Abstract:
The invention provides a welding device ( 10 ) using a power laser beam ( 22 ), the device ( 10 ) being miniaturized, the device ( 10 ) being suitable for being moved very close to the surface ( 52   a ) of the part ( 52 ) to be welded and of reaching welding zones that are difficult to access, but without vapor and particles of molten metal ( 56 ) being capable of penetrating into the inside and dirtying the optical components ( 28, 48, 46 ). Such a device is remarkable in that it includes a feed ( 26 ) of gas under pressure suitable for producing a primary flow of gas ( 50 ) leaving via the front opening ( 20 ) together with the laser beam ( 22 ), and in that it also includes a nozzle ( 60 ) connectable to a source of gas under pressure, said nozzle ( 60 ) producing a secondary flow of gas ( 62 ) sweeping the front opening ( 20 ) transversely, thereby deflecting the primary flow of gas ( 50 ) laterally.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The invention relates to welding by a power laser beam, e.g. by way of indication, at a power of 500 watts (W) to several kilowatts (kW), and more particularly it relates to a miniaturized laser beam welding device suitable for being brought very close to the surfaces of parts to be welded and capable of reaching surfaces for welding that are difficult to access because of obstacles situated over them.  
         STATE OF THE ART AND PROBLEM POSED  
         [0002]    It is known to weld metals by means of a power laser beam. To do this, a welding device is used that comprises an enclosure of generally elongate shape in which a laser beam can travel and leave via one end of the enclosure in order to touch the part that is to be welded. The enclosure is light-tight in order to protect the operator from any accidental propagation of the laser beam outside the enclosure. The term “light-tight” is used to mean that the enclosure prevents any accidental exit of the laser beam, e.g. as a result of an optical component being wrongly adjusted.  
           [0003]    Inside the enclosure, the laser beam travels along a path comprising in succession: a power laser source, a converging optical system, and a “front” opening through which the laser beam can leave the enclosure going towards the part to be welded. The converging optical system concentrates the laser beam on a focus situated outside the enclosure, the part naturally being placed at said focus in order to be welded. The laser source may be a laser generator, or it may be the end of an optical fiber bringing the laser beam into the enclosure from a remote generator. In more elaborate versions, the enclosure may be articulated and/or include one or more optical mirrors disposed on the path of the laser beam so as to deflect the laser beam appropriately.  
           [0004]    To simplify the use of language, an object is said to be situated “in front of” the front opening when it is outside the device and in register with the front opening. Conversely, an object is said to be “behind” the front opening when it is on the other side of the front opening.  
           [0005]    During welding, the optical components need to be protected from vapor and droplets of molten metal. The enclosure normally performs this protection function, however vapor and droplets of molten metal can pass through the front opening, penetrate into the enclosure, and dirty the optical components, in particular those which are close to the front opening. Various means are used to prevent this:  
           [0006]    reducing the diameter of the front opening to the strict minimum needed for passing the laser beam;  
           [0007]    injecting an inert gas under sufficient pressure into the enclosure, the gas leaving through the front opening and thus constituting an obstacle to vapor and droplets of molten metal; and  
           [0008]    locating the optical components at a distance from the front opening or locating the front opening at a distance from the part to be welded.  
           [0009]    Patent EP 0 514 235 discloses a device making it possible to weld surfaces having obstacles overlying them, that welding device comprising an elongate tubular enclosure, a laser source being disposed at one end, the front opening being disposed laterally at the other end, a mirror being disposed behind the front opening and deflecting the laser beam from the laser source towards the opening, the working end of the welding device being passed between the obstacles and the surfaces to be welded, specifically the inside wall of a nuclear power station tube. An inert gas leaves through the front opening and protects the surface that is being welded from ambient air. That welding device nevertheless presents the drawback of exposing the mirror to vapor and droplets of molten metal because:  
           [0010]    the mirror is very close to the molten metal in the welding zone; and  
           [0011]    the pressure of the protective gas is necessarily limited for reasons of economy and so as to avoid dispersing the molten metal. It can be seen in the figure that the opening is much larger than the laser beam passing through it. This enables the zone that is being welded to be thoroughly surrounded by inert gas, without creating a jet of gas that could disperse the molten metal. Unfortunately, the protective gas is traveling only slowly when it passes through the front opening. It therefore constitutes only a minor obstacle to vapor and droplets of liquid metal, and consequently the mirror is quickly degraded.  
