EP2667997A1

EP2667997A1 - Weld cooling apparatus and method using an expansible coolant and a refractory seal

Info

Publication number: EP2667997A1
Application number: EP12703328.0A
Authority: EP
Inventors: Walter Mark Veldsman; Geoffrey Bernard MELTON; Roger William O'BRIEN
Original assignee: BOC Group Ltd; Tallent Automotive Ltd; TWI Ltd
Current assignee: BOC Group Ltd; Gestamp Tallent Ltd; TWI Ltd
Priority date: 2011-01-26
Filing date: 2012-01-24
Publication date: 2013-12-04
Also published as: GB201101379D0; WO2012101399A1

Abstract

A weld cooling apparatus (1) for the forced cooling of a heated weld zone in a workpiece (10) being welded is described. The cooling apparatus (1) comprises at least one nozzle (6) for ejecting expansible liquid coolant and a compliant seal (20). The compliant seal (20) extends from the cooling apparatus (1) and is arranged such that, in use, the seal (20) make sealing engagement with the surface of the workpiece (10). In use, the seal (20) forms a barrier between the coolant and the weld pool. The seal (20) comprises a refractory material, typically a ceramic cloth, for example, a woven alumino silica based fibre cloth, a flexible graphite, or a fibre glass reinforced silicone rubber.

Description

WELD COOLING APPARATUS AND METHOD USING AN EXPANSIBLE

COOLANT AND A REFRACTORY SEAL

The present invention relates to apparatus for, and a method of, forced cooling of a heated weld zone in a workpiece being welded.

During the thermal welding of metallic workpieces a high heat input is required to generate an acceptable weld. However, this high heat input has a disadvantage that the thermal stresses generated by the welding process can cause significant levels of distortion of the workpiece being welded. Sheet workpieces of relatively soft metals and alloys such as mild steel, titanium, titanium alloys and stainless steels are particularly prone to distortion and reduced tensile strength.

It is known to provide forced cooling to the heated weld zone of a work during thermal joining processes (including solid phase welding, for example, friction stir welding), particularly arc welding processes, so as to reduce or eliminate residual stresses generated by the welding process and/or to reduce or eliminate distortion and/or to modify the microstructure of the weld metal.

Particularly, it is known to provide forced cooling by using a jet of cryogen, for example carbon dioxide, from a nozzle head. The requirements of the cooling nozzle head are to typically provide a jet of liquid carbon dioxide of sufficient quantity to a spot, at a required distance behind the welding arc. The distance behind the welding arc is limited by the need to avoid or minimise disturbance to the weld pool and/or welding arc so that the weld quality is not compromised.

It is particularly desirable to provide forced cooling on a top-side/same-side arrangement, for example by delivering coolant through a nozzle immediately behind the welding gun. Such an arrangement enables the integration of the cooling nozzle and welding gun onto a robotic welding system. However, it has been found that the expansion of the cryogenic coolant, for example carbon dioxide, from solid to liquid is such that it causes the weld shielding gas to be blown away. As a result, the welding arc is disturbed directly causing weld defects on the workpiece. The weld defects can be so severe that such weld cooling processes are unsuitable for top side/same side welding and cooling.

According to the present invention there is provided weld cooling apparatus for the forced cooling of a heated weld zone in a workpiece being welded, wherein the cooling apparatus comprises: at least one nozzle for ejecting at the weld zone expansible coolant that undergoes a change of state from liquid or solid to gas; and a compliant seal extending from the cooling apparatus and arranged, in use, to make sealing engagement with the surface of the work to form a barrier between the coolant and the weld pool; wherein the seal comprises a refractory material.

The invention also provides a method of forced cooling of a heated weld zone in a workpiece being welded, the method comprising: a. directing a welding tool at an area of the workpiece to form a weld; b. directing at a weld zone adjacent welding tool a flow of expansible liquid or solid coolant that undergoes a change of phase to gas; and c. providing a compliant seal in sealing engagement with the surface of the workpiece to form a barrier between the coolant flow and the weld; wherein the seal comprises refractory material. The terms "welding" and "weld" as used herein refer to any thermal joining method. The term "weld zone" as used herein refers to the at least the region of the workpiece comprising the solidified weld metal and may further comprise some or all of the heat-affected zone.

