CA1292641C - Construction of large sandwich structures - Google Patents

Abstract

ABSTRACT
Composite metal panels comprise two parallel plates each laser-welded to an internally sandwiched corrugated stiffener plate . Typically, all the welds are in the same sense: firstly, penetrating laser welds 6b are made along and through the troughs of the corrugations into (or, less preferably, through) an underlying plate and secondly welds are made along and through an overlying plate along and into or through the peaks of the corrugations . Such a panel can be readily fabricated into large-scale metal constructions, especially ships.

Description

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M~C FOLIO: 230P49900 WANGDOC: 0206r CONSTRUCTION OF LARGE SANDWICH STRUCTURES

This invention relates to the construction of large sandwich metal structures from plate material.

It has particular application to ship building, in which composite metal structures incorporating metal plate are used in hull, superstructure, deckhouse, bulkhead or hatch fabrication etc, and accordingly the invention will be described below primarily in such a context.
However, the invention also has utility in respect of 10 other structures such as linkspans,bridges, oil rigs, offshore structures, platforms, containers, buildings, columns, pontvons, tubes, pipes, and like large welded constructions.

In the construction of ships, the basic unit of the hull 15 or bulkhead construction and other parts of ship construction is conventionally a composite pansl. Such a panel is typically made by (a) cutting up plate material into predetermined sizes (b)butt-welding together the edges of a number of such plates, and (c) 20 applying stiffener bars parallel to (or across) the butt welds. This forms a stiffened panel, stiffened by `` ~29Z6 ~i the bars which are of various cross sections but often rather L-shaped in cross-section with a short foot portion integral with a long shank portion projecting from the panel. Such panels can be further fabricated using large dimension connecting webs welded to the plates at right angles and/or parallel to the stiffeners, and possibly also mounting another such stiffened plate opposed to the first to form a double skin compartment.

Conventionally, a number of different weld techniques are used in this construction such as butt welding, fillet welding, overhead welding and the like. The process can moreover involve major lifting and turning of the panel during its construction and/or can involve the need for welding operatives to work in confined internal spaces of a double skin. Our own Canadian Patents Nos. 1,226,336 and 1,224,534 issued September 1, 1987 and April 19, 1984 respectively discuss weld techniques involving lasers to rationalise and facilitate these known methods.

These earlier laser techniques were however confined to utilizing lasers in such a way as to produce a composite article of more or less conventional overall appearance (that is to say conventional apart from details of the weld) which can be utilised as it stands in existing structures.

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Accordingly a first aspect of the invention provides a composite metal panel comprising two parallel plates secured to the peakc and troughs respectively of a sandwiched corrugated metal stiffener by weld lines extending along the peaks and troughs and constituted by laser through-welds of the type provided by passing a high-intensity laser beam into or through both layers of metal along the peak or trough, all of said through-welds being in the same sense.

We have now discovered a generally new construction of composite panel; methods for its fabrication; methods for its onward utiiization (that is to say, joining up with similar such composites); details of fixing, repair and transition techniques using such composites; and, in general, a new type of sandwich large-scale metal construction, especially of ship construction using such a composite.

The invention also provides a panel as claimed in claim 1 in which the parallel plate portions are thicker than the one corrugated plate portion.

In one aspect the invention provides a ship or like large-scale metal construction in which structural, sub-dividing, or enclosing, hulls, bulkheads, deckhouses or like structures comprise composite metal panels consisting of two parallel plates secured to the peaks and troughs respectively of a corrugated metal stiffener plate arranged between the parallél plates.

The type of structure defined above can be used alone, for example, for bulkheads, or like sub-division or cladding elements. However, two or more such structures can themselves be linked together and form a parallel composite to define a hull compartment including sub-divisions.

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l~9Z6'~1 In another aspect the invention comprises a composite metal panel comprising two parallel plates secured to the peaks and troughs respectively of a sandwiched : - 3a -1292~

corrugated metal stiffener plate by weld lines extending along the peaks and tcoughs.

Such a panel is a preferred constructional element of the ship construction and others as defined above .

Although the techniques of the invention can be used over a wide range of thicknesses, generally speaking plates from 1 to 25 millimetres thickness are envisaged - for the component parts of the composite panel applied to ships. For other applications greater thickness may 10 be required. It will be found usually preferable to have the two parallel plate portions each thicker than the metal thickness of the corrugated plate portion.

