This invention relates to a welding head used in electron beam and laser welding, and in particular a welding head for external welding of large items such as large tubular steel sections used for wind turbines and oil pipelines.

Background to the Invention

Electron beam welding and laser welding create a high quality weld and are often used to weld small items where rapid throughput is required or to weld complex workpieces. Welding takes place in an evacuated chamber containing the workpiece.

Conventionally large thick sectioned tubular cylinders for pipelines and the like have been welded using arc welding, but multiple passes of a welding head are required to weld the full depth of the joint. After each pass, non-destructive testing is required to ensure the weld meets quality standards with no inclusions or fractures that might cause failure of the weld. Arc welding is thus a slow and laborious process for welding such tubular structures.

When conducting vertical welds on large circumference objects, also known as circular welds, the quality of the weld joint can be impaired due to gravity acting on the molten material.

It is the aim of the present invention to provide a weld head to allow electron beam and laser welding to take place under vacuum on large metal structures.

Summary of the Invention

In accordance with a first aspect of the present invention, there is provided a welding head comprising an outer face attachable to a welding device, such as an electron beam gun or laser, an inner face sealable to a workpiece, a beam channel for directing a welding beam to a workpiece and at least one exit channel, both the beam channel and the at least one exit channel extending between the outer and inner face, wherein the at least one exit channel is positioned relative to the beam channel so as to be capable of removing molten material. Typically the workpiece will be a large tubular section having a wall thickness of between 40 to 300mm.

Preferably the at least one exit channel is located 80 to 200mm below the beam channel. This ensures that molten material is removed before it can cause substantial damage to a weld joint.

A pair of first and second exit channels may be provided, both positioned below the beam channel so as to provide enhanced removal of molten material.

If desired, exit channels may be disposed both above and beneath the beam channel so that the molten material can be removed regardless of orientation of the welding head.

Preferably the at least one exit channel, and any additional exit channels, are connected to pumping means, typically a vacuum pump, so as to ensure the exit channel is evacuatable and to assist with removal of molten material.

The welding head may further comprise a receptor for receiving a welding device, the receptor pivotally connected to the outer face. This allows variations in surface height to be accommodated, particularly when undertaking welds around the circumference of large tubular sections.

Preferably the welding head further comprises sealing means between the receptor and outer face so as to ensure that a vacuum can be maintained within the beam channel.

The receptor may be connected to the outer face by one or more adjustable joints so as to limit the range of pivotal movement.

The beam channel may be lined with a replaceable metal tubular section, typically made from steel, so as to improve the working lifetime of the head. A sacrificial protective layer, such as a metal plate, may be disposed over at least part of the inner plate proximal to the beam channel. The region close to the beam channel becomes extremely hot during welding and by providing a sacrificial layer, the usable lifetime of the head can be improved.

The welding head may further comprise one or more regions made of rigid PTFE so as to reduce the resistance of the head to movement of the workpiece.

The inner face is preferably substantially rectangular and the beam channel is positioned centrally within the inner face.

The welding head may further comprise first and second annular channels formed in the inner face so as to surround the beam channel, wherein an outer sealing ring is located in the first annular channel and an inner sealing ring is located in the second annular channel, the region between the first and second annular channels being evacuatable.

The inner and outer seals may be formed from self-lubricating material so as to assist movement of the welding head relative to a workpiece.

Both sealing rings may be formed from High Temperature High Tear silicone.

If desired, the inner and/or outer sealing ring may be formed with a heel portion to provide a region of increased wall width extending across the inner face and so increase resistance to heat from a weld.

The inner and outer seals may be inflatable, with pressure applied to the seals altered to adjust the fit of the seals to a surface to be welded. This may be achieved by each of the inner and outer seals being formed from upper and lower seals overlying each other, the lower seal being inflatable, typically in response to an actuator which continually adjusts the inflation pressure of the lower seal. Either or both of the inner and outer seals may be formed from overlying upper and lower seal elements having different material properties and preferably the upper sealing element is formed from a material with a Shore hardness of between 50 to 70, with the material of the lower sealing element preferably being formed from a material with a Shore hardness of 20 to 40, and more preferably a Shore hardness of 34, such as High Temperature High Tear silicone. This ensures the upper sealing element, which in use contacts the workpiece, is more robust and resistive to damage with the lower sealing element providing flexibility to allow the seals to deform in response to surface imperfections of a workpiece.

