Patent Publication Number: US-2022219226-A1

Title: Improved foundry mould for the formation of turbine blade ceramic cores

Description:
TECHNICAL FIELD 
     This summary relates to the field of turbomachines, and specifically relates to techniques for filling casting core molds for the production of cores for the purpose of producing turbomachine components. 
     PRIOR ART 
     The production of ceramic cores for the formation of turbomachine components, and more specifically for the formation of high—pressure turbine blades and high—pressure nozzle guide vanes of turbomachines poses a problem, particularly in relation to the specific geometry of such parts. 
     Ceramic cores are commonly produced by injection at medium or high pressure. A paste comprising a ceramic filler incorporated into a polymer binding agent is injected into a mold, thus making it possible to produce a raw body. After unmolding, the latter is then baked, which makes it possible to perform steps of debinding and sintering, respectively consisting in the elimination of the binding agent and the consolidation of the core. After these steps, the wax forming the model of the metallic part is injected around the core, the latter thus making it possible to delineate the inner cavity of the part. 
     The most recent core geometries comprise trailing edges including narrow and very fine teeth, for example up to 0.4 mm. On this type of geometry, cracks leading to the rupture of the cores have been observed virtually systematically, particularly after the baking of the core. They are often located in the teeth of the trailing edge, or in immediate proximity to the latter. It has been supposed that one of the phenomena contributing to these cracks/ruptures is the separation of the phases (binding agent and ceramic) when the paste is injected during the filling of the cavity under pressure. Interrupted injections have shown the formation of jetting in these teeth, which can prove detrimental to the quality of the front joints and to the homogeneity of the spatial distribution of binding agent/ceramic, once that material has traversed the teeth. It has also been assumed that this is linked either to the local geometry, which is characterized by passages in which the exchange surface with the mold in relation to the volume increases quickly, which causes a quick cooling of the paste, and therefore, potentially, the formation of fragile joints, and also by a shape that may locally promote convergent flows, the latter then increasing the risk of a poorly controlled increase in the flow speed of the paste. Moreover, local cooling, potentially quick, can be attenuated by increasing the speed of passage of the paste, but this then accentuates the segregation problem. 
     SUMMARY OF THE INVENTION 
     This summary thus aims to at least partly respond to the problems mentioned previously. 
     This summary thus relates to a mold for producing a ceramic core by injecting a ceramic composition, the mold comprising a body in which a casting core cavity is formed, said cavity comprising a primary cavity and a secondary cavity connected by a plurality of segments allowing the passing of fluid between the primary cavity and the secondary cavity, the primary cavity defining recesses of a turbomachine blade, said mold being characterized in that it comprises a primary arm configured in such a way as to allow an injection of ceramic composition into the primary cavity, and a secondary arm configured in such a way as to allow an injection of ceramic composition into the secondary cavity, and in that the ratio of the volume of the primary cavity to the volume of the secondary cavity is equal, plus or minus 15%, to the ratio of the volume of the primary arm to the volume of the secondary arm. 
     According to an example, the ratio of the contact surface of the primary arm with the body of the mold with respect to the volume of the primary arm is equal, plus or minus 20%, to the ratio of the contact surface of the secondary arm with the body of the mold with respect to the volume of the secondary arm. 
     According to an example, the primary arm and the secondary arm are connected to a common injection orifice of the mold. 
     The primary arm and the secondary arm are then typically connected to a feed cavity, the feed cavity being connected to an injection orifice of the mold. 
     According to an example, the segments comprise a plurality of segments each having a section having a dimension less than or equal to 1 mm 2 . 
     According to an example, the primary arm and the secondary arm are embodied in a joint plane of the mold. 
     According to an example, the primary arm and the secondary arm each have a portion of reduced section at the level of their connection with the primary cavity and the secondary cavity, respectively. 
     This summary also relates to a method for producing a ceramic casting insert for producing a turbine blade, wherein
     a mold is supplied as previously defined,   a ceramic composition is injected into the mold in such a way as to fill the primary cavity and the secondary cavity via the primary arm and the secondary arm, respectively.   

     According to an example, a subsequent step is performed of finishing the ceramic insert wherein the ceramic material corresponding to the secondary arm (or to the volume of the secondary arm) is removed. 
