Patent Publication Number: US-9833834-B2

Title: Turbine engine blade preform

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage of International Application No. PCT/FR2014/051324, filed on Jun. 4, 2014, which claims the benefit of French Patent Application 1355177, filed Jun. 5, 2013, the contents of each of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a turbine engine blade preform, such as in particular a turbine blade preform in a turbine engine, and a mold and a method for obtaining the blade preform. 
     In the current technique, the turbine blades are one-piece parts obtained by molding, using a mold comprising a cavity, the three-dimensional shape of which makes it possible to obtain the desired shape in a form-fitting manner. In practice, the part obtained after the step of molding consists of a blade preform which has to be machined to reach the desired final shape and dimensions. 
     Thus, a turbine blade preform comprises a strut connecting a blade root to a plat-form. The blade preform also has two transverse upstream and downstream webs formed at the upstream and downstream ends of the strut and connecting the upstream and downstream edges of the plat-form to the upstream and downstream ends of the blade root, respectively. It should be noted that the terms “upstream” and “downstream” are to be considered relative to a general upstream to downstream fluid flowing direction around the blade when the latter is mounted in a turbine engine. 
     When viewed in an upstream/downstream direction, each web of the blade comprises side edges connected to the blade root flanks through concave curved walls which extend towards each other and form a throat area the width of which measured in a transverse direction is smaller than that of the blade root. 
     The presence of a throat at the junction between each web and the blade root makes it possible to limit the time required for the final machining of the blade root. To obtain such above-mentioned throats, the mold must also comprise the matching throat areas. 
     However, when the liquid material flows into the mold, the mold throat areas slow down the flow of material at these locations, which may lead to interrupted solidification and cold lap which may lead to a poor solidification of the material and the forming of mechanical defects, such as cracks in the connecting areas of the side webs to the preform of the blade root. 
     SUMMARY OF THE INVENTION 
     The present invention provides a simple, efficient and economical solution to such problems. 
     To this end, it provides a turbine engine blade preform, with the preform comprising a strut connecting a platform to a blade root portion extending longitudinally in an upstream-downstream direction, two upstream and downstream webs, which extend in a direction substantially perpendicular to the longitudinal direction of the blade root and are formed at the upstream and downstream ends of the strut, with such upstream and downstream webs connecting the upstream and downstream ends of the platform to the upstream and downstream ends of the blade root, characterized in that the blade root extends in a direction perpendicular to the longitudinal direction of the blade root over a distance smaller than that over which the upstream and downstream webs extend and in that the side edges are extended by walls that converge at the flanks of the blade root. 
     Forming side edges converging towards each other at the connection areas of each web at the blade root makes it possible, when casting the material into an appropriate mold, to obtain optimum metallurgical quality at such connecting regions since the material flows into the mold without any throat zone like in the prior art. 
     Thus, the rate of rejection of blade preforms obtained by molding is greatly reduced, which reduces the manufacturing costs of the blades. 
     Said converging walls are preferably formed by plane faces inclined with respect to a median plane of the blade root extending longitudinally in an upstream/downstream direction. 
     According to a particular embodiment of the invention, the above-mentioned faces are inclined at an angle of approximately 45° with respect to the median plane. 
     The invention also relates to a mold for manufacturing a casting blade preform as described above, comprising an internal cavity the three-dimensional shape of which determines, in a form-fitting manner, the three-dimensional shape of the blade preform, with the mold comprising at least two first and two second blocks arranged in pairs opposite each other along perpendicular axes, with the first two cavity retainer blocks each comprising a recess having a boss intended to define the inner surface of a platform of the preform and the blade defining, with recesses of the second cavity retainer blocks, spaces intended to form upstream and downstream side webs for connecting upstream and downstream edges of a plat-form of the blade preform to the upstream and downstream ends of a blade root, with the first cavity retainer blocks further comprising substantially parallel and opposed faces intended to form flanks of a root part of the blade preform, characterized in that the recess of each first block comprises first and second surfaces on either side of the boss connected to the face forming a flank of the blade root, with the first and the second surfaces of the first cavity retainer blocks converging toward the faces of the flanks of the blade root. 