Any one or more of a number of different (com)pliant refractory materials may be used to form the seal. The selection of the or each refractory material may be made according to the temperature at which the weld metal melts and they duration of an individual welding operation. It is preferred that the material is able to pass the fire testing standard of AS1055D. Preferred refractory materials are able to withstand continuous exposure to a temperature of 260 °C, exposure to a temperature of 1205°C for periods of up to 15 minutes, and short temperature excursions of up to about 1650°C.

The compliant seal of the invention provides a barrier to prevent the coolant, for example carbon dioxide, from disturbing/interfering with the welding arc and the shielding gas of the weld. Embodiments of the invention therefore reduce the risk of weld defects being formed in the workpiece. Embodiments of the invention therefore allow same side/top side cooling to take place by preventing the cooling gas from interfering with the shielding gas of the weld. Embodiments of the invention also allows welding to take place on thin-sheet materials such as aluminium and titanium alloys with reduced residual distortion and/or with reduced risk of premature failure due to the welded structure.

The compliant seal is typically composed of one or more refractory materials (i.e. having high temperature resistance) so that the seal is not compromised during the welding process. The refractory material may, for example, be of a flexible graphite which has a melting temperature of about 3650°C. The refractory material may alternatively be a ceramic cloth, typically woven. Examples of ceramic cloths include fibre glass cloth, alumina - silica cloth fabric and silica ceramic fibre glass cloth. Such materials can be given further temperature resistance by being coated with a particulate ceramic material. For example, vermiculite - coated high temperature resistant fibre glass cloth can be used at temperatures up to 1645°C.

If desired, the refractory material may be based on silica, The Silica refractory materials can be used continuously at temperatures of up to 2300 °F/ 1260 °C (with occasional excursions to 1650°C )without degradation. Silica - alumina refractory materials have similar properties.

Another alternative refractory material is a fibre-glass reinforced silicone rubber.

In use, the compliant seal conforms to the shape/profile of the surface of the workpiece by deforming against the surface. The seal may be arranged to form sealing engagement with the surface of the workpiece so as to form either a localised or complete barrier around the nozzle. For example, the compliant seal may completely surround the nozzle. The compliant seal may for example be an annular seal.

Alternatively, the compliant seal can provide one or more openings away from the weld to allow the cooling gas to escape in a direction away from the weld. For example, the compliant seal may be horseshoe shaped, a semi-annular seal or a flat seal or may generally confirm to the profile of the weld. It will also be appreciated that a seal which completely surrounds the nozzle, such as an annular seal may be used as either a complete or localised barrier depending upon its orientation and engagement with the workpiece.

The seal may have a three-dimensional profile which substantially matches the surface profile of the workpiece, such that the seal needs only comply to local variations, for example due to build tolerances in the work. For example, the profile of the seal may be flat, concave or convex.

The seal may take any suitable form (for example, a gasket or bellows) for making for providing a barrier between the cooling apparatus and the welding nozzle. The refractory material may be in the form of, for example, a sleeve, a braid, a rope, a tape or fabric.

Preferably, the seal may extend from a first end proximal to the cooling apparatus to a second end distal to the cooling apparatus and the maximum cross-sectional thickness is at, or close to, the distal end. Such an arrangement allows for convenient attachment at the nozzle body while maximising the contact area at the distal end for sealing engagement with the workpiece. For example, the seal may be bulbous (for example. P-shaped, tadpole - or teardrop - shaped) or tapered in shape.

The seal may have an outer skin arranged to form a pocket at the distal end of the seal. The seal may further comprise a flexible member within the pocket. The flexible member may comprise a substantially cylindrical bead extending around the periphery of the seal. The flexible member may comprise a bundle of fibres extending around the periphery of the seal. These fibres may be in the form of a rope, for example a braided rope. The flexible member may comprise a refractory material. One example is a tadpole tape with a braided silica ceramic fibre bulb. The seal may comprise an outer skin comprising a fabric, tape, braid, rope or mesh. The outer skin may comprise any suitable material, such as for example silica, ceramic tape or cloth, silica fabric, stainless steel mesh, or Inconel(RTM) mesh. The choice of outer skin may depend on the temperature of the welding application and the required durability of the seal. The outer skin may, for example comprise, a flexible outer skin composed of a single or a combination of fibre glass woven materials. The outer skin may include a high temperature resistant polymer coating of, for example, reinforced silicone rubber.

