Abstract:
The invention relates to a method for the production of a pattern for precision casting representation of turbine component comprising at least one cavity, whereby the prepared pattern comprises at least one core and an outer contour pattern, at least partly enclosing the core and at least partly defining the outer contour of the turbine component. The core is made from a hardening core material, which hardens during the course of the method and the outer contour pattern is made from a combustible or fusible material. The outer contour model is first produced with at least one cavity corresponding to the at least one cavity of the turbine component and subsequently, in order to form the at least one care, the hardening core material is introduced into the at least one cavity and hardened.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2005/055769, filed Nov. 4, 2005 and claims the benefit thereof. The International Application claims the benefits of European application No. 04027895.4 filed Nov. 24, 2004, both of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates to a process for producing a model for the production by investment casting of a component that has a cavity, and to a process for producing a casting mold for a component that has at least one cavity, in which a model is used for producing the component by investment casting. 
       BACKGROUND OF THE INVENTION 
       [0003]    When producing hollow cast components, such as for example turbine components, the production of the cavity is of particular importance. By way of example, turbine blades or vanes for gas turbines have a main blade or vane part, which has a leading edge and a trailing edge. Leading and trailing edges are connected to one another via a suction-side wall and a pressure-side wall. At least one cavity, which extends through a large part of the main blade or vane part and is used to supply a cooling fluid, for example air or steam, by which the main blade or vane part is cooled when the turbine is operating, is arranged between the suction-side wall and the pressure-side wall. The cooling action is dependent on the configuration of the cavity and its precise positioning within the main blade or vane part. Even relatively minor deviations in the positioning of the cavity can lead to a considerable variation in the cooling action. 
         [0004]    The shape and position of the cavity within the main blade or vane part therefore represents a particular challenge in the design of turbine blades and vanes. It is by no means unusual for a whole range of design changes to be required in order to optimize the position and shape of the cavity in relation to the external contour of the turbine blade or vane before the final design is fixed. 
         [0005]    Turbine blades and vanes of different designs are produced and tested as part of the development process before the final design is fixed. 
         [0006]    The production of for example hollow cast turbine blades or vanes for gas turbines is realized by means of a ceramic investment casting technique, in which a core is injection-molded or cast from a ceramic material in order to define the cavity. Then, this core is placed into a mold for the injection molding or casting of a wax model and the wax is injected or poured into the mold. After the wax has cooled, the finished wax model, together with the ceramic core, forms a model for producing the turbine blade or vane by investment casting, and as the process then continues this model is used to produce a ceramic mold for the casting of the turbine blade or vane. To produce the ceramic mold, a ceramic sleeve is fitted around the wax model. After the ceramic sleeve has been hardened, the wax of the wax model is melted out, so that what remains is a mold used to cast the hollow turbine blade or vane. This mold comprises firstly the ceramic sleeve and secondly the ceramic core. A process of this type is disclosed for example in U.S. Pat. No. 5,465,780. 
         [0007]    Since, despite computer-aided technology used to simulate flow and cooling properties, the core design none the less has to be corrected by means of tests in the final stage of product development, the process described is relatively complex in product development, since new casting or injection molds for the core and wax model have to be produced for each design. 
         [0008]    Therefore, in DE 101 29 975 A1 it has been proposed for the casting or injection molds for the casting of the core to be equipped with exchangeable inserts, in order in this way to allow the core design to be changed without a completely new casting or injection mold having to be produced for the core. However, even this procedure only allows local corrections but does not permit an overall correction to the core design. Moreover, in the process described in DE 101 29 975 A1, corrections to the design of the external contour of the turbine blade or vane are not possible without producing new tools, such as for example new casting molds. 
         [0009]    The production of tools for the manufacture of the ceramic cores and of the wax models continues to be complex and expensive. For example, during the production process development for hollow cast turbine blades or vanes, a large proportion of the development time and development costs are attributable to the production of the tools. Moreover, the tools can only be released for series production once the design of the hollow cast turbine component has been released. Otherwise, changes to the design can lead to a considerable time delay and high costs. 
       SUMMARY OF INVENTION 
       [0010]    Therefore, it is an object of the present invention to provide an improved process for producing a model for the production by investment casting of a turbine component that has at least one cavity, and also an improved process for producing a casting mold for a turbine component that has at least one cavity. 
         [0011]    This object is achieved by the process for producing a model for the production by investment casting of a component that has at least one cavity as claimed in the claims and by the process for producing a casting mold or an injection mold for a component that has at least one cavity as claimed in the claims. 
