BACKGROUND OF THE INVENTION
The present invention relates to the field of 5 casting, and more particularly to a pattern for lost-pattern casting, and also to methods of fabricating shell molds, and to methods of casting using such a pattern.
So-called "lost-wax" or "lost-pattern" casting methods have been known since antiquity. They are
10 particularly suitable for producing metal parts that are complex in shape. Thus, lost-pattern casting is used in particular for producing turbine engine blades.
In lost-pattern casting, the first step normally comprises making a pattern out of a material having a
15 melting temperature that is comparatively low, such as
for example out of wax or resin, and then overmolding the mold onto the pattern. After removing the material of the pattern from the inside of the mold, whence the name of such methods, molten metal is cast into the mold in
20 order to fill the cavity that the pattern has formed inside the mold by being removed therefrom. Once the metal has cooled and solidified, the mold may be opened or destroyed in order to recover a metal part having the shape of the pattern. In the present context, the term
25 "metal" should be understood to cover not only pure metals but also, and above all, metal alloys.
In order to be able to make a plurality of parts simultaneously, it is possible to unite a plurality of patterns in a single assembly in v-jhich they are connected
30 together by a tree that forms casting channels in the mold for the molten metal.
Among the various types of mold that can be used in lost-pattern casting, so-called "shell" molds are known that are formed by dipping the pattern or the assembly of
35 patterns into a slip, and then dusting refractory sand onto the pattern or the assembly of patterns coated in the slip in order to form a shell around the pattern or

the assembly, and then baking the shell in order to solidify the slip and thus consolidate the slip and the sand. Several successive operations of dipping and dusting may be envisaged in order to obtain a shell of 5 sufficient thickness prior to baking it. The term "refractory sand" is used in the present context to designate any granular material of grain size that is sufficiently small to satisfy the desired production tolerances, that is capable, while in the solid state, of
10 withstanding the temperature of the molten metal, and
that is capable of being consolidated into a single solid piece by the slip during baking of the shell.
In order to obtain particularly advantageous thermomechanical properties in the part produced by
15 casting, it may be desirable to ensure that the metal undergoes directional solidification in the mold. The term "directional solidification" is used in the present context to mean that control is exerted over the nucleation and the growth of solid crystals in the molten
20 metal as it passes from the liquid state to the solid
state. The purpose of such directional solidification is to avoid the negative effects of grain boundaries within the part. Thus, the directional solidification may be columnar or monocrystalline. Columnar directional
25 solidification consists in orienting all of the grain boundaries in the same direction so that they cannot contribute to propagating cracks. Monocrystalline directivity solidification consists in ensuring that the part solidifies as a single crystal, so as to eliminate
30 all grain boundaries.
Directional solidification is particularly desirable when producing parts that are to be subjected to high levels of thermomechanical stress, such as turbine engine blades. Nevertheless, the complex shapes of such blades
35 can interfere with directional solidification, giving
rise to unv/anted grains, in particular in the proximity of sharp corners in the blade. In particular, in a

turbine engine blade v;ith a root and a body on either side of a platform extending substantially perpendicularly to a main axis of the blade, said body presenting a pressure side, a suction side, a leading 5 edge, and a trailing edge, the sudden transition betv/een the body of the blade and the platform can cause such unwanted grains to form, in particular in the vicinity of the trailing edge.
In order to reduce the weight of turbine engine
10 blades, and above all in order to enable them to be
cooled, it is common practice to embed refractory cores in the non-permanent pattern. Such a refractory core remains inside the shell mold after the material of the pattern has been removed, and after the metal has been
15 cast and allowed to cool, thereby forming a hollow volume in the metal part. In particular, in order to provide good cooling of the trailing edge, given that its small thickness makes it particularly vulnerable to high temperatures, it is common practice for such a core to be
20 flush with the surface of the pattern at the trailing
edge so as to form a cooling slot for the trailing edge. Nevertheless, the small thickness of the core in this location makes it fragile. In addition, in order to keep the core in the correct position inside the shell mold
25 during casting and cooling of the metal, it is desirable to guide its thermal expansion. For this purpose, the pattern may include a guide strip adj acent to the trailing edge and having a varnished surface of the refractory core flush v/ith each side of the pattern
30 between the trailing edge and the expansion strip. The varnish on these surfaces, which may be removed from the shell mold together with the material of the pattern, ensures that there is a small amount of clearance (of the order of a few hundredths of a millimeter) between the
35 refractory core and the shell mold, so as to guide the expansion of the core at this location in a direction perpendicular to its thickness. Inside the expansion