     This summary also relates to a ceramic core comprising
     a primary body and a secondary body connected by a plurality of teeth,   a tertiary body extending from a proximal end of the secondary body,   a primary branch extending from the primary volume,   a secondary arm extending from a distal end of the secondary body, opposite the proximal end,   a tertiary branch, connected to the primary branch and to the secondary branch, and   a chimney, extending in the extension of the tertiary branch, in which the ratio of the volume of the primary body to the volume of the secondary body is equal to the nearest 15% to the ratio of the volume of the primary branch to the volume of the secondary branch.   

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention and its advantages will be better understood on reading the detailed description given below of different embodiments of the invention given by way of non—limiting example. 
         FIG. 1  shows an example of a casting core produced by means of a mold and a method according to an aspect of the invention. 
         FIG. 2  illustrates a step of filling the mold according to an aspect of the invention. 
         FIG. 3  illustrates a step of filling the mold according to an aspect of the invention. 
         FIG. 4  illustrates a step of filling the mold according to an aspect of the invention. 
     
    
    
     On all the figures, common elements bear identical numeral references. 
     DESCRIPTION OF THE EMBODIMENTS 
     The figures schematically represent a view of a mold  1 , which is here represented along a section plane of the mold  1 . The mold  1  may have a rectangular parallelepipedal shape, it being understood that such an embodiment is not limiting, and that the mold can have any suitable shape, particularly as a function of the geometry of the associated core. 
     The mold  1  comprises a body  10  having an inner recess  2  defining a cavity of a casting core. The figures represent this recess  2  via the corresponding casting core. This is because it would lack clarity to represent a casting mold, which is by nature closed. The figures thus represent the casting core associated with each mold  1 , these cores being the complementary volumes to the inner recess  2  of the mold  1  under consideration. The mold  1  is an injection mold, typically a mold used to inject ceramic paste. 
     In the example represented in  FIG. 1 , the recess  2  forms an cavity  20  defining the geometry of a core. In the example illustrated in the figures, the cavity  20  comprises a primary volume  20 A and a secondary volume  20 B connected by a plurality of segments  20 C ensuring the passage of fluid between the primary volume  20 A and the secondary volume  20 B. 
     The primary volume  20 A typically defines the inner cavities for a turbine blade, while the secondary volume  20 B typically makes it possible to define a volume internal to a bar formed with the turbine blade when the material is injected, the bar being a reservoir of material aiming to avoid misruns in the manner of a flyweight. The primary volume  20 A may thus be qualified as the functional part of the core, while the secondary volume  20 B corresponds to an accessory volume, which will be removed during a machining operation to produce a finished part. 
     The primary volume  20 A typically comprises several sub—volumes which can be disjointed or connected by segments having a reduced section with respect to the sub—volumes in question. 
     The primary volume  20 A and the secondary volume  20 B have inner volumes which are substantially equal, typically plus or minus 15%. 
     The segments  20 C are commonly denoted as the “teeth” of the core, and are part of the functional volume of the core. These segments  20 C comprise a plurality of ducts connecting the primary volume  20 A and the secondary volume  20 B. These ducts each having small dimensions in relation to the primary volume  20 A and to the secondary volume  20 B. Each of the segments  20 C thus typically has a section of less than 1 mm 2 . 
     The secondary volume  20 B is surmounted by a tertiary volume  20 D in the extension of the secondary volume  20 B, the limit between the secondary volume  20 B and the tertiary volume  20 D substantially corresponding to the upper limit of the last segment  20 C (or tooth) of the core. The tertiary volume  20 D generally corresponds to the parts of the core defining the squealer and the dome of a blade produced using the core. 
     In the example represented, the recess  2  makes it possible to define the geometry of a core typically produced from ceramic for the purpose of producing a mobile wheel or a nozzle guide vane of a high—pressure turbine of a turbomachine by loss wax casting method. Note that the geometry of the recess  2  is not limiting, and that this summary can be applied to different core geometries. 
     The body  10  of the mold  1  comprises a primary arm  30 A and a secondary arm  30 B, respectively opening into the primary volume  20 A and in the secondary volume  20 B. 
