SHELL MOULD HAVING A HEAT SHIELD
Background of the invention
' The present invention relates to the field of
5 casting, and more particularly to a shell mold, and to
methods of manufacturing and using such a shell mold.
So-called "lost-wax" or "lost-pattern" casting
methods have been known since antiquity.. They are
particularly suitable for producing metal parts that are
10 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
melting temperature that is comparatively low, such as
15 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
order to fill the cavity that the pattern has formed
20 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 order to be able to make a plurality of parts
25 simultaneously, it is possible to unite a plurality of
patterns in a single assembly in which they are connected
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
30 lost-pattern casting, so-called "shell", molds are known
that are formed by dipping the pattern or the assembly of
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
35 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
2
dusting may be envisaged in order to.obtain a shell of
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
5 small enough to satisfy the desired production
tolerances, that is capable, while in the solid state, of
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.
10 The term "metal" is used in the present context to
designate both pure metals and metal alloys, and in
particular metal alloys known as monocrystalline alloys
such as those developed since the end of 1970s in order
to enable parts to be cast in the form of a single grain.
15 Conventional metal alloys are equiaxed polycrystallines:
in their solid state, they form a plurality of grains of
substantially identical size, typically about
1 millimeter (mm), but of more or less random
orientation. The boundaries between grains constitute
20 weak points in a metal part made out of such an alloy.
However, using additives to strengthen these inter-grain
boundaries presents the drawback of reducing the melting
point temperature, which is a disadvantage, particularly
when the parts produced in this way are for use at high
25 temperature. Typically, monocrystalline alloys are
nickel alloys with a concentration of titanium and/or
aluminum that is lower than 10 molar percent (%mol).
Thus, after solidifying, these alloys form two-phase
solids, with a y first phase and a y' second phase. The y
30 phase presents a face-centered cubic crystal lattice, in
which the nickel, aluminum, and/or titanium atoms may
occupy any position. In contrast, in the y1 phase, the
aluminum, and/or titanium atoms form a cubic
configuration, occupying the eight corners of the cube,
35 while the nickel atoms occupy the faces of the cube.
One of these alloys is the nickel alloy "AMI"
developed.jointly by SNECMA and the ONERA laboratories,
3
the Ecole des Mines in Paris, and IMPHY SA. Parts made
of such an alloy can not only achieve mechanical strength
that is particularly high along all stress axes, but can
: also achieve improved thermal resistance, since additives
5 for binding the crystalline grains together more strongly
may be omitted. Thus, metal parts made of such
monocrystalline alloys may advantageously be used, e.g.
in the hot parts of turbines.
Nevertheless, in order to benefit fully from the
10 advantages of monocrystalline alloys in order to obtain
advantageous thermomechanical properties in a part made
by 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
15 context to mean that control is exerted over the
nucleation and the growth of solid crystals in the molten
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
20 the part. Thus, the directional solidification may be
columnar or monocrystalline. Columnar directional
solidification consists in orienting all of the grain
boundaries in the same direction so that they cannot
contribute to propagating cracks. Monocrystalline
25 directional solidification consists in ensuring that the
part solidifies as a single crystal, so as to eliminate
all grain boundaries.
The published specification of French patent
application FR 2 874 340 describes a shell mold that is
30 particularly adapted to implementing a casting method
with directional solidification. That shell mold of the
prior art includes a central cylinder extending, along a
main axis, between a casting cup and a base, and a
plurality of molding cavities arranged as an assembly
35 around the central cylinder, each one connected to the
casting cup by a feed channel. In order to enable
directional solidification of molten metal in the molding
4
cavities, each of them is also connected via a baffleselector
to a starter adjacent to the base. Furthermore,
the shell mold also includes at least one heat shield
; that is. substantially perpendicular to said main axis.
