Abstract:
A method for manufacturing a shell mold for the production by lost-wax casting of bladed elements ( 1 ) of an aircraft turbine engine, including the following steps: creating an assembly ( 200 ) including a wax pattern ( 100 ) as well as a device for forming a cup for pouring metal ( 32   b ) and having an end surface ( 40   a ); depositing a hot wax coating layer on at least one portion of the end surface ( 40   a ); forming the shell mold around the assembly ( 200 ). In addition, the method includes, between steps b) and c), the implementation of a step of structuring the coating layer intended for reinforcing the adhesion between the layer ( 46 ) and the shell mold, and including the production of recesses ( 62 ) and projections ( 60 ) on the still-malleable coating layer.

Description:
FIELD OF THE INVENTION 
     The invention relates to the field of clustered manufacturing of bladed elements of an aircraft turbine engine, by the lost-wax casting technique. Each bladed element may be a sector comprising a plurality of blades, such as a low-pressure dispenser sector, or be an individual blade, such as a mobile compressor or turbine impeller blade. 
     The invention relates more particularly to the manufacture of the shell mold in cluster form, wherein the metal is intended to be cast to obtain the bladed elements of the turbine engine. 
     The invention relates to all types of aircraft turbine engines, in particular turbojets and turboprops. 
     STATE OF THE RELATED ART 
     From the prior art, the use of the lost-wax casting technique is known for simultaneously manufacturing a plurality of aircraft turbine engine bladed elements, such as mobile blades. Such a technique is for example described in the document FR 2 985 924. 
     As a reminder, lost-wax precision casting consists of creating in wax, by injecting into tools, a pattern of each of the bladed elements sought. Assembling these patterns on casting arms also made of wax, in turn connected to a metal dispenser made of wax, makes it possible to create a cluster which is subsequently dipped in various substances in order to a form a ceramic shell mold of substantially uniform thickness around same. 
     The method is continued by melting the wax, which then leaves the exact imprint thereof in the ceramic, wherein the molten metal is poured, via a casting cup assembled on the metal dispenser. After cooling the metal, the shell mold is destroyed and the metal parts are separated and finished. 
     This technique offers the advantage of dimensional precision, making it possible to reduce or even do away with some machining operations. Furthermore, it offers a very good surface finish. 
     In practice, the shell mold is created not only around the wax pattern, but also around the casting cup assembled with this pattern. The pattern generally has an end surface situated on a cover, this surface facing downward during the passage through the drying tunnel intended to solidify the shell mold. During this drying, the assembly moving in the tunnel is subject to vibrations. Due to these vibrations and the significant mass of the portion of the shell mold covering the cover of the cup, falls of shell mold blocks are frequently observed. These blocks are then found on the floor and need to be removed, for example using costly conveyor belts. Alternatively, for the removal of these blocks outside the facility, frequent cleaning operations may be carried out. However, these operations are also costly, and liable to involve risks in respect of health, safety and the environment (HSE risks). 
     OBJECT OF THE INVENTION 
     The aim of the invention is thus that of remedying at least partially the drawbacks mentioned above, relative to the embodiments of the prior art. 
     For this purpose, the invention first relates to a method for manufacturing a shell mold for the production by lost-wax casting of bladed elements of an aircraft turbine engine, said shell mold in cluster form comprising a plurality of bladed shell mold elements each intended to obtain one of said bladed turbine engine elements, said method comprising the following steps: 
     a) creating an assembly about which the shell mold is intended to be formed, the assembly including a wax pattern as well as a device for subsequently forming a cup for pouring metal, said device having an end surface; 
     b) depositing a hot wax coating layer about at least one portion of said assembly, such that said coating layer covers at least one portion of the end surface of the device intended to subsequently form the cup for pouring metal; and 
     c) forming the shell mold about said assembly. 
     According to the invention, the method further includes, between steps b) and c), the implementation of a step of structuring the coating layer intended for reinforcing the adhesion between this layer and the shell mold to be formed, and including the production of recesses and projections on the still-malleable coating layer. 
     As such, the invention cleverly envisages carrying out a structuring of the coating layer after the deposition thereof, in order to create a raised surface favorable for superior adhesion of the shell mold intended to be formed about this coating layer. 
     The risks of shell mold blocks falling are thus considerably reduced. For this reason, it is no longer necessary to use costly means for removing the blocks that have fallen on the floor, such as conveyor belts as proposed in the prior art. This advantageously results in a reduction in the costs of the facility devoted to the implementation of the method for manufacturing the shell mold. 
     The invention further has at least one of the following optional features, taken alone or in combination. 
