Abstract:
Welding repairs are often carried out on directionally solidified components that nevertheless do not possess the desired crystallographic surface alignment, which reduces mechanical strength. The method provided selects the direction of travel depending on the crystallographically preferred direction of the substrate such that no more misorientations occur. A laser beam may be used for remelting.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2009/054306, filed Apr. 9, 2009 and claims the benefit thereof. The International Application claims the benefits of German application No. 10 2008 018 708.9 DE filed Apr. 14, 2008. All of the applications are incorporated by reference herein in their entirety. 
     FIELD OF INVENTION 
     The invention relates to a process for welding a substrate having a preferred direction. 
     BACKGROUND OF INVENTION 
     Welding is a repair process which is frequently used to close cracks or to apply material. In this case, a laser is often used as the energy source. The laser welding process is also used to repair directionally solidified components, for example turbine blades or vanes of the largest gas turbines, after they have been used, which possibly have cracks as a result of extraordinarily severe loading. These can be components with grains solidified in columnar form (DS) or else single crystals (SX). 
     The component therefore has a defined preferred crystallographic direction in the crystal structure. The solidification behavior of the material, which should obtain the same orientation as the substrate during the laser welding, depends on the composition of the alloy, the temperature gradient and the solidification rate. For a defined alloy, there are graphs showing how the structure developed depending on the temperature gradient and the solidification rate. 
     Nevertheless, grains frequently grow in an undesirable direction. 
     SUMMARY OF INVENTION 
     It is therefore an object of the invention to overcome this problem. 
     The object is achieved by a process as claimed in the claims. 
     The dependent claims list further advantageous measures which can be combined with one another, as desired, in order to obtain further advantages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-6  show a substrate during laser remelting, 
         FIG. 7  shows a gas turbine, 
         FIG. 8  shows a perspective view of a turbine blade or vane, 
         FIG. 9  shows a perspective view of a combustion chamber, and 
         FIG. 10  shows a list of superalloys. 
     
    
    
     The figures and the description represent only exemplary embodiments of the invention. 
     DETAILED DESCRIPTION OF INVENTION 
       FIG. 1  is a cross-sectional view of a component  1 ,  120 ,  130  ( FIGS. 8 ,  10 ),  155  ( FIG. 9 ) having a substrate  4  which, in particular in the case of turbine blades or vanes for gas turbines  100  ( FIG. 7 ) or steam turbines, has a superalloy according to  FIG. 10 . 
     The substrate  4  has a directionally solidified structure, i.e. it can consist of columnar grains solidified in columnar form (DS) or of a single crystal (SX). The arrows  7 ,  22  indicate the preferred crystallographic directions of the substrate  4 , i.e. of the single crystal or of the columnar grains (e.g.: [001]=7, [010]=22). 
     The substrate  4  has a crack (not shown). The substrate  4  is therefore melted (remelted) in the region of the crack, where the molten region (melt  19 ,  FIGS. 3 ,  4 ) should again solidify directionally in a DS or SX structure. 
     The substrate  4  may likewise have a point (excessively thin wall, not shown) which is to be strengthened by build-up welding (i.e. the supply of material is required), in particular laser build-up welding. 
       FIG. 2  shows a line  10  of a solidification front, which represents a surface and, in the plane of the drawing, shows a transition between a melt  19  and the zone  24  which has already solidified from a melt and also a region  23  still to be remelted. 
     In the figures, the line  10  always shows only a section of the solidification front. 
     The substrate  4  moves along a direction  25  from left to right in the drawing, such that the solidification front  10  propagates from right to left in the drawing counter to the direction  25 . 
     It is likewise possible for only the welding appliance  31  to move instead of the substrate  4 . 
     The solidification front  10  is then that part of the elliptical line  10 , on the right in  FIG. 2 , which comprises the melt  19 . The line  10  is only exemplary. The line  10  may also have other forms. 
     Depending on the depth t along the direction  28  (perpendicular downward to the surface  16 ) of the line  10 , there are differently oriented temperature gradients  13 ,  13 ′, depending on the vicinity of the surface  16  of the substrate  4 . Here, the temperature gradient  13 ,  13 ′ is virtually perpendicular on the solidification front  10 . 