     According to the invention, the integration of first and second converging surfaces prevents the forming of a throat in the connecting regions of the side webs to the blade root, thereby limiting the forming of defects in these areas. 
     The invention also relates to a method for manufacturing a turbine blade using the mold described above, comprising:
         a. positioning the mold so that the first and second cavity retainer blocks are arranged in an upper position relative to a lower mold part;   b. gradually introducing a liquid material into the lower part of the mold so that the liquid level gradually increases inside the mold and forms a blade preform, in a form-fitting manner; then   c. performing a finishing machining of the blade preform to the desired final dimensions of the blade.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood, and other details, characteristics and advantages thereof will appear upon reading the following description given by way of a non-restrictive example while referring to the appended drawings wherein: 
         FIG. 1  is a schematic view of a turbine engine blade preform according to the prior art; 
         FIG. 2  is a view from upstream of the area enclosed in dotted lines in  FIG. 1 ; 
         FIG. 3  is a schematic view, in perspective, of a turbine engine blade preform according to the invention; 
         FIGS. 4 to 6  are schematic representations in perspective of several cavity retainer blocks of a tool for making a preform according to the invention; 
         FIG. 7  is a view from upstream simultaneously representing in superposition a turbine blade preform of the invention and a turbine engine blade obtained after machining the preform; 
         FIG. 8  is a schematic view, in perspective, simultaneously showing in superposition a turbine blade preform according to the invention and a turbine engine blade obtained after machining the preform. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will first be made to  FIG. 1  which shows a one-piece turbine engine blade preform  10  according to the prior art obtained by molding in a mold and comprising, along the axis  12 , a portion of the blade root  14  which extends longitudinally in an upstream/downstream direction and having, in cross-section, a substantially rectangular shape, a plat-form  16 , a blade  18  and a blade root  20 . From upstream or downstream, the outline of the root is U-shaped, with the branches of the U being substantially parallel and being formed by the flanks  42  of the blade root. The blade shown in  FIG. 1  more particularly represents a blade mounted in a turbine of the turbine engine. 
     The mold  22  comprises an internal cavity  24 , the three-dimensional shape of which is so determined that the desired three-dimensional shape of the blade preform  10  is obtained by filling the mold with the liquid material. In practice, the liquid material is injected into the mold  22  from a lower portion  23  of the mold  12  corresponding to the one making it possible to obtain the blade root  20 . The liquid material progressively fills the mold (A arrow) up to an upper portion  25  of the mold corresponding to the one making it possible to obtain a blade root portion  14 . In  FIG. 1 , and for a usual representation of the blade, the mold  22  is thus shown in the reversed position relative to its position of use. 
       FIG. 2  shows the part in dotted lines in  FIG. 1 , i.e. the internal part of the blade preform comprising two upstream  26  and downstream  28  webs extending in a direction substantially perpendicular to the longitudinal direction of the blade root  14 , with the two upstream  26  and downstream  28  webs connecting the root portion of the blade root  14  to the plat-form  16  (only the upstream  26  web is shown in  FIG. 2 , with the downstream  28  web being visible in  FIG. 3  illustrating the invention).  FIG. 2  also schematically shows two cavity retainer blocks  30 ,  32  of the mold  22 , arranged in facing relation and each comprising a protruding portion  34  towards one another, with such portions  34  comprising convex curved surfaces  36  making it possible to form, in a form-fitting manner, concave curved walls  38  connecting the side edges  40  of the webs  26 ,  28  to the portion of the blade root  14 . 
     As explained above, this type of blade preform may have mechanical defects such as cracks, at the connecting areas  38  of the webs  26 ,  28  to the flanks  42  of the root portion  14  of the blade preform  10  due to the presence of a throat zone in the mold between the two parts  34  which slows down the flow of material (A arrow). 
