Manufacture of parts using the lost wax method

Abstract

The invention relates to the making, on a support plate (34), of an annular space (76) in a ceramic paste covering this plate, in order, by successive deposits and firing of layers of said ceramic paste, to create a base of a ceramic shell (40) for the moulding of parts, the base having said annular space (76). For this purpose, between two deposits of said ceramic paste, and on the plate, said deformable annular element (82) will be deformed in order to break the ceramic layer.

Claims

1. An assembly for moulding parts, the assembly comprising: a) a wax cluster comprising: models of the parts, a central barrel around which the models are arranged, segments that connect the models to the central barrel, a plurality of grain selection areas arranged respectively under the models, and b) a support plate for supporting the wax cluster, wherein said assembly further comprises a deformable annular element disposed on the support plate, between the central barrel and the grain selection areas, for creating an annular space at a location of a ceramic paste coating covering the deformable annular element.

2. An assembly according to claim 1, which comprises said ceramic paste coating which covers the wax cluster.

3. An assembly according to claim 2, wherein the annular space extends on a closed perimeter.

4. An assembly according to claim 2, wherein the deformable annular element has end pieces that pass through the support plate.

5. An assembly according to claim 1, wherein the annular space extends on a closed perimeter.

6. An assembly according to claim 5, wherein the deformable annular element is defined by an inflatable bladder.

7. An assembly according to claim 1, wherein the deformable annular element is defined by an inflatable bladder.

8. An assembly according to claim 7, wherein the deformable annular element has end pieces that pass through the support plate.

9. An assembly according to claim 8 wherein the inflatable bladder is connected to an inflating fluid source for feeding the deformable annular element with the inflating fluid through the support plate.

10. An assembly according to claim 7 wherein the inflatable bladder is connected to an inflating fluid source for feeding the deformable annular element with the inflating fluid through the support plate.

11. An assembly according to claim 1, wherein the deformable annular element has end pieces that pass through the support plate.

12. A method for making, on the support plate of the assembly according to claim 1, said annular space in the ceramic paste covering said support plate, in order to, by successive deposits and firing of layers of said ceramic paste, to create a base of a ceramic shell for the moulding parts, the base having said annular space, wherein said deformable annular element is arranged on the support plate and deformed between two deposits of said ceramic paste in order to break the deposited ceramic layer.

13. A method according to claim 12, wherein said deformable annular element is an inflatable bladder inflated between two deposits of said ceramic paste.

14. A method according to claim 13, in which the bladder is inflated after firing each deposit of said ceramic paste.

15. An assembly comprising: a) a ceramic moulding shell for moulding parts made in a material, the ceramic moulding shell comprising: an upper neck, for entering the material in the ceramic moulding shell, a first central tube, erected and located under the upper neck, a plurality of peripheral moulding cavities having in hollow the shape of the parts to be moulded, the peripheral moulding cavities being arranged around the first central tube and connected to the first central tube and the upper neck by material circulation channels, as well as to lower material flow portions, the upper neck, the first central tube and the moulding cavities communicating together, and a bottom extending between a base of said first central tube and said lower material flow portions, the bottom having an annular space, b) a support plate for supporting the moulding shell, wherein said assembly further comprises a deformable annular element provided at the location of said annular space.

16. An assembly according to claim 15, wherein the annular space extends on a closed perimeter.

17. An assembly according to claim 15, wherein the deformable annular element is defined by an inflatable bladder.

18. A method for making a moulding assembly according to claim 15, wherein the moulding shell is formed by successively depositing and firing layers of ceramic paste, and wherein said deformable annular element is arranged on the support plate and deformed between two deposits of said ceramic paste in order to break the deposited ceramic layer.

19. A method according to claim 18, wherein said deformable annular element is an inflatable bladder inflated between two deposits of said ceramic paste.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a vertical section of the essential part of a wax cluster covered with a ceramic envelope which, stacked layer after stacked layer, must correspond to the shell of FIG. 3;

(2) FIG. 2 is a local view in perspective of the bottom (lower) part of the wax cluster of FIG. 1; and

(3) FIG. 3 is, seen as FIG. 1, the ceramic shell from the wax cluster in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

(4) The examples shown in the figures are representative of the manufacture of turbojet blades. They relate to a single crystal directional solidification, but could relate to a columnar directional solidification without changing the essential characteristics of the invention.

(5) FIG. 1 shows, partially and in vertical section, a wax cluster 10 covered with a ceramic shell which will correspond below to the shell 40.

(6) The cluster 10 consists, in a single piece: of models 12 of the parts to be manufactured, made of wax, of a central barrel (also called a descending barrel) 14, also made of wax, around which the models 12 are placed, of segments 16, also in wax, which connect models 12 to the central barrel 14, of grain selection areas 18, also made of wax, respectively arranged under the models 12, these grain selection areas 18 having a substantially U-shaped intermediate part 20 (substantially crankshaft-shaped), a lower part 22 with an enlarged cross-section serving as a base, and an upper part 24 also having a substantially cone-shaped enlarged cross-section whose tip is at the bottom (marked B), the axis 26 cluster 10 being considered vertical, possibly of one or more additional internal support(s) 19, also made of wax, respectively arranged in circumferential projection around the central barrel 14.

