LOST WAX MOLD MAKING PROCESS

Description

Technical field of the invention

[0001] This presentation relates to the field of turbines, in particular that of turbomachine turbines, and aims at a manufacturing process by lost wax casting of parts, in particular blades, said parts comprising cavities intended for the circulation of cooling fluids. State of the prior art

[0002] Turbine blades are subject to high thermal stresses and include means of cooling by circulating a heat transfer fluid, generally air in the case of a gas turbine engine, inside cavities formed in the blade.

[0003] Turbine blades are generally manufactured by casting, particularly by lost-wax casting. This technique involves making a wax model of the part to be manufactured. The wax model is enclosed in a ceramic shell mold. This is made by successively dipping the wax model in slips containing a ceramic material and stuccoing the layer formed between each dipping. The wax model is removed during an initial firing at a suitable temperature. The shell mold is then fired at a high temperature to give it the strength necessary for casting the metal.

[0004] The shell mold then has a cavity forming the image of the hollow model, molten metal being poured into said cavity. After cooling, the shell mold is broken to release the part. This then undergoes a finishing treatment.

[0005] When the blade includes cavities for the circulation of the cooling fluid, one or more core elements must be incorporated into the model. This phase of the process first includes the separate manufacture of the core element(s) by molding a ceramic material consolidated by a binder, their assembly if necessary, and the placement of the core element(s) in a wax mold. A wax model is thus produced by injecting wax into the wax mold. This model is a replica of the part to be obtained.

[0006] There figure 1 represents, in section perpendicular to the axis of the part, a wax mold 10 whose internal wall is the image of the part to be obtained. This wax mold is here in two parts 10A and 10B. A core 13 composed of a plurality of branches 13A to 13G parallel to each other is arranged in the mold 10. The branches define between them spaces which will form partitions after the casting of the metal. To ensure the wedging of the core inside the mold, platinum pins 20 are arranged between the core 13G and the wall 17 of the mold, the pins 20 ensuring the spacer function. A ceramic shell mold is then obtained after the extraction of the wax model. Molten metal is then poured into the space between the walls of the ceramic mold and those of the core. The pins are then melted and dissolved in the metal. The ceramic core is then removed by an appropriate treatment.

[0007] The function of the pins is to maintain a space between the walls of the core and the inner wall of the mold, particularly when injecting wax into the wax mold. This ensures that the dimensions of the wax model, and therefore of the ceramic shell mold and the metal part to be manufactured, are respected.

[0008] However, in the case of a part to be manufactured comprising cavities with complex geometries, it is necessary to use numerous core elements. This increases the complexity of the process and makes it difficult to maintain the position of the core elements in the wax model. Increasing the number of pins also increases the risk of pins being forgotten by the manufacturer, which would locally modify the geometry of the core. In addition, the cost of the manufactured part is higher, due to the complexity of the manufacturing process. Documents FR2889088 and WO2021038174 present examples of cores for lost wax casting assembled using rods. Summary of the invention

[0009] The aim of this presentation is to propose a method for manufacturing a cavity part by wax casting, ensuring precise thicknesses of the walls of the part, in a reliable and inexpensive manner.

[0010] To this end, the present disclosure proposes a method for producing a wax model for manufacturing a part, such as a turbine blade, said part comprising at least one cavity, the method comprising the steps of: arranging at least two core elements made of ceramic material in a wax mold, said core elements having at least in part a shape complementary to the cavity of the part to be manufactured, and injecting wax into the wax mold, around the core elements, so as to form the wax model, in which the two core elements are assembled prior to the injection of wax by at least one staple, said staple comprising a central part from which two branches extend, each branch being engaged in an orifice provided in one of the core elements.

[0011] Thus, the clip allows the core elements to be held in position relative to each other and / or relative to the internal surface of the wax mold, in particular during the injection of wax into the wax mold. The wax model obtained thus forms a replica whose dimensions are controlled, so that the part manufactured on the basis of this model corresponds to the dimensional specifications. One or two clips can be used to hold two core elements in place relative to each other, which reduces the cost of the manufacturing process. In addition, once the clip is in position, for example after gluing, it will remain in place throughout the process without the risk of losing it or moving it slightly, which would cause poor positioning. Thus, the clip helps to guarantee the wall thickness around the core.