The outer and inner sealing rings may be selected to have different hardnesses. The outer sealing ring may be formed from a material with a Shore hardness of between 50 to 70, with the material of the inner sealing ring preferably being formed from a material with a Shore hardness of 20 to 40, and more preferably a Shore hardness of 34, such as High Temperature High Tear silicone. The outer ring is more resistant to surface imperfections and debris which the outer ring encounters before the inner ring as the head travels along a workpiece. The less hard inner sealing ring is able to form to a workpiece surface, so providing enhanced sealing and making it easier to obtain a vacuum in the evacuatable region.

The beam channel extending through the outer and inner face is preferably evacuatable so that air can be pumped from the channel and a vacuum created of the order of 10 ^"2 mBar. This is particularly appropriate where the welding device is an electron beam device or laser requiring a vacuum for welding to take place.

An elongate groove may extend across the inner face from the beam channel to at least the inner sealing ring. This allows a weld bead to locate within the groove as the head travels along the weld joint and ensures the inner sealing ring is not subjected to sudden changes in the workpiece surface profile which might tear the inner sealing ring or affect sealing to the workpiece. In accordance with another aspect of the invention, there is provided welding apparatus comprising a welding head as aforesaid attached to a welding device such as an electron beam gun or laser.

In such an apparatus, preferably the welding head is moveable relative to a workpiece, to allow circumferential welding of tubular workpieces. Typically the workpiece will rotate whilst the welding head remains stationary.

The invention will now be described, by way of example, and with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of pipe welding apparatus embodying the invention;

Figure 2 is a side view of a welding head;

Figure 3 is a cross-section of the welding head;

Figure 4 is an end view of the welding head;

Figure 5 is a perspective from one side of the welding head; and

Figure 6 is a side view of the welding head.

Description

Figure 1 shows welding apparatus 10 used to weld a large thick sectioned tubular cylinder 12 which requires welding around its circumference along vertical joint 14. The vertical circumferential weld is often referred to as a circular weld. Electron beam gun 16 is mounted on a carriage 18 and is connected to sealing head 20 which slideably seals to an outer surface 22 of pipe 12 allowing a vacuum to be created at weld joint 14 so that electron beam welding can take place. If desired an internal back box seal can extend along joint 14 on internal wall 26 so that a vacuum can be maintained at the rear of weld joint 14.

The present invention is particularly useful for large tubular sections of 50 to 300mm wall thickness and of large diameter, typically 0.5 to 3m. The electron beam gun attached to the welding head is able to achieve the full depth of the weld in one pass, welding from one side only, and is capable of continuous welding without interruption. Such large tubular sections are typically used in oil pipelines, wind turbines and other heavy engineering applications. In electron beam welding, the metal either side of the joint is fused together without the need for any fillers or flux. A joint extending through the full depth of the wall is achieved in a single pass, contrasting with arc welding where ninety passes of the same joint are required to weld a region 100 mm deep. This enables the weld to be completed quicker, particularly given the electron beam welds do not need to be tested for hydrogen cracking or flux particles in the weld. With an electron beam weld, a linear section of around 1.3m in length with a wall thickness of 60mm can be welded in six minutes and a weld speed of 200mm per minute can be achieved for 150mm thick steel.

Figure 2 shows a side view of sealing head 20 when sealed to cylinder 12 with a cross-section shown in Figure 3. Sealing head 20 comprises sealing plate 30 pivotally sealed to tubular receptor 32 which secures to electron beam gun 16 as shown in Figure 1, and provides a channel 33 along which an electron beam travels. Receptor 32 is located centrally on sealing plate 30 so as to dispose channel 33 directly over aperture 34 in sealing plate 30 and allow unimpeded access of electron beam 35 to cylinder 12, see Figure 4. Hinge pins 36, 36' are attached to receptor 32 at diametrically opposed positions and secure receptor 32 to sealing plate 30. Variable length joint mechanisms 40, 40' connected between receptor 32 and an outer face 41 of plate 30 accommodate pivoting of receptor 32 relative to sealing plate 30 and ensure that pivotal movement is limited.

Central channel 33 defined by receptor 32 is evacuatable through port 44 when electron beam gun 16 is secured in position. A high vacuum of around 10 ^"2 mBar is required and to ensure such a vacuum is maintainable along channel 33 through to workpiece 12, seals 46, 48 are provided where receptor 32 meets sealing plate 30. Typically seals 46, 48 will be provided by a resilient annular seal 46 located at one end of channel 42 closest to plate 30 and a resilient apertured disc seal 48 located on the outer face 41 of sealing plate 30, the two seals 46, 48 remaining in contact throughout the range of pivotable movement allowed by joint mechanisms 40, 40' .