     The step for structuring the coating layer is implemented by inserting a plurality of imprinting elements in said still-malleable coating layer, causing the formation of said projections about the imprinting elements, then by removing the latter revealing recesses, each surrounded by one of said projections. 
     The imprinting elements are studs, preferably with an external surface head having a general spherical cap shape, for example a general hemispherical shape. 
     The ratio between the maximum external diameter of each stud, and the external diameter of the end surface of the device, is less than 20. 
     The number of studs is between 3 and 20. 
     The step for structuring the coating layer is implemented by applying a pressure from a supporting member bearing the plurality of imprinting elements, against said still-malleable coating layer. Said application of pressure is performed by moving said assembly, against the supporting member remaining stationary. Alternatively, the supporting member could be moved in order to come into contact with the coating layer, without leaving the scope of the invention. 
     The step for forming the shell mold about said assembly includes at least one drying operation performed at least in part with said end surface facing downward, and preferably with said shell mold, surrounding the assembly, moved inside a drying station. 
     The step for forming the shell mold is performed by dip coating. 
     The invention also relates to a method for manufacturing by lost-wax casting a plurality of bladed elements of an aircraft turbine engine, this method including the production of a shell mold using a method as described above, followed by casting of metal in the shell mold. 
     Further advantages and features of the invention will emerge in the non-limiting detailed description hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       This description will be given with reference to the appended drawings wherein: 
         FIG. 1  represents a perspective view of a bladed element of a turbine engine intended to be obtained by implementing the method according to the present invention, said bladed element being in the form of a mobile high-pressure turbine blade; 
         FIG. 2  represents a perspective view of a wax pattern used for manufacturing a shell mold for the production, by lost-wax casting, of blades such as that shown in  FIG. 1 ; 
         FIGS. 3 to 10  represent schematically different steps of the method for manufacturing the shell mold; and 
         FIG. 11  represents a schematic view of such a shell mold obtained by implementing the manufacturing method represented schematically in the preceding figures. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to  FIG. 1 , an example of a mobile high-pressure turbine blade  1  for an aircraft turbine engine is represented. Conventionally, this blade  1  includes a blade  2  extending from one end  4  forming a blade root, and including a platform  8  intended to define a main gas flow jet. 
     The aim of the invention is that of manufacturing the mobile blade  1  from a shell mold intended to be produced using a method specific to the invention, one preferred embodiment whereof will now be described with reference to  FIGS. 2 to 10 . Nevertheless, it is noted that the invention may also be applied to the manufacture of mobile compressor blades, or to the manufacture of compressor or turbine stator blades, produced separately or in sectors including a plurality of blades. 
     For the manufacture of the shell mold, a wax pattern is first created, also known as a replica, about which a ceramic shell mold is intended to be subsequently formed. 
     In  FIG. 2 , the wax model  100  is represented in an inverted position with respect to the position wherein the shell mold is subsequently filled with metal. This inverted position facilitates the assembly operation of the various constituent elements of the wax pattern, which will now be described. 
     The model  100  firstly includes a portion for dispensing metal, referenced  12   a . It adopts a solid revolutionary, cylindrical or conical shape, having a central axis  14   a  aligned with the central axis of the assembly of the wax pattern  100 . This axis  14   a  is oriented vertically, and thus considered to represent the direction of the height. This dispensing portion  12   a  is attached directly to a specific tool  16 , above which it is situated. 
     The portion  12   a  is terminated at the top by an end  18   a  of greater diameter, from which a plurality of portions  20   a  extend radially for the formation of a plurality of casting arms. The portions  20   a  are herein three in number, distributed at 120° about the axis  14   a . Each portion  20   a  thus includes a first end  21   a  connected to the enlarged end  18   a  of the dispensing portion  12   a , and extends in a straight or slightly curved manner up to the second end  22   a.    
     For each portion forming an arm  20   a , a wax/ceramic securing reinforcement  23   a  may be envisaged between the dispensing portion  12   a  and the second end  22   a  of the portion  20   a.    
     Furthermore, from each second end  22   a , a wax replica  1   a  of the turbine blade represented in  FIG. 1  is attached. This replica  1   a  thus includes a blade  2   a , extending from an end  4   a  forming a blade root, and comprising a platform  8   a . In  FIG. 2 , the blade replicas  1   a  were only represented schematically. 
     It is noted that while the replicas  1   a  have been represented with the blade root  4   a  arranged at the bottom with respect to the blade  2   a  in the position in  FIG. 3 , this root  4   a  could alternatively be arranged at the top, such that, once the shell mold has been inverted to cast the metal, the metal only reaches the root after having passed through the blade portion. 
