Patent Publication Number: US-2009230099-A1

Title: Method and Apparatus in Connection with Laser Use

Description:
The invention relates to a method in connection with laser use, wherein one or more laser beams emitted by one or more laser sources are focused by means of beam guiding elements on a fusing spot, whereat a filler delivered thereto is melted, especially for performing a welding, coating, piece manufacturing and/or the like process. The employed filler comprises a substantially solid-state, elongated filler material, such as a wire or the like, which is fed by means of a delivery system to the fusing spot centrally relative to said one or more laser beams focusing on the fusing spot. 
     For example LENS (Laser Engineered Net Shaping) rapid manufacturing technique (RP, Rapid Prototyping), as shown for example in  FIG. 1 , uses a single laser beam, focused in an optically centralized manner on a focal point, for producing a fused spot on a substrate material, whereby a powder to be added thereto is sprayed from opposite sides of the laser beam while being heated with the laser beam. Consequently, the spraying powder melts and bonds with the spot fused in the substrate material. However, the delivery of a filler material in the form of powder is wasteful in terms of energy, the efficiency being typically about 70%, and even that high on the condition that the sprayed powder can be fed in its entirety inside a laser beam. Therefore, the efficiency in a normal case is substantially poorer, since generally as much as most of the powder is blown away to the environment. The efficiency, in turn, has a direct impact on the speed of a laser process and on the price of an article manufactured by the discussed process. 
     On the other hand, when laser is used e.g. in welding or coating, the feed of a filler supplied in solid state is in current technology generally performed, as e.g. in covered electrode metal arc welding or flux cored arc welding, by continuously feeding filler wire from ahead of the moving burn spot, the practical implementation of which is highly inconvenient, especially with an unsteady processing direction. In practice, what is called for in this context is highly advanced automatics and sophisticated basic equipment in the relative shifting of a burning apparatus and an x-y plane used as the substrate. 
     It is further prior known from patent publications JP2003311456, EP 1 179 382 and U.S. Pat. No. 6,269,540 to utilize, in association with a laser source, a substantially solid-state filler which is delivered to a fusing spot centrally with respect to one or more laser beams focused thereon. 
     The first of the above-cited solutions is based on providing the apparatus with laser sources disposed along the circumference in radial direction, the laser beams emitted thereby being focused by optical guide elements on a fusing spot from around a supplied filler. A practical implementation of such solution is difficult as the size of laser sources inherently limits the number of circumferentially disposed laser sources or increases excessively the total diameter of the discussed apparatus. On the other hand, the implementation consistent with the last-cited solution, at least from the viewpoint of a person skilled in the art, is not per se applicable, with technical solutions presented therein, particularly to the use of carbon dioxide laser, which is why the utility of this particular solution is highly restricted in practice. 
     Further, an objective in the solution disclosed in patent publication EP 1 179 382 has been to correct “a shadow problem” occurring in a laser beam whose intensity distribution is consistent with a so-called Gaussian curve, which means in practice that, when implemented with technology shown in principle in  FIG. 5 , the centrally supplied filler wire causes a shadow in the intensity distribution applied to the fusing spot. Hence, an objective in this particular solution has been to cut the mid-portion of a maximum intensity distribution off the laser beam and to return it to the fusing spot wherever there is a shadow caused by the filler wire. 
     This type of implementation breaks up the original intensity distribution of a laser beam in such a way that part of a fusing spot receives a per se homogeneous intensity distribution, yet part of it assumes a more powerful intensity distribution as the latter has been cut off the laser beam&#39;s maximum intensity range. Thus, the discussed solution is not sufficiently functional in practice, because the final result regarding the quality of processing is then dependent e.g. on a processing direction (homogeneous/inhomogeneous intensity distribution). Hence, the discussed solution has not been successful in providing a decisive improvement in terms of eliminating a “shadow” problem, as it has merely changed the nature of the problem. 
     A method according to the present invention has an objective to provide a decisive improvement regarding the foregoing drawbacks and thereby to raise substantially the available prior art. In order to accomplish this objective, a method of the invention is principally characterized in that one or more laser beams are diverged with a multi-segment mirror, especially for maintaining the symmetry of the intensity distribution thereof, whereby beams or divisional beams, reflecting from its various segment elements in substantially divergent directions, are converged on a fusing spot by means of a condenser system included in beam guiding elements. 
     The most important benefits offered by a method of the invention include the simplicity and efficiency of the method itself and the equipment population applicable thereto, enabling a laser processing which is significantly faster and more versatile than what is used at present. In addition, a method of the invention serves to minimize waste of material, as it enables a hundred percent supply of the filler wire to a fusing spot and processing the same with an all-round homogeneous laser beam without a so-called shadow effect, by virtue of which the method also enables processing that is remarkably easier to control and cleaner than currently employed solutions. In addition, by virtue of precisely focused homogeneous fusion, the method produces a surface finish which is remarkably better than what is obtainable by currently available methods. In a method of the invention, it is further possible to promote the melting of a filler wire by simultaneously reducing the power demand of a laser source by preheating the filler material almost to its melting point prior to its delivery to the fusing spot. 
     Preferred applications for a method of the invention are presented in dependent claims directed thereto. 
     The invention relates also to an apparatus as defined in the independent claim for applying the method, the features principally characteristic of said apparatus being presented in the characterizing clause of the same claim. 
     The most important benefits offered by an apparatus of the invention include the simplicity and efficiency of the equipment population applicable thereto, as well as an optimal surface finish obtainable thereby. The efficiency provided by an apparatus of the invention is remarkably higher than what is obtained in currently available solutions, based e.g. on the use of a powdered filler material, since by a continuous feed of solid wire, effected by a totally homogeneous laser beam, it is possible to ensure that the intensity distribution of the original laser beam be focused by a hundred percent on a fusing spot, whereby the processing direction can be completely arbitrary. An apparatus of the invention is also readily variable for a particular application, such as e.g. for welding, coating, piece manufacturing, whereby it is possible to make use of an xy-plane associated with the laser apparatus. On the other hand, the discussed plane and the head of a laser apparatus are still preferably movable relative to each other also in z-direction, which enables the manufacture of e.g. three-dimensional articles by adding vertically successive layers of material to the substrate material. In an apparatus of the invention, it is naturally possible to employ as a filler material not only metal-based ingredients but also but also other materials such as, for example, plastics, resin, glass, etc. In addition, the apparatus makes it possible to assemble an article partially from e.g. an inert material, the removal of which is possible in further processing, e.g. in the production of articles including hollow or negative inclined surfaces, which is very difficult, if not outright possible to achieve e.g. with LENS type of rapid prototyping methods as described before. Furthermore, an apparatus of the invention enables the manufacture of articles, wherein integrally interfused material layers of various natures are utilized by switching a wire delivered during the manufacturing process and/or a nozzle head involved in the process at a given time. The dependent claims directed to an apparatus of the invention set forth preferred embodiments for an apparatus of the invention. 
    