The spacing of the corrugations ( that i5 to say. the distance between corresponding points of one corrugation 15 and the next) taken as a ratio to the separation of the parallel plates can vary again over wide ranges but is preferably within the range of 1.5:1 to 1:1.5. More preferably it lies within a closer range, of 1.1:1 to 1:1.1, that is to say about unity from a production 20 standpoint. Weight and strength considerations in different applications may require a wider range of this ratio.

It is a preferable feature of the invention that the lZ~26'~1 corrugated sheet between the panels has flat peaks and teoughs . The external width measurement of the flat peak where adopted is not limited but from a production standpoint preferably lies between the ranges of 1:3 to 1:7, that is to say, an average of about 1:5, in relation to the plate spacing. Again, weight and strength considerations in different applications may require a wider range of this ratio.The area in contact of course will be narrower by virtue of the thickness 10 of, and the radius of any bend in, of the corrugated plate.

The weld lines fixing the parallel plates and corrugated plates together are most preferably those formed by laser through-welds, of the type provided by passing a 15 high-intensity laser beam into or through both layers of metal and along the position of the peak or trough. Most valuably, the through welds are provided all in one sense, that is to say so that successive welds pass through (a) trough material then surface plate material 20 and (b) plate material then peak material. To facilitate this, and as a further aspect of the invention a design of plasma control device has been produced which will allow a gas-supplying shoe into the valley of a corrugated profile to improve laser through-welding of 25 the corrugated stiffener plate of the lower parallel plate.Such an arrangement, as described in more detail below, allows a single weld orientation to be used and - lZ926'11 the avoidance of turning over of the composite panel structure halfway through its manufacture.

More particularly, the plate preferably has along one surface a number of weld lines which pass through the surface plate and into an underlying peak; and on the other surface no visible weld lines but an internal weld line structure each line of which passes through a trough and into the material of the plate without however totally penetrating that material. This gives a composite panel with weld lines on one face but smooth surfaces on the other face. Of course, in a particular application it may be re~uired that a weld should completely penetrate through a facing plate.

The construction of the sandwich structure will not (in normal workshop or site conditions) prevent gaps occurring between, on the one hand, t.he troughs and peaks of the corrugated plate, and, on the other, the face plates. The preferred method of welding using a high-power density laser beam also however allows the addition of material to run into and fill the gaps between the plates making the sandwich panel. One method of adding material to fill the gap is by wire feed. The addition of wire feed also makes it possible to obtain a fine control over production as described in our own Canadian Patents Nos. 1,226,336 and 1,224534 issued September 1, 1987 and April 19, 1984 respectively.

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The product of the present invention is referred to as a sandwich composite panel of plate material. It is an understood terminology that plate material, in the metallurgical art, is a thicker, denser and more dimensionally stable and stiff grade of material than sheet material. Typically, plate thickness lies within the range of l to 25 millimetres as defined above, although in certain circumstances may extend above or below these limits.

Most typically, but not by way of limitation the sandwich composite is of a thickness such that the spacing between the external plates lies between 25 and 200 millimetres, more preferably between 50 and 150 millimetres or, in a preferred standardised embodiment about 50 and 100 millimetres. The surface plate in such an instance is between 5 and 15 millimetres thick in typical embodiment, and the corrugated plate is preferably similar and within the range of 2 to 10 millimeters thick.

The application of the invention to plate materials and minimisation of the use of rolled stiffener sections is of considerable importance.

:. j ~ - 7 -It is known for example in aircraft construction to form a sandwich material of thin lightweight metal sheet with corrugated metal reinforcement inside. Such material is normally made by electron beam welding, bue there has 5 been a prior proposal to produce the material by a form of spot welding,known as pulse trains, using lasers.
However, the overall composite product is very thin (the only example known i8 8.7 millimetres thick external dimensions ) and is made of extremely thin 10sheet certainly not exceeding Lmm in thickness. Also, it is always welded by through welds from the outside inwards, in contrast to the much preferred embodiment of the present invention which welds in one sense, that is to say alternately from the inside outwards and from 15the outside inwards.

The present invention does not represent therefore a mere change of scale as compared to the above invention.
More specifically, the differences are (a) A different type of welding in the prior art, that 20is to say spot (pulse train) welding as against continuous seam welding,also by a small scale 300w YAG
laser as against a multi-kilowatt Co2 laser, giving two-dimensional welding problems as against three-dimensional in the large scale now proposed.