The seal may for example comprise one or more ceramic tadpole tapes formed into full or partial rings. The core of the tadpole tapes may be filled with a flexible member such as for example ceramic ropes or fibres.

The apparatus may include more than one seal, for example a plurality of compliant seals. The apparatus may include a plurality of seals in accordance with the invention or a combination of at least one single seal in accordance with the invention and other forms of seal, for example, a wire brush seal.

A wire brush seal is particularly robust as it can cope with high temperatures and at high gas pressure giving an effective seal insulating the welding arc from the cryogenic coolant for various different component geometries. Although the wire brush seal may have limitations if a robotic welder is used, it may be suitable for other automated systems in which a pressure is exerted on the wire brush to obtain effective sealing. A wire brush seal is less effective on robotic systems that are sensitive to back pressure. We have found that in a robotic system, the downward pressure required to create an effective seal was large enough to cause a collision detection device to trip, the collision detection device forming part of a welding torch mounting apparatus. The wire brush seal may be designed and manufactured so as to fit the component geometry which is intended to seal. Thus, the wire mesh assembly is positioned on the components so that the wire ends conform to the shape of the joint to be made. Then the wire brush is moulded in place by applying a suitable adhesive, for example an epoxy resin, to permanently fix the wires to a support ring.

The apparatus may include a plurality of seals composed of the same materials. Alternatively, the apparatus may include a plurality of seals composed of different materials.

The apparatus may include a plurality of seals in layers to simulate flexibility. The plurality of seals may all be of the same shape. Alternatively, one or more of the plurality of seals may be of a different shape. The plurality of seals may be concentric. The plurality of seals may be closed (for example annular in shape), have an opening directed away from the weld zone, or a combination thereof.

Preferably, the compliant seal is provided by double folding a material composed of one or more suitable refractory materials. The double folding of the polymer material increases the compliance and the sealing efficiency of the seal.

The nozzle and compliant seal may be mounted on a movable head of the

apparatus. In use the movable head may provide a force to compress the seal against the workpiece. The movable head may exert a downward pressure on the compliant seal so as to form an effective seal against the workpiece. 64

- 8 -

Preferably, the coolant is a cryogenic liquid or cryogenic solid that undergoes a change of state to a gas in operation of the weld cooling apparatus according to the invention. The term "gas" as used herein includes "vapour". The cryogenic coolant may be any solid or liquid substance having a temperature of minus 50°C or less which is gaseous or vaporous at ambient temperatures, and is preferably solid carbon dioxide. A jet of particles of solid carbon dioxide, in the form of a "snow" can be formed by passing a stream of liquid carbon dioxide at a pressure above its critical pressure through the nozzle. On expansion from the nozzle the liquid carbon dioxide undergoes a change of phase into a gas and a solid snow, the latter having a temperature of minus 78°C. This snow collects on the heat affected zone associated with the weld and extracts heat therefrom causing the snow to sublime. Because it collects on the surface of the heated weld zone and takes its enthalpy of sublimation from the heated weld zone, the solid carbon dioxide tends to be far more effective than lower temperature cryogenic liquids such as liquid argon, liquid air, liquid nitrogen and liquid helium, which all tend to vaporise above the heat affected zone of the weld and are not able therefore to take much of their enthalpy of vaporisation directly from the heat affected zone.

The apparatus may further include an extractor for removing spent cooling gas. The extractor inlet may be provided adjacent to the nozzle. For example the extractor may be a through head extraction device with an inlet which may for example surround the nozzle. The extractor inlet may be provided between the nozzle and the compliant seal. The apparatus may further include a sensor to activate or control the extraction of spent cooling gas.