[0012] The core elements may partly have a shape complementary to the cavity and their assembly may entirely have the shape complementary to the cavity.

[0013] The staple may have a general shape of U, C, V or M.

[0014] The part to be manufactured may comprise at least two cavities each formed by a core whose shape may depend on the shape of the cavity to be produced.

[0015] The method may comprise, prior to assembling the two core elements by the clip, a step of positioning at least one spacer between the two core elements to maintain said core elements at a predetermined distance from each other. Said spacer may be removed after fixing the clip. The two core elements may be assembled directly by the clip without using the spacer.

[0016] At least one, in particular each, of the legs of the staple can be held in the hole by gluing.

[0017] The bonding of one of the branches of the staple can be carried out using glue, for example of the Loctite ® type, wax or any other suitable means for holding said branch in the corresponding hole.

[0018] At least one, in particular each, hole in one of the core elements may be a blind hole. This makes it possible to limit the length of the staple branches.

[0019] At least one, in particular each, orifice of one of the core elements may be a through orifice, the corresponding branch of the clip passing through said orifice and the free end of said branch being folded back onto the corresponding core so as to lock the clip in position. This makes it possible to ensure that the core elements are held in relative position more reliably.

[0020] The central part and the two branches of the clip may have a circular section. At least one, in particular each, orifice of one of the core elements may have a diameter of between 0.5 mm and 1 mm, in particular equal to 0.8 mm. The clip may have a diameter substantially identical to the diameter of the corresponding orifice(s), with a mounting clearance which is for example of the order of 0.1 mm.

[0021] The clip can be made of platinum or another material, depending on the metal material intended for casting.

[0022] The cores may have first side surfaces facing the same side. Said staple may be placed so that the central branch is against said first side faces of the cores.

[0023] The staple branches can extend in the same direction across the cores.

[0024] The distal portions of the staple branches may be folded onto second lateral surfaces of the cores opposite the first lateral surfaces. In particular, the staple branches may be folded onto said lateral surfaces in two opposite directions.

[0025] The present disclosure also relates to a method of manufacturing a part, such as a turbine blade, comprising at least one cavity comprising the steps of: making a wax model according to the above-mentioned process, forming a ceramic shell mold from the wax model, removing the wax model, pouring a molten metallic material into the shell mold.

[0026] The shell mold can be made by successive dippings of the wax model into slips containing a ceramic material and by stuccoing the layer formed between each dipping.

[0027] The wax model can be removed from the shell mold by baking at a suitable temperature.

[0028] The shell mold can be fired at a high temperature after removal of the wax model. The shell mold can be removed after casting the metal material by appropriate mechanical and / or chemical treatment.

[0029] The part can be a moving blade or a turbine stator blade. Brief description of the figures

[0030] [ Fig. 1 ] there figure 1 , already described, represents a schematic view of a wax mold for making a wax model, [ Fig. 2 ] THE figures 2a et 2b are schematic perspective views of two core elements intended to each form a corresponding cavity in a blade to be manufactured. Fig. 3 ] there figure 3 shows a first stage of assembly of the core elements of the figure 2 . [ Fig. 4 ] there figure 4 shows a second stage of assembly of the core elements of the figure 2 . [ Fig. 5 ] there figure 5 shows a schematic perspective view of a staple connecting the core elements of the figure 2 . [ Fig. 6 ] there figure 6 shows a schematic sectional view of a staple connecting the core elements of the figure 2 . Detailed description of the invention

[0031] In reference to the figures 2 à 6 , a wax model for manufacturing a blade is obtained by injecting wax into the wax mold 10. Prior to the wax injection, core elements 102 and 104 are arranged in the wax mold 10 in place of the core elements 13. The core elements 102 and 104 are made of a ceramic material. An external face 106 of the core 102 defines the leading edge of the blade to be produced. A portion 108 of the core 104 defines the trailing edge of the blade to be produced.