     Proceeding from  FIG. 2 , angles Ψ 1 , Ψ 1 ′ and Ψ 2 , Ψ 2 ′ are then additionally shown in  FIG. 3  (and also in  FIG. 4 ), where Ψ 1 , Ψ 1 ′ are the angles between the preferred direction  7  and the temperature gradients  13 ,  13 ′ and Ψ 2 , Ψ 2 ′ are the angles between the temperature gradients  13 ,  13 ′ and a second crystallographic direction  22  (perpendicular to the preferred direction  7 ). 
     Here, the substrate  4  moves from left to right in the drawing. 
     In  FIG. 3 , the direction of dendrite growth is changed during growth from the melt  19 , since Ψ 2 &lt;Ψ 1  holds true at the surface  16 , such that the crystallographic direction  22  directed downward from the surface  16  is energetically promoted, and the dendrites grow in a second crystallographic direction  22  from the surface  16 , such that secondary grains form in the region of the surface. 
     At a greater depth, it may hold true that Ψ 2 ′&gt;Ψ 1 ′ and the direction  7  is preferred. 
     The problem first arises when a direction of dendrite growth directed from the surface  16  into the melt  19  is favored at the surface  16 . By definition, epitaxial growth from the surface  16  is not possible, because a substrate which can act as a nucleus for the dendrites is not present there. Instead, the progression of the solid/liquid phase boundary at the surface  16  is realized under these conditions via the formation of secondary arms, tertiary arms, etc. This is too slow compared to the rate of growth of the nuclei before the solidification front. At some point in time, one of these nuclei prevails with respect to the epitaxially grown dendrites, and directions of dendrite growth which are not correlated with those in the substrate  4  are formed. 
     The problem of epitaxy loss therefore always arises whenever the crystal directions  7 ,  22  favored at the surface  16  are not oriented parallel to the surface  16 . These crystal directions  7 ,  22 , favored for the dendrite growth, are independent of the direction of movement  25 . However, these crystal directions can be utilized by the dendrites for their growth in two directions. 
     In order to avoid epitaxy loss, the direction of movement  25  has to be selected in such a manner that of the crystal directions  7 ,  22  (here  22 ) favored at the surface  16  on the solidification front  10 , a direction of dendrite growth which has a projection (vectors P 22 , P 7 =projections of  7 ,  22  to surface normal {right arrow over (n 0 )}) in the direction of the surface normal {right arrow over (n 0 )}( FIG. 5 ) is initialized. 
     By selecting the direction of movement  25  in  FIG. 4 , specifically from right to left in the drawing, that crystallographic direction, here  22 , which is not directed downward from the surface  16  is preferred. 
     This applies with preference to the entire solidification front  10 , i.e. the line  10  between the melt pool  19  and the region  24  which has already solidified. 
     Both of the crystallographic directions  7 ,  22  are permissible and desirable. This actually involves the loss of epitaxial growth, which has the effect that the crystal orientation is lost completely in the weld metal ( FIG. 6 : vector P 22  opposed to {right arrow over (n 0 )}= FIG. 3 ). This can be avoided by preventing the promotion of a direction of dendrite growth directed downward from the surface  16 . 
       FIG. 7  shows, by way of example, a partial longitudinal section through a gas turbine  100 . 
     In the interior, the gas turbine  100  has a rotor  103  with a shaft  101  which is mounted such that it can rotate about an axis of rotation  102  and is also referred to as the turbine rotor. 
     An intake housing  104 , a compressor  105 , a, for example, toroidal combustion chamber  110 , in particular an annular combustion chamber, with a plurality of coaxially arranged burners  107 , a turbine  108  and the exhaust-gas housing  109  follow one another along the rotor  103 . 
     The annular combustion chamber  110  is in communication with a, for example, annular hot-gas passage  111 , where, by way of example, four successive turbine stages  112  form the turbine  108 . 
     Each turbine stage  112  is formed, for example, from two blade or vane rings. As seen in the direction of flow of a working medium  113 , in the hot-gas passage  111  a row of guide vanes  115  is followed by a row  125  formed from rotor blades  120 . 
