Patent Publication Number: US-2012031579-A1

Title: Method for producing a negative mold for casting a turbine blade and mold for producing a wax model of a gas turbine

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2010/054327, filed Mar. 31, 2010 and claims the benefit thereof. The International Application claims the benefits of European Patent Office application No. 09005327.3 EP filed Apr. 14, 2009. All of the applications are incorporated by reference herein in their entirety. 
    
    
     FIELD OF INVENTION 
     The present invention relates to a method for producing a negative mold for casting a turbine blade or vane, in which method a wax model of the turbine blade or vane is produced and the negative mold is created on the basis of the wax model. In addition, the invention relates to an aid used in the method and to a mold for producing a wax model of a turbine blade or vane. 
     BACKGROUND OF INVENTION 
     By way of example, DE 197 26 111 C1 discloses a method for producing a turbine rotor blade by casting, in which method a wax model of the blade is produced, the blade root being produced by filling a permanent mold having the blade contour. This is followed by the production of a mold shell for casting the turbine blade, the permanent mold remaining on the blade root. Once the wax model has been melted out, the inner contour of the permanent mold defines the outer contour of the turbine blade to be cast. In other words, the permanent mold represents part of the casting mold for producing the turbine blade by casting. Within the context of this method, the permanent mold should improve the surface accuracy for shaping the blade root, such that complex reworking of the blade root once the turbine blade has been cast is not necessary. The permanent mold is either produced from an alloy material which is resistant to high temperatures and has sufficient oxidation resistance, such that it can be reused, or it is produced as a disposable permanent mold made of a low-alloy steel. 
     SUMMARY OF INVENTION 
     With respect to this prior art, it is an object of the present invention to provide an advantageous method for producing a negative mold for casting a turbine blade or vane, with which method it is possible to realize improved shaping even without the use of a permanent mold in critical blade or vane regions. 
     It is a second object of the present invention to provide an advantageous method for producing a turbine blade or vane, with which method it is possible to achieve improved shaping of the turbine blade or vane without the use of a permanent mold. 
     It is a further object of the present invention to provide an advantageous aid for producing a negative mold of a turbine blade or vane. 
     The first object is achieved by a method for producing a negative mold for casting a turbine blade or vane as claimed in the claims, the second object is achieved by a method for producing a turbine blade or vane as claimed in the claims and the third object is achieved by a spacer for a wax mold as claimed in the claims. The dependent claims contain advantageous configurations of the invention. 
     In the method according to the invention for producing a negative mold for casting a turbine blade or vane, the turbine blade or vane to be cast has a blade or vane platform and a fastening element for fastening the blade or vane to a blade or vane mount, wherein at least one surface of the fastening element is located opposite a platform surface of the blade or vane platform at a distance. In the method according to the invention, a wax model of the turbine blade or vane to be produced is produced, and a negative mold of the turbine blade or vane is created on the basis of the wax model once the wax model has hardened. The wax model is melted out once the negative mold has been completed. According to the invention, during hardening and/or storage and/or transport of the wax model, at least one spacer is arranged between the platform surface of the blade or vane platform and the at least one opposite surface of the fastening element of the wax model. 
     The invention is based on the knowledge that deviations in the shaping in particular of the fastening elements of turbine blades or vanes occur on account of deformations of the wax model during hardening and/or storage up to the production of the negative mold and/or transport. These deformations can be particularly serious in the region of the blade or vane suspension, which is generally formed by hook-shaped elements spaced apart from the blade or vane platform. However, these hooks are particularly critical with respect to the dimensioning of the distance between the hooks and the blade or vane platform. Specifically, this distance defines the position of the platform in the hot gas path of the turbine. 
     By using the spacer, it is possible for the particularly critical distance between the hooks and the blade or vane platform to be kept constant very effectively, and therefore reworking of a cast turbine blade or vane is generally no longer necessary. Since the spacer is removed from the wax model before the negative mold is produced, it does not need to consist of a material which withstands the casting process for casting the turbine blade or vane. Compared to the permanent mold mentioned in the introduction, it is therefore possible to use a material which is very much less resistant to high temperatures and therefore a less expensive material. 
