CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2006/069157, filed Nov. 30, 2006 and claims the benefit thereof. The International Application claims the benefits of European application No. 05027790.4 filed Dec. 19, 2005, both of the applications are incorporated by reference herein in their entirety. 
     FIELD OF INVENTION 
     The invention relates to a welding method and to a welding device as claimed in the claims. 
     BACKGROUND OF THE INVENTION 
     In a welding method the substrate of a component is melted so that cracks in the substrate are sealed, optionally with the addition of an auxiliary welding material, particularly in the form of powder, which is also melted and allowed to solidify during the welding. 
     Particularly for directionally solidified components, which have a longitudinally directed grain structure or consist of a single crystal, such welding methods are also used in order to set up a directionally solidified structure in the welded region. 
     The problem with the welding method, however, is that the process does not already take place stably from the start, so that the sites which are welded first often have inferior properties compared with the other welded regions and they often have to be melted once more. 
     SUMMARY OF INVENTION 
     It is therefore an object of the invention to overcome the aforementioned problem. 
     The object is achieved by a welding method as claimed in the claims and a welding device as claimed in the claims. 
     The measures listed in the dependent claims may be combined with one another in any desired way, in order to achieve further advantages. 
     Here, according to the invention, the method start point of the welding method thus lies on the precursor plate, so that the welding process is stable when the cavity start point is reached and high-quality uniform properties of the entire welded region are achieved from the start. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention will be explained in more detail below with reference to the drawings, in which: 
         FIG. 1  shows a welding device according to the prior art, 
         FIG. 2  shows a device according to the invention, 
         FIGS. 3 ,  4  show special components of the welding device according to the invention, 
         FIG. 5  shows the conduct of the welding method according to the invention, 
         FIG. 6  shows a perspective view of a turbine blade, 
         FIG. 7  shows a perspective view of a combustion chamber, and 
         FIG. 8  shows a perspective view of a gas turbine. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
       FIG. 1  shows a welding device according to the prior art. 
     An auxiliary welding material  13  is supplied to a site  10  to be repaired in a substrate  4  via a powder feed  31  from a powder reservoir  28 , which  13  is melted by means of a heat source  34 , in particular by an electron beam gun, a laser  34  or by plasma welding, and allowed to solidify. Either the substrate  4  or the powder feed  31 , or the beams  37  from the heat source  34  travel in a welding direction  40 , in which  40  for example an elongate crack  10  or a cavity  10  propagates as a site  10  to be repaired. The method begins at a start point  22  of the cavity  10  and an end point  25  of the site  10  to be repaired. 
       FIG. 2  shows a device  1  according to the invention, with which the welding method according to the invention can be carried out. 
     The substrate  4  of a component  120 ,  130 ,  138 ,  155  has a surface  7 , a subregion of which, i.e. the site  10  to be repaired, is intended to be welded. This may be a crack  10  or a cavity  10  (discussed below by way of example) which needs to be filled. 
     Material may likewise be applied over a large area in order to achieve thickening of a wall, particularly in the case of hollow components  120 ,  130 . 
     The auxiliary welding material  13  is used for this, which fills the crack  10  or the cavity  10 , or thickens the surface  10 . 
     The auxiliary welding material  13  may be supplied constantly in the form of powder during the welding, or it may have been introduced into the cavity  10  in the form of a strip or wire. 
     According to the invention, the welding method does not begin at the start point  22  of the site  10  to be repaired, beyond which the supply or presence of the auxiliary welding material  13  would be necessary, but instead previously on a separate precursor plate  19  which either rests fully on the surface  7  (indicated by dashes) or bears on the surface  7  with a particular angle α at the start point  22 . 
     “Separate” means that the precursor plate  19  is not part of the component  120 ,  130 ,  138 ,  155  or of a welded region  14 . 
     There may be a gap between the precursor plate  19  and the surface  7 . 
     Here, according to the invention, the method start point  43  of the welding method thus lies on the precursor plate  19  so that the welding process is stable when the cavity start point  22  is reached, and high-quality uniform properties of the entire welded region are achieved from the start. 
     There may likewise be a separate follower plate  16  at the end point  25 , although there does not have to be one, which bears on the end point  25 , the follower plate  16  likewise being provided with the auxiliary welding material  13  and the method being concluded at the method end point  46 . 
     A homogeneous structure of the welded region  14  is thus achieved throughout the site  10  to be repaired. 
     The precursor or follower plate  16 ,  19 , which are preferably designed in the form of plates, bears on the surface  10  of the substrate  4  so that as far as possible no gap, or preferably only a small gap, is formed between the surface  10  and an edge, i.e. the contact surface  49  of the precursor plate  16  or follower plate  19  ( FIG. 3 ), i.e. the front edge of the precursor plate  16  is configured obliquely. 
     Optionally, the touching surfaces  49  of the plates  16 ,  19  are curved like a curved surface  10  of a component  4 , as is the case for example with turbine blades  120 ,  130  ( FIG. 4 ). 