     The guide vanes  130  are secured to an inner housing  138  of a stator  143 , whereas the rotor blades  120  of a row  125  are fitted to the rotor  103  for example by means of a turbine disk  133 . 
     A generator (not shown) is coupled to the rotor  103 . 
     While the gas turbine  100  is operating, the compressor  105  sucks in air  135  through the intake housing  104  and compresses it. The compressed air provided at the turbine-side end of the compressor  105  is passed to the burners  107 , where it is mixed with a fuel. The mix is then burnt in the combustion chamber  110 , forming the working medium  113 . From there, the working medium  113  flows along the hot-gas passage  111  past the guide vanes  130  and the rotor blades  120 . The working medium  113  is expanded at the rotor blades  120 , transferring its momentum, so that the rotor blades  120  drive the rotor  103  and the latter in turn drives the generator coupled to it. 
     While the gas turbine  100  is operating, the components which are exposed to the hot working medium  113  are subject to thermal stresses. The guide vanes  130  and rotor blades  120  of the first turbine stage  112 , as seen in the direction of flow of the working medium  113 , together with the heat shield elements which line the annular combustion chamber  110 , are subject to the highest thermal stresses. 
     To be able to withstand the temperatures which prevail there, they may be cooled by means of a coolant. 
     Substrates of the components may likewise have a directional structure, i.e. they are in single-crystal form (SX structure) or have only longitudinally oriented grains (DS structure). 
     By way of example, iron-based, nickel-based or cobalt-based superalloys are used as material for the components, in particular for the turbine blade or vane  120 ,  130  and components of the combustion chamber  110 . 
     Superalloys of this type 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. 
     The blades or vanes  120 ,  130  may likewise have coatings protecting against corrosion (MCrAlX; M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon, scandium (Sc) and/or at least one rare earth element, or hafnium). Alloys of this type are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1. 
     It is also possible for a thermal barrier coating to be present on the MCrAlX, consisting for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. 
     Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
     The guide vane  130  has a guide vane root (not shown here), which faces the inner housing  138  of the turbine  108 , and a guide vane head which is at the opposite end from the guide vane root. The guide vane head faces the rotor  103  and is fixed to a securing ring  140  of the stator  143 . 
       FIG. 8  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 generating electricity, a steam turbine or a compressor. 
     The blade or vane  120 ,  130  has, in succession along the longitudinal axis  121 , a securing region  400 , an adjoining blade or vane platform  403  and a main blade or vane part  406  and a blade or vane tip  415 . 
     As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
     A blade or vane root  183 , which is used to secure the rotor blades  120 ,  130  to a shaft or a disk (not shown), is formed in the securing region  400 . 
     The blade or vane root  183  is designed, for example, in hammerhead form. Other configurations, such as a fir-tree or dovetail root, are possible. 
     The blade or vane  120 ,  130  has a leading edge  409  and a trailing edge  412  for a medium which flows past the main blade or vane part  406 . 
     In the case of conventional blades or vanes  120 ,  130 , by way of example solid metallic materials, in particular superalloys, are used in all regions  400 ,  403 ,  406  of the blade or vane  120 ,  130 . 
     Superalloys of this type 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. 
     The blade or vane  120 ,  130  may in this case be produced by a casting process, by means of directional solidification, by a forging process, by a milling process or combinations thereof. 
     Workpieces with a single-crystal structure or structures are used as components for machines which, in operation, are exposed to high mechanical, thermal and/or chemical stresses. 
     Single-crystal workpieces of this type are produced, for example, by directional solidification from the melt. This involves casting processes in which the liquid metallic alloy solidifies to form the single-crystal structure, i.e. the single-crystal workpiece, or solidifies directionally. 
     In this case, dendritic crystals are oriented along the direction of heat flow and form either a columnar crystalline grain structure (i.e. grains which run over the entire length of the workpiece and are referred to here, in accordance with the language customarily used, as directionally solidified) or a single-crystal structure, i.e. the entire workpiece consists of one single crystal. In these processes, a transition to globular (polycrystalline) solidification needs to be avoided, since non-directional growth inevitably forms transverse and longitudinal grain boundaries, which negate the favorable properties of the directionally solidified or single-crystal component. 