         [0012]    In the process according to the invention for producing a model for the production by investment casting of a component that has at least one cavity, the finished model comprises at least one core and an external contour model which at least partially surrounds the core and at least partially defines the external contour of the component. The core is produced from a hardenable core material that is hardened during the process. The external contour model is produced from a material that can be burnt or melted out. In the process according to the invention, first of all the external contour model is produced with at least one cavity corresponding to the at least one cavity of the component. Then, to produce the at least one core the hardenable material is introduced into the at least one cavity and hardened. 
         [0013]    Therefore, in the process according to the invention, the external contour model simultaneously serves as a casting or injection mold for the core, and consequently there is no need for a separate injection or casting mold to be present for the core. It is in this way possible to dispense with the expensive production of a dedicated casting or injection mold for the core for process development all the way to prototype production. 
         [0014]    The external contour model used may be a resin model. A rapid prototype process, in particular a stereolithography process, can be used to produce the resin model. A stereolithography process uses a resin that hardens when irradiated with a laser. To produce the external contour model, the previously free-flowing resin is hardened layer by layer by means of the laser, until the external contour model is complete in its desired contour. The laser hardening can in this case in particular be computer-controlled, so that designs which have already been computer-simulated can be converted relatively quickly into a model used to produce the component by investment casting. 
         [0015]    When the hardenable material is being introduced and/or hardened, the external contour model may be surrounded by a stabilizing casing, known as a setter. The stabilizing casing can also be produced by means of a rapid prototype process, for example by being produced from resin by means of a stereolithography process. It is particularly advantageous if the casing contains a mechanically stabilizing material, for example a metal powder. Finally, it is also possible for the casing to be produced entirely from metal powder, in which case suitable rapid prototype processes, for example rapid laser sintering, can be used to consolidate the metal powder. 
         [0016]    The stabilizing casing holds the external contour model in shape when the hardenable material is being introduced, with the result that the pressure on the external contour model that occurs during this filling operation does not cause any design deviations in the core. 
         [0017]    A particularly suitable core material is a material that is free-flowing prior to hardening and is poured or injected into the at least one cavity of the external contour model. On account of their high thermal stability, ceramic-based materials are particularly suitable for use as the core material. 
         [0018]    The process according to the invention for producing a model for the production by investment casting of a component that has at least one cavity offers greater variety and reduced cost compared to the prior art processes, in which production tools, in particular casting or injection molds, have to be produced for process development and for qualification of turbine components. 
         [0019]    In particular if rapid prototype processes, such as stereolithography processes or rapid laser sintering processes, are used to produce the external contour model and/or the stabilizing casing, the process according to the invention also results more quickly in a model for the investment casting production of the component compared to the prior art processes. 
         [0020]    With the process according to the invention, the expensive actual production tools can be dispensed with for process development all the way to prototype production, which allows the process development to start at a significantly earlier time, considerably shortens development time and greatly reduces the risk of corrections to production tools caused by design changes. The overall tooling costs can be correspondingly reduced. 
         [0021]    Finally, the process according to the invention allows significantly earlier introduction of new designs to the market and also makes it possible to react more quickly to service-related design changes. 
         [0022]    The process according to the invention for producing a model for the production by investment casting of the component is used in a process according to the invention for producing a casting mold or an injection mold for a component that has at least one cavity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0023]    Further features, properties and advantages of the present invention will emerge from the following description of an exemplary embodiment with reference to the accompanying figures, in which: 
           [0024]      FIG. 1  shows a diagrammatic illustration of an external contour model for use in the process according to the invention, in the form of a sectional perspective view. 
           [0025]      FIG. 2  shows the external contour model from  FIG. 1  while a hardenable material is being introduced into openings in the external contour model. 
           [0026]      FIG. 3  shows an example of a partial longitudinal section through a gas turbine. 
           [0027]      FIG. 4  shows a perspective view of a rotor blade or guide vane of a turbomachine. 
           [0028]      FIG. 5  shows a combustion chamber of a gas turbine. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0029]      FIG. 1  shows a somewhat simplified illustration of a model  1  of a turbine blade or vane as an example of a component with a cavity, in the form of a perspective, sectional view. The model  1  has an outer surface  3 , which reproduces the external contour of a turbine blade or vane. The external contour is divided into a pressure-side contour  4  and a suction-side contour  5 . The edges at which the pressure-side contour  4  and the suction-side contour  5  merge into one another form the leading edge (edge  6 ) and the trailing edge (edge  7 ), respectively, in the subsequent turbine blade or vane. 