     It is advantageous if the spacer is arranged between the platform surface of the blade or vane platform and the at least one opposite surface of the fastening element as early as during the hardening of the wax model, but also remains in the wax model until immediately before the negative mold is produced. It is thereby possible for deformations of the wax model which influence the distance between the blade or vane platform and the fastening element to be avoided without interruption from production up to the use thereof. 
     In order to distribute any forces which occur as uniformly as possible, it is advantageous if the at least one spacer completely fills the space between the platform surface of the blade or vane platform and the at least one opposite surface of the fastening element. 
     In order to prevent not only displacements in the distance between the fastening element and the blade or vane platform, but also lateral movements between the blade or vane platform and the fastening element, the spacer can rest against at least one further surface of the fastening element. In addition, it is also possible to use a spacer which has a receptacle, matched to the shape of the fastening element, for receiving at least part of the fastening element. The fastening element can thereby be secured against deformations in all relevant directions. In particular, the spacer can also be part of a mold for producing the wax model of the turbine blade or vane. 
     The spacer used can be produced, in particular, from metal or ceramic. If the spacer is produced from metal, steel or aluminum are suitable, for example, in which case aluminum involves advantages on account of its low weight. 
     In the method according to the invention for producing a turbine blade or vane, a turbine blade or vane having a blade or vane platform and a fastening element for fastening the blade or vane to a blade or vane mount is produced, wherein at least one surface of the fastening element is located opposite a platform surface of the blade or vane platform at a distance. The turbine blade or vane is cast with the aid of a negative mold of the turbine blade or vane. According to the invention, the negative mold is created according to the method according to the invention for producing a negative mold. 
     The method according to the invention for producing a turbine blade or vane makes it possible to produce the turbine blade or vane with surfaces and dimensions which are readily controllable in the particularly relevant regions of the suspension, without this requiring the use of a permanent mold. 
     The invention also provides a spacer for a wax mold of a turbine blade or vane having a blade or vane platform and a fastening element for fastening the blade or vane to a blade or vane mount, wherein at least one surface of the fastening element is located opposite a platform surface of the blade or vane platform at a distance. The spacer according to the invention has a receptacle, matched to the shape of the fastening element, for receiving at least part of the fastening element. Within the context of the method according to the invention for producing a negative mold, such a spacer can advantageously be used for the production of a turbine blade or vane. It can be produced, in particular, from metal, for example from steel or aluminum. It can also be produced from ceramic. 
     A further aspect of the invention provides a mold for producing a wax model of a turbine blade or vane. The mold comprises a removable or detachable part which remains on the wax model as the wax model is being removed from the mold and forms a spacer according to the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features, properties and advantages of the present invention will become apparent from the following description of exemplary embodiments with reference to the accompanying figures. 
         FIG. 1  shows a partial longitudinal section through a gas turbine. 
         FIG. 2  shows a perspective view of an example of a turbine blade or vane. 
         FIG. 3  shows an example of a combustion chamber of a gas turbine. 
         FIG. 4  shows a highly diagrammatic view of the suspension of a turbine blade or vane. 
         FIG. 5  shows the sequence of a method for producing a turbine blade or vane by casting. 
         FIG. 6  shows a section of a wax model of a turbine blade or vane. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
       FIG. 1  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; these documents form part of the disclosure with respect to the chemical composition of the alloys. 
     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. 2  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;these documents form part of the disclosure with respect to the solidification process. 
     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, which are intended to form part of this disclosure with respect to the chemical composition of the alloy. 
     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. 3  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 . 
       FIG. 4  shows a highly diagrammatic illustration of the suspension of a guide vane  1  on a holding ring  3 , which extends in the circumferential direction of the turbine at least around part of the circumference. The holding element  3  has holding projections  5 , on which the turbine blade or vane  1  is fastened. 
     The turbine blade or vane  1  has a main blade or vane part  9  and at least one blade or vane platform  11  arranged at the radially outer end of the main blade or vane part  9 . A corresponding blade or vane platform can also be arranged at the radially inner end of the main blade or vane part  9  (not shown in  FIG. 4 ). The blade or vane platform  11  has a radially inner platform surface  13 , which forms part of the wall of the flow path for the hot combustion gases in the gas turbine. Holding hooks  7  are foamed on the radially outer platform surface  15  located opposite the radially inner platform surface  13  and can be used to suspend the turbine blade or vane on the holding projections  5  of the holding element  3 . Corresponding holding hooks can also be present on the radially inner platform. 