     Where the text refers in general terms to directionally solidified microstructures, this is to be understood as meaning both single crystals, which do not have any grain boundaries or at most have small-angle grain boundaries, and columnar crystal structures, which do have grain boundaries running in the longitudinal direction but do not have any transverse grain boundaries. This second form of crystalline structures is also described as directionally solidified microstructures (directionally solidified structures). 
     Processes of this type are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1. 
     The blades or vanes  120 ,  130  may likewise have coatings protecting against corrosion or oxidation e.g. (MCrAlX; M is at least one element selected from the group consisting of 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)). Alloys of this type are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1. 
     The density is preferably 95% of the theoretical density. 
     A protective aluminum oxide layer (TGO=thermally grown oxide layer) is formed on the MCrAlX layer (as an intermediate layer or as the outermost layer). 
     The layer preferably has a composition Co-30Ni-28Cr-8Al-0.6Y-0.7Si or Co-28Ni-24Cr-10Al-0.6Y. In addition to these cobalt-based protective coatings, it is also preferable to use nickel-based protective layers, such as Ni-10Cr-12Al-0.6Y-3Re or Ni-12Co-21Cr-11Al-0.4Y-2Re or Ni-25Co-17Cr-10Al-0.4Y-1.5Re. 
     It is also possible for a thermal barrier coating, which is preferably the outermost layer and consists for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide, to be present on the MCrAlX. 
     The thermal barrier coating covers the entire MCrAlX layer. Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
     Other coating processes are possible, for example atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may include grains that are porous or have micro-cracks or macro-cracks, in order to improve the resistance to thermal shocks. The thermal barrier coating is therefore preferably more porous than the MCrAlX layer. 
     Refurbishment means that after they have been used, protective layers may have to be removed from components  120 ,  130  (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the component  120 ,  130  are also repaired. This is followed by recoating of the component  120 ,  130 , after which the component  120 ,  130  can be reused. 
     The blade or vane  120 ,  130  may be hollow or solid in form. 
     If the blade or vane  120 ,  130  is to be cooled, it is hollow and may also have film-cooling holes  418  (indicated by dashed lines). 
       FIG. 9  shows a combustion chamber  110  of a gas turbine. The combustion chamber  110  is configured, for example, as what is known as an annular combustion chamber, in which a multiplicity of burners  107 , which generate flames  156 , arranged circumferentially around an axis of rotation  102  open out into a common combustion chamber space  154 . For this purpose, the combustion chamber  110  overall is of annular configuration positioned around the axis of rotation  102 . 
     To achieve a relatively high efficiency, the combustion chamber  110  is designed for a relatively high temperature of the working medium M of approximately 1000° C. to 1600° C. To allow a relatively long service life even with these operating parameters, which are unfavorable for the materials, the combustion chamber wall  153  is provided, on its side which faces the working medium M, with an inner lining formed from heat shield elements  155 . 
     On the working medium side, each heat shield element  155  made from an alloy is equipped with a particularly heat-resistant protective layer (MCrAlX layer and/or ceramic coating) or is made from material that is able to withstand high temperatures (solid ceramic bricks). 
     These protective layers may be similar to the turbine blades or vanes, i.e. for example MCrAlX: M is at least one element selected from the group consisting of 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). Alloys of this type are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1. 
     It is also possible for a, for example, ceramic thermal barrier coating to be present on the MCrAlX, consisting for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. 
     Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
     Other coating processes are possible, e.g. atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may include grains that are porous or have micro-cracks or macro-cracks, in order to improve the resistance to thermal shocks. 
     Refurbishment means that after they have been used, protective layers may have to be removed from heat shield elements  155  (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the heat shield element  155  are also repaired. This is followed by recoating of the heat shield elements  155 , after which the heat shield elements  155  can be reused. 
     Moreover, a cooling system may be provided for the heat shield elements  155  and/or their holding elements, on account of the high temperatures in the interior of the combustion chamber  110 . The heat shield elements  155  are then, for example, hollow and may also have cooling holes (not shown) opening out into the combustion chamber space  154 .