     The invention thus provides a modification of the connecting regions of the side edges of the webs  26 ,  28  to the flanks  42  of the blade root by extending the side edges  40  of the webs  44  by walls converging towards one another up to the flanks  42  of the blade root  14  as shown in  FIG. 3 . The blade root  14  thus extends in a direction substantially perpendicular to the longitudinal upstream/downstream direction of the blade root over a distance smaller than that of the upstream  26  and downstream  28  webs. Unlike the prior art, the preform blade  46  no longer comprises a throat area, which greatly reduces the risks of formation of defects at the junction of the webs  26 ,  28  with the portion of the blade root  14 . 
     In the embodiment shown in  FIG. 3 , the walls  44  are formed by flat surfaces inclined at an angle of approximately 45° relative to a median plane of the blade root extending from upstream to downstream, i.e. a plane extending longitudinally through the centre of the blade root  14 . 
     In other not shown embodiments, the walls may be curved, concave or convex, while converging towards the flanks of the blade root. 
     Reference is now made to  FIGS. 4 to 6  showing a portion of a mold  48  according to the invention for forming the connecting regions with convergent faces as described with reference to  FIG. 3 . 
     This mold comprises two first  50 A,  52 A and two second  54 A,  56 A cavity retainer blocks arranged in pairs opposite one another along perpendicular axes  58 ,  60  ( FIG. 4 ). Each cavity retainer block  50 A,  52 A,  54 A,  56 A comprises a recess  50 B,  52 B,  54 B,  56 B delimiting a cavity intended to form, in a form-fitting manner, a portion of the turbine blade preform  46  in a form-fitting manner, after assembling the cavity retainer blocks  50 A,  52 A,  54 A,  56 A. 
     In particular, the recess  50 B of the block  50 A comprises a boss  50 C an outer surface  50 D of which is arranged with some clearance opposite an outer surface  52 D of a matching boss  52 C of the recess  52 B of the block  52 A so as to form the strut of the blade preform  46 . Each outer surface  50 D,  52 D of a boss  50 C,  52 C is connected to the rest of the recess  50 B,  52 B by a periphery an upper surface  50 E of which is intended to delimit an inner face of the plat-form  16 , two side surfaces  50 F are intended to form the upstream  26  and downstream  28  side webs of the blade preform  46  with the recesses  54 B,  56 B of the second cavity retainer blocks  54 A,  56 A. 
     The boss  50 C,  52 C of each recess  50 B,  52 B of a first block protrudes from a surface  50 G,  52 G intended to form the side edges of the plat-form  16  and the side edges  40  of the upstream  26  and downstream  28  webs of the plat-form  16 . This area  50 G,  52 G is connected to a first  50 H,  52 H and a second  50 I,  52 I surfaces extending on either side of the boss  50 D,  52 D and intended to form the connecting regions to the blade root  14  according to the invention. For this purpose, the first surfaces  50 H,  52 H of the cavity retainer blocks  50 B,  52 B converge towards one another in a direction oriented from the impeller  18  to the blade root  14 . Similarly, the second surfaces  50 I,  52 I converge towards one another in a impeller to blade root direction. The first surface  50 H,  52 H and the second surface  50 I,  52 H of each cavity retainer block  50 A,  52 A are connected to the same surface  50 J,  52 J intended to form a flank  42  of a portion of the blade root  14 . These surfaces of the first cavity retainer blocks  50 A,  52 A forming the above-mentioned flanks are substantially parallel. 
     Forming converging faces opening onto parallel faces  50 J,  52 J on the first and second cavity retainer blocks facilitates the flow of liquid material into the mold (A arrow). 
     It should be noted that the second cavity retainer blocks  54 A,  56 A each comprise a slot  54 C for forming a spoiler  62  extending from a side web  26 ,  28  of the blade preform  46  and opposite the plat-form  16  relative to the web. 
     After molding the preform  46  using the mold described above, a machining operation of the preform is carried out so as to obtain a turbine blade  64  to the desired dimensions. A machining of the part of the blade root and the connection zones of the side webs to the part of the blade root is executed so as to form a dovetail blade root  66  adapted to be axially engaged and radially retained in a cavity of a turbine disk in a well known manner. The step of machining also consists in machining the side edges of the plat-form.