Method of welding metal plates

Abstract

In joining large metal plates 22, 22’; initially friction stir welding (FSW) 26 is used underneath the joint; and subsequently electron beam welding (EBW) 28 occurs above the welding line. The metal plates 22, 22’ are in excess of 50 tonnes. A vacuum chamber 36 may be used. The heavy metal plates 22, 22’ may be supported on a conveyor, with electron beam welding 28 occurring downstream of the friction stir welding 26 (figure 2). The initial FSW 26 root pass weld may seal the bottom of the joint, before EBW 28. Multiple metal plates 22, 22’ may be rolled and joined to form a large metal tube, having a diameter in excess of 11 m, e.g. for pressure vessels, reactors and wind turbines. The metal plates 22, 22’ may comprise C-Mn structural steel, steel alloys, nickel, titanium, or aluminium.

Description

Figure 1 shows a prior art arrangement for electron beam welding together two rolled metal plates 10, 12 to form a hollow cylindrical tube also known as a can, such as is used to form transition pieces in wind turbine monopiles or to create walls of a pressure vessel or reactor. Typically each plate is around 10m long and around 4m wide, with a thickness of between 90 to 130mm. An electron beam gun within housing 14 is used to weld longitudinal seam 16 where the plates 10, 12 adjoin, seam 16 being held horizontal so as to avoid weld material dripping out of the seam during formation. In Figure 1, one seam position is shown with the second seam position obscured by housing 14. A human operative 18 is shown to give an idea of scale. This type of arrangement can be used to weld tubes of many meters in diameter but for diameters above 11m more than two metal plates are needed and it becomes very difficult to handle the plates and achieve good quality welds. In general, when welding adjoining metal plates, welds along vertical edges result in liquid metal created during the welding process falling out due to gravity, producing a porous low quality weld. In prior art arrangements a backing strip can be secured between the lowermost edges of adjoining plates to prevent this happening, the backing strip being removed after welding. Another option is to use arc welding to create a plug extending along the lower edges to seal the bottom of the joint welding line which is distal from the incident electron beam. However these arrangements compromise the qualities of the final weld and / or lead to delays in weld testing. Figure 2 shows apparatus 20 for welding a plurality of metal plates 22 together for subsequent rolling into a cylinder greater than 11m before final welding. Typically each plate 22 is around 10m long and around 4m wide, with a wall thickness of between 90 to 130mm and has a weight in excess of 50 tonnes, generally in the range 50 to 100 tonnes. Typically three or more plates are to be welded together and after welding are to be rolled into a cylinder in excess of 1 Im diameter. The metal plates are formed from steel such as C-Mn structural steel, steel alloys or any other metallic material requiring welding, such as Nickel, Titanium, Aluminium and their alloys. Apparatus 20 comprises an electron beam welding assembly 24 and a friction stir welding machine 26. Electron beam assembly 24 comprises an electron beam gun 28 disposed above plates 22 and movable across the metal plates so as to be able to weld a longitudinal seam along a joint welding line between adjacent plates. Electron beam gun 28 moves perpendicular to the direction of travel 30 of metal plates 22 along roller conveyor 32, generally moving in a horizontal plane parallel to upper surface 34 of plates 22. Electron beam gun 28 is within a sealed housing 36 evacuatable to create a vacuum, with plates 22 moving along roller conveyor 32 into this evacuatable region 36 for welding. Friction stir welding machine 26 is also movable across metal plate 22 using a linear drive arrangement and is disposed beneath plates 22 proximal lower plate surface 38 so as to perform overhead welding. Before electron beam welding takes place, friction stir welding machine 26 is used to create a continuous root pass weld 42 extending along the entire length of joint welding line 40 between adjacent plates so as to join plates 22, 22’ and seal the bottom of joint welding line 40 before electron beam welding takes place, see Figures 3 and 4. The root pass weld 42 is performed from beneath the plate as an overhead weld, with typically Argon used as a shielding gas to reduce oxidisation during the weld process. The root pass weld 42 typically has a depth of around 12 to 20mm. After root pass weld 42 has been created along the width of adjoining plates 22, 22’, electron beam welding takes place along an upper edge of joint welding line 40 so as to create a fully welded seam along the width of plates 22, 22’ and extending from upper surface 34 to lower surface 38. The seam is cool to the touch immediately after welding and so can be tested straightaway, unlike when arc welding is used and cooling time of some hours is required before testing can take place. By creating a root pass weld before electron beam welding, it is possible to weld vertical edges of adjoining metal plates using electron beam welding without compromising the quality of the electron beam weld. The resulting seam being a combination of a friction stir weld and an electron beam weld is sufficiently resilient to ensure conjoined plates can be rolled into a hollow cylindrical shape ready for final welding without the welded seams failing. Thus at least three adjoining metal plates are weldable together for formation into a tube with a diameter in excess of 1 Im. In Figure 2, friction stir welding machine 26 is positioned upstream of electron beam assembly 24 so that any given joint welding line encounters friction stir welding machine 26 before electron beam gun 28. This allows one joint welding line 40 to be friction stir welded to create a root pass weld whilst simultaneously a joint welding line already processed to have a root pass weld can be welded by electron beam assembly 24, so allowing for faster overall processing of plates 22. If desired, a welding head of friction stir welding machine 26 can be replaced with a milling cutter 50, see Figure 5, with the edges of joint welding line 40 milled from underneath along the width of sheets 22, 22’ in the direction of arrow 51 using a linear drive to ensure any burrs or rough edges are removed in the region of the root pass weld, and ensure metal plates 22, 22’ can be butted up close to each other. After milling, milling cutter 50 is replaced with stir friction welding head 52, plates 22, 22’ are butted closely together, and the friction stir welding root pass weld 42 created, see Figure 6, before electron beam welding takes place. By milling and then subsequently friction stir welding with the same machine, there is no need to move the metal plate between milling and friction stir welding. The process of creating a root pass weld along the joint weld line before electron beam welding, ensures that metal plates with vertically adjoining faces can be welded together without loss of welding material. Replacing friction stir welding head 52 with milling cutter 50 so as to remove burrs and rough edges ensures that adjoining plates can butt close together improving the quality of the root pass weld and ensuring gaps do not occur around the root pass weld if the plates have irregularly shaped edges. The creation of a root pass weld 42 ensures that large heavy metal plates can be processed in a cost effective and time efficient manner by electron beam welding prior to rolling. Further the quality of the root pass weld can be checked and defects corrected before the metal plates are moved along roller conveyor 32 to the electron beam welding machine as the friction stir welding generates little heat. If desired and as shown in Figure 7, friction stir welding machine 26 can be incorporated into a vacuum housing, and if desired in the same vacuum housing 36 as electron beam assembly 20, with this preventing the friction stir welding head oxidising, so extending the life of the friction stir welding tool. Electron beam gun 28 and friction stir welding machine 26 weld along the same joint line from opposing sides, with friction stir welding machine 26 completing a root pass weld before the electron beam gun completes the weld. 5 When welded metal plates are rolled to form a final cylinder 58 that requires welding along a final seam 60, as shown in Figure 8, optionally edges 62, 64 are milled using milling cutter 50, before being abutted together and friction stir welded as in Figure 9. Whilst the embodiment shown in Figure 9 uses argon shielding, if desired an electron io beam assembly 70 and friction stir welding machine 26 positioned within evacuatable housing 74 can be used as shown in Figure 10, housing 74 evacuatable using vacuum ports 76, 76’. Friction stir welding machine 26 creates a root pass weld along joint welding line 60 before electron beam welding takes place.