           [0012]    The mirror therefore needs to be changed frequently. Such a welding device,is therefore suitable for repairing thin parts that require low power only, but it is not suitable for industrial use in a workshop on thick parts using a high power laser.  
           [0013]    A first problem to be solved is that of providing a welding head whose front opening is suitable for being brought very close to the surface to be welded without the optical components being dirtied by the vapor and droplets of molten metal and without the molten metal being disturbed, e.g. by a flow of gas passing out through the front opening.  
           [0014]    A second problem to be solved is that of providing a welding head suitable for being passed between the surfaces to be welded and nearby obstacles situated above the surfaces to be welded.  
         SUMMARY OF THE INVENTION  
         [0015]    To solve the first problem, the invention provides a miniaturized laser beam welding device, the device comprising a welding head constituted by a wall surrounding a cavity, the wall having an “admission” first opening and a “front” second opening, a laser beam being suitable for entering into the welding head via the admission opening and leaving it via the front opening by following a predefined light path. The welding head is fed with gas under pressure suitable for producing a “primary” gas flow in the cavity and for leaving the welding head via the front opening. Such a device is remarkable in that the welding head includes a nozzle suitable for being connected to a source of gas under pressure, said nozzle being outside the welding head against its wall. The nozzle produces a “secondary” flow of gas under pressure transversely sweeping the space situated immediately in front of the front opening.  
           [0016]    The term “transversely” is used to mean that the secondary flow is directly parallel to the surface of the front opening. With such a disposition:  
           [0017]    a) the primary flow pushes back the vapor and droplets of molten metal heading towards the front opening, thereby preventing said vapor and droplets from penetrating inside the welding head; and  
           [0018]    b) the secondary flow strikes the primary flow perpendicularly at the instant it leaves through the front opening, thereby deflecting the primary flow and thus preventing it from reaching the liquid metal which is being formed a little further away in the vicinity of the focus.  
           [0019]    Thus, the vapor and droplets of molten metal cannot penetrate into the welding head through the front opening and the molten metal of the weld is not disturbed, deformed, or even dispersed by the primary flow of gas, thus making it possible to place the welding head very close to the surface to be welded, thereby solving the first problem.  
           [0020]    The operator gives the primary flow sufficient force to enable it to push back the vapor and droplets of molten metal that approach the front opening. Similarly, the operator gives sufficient force to the secondary flow to deflect the primary flow before it reaches, at least directly, the liquid metal that forms in the vicinity of the focus in front of the front opening.  
           [0021]    Advantageously, the welding head also has a shield positioned in front of the front opening, said shield being substantially flat and parallel to the wall of the welding head surrounding the front opening, said shield being made of a rigid material that withstands high temperatures, i.e. niobium, the shield being pierced by a hole positioned on the axis of the front opening, the nozzle being positioned between the shield and the wall of the welding head. The shield enables the primary and secondary flows to be channeled transversely to the front opening, thereby improving protection of the molten metal against the primary flow.  
           [0022]    The shield also constitutes a mechanical obstacle to the vapor and droplets of molten metal heading towards the welding head and around the front opening, thereby reducing dirtying of the welding head. The shield acts in particular to form an obstacle to the vapor and droplets of molten metal traveling between the front opening and the nozzle, which droplets would, without the shield, be likely to be moved back in front of the front opening by the secondary flow. Thus, and in spite of the presence of the hole situated between the molten metal and the front opening, this obstacle effect further improves the integrity of the inside of the welding head against vapor and droplets of molten metal.  