5 In a casting method using said shell mold, after
casting the molten metal through the casting cup, the
molten metal is cooled progressively, along said main
axis from the base towards the casting cup. By way of
example, this may be performed by gradually extracting
10 the shell mold from a heater chamber, along the main
axis, towards the base, while cooling the base.
Because the molten metal is cooled progressively
going away from the plate, the first solid grains
nucleate in the starters adjacent to the plate. The
15 configuration of the baffle-selectors then prevents
propagation of more than a single grain towards each
molding cavity.
The purpose of using at least one heat shield is to
try to ensure that the propagation front of the
20 crystallization in each molding cavity remains
substantially perpendicular to the main axis. A sloping
propagation front would be likely to cause unwanted
grains to nucleate in the molding cavity. However, it is
nevertheless found to be difficult to prevent such
25 sloping, in particular in molding cavities that are
complex in shape.
Object and summary of the invention
The invention seeks to overcome those drawbacks, and
30 in particular to provide a shell mold that makes it
possible to ensure directional solidification of the
molten metal in the molding cavities of the shell mold,
and to do so in a general manner.
In at least one embodiment, this object is reached
35 by means of the fact that at least one heat shield
completely surrounds each molding cavity in a plane that
is substantially perpendicular to said main axis.
5
By means of these provisions, it is possible to
obtain temperatures that are substantially uniform over
the periphery of each molding cavity in each plane that
is perpendicular to the main axis, thus contributing to
5 maintaining the orientation of the propagation front of
the crystallization inside the molding cavity in such a
manner as to avoid unwanted grains forming.
In order to maintain the orientation of the
propagation front in each molding cavity, in particular
10 when they are relatively long, the shell mold may
comprise at least two heat shields that are substantially
perpendicular to said first direction, with an offset
between them in said first direction, and each completely
surrounding each said molding cavity in a plane that is
15 substantially perpendicular to said main axis. In order
to facilitate production of the shell mold, these heat
shields may, in particular, be substantially identical,
i.e. sufficiently similar as to be interchangeable.
The at least one heat shield may include stiffeners
20 so as to support it in the direction of the main axis of
the shell mold. In order to adapt the molding cavities,
the at least one heat shield may present a through
orifice around each molding cavity.
The base of the shell mold may form a plate for
25 supporting it and also for providing a metal that is cast
into the shell mold with good thermal contact with a
cooled soleplate under the shell mold. This then enables
the metal to be cooled from below while the shell mold is
being extracted from a heater chamber, so as to ensure
30 directional solidification of the molten metal inside the
shell mold. Furthermore, the shell mold may also include
additional stiffeners, connecting the molding cavities to
a tip of the casting cup.
The present invention also provides a method of
35 fabricating such a shell mold, and in particular a method
comprising: making a non-permanent assembly comprising a
plurality of models connected together by a tree; dipping
6
the assembly in a slip; dusting the slip-coated assembly
with refractory sand to form a shell around the assembly;
removing the assembly; and baking the shell. The steps
: of dipping and dusting may be repeated several times in
5 order to obtain a desired thickness of the shell. The
non-permanent assembly may be removed in conventional
manner by melting the material of the assembly, said
material having a melting point that is comparatively
low.
10 In particular, in order to facilitate forming each
heat shield, each heat shield may also be formed around a
non-permanent disk, e.g. made out of a material having a
low melting point, like the assembly.
The present invention also relates to a casting
15 method using such a shell mold and comprising: casting
molten metal into the shell mold through the casting cup;
and progressively cooling the molten metal along said
main axis, from the base towards the casting cup. In
particular, the step of progressively cooling the molten
20 metal is performed by gradually extracting the shell mold
from a heater chamber, along the main axis, in the
direction of the plate, while cooling the base.