Claims

1. A method of welding metal plates comprising:i) undertaking friction stir welding using a friction stir welding machine along a lower edge of a joint welding line between adjoining metal plates weighing in excess of 50 tonnes so as to create a root pass weld along the joint welding line, the friction stir welding machine disposed beneath the plates so as to weld in an overhead position; andii) after friction stir welding, undertaking electron beam welding using an electron beam gun along an upper edge of the joint welding line so as to create a welded seam between adjoining plates, the electron beam gun disposed above the plates.

2. A method according to Claim 1, wherein each metal plate has dimensions of each around 10m long and around 4m wide, with a wall thickness of between 90 to 130mm.

3. A method according to Claim 1 or Claim 2, wherein at least three adjoining metal plates are welded together and formed into a tube with a diameter in excess of 1 Im.

4. A method according to any of the preceding claims, further comprising replacing a welding head of the friction stir welding machine with a milling head and milling along the joint welding line before friction stir welding takes place.

5. A method according to any of the preceding claims, wherein the root pass weld is 12 to 20mm in depth.

6. A method according to any of the preceding claims, wherein the electron beam welding takes place within a vacuum.

7. A method according to any of the preceding claims, wherein the friction stir welding takes place within a vacuum.

8. A method according to any of the preceding claims, wherein the electron beam welding and the friction stir welding take place within a common evacuatable housing.5   9. A method according to any of Claims 1 to 7, wherein the metal plates aremoveable along a conveyor and the friction stir welding machine and the electron beam gun are spaced apart with respect to the direction of travel of the metal plates, the friction stir welding machine upstream of the electron beam gun.io 10. A method according to any of the preceding claims, wherein the metal plates are be formed from one or more of the following: steel, C-Mn structural steel, steel alloys, Nickel, Titanium, Aluminium, Nickel alloys, Titanium alloys, Aluminium alloys.

11. A metal tube made in accordance with the method of any of Claims 1 to 10.1512. A metal tube according to Claim 11 having a diameter in excess of 1 Im.

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