           [0023]    The shield is advantageously removable so as to allow it to be cleaned or replaced once it has become dirty.  
           [0024]    Also advantageously, the welding head includes a skirt surrounding the front opening, the nozzle, and the shield, the skirt being open and flaring in front of the shield, the skirt being made of a material that is thin, flexible, and gas-proof, a gap being left between at least one edge of the shield and the skirt. The skirt thus co-operates with said gap to bring the primary and secondary flows of gas over the zone being welded, thereby protecting the welding zone from ambient air while consuming less gas. The skirt is of a length suitable for coming into contact with the surface of the part to be welded, said skirt also enabling the gas of the primary and secondary flows to be retained above the welding zone and consequently simultaneously reducing gas consumption and oxidation of the part that is being welded.  
           [0025]    Also advantageously, the welding head may be provided with a mirror positioned immediately behind the front opening and deflecting the laser beam through 90° towards said front opening. This disposition enables the size of the welding head behind the front opening to be reduced, thereby enabling the welding head to be passed between the surfaces to be welded and a nearby obstacle, with this disposition enabling the second problem to be solved.  
           [0026]    In particular, the welding head has a thickness E 1  between the front opening and the wall opposite from the front opening that is less than 50 millimeters (mm). 
       
    
    
     DESCRIPTION OF THE FIGURES  
       [0027]    The invention will be better understood in view of a detailed embodiment and from the accompanying figures.  
         [0028]    [0028]FIG. 1 shows a welding head mounted at the end of a welding device fed with light by an optical fiber.  
         [0029]    [0029]FIG. 2 shows the welding head. 
     
    
     DETAILED DESCRIPTION  
       [0030]    Reference is made initially to FIG. 1. The welding device  10  comprises a welding head  12  constituted by a wall  14  defining a closed cavity  16 , the wall  14  being light-tight, the wall  14  nevertheless having an “admission” opening  18  and a “front” opening  20 , a laser beam  22  being capable of penetrating into the cavity  16  via the admission opening  18  and of leaving the cavity  16  via the front opening  20  by following a light path  24 . The welding head  12  also has a feed  26  suitable for being connected to a source of inert gas under pressure, e.g. argon, said feed  26  in this example being a duct passing through the wall  14  so as to open out inside the cavity  16 .  
         [0031]    In this example, the admission opening  18  and the front opening  20  are plane, circular, and centered on the light path  24  which constitutes the axes thereof. The admission opening  18  and the front opening  20  are perpendicular. A mirror  28  is placed in the cavity  16  in the light path  24 . The mirror  28  deflects the laser beam though 90° so as to direct it towards the front opening  20 . The mirror must naturally be capable of withstanding high temperatures. By way of example, it can be made of ZnSe, of copper with a cooling circuit, or it can be of the “dielectric” type.  
         [0032]    The welding device  10  also comprises an enclosure  30  of generally elongate shape with its opposite ends being referenced  30   a  and  30   b . A laser light source  32  is placed at a first end  30   a  and produces the laser beam  22  which travels inside the enclosure  30  along the light path  24 . The laser beam  22  reaches the other end  30   b  having the welding head  12  attached thereto, the end  30   b  surrounding the admission opening  18  so as to allow the laser beam  22  to pass from inside the enclosure  30  into the cavity  16  of the welding head  12 .  
         [0033]    In this example, the laser source  32  is a point source constituted by an optical fiber which delivers laser light from a remote generator (not shown) via its end  32   a  inside the enclosure  30 . Also in this example, the end  30   a  of the enclosure  30  is constituted by a socket  34  that supports the laser source  32 , the socket  34  being extended to the other end  30   b  by a straight tube  36 , the straight tube  36  being attached to the socket by, for example, screws (not referenced). The welding head  12  is also attached to the end of the straight tube  36 . In a preferred embodiment, the connection between the welding head  12  and the enclosure  30  is releasable so as to make it possible to combine welding heads  12  of different shapes with straight tubes  36  of different lengths.  