Brief description of the drawings
25 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 drawings, in which:
30 • Figure 1 is a diagram showing a step of
progressively cooling molten metal in a directional
solidification casting method;
• Figures 2A and 2B show, respectively, a desirable
progression.and a non-desirable progression of the
35 propagation front of the crystallization of the metal in
a molding cavity during the progressive cooling of
Figure 1;
7
/
• Figure 3 is a longitudinal section view of a shell
mold in an embodiment of the invention;
• Figure 4 is a side view of the shell mold of
: Figure1 3; •
5 • Figure 5 is a perspective view of a non-permanent
core for forming a heat shield of the shell mold of
Figures 3 and 4; and
• Figure 6 is a perspective view of a non-permanent
assembly that is used to form the shell mold of Figures 3
10 and 4.
Detailed description of the invention
Figure 1 shows how progressive cooling of molten
metal in order to obtain directional solidification can
15 typically be performed in a casting method. In this
progressive cooling step, following casting of the molten
metal into a shell mold 1, said shell mold 1, supported
by a cooled and movable support 2 is extracted from a
heater chamber 3 downwards along a main axis X.
20 The shell mold 1 comprises a central cylinder 4
extending along the main axis X between a casting cup 5
and a plate-shaped base 6. During removal of the shell
mold 1 from the heater chamber 3, this base 6 is directly
in contact with the support 2. The shell mold 1 also
25 includes a plurality of molding cavities 7 arranged as an
assembly around the central cylinder 4. Each molding
cavity 7 is connected to the casting cup 5 by a feed
channel 8 through which the molten metal was introduced
during casting. Each molding cavity 7 is also connected
30 at the bottom via a baffle-selector 9 tp a starter 10
formed by a smaller cavity in the base 6.
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 starters 10 and it propagates upwards
35 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,
8
nevertheless serve to ensure that only one of the grains
that nucleates initially in each of the starters 10 is
capable of continuing so as to extend to the
; corresponding mold cavity 7.
5 Figure 2A shows the desirable progression of the
propagation front 11 of the crystallization of molten
metal in a molding cavity 7 in the shape of a turbine
engine fan blade. In order to obtain a monocrystalline
turbine engine blade, it is desirable for said
10 crystallization to advance in regular manner along the
main axis of the molding cavity 7. In contrast, if the
propagation front 11 is sloping while it is advancing
into the molding cavity 7, as shown by way of comparison
in Figure 2B, the risk of generating unwanted grains 12
15 in "certain areas of the molding cavity 7 increases
substantially. Unfortunately, temperature gradients
perpendicular to the main axis of the molding cavity 7
Can easily cause the propagation front 11 to slope in
this way. It is thus desirable in particular to control
20 the way heat is radiated from the various elements of the
shell mold 1.
Figures 3 and 4 show a shell mold 1 in an embodiment
of the invention. This shell mold 1 includes two heat
shields 13 that extend perpendicularly to the main axis X
25 starting from the central cylinder 4. .The two heat
shields 13 are situated at the height of the molding
cavities 7 with a longitudinal offset d along the main
axis X. The diameter of each of the two heat shields 13
is such that they extend radially beyond the walls of
30 each molding cavity 7. Thus, each heat, shield 13
completely surrounds each molding cavity 7 in a
transverse plane that is perpendicular to the main axis
X. However, to prevent heat being conducted directly
between the walls of the molding cavity 7 and the heat
35 shield 13, a transverse gap may separate said walls from
each heat shield 13 in said transverse plane all around
each molding cavity.
9
In the embodiment shown, each heat shield 13 was
formed around a disk 14 made of wax, such as that shown
in Figure 5. The two disks 14 may be substantially
:; identical. The disk 14 shown presents a plurality of
5 through orifices 15, each corresponding to a molding
cavity 7, a central cylinder with a positioning and
holding spline 17, and webs 18 extending radially, and
perpendicularly to the transverse plane in order to form
stiffeners for ensuring that each heat shield 13 is rigid
10 along the main axis X.
The base 6 of the shell mold 1 is plate-shaped.
Furthermore, stiffeners 20 in the shape of sloping
columns connect the top of each molding cavity 7 to that
of the casting cup 5.