    
     
       The invention will be explained in detail in the following specification with reference to the accompanying drawings, in which 
         FIG. 1  depicts a general operating principle for a LENS manufacturing method representing the prior art, 
         FIGS. 2   a - 2   d  show one preferred apparatus, operating according to a method of the invention on symmetrical principle, in a perspective view, an overhead view and a cross-sectional view, and a multi-segment mirror included therein, in a perspective view, 
         FIGS. 3   a - 3   e  show one preferred apparatus, operating according to a method of the invention on asymmetrical principle, in perspective views from various directions, 
         FIGS. 4   a - 4   d  show sections and enlargements from  FIGS. 2   c,    3   b,    3   c  and  3   e,  and 
         FIG. 5  depicts a so-called shadow effect problem in a laser use based on central wire delivery. 
     
    
    
     The invention relates to a method in connection with laser use, wherein one or more laser beams R emitted by one or more laser sources Y are focused by means of beam guiding elements  1  on a fusing spot S, whereat a filler L delivered thereto is melted, especially for performing a welding, coating, piece manufacturing and/or the like process. The employed filler comprises a substantially solid-state, elongated filler material L; L 1 , such as a wire or the like, which is fed by means of a delivery system X to the fusing spot S centrally relative to a single laser beam R focusing thereon as shown in  FIG. 3   a  or several. laser beams R focusing thereon as shown in  FIG. 3   b.  The one or more laser beams R are diverged I with a multi-segment mirror  1   a,  especially for maintaining the symmetry of the intensity distribution thereof, whereby beams or divisional beams R′, reflecting from its various segment elements  1   a ′ in substantially divergent directions, are converged II on the fusing spot S by means of a condenser system  1   b  included in the beam guiding elements  1 . 
     In a type of solution shown e.g. in  FIG. 5 , the above-described procedure makes it possible to obviate a shadow cast by the filler wire L; L 1  on the fusing spot S, especially when using a laser beam intensity distribution making use of a conventional Gaussian curve G. 
     In reference to the arrangements shown in  FIGS. 2   a - 2   c  and  3   a - 3   e,  the condenser system  1   b  in a preferred embodiment comprises a mirror array  1   b ′,  1   b ″, a lens array and/or the like, whereby diverged beams or divisional beams R′ are converged, as shown in  FIGS. 4   a  and  4   b,  with an intensity distribution substantially equal to the one or more original laser beams R on the fusing spot S symmetrically relative to the filler material L; L 1  supplied thereto. 
     In a further preferred embodiment, the one or more laser beams R are diverged I by means of the segment elements  1   a ′ of a multi-segment mirror, polished for reflections in directions substantially divergent from each other, and converged by means of a focusing lens FL and substantially flat and/or adaptive mirrors  1   b ,  1   b ″ included in the condenser system. This type of solution is feasible especially in a so-called symmetrical implementation. In a solution optional to what is described above, it is also possible to execute the method by a so-called asymmetrical arrangement, which does not necessarily comprise a separate focusing lens but, instead, makes use of focusing paraboloidal mirrors  1   b;    1   b′.    
     Accordingly, a method of the invention is capable of being applied by means of the symmetrical multi-segment mirrors  1   a  and the condenser system  1   b  disposed concentrically relative to the fusing spot S, as shown especially in  FIGS. 2   a - 2   c.  Thus, it is further possible to set up the condenser system by first of all utilizing substantially flat mirror surfaces  1   b ″ and one or more focusing lenses FL. In such a solution, it is further possible to make use of so-called adaptive mirrors, having a focal distance whose adjustment is possible through the action of a pressure fluid by convexing/concaving the mirror surface thereof. A symmetric configuration established like this is first of all beneficial in the sense that the size of an apparatus consisting thereof is minimized in radial direction. 
     In a solution optional to what is described above, it is also possible to utilize an asymmetric condenser system consisting of a multi-segment mirror  1   a , eccentric relative to the fusing spot S, and of focusing paraboloidal mirrors  1   b ′, as shown in  FIGS. 3   a - 3   e.    