129Z6~1 (b) Different weld directions on different faces in the present invention thus obviating the need to turn over the composite. This is particular importance with a large scale fabrication and also permits a different design of the whole production line with consequent equipment savings (c) A different field of eventual use in ships, buildings, offshore structures, containers, platforms, linkspans, oil rigs, pipelines and other large scale 10 users.The use of sandwich structures in ships etc results in a superior end product from the aspects of weight, strength, headroom,damage from various causes, intactness, cleaning, painting, liquid flow and the transmission of vibration, heat and noise, etc.

15 (d) The design of joints between sandwich composite panels end-on, side-on, and perpendicular to one another.

(e) The use of sandwich structures in cylindrical or non-cylindrical arcuate shapes as against flat shapes.

(f) The design of penetrations and holes in sandwich 20 structures.

(g) Repair procedures lZ9Z64~

(h) Protective measures against corrosion.

(i) The design of a laser-equipped sandwich panel line.

(j) Different characteristics of welding procedure including the use of paint-primed materials.

As to the last of these laser weld procedure depends upon the total energy input; extent and type of focusing; loss of metal if any by evaporation: plasma formation, or retention, or re-shaping; heat conduction away from the weld; surface tension of molten metal 0 shape: and similar ehysical or chemical characteristics in the extreme conditions at the point of welding. Most if not all of these conditions are not applicable in at all the same fashion in different scales of working. The Applicants have nonetheless found that through welding 15 on a seam basis can be used and can be used moreover in either direction, that is to say, with the thick sheet before the thin sheet, or with the thin sheet before the thick sheet.

On the large scale it is not practical to depend upon 20 the pressing together of flat plates and corrugated stiffener plate. The through-welding procedure has been developed to cater for gaps between these two component parts. A wire feed system is used to account for such 129Z6~1 gaps.

Moreover, an option in the finished product is of obtaining one surface free from weld lines and thus being particularly suitable to form the inne~ surface of containing tanks or holes,or the outer surface of hulls.

In a further aspect welds can be made from inside the sandwich panel and thereby obtain two outer surfaces-free from weld lines.

The invention will be further described with reference 10 to the accompanying drawings in which:-Figure 1 is a cross-section through part of a composite double-skinned panel according to the invention, Figures 2 and 2a show forms of transverse assembly of the composite of Figure 1, in perspective view, 15 Figure 3 shows one form of longitudinal assembly of the composite of Figure 1, in perspective view.

Figure 4 shows a perspective view of a preferred corne~
configuration of the panel facilitating fi~ing as in Figures 2 and 3, lZS~Zt;~l Figure 5 shows a flow sheet of a flat fabrication sequence, Figure 6 shows a diagram of one arrangement of a production line for making such composites; in a particular application other design solutions may be adopted.

Figure 7 shows a hole for ducting or hatchway access formed through a composite panel according to the present invention. and Figure 8 shows a laser-welded butt-through joint.

Figure l shows a section through a portion of composite panel in accordance with the invention~The composite panel comprises a top plate l, a bottom plate 2, and an internal corrugated plate 3, the corrugations of which have, in the instance shown, flattened peaks and troughs 4 and 5 respectively. The plates l and 2 are-secured to the corrugated plates 3 by laser through-welds referenced at 6a at the upper plate, and 6b at the lower plate. Such laser welds can be made by the methods described in our earlier Patent Applications, namely by continuous tracking of for example a lO kilowatt focussed laser beam. As shown, the various welds 6 do not penetrate the underside of the lower plate. It would 1;Z~26~1 appear that there is no 106s in weld strength if the underside is penetrated, but it is preferable if, at least for the lower welds 6b, complete penetration is avoided so that an unmarked surface is provided on at S least one side of the composite.

The composite as shown can readily be made by such welding without turning over, utilising the weld beam in a single vertical direction as shown by the arrows.
- To make such a composite, the lower plate 2 is positioned as required, the corrugated plate 3 is placed upon it and welded along each trough 5 to form the weld 6b, and the plate 1 is placed over the corrugated plate and welded along each peak 4 to form the weld 6a. Since the peaks and troughs are flat they readily lend themselves to such welding, and since they are regularly spaced it is simple to arrange the laser welding head to follow the underlying course of the flattened peak 4 even though the peak itself may be covered by plate 1.