strip, the core can be of greater thickness, thereby
making it more robust.
Nevertheless, the complexity betv;een the shape of
the mold cavity at the intersections of the trailing edge
5 or the expansion strip with the blade platform
significantly increases the risk of grains being
generated.
OBJECT AND SUMMARY OF THE INVENTION
10 The present invention seeks in particular to remedy these drawbacks. In particular, the invention seeks to provide a pattern that makes it possible to avoid unwanted grains forming in the proximity of intersections between the trailing edge or the expansion strip with the
15 platform of a turbine engine blade as produced from the pattern in a lost-pattern casting method.
In at least one embodiment of the present invention, this object is achieved by the fact that the pattern also includes a web extending between the platform and said
20 expansion strip and presenting a free edge between them. The term "web" is used in the present context to designate a wall that is very fine, i.e. of thickness that is substantially less than its other dimensions. The thickness of the web is nevertheless not necessarily
25 less than the thickness of the expansion strip.
By means of these provisions, it is possible to ensure a transition betv/een the trailing edge and the platform that is more gradual, avoiding sharp corners that might be at the origin of unwanted grains. Since
30 the raw casting that results from the casting method using such a pattern must in any event be machined subsequently in order to eliminate the expansion strip, this web can be eliminated in the same machining step without giving rise to additional operations.
35 Advantageously, the free edge of the v;eb may extend from one edge of the platform to the expansion strip, so as to avoid unwanted grains nucleating not only between

the platform and the trailing edge^ but also at the edge of the platform.
For better avoidance of unwanted grains forming, the pattern may present a progressive transition between a 5 free edge of the expansion strip and the free edge of the web. In addition, the web may be of thickness that is less than or equal to a thickness of the expansion strip, and the free edge of the web may be rounded in a transverse plane.
10 The pattern may also include an out-of-part segment extending the body from an end remote from the blade root, in particular in order to provide a smooth transition betv/een a selector channel and the body of the blade. Under such circumstances, the web may present
15 height that is no greater than half the height of the body together with the out-of-part segment.
In order to limit the number of corners that might generate unwanted grains, a junction between the web and the platform may extend a junction between the pressure
20 side and the platform.
In order to facilitate directional solidification, this casting pattern may also have a selector channel pattern that is connected to an end of the blade body that is opposite from the blade root. In a casting
25 method using a mold formed around this casting pattern,
it is possible, by progressively cooling the molten metal inside the mold from a starter cavity connected to the blade-shaped cavity via a selector channel, e.g. a baffle-shaped channel, to ensure that only one of the
30 grains that has nucleated in the starter cavity propagates into the blade-shaping cavity.
The invention also provides an assembly comprising a plurality of said casting patterns connected together by a tree so as to be capable of producing a plurality of
35 blades simultaneously.
The invention also provides a method of fabricating a shell mold, the method comprising the steps of dipping

at least one such casting pattern in a siip^ powdering the at least one slip-coated pattern with refractory sand in order to form a shell around at least one pattern, removing the at least one pattern, and baking the shell. 5 In addition, the invention also provides a casting method in which such fabrication of a shell mold is follov/ed by casting molten metal into the shell mold, cooling the metal with directional solidification thereof, knocking out in order to recover the rav; metal casting, and 10 finishing the raw casting. This finishing step may in particular include machining away the out-of-part elements from the rat-j casting.
BRIEF DESCRIPTION OF THE DRAWINGS
15 The invention can be well understood and its advantages appear better on reading the following detailed description of an embodiment given by way of non-limiting example. The description refers to the accompanying drav/ings, in which:
20 ■ Figure 1 is a diagram showing the implementation of a directional solidification casting method;
■ Figure 2 is a diagram showing an assembly of
casting patterns;
■ Figure 3 is a side view of a casting pattern in an
25 embodiment;
- Figure 4 is a view of an opposite side of the Figure 3 pattern;
• Figure 5 is a cross-section of the pattern of Figures 3 and 4 on line V-V; 30 • Figure 6 is a cross-section of the pattern of Figures 3 to 5 on line Vl-Vl; and
■ Figure 7 is a longitudinal section view on line
VII-VII of the portion of the pattern shown in Figures 3
to 6.
35
DETAILED DESCRIPTION OF THE INVENTION
Figure 1 shows hov/ progressive cooling of the molten

