Abstract:
A first ( 5 ) of the sheets has, near the zone ( 6 ) of overlap, stiffening parts ( 74 ) designed to resist the bending of the sheet along the zone of overlap. At least one region of the first sheet ( 5 ) is made to project, cantilever fashion, into the zone ( 6 ) of overlap. The stiffening parts are located near this cantilevered region. A pressing mechanism of a laser welding installation ( 2 ) is made to press against another sheet ( 4 ) to hold the sheets in contact with one another at the zone ( 6 ) of overlap. The laser welding installation is used to weld the sheets together along the zone of overlap. Application to the welding together, by transparency-welding, of sheets of the external skin of railway vehicle bodies.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a sheet to be welded to another sheet along a zone of overlap between the two sheets. 
   The invention applies in particular to the transparency-welding of external body skin panels made of steel sheet for railway vehicles. 
   Laser transparency-welding allows metal parts to be welded together at their surfaces that face one another in a zone of overlap. 
   To form a weld between these opposing surfaces in this zone where they are inaccessible, the laser beam impinging on one of the parts passes right through this part and melts the other part. The molten metal solidifies after the laser beam has passed, to form a welded seam. 
   The use of this transparency-welding technique requires the clearance between the opposing surfaces of the parts that are to be welded to be less than 10% of the total thickness of the zone of overlap. 
   This constraint on the clearance poses problems in applying this welding technique to railway vehicle exterior body skin panels because of the long lengths over which they need to be welded, it being possible for these lengths typically to be as much as 30 m. 
   In other fields and for small parts, uniformly spaced rams which clamp the two parts together along the entire length to be welded of the zone of overlap are used in order to comply with this constraint on clearance. 
   However, this solution would be particularly expensive for welding parts together over relatively long lengths, because of the proliferation in the number of rams. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to solve this problem by supplying a sheet which can be welded relatively simply and economically at a relatively long zone of overlap. 
   To this end, the subject of the invention is a sheet that is to be welded to another sheet along a zone of overlap between the two sheets, characterized in that the sheet comprises, near the zone of overlap, stiffening means designed to resist the bending of the sheet along the zone of overlap when a pressing mechanism of a laser welding installation presses against the other sheet. 
   According to particular embodiments, the sheet may have one or more of the following features, taken in isolation or in any technically possible combination:
         the stiffening means comprise a profiled part formed as an integral part of the first sheet;   the profiled part is formed by bending the sheet away from the zone of overlap, close thereto; and   the profiled part is a thickened part of the sheet.       

   Another subject of the invention is a collection of sheets which are intended to be welded together along a zone of overlap, characterized in that at least one of the sheets is a sheet as defined hereinabove. 
   A further subject of the invention is a method of laser welding at least two sheets along a zone of overlap of these sheets, characterized in that at least a first of the sheets is are sheet as defined hereinabove, in that at least one region of the first sheet is made to project, cantilever fashion, into the zone of overlap, the stiffening means being located near this cantilevered region, in that a pressing mechanism of a laser welding installation is made to press against another sheet to hold the sheets in contact with one another in the region of the zone of overlap, and in that the laser welding installation is used to weld the sheets together along the zone of overlap. 
   According to an alternative form, a laser beam emitted by the welding installation passes through the other sheet to weld it to the sheet at those of their surfaces which face one another in the zone of overlap, the method thus constituting a laser transparency-welding method. 
   A final subject of the invention is a railway vehicle body comprising at least one support framework and an external skin, the external skin comprising a collection of sheets welded together at zones of overlap and welded to the support structure, characterized in that welds connecting the sheets of the skin together are produced by a method as defined hereinabove. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood from reading the description which will follow, given merely by way of example and made with reference to the appended drawings, in which: 
       FIG. 1  is a diagrammatic view in elevation of a laser welding installation according to the invention, 
       FIG. 2  is an enlarged diagrammatic view in section on the line II—II of  FIG. 1 , 
       FIG. 3  is a partial plan view of two overlapping sheets which diagrammatically illustrates one use of the installation of  FIG. 1 , 
       FIG. 4  is a diagrammatic view from below which illustrates an alternative form of the pressing mechanism of the installation of  FIG. 1 , 
       FIG. 5  is a partial perspective view with cutaway of two overlapping sheets that are to be welded together, 
       FIG. 6  is a diagrammatic view in section along the line VI—VI of  FIG. 5  which illustrates the use of the installation of  FIG. 1  for welding the sheets of  FIG. 5 , 
       FIG. 7  is a diagrammatic section of a railway vehicle body, and 
       FIG. 8  is an enlarged sectioned view of the part ringed VIII in  FIG. 7 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  depicts a laser welding installation  2  for welding together two steel sheets  4  and  5  which overlap along a zone  6  of overlap. The sheet  4  is placed on top (at the top in  FIG. 1 ) of the sheet  5  in the zone  6  of overlap. The underside  7  (at the bottom in  FIG. 1 ) of the sheet  4  faces the upper surface  8  of the sheet  5  in the zone  6  of overlap. The sheets  4  and  5  are immobilized one with respect to the other by means which have not been depicted. 