Sealing plate 30 is substantially rectangular in cross-section, as can be seen in detail in Figure 4 where a plan view is shown of inner face 50 which seals to workpiece 12. In use, the longest axis of plate 30 is orientated substantially parallel to weld joint 14. Inner and outer annular channels or depressions 52, 54 are integrally formed within inner face 50 and receive annular seals 56, 58.

By adopting the substantially rectangular shape, a newly welded region is able to cool slightly before seals 56, 58 pass over the weld. This reduces the amount of heat that seals 56, 58 are exposed to and extends the welding distance they can remain operational for.

Sealing plate 30 is shaped to approximately match the curvature of pipe 12 and so has a slight curve to internal face 50, see Figure 5. This curve does not need to match the profile of tubular section 12 exactly as seals 56, 58 can accommodate a slight variation between the profiles of internal face 50 and tubular section 12. Typically tubular section 12 will be of such a great diameter that internal face 50 is almost planar.

To assist with movement of plate 30 relative to a workpiece, regions 60, 61 of the inner face 50 are covered by rigid PTFE material, this typically extending over the region between outer seal 58 to the edge of face 50 and an evacuatable region between seals 56 and 58. Attaching rigid PTFE to regions of face 50 assists head 20 to glide across the work surface and helps absorb downward forces from the vacuum, ensuring the force on the seals is reduced.

A sacrificial metal plate 62, typically made of Aluminium, is disposed around central aperture 34. This area experiences high levels of heat from weld beam 35 and having a replaceable sacrificial metal plate allows head 20 to remain operational for longer.

Typically plate 62 is obtained as a flat section and then rolled to match the curvature of inner face 50, see Figure 6.

In the embodiment shown, upper and lower pairs 64, 66 of inner and outer exit channels 68, 70 are disposed along the central axis of head 20 to ensure head 20 is symmetrical. The exit channels ensure that excess molten liquid is able to flow away from the weld joint and so restricts the damage this molten material can cause to the weld. Typically the exit channels are connected to a pump, such as a vacuum pump, to ensure the exit channels are evacuatable and to aid with removing molten material. Molten material either solidifies as it leaves the exit channels or is caught in traps and filters within a pumping line connecting the exits to the pump. If desired, a single exit channel can be used, positioned below channel 34. The closest exit channel is preferably located around 80 to 200mm beneath the weld centre, with the outer exit channels 70 disposed around 40 to 60mm from the inner exit channels 68.

Inner face 50 can be formed with an elongate channel 72 extending along the central long axis, and so extending along the direction of travel of workpiece 12 relative to head 20. The weld sits within the depression formed by channel 72 as head 20 travels relative to the workpiece surface and so the weld bead does not exert pressure on seals 56, 58. Channel 72 can be formed in sacrificial metal liner 62 if desired.

To further improve the useable lifetime of head 20, a cylindrical stainless steel liner 74 can be used to line channel 34. Steel has a high melting point and so is resistant to weld beam 35. Liner 74 can readily be replaced when damaged.

When positioned against outer wall 22 of pipe 12, seals 56, 58 form an airtight seal allowing air to be continually pumped from between seals 56, 58 by a pumping conduit and a rough vacuum obtained within gap 80 between seals 56, 58. Typically inner exits 68 will be connected to the pumping conduit associated with seals 56, 58. Channel 34 also has air pumped from it, such that in use there are two regions of vacuum, a high vacuum of around 10 ^"2 mBar associated with channels 34 and 33 and electron gun 16 and a coarse or rough vacuum of 0.1 to lOmBar associated with region 80 defined between seals 56 and 58.

Seals 56 and 58 are formed from plastics materials or rubber and if desired can be selected to have different material properties, and in particular different hardnesses. Outer seal 58 can be selected to be robust as the forces on the travelling front edge of plate 30 cause a lot of drag tearing at the seal and also seal 58 will encounter roughness and imperfections along the surface of tube 12 as head 20 moves relative to cylinder 12. Inner seal 56 encounters fewer imperfections as debris and some surface roughness will have been removed or abraded by outer seal 58 passing over the surface first. The material for seal 58 can thus be selected to be harder and more rigid than that of seal 56, with the material for seal 56 being more pliable and flexible. Seals 56, 58 are typically between 10 to 30mm wide, with seal 58 having a Shore hardness of between 50 to 70, and desirably 60. Seal 56 can be selected to have a Shore hardness of around 20 to 40, and in particular selected to be closed cell silicone with a Shore value of 34 and in particular high temperature high tear (HTHT) silicone rubber sponge. Too low a Shore value would result in seal 56 collapsing after a short weld distance. Desirably the material of the seals is self-lubricating which improves ease of movement of head 20, and thus each seal is typically made of self-lubricating HTHT silicone.

