Abstract:
Thick-walled parts made via a casting method often exhibit, in those thick zones, the worst mechanical properties since the solidification speed in the zones is reduced relative to the thin-walled zone and frequently induces the worst mechanical properties. There is described a method incorporating solidification control elements in a melting charge, the elements increase locally the solidification speed of the melting charge.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2005/055766, filed Nov. 4, 2005 and claims the benefit thereof. The International Application claims the benefits of European application No. 04027556.2 EP filed Nov. 19, 2004, both of the applications are incorporated by reference herein in their entirety. 
     FIELD OF INVENTION 
     The invention relates to a casting process. 
     BACKGROUND OF INVENTION 
     Nowadays, complex casting processes can be successfully managed using modern modeling and simulation tools for casting solidification. This allows better and targeted setting of microstructures and properties. For critical component regions, better mechanical properties can be set with a higher reproducibility in the casting process. For thick-walled regions of cast components, for example in flange regions of housings for gas turbines or steam turbines, it is difficult in casting processes to set the homogenous globular microstructure, which may be required by way of example, during the graphite formation. This is because of the poor dissipation of heat and solidification energy. The result is a drop in the mechanical characteristic values as the wall thickness of these highly stressed component regions increases. 
     U.S. Pat. No. 5,314,000 discloses a process for controlling the grain size during a casting process. 
     SUMMARY OF INVENTION 
     Therefore, it is an object of the invention to overcome the abovementioned problem. 
     This object is achieved by the casting process as claimed in the independent claims. 
     The subclaims list further advantageous measures which can be combined with one another in any desired, advantageous way. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawing: 
         FIG. 1  shows a casting mold together with melt and solidification control  Elements, 
         FIG. 2  shows the operating principle of the process according to the invention, 
         FIG. 3  shows a component which is produced using the process according to the invention, 
         FIG. 4  shows a turbine blade or vane, 
         FIG. 5  shows a combustion chamber, 
         FIG. 6  shows a gas turbine, 
         FIG. 7  shows a steam turbine. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
       FIG. 1  illustrates an apparatus  1  comprising a casting mold  10  with a melt  4  and at least one, and in this case for example two, solidification control elements  7 . The melt  4  is introduced into the casting mold  10 . At least one or a plurality of, in this case for example two, solidification control elements  7  are introduced into the casting mold  10  either before, during or after the introduction of the melt  4 . The solidification control elements  7  consist in particular of an identical material to the melt  4 . It is also possible for the material of the solidification control elements  7  to be of a similar type to the material of the melt  4 , i.e. the solidification control element  7  includes all the elements of the melt  4  but with deviations in respect of the individual elements, in particular to an extent of ±20% and in particular ±10% for the individual elements (at least of similar type means of similar type or identical). It is preferable for the solidification control element  7  to contain the chemical alloying elements of the melt  4 . In the abovementioned examples, it is also possible for elements of the melt  4  with low contents by weight (&lt;5 wt %, in particular &lt;1 wt %) not to be present in the material of the solidification control elements  7 . The solidification control element  7  preferably consists of the chemical alloying elements of the melt  4 . The melting temperature of the solidification control elements  7  may therefore be less than, equal to or greater than the melting temperature of the material of the melt  4 . The solidification control elements  7  may therefore be metallic, ceramic or made from glass. 
     The temperature of the solidification control elements  7  can be preset before they come into contact with the melt  4 . This can be achieved by heating or cooling as required. It is also possible for the solidification control elements  7  to be actively cooled, by a coolant being passed for example through the solidification control elements  7  or being brought into contact with at least one solidification control element  7  at one end, so as to impose forced cooling. The solidification control elements  7  are not yet melted at the outset. In particular, the solidification control elements  7  may but need not be at least partially or completely melted after they have come into contact with the melt  4 , during the liquid phase of the melt  4  (i.e. the phase in which the melt is present) or during the solidification of the melt  4 . It is preferable for the solidification control elements  7  to be at most partially melted, i.e. part of the solidification control elements  7  does not melt. 
     The solidification control elements  7  are not made from the same material as the casting mold  10 , but rather are used for the additional dissipation of heat from the melt. The solidification control elements  7  are therefore also not casting cores. After solidification, their material forms an integral part of the cast component  13 . The solidification control elements  7  are in particular a solid crystalline body and are not, as in the case of a casting mold used in a casting process, composed of individual grains (sand mold) which are joined together for example by a binder. The solidification control element  7  is for example a sintered body comprising a large number of grains. 
     The casting process according to the invention therefore does not constitute an injection-molding process in which a molten or soft material is injection-molded around another material. 
     The solidification control elements  7  may be of identical or different sizes. 
     The solidification control elements  7  are of elongate shape and are in particular symmetrical, in particular cylindrical, in form. 
     A component  13  which is produced by the casting process may for example represent a component of a steam turbine  300 ,  303  or a gas turbine  100  for an aircraft or for power generation, in which case it then in particular represents a housing component. 
     In this case, high-grade steels or nickel-, cobalt-, or iron-base superalloys are used. 
       FIGS. 2   a, b  diagrammatically depict the way in which the casting process according to the invention works. 
       FIG. 2   a  illustrates a for example cuboidal wall element of a component in a casting process according to the prior art. The dissipation of thermal energy over time dQ/dt is denoted here by {dot over (Q)}. In particular in the case of thick-walled components with a considerable width b, it takes a very long time before the melt  4  has cooled, i.e. {dot over (Q)}=0. 