metal in order to obtain directional solidification can typically be performed in a casting method.
The shell mold 1 used in this method comprises a central descender 4 extending along the main axis X 5 betv/een a casting cup 5 and a plate-shaped base 6. While the shell mold 1 is being extracted from the heater chamber 3, the base 6 is directly in contact with a soleplate 2. The shell mold 1 also has a plurality of molding cavities 7 arranged as an assembly around the
10 central descender 4. Each molding cavity 7 is connected to the casting cup 5 by a feed channel 8 through which the molten metal is inserted while it is being cast. Each molding cavity 7 is also connected at the bottom via a baffle-selector channel 9 to a starter 10 formed by a
15 smaller cavity adjacent to the base 6.
The shell mold 1 may be produced by the so-called "lost-wax" or "lost-pattern" method, A first step of such a method is creating a non-permanent assembly 11 comprising a plurality of patterns 12 connected together
20 by a tree 13, as shovm in Figure 2. Both the patterns 12 and the tree 13 are for forming hollow volumes in the shell mold 1 and so they are made of a material having a low melting temperature, such as a patterning resin or wax. When it is intended to produce large numbers of
25 parts, it is possible in particular to produce these
elements by injecting the patterning resin or wax into a permanent mold,
In this implementation, in order to produce the shell mold 1 from the non-permanent assembly 11, the
30 assembly 11 is dipped in a slip, and then dusted with
refractory sand. These dipping and dusting steps may be repeated several times, until a shell of slip-impregnated sand of desired thickness has been formed around the assembly 11.
35 The assembly 11 covered in this shell can then be
heated so as to melt the low melting-temperature material of the assembly 11 and remove it from the inside of the

shell. Thereafter, in a higher temperature baking step, the slip is solidified so as to consolidate the refractory sand and form the shell mold 1.
The metal or metal alloy used in this casting method 5 is cast v/hile molten into the shell mold 1 via the
casting cup 5, and it fills the molding cavities via the feed channels 8. During this casting, the shell mold 1 is kept in a heater chamber 3, as shov/n in Figure 1. Thereafter, in order to cause the molten metal to cool
10 progressively, the shell mold 1 supported by a cooled and movable support 2 is extracted from the heater chamber 3 dovmwards along the main axis X. Since the shell mold 1 is cooled via its base 6 by the support 2, the solidification of the molten metal is triggered in the
15 starters 10 and it propagates upwards during the
progressive downward extraction of the shell mold 1 from the heater chamber 3. The constriction formed by each selector 9, and also its baffle shape, nevertheless serve to ensure that only one of the grains that nucleates
20 initially in each of the starters 10 is capable of
continuing so as to extend to the corresponding mold cavity 7.
Among the metal alloys that are suitable for use in this method, there are to be found in particular
25 monocrystalline nickel alloys such as in particular AMI and AM3 from Snecma, and also other alloys such as CMSX-2®, CMSX-4®, CMSX-6®, and CMSX-10® from C-M Group, Rene® N5 and N6 from General Electric, RR2000 and SRR99 from Rolls-Royce, and PWA 1480, 1484, and 1487 from Pratt
30 & Whitney, amongst others. Table 1 summarizes the compositions of these alloys:

Table 1: Monocrystalline nickel alloys in weight percentages

Ailoy Cr Co Mo W Al Tl Ta Nb Re Hr C B Ni
CMSX-2 8.0 5.0 0.6 8.0 5.6 1.0 6.0 _ . . . . Bal
CMSX-4 6.5 9.6 0.6 6.4 5.6 1.0 6.5 . 3.0 0.1 _ , Baf
CMSX-6 10.0 5.0 3.0 _ 4.8 4.7 6.0 . _ 0.1 . . Bal
CMSX-10 2.0 3.0 0.4 5.0 5.7 0.2 8.0 _ 6.0 0.03 _ _ Bal
Rene N5 7.0 8.0 2.0 5.0 6.2 . 7.0 3.0 0.2 . _ Bal
Rene N6 4.2 12.5 1.4 6.0 5.75 _ 7.2 _ 5.4 0.15 0.05 0.004 Bal
RR2000 10.0 15.0 3.0 _ 5.5 4.0 . . _ . . . Bal
SRR99 8.0 5.0 _ 10.0 5.5 2.2 12.0 _ _ . _ _ Bal
PWA1480 10.0 5.0 4.0 5.0 1.5 12.0 . . 0.07 Bal
PWA1484 5.0 10.0 2.0 6.0 5.6 . 9.0 . 3.0 0.1 . . Bal
PWA1487 5.0 10.0 1.9 5.9 5.6 _ 8.4 _ 3.0 0.25 _ _ Bal
AMI 7.0 8.0 2.0 5.0 5.0 1.8 8.0 1.0 . Bal
AM3 8.0 5.5 2.25 5.0 6.0 2.0 3.5 - - - - - Bal
5 After the metal has cooled and solidified in the
shell mold, the mold can be knocked out so as to release the metal parts, which can then be finished by machining and/or surface treatment methods.
When the parts for molding are of complex shapes,
10 they can nevertheless make the directional solidification of the metal in each mold cavity 7 more complicated. In particular, the sharp corners in the cavity 7 can lead to unwanted grains that v/eaken the part. In order to avoid such unv/anted grains forming, the patterns 12 in this
15 embodiment receive added elements that smooth certain sharp angles in the mold cavities 7. One such casting pattern 12 for producing a turbine engine blade is shown in Figures 3 and 4. This casting pattern 12 is thus in the shape of a turbine engine blade with a blade body 14
20 and a blade root 15 for fastening the blade to a turbine engine rotor. The blade body 14 has a suction side 16

10
and a pressure side 17 that meet along a leading edge 18 and a trailing edge 19. A platform 20 lies betv/een the blade body 14 and the blade root 15. The pattern 12 also has out-of-part elements, and in particular an expansion 5 strip 21 adjacent to the trailing edge 19 and an out-of-part segment 22 extending the blade body 14 at an end opposite from the blade root 15. This out-of-part segment 22 is for connection to the selector channel 9, and the blade root 15 is for connection to the feed
10 channel 8 so that in the mold cavity 7 formed by the
pattern 12 in the shell mold 1, the molten metal flows from the root of the blade 15 towards the blade body 14 during casting, and subsequently solidifies in the opposite direction during its directional solidification.
15 The pattern 12 also has a refractory solid core 23
for the purpose of forming a cavity in the turbine engine blade. On each side of the pattern 12, a varnished surface 31 of the core 23 is flush with the surface of the pattern 12 between the trailing edge 19 and the strip
20 21, as shown in Figures 5 and 6. During the steps of
dipping and dusting the pattern 12, the slip-impregnated sand shell forms on the exposed surfaces of the pattern 12, including on these varnished surfaces 31 of the core 23. During removal of the pattern and/or baking of the
25 shell, the varnish covering these surfaces 31 is also
eliminated, thereby leaving a small amount of clearance, typically lying in the range two to three hundredths of a millimeter, betv/een these surfaces 31 of the core and the corresponding inside surfaces of the shell mold 1. At
30 this location, this small clearance allov/s the core 23 to move perpendicularly to its thickness relative to the shell mold 1, thereby guiding the thermal expansion of ■ the core 23 during casting and cooling of the metal. Nevertheless, the small size of this clearance prevents
35 the molten metal from running between the core 23 and the shell mold 1 at this location. Thus, in the raw casting, the trailing edge and the strip are separated by a gap