[0032] THE figures 3 et 4 show steps of assembling the core elements 102 and 104 before positioning them in the wax mold 10.

[0033] The core elements 102 and 104 are first positioned relative to each other using an axial or longitudinal stop 110 and two first spacers 112. The stop 110 allows the core elements 102 and 104 to be aligned in an axial or longitudinal direction. The first spacers 112 allow a distance to be maintained between the core 102 and the core 104 in a direction perpendicular to the longitudinal direction. This distance between the core elements 102 and 104 allows a wall to be formed between two cavities in the blade, after the wax model has been made and a molten metallic material has been poured into a shell mold made from the wax model.

[0034] It is important to prevent the core elements from moving relative to each other when injecting wax into the wax mold or when pouring the molten metallic material into the shell mold, so that the manufactured blade meets the dimensional specifications.

[0035] For this, one or more clips 114 are configured to hold the core elements 102 and 104 relative to each other. Each clip 114 comprises a central portion 116, two branches 118 extending perpendicular to the central portion 116, from each of the ends of said central portion 116. The branches 118 of the clips 114 are engaged in orifices 120 of the core 102 and in orifices 122 of the core 104.

[0036] The clip 114 may be held in the holes 120 and 122 by a bonding step, for example by means of a suitable glue or wax.

[0037] The staple is made of platinum or any other material suitable for being melted when casting the metallic material.

[0038] Once the core members 102 and 104 are held together using the clips, the spacers 112 are removed and the core members 102 and 104 are positioned in the wax mold 10.

[0039] In another embodiment, the core members 102 and 104 are positioned relative to each other without the use of the spacers 112. In this case, the core members 102 and 104 held together are directly disposed in the wax mold 10. The orifices 120 or 122 may be blind orifices.

[0040] The orifices 120 or 122 may be through orifices as shown in the figure 6In this case, the ends of the branches 118 can be folded onto a surface 103 of the core 102, respectively a surface 105 of the core 104, opposite the central part 116. In this way, gluing the staple 114 is no longer necessary to hold the staple 114 in position.

Claims

1. A method for making a wax model for manufacturing a part, such as a turbine vane, said part comprising at least one cavity, the method comprising the steps of: arranging at least two core elements (102, 104) made of a ceramic material in a wax mould (10), said core elements having, at least partially, a shape complementary to the cavity of the part to be manufactured, and injecting wax into the wax mould, around the core elements (102, 104), so as to form the wax model, wherein the two core elements (102, 104) are assembled prior to the injection of wax by at least one staple (114), said staple including a central portion (116) from which two branches (118) extend, each branch (118) being fitted into an orifice (120, 122) formed in one of the core elements (102, 104), and wherein at least one orifice (120, 122) of one of the core elements is an open-through orifice, the corresponding branch (118) of the staple (114) passing through said orifice (120, 122) and the free end of said branch being folded over the corresponding core (102, 104) so as to block the staple (114) in position.

2. The method according to claim 1, wherein at least one of the branches (118) of the staple (114) is held in the orifice (120, 122) by gluing.

3. The method according to one of claims 1 or 2, wherein the central portion (116) and the two branches (118) of the staple (114) have a circular cross section.

4. A method for manufacturing a part, such as a turbine vane, comprising at least one cavity including the steps of: - making a wax model according to the method according to one of claims 1 to 3, and - forming a shell mould made of a ceramic material from the wax model, - eliminating the wax model, and - casting a molten metallic material into the shell mould.

5. The method according to claim 4, wherein the part is a movable vane or a turbine stator vane.

6. The method according to one of the preceding claims, wherein said cores have first lateral surfaces facing the same side, and wherein said staple is placed so that the central branch is against said first lateral faces of the cores.

7. The method according to the preceding claim, wherein the distal portions of the branches of the staple are folded over second lateral surfaces of the cores opposite to the first lateral surfaces, said branches of the staple being folded over said lateral surfaces along two opposite directions.