       FIG. 2   b  illustrates the corresponding solidification control element  7  in a casting process according to the invention, in which for example a solidification control element  7  is present in the melt  4 . As a result of the solidification control element  7  being at a lower temperature than the melting temperature, the solidification control element  7  absorbs heat, or if the solidification control element  7  even melts, it also withdraws melting energy from the melt  4 . This increases the cooling rate of the melt, i.e. {dot over (Q)} is significantly higher. This prevents slower solidification, which often leads to graphite degeneration or to porosity and voids, from occurring in relatively thick regions and thick components. The introduction of solidification control elements  7  into the melt  4  for example results in a homogenous modular graphite formation, in particular in the case of gray cast iron parts. The width b, i.e. the extent of the melt  4 , is in effect divided into two smaller widths b 1 , b 2  (b 1 +b 1 =b) and the desired cooling properties of thin-walled (b 1 , b 2 ) walls manifest themselves within the widths b 1 , b 2 , which are thin. 
       FIG. 3  shows a cast component  13  according to the invention. 
     The component  13  has been formed from a melt  4  and includes the solidification control elements  7 , which are surrounded by the solidified melt  4 . The solidification control elements  7  have in this case been introduced for example in a thick-walled region  16  of the component  13 . Such thick-walled regions  16 , constitute for example the flanges of a housing part. In this context, the term thick is to be understood as meaning a wall thickness of at least 200 mm. It is preferable for the solidification control elements  7  to be introduced at a location where holes  19  are subsequently introduced into the flange  16 , i.e. where material is removed. This reduces the risk of defects being introduced into the component as a result of bonding defects or inadequate melting of the solidification control elements  7 , since these regions are in any case removed during the subsequent machining of the component. The solidification control elements  7  do not form part of the casting mold  10  and are for example metallic but may also be ceramic or vitreous. 
       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 . 
     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 . 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 has, for example, thick-walled regions  16  and 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; 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. 
     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. 
     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. 
     It is also possible for a thermal barrier coating, consisting for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. unstabilized, partially stabilized or completely 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). 
     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. 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 around the axis of rotation  102  in the circumferential direction open out into a common combustion chamber space. 
     For this purpose, the combustion chamber  110  overall is configured as an annular structure 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 operating time to be achieved even under these operating parameters, which are unfavorable for the materials, the combustion chamber wall  153  is provided, on its side facing the working medium M, with an internal lining formed from heat shield elements  155 . 
     On the working medium side, each heat shield element  155  is provided with a particularly heat-resistant protective layer or is made from material that is able to withstand high temperatures. This may mean 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. 
     Moreover, a cooling system may be provided for the heat shield elements  155  and/or for their holding elements, on account of the high temperatures in the interior of the combustion chamber  110 . 
     The heat shield elements may also have thick-walled regions  16  and can therefore be produced by the process according to the invention. 
       FIG. 6  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  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  having for example thick-walled regions  16  follow one another along the rotor  103 . 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 . 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  (having for example thick-walled regions  16 ) 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  (having for example thick-walled regions  16 ) 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 bricks which line the annular combustion chamber  106 , are subject to the highest thermal stresses. To be able to withstand the temperatures which prevail there, they can 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-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. 
     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. 
     A thermal barrier coating, consisting for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. unstabilized, partially stabilized or completely 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). 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. 7  illustrates, by way of example, a steam turbine  300 ,  303  with a turbine shaft  309  extending along an axis of rotation  306 . The steam turbine has a high-pressure part-turbine  300  and an intermediate-pressure part-turbine  303 , each with an inner casing  21  (having for example thick-walled regions  16 ) and an outer casing  315  (having for example thick-walled regions  16 ) surrounding it. The high-pressure part-turbine  300  is, for example, of pot-type design. The intermediate-pressure part-turbine  303  is of two-flow design. 
     It is also possible for the intermediate-pressure part-turbine  303  to be of single-flow design. Along the axis of rotation  306 , a bearing  318  is arranged between the high-pressure part-turbine  300  and the intermediate-pressure part-turbine  303 , the turbine shaft  309  having a bearing region  321  in the bearing  318 . The turbine shaft  309  is mounted on a further bearing  324  next to the high-pressure part-turbine  300 . In the region of this bearing  324 , the high-pressure part-turbine  300  has a shaft seal  345 . The turbine shaft  309  is sealed with respect to the outer casing  315  having for example thick-walled regions  16  of the intermediate-pressure part-turbine  303  by two further shaft seals  345 . Between a high-pressure steam inflow region  348  and a steam outlet region  351 , the turbine shaft  309  in the high-pressure part-turbine  300  has the high-pressure rotor blading  354 ,  357 . This high-pressure rotor blading  354 ,  357 , together with the associated rotor blades (not shown in more detail), constitutes a first blading region  360 . The intermediate-pressure part-turbine  303  has a central steam inflow region  333 . Assigned to the steam inflow region  333  the turbine shaft  309  has a radially symmetrical shaft shield  363 , a cover plate, on the one hand for dividing the flow of steam between the two flows of the intermediate-pressure part-turbine  303  and also for preventing direct contact between the hot steam and the turbine shaft  309 . In the intermediate-pressure part-turbine  303 , the turbine shaft  309  has a second blading region  366  comprising the intermediate-pressure rotor blades  354 ,  342 . The hot steam flowing through the second blading region  366  flows out of the intermediate-pressure part-turbine  303  from an outflow connection piece  369  to a low-pressure part-turbine (not shown) which is connected downstream in terms of flow.