By using seals with different hardnesses, sealing around weld joint 14 by seal 56 is optimised as the lower Shore value seal is compliant and able to shape itself to the cylinder surface, ensuring a high vacuum can be achieved in central channel 34 without needing to make a seal with a profile matching that of wall 22. The harder outer seal 58 is able to resist imperfections, is resistant to tearing and has a protective effect for the softer seal.

By having a pair of spaced apart annular seals with different material properties, consistent vacuum levels in the 10 ^"2 mBar range are achievable in channels 34 and 33 during welding.

Seals 56 and 58 are responsive to an air cylinder actuator [not shown] to adjust their position relative to the surface being welded. Typically this is achieved by annular inflatable seals 76, 78 located beneath seals 56, 58. The actuator detects the pressure exerted on the seals 56, 58 by any irregularities in the workpiece surface and adjusts the pressure applied to inflatable seals 76, 78 so as to alter the position of seals 56, 58 relative to the surface. This ensures seals 56, 58 form to the surface whilst retaining their ability to slide along the surface Thus seals 56, 58 are active seals, continually adjusting to the surface profile of the workpiece as head 20 moves. If desired, seals 56, 58 can either or both consist of a double seal formed by two overlying seals of different Shore values. Thus inner seal 56 can be located on a lower seal disposed above inflatable seal 76, with inner seal 56 formed from a material with a Shore hardness of between 50 to 70, and the lower seal formed from a material with a Shore hardness of 20 to 40, and typically a Shore hardness of 34, such as High Temperature High Tear silicone. This ensures the outer part of the seal, seal 56, which in use contacts the workpiece, is more robust and resistive to damage with the lower sealing element providing flexibility to allow the seals to deform in response to surface imperfections of a workpiece and in response to adjustment of inflatable seal 76. Similarly seal 58 can be formed with an outer harder seal with a Shore hardness of between 50 to 70 and a more pliable lower seal with a Shore hardness of 20 to 40.

Typically the inner seal 56 is located at least 100mm from weld centre 35, and more preferably around 185mm.

Seals 56, 58, and the corresponding channels within which they are located, can be provided with a heel section 80, 82 to increase the surface area of the seal across the inner face and in the region where it meets the weld joint and so improve longevity.

In use to weld a workpiece, cylinder 12 is typically rotated whilst head 20 remains stationary, sliding over the surface of the rotating cylinder with the longest axis of head 20 parallel to the direction of rotation. Excess molten material from the weld joint falls downwards under the influence of gravity and can cause damage to the weld joint after formation, such as inclusions. By providing exit regions beneath weld beam 35, the molten material can be removed from the workpiece surface. Desirably inner exits 68 are located between 80 to 200mm from the outer edge of aperture 34 as if they are positioned too far from the point of origin of the weld joint, then the molten material will already have damaged the joint. Inner exits 68 can be surrounded by heel section 80, 82 as shown. If seals 56, 58 have a constant wall width with no heel section, then inner exits 68 will typically be positioned between aperture 34 and inner seal 56 and outer exits 70 positioned between the edge of inner face 50 and outer seal 58. By providing a sealed pivotal connection between electron beam gun receptor 32 and plate 30, distortions in the workpiece surface can be accommodated without compromising the internal vacuum of the channel through which the electron beam passes to reach the workpiece. This is particularly of use when the end point of the weld is reached, and a circumferentially complete weld is achieved, as the head will need to be able to accommodate the slightly raised region where the start and finish points of the weld overlap. The active seals with their adjustable position relative to the workpiece surface and the pivotal arrangement ensure that sealing can be maintained at all times, and thus appropriate vacuums maintained for high quality electron beam welding.

By having a local head for welding, the welding apparatus is portable, having a relatively small footprint compared to arc welding apparatus. Also the head profile can readily be customised to accommodate a variety of different cylinder shapes and profiles.

Whilst described with reference to electron beam welding, the weld head and technique described is applicable to laser welding which also preferably requires a vacuum at the weld site. The invention enables welding techniques previously used for smaller scale welds to be adapted for use with large scale workpieces given it enables a vacuum to be applied and maintained as a weld progresses over many metres.