1
FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
& The Patent Rules, 2003
COMPLETE SPECIFICATION
1. TITLE OF THE INVENTION:
IMPROVED METHOD FOR MANUFACTURING A SHELL MOLD FOR PRODUCTION BY
LOST-WAX CASTING OF BLADED ELEMENTS OF AN AIRCRAFT TURBINE ENGINE
2. APPLICANT:
Name: SAFRAN AIRCRAFT ENGINES
Nationality: France
Address: 2 boulevard du Général Martial Valin, 75015 Paris, France.
3. PREAMBLE TO THE DESCRIPTION:
The following specification particularly describes the invention and the manner in which it
is to be performed:
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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.
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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.
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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 stillmalleable
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.
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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:
- figure 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;
- figure 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 figure 1;
- figures 3 to 10 represent schematically different steps of the method for
manufacturing the shell mold; and
- figure 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 figure 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.
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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 figures 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 figure 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 12a. It
adopts a solid revolutionary, cylindrical or conical shape, having a central axis 14a aligned
with the central axis of the assembly of the wax pattern 100. This axis 14a is oriented
vertically, and thus considered to represent the direction of the height. This dispensing
portion 12a is attached directly to a specific tool 16, above which it is situated.
The portion 12a is terminated at the top by an end 18a of greater diameter, from
which a plurality of portions 20a extend radially for the formation of a plurality of casting
arms. The portions 20a are herein three in number, distributed at 120° about the axis 14a.
Each portion 20a thus includes a first end 21a connected to the enlarged end 18a of the
dispensing portion 12a, and extends in a straight or slightly curved manner up to the
second end 22a.
For each portion forming an arm 20a, a wax/ceramic securing reinforcement 23a
may be envisaged between the dispensing portion 12a and the second end 22a of the
portion 20a.
Furthermore, from each second end 22a, a wax replica 1a of the turbine blade
represented in figure 1 is attached. This replica 1a thus includes a blade 2a, extending
from an end 4a forming a blade root, and comprising a platform 8a. In figure 2, the blade
replicas 1a were only represented schematically.
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It is noted that while the replicas 1a have been represented with the blade root 4a
arranged at the bottom with respect to the blade 2a in the position in figure 3, this
root 4a 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 1a extend upward, being arranged about the axis 14a, and also
about a central wax supporting member 24a extending along the same axis from the
end 18a of the dispensing portion 12a. The supporting member 24a is preferentially in the
form of a rod having the axis 14a, which extends up to the vicinity of the blade heads 2a.
As seen in figure 2, for each wax blade 1a, a wax/ceramic securing
reinforcement 25a may be envisaged between the upper end of the central support
rod 24a, and the blade head. Similarly, wax/ceramic securing reinforcements (not shown)
may interconnect adjacent blade heads of the different blades 1a.
The wax blades 1a 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 14a, wherein the central support rod 24a is thus situated.
As represented schematically in figure 3, once the wax replica 100 has been
produced, a device 32a is assembled thereon intended to subsequently form a cup for
pouring metal into the shell mold. The device 32a includes a conical element 34a
centered on the axis 14a and flaring at the bottom from a small-sized section rigidly
connected to the lower end of the dispensing portion 12a. The conical element 34a is
preferably produced hollow, and closed at the lower end thereof by a cover 36a, the
external surface 40a whereof forms an end surface of the device 32a. Alternatively, the
device 32a could be produced solid, in a wax intended to be subsequently removed when
removing the wax pattern 100.
Optionally, reinforcement elements 42a may subsequently be produced between
the device 32a and the arms 20a.
The wax pattern 100 and the device 32a 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
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schematically in figure 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 32a 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 40a defined by the cover 36a of the
device 32a, as represented schematically in figure 5. A hot wax coating layer 46 also
covers the external surface of the conical element 34a.
One of the specificities of the invention consists of structuring at least the layer 46
covering the end surface 40a, when this layer is still malleable, i.e. before it has
completely cooled.
For this purpose, a tool as shown in figures 5, 5a, 5b 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 D1 thereof
and the external diameter D2 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 figure 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 40a 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 figure 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
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covering the end surface 40a of the cover 36a, 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 40a of the cover 36a, for example using goffering 64 as seen in figure 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 figures 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 figure 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 36a.
During the formation of the shell mold 300, at least one drying operation is
performed intended to dry same. This operation, represented schematically in figure 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 40a 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 40a.
After drying, the shell mold 300 which is obtained is represented schematically in
figure 11. It also has a general cluster shape, and obviously includes similar elements to
those of the wax replica 100 and the device 32a 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 32b, followed by the metal dispenser, referenced 12b.
The latter thus has a hollow revolutionary, cylindrical or conical shape, having a central
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axis 14b which is aligned with the central axis of the shell mold 300. This axis 14b is
oriented vertically, and thus considered to represent the direction of the height.
The dispenser 12b is terminated at the top with a hollow end 18b of greater
diameter, from which a plurality of metal casting arms 20b extend radially. The arms 20b
are herein three in number, distributed at 120° about the axis 14b. Each arm 20b thus
includes a first end 21b connected to the enlarged end of the dispenser 12b, and extends
in a straight or slightly curved manner up to a second end 22b.