         [0030]    The model  1  is not solid, but rather has cavities, in the present exemplary embodiment five cavities  8   a  to  8   e , which represent the subsequent cooling air passages in the turbine blade or vane. The inner surfaces  9   a  to  9   e , which delimit the cavities  8   a  to  8   e , of the model  1  correspondingly represent the internal contour of the subsequent turbine blade or vane. In the region of the edge  7 , the fifth cavity  8 e has an opening  10  which extends parallel to the edge and represents an outlet opening for the emergence of cooling fluid in the subsequent turbine blade or vane. 
         [0031]    As explained above, the model  1  already represents both the external contour and the internal contour of the subsequent turbine blade or vane. The model is made from synthetic resin which melts or burns under the action of heat and is used during production of a model for the investment casting production of the turbine blade or vane that is to be produced. 
         [0032]    The synthetic resin model described above, in the model for the investment casting production of the turbine blade or vane, merely constitutes a model for the external contour of the turbine blade or vane, and is therefore referred to below as the external contour model  1 . The internal contour of the cavities of the turbine blade or vane, by contrast, is formed using what is known as a core, the outer surfaces of which represent the internal contour of the cavities of the turbine blade or vane. The external contour model  1  and the cores which are yet to be described together form the model for the investment casting production of the turbine blade or vane. 
         [0033]    In the exemplary embodiment, the external contour model  1  is produced by means of a stereolithography process, in which a photoreactive liquid resin located in a container is locally irradiated with laser radiation of a suitable wavelength. This irradiation leads to hardening of the resin at the irradiated location. By suitable guidance of the laser beam, it is possible to control the hardening of the resin in such a way that structures of any desired shape can be realized from hardened synthetic resin. Stereolithography processes are known from the prior art and therefore require no further explanation at this point. 
         [0034]    The stereolithography process is used to produce the external contour model  1  from a liquid synthetic resin by controlled local hardening. The laser is controlled by means of a computer, with the result that the external contour model  1  can be produced on the basis purely of a computer model. 
         [0035]    After the external contour model  1  has been produced, the cores which define the cavities in the subsequent turbine blade or vane are produced. For this purpose, a free-flowing ceramic material, known as the core compound  11 , is introduced into the cavities  8   a  to  8   e  of the external contour model  1 . This introduction can be realized for example, as illustrated in  FIG. 2 , by pouring. However, other introduction processes are also possible. By way of example, the core compound  11  can also be spread or injected into the cavities. 
         [0036]    To prevent deformation of the external contour model  1  while the core compound  11  is being introduced, on account of the resultant pressure, the external contour model  1  is surrounded by a stabilizing casing  12 ,  13  prior to the introduction of the ceramic core compound  11 . This stabilizing casing  12 ,  13  is of two-part design. One part  12  of the stabilizing casing has a surface that is the inverse of the pressure-side contour  4  of the external contour model  1 , while the other part  13  of the external contour model  1  has a surface that is the inverse of the suction-side contour  5  of the external contour model  1 . The surfaces of the stabilizing casing that are the inverse of the external contour  4 ,  5  are surrounded by abutment surfaces, at which the two parts  12 ,  13  abut one another when they are surrounding the external contour model  1  in a stabilizing manner. Accordingly, the abutment surfaces are then located in the region  15 ,  16  of the edges  6 ,  7  of the external contour model  1 . 
         [0037]    In the region  16  of the edge  7  of the external contour model  1  there is also, for example, a widening  17  of the abutment surfaces, so that they do not directly abut one another in the immediate vicinity of the edge  7 . The widened region  17 , together with the cavity  8   e , forms the mold for the core that subsequently defines the internal contour of the corresponding cavity of the turbine blade or vane. 
         [0038]    In the present exemplary embodiment, the stabilizing casing  12 ,  13 , like the external contour model  1 , is produced by means of a stereolithography process. It is advantageous if the resin or synthetic resin compound which is hardened in the stereolithography process contains a stabilizing component, for example a metal powder. A stabilizing casing  12 ,  13 , once produced, can be reused, provided that there are no design changes made to the external contour of the turbine blade or vane. In a modification to the variant embodiment described, the stabilizing casing may also consist entirely of metal. In this case, it can be produced for example by means of rapid laser sintering from metal powder. 
         [0039]    After all the cavities  8   a  to  8   e  of the external contour model  1  have been filled with the ceramic core compound  11 , the compound is hardened. After the compound has hardened, the stabilizing casing  12 ,  13  is removed, so that what remains is the external contour model  1  with ceramic cores located in its cavities. The external contour model  1 , together with the ceramic cores, then forms a model for the investment casting production of the turbine blade or vane. 