     In combination with the position of the holding projections  5  of the holding element  3 , the distance d between the radially outer platform surface  15  and a hook surface  17  located opposite this platform surface determines the positioning of the radially inner platform surface  13  in the hot gas path of the gas turbine, and is therefore of great importance for the formation of a hot gas path which is optimized in terms of flow. Within the context of the production of a turbine blade or vane by casting, it is therefore of great importance to ensure a defined distance d between the radially outer platform surface and the hook surface  7 . 
     A method for producing the turbine blade or vane shown in  FIG. 4  by casting is described below with reference to  FIG. 5 . 
     In a first method step  19 , a casting mold is created for a wax model of the turbine blade or vane  9  to be produced. Since no particularly high temperatures prevail during casting of the wax model, the material for the wax model casting mold can be selected so that it is optimized in terms of processability, such that the contour of the wax model can be produced precisely as a negative mold. A casting mold once produced for the wax model can also be reused, and therefore the step of producing a casting mold for a wax model does not necessarily have to be present in each method for producing a negative model for a turbine blade or vane. Only when a turbine blade or vane with a new geometry is to be produced for the first time is it necessary to produce a new casting mold for the corresponding wax model. 
     In step  21 , the wax model of the turbine blade or vane is cast by means of the casting mold for the wax model. Once the wax model has hardened to such an extent that the casting mold can be removed, spacer elements are inserted into the region of the holding hooks  7  in step  23 . Such a spacer element  219  is shown schematically in  FIG. 6 . The figure shows a section of a wax model  201  of the turbine blade or vane  1  shown in  FIG. 4 . The wax model has a model main blade or vane part  209 , a model platform  211  with a radially inner model platform surface  213  and a radially outer model platform surface  215 . A model holding hook  207  extends from the model platform  211  and has a model hook surface  217  which faces toward the outer model platform surface  215  and is spaced apart therefrom. 
     As soon as the wax model  209  can be taken from the mold, spacer elements  219  are pushed into the intermediate space between the radially outer model platform surface  215  and the opposite model hook surface  217 . The spacer  219  is produced from a dimensionally stable material, for example metal or ceramic. Metallic materials in particular, for instance steel or aluminum, can be produced with precise dimensions, such that the thickness of the spacer can be matched very accurately to the distance d to be observed between the radially outer model platform surface  215  and the opposite model hook surface  217 . 
     As soon as the spacer  219  has been pushed into the intermediate space between the radially outer platform surface  215  and the opposite model hook surface  217 , the distance between these two surfaces is secured against a change on account of deformations in the wax model during further hardening and/or during transport and/or storage of the wax model. The spacer  219  additionally has a thickened portion  221 , which serves as an edge located opposite the end face  223  of the model hook  207 . It is thereby also possible to reliably counteract displacement of the end face  223  by deformation, in particular, of the model hook  207 . 
     Even though the spacer  219 , in the present exemplary embodiment, is inserted after the wax model has been removed from the casting mold, the spacer can also be designed such that, during the casting process, it forms part of the casting mold which can subsequently be taken from the casting mold together with the wax model or can be separated from the casting mold. Since, in this alternative configuration of the method, the spacer is already located in its position during casting, in the region of the model hook  207  the wax model is also secured against deformations which might occur as the model is being removed from the mold. 
     The spacer  229  advantageously remains in the wax model  201  of the turbine blade or vane  1  until a negative mold is created for the turbine blade or vane  1  on the basis of the wax model  201 . Once the spacer has been removed in step  25  of the method, the negative mold is formed around the wax model, for example by means of a ceramic material (step  27 ). Once the negative mold has set, the wax is melted out of the negative mold, and therefore the cavity thereby created is available for casting the turbine blade or vane in step  29 . Once the turbine blade or vane has been cast with the aid of the negative mold, the negative mold is removed and the method for producing the turbine blade or vane is complete. 
     The present invention provides a simple means with which it is possible to counteract deformation of the wax model used during a casting process for producing a turbine blade or vane. It is thereby possible to achieve improvements in the observance of a precise geometry of the wax model, as a result of which it is possible to reduce the number of erroneously cast turbine blades or vanes, which are to be reworked or, in the worst case, represent rejects.