The sandwich composite as shown can be made in a number of different relatively large scale sizes, and is not to be confused with the very thin metal skin, typically less than lO millimetres in overall thickness, produced for aeco8pace purposes. Typically, the sandwich composite of the invention has an internal spacing dimension between the opposed inner faces of plates 1 lZ~

and 2 of 50 or lOO millimetres, and a corrugation pitch of an equivalent amount, from, for example, the mid points of each peak to the next peak. The thicknesses of plate can vary, as previously indicated,depending upon the strength of sandwich composite that is required. In the example the thicknesses are for plates 1 and 2 are 8 millimetres thick, and plate 3 is 3 millimetres thick .

Figures 2 and 2a shows methods of joining such a composite panel to an adjacent such panel by welding of 10 a transverse connection. While several methods are possible, it has been found by the Applicants that one of the methods as shown is to be preferred.

With reference to Figure 2, Alternative l:-in this attachment procedure, composite panels A and B are utilised. As before, each possesses a top plate 1, a corrugated plate 3 and a lower plate 2. The end configuration of the panels will be apparent from Figure 2, being such that the corrugations 3 project by a distance (a) beyond the edge of plate 1, and that plate 2 projects by a distance (b) beyond the ends of the corrugations. The two panels A and B, of these particular end configurations, are united with the help of a cocrugated insert C and a closing plate D. The insert C has a total width of twice the distance ~b) tminUS any weld gap requirements) and is used to fill ~:926~i the gap between the ends of the corrugations of panels A
and B. As shown in dotted lines, backing corrugation strips (not shown) may be used depending upon the welding process adopted. The closing plate D has a total width of (2b + 2a) (again, minus any weld gap requirements) and is used to fill the gap between two top plates on panels A and B. The plates may have such edge preparations as necessary for the welding process used.

10 To assemble two such panels, the first weld to be made would be the lower face plate butt weld, one end of which is shown at 7, which can be done by laser or by a conventional process such as a manual metal arc or submerged arc if necessary. The second welds to be made 5 would be the two sets of butt welds between the corrugated plates 3 and insert C. The final welds would be two further butt welds at 8 and 9 when the closing plate D is placed in position. If appropriate in a particular application laser welds could be incorporated along some or all of the peaks and troughs of such an intermediate structure. In between the various welding operations the activities of cleaning, inspection and anti-corrosion treatment may be carried out as known per se.

25 With reference to Figure 2i, Alternative ~, there is shown a second method of joining such a composite panel to an adjacent such panel by the welding in of a 129Z6~1 transverse connection.

In this attachment procedure composite panels A and B
are utilised as in Alternative L. The two panels A and B
of this particular end configuration, are again united with the help of a corrugated insert C, closing plate D, but additionally stopper plates El and E2 are utilised.
The stopper plates have a height equal to the depth of the corrugated plate minus the thickness of the corrugated plate.

10 The insert C has a total width of twice the distance (b) minus twice the thickness of the stopper plate plus any gaps required by processes. The closing plate D has a total width (2b + 2a) minus any gaps required by processes and is used to fill the gap between the two 5 top plates on panels A and ~.

To assemble two such panels, the first weld to be made would be the lower face plate butt, one end of which is shown at 7, which can be done by high power density laser or conventional welding processes. The second 20 substantive welds to be made would be the two sets of butt welds between the corrugated insert C and the stopper plates E~ and E2, which would have been previously laser welded to the corrugated plate 3 during the fabrication sequence. The final welds would be two 25 further butt 9Z6~1 welds at 8 and 9 when the closing plate D is placed in position. If appropriate in a particular application laser welds could be incorporated along some or all of the peaks and troughs of such an intermediate structure.

In between the various welding operations the activities of cleaning, inspection and anti-corrosion may again be carried out.

Figure 3 shows the joining of two composite panels A
and B in a longitudinal manner. The procedure to make 0 such a connection is considerably simpler than that for a transverse connection as shown in Figures 2 and 2a, the first weld being a butt weld lO and the second weld being two similar butt welds 11 and 12 when the closing plate E is in position. It will be apparent from Figure 3 therefore, that both the upper and lower plates 1, 2 prefecably terminate approximately halfway across a peak or trough respectively. The plates may have edge preparations as required by the welding processes.