   The installation  2  essentially comprises:
         a stand  10  secured to the ground  12  and raised off the latter,   a support structure  14  mounted on the stand  10 ,   a laser source  16  mounted on the stand  10 ,   a head  18  for focusing a laser beam, the head being optically coupled to the laser source  16  by optical-coupling means  19  (shown in chain line),   a source  20  of inert gas,   a nozzle  22  for spraying inert gas, this nozzle being borne by the head  18  and connected to the source  20  of inert gas by connecting means  21  (depicted in double-dash chain line),   a pressing mechanism  24  which is borne by the structure  14 , and   a base  26  for retaining the second sheet  5  and which is secured to the ground  12  and arranged at a lower level than the stand  10 .       

   The structure  14  comprises two substantially mutually orthogonal rods  28  and  30 , a carriage  32  and a mount  34 . 
   The rod  28 , which is substantially horizontal, is mounted on the stand  10  so that it can slide in a horizontal direction orthogonal to the plane of  FIG. 1 . The carriage  32  is mounted so that it can slide horizontally along the rod  28 , as depicted diagrammatically by the arrows  36 . 
   The rod  30  extends substantially vertically from the carriage  32  downwards and towards the zone  6  of overlap. The rod  30  is mounted so that it can slide vertically along the carriage  32 , as depicted diagrammatically by the arrows  38 . 
   The mount  34  extends the rod  30  downwards. This mount  34  is mounted so that it can rotate on the rod  30  about the axis X—X of the rod  30 , as depicted diagrammatically by the arrows  40 . 
   The structure  14  bears the optical-coupling means  19  and the connecting means  21 . 
   The head  18  and the nozzle  22  which surrounds the head  18  are coaxial with the rod  30  and with the mount  34 . The axis X—X of the rod  30  is substantially orthogonal to the zone  6  of overlap and the head  18  and the nozzle  22  are orientated towards this zone  6 . 
   The head  18  and therefore the nozzle  22  are mounted so that they can slide vertically on the mount  34 , as depicted diagrammatically by the arrows  42 , while the pressing mechanism  24  is secured to the mount  34 . Thus, the head  18  and the nozzle  22  can be moved up closer to the zone  6  of overlap independently of the pressing mechanism  24 . 
   As illustrated more particularly in  FIG. 2 , the pressing mechanism  24  comprises a hollow tube  44 , with a circular base, which is coaxial with the rod  30  and which surrounds the head  18  and the nozzle  22 . This tube  44 , borne by the mount  34 , extends the latter downwards and therefore towards the zone  6  of overlap. 
   The lower edge face  46  of this tube  44  is equipped with an open-faced annular runway  48  in which a ball  50  is retained. The ball  50  can travel freely in this runway  50 . A lower part of the ball  50  projects downwards from the edge face  46 . 
   The edge face  46  of the tube  44  and the runway  48  are inclined with respect to a plane that is transverse to the axis X—X of the tube  44  and are therefore inclined with respect to the zone  6  of overlap. 
   The ball  50  is pressed, by the tube  44  and the structure  14 , against the sheet  4  in the region of the zone  6  of overlap. 
   The sheet  5  rests on the base  26  which extends, underneath this sheet- 5 , substantially along the entire length of a straight welded seam  51  to be produced orthogonally to the plane of  FIG. 1 . 
   To weld the sheets  4  and  5  together at their surfaces  7  and  8  in the zone  6  of overlap, the head  18  focuses a laser beam  52  emitted by the source  16  in the zone  6  of overlap. This laser beam  52  strikes the zone  6  of overlap substantially orthogonally at an impingement region  53 . The beam  52  melts the sheet  4  as it passes through it, then reaches the sheet  5  where it locally melts the steel without passing through the sheet  5 . 