         [0034]    The welding device  10  includes an optical system  42  on the light path  24 , which system concentrates the laser beam  22  on a focus  44  in front of the front opening  20 , outside the welding head  12 , the focus  44  being on the light path  24 . When the laser source  22  is a point source, the optical system  42  is a converging system and delivers a real image of the laser source  32  at the focus  44 . The converging optical system  42  comprises two converging lenses. A first lens is a “collimator” lens  46  which transforms the diverging laser beam that emerges from the end  32   a  of the optical fiber into a parallel beam, and the second lens  48  is a “focusing” lens transforming the parallel beam into a beam that converges on the focus  44 . This disposition makes it possible to use straight tubes  36  of different lengths without changing the position of the focus  44  relative to the front opening  20  of the welding head  12 , so long as the position of the collimator lens relative to the end  32   a  of the optical fiber and the position of the focusing lens  48  relative to the welding head  12  both remain unchanged. In this example, the optical system  42  also comprises a prism  49  disposed between the collimator lens  46  and the focusing lens  48  in the vicinity of the focusing lens, the prism  49  having a cylindrical portion intersecting the laser beam  22  over half of its section, and serving to deflect half of the laser beam  22  slightly to a secondary second focus (not referenced) that is slightly offset relative to the focus  44 , where this disposition is itself known.  
         [0035]    The welding head is described below in greater detail with reference simultaneously to FIGS. 1 and 2.  
         [0036]    The feed  26  produces a “primary” flow of gas  50 .  
         [0037]    The welding head  12  is positioned above the surfaces  52   a  of the parts to be welded  52 , the focus  44  being on said surfaces  52   a , the welding head  12  being subjected to displacement parallel to the surfaces  52   a , said displacement being represented by a speed vector  54 . Under the heating effect of the laser beam  52 , the metal melts in the vicinity of the focus  44  and subsequently solidifies in order to form a welding bead  58 . The liquid metal that forms in the vicinity of the focus  44  during welding is referenced  56 . The liquid metal gives off vapor and droplets that might pass through the front opening  20 , penetrate into the cavity  16  of the welding head  12 , and touch the mirror  28  which is immediately behind the front opening  20 . It will be understood that if the intensity of the primary flow  50  is increased in order to prevent the vapor and droplets of liquid metal  56  penetrating into the cavity  16 , the primary flow  50  will reach the liquid metal  56  at a speed that is too fast, thereby causing it to be deformed or even  25  dispersed, and thus deforming the welding bead  58  that is being formed.  
         [0038]    The welding head  12  has a nozzle  60  suitable for being connected to a source of gas under pressure (not shown), said nozzle being positioned to deliver a “secondary” flow of gas  62  passing in front of the front opening  20  transversely relative thereto. The secondary flow  62  sweeps the entire surface of the front opening  60  but without penetrating inside the cavity  16  through said front opening  20 . The secondary flow  62  thus collides with the primary flow  50  substantially perpendicularly to the light path  24  in front of the front opening  20 , the secondary flow  62  thus deflecting the primary flow  50 , which can thus no longer arrive directly on the liquid metal  56 , which it would disperse. The term “front wall”  64  is applied to the outside surface of the wall  14  of the welding head situated around the front opening  20 . In practice, the front wall  64  is planas and is perpendicular to the light path  24  when the light passes through the front opening  20 . The nozzle  60  is fixed against the front wall  64  but is eccentric relative to the front opening  20 . The nozzle  60  nevertheless points towards the front opening  20  and produces the secondary flow  62  transversely relative to the front opening  20  and parallel to the front wall  64 .  