Figure 4 shows a corner of a composite panel in accordance with the invention, showing a preferred configucation of upper and lower plates 1 and 2, in relation to corrugated plate 3, at this location. As will be apparent from a consideration of Figures 2, and 2a and 3, it is preferred for the top plate 1 to 12926 ~1 terminate at a distance (a) from the ends of the corrugations, and for the bottom plate 2 to project for a distance (b) beyond such corrugations. It is also preferred,but not essential, for the top plate 1 to finish approximately halfway across a peak 4, as shown and it is preferred for the bottom plate 2 to finish similacly approximatately halfway across a trough 5.

It will be appreciated therefore that a composite panel of the type of cross-section generally as shown in 0 FiguIe 1, and having the corner configurations generally as shown in Figure 4, is a particular valuable embodiment of the present invention. It will also be appreciated that Alternative 2 (Figure 2a) simply adds a stopper plate to the end of the corrugated plate 3.

Figure 5 is a flow chart showing a typical composite sandwich panel fabrication sequence.

The example sequence shown is developed for the fabrication of a standard unit panel and is designed to permit a throughput capacity of a composite panel at regular intervals using an appropriate laser power per unit with sufficient units arranged as necessary.

The 3-millimetre and 8 millimetre plates are taken from the stock yard, levelled and cleane~. Following this lZ9Z6~i stage they are sent on separate paths. The 3-millimetre plate is sent to cold-rolling or other process for formation of the necessary corrugations.

The 8-millimetre plate is cut to precise size, and butt-welded to form a panel of plate.The eanels are split into two streams. One face plate panel stream, destined to be the lower panel of plate, receives the corrugated plate, and after tack welding is welded through the flat troughs of the corrugated plate to form the initial part of the composite. Butt through welds (see Figure 8) may be required to join corrugations to face plate if more than one corrugated plate is to be used, unless of course they are already welded together prior to fixing on the lower face plate.The other plate panel stream gives the other plate panels which are positioned on top of the partly formed composite and laser through-welded through the plate along the flattened peaks of the corrugations. This gives the complete composite panel. During or after such fabrication various corrosion resistance measures may be applied. Some of these may necessitate action prior to welding the upper face plate into position, and some after.

Figure 6 shows diagrammatically a sample production line for such panels. Plates are butt welded into upper and lower panels at 13, held in a buffer store at 14, and then moved to weld station 15. Further panels are also fed, to overlie the flat panels, from the corrugation generation equipment 16 via a buffer 17. The aligned corrugated stiffeners are at this point held down on the 5 underlying plate and the subassembly laser welded in the troughs of the corrugations. Attention is drawn to the projection of the lower plate beyond the end of the corrugations, as shown in Figure 4 above. The sub assembly moves to inspection 18.Top panels are then 10 placed upon the corrugations (again to leave a certain proportion of corrugation projectinq, as shown at station 19) and welding is effected along the pea~s through the uppermost plate panel. Finally, the assembly moves to inspection station 20.

15 Provision is made, as shown in Figure 6, for an overhead lifting facility for the individual panels and corrugated stiffening. Nonetheless, this facility can consist in a simple lifting and transfer arrangement, and it is not normally necessary to turn over the panels 20 at any stage in their fabrication. The whole sandwich line may be mechanised and automated.

Attention is dcawn to the laser beam ducts 21. In the practice of this invention, as described in their earlier Application, it is envisaged to have a central lZ9Z641 laser generator and to transmit an unfocused laser beam down a duct common to all of the processing installations, the beam being selectively intercepted and thereafter directed along a duct on a gantry to be 5 focused by optics on to a workpiece.

Figure 8 shows how a high power density laser beam may be used to produce a weld which butts together two corrugated plates 3 whilst at the same time creating-a through connection between the corrugated plates 3 and 10 the lower face plate 2.

This joint configuration "Butt-through weld~, may be used to join corrugated plates in order to manufacture a sandwich composite structure.

The composite panels made in accordance with the 15 invention have generally smooth external surfaces, are smaller in overall thickness dimension than existing forms of large structures, and in the end product i.e.
ship or other are of considerably quicker fabrication f time and prime cost. In addition to these advantages, 20 they are of a structure considerably simplified for cleaning and painting processes. Thus, the external surfaces are smooth apart from weld configurations and present fewer problems of corrosion protection.
Moreover, the internal surfaces are in effect simple 9Z~

polygonally sectioned tubed surfaces and can be filled with a paint or anti-corrosion material such as a foam, vapour, gas etc.

