directs an arc or jet of hot plasma 58 at the workpiece 12 through the aperture 38. The plasma 58 heats the workpiece 12 to form the weld pool 40. When the weld pool 40 has been established and stabilised, the plasma arc device 56 is deactivated, the shutter 46 is closed and the primary welding device 14 is activated to start a weld that incorporates the weld pool 40.

[0064] FIGS. 3a and 3b show a third embodiment of the invention in which, again, like numerals are used for like features. The third embodiment is shown here at the same stages as shown respectively in FIGS. 1c and 1d for the first embodiment. Thus, the stages shown in FIGS. 1a and 1b have already happened, so that the welding apparatus 10 has been positioned and the flow of shielding gas 28 has been turned on.

[0065] In FIG. 3a, the flow of shielding gas 28 is continuing and the weld pool 40 is being formed in the workpiece 12 by the secondary melting device 16 while the primary welding device 14 remains inactive. Conversely, FIG. 3b shows the primary welding device 14 activated, the secondary melting device 16 turned off and the shutter 46 closed.

[0066] The third embodiment shown in FIGS. 3a and 3b differs from the first and second embodiments in that the secondary melting device 16 comprises a retractable tungsten rod 60 of a type used as a non-consumable electrode in GTAW or TIG operations. The rod 60 is movable telescopically along its longitudinal axis by a drive mechanism 64 within a friction-fitted sleeve 62 in the surrounding body of the secondary melting device 16.

[0067] The rod 60 is extended by the drive mechanism 64 through the aperture 38 toward the initiation point 24 as shown in FIG. 3a. When the extended rod 60 is energised, a secondary arc 66 is established between the distal end of the rod 60 and the workpiece 12 to form a weld pool 40 as shown.

[0068] When the weld pool 40 has been established and stabilised, the rod 60 is de-energised to extinguish the secondary arc 66 and is then quickly retracted by the drive mechanism 64 into the surrounding body of the secondary melting device 16. The shutter 46 is then closed and the primary welding device 14 is activated to start a weld that incorporates the weld pool 40. It may, however, be possible to activate the primary welding device 14 before retracting the rod 60.

[0069] Advantageously, the rod 60 can retract very quickly, for example in less than 0.3 seconds, to allow the shutter 46 to be closed so that welding can begin before the weld pool 40 cools significantly. For example, the drive mechanism 64 may extend the rod 60 against the bias of a mechanical spring or a gas spring and latch the extended rod 60 against that bias, so that the rod 60 snaps back into a retracted position upon being unlatched.

[0070] Those skilled in the art will know that a tungsten electrode used in GTAW requires particular shielding gases such as pure argon. However, such shielding gases are not necessarily appropriate for forming the weld itself. Consequently, the composition of he shielding gas supplied through the primary welding device 14 could be changed as the welding apparatus 10 switches from forming the weld pool 40 to forming the weld. Alternatively, while forming the weld pool 40, a shielding gas appropriate for a tungsten electrode could be supplied through the secondary melting device 16, or indeed along the tungsten rod 60, before a different shielding gas is supplied through the primary welding device 14 for forming the weld.

[0071] Turning finally to FIG. 4 of the drawings, this block diagram of the welding apparatus 10 shows how a controller 68 and power supply 70 interact with other aforementioned components and functions. In particular, it will be apparent that the controller 68 takes input from the weld pool sensor 42 and an arc monitoring device 72 to control the primary welding device 14 and the secondary melting device 16 in addition to the shutter drive 48 and a shielding gas supply 74. It will also be apparent that the common power supply 70 conveniently provides electrical power to both the primary welding device 14 and the secondary melting device 16.

[0072] Many variations are possible within the inventive concept. For example, with the primary welding device turned off, the secondary melting device could be reactivated upon termination of a weld. In this way, the secondary melting device could improve the condition of all or part of the weld, such as a termination region of the weld.