         [0039]    A thin flat shield  66  is placed in front of the front opening  20  extending parallel to the front wall  64 , i.e. perpendicular to the light path  24 , with the nozzle  60  being located between the front wall  64  and the shield  66 , the shield  66  being pierced by a hole  68  centered on the light path  24 , the shield  66  being closer to the front wall  64  than is the focus  44 . The shield is made of a material that withstands high temperature, for example niobium. Such a shield presents several advantages:  
         [0040]    firstly, it channels the secondary flow  62  and the primary flow  50  parallel to the front wall  64 , thereby providing the liquid metal  56  with better protection against the primary flow  50 ;  
         [0041]    secondly it constitutes an obstacle to vapor and droplets of liquid metal  56 , thereby serving to keep the welding head itself becoming dirtied;  
         [0042]    finally it constitutes an obstacle to vapor and droplets of liquid metal which would otherwise go between the front opening  20  and the nozzle  60  and could then be deflected towards the front opening  20  by the secondary flow  62 . Thus, by providing an additional obstacle and in spite of the holes situated on the light path, the shield  66  further improves the protection of the optical components, and in particular of the mirror  28  against vapor and droplets of liquid metal  56 .  
         [0043]    The shield may be made of for example, niobium, of a metal alloy in which niobium is the major component, or a nickel-based superalloy. The hole  68  and the front opening  20  are preferably projections of each other relative to the focus  44 , thus enabling their dimensions to be restricted to the minimum required for passing the laser beam  22 .  
         [0044]    A skirt  70  is placed around the laser beam  22  between the front opening  20  and the focus  44 , one end of the skirt extending up to the front wall  64  and forming an opening surrounding the front opening  20 , the other end of the skirt forming a flared opening around the focus  44 . The skirt  70  is made of a material that withstands heat, in particular from droplets of liquid metal  56 . The skirt is gas-proof laterally, and also surrounds the nozzle  60  and the shield  66 , leaving a gap  74  relative to the edge  66 a of the shield  66  opposite from the nozzle  60 . The skirt is of a height that is suitable to ensure that, during welding, its flared end  70   a  is flush with the surface  52   a  of the part to be welded  52 . The skirt  70  brings the primary and secondary flows  50  and  62  over the liquid metal  56  and keeps them there, causing them to pass via the above-defined gap  74 . The gas retained in this way is effective in protecting the surface  52   a  for welding around the liquid metal  56 . This gas then leaks out between the skirt  70  and the surfaces  52   a  that are to be welded together.  
         [0045]    During welding, the welding head is moved, preferably in the same direction as the secondary flow  62  leaves the nozzle  60 . Thus, the combined primary and secondary flows  50 ,  62  arrive at slower speed parallel to the welding bead  58  that is being formed, thus avoiding deforming it.  
         [0046]    In practice, the shield is removable so that it can be cleaned or replaced once it becomes too dirtied by vapor and droplets of liquid metal. For a welding head that is greatly miniaturized, it can be held by two screws screwed into the front wall, the nozzle being sandwiched between the shield and the front wall.  
         [0047]    The skirt  70  is, for example, cut out from closely-woven fiberglass cloth, and its outside face is covered in a silicone elastomer layer that withstands high temperatures, with these two materials being commonly available in trade. Such a skirt is both flexible and resistant to tearing. In addition, the silicone elastomer makes the skirt gas-proof and the fiberglass cloth protects the silicone elastomer from the vapor and droplets of molten metal and also from the heat radiation.  
         [0048]    The invention enables the welding device to be greatly miniaturized. That is why the Applicant also claims a welding device comprising a welding head of the invention of size E 1  along the axis of the front opening  24   a  no greater than 50 mm. For a welding head  12  with a skirt  70 , this size corresponds in practice to the distance between the flared opening  70   a  of the skirt  70  and the outside face  76  of the wall  14  behind the front opening  20 . Thus, the welding device  10  enables the surfaces  52   a  of parts  52  to be welded in spite of the presence of an obstacle  78  situated at a distance E 1  above the surfaces to be welded  52   a.    
         [0049]    The present invention has made it possible in particular to provide a welding head that is powered by a 4 kW yttrium aluminum garnet (YAG) laser, while requiring a space of no more than 24 mm above the surfaces  52   a  to be welded.