     In the illustrated example, the primary arm  30 A and the secondary arm  30 B are both connected to a feed cavity  40 , which is connected to an injection orifice  50  via an injection duct  45 , in such a way as to make it possible to fill the cavity of the mold  1  with material. Note that unlike the secondary arm  30 B which fulfils only one function of the material feed cavity, the primary arm  30 A here fulfils a function for feeding the cavity of the mold  1  with material and also forms a functional part of the ceramic insert thus produced by defining a cavity in the cooling system of a blade formed using the ceramic insert. Thus, the secondary arm  30 B which is not a functional part of the core is removed after producing the ceramic insert and before positioning the ceramic insert in a wax injection mold, while the primary arm  30 A is preserved. 
     The nozzle guide vane  40  is an intermediate volume, connecting the primary arm  30 A and the secondary arm  30 B to the injection duct  45 . The feed cavity  40  is configured in such a way as to distribute a suspension or a fluid which is injected via the injection duct between the primary arm  30 A and the secondary arm  30 B. 
     The injection duct  45  is connected to the injection orifice  50 , the latter being suitable for being connected to a feed source of a suspension or a pressurized fluid. 
     The primary arm  30 A is used to fill the primary volume  20 A with material, for example a ceramic suspension. 
     The secondary arm  30 B is used to fill the secondary volume  20 B with material, for example a ceramic suspension, avoiding the latter from filling the cavity exclusively by traversing the teeth, and reducing the cooling of the injected material. 
     The primary arm  30 A and the secondary arm  30 B typically extend in a joint plane of the mold  1 . The injection duct  45  and the feed cavity  40  also typically extend in a joint plane of the mold  1 , which makes it possible to facilitate the opening of the mold  1  after the injection of material. 
     In order to ensure the proper filling of the cavity  20 , the secondary arm  30 B is advantageously dimensioned in such a way as to ensure the distribution of material between the primary volume  20 A and the secondary volume  20 B of the cavity  20 . Specifically, since the primary arm  30 A is a functional part of the core, it cannot be freely dimensioned. 
     Thus, considering that the primary volume  20 A has a volume V 1 , the secondary volume has a volume V 2 , the primary arm  30 A has a volume V 3  and the secondary arm  30 B has a volume V 4 , these volumes are typically dimensioned such that the ratio V 1 /V 2  is equal to the ratio V 3 /V 4 , typically to the nearest 20%, or for example to the nearest 15%, or for example substantially to the nearest 15%, or for example to the nearest 10%, or for example to the nearest 5%. 
     In the case where the volumes V 1  and V 2  are equal or substantially equal, then the volumes V 3  and V 4  are typically equal or substantially equal. 
     Advantageously the secondary arm  30 B is also dimensioned in such a way that the primary arm  30 A and the secondary arm  30 B have equal or substantially equal ratios between their contact surfaces with the body  10  of the mold  1  and their respective volumes. 
     More precisely, for the primary arm  30 A and for the secondary arm  30 B a contact surface, respectively S 3  and S 4 , is defined corresponding to the peripheral surface of each of the feed cavities. This contact surface corresponds to the surface of the body  10  of the mold which will be in contact with the material injected into the primary arm  30 A or secondary arm  30 B. 
     Thus, the secondary arm  30 B is typically dimensioned such that the ratios S 3 /V 3  and S 4 /V 4  are equal or substantially equal, or typically equal to the nearest 20%. 
     Specifically, this ratio determines in particular the heat exchanges occurring between the material injected into the mold  1  and the body  10  of the mold  1 . Thus, preserving a similar or identical ratio S/V allows the injected material to reach the primary volume  20 A and the secondary volume  20 B of the cavity  20  in substantially identical states, which improves material joints within the cavity  20 . 
     Should the primary arm  30 A and the secondary arm  30 B be revolution cylinders, the ratio S/V then depends solely on the radius of the revolution cylinder section, which makes it possible to simplify the dimensioning of the primary arm  30 A and of the secondary arm  30 B. 