     The wax blades  1   a  extend upward, being arranged about the axis  14   a , and also about a central wax supporting member  24   a  extending along the same axis from the end  18   a  of the dispensing portion  12   a . The supporting member  24   a  is preferentially in the form of a rod having the axis  14   a , which extends up to the vicinity of the blade heads  2   a.    
     As seen in  FIG. 2 , for each wax blade  1   a , a wax/ceramic securing reinforcement  25   a  may be envisaged between the upper end of the central support rod  24   a , and the blade head. Similarly, wax/ceramic securing reinforcements (not shown) may interconnect adjacent blade heads of the different blades  1   a.    
     The wax blades  1   a  form the peripheral wall of the wax replica  100 . They are spaced circumferentially from one another, and define an internal space centered on the axis  14   a , wherein the central support rod  24   a  is thus situated. 
     As represented schematically in  FIG. 3 , once the wax replica  100  has been produced, a device  32   a  is assembled thereon intended to subsequently form a cup for pouring metal into the shell mold. The device  32   a  includes a conical element  34   a  centered on the axis  14   a  and flaring at the bottom from a small-sized section rigidly connected to the lower end of the dispensing portion  12   a . The conical element  34   a  is preferably produced hollow, and closed at the lower end thereof by a cover  36   a , the external surface  40   a  whereof forms an end surface of the device  32   a . Alternatively, the device  32   a  could be produced solid, in a wax intended to be subsequently removed when removing the wax pattern  100 . 
     Optionally, reinforcement elements  42   a  may subsequently be produced between the device  32   a  and the arms  20   a.    
     The wax pattern  100  and the device  32   a  form collectively an assembly  200  about which the shell mold is intended to be formed. Nevertheless, before the step for forming the shell mold, a step is envisaged for depositing a hot wax coating layer, as represented schematically in  FIG. 4 . This depositing step is also referred to as “dip seal”. It is intended to partially dip-coat the assembly  200  in a vat  44  of liquid hot wax  46 , so as to enable good adhesion of the shell mold subsequently formed. As an indication, the dip coating is herein performed so as to immerse the entire device  32   a  in the hot wax  46 , and optionally a lower part of the wax model  100 . In addition, after this dip coating step, a hot wax coating layer  46  covers the entire end surface  40   a  defined by the cover  36   a  of the device  32   a , as represented schematically in  FIG. 5 . A hot wax coating layer  46  also covers the external surface of the conical element  34   a.    
     One of the specificities of the invention consists of structuring at least the layer  46  covering the end surface  40   a , when this layer is still malleable, i.e. before it has completely cooled. 
     For this purpose, a tool as shown in  FIGS. 5, 5   a ,  5   b  and  6  is envisaged. It consists of a supporting member  50  bearing a plurality of imprinting elements  52  in the form of studs, with a hemispherical external surface head  54 . The number of these studs  52 , the size and arrangement thereof are selected according to the needs encountered. By way of indicative example, the number of studs  52  projecting from the supporting member  50  may be between 3 and 20, whereas the ratio between the external diameter D 1  thereof and the external diameter D 2  of the cover is preferentially less than 20. 
     In order to perform the step for structuring the coating layer  46 , the assembly  200  is moved against the supporting member  52  remaining stationary on a specific station  58 , represented schematically in  FIG. 6 . The movement of the assembly  200  against the supporting member  50  bearing the studs  52  is preferably performed vertically downward, with the end surface  40   a  oriented horizontally. The pressure applied results in the studs  52  being inserted into the layer  46 , creating an expulsion of wax about same. This expulsion, in the form of a bead surrounding each stud  52 , generates a projection  60 . After removing the studs  52 , the latter give way to recesses  62  shown in  FIG. 7 , each recess being surrounded by a projection  60 . 
     The depth of the recesses  62  is less than the thickness of the coating layer  46 , such that wax is found at the bottom of each recess. The structuring performed makes it possible, clearly and inexpensively, to reinforce the adhesion between the layer  46  covering the end surface  40   a  of the cover  36   a , and the shell mold intended to be formed subsequently. This structuring is added to the optional presence of an initial structuring of the end surface  40   a  of the cover  36   a , for example using goffering  64  as seen in  FIG. 7 . It should however be specified that this goffering  64  is covered with the coating layer  46 , which tends to attenuate the raised surfaces of the goffering, and thus lower the adhesion power thereof. The structuring according to the invention, generated after the deposition of the coating layer  46 , makes it possible to effectively reinforce the adhesion power of this layer to the shell mold subsequently formed. 