   To form the straight welded seam  51  which does not go all the way through, the structure  14  which bears the head  18  is displaced horizontally along the edges of the plates  4  and  5 , as depicted diagrammatically by the arrow D in  FIG. 2 , that is to say orthogonally to the plane of  FIG. 1 . The head  18 , the nozzle  22  and the pressing mechanism  24  are secured to the structure  14  during this displacement parallel to the zone  6  of overlap. The beam  52  and therefore the impingement region  53  are displaced in the sense and direction of the arrow D. After the laser beam  52  has passed, the molten metal solidifies to form the welded seam  51 . 
   During the rectilinear displacement of the structure  14 , the ball  50 , subject to the displacement of the head  18 , runs along the sheet  4  and remains ahead of the impingement region  53 , with respect to the sense of displacement of this region  53 . Thus, the ball  50  lies to the right of the region  53  in  FIG. 2 . The ball  50  which rolls along the sheet  4  presses the surface  7  of the sheet  4  onto the surface  8  of the sheet  5 , this sheet  5  being retained by, the base  26 . Thus, the constraint on the clearance between the parts  4  and  5  in the zone  6  of overlap is complied with and it is possible to form the seam  51 . 
   Behind the impingement region  53 , with respect to the sense of displacement D, that is to say to the left in  FIG. 2 , the sheets  4  and  5  are held pressed together by the welded seam  51 . 
   The nozzle  22  forms a stream  58  of inert gas around the laser beam  52 . This stream strikes the upper surface  59  of the sheet  4  to protect the weld pool formed. 
   The nature of the support structure  14  allows the head  18  and the pressing mechanism  24  to be brought into a predetermined welding position and then allows these items to be displaced along a predetermined welding path. The pressing mechanism  24  is subject to the displacements of the head  18  and therefore of the impingement region  53 , in directions parallel to the zone  6  of overlap. 
   The position of the head  18  in a direction orthogonal to the zone  6  of overlap may be altered independently of that of the pressing mechanism  24 , as indicated by the arrows  42 . 
   The inclination of the runway  48  gives the ball  50  a preferred position, because this ball tends to locate itself at the lowermost point on this runway  48 , that is to say to the right in  FIG. 2 . Thus, by making the tube  44  pivot about the axis X—X of the rod  30 , as depicted diagrammatically by the arrows  40  in  FIG. 1 , it is possible for the ball  50  always to be placed ahead of the zone  53  of impingement of the beam  52  irrespective of the shape of the welded seam  51  to be produced. 
   By way of example,  FIG. 3  illustrates the production of a welded seam  51  containing a right angle. 
   The pressing mechanism  24  is depicted in solid line at this right angle after a first rectilinear part  51 A of the seam  51  has been produced and before a second part  51 B of the seam  51 , orthogonal to the part  51 A, is produced. The pivoting of the tube  44  through a quarter turn in the clockwise direction in  FIG. 3 , as depicted diagrammatically by the arrow  59 , allows the ball  50  to be brought into the appropriate position (at the bottom in  FIG. 3 ) to form the second part  51 B (in broken line) of the seam  51 . The pressing mechanism  24  is depicted in chain line in its two positions corresponding respectively to the formation of the parts  51 A and  51 B of the welded seam. 
   The installation  2  therefore makes it possible, by transparency-welding, to produce laser-welded seams of varying shapes on sheets which overlap over relatively long lengths. The local pressing mechanism  24  allows these welded seams to be produced relatively economically without using a large number of rams for clamping the parts  4  and  5 . 
   The displacements of the structure  14 , of the various elements of which it is composed, and of the head  18  may be produced by conventional means such as rams and/or electric motors. 
   The operation of such an installation may be carried out entirely by robots, or automated using a control unit connected to the various motors and rams. 
   According to an alternative form which has not been depicted, the mount  34  and therefore the tube  44  may be mounted so that they are free to rotate about the axis X—X on the rod  30 . In this case, the ball  50  automatically positions itself behind the region  53  with respect to the sense of displacement of the head  18  in a direction parallel to the zone  6  of overlap. Thus, the ball  50  is automatically orientated behind the region  53  in the direction of displacement of the head  18 , whatever this direction may be, and without there being a need for means for making the mount  34  rotate about the axis X—X. This result is obtained by automatic rotation of the tube  44  about its axis X—X when there is a change in sense and/or direction of displacement of the head  18 . 
   According to another alternative form which is illustrated in  FIG. 4 , the lower edge face  46  of the tube  44  is substantially orthogonal to the axis X—X of the tube  44 . Three housings  60 A,  60 B and  60 C are formed in this edge face  46 . Each of these housings holds a ball  50 A,  50 B and  50 C, respectively, and is of a shape that mates with that of the corresponding ball. These balls are intended, like the ball  50  of  FIGS. 1 to 3 , to roll along the sheet  4  ahead of the impingement region  53 . 