       FIG. 4  shows the conduct of the welding method according to the invention. 
     A precursor plate  19  is used, on which the welding method is started at a method end point  43 . 
     At least at the start point  22 , the welding method takes place stably ( FIG. 4   a ). The distance between the method start point  43  and the start point  22  is preferably 2 mm-10 mm. 
     The method then continues by applying the auxiliary welding material  13  onto or into the site  10  to be repaired ( FIG. 5   b ). 
     The precursor plate  19  may be displaced, although it does not have to be, so that it also lies over a previously welded region  14 . This has the advantage that already welded regions  14  are no longer contaminated, for example by a vapor cloud from the region  13  currently to be welded ( FIG. 5   c ). 
     Instead of displacing the precursor plate  19 , a further shielding plate (not shown) may also be used and displaced. 
     The welding method is carried out as far as the cavity end point  25 , and then preferably continuous over a follower plate  16  until a method end point  46  is reached ( FIG. 5   d ). 
     The precursor plates  16 ,  19  are removed, so that the fully repaired component  1  is then provided ( FIG. 5   e ). 
       FIG. 6  shows a perspective view of a rotor blade  120  or guide vane  130  of a turbomachine, which extends along a longitudinal axis  121 . 
     The turbomachine may be a gas turbine of an aircraft or of a power plant for electricity generation, a steam turbine or a compressor. 
     Successively along the longitudinal axis  121 , the blade  120 ,  130  comprises a fastening zone  400 , a blade platform  403  adjacent thereto as well as a blade surface  406 . 
     As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
     A blade root  183  which is used to fasten the rotor blades  120 ,  130  on a shaft or a disk (not shown) is formed in the fastening zone  400 . 
     The blade root  183  is configured, for example, as a hammerhead. Other configurations as a fir tree or dovetail root are possible. 
     The blade  120 ,  130  comprises a leading edge  409  and a trailing edge  412  for a medium which flows past the blade surface  406 . 
     In conventional blades  120 ,  130 , for example solid metallic materials, in particular superalloys, are used in all regions  400 ,  403 ,  406  of the blade  120 ,  130 . 
     Such superalloys are known for example from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949; with respect to the chemical composition of the alloy, these documents are part of the disclosure. 
     The blades  120 ,  130  may in this case be manufactured by a casting method, also by means of directional solidification, by a forging method, by a machining method or combinations thereof. 
     Workpieces with a monocrystalline structure or structures are used as components for machines which are exposed to heavy mechanical, thermal and/or chemical loads during operation. 
     Such monocrystalline workpieces are manufactured, for example, by directional solidification from the melts. These are casting methods in which the liquid metal alloy is solidified to form a monocrystalline structure, i.e. to form the monocrystalline workpiece, or is directionally solidified. 
     Dendritic crystals are in this case aligned along the heat flux and form either a rod crystalline grain structure (columnar, i.e. grains which extend over the entire length of the workpiece and in this case, according to general terminology usage, are referred to as directionally solidified) or a monocrystalline structure, i.e. the entire workpiece consists of a single crystal. It is necessary to avoid the transition to globulitic (polycrystalline) solidification in these methods, since nondirectional growth will necessarily form transverse and longitudinal grain boundaries which negate the beneficial properties of the directionally solidified or monocrystalline component. 
     When directionally solidified structures are referred to in general, this is intended to mean both single crystals which have no grain boundaries or at most small-angle grain boundaries, and also rod crystal structures which, although they do have grain boundaries extending in the longitudinal direction, do not have any transverse grain boundaries. These latter crystalline structures are also referred to as directionally solidified structures. 
     Such methods are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1; these documents are part of the disclosure. 
     The blades  120 ,  130  may likewise have coatings against corrosion or oxidation, for example (MCrAlX; M is at least one element from the group iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon and/or at least one rare earth element, or hafnium (Hf)). Such alloys are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1 which, with respect to the chemical composition of the alloy, are intended to be part of this disclosure. 
     On the MCrAlX layer, there may furthermore be a thermal insulation layer which consists for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. it is not stabilized or is partially or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. 
     Rod-shaped grains are produced in the thermal insulation layer by suitable coating methods, for example electron beam deposition (EB-PVD), or for example atmospheric plasma spraying (APS) produces porous grains affected by microcracks and macrocracks in the thermal insulation layer. 
     Refurbishment means that components  120 ,  130  may need to have protective layers taken off (for example by sandblasting) after their use. Then the corrosion and/or oxidation layers or products are removed. Optionally, cracks in the component  120 ,  130  are also repaired by the method according to the invention. The component  120 ,  130  is then recoated and the component  120 ,  130  is used again. 
     The blade  120 ,  130  may be designed to be a hollow or solid. If the blade  120 ,  130  is intended to be cooled, it will be hollow and optionally also comprise film cooling holes  418  (indicated by dashes). 
       FIG. 7  shows a combustion chamber  110  of a gas turbine  100  ( FIG. 8 ). 