15 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 21
comprising a plurality of patterns 22 connected together
by a tree 23, as shown in Figure 6. The parts of the
20 tree 23 for forming hollow volumes in the shell mold 1,
such as in particular the casting cup 5, the feed
channels 8, the stiffeners 20, the selectors 9, the heat
shields 13, and the starters 10 are made of a material
having a low melting point, such as a modeling wax or
25 resin. The models 22, that are to form the molding
cavities 7, are made of a material having a low melting
point. When it is intended to produce large numbers of
parts, it is possible in particular to produce these
elements by injecting the patterning resin or wax into a
30 permanent mold. By means of the positioning and holding
spline 17, each disk 14 can be correctly positioned, with
its orifices 15 in alignment with the models 22.
In this implementation, in order to produce the
shell mold 1 from the non-permanent assembly 21, the
35 assembly 21 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
10
sand of desired thickness has been formed around the
assembly 21.
The assembly 21 covered in this shell can then be
,; heated' so as to melt the low melting-temperature material
5 of the assembly 21 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 of Figures 3
and 4.
10 The shell mold 1 can then be used in a casting
method in which molten metal is initially cast in the
shell mold 1 through the casting cup 5, so as to then be
subjected to directional solidification in the manner
shown in Figure 1. The metal alloys that are suitable
15 for use in this method include in particular
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
20 from Rolls-Royce, and PWA 1480, 1484, and 1487 from Pratt
& Whitney, amongst others. Table 1 summarizes the
compositions of these alloys:
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.
Although the present invention is described with
10 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 as defined by the claims. Consequently, the description and the drawings should be considered in an illustrative sense rather than in a restrictive sense.

CLAIMS
1. A shell mold (1) comprising:
• a central cylinder (4) extending, along a main
axis (X), between a casting cup (5) and a base (6);
• a plurality of molding cavities (7) arranged as a
cluster around the central cylinder (4), each one being
connected to the casting cup (5) by at least a feed
channel (8), and via a baffle-selector (9) to a starter
(10) in the base (6); and
• at least one heat shield (13) that is
substantially perpendicular to said main axis (X);
the shell mold (1) being characterized in that said
at least one heat shield (13) completely surrounds each
said molding cavity (7) in a plane that is substantially
perpendicular to said main axis (X).
2. A shell mold (1) according to claim 1, comprising at
least two heat shields (13) that are substantially
perpendicular to said first direction, with an offset
between them in the direction of said main axis (X), and
each completely surrounding each said molding cavity (7)
in a plane that is substantially perpendicular to said
main axis (X).
3. A shell mold (1) according to claim 1 or claim 2,
wherein at least one heat shield (13) includes stiffeners
(18) .
4. A shell mold (1) according to any one of claims 1 to
3, wherein at least one heat shield (13) presents a
through orifice (15) around each molding cavity.
5. A method of fabricating a shell mold (1) according to
any preceding claim, comprising the following steps:
• making a non-permanent cluster (21) comprising a
plurality of models (22) connected by a tree (23);
• dipping the cluster (21) in a slip;
:13
• dusting the^ slip-coated cluster (21) with
refractory sand in order to form a shell around the
assembly (21);
• removing the cluster (21); and
•"baking the shell.
6. A method of fabricating a shell mold (1) in accordance
with claim 5, wherein each heat shield (13) is formed
around a non-permanent disk (14).
7. A casting method using a shell mold (1) according to
any one of claims 1 to 4, comprising the following steps:
• casting molten metal into the shell mold (1)
through the casting cup (5); and
• progressively cooling the molten metal along said
main axis (X), from the base (6) towards the casting cup.
(5) .
8. A method according to claim 1r wherein the step of
progressively cooling the molten metal is performed by
.gradually extracting the shell mold (1) from a heater
chamber (2), along the main axis (X), in the direction of
the base (6), while cooling the base (6).