The invention also provides a welding apparatus comprising a welding tool (for example a welding torch) and a cooling apparatus as discussed above. The position of the cooling apparatus can be varied in order to optimise the cooling effect while minimising any disruption of the arc. The welding apparatus may, for example, be robot mountable.

Embodiments of the invention will now be described by way of example with reference to the accompanying figures, in which:

Figure 1 is a sectional schematic of a welding apparatus utilising a cooling apparatus according to an embodiment of the invention;

Figure 2 shows an experimental welding apparatus in accordance with an

embodiment of the invention;

Figure 3 shows a nozzle for use with embodiments of the invention;

Figure 4 shows a double annular seal arrangement according to an alternate embodiment of the invention; and

Figure 5 shows a welding apparatus according to a further embodiment of the invention.

Referring to Figure 1 , a welding apparatus 1 comprises an arc welding torch 4 (for example a MIG welding torch) and a cooling apparatus 1 in the form of a cooling head 2. The welding apparatus 1 is mounted to an industrial robot (not shown). The cooling head 2 is symmetrical about its longitudinal axis and located above work 10 to be welded. The cooling head 2 is mounted on a carriage 3 behind the arc welding torch 4. The cooling head 2 is mounted behind the welding torch 4 at a variable distance of generally between 40 to 80 mm (weld tip to cooling jet centreline to centreline distances) depending on nozzle and joint configuration. The cooling head 2 comprises a central nozzle 6 which is coaxial with the axis of the head 2. An annular passage 8 is provided in the head 2, which surrounds and is coaxial with the nozzle 6. The annular passage 8 is adapted to, in use extract the vapour of the coolant. The nozzle 6 typically has a relatively small radius, for example of 1.2 mm. In use, this allows the coolant to be directed with a high degree of accuracy at the weld zone. The head may, for example, be of the type described in WO

2007/144673.

The cooling head 2 further comprises an annular compliant refractory seal 20 which surrounds and is coaxial with the nozzle 6. The seal 20 also surrounds and is coaxial with the annular passage 8. The seal 20 extends from a first end 21 proximal to the nozzle 6 to a second end 22 distal to the nozzle. The seal 20 is attached to, and substantially sealed with the cooling head 2 at the first end 21 (for example, the first end 21 may be clamped to the outside of the cooling head 2). The seal 20 has a generally bulbous cross sectional shape with a maximum thickness close to the second end 22. The seal 20 has an outer skin 24 formed from a high temperature resistant material. The outer skin 24 is arranged to form a pocket containing a high temperature flexible member 26 at the distal end 22. The high temperature flexible member 26 provides an increased contact area with the workpiece 10 while allowing the seal to be highly conformable to the surface and profile of the workpiece 10. The bulb material can be of alumina-silica rope, stainless steel mesh or Inconel® mesh. The outer skin 24 can be of flame and fire resistant ceramic, woven, alumino-silica based fibre cloth. Such a seal so can be used continuously at temperatures up to 1093°C with occasional excursions to 1650°C. Figure 2, shows an experimental embodiment of the invention in which the seal 20 comprises an outer skin 24 of refractory ceramic cloth secured to the cooling head 2 by means of a jubilee clip 30. The seal 20 has a tadpole-shaped cross-section with a bulb diameter of 10 mm at the distal end and a tail length of 15 mm. The distal end 22 of the seal contains a flexible member (not visible) in the form of a silica/ceramic braid.

In operation, the arc welding torch 4 and the cooling head 2 are moved in unison over the work 10, with the cooling head 2 trailing the arc welding torch 4. The welding torch 4 is positioned relatively close to the work 10 being welded. Typically, the cooling head 2 has a stand off position of from 30 mm to 50 mm from the heated weld zone (not shown). An advantage of such a relatively short stand-off distance is that the jet of cryogenic coolant issuing from the nozzle 6 diverges and loses momentum only to a limited extent.