     The primary arm  30 A typically has a portion of reduced section at the level of its joint with the primary volume  20 A. Similarly, the secondary arm  30 B typically has a portion of reduced section at the level of its joint with the secondary volume  20 B 
     Thus forming a shrinkage of the secondary at its end makes it possible to concentrate the thermomechanical stresses in the latter, which can facilitate the separation of the secondary with respect to the injected part, and therefore the finishing of the core, while reducing the risks of damage to the functional part of the core. However, this shrinkage is typically not very pronounced in order to avoid the segregation and/or formation of jetting at the level of the joint between the secondary volume  20 B and the secondary arm  30 B. 
     Once the secondary arm  30 B has been dimensioned, a test is typically performed of the mold thus dimensioned by digital simulation, particularly in order to check the correct balance of the volumes of the different areas during the filling of the mold before producing the mold. 
       FIGS. 2, 3 and 4  illustrate different steps of the filling of such a mold for producing a ceramic casting insert with the aim of producing a hollow turbine blade. 
       FIG. 2  represents the entry of the material into the primary volume  20 A and into the secondary volume  20 B of the cavity  20 , which is thus filled along two separate fronts. 
       FIGS. 3 and 4  illustrate the joint between the two material fronts M, as the cavity  20  of the mold  1  is filled. 
     In these figures it can be seen that the different regions of lesser thickness, particularly the segments  20 C and where applicable portions of lesser thickness within the primary volume  20 A, are filled with material directly after the material penetrates the primary volume  20 A or the secondary volume  20 B. Such a configuration greatly reduces or cancels the risk of misruns and segregation and/or formation of jetting, particularly by avoiding material coming from the same arm having to pass through two restrictions of the mold  1 . 
     In addition, it can be seen that the joint between the two material fronts is done quickly after the entry of the material into the primary volume  20 A and into the secondary volume  20 B, typically at the level of the segments  20 C. 
     The joint is thus made while the material has a relatively high temperature which ensures good mixability and avoids or limits the formation of discontinuities or other defects due to rapid variations in viscosity within the material. 
     The core thus produced then comprises a primary body, a secondary body and a tertiary body respectively corresponding to the primary volume  20 A, to the secondary volume  20 B and to the tertiary volume  20 D, the primary body and the secondary body being connected by teeth corresponding to the volume of the segments  20 C. The primary arm and the secondary arm respectively define a primary branch and a secondary branch extending from the primary body and the secondary body respectively, the secondary branch extending from a distal end of secondary body, opposite a proximal end of the secondary body from which the tertiary body extends. The primary branch and the secondary branch meet at the level of a tertiary branch corresponding to the volume defined by the feed cavity  40 , and extending by a chimney corresponding to the volume of the injection duct  45 . 
     Once the mold is filled and the ceramic insert formed, the part of the insert corresponding to the volume of the secondary arm  30 B is removed, for example by machining. 
     The mold structure as proposed has several advantages over a conventional mold for the production of such a core which comprises only a single feed cavity. 
     As previously indicated, the proposed mold makes it possible to limit the risks of misruns as well as the formation of segregated areas and jetting in the part, and also the risks of incorrect joining of material. By limiting the formation of segregated areas, the proposed mold makes it possible to limit differential removal causing the deformation and rupture of the core. Moreover, the proposed mold makes it possible to limit residual stresses as well as the pressure observed within the injected material, which thus makes it possible to obtain a casting core with better properties, and which is therefore less liable to break during use, for the purpose of forming a turbine blade of a turbomachine. The reduction of the pressure during the injection in particular makes it possible to reduce wear of the mold  1 . In the same way, the reduction of the shear rates makes it possible to reduce the risk of phase separation. 
     The use of such a structure of mold  1  has never been proposed for the production of ceramic cores for the purpose of producing turbine blades of a turbomachine. 
     Although this invention has been described with reference to specific exemplary embodiments, it is obvious that modifications and changes can be made to these examples without departing from the general scope of the invention as defined by the claims. In particular, individual features of the different illustrated/mentioned embodiments may be combined in additional embodiments. Consequently, the description and drawings must be considered in an illustrative sense rather than a restrictive one. 
     It is also obvious that all the features described with reference to a method are transposable, alone or in combination, to a device, and conversely, all the features described with reference to a device are transposable, alone or in combination, to a method.