The type of composite panels shown can be used as it stands for the tank top of a dry cargo ship, or for similar structures . In principle, the sandwich form could be applied to bottom shell, side shell, decks, longitudinal bulkheads, transverse bulkheads, flats, platforms , superstructures, deck houses, etc. These 0 main and minor structural members may require further supporting members, in which case these may be conventional stiffened webs or a second such sandwich panel structure spaced from the first. For example, initial studies have indicated that a flat 12 millimetre transverse floor structure supported by 180 millimetre by 12 millimetre flat bars spaced at 600 millimetres could be replaced by a 50 millimetre deep sandwich as shown comprising 3-millimetre corrugated central plate welded between 6-millimetre face plates.

Composite panels of the invention as shown could also be made to follow a curved or part cylindrical path. This would require the production of arcuate surface plates and the formation of the corrugated stiffener plates to follow the same arcuate line. While the former plates are within the ambit of conventional fabrication 1'~9Z6 ~i techniques , the latter represent a less common fabrication, which may be carried out,for exa~ple, by providing tapered corrugations and pressing them around a former to give a conic development or by the use of tapered ihserts.

Figure 7 shows three assembled corrugated panels ~the upper closing plates E of which have been omitted for clarity) provided at the fabrication stage with an elliptical access manhole 17. Such panels can be fabricated as necessary by a variant of the production line described above and be incorporated into the eventual steucture where desired. It is alternatively possible to provide postcutting of holes, since the structure is overall sufficiently rigid to withstand reasonable loss of integrity in this fashion.

Claims (8)

1. A composite metal panel comprising two parallel plates secured to the peaks and troughs respectively of a sandwiched corrugated metal stiffener by weld lines extending along the peaks and troughs and constituted by laser through-welds of the type provided by passing a high-intensity laser beam into or through both layers of metal along the peak or trough, all of said through-welds being in the same sense.

2. A panel as claimed in claim 1 in which the parallel plate portions are thicker than the corrugated plate portion.

3. A panel as claimed in claim 2 in which the corrugated plate has flat peaks and troughs.

4. A panel as claimed in claim 1 in which the trough through-welds at least pass into but not through the underlying plate layer, to give a composite panel with weld lines on one face and smooth on the other.

5. A composite metal panel as claimed in claim 1 said panel being assembled to the end of a like panel with the corrugations of both panels aligned: wherein facing and opposed straight edges of each lower parallel plate extend parallel to and forwardly of a plane containing the ends of the corrugations, and facing and opposed straight edges of each upper parallel plate extend parallel to and rearwardly of said plane, and (a) a butt weld joins the opposed lower plats edges (b) a corrugated insert piece is joined at each face by butt welds to the ends of the corrugations on the respective panels and (c) a plate insert is butt-welded at parallel edges thereof to the respective straight edges of said opposed upper plate edge.

6. A composite metal panel as claimed in claim 1 said panel being assembled to the side of a like panel, with corrugations parallel wherein facing and opposed straight edges of each lower parallel plate are spaced forwardly of facing and opposed straight edges of each upper parallel plate, and (a) a butt weld joins the lower parallel plate opposed edges (b) a plate insert is butt-welded at parallel edges thereof to the respective opposed straight edges of the upper parallel plates.

7. A composite metal panel as claimed in claim 2 in which (a) lower straight panel end edges transverse to the corrugations are each parallel to and spaced forwardly of a plane containing the ends of the corrugations, and upper straight panel end edges transverse to the corrugations are each parallel to and spaced rearwardly of the said plane and (b) lower straight panel side edges extend beneath and substantially along the centre of a trough in a corrugation and upper straight panel side edges extend above and substantially along the centre of a peak in a corrugation.

8. A method of fabricating a composite metal panel comprising the steps of:

supplying to a weld station a first flat steel plate;

corrugating an initially flat steel plate to exhibit parallel peaks and troughs, immobilising the corrugated steel plate over and in contact with the first flat plate traversing a focussed high-intensity laser beam under welding conditions along each trough, so that the metal in the trough and at least some of the thickness of metal of the underlying flat plate are melted and fuse together as a weld line;

immobilising over the welded plates or second flat steel plate, to lie upon the peaks of the corrugations parallel to the first steel plate; and traversing a focussed high-intensity laser beam under welding conditions along the known position of each peak so that the metal of the second plate and at least some of the thickness of metal of the underlying corrugated plate are melted and fuse together as a weld line.