11
that facilitates subsequent machining of the strip v/hile finishing the raw casting.
A particularly critical location for the formation of unwanted grains is in the proximity of the 5 intersection between the trailing edge 19 and the
platform 20. A plurality of sharp corners can meet at this location, thereby increasing the danger of unwanted grains forming. To avoid that, in the embodiment shown, the pattern 12 also has a fine web 24 between the strip
10 21 and the platform 20. This v/eb 24 presents a free edge 25 extending between the strip 21 and an end 26a of an edge 26 of the platform 20. The web 24 is of thickness el equal to or less than the thickness e2 of the adjacent strip 21. The height hi of the v/eb 24 is approximately
15 half the raw height h2 of the blade body 14 including the out-of-part segment 22. So long as the free edge 25 of the web 24 and the outside edge 27 of the strip 21 are rounded, as shown in Figures 5 and 6, the transition 28 between them is very progressive. The strip 21 and the
20 web 24 both follow the curvature, if any, of the trailing edge 19. The transition 29 between the web 24 and the platform 20 is rounded in the longitudinal plane, as shown in Figure 7, and runs on from the line of transition 30 between the suction side 17 and the
25 platform 20.
In the casting method used for producing at least one turbine engine blade from such a pattern, the web and the strip in the rav/ casting can easily be eliminated simultaneously by machining v/hile finishing the raw
30 casting. This makes it possible to obtain a clean part without it being necessary to perform more machining operations than would be required with a pattern that does not have the web 24.
Although the present invention is described with
35 reference to a specific embodiment, it is clear that various modifications and changes may be made thereto without going beyond the general ambit of the invention

12
as defined by the claims. In addition, individual characteristics of the various embodiments mentioned may be combined in additional embodiments. Consequently, the description and the drawings should be considered in an 5 illustrative sense rather than in a restrictive sense.

WE CLAIMS:-
1. A pattern (12) for lost-pattern casting, the pattern
being in the shape of a turbine engine blade v/ith a root
(15) and a body (14) on either side of a platform (20) 5 that is substantially perpendicular to a main axis of the blade, said blade body (14) presenting a pressure side (17), a suction side (16), a leading edge (18), and a trailing edge (19), the pattern (12) also including an expansion strip (21) adjacent to the trailing edge (19), 10 and a refractory core (23) embedded in the pattern (12)
but presenting, both on the pressure side (17) and on the suction side (16), a respective flush varnished surface (31) betv/een the trailing edge (19) and the expansion strip (21), the pattern (12) being characterized in that 15 it also includes a v/eb (24) extending between the platform (20) and said expansion strip (21) and presenting a free edge (25) between them.
2. A pattern (12) according to claim 1, wherein the free
20 edge (25) of the web (24) extends from one edge (26) of
the platform (20) to the expansion strip (21).
3. A pattern (12) according to any preceding claim,
presenting a progressive transition (28) between a free
25 edge (27) of the expansion strip (21) and the free edge (25) of the web (24).
4. A pattern (12) according to any preceding claim,
wherein the v/eb (24) is of thickness that is less than or
30 equal to a thickness of the expansion strip (21).
5. A pattern (12) according to any preceding claim,
wherein the free edge (25) of the web (24) is rounded in
a transverse plane.
35
6. A pattern (12) according to any preceding claim, also including an out-of-part segment (22) extending the body

14

10
15

(14) at an end opposite from the root (15) of the blade^ and wherein the vjeb (24) presents a height that is not greater than half the height of the body (14) including the out-of-part segment (22).
7. A pattern (12) according to any preceding claim, wherein a junction (29) between the web (24) and the platform (20) extends a junction (30) betv/een the suction side (16) and the platform (20).
8. A pattern (12) according to any preceding claim, v/ith a selector channel pattern (9) that is connected to an end of the blade body (14) that is opposite from the blade root (15).
9. An assembly (11) comprising a plurality of patterns (12) according to the preceding claims that are connected
together by a tree (13).

20 10. A method of fabricating a shell mold (1), the method comprising the following steps:
dipping at least one casting pattern (12) according to any one of claims ItoSinaslip
powdering the at least one slip-coated pattern (12) 25 v/ith refractory sand in order to form a shell around at least one pattern (12);
removing the at least one pattern (12); and baking the shell.
30 11. A casting method comprising at least the following steps:
fabricating a shell mold (1) in accordance with claim 10;
casting molten metal into the shell mold (1); 35 cooling the metal with directional solidification thereof;

15
knocking out the shell mold (1) in order to recover the rav; metal casting; and
finishing the raw casting.