     In a further preferred embodiment of the method, the laser source Y is provided by one or more CO2-, NdYAG-, diode-, fiber laser sources Y and/or the like. 
     In a further preferred embodiment, melting of the filler wire L; L 1  at the fusing spot S is assisted by focusing the maximum intensity of the laser beam R on a section between its mid-portion and outer rim and by reducing the laser beam intensity in its mid-portion and outer rim by making use of a so-called donut beam D or the like as shown particularly in  FIG. 4 . Based on a three-dimensional study of the matter, it should be noted at this point that the above-explained shadow effect cannot be totally eliminated by means of a donut beam alone. 
     In a preferred embodiment, the filler L; L 1  is provided by using an essentially metal-based material, such as 0.1-1.5 gauge metal wire, which, in a further preferred embodiment, is preheated almost to the material&#39;s melting point upstream of the fusing spot S. In this context, it is possible to provide a useful filter material by using e.g. steel, aluminum or any appropriate metal or metal alloy. 
     Accordingly, the beam guiding elements  1  of configurations, shown e.g. in  FIGS. 2   a - 2   c.  and  3   a - 3   e  and making use of the above-described method, include a multi-segment mirror  1   a  for diverging (I) one ( FIG. 3   a ) or several ( FIG. 3   b ) laser beams R, especially for maintaining the symmetry of the intensity distribution thereof, and a condenser system  1   b  for converging II beams or divisional beams R′, reflecting from various segment elements  1   a ′ of the multi-segment mirror in substantially divergent directions, on the fusing spot S. 
     In a preferred embodiment, the condenser system  1   b  comprises a mirror array  1   b ′,  1   b ″, a lens array and/or the like for converging, as shown especially in  FIGS. 4   a  and  4   b,  the diverged beams or divisional beams R′, having an intensity distribution substantially equal to the one or more original laser beams R, on the fusing spot S symmetrically relative to the filler material L; L 1  supplied thereto. 
     In preferred embodiments, the segment elements of a multi-segment mirror consist of mirror surfaces  1   a ;  1   a ′, polished for reflections in directions substantially divergent from each other, and the condenser system, for example in a symmetric configuration as shown in  FIGS. 2   a - 2   c,  consists of a focusing lens (FL) and substantially flat and/or adaptive mirrors  1   b;    1   b ″ or, in an asymmetric configuration as shown in  FIGS. 3   a - 3   e,  of focusing paraboloidal mirrors  1   b;    1   b′.    
     In a further preferred embodiment, the laser source Y included in the apparatus comprises one or more CO2-, NdYAG-, diode-, fiber laser sources and/or the like. In addition, the apparatus can be provided with a heating assembly for preheating an essentially metal-based material, such as 0.1-1.5 gauge metal wire or the like, used as the filler L; L 1 , almost to the materials melting point upstream of the fusing spot S. 
     For example, the use of a fiber laser enables the use of laser sources of, for example, about 100 watts. Respectively, the wavelength of a laser beam in fiber is, for example, approximately 10,090 nm and the standard thereof is 0.3 nm·mrad. 
     It is obvious that the invention is not limited to the embodiments described or specified above, but it can be varied according to the original inventive concept within the scope defined in the appended claims. Thus, for example, when it is desirable to protect the flux formed in laser processing from ambient atmosphere, i.e. for example from air, and especially from nitrogen and oxygen present therein, which may have an embrittling effect on a metal solidifying in the wake of melting, it is possible to take advantage of a technique, known as such e.g. in welding technology, by using for example a shroud gas during the course of laser processing. Such shroud gases may include for example argon, helium, and carbon dioxide or combinations of the above. On the other hand, it is also possible to take advantage of an alloyed filler wire, the shroud being provided by slag separating in laser fusion. Another possibility in a method and apparatus of the invention is to make use of a hollow filler wire, through the inside of which a shroud gas and/or shielding agents or other possibly desired alloying elements can be delivered to the melt.