     In this regard, with reference to  FIGS. 8 and 9 , the step for forming the ceramic shell mold is then implemented, by dip-coating the assembly  200  in successive baths  68 , one whereof is represented schematically in  FIG. 8 . This step is known per se and will not be described further, apart from the fact that during the embodiment thereof, the shell mold  300  being formed is deposited in the recesses  62  and about beads  60  of the coating layer  46 . These layers act as anchor points of the shell mold, thus promoting the adhesion thereof to the cover  36   a.    
     During the formation of the shell mold  300 , at least one drying operation is performed intended to dry same. This operation, represented schematically in  FIG. 10 , consists of conveying one or a plurality of shell molds  300  inside a drying station also known as a drying tunnel  70 , with the shell molds  300  suspended above the floor  72 . During this movement, the end surface  40   a  of the cover is oriented horizontally, downward, but the risks of uncoupling of the shell mold blocks is reduced considerably by the structuring  60 ,  62  previously carried out on the coating layer  46  covering the end surface  40   a.    
     After drying, the shell mold  300  which is obtained is represented schematically in  FIG. 11 . It also has a general cluster shape, and obviously includes similar elements to those of the wax replica  100  and the device  32   a  cited above. These shell mold elements will now be described, with the shell mold represented in an inverted position with respect to the position wherein it is subsequently filled with metal. 
     It consists first of the cup  32   b , followed by the metal dispenser, referenced  12   b . The latter thus has a hollow revolutionary, cylindrical or conical shape, having a central axis  14   b  which is aligned with the central axis of the shell mold  300 . This axis  14   b  is oriented vertically, and thus considered to represent the direction of the height. 
     The dispenser  12   b  is terminated at the top with a hollow end  18   b  of greater diameter, from which a plurality of metal casting arms  20   b  extend radially. The arms  20   b  are herein three in number, distributed at 120° about the axis  14   b . Each arm  20   b  thus includes a first end  21   b  connected to the enlarged end of the dispenser  12   b , and extends in a straight or slightly curved manner up to a second end  22   b.    
     Each arm  20   b  is thus envisaged to be hollow and form a metal supply duct after removing the wax  20   a . Herein also, a securing reinforcement  23   b  may be envisaged between the dispensing portion  12   b  and the second end  22   b  of each arm  20   b.    
     From each second end  22   b , a bladed shell mold element  1   b  is situated. These elements  1   b  are referred to as bladed as, after removing the wax replica  1   a , they each form internally an imprint corresponding to one of the blades  1 . 
     The bladed element  1   b , also referred to as shell mold blade, thus includes a blade portion  2   b  defining adjacent blade imprints, this portion  2   b  extending from one end  4   b  forming a blade root, and including a platform  8   b . In  FIG. 11 , the shell mold blades  1   b  have been represented only schematically. 
     The bladed elements  1   b  thus extend upward, being arranged about the axis  14   b , and also about a central supporting member  24   b  extending along said axis from the end  18   b  of the dispenser  12   b . The supporting member  24   b  preferentially takes the form of a hollow cylinder having the axis  14   b , which extends up to the vicinity of the ends  6   b  of the bladed elements  1   b.    
     Furthermore, for each bladed element  1   b , a securing reinforcement  25   b  may be envisaged between the upper end of the central support rod  24   b , and the blade head. Similarly, wax/ceramic securing reinforcements (not shown) may interconnect adjacent blade heads of the different shell mold blades  1   b . Finally, reinforcing elements  42   b  are arranged between the cup  32   b  and the casting arms  20   b.    
     After obtaining the shell mold  300  and removing the wax replica  100  contained therein, and removing the cover initially closing the cup, the shell mold is preheated at a high temperature in a dedicated furnace, for example at 1150° C., in order to promote the fluidity of the metal in the shell mold during casting. 
     At the shell mold preheating outlet, metal from a melting furnace is cast in imprints via the cup  32   b  shown, with the shell mold in the inverted position with respect to that shown in  FIG. 11 , i.e. with the cup  32   b  open at the top and once again the axis  14   b  oriented vertically. 
     The molten metal thus successively travels through the cup  32   b , the dispenser  12   b , the casting arms  20   b , and the bladed shell mold elements  1   b , merely flowing gravitationally. It is noted that prior to casting, the central supporting member  24   b  preferably has the end thereof sealed so as to not be filled with metal, and such that the metal cast necessarily passes through the arms  20   b  before entering the bladed elements  1   b . The reinforcements  23   b ,  25   b ,  42   b  are preferentially solid, made of ceramic, thus not traversed by the molten metal during the casting in the shell mold  300 . 
     After cooling the metal, the shell mold is destroyed, and the mobile blades  1  are separated from the cluster for any machining and finishing and inspection operations required. 
     Obviously, various modifications may be made by those skilled in the art to the invention described above, merely by way of non-limiting examples.