     The combustion chamber  110  is designed for example as a so-called ring combustion chamber in which a multiplicity of burners  107 , which produce flames  156  and are arranged in the circumferential direction around a rotation axis  102 , open into a common combustion chamber space  154 . To this end, the combustion chamber  110  as a whole is designed as an annular structure which is positioned around the rotation axis  102 . 
     In order to achieve a comparatively high efficiency, the combustion chamber  110  is designed for a relatively high temperature of the working medium M, i.e. about 1000° C. to 1600° C. In order to permit a comparatively long operating time even under these operating parameters which are unfavorable for the materials, the combustion chamber wall  153  is provided with an inner lining formed by heat shield elements  155  on its side facing the working medium M. 
     Each heat shield element  155  made of an alloy is equipped with a particularly heat-resistant protective layer (MCrAlX layer and/or ceramic coating) on the working medium side, or is made of refractory material (solid ceramic blocks). 
     These protective layers may be similar to the turbine blades, i.e. for example MCrAlX means: M is at least one element from the group iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon and/or at least one rare earth element, or hafnium (Hf). Such alloys are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1 which, with respect to the chemical composition of the alloy, are intended to be part of this disclosure. 
     On the MCrAlX, there may also be an e.g. ceramic thermal insulation layer which consists for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. it is not stabilized or is partially or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. 
     Rod-shaped grains are produced in the thermal insulation layer by suitable coating methods, for example electron beam deposition (EB-PVD). 
     Refurbishment means that heat shield elements  155  may need to have protective layers taken off (for example by sandblasting) after their use. The corrosion and/or oxidation layers or products are then removed. Optionally, cracks in the heat shield element  155  are also repaired by the method according to the invention. The heat shield elements  155  are then recoated and the heat shield elements  155  are used again. 
     Owing to the high temperatures inside the combustion chamber  110 , a cooling system may also be provided for the heat shield elements  155  or for their retaining elements. The heat shield elements  155  are then hollow, for example, and optionally also have film-cooling holes (not shown) opening into the combustion chamber space  154 . 
       FIG. 8  shows a gas turbine  100  by way of example in a partial longitudinal section. 
     The gas turbine  100  internally comprises a rotor  103 , which will also be referred to as the turbine rotor, mounted so as to rotate about a rotation axis  102  and having a shaft  101 . 
     Successively along the rotor  103 , there are an intake manifold  104 , a compressor  105 , an e.g. toroidal combustion chamber  110 , in particular a ring combustion chamber, having a plurality of burners  107  arranged coaxially, a turbine  108  and the exhaust manifold  109 . 
     The ring combustion chamber  110  communicates with an e.g. annular hot gas channel  111 . There, for example, four successively connected turbine stages  112  form the turbine  108 . 
     Each turbine stage  112  is formed for example by two blade rings. As seen in the flow direction of a working medium  113 , a guide vane row  115  is followed in the hot gas channel  111  by a row  125  formed by rotor blades  120 . 
     The guide vanes  130  are fastened on an inner housing  138  of a stator  143  while the rotor blades  120  of a row  125  are fastened on the rotor  103 , for example by means of a turbine disk  133 . Coupled to the rotor  103 , there is a generator or a work engine (not shown). 
     During operation of the gas turbine  100 , air  135  is taken in and compressed by the compressor  105  through the intake manifold  104 . The compressed air provided at the turbine-side end of the compressor  105  is delivered to the burners  107  and mixed there with a fuel. The mixture is then burnt to form the working medium  113  in the combustion chamber  110 . From there, the working medium  113  flows along the hot gas channel  111  past the guide vanes  130  and the rotor blades  120 . At the rotor blades  120 , the working medium  113  expands by imparting momentum, so that the rotor blades  120  drive the rotor  103  and the work engine coupled to it. 
     During operation of the gas turbine  100 , the components exposed to the hot working medium  113  experience thermal loads. Apart from the heat-shield elements lining the ring combustion chamber  110 , the guide vanes  130  and rotor blades  120  of the first turbine stage  112 , as seen in the flow direction of the working medium  113 , are heated the most. 
     In order to withstand the temperatures prevailing there, they may be cooled by means of a coolant. 
     Substrates of the components may likewise comprise a directional structure, i.e. they are monocrystalline (SX structure) or comprise only longitudinally directed grains (DS, structure). 
     Iron-, nickel- or cobalt-based superalloys are for example used as material for the components, in particular for the turbine blades  120 ,  130  and components of the combustion chamber  110 . 
     Such superalloys are known for example from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949; with respect to the chemical composition of the alloy, these documents are part of the disclosure. 
     The guide vanes  130  comprise a guide vane root (not shown here) facing the inner housing  138  of the turbine  108 , and a guide vane head lying opposite the guide vane root. The guide vane head faces the rotor  103  and is fixed on a fastening ring  140  of the stator  143 .
Conventional welding methods have the disadvantage of producing worse welded joints at the beginning of the process than in the later course of the method. The method in question uses an initializing plate on which the welding process is begun and then once a consistent quality can be achieved for the welded joint continues onto the points for repair.