The cooling head 2 provides a downward force to compress the compliant seal 20, and in particular the bulbous end of the seal 22 containing the flexible member 26, against the work 10. The downward pressure causes the compliant seal 20 to conform to the shape/profile of the work 10 and form an effective seal. If desired, in order to ensure that the compliant seal 2 forms an effective seal on the work 10, the compliant seal 20 may be run along the work in the direction of the weld before the start of the welding process. This movement of the seal 20 along the work 10 advantageously allows the seal to flex and deform to the surface of the workpiece 10 prior to the weld. A heated weld zone (not shown) is formed by the deposition of weld metal from the arc welding torch 4 on to the work 10. A stream of liquid carbon dioxide, or other coolant, under pressure (typically in the range of 15 to 25 bar) is passed from a source thereof (not shown) to the cooling head 2. Control of the coolant can be achieved by any suitable means, such as for example by a needle valve. A suitable flow rate of the coolant, for example a cryogenic coolant can be for example 0.37 kg/min. As the liquid carbon dioxide passes through the outlet of the nozzle 6, so it is converted into a jet of gas carrying particles of solid carbon dioxide. This jet deposits solid carbon dioxide on the heated weld zone (not shown) as the head 2 follows the welding tool 4. The cooling region is typically between 10 and 60 mm behind the welding arc, for example from 20 to 30 mm.

The liquid carbon dioxide is supplied at a pressure such that solid carbon dioxide will not be formed until the pressure is released as a consequence of the ejection of the carbon dioxide through the nozzle 6. The nozzle 6 is arranged to eject the pressurised stream of carbon dioxide at a high, preferably supersonic, velocity.

Accordingly, the nozzle 6 is preferably a Laval nozzle which has a characteristic convergent-divergent bore at its distal end. It is as the liquid carbon dioxide is expanded rapidly in the divergent section of the Laval nozzle that a part of it solidifies. At optimal conditions about 40% is converted to solid and the rest to gas. The solid particles are accelerated by the gaseous component. One advantage of employing a high velocity jet of cryogenic coolant is that it has the momentum necessary to penetrate any blanket of vapour that forms in operation over the heated welded zone (not shown).

The compliant seal 20 provides a barrier to prevent the cooling gas, carbon dioxide, from disturbing/interfering with the shielding gas of the arc welding tool 14. An advantage of such a compliant seal 20 is that the risk of weld defects being formed in the work 10 is significantly reduced. The cooling apparatus can be used for top side/same side welding and cooling with reduced induced distortion and/or residual stress.

Figure 3 shows a modified nozzle arrangement 100, which is specifically adapted for use with embodiments of the invention. The modified nozzle arrangement 100 is provided with a clamping device 110, which surrounds and is coaxial with the nozzle 106. The clamping device 110 provides a convenient attachment means for a compliant seal and may take the form of any kind of retaining ring.

With reference to Figure 4, multiple seals 220, 225 may be provided to further increase the shielding effect of the invention. In this embodiment, the inner annular seal 220 surrounds and is coaxial with the nozzle 206 and the outer annular seal 225 surrounds and is coaxial with the inner seal 220 (and the nozzle 206). Both the inner seal 220 and the outer seal 225 are formed from a ceramic cloth and have a construction as described above with reference to Figures 1 and 2. It will be appreciated that the inner and outer seals 220, 225 may be of different shapes, for example one may be annular while the other may be U-shaped (with the opening away from the welding torch). Other possible configurations include a half moon - shaped tadpole - type seal applied to a steel wire brush seal. In such an

arrangement the wire brush may be associated with a thin half moon - shaped steal strip support.

Figure 5 shows a welding apparatus 301 according to a further embodiment of the invention. The welding apparatus 301 comprises a welding torch 304 and a cooling head 302. In this embodiment the seal 320 is in the form of a skirt of high

temperature resistant (i.e. refractory) polymer. The compliance of the polymer skirt seal 320 may be increased by double folding the polymer. The polymer may be a silicone rubber coated fibre glass. Such a seal is useful for hold times up to and in the order of one minute if a flat, planar weld is to be produced. It withstands weld splatter and molten splash and may be continuously exposed to temperatures of up to 260°C; and for periods of up to 15 minutes to temperatures of up to 1205°C with excursions to 1650°C. The skirt seal 320 typically needs replacing after about eight welding operations.

Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

For example, while the preferred embodiments of the invention have been described for use in conjunction with conventional arc welding methods, (such as MIG welding, TIG welding and plasma arc welding) they may also be used to provide cooling to any other thermal joining process. For example the invention may also be used in conjunction with solid phase welding (FSW), such as friction stir welding, so as to reduce or eliminate distortion or to modify the microstructure of the weld metal.

Claims

1 . A weld cooling apparatus for the forced cooling of a heated weld zone in a workpiece being welded, wherein the cooling apparatus comprises at least one nozzle for ejecting at the weld zone expansible coolant that undergoes a change of state from liquid or solid to gas; and a compliant seal extending from the cooling apparatus and arranged, in use, to make sealing engagement with the surface of the workpiece to form a barrier between the coolant and the weld pool; wherein the seal comprises a refractory material.

2. A weld cooling apparatus according to claim 1 , wherein the refractory material is a flexible graphite, a ceramic cloth or a fibreglass reinforced silicone rubber.

3. A cooling apparatus according to claim 1 or claim 2, wherein the seal

comprises an annular seal surrounding the nozzle.

4. A cooling apparatus according to any preceding claim, wherein the seal

extends from a first end proximal to the cooling apparatus to a second end distal to the cooling apparatus and the maximum cross sectional thickness is at or close to the distal end.

5. A cooling apparatus according to claim 4, wherein the seal has a substantially bulbous cross sectional profile.

6. A cooling apparatus according to any preceding claim wherein the seal has a high-temperature resistant polymer coating.

7. A cooling apparatus according to any preceding claim, wherein the seal has an outer skin comprising a fabric, tape or mesh.

A cooling apparatus according to claim 7, wherein the seal has an outer skin formed from high temperature ceramic cloth.

A cooling apparatus according to any preceding claim, wherein the seal has an outer skin arranged to form a pocket at the distal end of the seal and the seal further comprises a flexible member within said pocket.

A cooling apparatus according to claim 9, wherein the flexible member comprises a substantially cylindrical bead extending around the periphery of the seal.

1 1 . A cooling apparatus according to claim 9, wherein the flexible member

comprises a bundle of fibres extending around the periphery of the seal.

12. A cooling apparatus according to claim 9, 10 or 10, wherein the flexible member comprises a high temperature resistant silica, ceramic or polymer.

13. A cooling apparatus according to any preceding claim, wherein the seal has a profile which substantially matches the surface profile of the workpiece.

14. A cooling apparatus according to any preceding claim, comprising a plurality of compliant seals.

15. A cooling apparatus according to any preceding claim, wherein the nozzle and compliant seal are mounted on a movable head and, in use, the movable head provides a force to compress the seal against the workpiece.

16. A cooling apparatus according to any preceding claim, further comprising an extractor for removing spent cooling gas.

17. A welding apparatus comprising a welding tool and a cooling apparatus

according to any preceding claim.

18. A welding apparatus according to claim 16, wherein the welding apparatus is robot mountable.

19. A method of forced cooling of a heated weld zone in a workpiece being

welded, the method comprising: a. directing a welding tool at an area of the workpiece to form a weld; b. directing at a weld zone adjacent the welding tool, a flow of expansible liquid or solid coolant that undergoes a change of state to gas at; and c. providing a compliant seal in sealing engagement with the surface of the workpiece to form a barrier between the coolant flow and the weld; wherein the seal comprises a refractory material.

20. A method as claimed in claim 19, in which the welding tool is moved along the workpiece and the coolant flow tracks the movement of the welding tool.

21. A method as claimed in claim 20, in which the compliant seal is run along the workpiece prior to starting the welding process.

22. A method as claimed in any one of claims 19 to 21 , in which the coolant is ejected from a cooling head and the compliant seal extends from the cooling head for engagement with the surface of the workpiece.

23. A method as claimed in any one of claims 18 to 21 , wherein the coolant is liquid carbon dioxide which undergoes a change of state to solid and gas, wherein the solid is deposited on the heated weld zone and extracts enthalpy of sublimation therefrom.