         [0040]    The model produced in this way for the investment casting production of the turbine blade or vane can then be used to produce a casting mold for the turbine blade or vane. For this purpose, the model is surrounded with a ceramic compound which is then hardened. In the process, the ceramic compound is joined at selected locations to the ceramic cores located in the external contour model  1 . After the ceramic compound surrounding the external contour model  1  has been completely hardened, the resin forming the external contour model  1  is melted or burnt out. What remains is a casting mold for casting the turbine blade or vane. 
         [0041]    On account of the destruction of the external contour model  1  when it is burnt or melted out, the external contour model  1  is also known as a lost model. In the casting mold, the external contours of the ceramic cores define the internal contours of the subsequent turbine blade or vane, and the internal contour of the ceramic mold defines the subsequent external contour of the turbine blade or vane. 
         [0042]    On account of the direct conversion of a computer model into an external contour model  1  which is simultaneously used as a mold for the ceramic cores, it is possible to dispense with the complex and expensive production of tools, such as for example casting molds for the manufacture of the ceramic cores and of the wax models. The result is that a computer model can be converted much more quickly into a model that is suitable for the investment casting production of the turbine blade or vane. It is in this way possible to reduce outlay on production of a casting mold for a turbine blade or vane and the associated time. 
         [0043]    To provide a better understanding of the invention, there now follows a description of a typical gas turbine, a typical turbine blade or vane and a typical combustion chamber, with reference to  FIGS. 3 to 5 . 
         [0044]      FIG. 3  shows an example of a partial longitudinal section through a gas turbine  100 . In the interior, the gas turbine  100  has a rotor  103  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  106 , with a plurality of coaxially arranged burners  107 , a turbine  108  and the exhaust-gas housing  109  follow one another along the rotor  103 . 
         [0045]    The annular combustion chamber  106  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 . 
         [0046]    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 . 
         [0047]    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 . 
         [0048]    A generator (not shown) is coupled to the rotor  103 . 
         [0049]    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. 
         [0050]    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 bricks which line the annular combustion chamber  106 , are subject to the highest thermal stresses. 
         [0051]    To be able to withstand the temperatures which prevail there, they may be cooled by means of a coolant. 
         [0052]    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). 
         [0053]    By way of example, iron-base, nickel-base or cobalt-base 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. 
         [0054]    The blades or vanes  120 ,  130  may also have coatings which protect 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 represents yttrium (Y) and/or silicon 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, which are intended to form part of the present disclosure. 
         [0055]    A thermal barrier coating, consisting for example of ZrO 2 , Y 2 O 4 ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide, may also be present on the MCrAlX. Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
         [0056]    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 . 
         [0057]      FIG. 4  shows a perspective view of a rotor blade  120  or guide vane  130  of a turbomachine, which extends along a longitudinal axis  121 . 
         [0058]    The turbomachine may be a gas turbine of an aircraft or of a power plant for generating electricity, a steam turbine or a compressor. 
         [0059]    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 . 
         [0060]    As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
         [0061]    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 . 
         [0062]    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. 
         [0063]    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 . 
         [0064]    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; these documents form part of the disclosure. The blade or vane  120 ,  130  may in this case be produced by a casting process, also by means of directional solidification, by a forging process, by a milling process or combinations thereof. 
         [0065]    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. 
         [0066]    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. 
         [0067]    The blades or vanes  120 ,  130  may likewise have coatings protecting against corrosion or oxidation (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 represents 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 the present disclosure. 
         [0068]    It is also possible for a thermal barrier coating, consisting for example of ZrO 2 , Y 2 O 4 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. Columnar grains are produced in the thermal barrier coating by means of suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
         [0069]    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. 
         [0070]    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). 
         [0071]      FIG. 5  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  arranged circumferentially around the axis of rotation  102  open out into a common combustion chamber space. For this purpose, the combustion chamber  110  overall is of annular configuration positioned around the axis of rotation  102 . 
         [0072]    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 . 
         [0073]    On the working medium side, each heat shield element  155  is equipped with a particularly heat-resistant protective layer or is made from material that is able to withstand high temperatures. These may be solid ceramic bricks or alloys with MCrAlX and/or ceramic coatings. The materials of the combustion chamber wall and their coatings may be similar to the turbine blades or vanes. 
         [0074]    A cooling system may also 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 . 
         [0075]    The combustion chamber  110  is designed in particular to detect losses of the heat shield elements  155 . For this purpose, a number of temperature sensors  158  are positioned between the combustion chamber wall  153  and the heat shield elements  155 .