   The housings  60 A,  60 B and  60 C are formed uniformly in a region of the edge face  46  of the tube  44  that subtends an angle α. This alternative form allows the surfaces  7  and  8  of the sheets  4  and  5  to be pressed together over a broader region, ahead of the region  53  of impingement of the laser beam  52 . 
   The shape of the housings  60 A,  60 B and  60 C allows the balls  50 A,  50 B and  50 C to be positioned with precision ahead of the impingement region  53 . 
   In alternative forms which have not been depicted, the number of rolling balls may be higher and the subtended angle α may also be larger and be as much as 360°. In this last instance, balls are distributed uniformly over the entire edge face  46  of the tube  44  and the pressing mechanism  24  presses on the sheet  4  right around the region  53  of impingement of the laser beam  52 . 
   In other embodiments which have not been depicted, the structure  14  consists of an articulated arm system equipped with rams, thus making it possible to weld overlapping metal parts together, in varying positions, with greater freedom. 
     FIGS. 5 and 6  illustrate a collection  62  of two sheets  4  and  5  that are to be welded together along a zone  6  of overlap in which their respective front edges  64  and  65  overlap. 
   The sheets  4  and  5 , of rectangular overall shape, have already been welded along their respective lateral edges, with welded seams  66  to  69 , to two U-sections  70  and  72 . 
   These U-sections  70  and  72  are, for example, elements of a framework of a railway vehicle body, and the sheets  4  and  5  are intended to form the exterior skin of this body. The welded seams  66  to  69  have, for example, been produced by transparency-welding using the installation  2  of  FIGS. 1 to 3 , the U-sections  70  and  72  therefore being supported by the rest of the aforementioned framework rather than by the base  26 . 
   The sheets  4  and  5  are stretched between the U-sections  70  and  72 . The sheets  4  and  5  are thus immobilized one with respect to the other. 
   The front edge  65  of the sheet  5  is extended downwards in  FIGS. 5 and 6  by an L-section central part  74 . This part  74  has been formed by bending the sheet  5  away from the zone  6  of overlap. This part  74  comprises, starting from the edge  65 , two successive rectangular panels  76  and  78  which form an L orthogonal to the rest of the sheet  5 . This part  74  increases the bending stiffness of the edge  65  of the sheet  5 . 
   With the U-sections  70  and  72  retained by the aforementioned framework, to produce the welded seam  51  (depicted in broken line in  FIG. 5 ) the pressing mechanism  24  of the welding installation  2  is pressed against the plate  4  in the region of the zone  6  of overlap. The stiffening part  74  allows that region of the surface  8  that lies between the U-sections  70  and  72  and in the zone  6  of overlap to resist bending so that the surfaces  7  and  8  of the sheets  4  and  5  are pressed together in the zone  6 . 
   Thus, the stiffening part  74  makes it possible to form a welded seam  51  without placing a retaining support under that region of the edge  65  which projects cantilever fashion between the U-sections  70  and  72 . This feature makes the transparency-welding of overlapping sheets over relatively long lengths even easier. 
   In alternative forms which have not been depicted, the stiffening parts may differ in shape and may in particular be formed by thickened regions of sheet. 
   It is possible to join together in turn several sheets which have stiffening parts  74 . Thus, in the case of sheets which have stiffening parts  74  only at their front edge, a first sheet is connected at an overlap between its rear edge and the front edge of a second sheet, the rear edge of this second sheet being itself connected with overlap to the front edge of a third sheet, and so on. 
   Such sheets with stiffening parts  74  can be used to cover railway vehicle body frameworks. 
     FIG. 7  diagrammatically illustrates a railway vehicle body  81  which essentially comprises a chassis  82 , a top or roof  83  and two side walls  84 . 
   As illustrated more specifically in  FIG. 8 , the roof  83  comprises a support framework  840  comprising bowed transverse beams  85  which are arranged uniformly one after the other along the entire length of the body  81 , and over which an external skin  86  is fixed. 
   This skin  86  comprises corrugated sheets, two of which are depicted in  FIG. 8 . These two sheets  87  and  88  are welded along a zone  89  of overlap that runs longitudinally with respect to the body  81 , that is to say orthogonally to the plane of  FIGS. 7 and 8 . The sheets  87  and  88  have been welded together by virtue of parts  74  provided on the sheet  88 , in each space between beams  85 . As before, these stiffening parts have allowed the sheets  87  and  88  to be welded together in zones where they project cantilever fashion. 
   Such sheets equipped with stiffening parts may also be used to form the chassis  2 .