(7) The intermediate part 20 has a substantially crankshaft shape because the example shown is that of a directional monocrystalline solidification. The intermediate part 20 is used to create a baffle, as described below.

(8) Each model 12 can have one or more insert(s) or core(s), preferably made of ceramic, if it is desired to obtain one or more hollow part(s) inside.

(9) In the selected embodiment, the segments 16, which serve as a link between the central barrel 14 and the peripheral and concentric models 12, here erected parallel to axis 26, therefore vertically, are located in the upper part (top marked B) of the cluster 10.

(10) Staggered along the central barrel 14, the internal additional supports 19 can be made of graphite paper. They are individually presented as a solid annular disc.

(11) The central barrel 14 is defined by a solid rod 28a which passes axially through an upper central block 30 of the cluster 10 from which the segments 16 extend radially in a star pattern down to the bottom, where the rod 28a is extended by a screw 28b screwed with a nut 32, both supported on a plate 34 which can be made of aluminium. The nut is locked in rotation in the plate.

(12) The plate 34 is a support, like a plate, for the cluster 10, which rests and stands on this plate. In this case, the plate 34 is a flat and solid plate that extends all around the axis 26, under the whole cluster; see FIGS. 1 and 2.

(13) The plate 34 is evacuated when the wax is removed. It is then through its ceramic bottom 78 (see FIG. 3 and details below) that the cluster 10, placed in a so-called solidification furnace to achieve the expected solidification, will come into direct contact with the bottom of the furnace, this bottom 78 ensuring tightness.

(14) Before that, the wax cluster 10 will have been coated with slip and then covered with refractory sand, repeating these two actions a number of times until a satisfactory thickness of ceramic material constituting a shell is obtained, such as the one marked 40 in FIG. 3.

(15) The shell 40, obtained after this first operation, is an intermediate shell, which includes a first part of a shell 42, second parts of a shell 44, third parts of a shell 46, and fourth parts of a shell 48.

(16) The first part of a shell 42 corresponds to the central wax barrel 14. It is essentially in the form of a hollow erect, here vertical, tube 50, delimiting an elongated cavity. It ends at the lower end with a lower flared part delimiting a lower extended area 52 of the cavity 50. It ends at the upper end with an upper flared part delimiting an upper widened area of the cavity 50 forming a material inlet upper neck 54.

(17) The second shell parts 44 correspond to the wax models 12 and are in the form of hollow moulds delimiting the moulding cavities of the parts, thus having substantially the hollow shape of the parts to be manufactured. They are distributed around the first part of a shell 42.

(18) The third part 46 of the shell corresponds to the wax grain selection areas 18. They each have an intermediate part 58, which is substantially in the form of hollow and double-angled vertical tubes delimiting a cavity 60 in the shape of a baffle, a flared lower part delimiting an enlarged lower zone 62 of the cavity 60. The third parts of a shell 46 constitute selector assemblies and grain ducts.

(19) The fourth parts 48 of the shell correspond to the wax segments 16. They are in the form of channels 66 connected at each end to the highest part 44a of each second shell part 44 and the first shell part 42 respectively. They are inclined so that their end connected to the first shell part 42 is higher than their end connected to the top of the second shell part 44.

(20) The cavities 56 (also called second peripheral hollow tubes) distributed around the cavity 50 (also called first central tube) are therefore connected to the cavity 50 and the upper neck 54 by the material flow channels 66, as well as to said third shell parts 46 which therefore form lower material flow parts up to the plate 34. The first central tube 50 is plugged by the solid rod 28a; thus, the metal cannot flow into it.

(21) Such an intermediate shell 40 is made around the wax cluster 10 including the central barrel 14, models 12, grain selection areas 18 and segments 16.

(22) Therefore, the first, second, third and fourth shell parts 42, 44, 46, 46, 48 are in communication with each other and delimit a global cavity combining the cavities and/or parts of cavities 50, 52, 54, 56, 60, 62, 66.

(23) If additional internal supports 19 exist, the shell 40 has additional parts of shell 68 similar to shells of a substantially annular shape. Individually their section has a significant U shape. They are made of the same material as the rest of the shell.

(24) The additional shell parts 68, all located inside the internal recess 70 peripherally delimited by the models 12 extended by the grain selection areas 18 and, at the top, by the segments 16, act on the thermal parameters of the solidification.

(25) The additional parts of the shell 40 can: act as obstacles to thermal radiation, limit the cooling of the shell 10, facing the internal recess 70, behave like thermal lenses, allow to locally control and modify the shape of the solidification front, and therefore to avoid porosity defects at the end of the solidification.

(26) Once such a shell 40 has been made on the plate 34, they are both placed in a solidification furnace. It can be an induction furnace with hot and cold zones separated from each other by an insulating screen, the walls of the hot zone being equipped with devices capable of generating thermal radiation towards this hot zone.

(27) The plate 34 is able to move in a direction parallel to the vertical axis 26, inside the cold zone of the solidification furnace.