Each arm 20b is thus envisaged to be hollow and form a metal supply duct after
removing the wax 20a. Herein also, a securing reinforcement 23b may be envisaged
between the dispensing portion 12b and the second end 22b of each arm 20b.
From each second end 22b, a bladed shell mold element 1b is situated. These
elements 1b are referred to as bladed as, after removing the wax replica 1a, they each
form internally an imprint corresponding to one of the blades 1.
The bladed element 1b, also referred to as shell mold blade, thus includes a blade
portion 2b defining adjacent blade imprints, this portion 2b extending from one end 4b
forming a blade root, and including a platform 8b. In figure 11, the shell mold blades 1b
have been represented only schematically.
The bladed elements 1b thus extend upward, being arranged about the axis 14b,
and also about a central supporting member 24b extending along said axis from the
end 18b of the dispenser 12b. The supporting member 24b preferentially takes the form
of a hollow cylinder having the axis 14b, which extends up to the vicinity of the ends 6b of
the bladed elements 1b.
Furthermore, for each bladed element 1b, a securing reinforcement 25b may be
envisaged between the upper end of the central support rod 24b, and the blade head.
Similarly, wax/ceramic securing reinforcements (not shown) may interconnect adjacent
blade heads of the different shell mold blades 1b. Finally, reinforcing elements 42b are
arranged between the cup 32b and the casting arms 20b.
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
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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 32b shown, with the shell mold in the inverted position with respect
to that shown in figure 11, i.e. with the cup 32b open at the top and once again the
axis 14b oriented vertically.
The molten metal thus successively travels through the cup 32b, the dispenser 12b,
the casting arms 20b, and the bladed shell mold elements 1b, merely flowing
gravitationally. It is noted that prior to casting, the central supporting member 24b
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 20b before entering the bladed
elements 1b. The reinforcements 23b, 25b, 42b 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.
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WE CLAIM:
1. Method for manufacturing a shell mold (300) for the production by lost-wax
casting of a plurality of bladed elements (1) of an aircraft turbine engine, said shell mold
in cluster form comprising a plurality of bladed shell mold elements (1b) each intended to
obtain one of said bladed turbine engine elements (1), said method comprising the
following steps:
a) creating an assembly (200) about which the shell mold is intended to be formed,
the assembly including a wax pattern (100) as well as a device (32a) for subsequently
forming a cup for pouring metal (32b), said device having an end surface (40a);
b) depositing a hot wax coating layer (46) around at least one portion of said
assembly (200), such that said coating layer (46) covers at least one portion of the end
surface (40a) of the device intended to subsequently form the cup for pouring metal; and
c) forming the shell mold (300) about said assembly (200);
characterized in that the method further includes, between steps b) and c), the
implementation of a step of structuring the coating layer (46) covering said end
surface (40a), this structuring step being intended to reinforce the adhesion between this
layer (46) and the shell mold to be formed, and including the production of recesses (62)
and projections (60) on the still-malleable coating layer.
2. Method according to claim 1, characterized in that the step for structuring the
coating layer (46) is implemented by inserting a plurality of imprinting elements (52) in
said still-malleable coating layer, causing the formation of said projections (60) about the
imprinting elements, then by removing the latter revealing recesses (62), each
surrounded by one of said projections (60).
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3. Method according to claim 2, characterized in that the imprinting elements are
studs (52), preferably with an external surface head (54) having a general spherical cap
shape.
4. Method according to claim 3, characterized in that the ratio between the
maximum external diameter (D1) of each stud (52), and the external diameter (D2) of the
end surface (40a) of the device (32a), is less than 20.
5. Method according to claim 3 or claim 4, characterized in that the number of
studs (52) is between 3 and 20.
6. Method according to any one of claims 2 to 5, characterized in that the step for
structuring the coating layer (46) is implemented by applying a pressure from a
supporting member (50) bearing the plurality of imprinting elements (52), against said
still-malleable coating layer (46).
7. Method according to claim 6, characterized in that said application of pressure
is performed by moving said assembly (200), against the supporting member (50)
remaining stationary.
8. Method according to any one of the preceding claims, characterized in that the
step for forming the shell mold (300) about said assembly (200) includes at least one
drying operation performed at least in part with said end surface (40a) facing downward,
and preferably with said shell mold, surrounding the assembly, moved inside a drying
station (70).
9. Method according to any one of the preceding claims, characterized in that the
step for forming the shell mold (300) is performed by dip coating.
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10. Method for manufacturing by lost-wax casting a plurality of bladed
elements (1) of an aircraft turbine engine, characterized in that it includes the production
of a shell mold (300) using a method according to any one of the preceding claims,
followed by casting of metal in the shell mold.
Dated this 02nd day of January, 2017.

(CHETAN CHADHA)
PATENT AGENT
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ABSTRACT
IMPROVED METHOD FOR MANUFACTURING A SHELL MOLD FOR PRODUCTION BY LOSTWAX
CASTING OF BLADED ELEMENTS OF AN AIRCRAFT TURBINE ENGINE
The invention relates to 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:
a) creating an assembly (200) including a wax pattern (100) as well as a device for
forming a cup for pouring metal (32b) and having an end surface (40a);
b) depositing a hot wax coating layer on at least one portion of the end
surface (40a);
c) 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 said
layer (46) and the shell mold, and including the production of recesses (62) and
projections (60) on the still-malleable coating layer. Figure 8 is the representative figure.