(28) Resting on the plate 34, the ceramic shell 40 will then receive molten metal during the so-called casting stage.

(29) However, during the solidification phase when casting the alloy concerned into the shell 40 for the manufacture of parts (in particular monocrystalline parts), a shrinkage occurs on the parts containing the alloy and not (less) on the central part (the first part of the shell 42). This results in different mechanical pulls and leads to: deformations on the models (quality defect), grains recrystallized on the models (quality defect), model ruptures and breaks (scrap), leaks during casting (impact on production; shutdown of equipment for cleaning).

(30) This is even accentuated in the case of multi-storey clusters (3 stages in the case in point) and/or when the cavities 56 having substantially the hollow shape of the parts to be manufactured are very long, especially since it is necessary, after solidification of the alloy, to uncheck the shell 40 with a hammer in order to release the metal parts, here stepped together in a cluster, at the place of the cavities 56.

(31) In fact, there is often a mechanical stress problem between the centre of the cluster (down 14) and the rest of the cluster (model part 12 and power supply; channels 16). Dissociating the two at the location of a space 76, as shown in FIG. 3 (circled area), is one solution. An annular space at the bottom 78 that extends between the base of the central cavity 50 and that of each enlarged low zone 62 of the shell 40 is suitable. The difference in shrinkage between the center of the cluster (down 14) and the rest of the cluster, and thus the impact on the models, can then be controlled.

(32) However, there is still a problem concerning the way in which the space 76 is to be created, with a dual purpose: ensure the ability to produce clusters of parts of high heights, while integrating into conventional production processes, limit the mechanical forces on the shell 40 that can generate non-quality (scrap, etc . . . ).

(33) The simple and efficient solution of the invention is to use a deformable annular element placed on the plate 34, between the central barrel 14 and the grain selection areas 18, so that the annular space 76 is created there, at a place (location) therefore of the coating of ceramic paste whose cluster 10 will have been covered with wax. The paste coating is formed by a series of layers of ceramic paste.

(34) Thus, once coated, the ceramic paste will cover the wax cluster 10, except at the location of the annular space 76.

(35) The term annular covers the case of a ring extending by sectors (element then in several parts, which can communicate with each other if necessary) or an open ring, it is specified that, in at least a number of cases, the deformable annular element will still extend over a closed perimeter (see FIG. 2), as long as the annular space itself extends over a closed perimeter. It will therefore be a totally closed ring, which can favour thermal and mechanical stress control in the cluster 10 and the shell 40, and therefore in the final moulded parts.

(36) Defining the annular element 82 as an inflatable bladder 821 (or inner tube) will allow the deformation(s) of this annular element 82 to be carried out in a relevant manner, at a distance and at will.

(37) For its maintenance, whatever its condition, the deformable annular element 82 can be provided with end pieces 84 which cross the plate 34, to open at the opposite side of the face where the cluster 10 or the shell 40 stands. This avoids interfering with the bottom realization area 78.

(38) An annular notch 85 on the upper side of the plate 34 (FIG. 1) may also be used to stabilize the position of the element 82.

(39) For similar considerations, it is proposed that the inflatable bladder 82 should be connected, through the plate 34 and through a connection 820, to a source of inflation fluid 86; see FIG. 2.

(40) Concerning the way to create space 76, it is recommended: first, to place said deformable annular element 82 on the support plate 34, then to cover the plate 34 with ceramic paste, by successive deposits of layers 41 (FIG. 1), in order to prepare the creation of said base 78, the ceramic paste being, at this time, deposited around the different parts 12,14,16,18,19 of the cluster 10, so that these parts are also covered with paste, at each layer deposit, to fire the layer of ceramic paste deposited, and, between two deposits of said ceramic paste, deforming said annular element 82 to break the ceramic layer.

(41) Thus, once the space 76 materialized by the annular element 82 placed on the plate 34, this space 76 can be maintained at each successive deposit and firing, since the ceramic layer formed around the element 82 is broken.

(42) In particular, to avoid as much as possible interfering with the ceramic areas forming around the element 82 and for efficiency, it is proposed that, if an inflatable bladder 821 is used, it should be inflated between two deposits of said ceramic paste.

(43) The surrounding mechanical stresses will then be very limited and the presence of the space 76 will ensure that the shell itself has mechanical stability free of such mechanical stresses.

(44) To further increase the effectiveness of the solution, it is recommended to inflate the bladder 821 after firing each deposit of said ceramic paste.

(45) In this way, during the moulding process and between two successive layers of dried ceramic, the bladder 821 will be blown into to break the deposited ceramic layer. Repeating the operation after each layer, after moulding and before waxing, will allow clean breaks to be made on a limited thickness of material.

(46) At the end of the moulding operation, and in the lower part, the central part 42 and the centre of the bottom 78 of the shell will be separated from the rest of the cluster (shell parts 46 and 44).

(47) After the wax removal operation, the element 82 is evacuated with the plate.

(48) A shell cut between the descendant 14 (and therefore the central part 42) and the model part (second shell parts 44) is thus obtained.